Evolutionary Trends and Future Directions in the Cyclization Synthetic Methodologies for Triphenylene Derivatives

Evolutionary Trends and Future Directions in the Cyclization Synthetic Methodologies for Triphenylene Derivatives | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 5 June 2025 V1 Latest version Share on Evolutionary Trends and Future Directions in the Cyclization Synthetic Methodologies for Triphenylene Derivatives Authors : Huicheng Cheng 0000-0003-2866-0820 [email protected] , Xu-Ming Zhou , Peng-Hu Guo , Jiao-Li Ma , and Ji-Cheng Shi Authors Info & Affiliations https://doi.org/10.22541/au.174911430.09572079/v1 248 views 184 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract π-Extended triphenylene frameworks, representing a privileged class of pericondensed polyarenes, serve as cornerstone structures in modern optoelectronic materials, bioactive molecule design, and supramolecular engineering. Their rigid C3-symmetric topology and delocalized π-surfaces enable unique charge transport characteristics and photonic responses distinct from lower-dimensional analogs like biphenyl or fluorene systems. While classical syntheses employing Friedel-Crafts trimerization or oxidative cyclodehydrogenation face challenges including step inefficiency, stoichiometric waste, and limited functional group tolerance, recent paradigm-shifting advances in transition-metal-catalyzed annulative cross-couplings have unlocked atom-economical routes to these fused polycycles. This review provides a comparative analysis of state-of-the-art catalytic strategies for triphenylene synthesis, with particular focus on palladium/nickel-mediated C-H activation protocols versus radical-based photoredox cascades. Mechanistic divergences between oxidative homocoupling of haloarenes and directing-group-assisted heterocoupling are examined through stereoelectronic arguments, addressing regiochemical control in π-extension processes. Substrate compatibility are critically mapped across metal catalysis, highlighting competing π- versus σ-activation pathways in fused-ring formation. Future directions propose the synergistic integration of machine learning-guided catalyst design, operando XAS/EPR spectroscopy, and electric-field-assisted assembly to transcend current synthetic limitations, ultimately enabling precision engineering of triphenylene-based quantum materials and bioresponsive nanosystems. Cite this paper: Chin. J. Chem. 2025 , 40 , XXX—XXX. DOI: 10.1002/cjoc.202500XXX Evolutionary Trends and Future Directions in the Cyclization Synthetic Methodologies for Triphenylene Derivatives Hui-Cheng Cheng*, Xu-Ming Zhou, Peng-Hu Guo, Jiao-Li Ma* and Ji-Cheng Shi College of Chemistry, Guangdong University of Petrochemical Technology, Maoming 525000, PR China. π-Extended triphenylene frameworks, representing a privileged class of pericondensed polyarenes, serve as cornerstone structures in modern optoelectronic materials, bioactive molecule design, and supramolecular engineering. Their rigid C3-symmetric topology and delocalized π-surfaces enable unique charge transport characteristics and photonic responses distinct from lower-dimensional analogs like biphenyl or fluorene systems. While classical syntheses employing Friedel-Crafts trimerization or oxidative cyclodehydrogenation face challenges including step inefficiency, stoichiometric waste, and limited functional group tolerance, recent paradigm-shifting advances in transition-metal-catalyzed annulative cross-couplings have unlocked atom-economical routes to these fused polycycles. This review provides a comparative analysis of state-of-the-art catalytic strategies for triphenylene synthesis, with particular focus on palladium/nickel-mediated C-H activation protocols versus radical-based photoredox cascades. Mechanistic divergences between oxidative homocoupling of haloarenes and directing-group-assisted heterocoupling are examined through stereoelectronic arguments, addressing regiochemical control in π-extension processes. Substrate compatibility are critically mapped across metal catalysis, highlighting competing π- versus σ-activation pathways in fused-ring formation. Future directions propose the synergistic integration of machine learning-guided catalyst design, operando XAS/EPR spectroscopy, and electric-field-assisted assembly to transcend current synthetic limitations, ultimately enabling precision engineering of triphenylene-based quantum materials and bioresponsive nanosystems. Triphenylene derivatives | Cyclization strategy |Transition metal catalysis | Synthetic methods. Left to Right: Jiao-Li Ma , Xu-Ming Zhou, Hui-Cheng Cheng Peng-Hu Guo (top), Ji-Cheng Shi (bottom) Huicheng Cheng is an associate professor in Guangdong University of Petrochemical Technology. He was born in 1985 in Henan, China. He obtained his M.S. Degree from Zhengzhou University in 2013 and received his Ph.D. degree from Nankai University in 2016. He joined Guangdong University of Petrochemical Technology in 2016. Currently, his research mainly focused on the development and application of new reactions for organic synthesis. Jicheng Shi is a Ph.D. from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, a Distinguished Professor of Guangdong Institute of Petrochemical Technology and a Ph.D. Supervisor of Fujian Normal University. He is a postdoctoral fellow at the Hong Kong Polytechnic University (supervisor: Prof. Xinzhi Chen), the University of Hong Kong (supervisor: Prof. Zhi Zhiming), the Technical University of Munich (supervisor: Prof. W. A. Herrmann; Humboldt Scholar), and Purdue University, USA (supervisor: Prof. E.-i. Negishi). He worked at Fujian Institute of Materials and Structures, Chinese Academy of Sciences, Dalian Institute of Chemical Physics and Fujian Normal University. He has published more than 100 papers in Angew. Chem. Int. Ed. and J. Catal. etc., and has been authorized more than 10 invention patents in China, the United States, Japan, Germany and France. Contents 1. Introduction Page 2-3. 2. Cyclization synthetic methodologies for triphenylene derivatives. Page 3-18. 2.1. Palladium-catalyzed cyclization synthetic methodologies for constructing triphenylene-based compounds. Page 3-14. 2.1.1 Palladium-catalyzed annulation of halogenated arenes for constructing triphenylene-based compounds. Page 3-6. 2.1.2 Palladium-catalyzed annulation of diaryliodide salts for constructing triphenylene-based compounds. Page 6-11. 2.1.3 Palladium-mediated bihalogenative cross-coupling methodology for constructing triphenylene-based compounds. Page 11-13. 2.1.4 Palladium-catalyzed annulation reaction of o-chloroaromatic carboxylic acids for constructing triphenylene-based compounds. Page 13-14. 2.2 Copper-catalyzed multiple oxidative methodology enabling the construction of triphenylene-fused polycyclic structures. Page 14-15. 2.3 Nickel-electrocatalyzed reductive strategy for constructing triphenylene-based compounds. Page 15-16. 2.4 Electronic oxidative coupling of the alkoxy substituents for constructing triphenylene-based compounds. Page 16. 2.5 Visible light-induced catalytic generation of tether-tunable distonic radical anions for constructing triphenylene-based compounds. Page 16-18. 3. Conclusion and outlook Page 18. 1. Introduction Triphenylene-based structures, distinguished by the fully conjugated phenylene frameworks with enforced planarity and threefold rotational symmetry, have emerged as pivotal building blocks in advanced materials chemistry. [1-9] These structurally rigid systems demonstrate exceptional electron delocalization and supramolecular ordering, enabling their extensive utilization in optoelectronic devices, functional coordination polymers, and energy storage systems. [10-14] The inherent donor-acceptor capabilities of oligomeric triphenylenes facilitate efficient photoinduced electron transfer processes with nitroaromatic quenchers, significantly enhancing fluorescence quenching dynamics (Figure 1, Compound 1). [15] In electroluminescent applications, BN-doped triphenylene derivatives (Compound 2) exhibit remarkable chromatic purity with CIE coordinates (0.26, 0.70) and a λmax at 528 nm, demonstrating exceptional potential for next-generation OLED displays. [16] Coordination chemistry applications leverage triphenylene-2,3,6,7,10,11-hexaamine as a hexatopic linker in conductive MOF structures for supercapacitor electrodes (Compound 3), [17] while its thiolated analog enables construction of wavy-layered 2D COFs (DUT-177) through dynamic S-Li interactions, creating redox-active sulfur sites for advanced Li-S battery technologies (Compound 4). [18] The molecular anisotropy of 6,7,10,11-tetrakis(dodecyloxy)triphenylene-2,3-diamine (Compound 5) drives spontaneous columnar mesophase formation through π-π stacking and alkyl chain interdigitation, establishing it as a prototype discotic liquid crystal. [19] These diverse implementations stem from triphenylene’s unique combination of electronic conjugation, topological control, and synthetic tunability-properties that continue to inspire innovations in molecular electronics and functional materials design. Figure 1 Potential triphenylene-based compounds in functional materials. Triphenylenes have garnered prominence as cornerstone components in materials science, enabling precise bottom-up engineering of functional π-extended frameworks that exhibit discotic mesomorphism and tunable optoelectronic properties. The strategic synthesis of functional polycyclic aromatic hydrocarbons (PAHs) has become paramount in contemporary materials design, driving intensive research into tailored PAH-based structures for next-generation organic electronics and supramolecular systems. [20-24] Notwithstanding the utility of biphenyl-based synthons, conventional Scholl oxidative cyclization approaches face three principal limitations: (i) reliance on multistep functionalization sequences to preorganize terphenyl precursors into cyclization-competent conformations, (ii) thermodynamically governed pathways resulting in kinetically persistent isomeric impurities via degenerate coupling modes, and (iii) mandatory implementation of labile directing motifs or transient arene activation strategies to surmount prohibitive energy barriers in dehydrogenative annulation. Three distinct synthetic frameworks have been rigorously analyzed through crystallographic and spectroscopic interrogation: (1) oxidative intramolecular coupling of conformationally constrained terphenyls for constructing ortho- fused triphenylene frameworks, [25] (2) sequence-controlled assembly via iterative Diels-Alder cycloaddition cascades between biphenyl-tethered dienes and quinoidal dienophiles, [26] and (3) threefold Csp²−Csp² homo-coupling of monosubstituted benzene derivatives guided by molecular symmetry principles to achieve trimeric annulation. [27] These methodologies collectively address the geometric and electronic constraints inherent to traditional PAH synthesis while expanding structural diversity. 2. Cyclization synthetic methodologies for triphenylene derivatives. 2.1 Palladium-catalyzed cyclization synthetic methodologies for constructing triphenylene-based compounds. 2.1 .1 Palladium-catalyzed annulation of halogenated arenes for constructing triphenylene-based compounds In 1998, Castedo and collaborators disclosed a palladium-catalyzed [2+2+2] cyclotrimerization protocol employing 2-trimethylsilylaryl triflates as bench-stable benzyne precursors (Figure 2). [28] This methodology has since been applied to the construction of diverse triphenylene structures through regioselective annulation pathways, though typically affording moderate isolated yields due to competing oligomerization processes and inherent steric constraints in the tricyclic product formation. [29-39] The mechanistic pathway is proposed to involve sequential oxidative addition of the triflate precursor, desilylation to generate the reactive aryne intermediate, followed by palladium-mediated cyclotrigomerization to establish the extended π-conjugated system. Figure 2 Palladium-catalyzed cyclization of 2-iodobiphenyl with 2-trimethylsilylaryl triflates. In 2005, Larock group reported a highly efficient palladium-catalyzed protocol for the in situ generation of polycyclic aromatic hydrocarbons, demonstrating its efficacy in synthesizing trimellitic benzene derivatives (Figure 3). [40] The methodology exhibited notable functional group tolerance, achieving up to 75% yield even with substrates bearing strong electron-withdrawing groups. Critical to the cyclization process was the controlled release kinetics of benzene precursors during CsF-mediated activation, where distinct directing groups manifested varying electronic and steric influences on reaction efficiency. This systematic investigation revealed that the CsF treatment’s role in modulating precursor reactivity significantly governed the regioselectivity and progression of annulation events. The robustness of the catalytic system was further evidenced by its compatibility with diverse functionalized substrates while maintaining synthetic practicality. Figure 3 Palladium-catalyzed cyclization of 2-iodobiphenyl with 2-trimethylsilylaryl triflates. The proposed reaction mechanism elucidating the aromatic cyclization process involves two distinct catalytic cycles (Figure 4), differentiated primarily by the initial oxidative activation pathway of palladium. In Cycle A, the Pd(0) catalyst engages in oxidative cyclization with aromatic substrate a , forming a strained metallacyclic intermediate b through concerted bond formation. Subsequent transmetallation with reagent 1a generates palladacycle d , which undergoes intramolecular C(sp²)-H activation via a concerted metalation-deprotonation (CMD) pathway to yield cyclopalladated complex e . Reductive elimination then releases the polycyclic product 3a while regenerating the active Pd(0) species. In contrast, Cycle B initiates through classical oxidative addition of 2-iodo-4’-methylbiphenyl to Pd(0), producing arylpalladium(II) intermediate c . This species undergoes migratory insertion with aromatic a to form intermediate d , followed by analogous C-H activation and reductive elimination steps as in Cycle A to complete the catalytic cycle. The mechanistic dichotomy lies in the divergent activation modes of the palladium center: direct metallacycle formation versus conventional oxidative addition, governing subsequent substrate engagement patterns. Figure 4 Proposed mechanism of palladium-catalyzed cyclization of 2-iodobiphenyl with 2-trimethylsilylaryl triflates. In 2013, Nishihara et al. developed a palladium-catalyzed protocol for synthesizing multisubstituted triphenylenes through the reaction of o- bromobenzyl alcohol derivatives with o- iodobiphenyl substrates in the presence of cesium carbonate (Figure 5). [41] This transformation involves a cascade process entailing the simultaneous formation of two C–C bonds and cleavage of C–H bonds, proceeding via a decarbonylative cross-coupling mechanism between aryl iodides and tertiary benzyl alcohols, followed by intramolecular cyclization. The palladium-phosphine catalytic system, employing a 1:2 palladium-to-ligand ratio, demonstrated superior efficiency in promoting the cyclization step. Notably, electron-deficient halogenated aromatic substrates afforded higher yields compared to the electron-rich counterparts, though both substrate classes-including those bearing strong electron-withdrawing and electron-donating groups – exhibited good reactivity under these conditions. The methodology displayed remarkable functional group tolerance, achieving high yields across diverse substituted biphenyl and benzyl alcohol precursors while maintaining operational simplicity. Figure 5 Palladium-catalyzed cyclization of 2-iodobiphenyl with o- bromobenzyl alcohols for triphenylenes. The proposed mechanistic pathway (Figures 6) involves a kinetically favored oxidative addition of aryl palladium iodide A compared to o- bromobenzyl alcohol 2a . A subsequent base-mediated ligand exchange between A and 2a generates the aryl(alkoxy)palladium(II) intermediate B . β-Carbon elimination of this species liberates acetone while forming the diarylpalladium complex C , which undergoes reductive elimination to yield compound 4a with concomitant regeneration of the Pd(0) catalyst. The resultant 4a re-enters the catalytic cycle through oxidative addition to Pd(0), proceeding via a concerted metalation-deprotonation (CMD) mechanism. Notably, the reaction efficiency is critically influenced by the acidity of the aromatic C-H bond undergoing activation. The CMD pathway facilitates formation of a seven-membered palladacycle intermediate E , which undergoes subsequent reductive elimination to afford product 3a . This tandem sequence, decarbonylative coupling followed by intramolecular C-H arylation, exhibits remarkable functional group tolerance. The methodology enables efficient construction of multisubstituted triphenylenes through palladium/phosphine-catalyzed cyclization between o-iodobiphenyls and o- bromobenzyl alcohols, demonstrating both mechanistic elegance and synthetic utility. Figure 6 Proposed mechanism of palladium-catalyzed cyclization of 2-iodobiphenyl with o- bromobenzyl alcohols. In 2016, Zhang et al. developed a Pd-catalyzed cross-coupling protocol between 2-iodobiphenyl and iodobenzene for triphenylene synthesis (Figure 7). [42] Through systematic optimization, the reaction achieved 71% yield using Pd(OAc)₂/DPPF as the catalytic system in DMF under inert N₂ atmosphere. Notably, the implementation of bidentate phosphine ligands provided marginal yield enhancement, while catalytic efficiency remained essentially unaffected upon reducing both Pd(OAc)₂ and ligand loading to 5 mol%. The methodology demonstrated compatibility with methyl- and phenyl-substituted substrates, though diminished efficacy was observed with electron-deficient iodobenzenes bearing trifluoromethyl or ester moieties. This limitation arises from competing homo-coupling pathways in electron-poor aryl iodides, leading to biphenyl byproduct formation. Mechanistic analysis suggests the process involves two distinct Pd-mediated C-H activation events followed by dual C-C bond-forming stages under catalytic control. The electronic modulation of aryl iodide substrates proves critical in suppressing undesired homo-dimerization side reactions. Figure 7 Pd-catalyzed coupling reaction of 2-iodobiphenyls with iodobenzenes. Figure 8 Proposed mechanism of Pd-catalyzed coupling reaction between 2-iodobiphenyls and iodobenzenes. A plausible mechanistic pathway is proposed (Figure 8), initiating with the oxidative addition of 2-iodobiphenyl to a Pd(0) center to form palladium(II) intermediate A . Subsequent intramolecular C–H bond activation via a concerted metalation-deprotonation (CMD) process generates a metastable Pd(IV) cyclometalated species B . This intermediate undergoes either (i) a second oxidative addition with iodobiphenyl to yield Pd(IV) complex C , followed by reductive elimination to produce Pd(II) species E , or (ii) an alternative pathway involving transmetallation with a phenylpalladium(II) species derived from oxidative addition of iodobenzene to Pd(0). The latter route forms a dinuclear μ-aryl Pd(II) complex D , which undergoes bimetallic reductive elimination to afford product E . In 2018, Yang et al.developed a novel tandem intermolecular decarboxylative cross-coupling protocol between o- bromobenzoic acid derivatives and aryl iodides for the construction of asymmetric triphenyl structures (Figure 9). [43] The implementation of a quinone catalytic system markedly enhanced reaction efficacy, though steric hindrance from adjacent substituents para to the bromine atom on the benzoic acid scaffold substantially diminished the reaction efficiency, leading to moderately reduced yields. Significantly, this methodology demonstrated compatibility with diverse ortho- substituted 1-iodo-2-methylbenzene substrates bearing electron-donating and electron-withdrawing groups, exhibiting pronounced regioselectivity and exceptional functional group tolerance. The observed substrate generality coupled with the operational simplicity of this transition metal-mediated transformation highlights its synthetic utility in biaryl synthesis. Figure 9 Palladium-catalyzed decarboxylation coupling between o- bromobenzoic acid and aryl iodides. In the same year, Shi and co-workers developed an efficient synthesis of benzophenanthrene-embedded polyaromatic systems through a palladium-catalyzed annulative dimerization strategy (Figure 10). [44] The protocol involves o-iododiaryl precursors undergoing concerted cleavage of dual C–I and C–H bonds under remarkably simple conditions, requiring neither exogenous ligands nor stoichiometric oxidants. This methodology demonstrates exceptional functional group tolerance across diverse coupling partners while maintaining both yield integrity and scalability. Notably, substrates bearing -Br or -OTf substituents exhibited complete inertness under optimized reaction parameters. Significantly, the synthetic utility was extended through late-stage diversification: tetrachloro-functionalized arene intermediates, accessible via sequential arylation with arylboronic acids and Scholl-type oxidative cyclization, served as versatile precursors for constructing fully fused graphene nanoribbons (GNRs) with precise structural control. This cascade approach provides a robust platform for bottom-up synthesis of extended π-conjugated materials. Figure 10 Palladium-catalyzed 2-iodobiphenyl dimerization. Figure 11 Proposed mechanism of palladium-catalyzed 2-iodobiphenyl dimerization. A plausible catalytic cycle was proposed (Figure 11), initiated by oxidative addition of substrate 1a to the Pd(0) center to generate σ-alkylpalladium(II) intermediate A . Subsequent kinetically favored proximal aryl C-H activation via concerted metalation-deprotonation (CMD) forms a strained five-membered palladacycle B . This undergoes secondary oxidative addition with a second equivalent of 1a, yielding bis-organopalladium(II) species C . Sequential reductive eliminations coupled with competitive C-H activation pathways then generate key palladacyclohexadiene intermediate E . The cycle culminates in stereoelectronic-controlled reductive elimination to release tetraphenylene product 2a while regenerating the catalytically active Pd(0) species. Mechanistic validation through parallel catalytic runs under divergent ligand environments demonstrated exclusive 2a formation, effectively eliminating metallacyclobutane intermediate D from the productive pathway. The observed system-dependent dichotomy originates from ligand-controlled differentiation of transient intermediates D and D’ at the catalytic branch point, where subtle electronic perturbations dictate selective evolution through either square-planar or trigonal-bipyramidal transition states. 2.1.2 Palladium-catalyzed annulation of diaryliodide salts for constructing triphenylene-based compounds In 2015, Zhu et al. developed an innovative palladium-catalyzed methodology for aromatic metathesis through organosulfur electrophilic activation (Figures 12). [45] The four-step protocol initiates with 4-chlorobutyl functionalization of dibenzothiophenes to generate sulfonium salts, followed by sodium tetraarylboronate-mediated arylation and ring-opening. Subsequent intramolecular S N 2 displacement induces C-S bond cleavage concomitant with C-H electrophilic cyclization. This transformation uniquely leverages the superior leaving group aptitude of the dibenzothiophene moiety, enabling rapid dealkylative aromatization. Notably, the reaction exhibits exceptional functional group tolerance: para-substituent electronic effects minimally influence catalytic efficiency, while steric encumbrance proximal to the sulfur center fails to impede cyclization kinetics. The methodology demonstrates remarkable regioselectivity for constructing extended π-conjugated systems, preferentially affording triphenylene derivatives with enhanced aromatic density in high yields. This represents a rare paradigm for alkylative aromatic coupling utilizing sulfur-containing electrophiles, circumventing conventional directing group requirements through strategic sulfur redox manipulation. Figure 12 Palladium-assisted“aromatic metamorphosis”of dibenzothiophenes into triphenylenes. In 2017, Wang et al. disclosed a palladium-catalyzed dual-activation strategy involving simultaneous cleavage of C-I bonds and functionalization of adjacent C-H bonds in diaryliodonium salts, enabling direct oxidative diarylation of 2-bromobiphenyls and bromobenzene derivatives (Figure 13). [46] This methodology exhibited exceptional functional group tolerance, successfully accommodating methyl, methoxy, halogen, nitro, cyano, ester, trifluoromethyl, and formyl substituents at ortho-, meta- , or para -positions of the biphenyl scaffold, irrespective of their electronic characteristics. Notably, sterically hindered 3-trifluoromethyl-substituted substrates underwent efficient transformation despite significant steric encumbrance, while 2-(2-bromophenyl)naphthalene achieved diarylation with moderate 49% yield. The system’s remarkable versatility stems from its ability to synergistically activate both electrophilic and directing groups, demonstrating unprecedented compatibility with diverse electronic environments and steric demands across aromatic systems. Figure 13 Palladium-catalyzed C-I and double activation of C-I bond and neighboring C-H bond of diaryl iodide salts. To further investigate the reactivity profile of trivalent iodonium species, the authors developed an optimized protocol employing homotrimethylene-bridged biphenyliodonium salts and bromobenzene under thermally enhanced conditions, achieving complete substrate conversion as evidenced by analytical data (Figures 14). The methodology demonstrated broad functional group tolerance, successfully accommodating various bromobenzene derivatives bearing both electron-donating substituents (para-methoxy, para-methyl, and 3,4-dimethyl) and electron-withdrawing groups (para-chloro, para-fluoro, and para-trifluoromethyl), with notable improvements in product yields compared to conventional approaches. This systematic exploration of substituent effects provides valuable insights into the electronic requirements of the coupling process involving hypervalent iodine reagents. Figure 14 Palladium catalyzed C-I and vicinal C-H dual activation of diaryliodonium salts for diarylations. Figure 15 Proposed mechanism of palladium catalyzed C-I and vicinal C-H dual activation of diaryliodonium salts for diarylations The catalytic cycle involves a sequence of organometallic transformations mediated by palladium’s variable oxidation states (0, II, IV) (Figure 15). Initiation occurs through oxidative insertion of Pd(0) into the C-Br bond of 2-bromobiphenyl, generating σ-aryl Pd(II) complex A . Base-assisted deprotonation triggers ligand sphere reorganization, producing a coordinatively unsaturated Pd(II) cation that enables nucleophilic capture through initial electrophilic aryl transfer to form metallacyclic intermediate B . Subsequent oxidative addition of diaryliodonium salt elevates the metal center to Pd(IV) in complex C , establishing a bis-aryl coordination environment. The system progresses through two distinct reductive elimination steps: first, Pd(IV)-to-Pd(II) reduction with concomitant biaryl bond formation yields intermediate D , followed by a second aryl transfer event to generate Pd(II) species E . Terminal reductive elimination from this intermediate restores the Pd(0) catalyst while forging the final C(sp²)-C(sp²) bond in the tetraarylated product. This catalytic cascade process highlights the distinctive capability of palladium to orchestrate alternating oxidative activation and aryl transfer steps via dynamic interconversion of its oxidation states, thereby facilitating successive bond-forming transformations without requiring intermittent catalyst recharging through discrete redox cycles. In 2017, Hong et al. reported a streamlined one-pot synthetic methodology for constructing functionalized triphenylene frameworks via a palladium-catalyzed cascade process (Figure 16). [47] The protocol employs Pd(dba)₂ as the catalytic species in conjunction with cyclic diaryliodonium salts to achieve both inter- and intramolecular site-selective tandem C-H arylation. This strategy eliminates the requirement for pre-functionalization of substrates while demonstrating broad functional group compatibility, delivering annulated products with satisfactory efficiency. Notably, cyclic diaryliodonium salts bearing electron-deficient (difluoro) and electron-rich (dimethyl) substituents exhibited enhanced reactivity, though inherent structural limitations impede the construction of non-planar or asymmetric structures. Mechanistic studies suggest that the electronic characteristics of amide directing groups (DGs) and their substituents critically influence cyclization kinetics through modulation of C-H activation barriers. Figure 16 Palladium-catalyzed an annulative synthetic strategy for triphenylene frameworks by multiple C-H bond activations. A plausible mechanistic pathway is proposed (Figures 17) wherein Cycle I initiates with sp² C-H activation-mediated facilitation of sequential single-electron transfer processes from Pd(0) to the cyclic diaryliodide 2, generating a Pd(II) intermediate A . This is followed by regioselective insertion of aromatic substrate 1 through cyclometalation to form complex C , which undergoes subsequent reductive elimination yielding iodinated intermediate D . In Cycle II, oxidative addition of D to Pd(0) initiates a cascade involving concerted metallation-deprotonation (CMD) of the activated C-H bond, culminating in final reductive elimination to restore the Pd(0) catalyst while delivering triarylated benzene derivatives. Both cycles maintain palladium redox shuttling between 0 and +2 oxidation states. Figure 17 Proposed mechanism of palladium-catalyzed an annulative synthetic strategy for triphenylene frameworks by multiple C-H bond activations. In 2018, Zhang et al. reported a ligand-free palladium-catalyzed cascade reaction between o- chlorobenzoic acid derivatives and cyclic diaryliodonium salts (Figure 18). [48] This methodology employed a one-pot process involving carboxylic acid-directed ortho-arylation followed by intramolecular decarboxylative cyclization to access triphenyl frameworks, representing the first demonstration of carboxylic acid-functionalized aryl halides participating in coupling reactions with diaryliodonium salts. Mechanistic studies revealed competitive pathways wherein the carboxylic acid moiety could either undergo direct ortho-arylation with the iodonium salt or initiate decarboxylative coupling. Notably, even when the initial ortho-arylation proceeded efficiently, subsequent in situ decarboxylation could generate linear coupling adducts rather than the anticipated cyclized products. The reaction exhibited broad functional group tolerance towards both electron-donating and electron-withdrawing substituents on the diaryliodonium salt. However, diminished yields observed with asymmetric diaryliodonium salts bearing strongly electron-rich aryl groups suggested a critical synergistic interaction between the carboxylic acid directing group and the ortho-chlorine substituent in facilitating the catalytic cycle. This work expanded the synthetic utility of aryl halides in transition metal-catalyzed cascade processes through strategic exploitation of directing group chemistry and decarboxylative pathways. Figure 18 palladium-catalyzed cascade reaction between o- chlorobenzoic acids and cyclic diaryliodonium salts. The proposed mechanism for carboxylate-directed ortho-arylation involves sequential cyclometalation, oxidative addition, and tandem decarboxylative cyclization processes mediated by palladium redox cycling (Figure 19). Initially, ortho-chlorobenzoic acid 1a undergoes cyclopalladation with Pd(0) to form a five-membered cyclometalated Pd(II) complex A . Subsequent oxidative addition with cyclic diaryliodonium 2a generates a Pd(IV) intermediate B , which facilitates ligand-directed C-C coupling. Concerted intramolecular decarboxylation then yields a Pd(II)-coordinated biaryl iodide D through reductive elimination. This intermediate undergoes conformational reorganization to form a seven-membered Pd(IV) metallocycle E via oxidative addition-assisted cyclization. Final reductive elimination liberates the triphenyl product 3a while regenerating Pd(II) species, which undergoes ligand exchange in NMP solvent to reform the active Pd(0) catalyst through triflate-assisted reduction, completing the catalytic cycle. Figure 19 Proposed mechanism of palladium-catalyzed cascade reaction between o- chlorobenzoic acids and cyclic diaryliodonium salts. In 2019, Zhang et al. demonstrated a dual-functionalization strategy involving Pd(II)-catalyzed tandem C-H arylation/decarboxylative cyclization for constructing trisubstituted benzene frameworks (Figure 20). [49] This one-pot methodology strategically employs cyclic diaryliodonium salts and benzoic acid derivatives, where the carboxylic acid moiety serves dual roles: first as a directing group for proximal C-H bond activation during arylation, then as a sacrificial functionality enabling subsequent decarboxylative cyclization. This cascade process efficiently assembles benzophenanthrene structures through dual cross-coupling/annulation sequences. Substrate scope evaluation revealed tolerance toward para-, meta-, and ortho- substituted phenolic hydroxyl, halogen, formyl, and nitro groups on benzoic acid precursors, generally affording trisubstituted polycycles in good yields. Notably, reduced efficiency with 2-nitrobenzoic acid was attributed to competing decarboxylation pathways. The methodology proved compatible with heteroaromatic carboxylic acids, yielding heterocyclic products effectively. Cyclic diaryliodonium salts bearing diverse substituents – including nitro, fluoro, chloro, and ester groups – exhibited excellent reactivity, with asymmetric variants demonstrating particular synthetic promise. Mechanistically, the preserved halogen substituents suggest compatibility with potential downstream functionalization strategies. Figure 20 Pd (II)-catalyzed tandem C-H arylation/decarboxylative annulation with cyclic diaryliodonium salts. The proposed reaction mechanism for this one-pot synthesis likely proceeds through an acid-mediated neighboring C-H activation/arylation pathway followed by intramolecular decarboxylative cyclization (Figure 21). The catalytic cycle initiates with the formation of a five-membered palladacycle A derived via directed C-H bond activation of the benzoic acid substrate. Subsequent oxidative addition of the cyclic diaryliodonium salt generates a Pd(IV) intermediate B , which undergoes concerted reductive elimination and decarboxylation to yield aryl iodide D . Oxidative addition of the C-I bond in D produces a seven-membered palladacycle E at the Pd(IV) oxidation state. Final reductive elimination liberates the target heterocyclic product while regenerating the Pd(II) catalyst. Alternatively, a Pd(II)-Pd(III) catalytic manifold involving bimetallic Pd(III) intermediates F may operate, suggesting potential participation of dinuclear palladium species during key redox steps. Figure 20 Proposed mechanism of palladium-catalyzed tandem C-H arylation/decarboxylative annulation with cyclic diaryliodonium salts. In 2019, Hong et al. developed a non-directed annulation strategy for inert simple arenes through palladium-catalyzed selective C-H arylation. This methodology employs cyclic diaryliodonium salts to facilitate tandem C-C bond formation, enabling efficient construction of phenyl-extended triphenylene frameworks (Figure 22). [50] The substrate-appointed ligand moiety directs the transition metal catalyst to spatially proximate C-H bonds through chelation assistance, enhancing regioselectivity and catalytic efficiency via formation of a six-membered metallocycle intermediate. This strategy achieves cyclic π-extension through either one- or two-step protocols with superior atom economy and step efficiency, circumventing conventional multi-step sequences. Notably, sterically demanding substituents (isopropyl, tert- butyl, cyclohexyl) induce exclusive (β+γ)-diarylation patterns unattainable through conventional directing group approaches. Crucially, the electronic characteristics of diaryliodonium salts demonstrate negligible correlation with reaction yields, suggesting a radical-involved pathway distinct from classical electrophilic aromatic substitution mechanisms. Figure 22 Annulative π-extension of palladium-catalyzed non-directed C-H arylation annulation. of unactivated benzene derivatives. 2.1.3 Palladium-mediated bihalogenative cross-coupling methodology for constructing triphenylene-based compounds In 2009, Hiyama and coworkers demonstrated a robust transition metal-catalyzed cross-coupling strategy for constructing regio- and stereodefined structures through reactions between organostannanes and organohalides (Figure 23). [51] The dual cross-coupling protocol employing 9-stannofluorophenanthrene with 1,2-dihaloarenes enabled efficient synthesis of diverse polycyclic aromatic hydrocarbons (PAHs), producing triphenylene-based triadic systems with exceptional yields. Mechanistic studies revealed that sterically demanding substituents (e.g.; iodine, butyl groups) impede transmetallation kinetics, whereas fluoride additives facilitate this rate-limiting step via generation of hypervalent stannate intermediates. This methodology exhibits remarkable functional group tolerance toward both electron-withdrawing and electron-donating moieties while enabling recovery of tin-containing byproducts through a closed catalytic cycle. The stereochemical fidelity throughout the coupling process is maintained through palladium-mediated oxidative addition and transmetallation sequence, demonstrating broad applicability in precision synthesis of extended π-conjugated systems. Figure 23 Palladium-catalyzed cross-coupling of organostannanes and organohalides. In 2018, Xu and co-workers developed a palladium-catalyzed Suzuki-Miyaura cross-coupling strategy employing aryl boronic acids and 2,2’-dibromobiphenyl substrates, integrated with intramolecular C-H functionalization, to efficiently construct functionalized triphenylene derivatives (Figure 24). [52] The synthetic protocol demonstrated enhanced cyclization efficiency at elevated temperatures, enabling cascade annulation reactions between substituted phenylboronic acids and the dibrominated biphenyl precursor to afford asymmetrically functionalized triphenylene structures. Notably, para-substituted phenylboronic acid derivatives bearing methyl, fluoro, chloro, cyano, and ester groups underwent smooth transformation with excellent yields, while substrates containing electron-withdrawing substituents exhibited pronounced regioselectivity during the cyclization process. This methodology features exceptional atom economy, minimized synthetic expenditure, and precise control over regio-chemical outcomes in the construction of polycyclic aromatic frameworks through sequential C-C bond formation and aromatic ring fusion. Figure 24 Palladium-catalyzed Suzuki-Miyaura coupling of aryl boronic acids and 2,2’-dibromobiphenyl substrates. In the mechanistic pathway delineated (Figure 25), the triphenylene scaffold is constructed through sequential palladium-catalyzed transformations involving arylboronic acid and 2,2’-dibromobiphenyl. The process initiates with a Suzuki-Miyaura cross-coupling between the arylboronic acid and dibromobiphenyl, generating intermediate I characterized by a residual C-Br bond. Subsequently, intermediate A participates in oxidative addition with a Pd(0) species to form palladium(II) complex B . The catalytic cycle culminates in a concerted sequence where intramolecular aromatic C-H bond activation, mediated by the palladium center C , facilitates cyclization followed by reductive elimination to liberate triphenylene while regenerating the Pd(0) catalyst. This cascade demonstrates three hallmark transition metal-mediated steps: cross-coupling, oxidative addition, and C-H functionalization-driven reductive elimination. Figure 25 Proposed mechanism of palladium-catalyzed Suzuki-Miyaura coupling of aryl boronic acids and 2,2’-dibromobiphenyl substrates. The structural ubiquity of polycyclic aromatic compounds (PACs) in cutting-edge optoelectronic materials has positioned them as strategic synthetic targets for molecular engineering. Contemporary advances in transition-metal catalysis have revolutionized PAC synthesis, with palladium-mediated [n]-annulation of o-halobiaryl precursors emerging as a paradigm-shifting methodology. Breaking new ground, the Shi team developed a dual reagent strategy employing bay-diiodinated aromatic cores and o-chloroarylcarboxylates under palladium catalysis (Figure 26). [53] This concerted activation mechanism enables precise construction of alkoxy-rich PAC frameworks – a structural class renowned for inducing columnar mesophases in discotic liquid crystals. Through a multitechnique analytical framework integrating TGA, DSC, POM, and XRD, the hexa-alkoxylated 5-azadibenzo[fg,op]tetracene derivative was rigorously characterized, exhibiting unprecedented ambient-temperature liquid crystallinity with long-range positional order. Such tailored mesophase engineering demonstrates critical implications for next-generation organic electronics requiring controlled charge transport structures. Figure 26 Palladium-catalyzed annulation of bay-diiodinated arenes with o- chloroaromatic carboxylic acids. A plausible reaction pathway involving five-membered C,C-pallada(II)cycle intermediate C has been proposed, as outlined in Figure 27. The catalytic cycle initiates through oxidative addition of bay-diiodinated arenes 1 or 3 to the Pd(0) center, generating Pd(II) complex A . Subsequent intramolecular oxidative addition induces valence expansion to form tetravalent palladacycle B , which undergoes spontaneous reduction to yield electron-rich divalent palladacycle C . Critical to the mechanism, the ortho-carboxyl substituent facilitates oxidative addition of o-chloroaromatic carboxylic acids 2 to intermediate C , producing metastable Pd(IV) species D. Sequential reductive elimination from D generates Pd(II) complex E , followed by decarboxylation to form Pd(II) intermediate F . The catalytic cycle concludes through final reductive elimination, liberating target products 4 or 5 while regenerating the Pd(0) catalyst. Notably, alternative pathways involving decarboxylative cross-coupling (Figure 24 b) remain mechanistically viable, as evidenced by control experiments in Scheme 6a that preclude definitive exclusion of this competing route. Figure 27 Proposed mechanism of palladium-catalyzed annulation of bay-diiodinated arenes with o -chloroaromatic carboxylic acids. 2.1.4 Palladium-catalyzed annulation reaction of o-chloroaromatic carboxylic acids for constructing triphenylene-based compounds Fluorescent structures constitute transformative components in modern photonic technologies, driving innovations that redefine human-centric applications. Polycyclic aromatic hydrocarbon (PAH)-based fluorophores, prized for their tunable optoelectronic signatures and biofunctional adaptability, occupy a pivotal nexus across interdisciplinary research frontiers. The first catalytic blueprint for NBE-CO₂Me-mediated palladium-catalyzed C–H activation/annulative diversification of bromo(hetero)aromatics—an unprecedented synthetic paradigm enabling systematic construction of π-extended PAH libraries was developed (Figure 28). [54] This platform demonstrates exceptional chemoselectivity across electronically divergent substrates, establishing a modular workflow for high-throughput discovery of advanced luminophores. Central to this advance is the pioneering realization of anti-Kasha dual-emissive PAH-based single-molecule white-light emitters, exploiting non-Kasha photophysical behavior to circumvent classical energy gap limitations. Engineered as mitochondria-targeting water-dispersible nanoparticles (NPs), these dual-channel emissive systems enable ratiometric biodetection with subcellular precision, resolving spatially encoded biomarker distributions beyond conventional intensity-limited imaging modalities. By synergizing catalytic C–H diversification with supramolecular nanoarchitectonics, this work bridges molecular design and functional biomaterial engineering, establishing a roadmap for intelligent optoelectronic systems with hierarchical information-encoding capabilities. Figure 28 Palladium/norbornene-catalyzed C–H activation and annulation for polycyclic aromatic hydrocarbon-based fluorescent materials. Figure 29 Proposed mechanism of palladium/norbornene-catalyzed C–H bond activation and annulation for fluorescent materials. A pioneering palladium/norbornene-catalyzed protocol for C–H bond activation and annulative coupling of brominated (hetero)aromatics has been established, utilizing NBE-CO₂Me as a cooperative catalytic mediator to systematically generate structurally diversified polycyclic aromatic hydrocarbons (PAHs) (Figure 29). This methodology demonstrates exceptional substrate generality while offering atom-economical access to PAH structures, thereby pioneering a transformative strategy for accelerated discovery of high-performance luminescent materials. Significantly, the inaugural development of anti-Kasha dual-emissive PAH-based single-molecule white-light emitters was successfully developed, wherein the unique two-channel emission profiles enable multidimensional cellular imaging with enhanced informational fidelity. Unlike conventional single-channel systems limited to textural analysis, our dual-intensity-ratio imaging platform permits simultaneous resolution of structural and microenvironmental biomarkers, advancing correlative microscopy paradigms. This work constitutes the first catalytic blueprint for anti-Kasha dual-emission PAH synthesis via C–H functionalization, unveiling an unprecedented route toward intelligent optoelectronic materials with ratiometric imaging capabilities and white-light emission at the single-molecule level. 2.2 Copper-catalyzed multiple oxidative methodology enabling the construction of triphenylene-fused polycyclic structures. In 2018, Zhao and co-workers developed a streamlined copper-catalyzed protocol for constructing asymmetric triphenylene structures through oxidative annulation of electron-rich biphenyl precursors (predominantly 3,3’,4,4’-tetraalkoxybiphenyls) with halogen-tolerant diaryliodonium salts (Figure 30). [55] This methodology leverages sequential C-H activation processes, demonstrating remarkable functional group compatibility with trihalogenated aryl iodonium species bearing bromine, chlorine, and fluorine substituents. The reaction exhibits particular efficacy for electron-donating aryl iodonium salts, affording comparable yields while preserving halogens for subsequent cross-coupling diversification. Mechanistic studies suggest the observed chemoselectivity arises from preferential activation of electron-rich biphenyl moieties, as neither electron-deficient biphenyl derivatives nor simple biphenyl substrates underwent productive cyclization. Despite its operational advantages, the current substrate scope remains constrained to specific alkoxy-functionalized biphenyl systems and catechol-derived aromatic partners, limiting general applicability across broader aromatic systems. Figure 30 Copper-catalyzed multiple C-H arylation for construction of unsymmetrical triphenylenes from electron-rich biphenyls and diaryliodonium salts. In 2020, Xiang et al. demonstrated a synthetic methodology for asymmetric triphenylene derivatives via electronically directed Scholl-type cyclization (Figure 31). [56] The substrate scope was restricted to electron-rich biphenyl derivatives bearing 3,4-dialkoxy substituents, which govern regioselectivity through electronic directing effects. Specifically, the protocol employed 4-benzoylamino-3,4′-dimethyl-1,1′-biphenyl and 3,4-dimethoxyphenyl (meta-methyl)iodotetrafluoroborate as key substrates, necessitating the use of exogenous oxidizing agents to drive the Scholl cyclization process. The methodology utilized aryl (meta-methyl)iodonium salts functionalized with methyl, tert-butyl, or methoxy groups as π-extending agents, with the APEX (Annulative π-Extension) reaction exhibiting good efficiency under optimized conditions. This approach highlights the critical role of electron-donating substituents in directing oxidative coupling pathways while underscoring the requirement for auxiliary oxidants in non-autocatalytic Scholl systems. Figure 31 Copper-catalyzed amide-directed bay-region two-step annulative π-extension of electron-rich biphenyls with diaryliodonium salts. In 2019, Song et al. developed an innovative copper-catalyzed strategy for constructing nitrile-functionalized trisubstituted benzenes through oxidative denitrogenative aromatization of 3-aminoindazole derivatives (Figure 32). [57] This methodology employs 3-aminoindazoles as versatile radical precursors, enabling dual C-N bond cleavage under aerobic conditions mediated by Cu(OAc)₂ in acetonitrile. The reaction demonstrates broad functional group tolerance, achieving moderate to excellent yields across diverse substrates, including electronically neutral and activated aryl systems. Notably, the protocol exhibits remarkable compatibility with heterocyclic frameworks bearing the 3-aminoindazole motif. Key advantages encompass operational simplicity under ambient atmosphere, cost-effective catalytic systems, and efficient generation of sterically congested aromatic structures through radical-mediated C-C bond formation. This oxidative dearomatization approach represents a significant advancement in transition-metal-catalyzed arene functionalization strategies. Figure 32 Copper-catalyzed strategy for constructing nitrile-functionalized triphenylenes. 2.3 Nickel-electrocatalyzed reductive strategy for constructing triphenylene-based compounds The Yamamoto coupling reaction involves the reductive homocoupling of aryl bromides mediated by a Ni(0) catalyst to construct aryl-aryl σ-bonds, serving as a robust methodology for synthesizing biaryl structures, [58, 59] conjugated polymers with extended π-systems, [60-62] and multidimensional macromolecular frameworks. [63] A seminal advancement was reported by Cheng et al. in 1983, who achieved a 25% yield of triphenylene through reductive coupling of o- dibromobenzene mediated by a highly reactive nickel amalgam catalyst under controlled conditions. [64] This approach was subsequently refined through systematic catalyst development, culminating in Yamamoto’s landmark 1991 discovery. By employing stoichiometric quantities of a Ni(0) complex derived from bis(1,5-cyclooctadiene)nickel(0) (Ni(cod)₂) coordinated with triphenylphosphine ligands, Yamamoto team realized a catalytic cycle enabling efficient trimerization of o- dihaloarene precursors. This optimized protocol delivered triphenylene derivatives in significantly enhanced yields of 60%, representing a critical advancement in the stereoelectronic control of aryl-aryl coupling processes (Figure 33). [65] Recent extensions of this strategy have enabled the synthesis of higher-order polycyclic aromatic hydrocarbons including trinaphthylene cores [66] and starphene derivatives with expanded π-conjugation. [67,68] Despite its synthetic potential, the application of Yamamoto coupling for preparing functionalized triphenylenes, particularly electron-deficient variants that are challenging to access through traditional oxidative trimerization or Scholl reactions, remains underexplored. [69-74] This transition metal-catalyzed cross-coupling approach offers distinct advantages for constructing such systems through rational monomer design, circumventing the electronic limitations inherent in electrophilic aromatic coupling methodologies. Figure 33 Nickel-catalyzed Yamamoto coupling of o- dibromobenzene for triphenylene. In 2022, Mei et al. developed a nickel-catalyzed electrochemical protocol enabling efficient aryl trimerization under mild conditions, employing readily accessible o- dibromobenzene or o-bromoaryl fluorosulfonate precursors (Figure 34). [75] This methodology delivered both symmetric and asymmetric trimellitic derivatives with excellent regioselectivity while demonstrating broad functional group compatibility, including compatibility with electron-deficient aromatic systems. Notably, when employing 1,2-dibromo-4-fluorobenzene as the substrate, the electronic disparity between substituents induced differential activation of the bromine sites. The enhanced electrophilicity of one aryl bromide facilitated preferential oxidative addition to the nickel center, thereby promoting sequential reductive elimination over competitive aromatic C-Br bond cleavage. This mechanistic pathway effectively suppressed undesired trimerization pathways through kinetic control of the elementary catalytic steps. The electrochemical approach circumvented traditional stoichiometric reductant requirements while maintaining precise control over coupling selectivity. Figure 34 Nickel-catalyzed electroreductive syntheses of triphenylenes using ortho- dihalobenzene-derived benzynes. Figure 35 Proposed mechanism of nickel-catalyzed electroreductive syntheses of triphenylenes using ortho- dihalobenzene-derived benzynes. The proposed mechanistic pathway (Figure 35) initiates with oxidative addition of 2,2’-dihalobiphenyl to a Ni(0) center, forming a Ni(II) complex A . Subsequent electrochemical reduction at the cathode generates a low-valent Ni(0) intermediate B with enhanced nucleophilic character, which promotes regioselective migratory insertion of transiently activated aromatic units into the nickelacyclic intermediate C . This electron transfer process enables iterative bond activation through precisely modulated nickel oxidation states (0 → II →0). The catalytic cycle terminates via reductive elimination from a hypervalent nickel species, affording structurally diversified triphenylene structures. This electrochemical platform exploits sequential nickel redox transitions (Ni⁰→Niᴵᴵ) to orchestrate directed C-C bond formation. The potential-dependent control of nickel oxidation states ensures spatiotemporal regulation of oxidative addition, migratory insertion, and elimination selectivity within the electrocatalytic manifold. 2.4 Electronic oxidative coupling of the alkoxy substituents for constructing triphenylene-based compounds The synthesis of peralkoxylated triphenylene derivatives, which represent the most extensively investigated subclass within the triphenylene family, predominantly employs oxidative cyclotrimerization as the most widely employed synthetic strategy. This methodology specifically involves the transition metal-catalyzed oxidative coupling of electron-rich 1,2-dialkoxybenzene precursors, which enables the efficient construction of symmetrically substituted alkoxytriphenylene systems through a [2+2+2] annulation pathway (Figure 36). [76-79] The electronic activation imparted by alkoxy substituents facilitates both the oxidative coupling process and subsequent aromatization steps, while maintaining strict regiochemical control essential for achieving molecular symmetry. Figure 36 Palladium-catalyzed cyclization of 2-iodobiphenyl with o- trimethylsilyl phenyl trifluorate aromatics. 2.5 Visible light-induced catalytic generation of tether-tunable distonic radical anions for constructing triphenylene-based compounds In 2015, Daigle et al. developed an innovative solution-phase methodology for constructing trisubstituted benzene structures through photochemically induced dechlorination cyclization of polychlorinated aryl precursors (Figure 37). [80] The protocol employed controlled photoirradiation in either acetone with stoichiometric base or neat benzene, facilitating sequential regioselective C-Cl bond activation and intramolecular C-C coupling to generate planarized π-conjugated systems. This ambient-temperature process achieved remarkable efficiency, delivering target molecules in high yields through facile filtration workup without chromatographic purification. The methodology demonstrated exceptional functional group tolerance, accommodating both electron-donating and electron-withdrawing substituents, while successfully extending to heteroaromatic systems including pyridine- and thiophene-annulated tricyclic benzenes. Notably, the reaction pathway enabled access to sterically congested, non-planar π-systems while preserving partial conjugation through controlled aromatization suppression. Kinetic analyses revealed ultrarapid reaction progressions under optimized conditions, with multiple substrates reaching full conversion within minutes. This mechanistically distinct approach represents a significant advancement in atom-economical polycyclic aromatic hydrocarbon synthesis, particularly for structurally constrained systems challenging to access through traditional cross-coupling strategies. Figure 37 Photochemical cyclodehydrochlorination forsynthesis of nanographenes. Polarity inversion strategies continue to revolutionize synthetic paradigms by unlocking unconventional reactivity manifolds. While radical-mediated umpolung typically engages in two-electron duality with closed-shell intermediates, precise regulation of radical-polar crossover mechanisms remains underexplored. The Shi group now discloses a visible light-driven benzannulation protocol exploiting tether-engineered distonic radical anions (TDRAs) as transient polarity modulators (Figure 38). [81] This methodology enables both intra- and intermolecular construction of 10-phenanthrenols and 1-naphthols through radical-anion-mediated regioselective C–C coupling. Notably, this transition metal-free platform operates under ambient visible light without requiring substrate preactivation or exogenous bases, circumventing conventional high-energy activation paradigms. Figure 38 Visible light-induced benzannulation of arylethanone derivatives enabled by tether-tunable distonic radical anions. Scheme 39 Proposed mechanism of visible light-induced benzannulation of arylethanone derivatives. Mechanistic investigations via operando NMR spectroscopy and density functional theory (DFT) modeling exclude iodinated intermediate pathways. Radical clock experiments combined with EPR spin-trapping and bond dissociation energy (BDE) correlations confirm TDRA generation via cesium iodide-mediated photoredox cycling (Figure 39). Multidimensional analysis–integrating intrinsic bond orbital (IBO) deconvolution, Hirshfeld spin distribution mapping, and asynchronicity metrics-elucidates a concerted yet asynchronous proton-coupled electron transfer (PCET) mechanism during hydrogen atom abstraction. This work establishes TDRA photoredox catalysis as a programmable platform for radical-polarity orchestration, merging mechanistic clarity with synthetic versatility in aromatic π-system engineering. 3. Conclusion and outlook Triphenylene-derived polycyclic aromatic hydrocarbons have become critical structural platforms in advanced organic functional materials, stimulating considerable fundamental interest in the catalytic assembly. Palladium-catalyzed [2+2+2] cyclotrimerization strategies demonstrate enhanced molecular complexity generation in single-step transformations, yet suffer from practical constraints such as excessive catalyst loading, prolonged reaction times, and suboptimal atom economy-factors incongruent with modern green chemistry principles. The paramount challenges lie in rational catalyst design to enable C-H activation under mild conditions while preserving aromatic stability, coupled with precise regioselective control for asymmetric triphenylene structures. Despite the prevalence of palladium-catalyzed C(sp²)-H activation methodologies utilizing directing groups for constructing trisubstituted PAH topologies, existing approaches remain constrained by their reliance on stoichiometric oxidants with inherent environmental and economic liabilities. While recent innovations in palladium-mediated annulation chemistry have enhanced access to these structures, persistent limitations in reaction sustainability and efficiency demand paradigm-shifting solutions. Current research emphasizes the development of transition metal catalytic systems that synergistically address these issues through ligand engineering and mechanistic optimization. Critical breakthroughs require innovative approaches to leverage commercially available functionalized benzene derivatives for direct, stereocontrolled assembly of tailored triphenylene systems, thereby eliminating pre-functionalization steps and maximizing synthetic efficiency. Three strategically distinct yet complementary research trajectories merit focused exploration to propel this field: 1) Photoredox-Electrochemical Synergistic Catalysis: Merging photoexcited charge-transfer phenomena with transition metal catalysis could bypass conventional substrate prefunctionalization requirements, facilitating direct C-H bond transformation under ambient conditions. This dual activation paradigm may enable oxidant-free, atom-economical pathways through controlled single-electron transfer (SET) processes. 2) Aqueous Electrochemical Dehydrogenative Coupling: Implementation of aqueous-compatible electrocatalytic systems could supplant toxic aprotic solvents in PAH synthesis. Such methodologies would exploit proton-coupled electron transfer (PCET) dynamics to orchestrate key dehydrogenative cyclization events, aligning with green chemistry imperatives while potentially enhancing functional group tolerance. 3) Preorganized Cyclic Iodonium Templates: Rational design of structurally constrained diaryliodonium(III) reagents incorporating latent directing moieties could enable regiodefined oxidative coupling with arylacetylenes. Design of cyclophane-type iodonium salts with inherent directing groups could facilitate regioselective coupling with alkyne precursors under base metal catalysis, enabling oxidant-free construction of strained polyarenes through formal [2+2+2] cyclization pathways. 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(The following will be filled in by the editorial staff) Manuscript received: XXXX, 2022 Manuscript revised: XXXX, 2022 Manuscript accepted: XXXX, 2022 Accepted manuscript online: XXXX, 2022 Version of record online: XXXX, 2022 Entry for the Table of Contents Information & Authors Information Version history V1 Version 1 05 June 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords cyclization strategy synthetic methods. transition metal catalysis triphenylene derivatives Authors Affiliations Huicheng Cheng 0000-0003-2866-0820 [email protected] Guangdong University of Petrochemical Technology View all articles by this author Xu-Ming Zhou Guangdong University of Petrochemical Technology View all articles by this author Peng-Hu Guo Guangdong University of Petrochemical Technology View all articles by this author Jiao-Li Ma Guangdong University of Petrochemical Technology View all articles by this author Ji-Cheng Shi Guangdong University of Petrochemical Technology View all articles by this author Metrics & Citations Metrics Article Usage 248 views 184 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Huicheng Cheng, Xu-Ming Zhou, Peng-Hu Guo, et al. Evolutionary Trends and Future Directions in the Cyclization Synthetic Methodologies for Triphenylene Derivatives. Authorea . 05 June 2025. 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