Photocatalytic Germylation via Direct Hydrogen Atom Transfer

Photocatalytic Germylation via Direct Hydrogen Atom Transfer | 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 Search Search ADVANCED SEARCH SCROLL Chinese Journal of Chemistry This is a preprint and has not been peer reviewed. Data may be preliminary. 12 February 2025 V1 Latest version Share on Photocatalytic Germylation via Direct Hydrogen Atom Transfer Authors : Wenshan Wang , Yan Liu , Gonghong Qiu , Igor B. Krylov , Alexander O. Terent’ev , Shuxia Cao , Kai Sun 0000-0003-2135-0838 , Xiaolan Chen , Lingbo Qu , and Bing Yu 0000-0002-2423-1212 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.173935655.59966816/v1 Published Chinese Journal of Chemistry Version of record Peer review timeline 436 views 292 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract An innovative visible-light-driven direct hydrogen atom transfer ( d -HAT) of Ge–H bond has been developed, wherein the photoexcited 9,10-phenanthraquinone ( PC HAT 9 ) serves as an efficient photocatalyst for the generation of germanium-centered radicals from triphenylgermane. By employing hypervalent iodine reagents as SOMOphiles, this protocol facilitates streamlined germylation through a mechanism involving germyl radical addition followed by β-cleavage of a carboxyl radical to yield a diverse array of ethynyl-, vinyl-, nitrile-, and phenyl-functionalized germanes. The methodological leap signifies a noteworthy departure from the previous photocatalytic indirect hydrogen atom transfer ( i -HAT) relying on combined usage of PC SET with abstractors, which not only advances the methodology for creating germanium radicals in a photocatalytic fashion but also provides access to structurally novel and pharmaceutically promising organogermanium compounds that are difficult to synthesize by routine methods. Cite this paper: Chin. J. Chem. 2024 , 42 , XXX—XXX. DOI: 10.1002/cjoc.202400XXX Photocatalytic Germylation via Direct Hydrogen Atom Transfer Wenshan Wang, a,b Yan Liu,* ,a,b,c Gonghong Qiu, a,b Igor B. Krylov, d Alexander O. Terent’ev, d Shuxia Cao, a Kai Sun, a Xiaolan Chen, a Lingbo Qu, a,e and Bing Yu* ,a a College of Chemistry, Zhengzhou University, Zhengzhou 450001, China. b Henan International Joint Laboratory of Rare Earth Composite Material, College of Materials Engineering, Henan University of Engineering, Zhengzhou 451191, China. c National Key Laboratory of Cotton Bio Breeding and Integrated Utilization, Institute of Cotton Research, Chinese Academy of Agricultural Sciences, Anyang 455000, China. d N. D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences, 47 Leninsky prosp., Moscow 119991, Russian Federation e Institute of Chemistry Henan Academy of Sciences, Zhengzhou 450002, China Germylation | Direct hydrogen atom transfer | Germyl radical | Photocatalysis Comprehensive Summary An innovative visible-light-driven direct hydrogen atom transfer ( d -HAT) of Ge–H bond has been developed, wherein the photoexcited 9,10-phenanthraquinone ( PC HAT 9 ) serves as an efficient photocatalyst for the generation of germanium-centered radicals from triphenylgermane. By employing hypervalent iodine reagents as SOMOphiles, this protocol facilitates streamlined germylation through a mechanism involving germyl radical addition followed by β-cleavage of a carboxyl radical to yield a diverse array of ethynyl-, vinyl-, nitrile-, and phenyl-functionalized germanes. The methodological leap signifies a noteworthy departure from the previous photocatalytic indirect hydrogen atom transfer ( i -HAT) relying on combined usage of PC SET with abstractors, which not only advances the methodology for creating germanium radicals in a photocatalytic fashion but also provides access to structurally novel and pharmaceutically promising organogermanium compounds that are difficult to synthesize by routine methods. Background and Originality Content The advancement of efficient methodologies for synthesizing organogermanium compounds holds paramount importance in expediting the discovery of functional materials, [1] alongside biologically and pharmaceutically active molecules, [2] which are endowed with distinct physical and chemical properties by germanium (Ge), including robustness, hydrophobicity, and nontoxicity. [3] Modern chemical synthesis significantly underscores the versatility of organogermanes as orthogonal coupling partners in bond-forming strategies, thereby opening up avenues to the wide applicability of organogermanium compounds in sophisticated molecular architectures. [4] As a result, the ongoing innovation in synthesizing organogermanium compounds is of continuous and growing interest within the scientific community. [5] Conventional synthetic repertoire predominantly relies on organometallic reactivity, particularly involving the substitution of Ge−electrophiles ( e.g . R 3 GeCl) by Grignard reagents or a Ge−metal species by electrophiles. However, the inherent basicity and nucleophilic nature associated with organometallic compounds may pose significant challenges to functionality compatibility and chemo-/regio-selectivity. [6] Over recent decades, exploration of photo-processes involving radicals derived from Group 14 elements, namely carbon, [7] silicon, [8] and tin, [9] has greatly enriched the toolkit available for synthetic chemists, presenting new strategies for developing environmentally benign synthetic methodologies. [10] Nevertheless, applying germanium-centered radicals warrants further investigation in photochemical systems when juxtaposed with its counterparts within Group 14. Historically, acylgermanes were identified as exceptional photoinitiators in radical polymerization. [11] Upon exposure to ultraviolet (UV) light, they are capable of occurring Ge–C bond homolysis to yield acyl and germyl radicals. Recent progress in this domain unveiled the feasibility of using ermacarboxylic acids, [12] hydrogermanes, [13] and chlorogermanes [14] as germyl radical precursors via photoredox and photoexcited-metal catalysis (Scheme 1A). Scheme 1 Photoinduced germanium radical chemistry. A : Typical methods for photo-initiation of Ge-centered radical; B : BDE of C–H, Si–H, Ge–H; C : Hydrogermylation via i -HAT; D : Envisaged mechanism of d -HAT of Ge–H; E : This work and its challenges. On the other hand, the evolution of photocatalytic hydrogen-atom transfer (HAT) has ushered in an era of manipulating inert hydrogen atoms bonded with carbon and silicon. [15] As compared in Scheme 1B, a notably lower bond dissociation energy (BDE) than C–H and Si–H bonds suggests a parallel reactivity pattern of hydrogermanes in the HAT reactions. [5b] This was well-exemplified by hydrogermylation of olefins via organophotoredox-mediated indirect HAT ( i -HAT), wherein a SET photocatalyst (PC SET ) collaborates with hydrogen abstractors (Abs.) to grant straightforward access to germyl radicals. However, from a mechanistic perspective, this “hydrogen-borrowing/returning” strategy is strictly limited to the hydrogermylation of unsaturated bonds rather than direct functionalization (Scheme 1C). [16] In this context, we envisioned a direct HAT ( d -HAT) of the Ge–H bond, in which the photoexcited HAT photocatalyst (PC HAT ) can initiate germanium-centered radicals effectively instead of the combined use of PC SET with Abs. By incorporating a radical leaving group (LG • ) into the terminal of unsaturated SOMOphiles, a streamlined germylation is thereby achieved through a sequence involving germyl radical addiction/β-cleavage of LG • . Meanwhile, the LG • captures the hydrogen atom from PC HAT –H to realize the photocata-lytic cycles without using any Abs. (Scheme 1D). The synthetic challenges of this protocol include (i) SET induced side processescompeting over the direct HAT; (ii) hydrogermylation of unsaturated C–C bonds instead of direct germylation; (iii) competitive HAT of C(sp 3 )–H instead of Ge–H as well as relatively low thermodynamic driving force for Ge–C bond formation. Herein, in this work, an unprecedented photocatalytic germylation via d -HAT was developed by reacting triphenylgermane with hypervalent iodine reagents, unlocking access to structurally unique and pharmaceutically potential organogermanium compounds that are difficult to synthesize by routine methods. Table 1 Optimization of reaction conditions. B Optimization of PC HAT a C Variation of standard conditions a Entry Variation of standard conditions Yield (%) b 1 none 78 2 without PC HAT 9 n.d. 3 without light n.d. 4 O 2 /air instead of N 2 n.d./n.d. 5 2b / 2c / 2d / 2e instead of 2a 35/30/20/42 a Standard reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol), and PC HAT (5 mol%) in DCE (dichloroethane, 1 mL) under irradiation of λ use under N 2 at rt for 12 h. Isolated yields of 3 were given. a Standard reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol), and PC HAT 9 (5 mol%) in DMSO (dimethyl sulfoxide, 1 mL) under irradiation of 10 W 430 nm blue LED light under N 2 at rt for 12 h. b Isolated yields of 3 were given. n.d. = not detected. Results and Discussion To validate our hypothesis, a range of PC HAT were selected specifically aiming at catalyzing the generation of germyl radical from hydrogermane. The PC HAT employed included sodium decatungstate (NaDT), tetrabutylammonium decatungstate (TBADT), acetophenone, xanthone, benzophenone, 9-fluorenone, anthraquinone, thioxanthone, 9,10-phenanthraquinone, 5,7,12,14-pentacenetetrone, and eosin Y, collectively termed as PC HAT 1 – 11 . These photocatalysts were categorized in accordance with the responsiveness to either visible or ultraviolet (UV) light, and corresponding light sources were matched based on their optimal excitation wavelengths ( λ use ) as documented in previous literature, [15] thus allowing a comprehensive evaluation of their efficiency under unified conditions (Table 1A). Subsequently, the model reaction was established as reacting triphenylgermane ( 1 ) with ethynylbenziodoxolone (EBX, 2a ) [17] in DCE in the nitrogen atmosphere at room temperature under irradiation for 12 h. To our delight, by varying the PC HAT , each group can afford the germylation product, triphenyl(phenylethynyl)germane ( 3 ), with isolated yields ranging from < 5% to 70%, among which PC HAT 1 , PC HAT 4 , and PC HAT 9 outperformed the others to obtain the comparable yields of 68%, 70%, and 68%, respectively. Considering the milder nature of visible light in comparison to ultraviolet light, PC HAT 9 was ultimately chosen as the optimal photocatalyst (Table 1B). After extensive experiments, the standard conditions of the model reaction were established as follows: 1 (0.1 mmol), 2a (0.2 mmol), and PC HAT 9 (5 mol%) in DMSO (1 mL) under irradiation of 10 W 430 nm blue LED light at room temperature for 12 h under N 2 (Table 1C, entry 1 and see Table S1 for details). Further variations of standard conditions were performed to assess the influence of reaction parameters including photocatalysts, light, atmosphere, and EBX reagents on yields of product 3 . The investigations revealed that no conversion of substrate to the desired product was observed in the absence of the photocatalyst PC HAT 9 or any form of light, unambiguously demonstrating the indispensable role played by both the photocatalyst and light in this photocatalytic process (Table 1C, entries 2 and 3). Additionally, reactions conducted in the presence of oxygen or air failed to deliver any product, probably due to the common quenching effect of triplet oxygen on photoexcited photocatalysts (Table 1C, entry 4). [18] When 2a was exchanged to substituted EBX reagents ( 2d – e ), a notable decrease in yield was obtained (Table 1C, entry 5), inferring that 2a itself might be the optimal SOMOphile under standard conditions. With optimized reaction conditions established, the substrate scopes of the newly developed protocol for photocatalytic germylation were comprehensively investigated (Scheme 2). Initially, various phenyl-substituted EBX reagents 2 featuring alkyl groups positioned on the benzene motif ( i.e. 4-Me, 3-Me, 2-Me, 3,5-Me, 3,4-Me, 4-Et, 4- i Pr, 4- t Bu) were reacted with triphenylgermane 1 under standard reaction conditions, resulting in the production of corresponding targets 3–11 with satisfactory yields ranging from 40–65%. It was worth emphasizing that no alkylation-type products were detected in the reaction system, highlighting the commendable chemical selectivity through this direct HAT strategy. When substrate 2 bearing electron-donating groups (EDGs) like 4-Ph and 4-OMe were tested, comparable yields of 50% were achieved, respectively (12 and 13). Altering the substitutes to halo groups, such as 4-F, 3-F, 2-F, 3,5-F, 4-Cl, 3-Cl-5-F, 2-F-4-Cl, 4-Br, led to the formation of desired products in good-to-moderate yields (14–21). The incorporation of electron-withdrawing groups (EWGs, i.e. 4-CF 3 , 3,5-CF 3 , 4-CN, 4-CHO, 4-CO 2 Et) in the benzene motif had no significant electronic effect on outcomes of generating products (22–25). Further use of polycyclic and heterocyclic aromatics in place of phenyl rings were well-compatible with this protocol, 2-naphthalenyl, 1-naphthalenyl, 9-phenanthrenyl, 2-thiophenyl, 3-thiopheny substituted ethynylgermane (26–30) were successfully synthesized. When product 3 was treated by azidotrimethylsilane, a triphenylgermyl-functionalized triazole 31 was synthesized via classic click reaction with a two-step total yield of 50% (see Scheme S9 for details). Additionally, bioactively and pharmaceutically relevant EBX-based ester derivatives originating from flurbiprofen, ibuprofen, naproxen, dicamba, ciprofibrate, oxaprozin, benzbromarone, sesamol, and guaiacol were tested in the established method to highlight its practical applicability in drug structure modification. The successful germylation of these compounds showcased the potential of this protocol for creating complex organogermanium compounds with a high degree of specificity and efficiency (32–40). To validate the synthetic versatility, more hypervalent iodine reagents were then evaluated with the intention of accessing structurally diverse organogermanium compounds. Pleasingly, this strategy could be extended to the selective germylation of vinyl (41), nitrile (42), and phenyl (43) groups to form corresponding products. To the best of our knowledge, our germylation strategy stands as one of the few photocatalytic methods available for synthesizing such organogermanium compounds. Among the targets obtained, the structure of 3 was further confirmed by X-ray crystallography (CCDC: 2391318, see Table S2 and Figure S8 for details). To probe a comprehensive understanding of reaction pathways, mechanistic studies were conducted on model reactions to elucidate the underlying mechanism. When (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) and 2,6-di- tert -butyl-4-methylphenol (BHT), and 1,1-diphenylethylene ( 1,1-DPE), three classical radical scavengers, were introduced to the reaction system under standard conditions, the model reaction was thoroughly suppressed, suggesting that this transformation might involve a radical pathway. Notably, TEMPO- germyl radical adduct 44 and 1,1-DPE-carboxyl radical adduct 45 ( m/z = 462.1847 and 427.0189 assigned to [C 27 H 34 NOGe] + and [C 21 H 16 O 2 l] + ) were detected by high-resolution mass spectrometry (HRMS) during the radical scavenging experiments, definitely aligning with our hypothesis proposed from generation of germyl radical and β-cleavage of LG radical (Scheme 3A and see Figure S2–3 for details). Further radical clocks and kinetic isotope effects (KIE) experiments were carried out by employing diethyl 2,2-diallylmalonate as a mechanistic probe and kinetic standard. [19] As outlined in Scheme 3B, a hydrogermylation product 46 was synthesized via radical 5- exo -trig cyclization with a yield of 64% under the standard conditions, suggesting the intervention of a hydrogen atom transfer (HAT) process within the transformation. To scrutinize the rate-determining step, an intermolecular KIE experiment was performed utilizing a 1:1 mixture of substrate 1 and its deuterium-labeled counterpart 1- d . A k H / k D ratio of 2.3 for products 46 and 46- d was observed, indicating that the HAT of the Ge−H bond is likely the rate-determining step. [20] Subsequently, to understand the interaction dynamics of PC HAT 9, fluorescence quenching experiments alongside Stern-Volmer analysis were carried out. The research findings revealed a linear attenuation in the fluorescence intensity of PC HAT 9 with the incremental concentration of either 1 or 2a. As expected, 1 demonstrated a more robust quenching effect on the luminescence emission of PC HAT 9 compared to 2a, implying that a specific interaction between PC HAT 9 and 1 exists toward germyl radical generation (Scheme 3A and Figure S6). As depicted in Scheme 3D, LED irradiation on/off experiments were utilized to explore the role of visible light in this transformation. The model reaction was subjected to period of 12 h, and the yields of 3 were recorded at the end of every interval. As can be seen, the yields were significantly increased during irradiation of 430 nm LED light. Even in the absence of light, slight increases in yield were still observed, demonstrating that a radical propagation process should be a complementary pathway within the mechanism (see Figure S7 for details). [21] In light of our experimental results, a tentative photocatalytic mechanism for this protocol was proposed as showcased in Scheme 3E. Initially, ground-state PC HAT 9 underwent photoactivation to its excited state, denoted as *PC HAT 9, resulting in an oxygen-centered diradical species that can facilitate the generation of the key germyl radical (Int. I) from triphenylgermane 1 via a d - Scheme 2 Substrate scope of photocatalytic germylation. HAT of Ge–H bond. This enabled *PC HAT 9 to form the corresponding phenanthrenol-type H-PC HAT 9. Subsequently, the radical addition of Int. I onto the SOMOphile 2a led to the formation of vinyl radical Int. II, which underwent spontaneous β-cleavage of carboxyl radical Int. III to produce the final germylation product 3. The successful implementation of the photocatalytic cycle was achieved by a retro-hydrogen atom transfer (RHAT) process, wherein the hydrogen atom from H-PC HAT 9 is circulating to carboxyl radical Int. III, regenerating the ground state PC HAT 9. Besides, a propagation process cannot be completely excluded at this stage, as HAT can also occur between the 1 and the in situ generated carboxyl radical Int. III to give germyl radical Int. I, which aligns with the observations obtained from LED irradiation on/off experiments. On a basis of previous references, [22] 9,10-phenanthraquinone (PQ, PC HAT 9) can also serve as PC SET in reactions, whose excited state and ground state reduction potential was calculated as \(\text{E}_{\text{1/2}}^{\text{red}}\) *(PQ*/PQ •− ) = +1.60 V, and \(\text{E}_{\text{1/2}}^{\text{red}}\) (PQ/PQ •− ) = −0.52 V (versus SCE). [22b, 22c] The oxidation potential ( \(\text{E}_{\text{1/2}}^{\text{ox}}\) ) for 1 and reduction potential ( \(\text{E}_{\text{1/2}}^{\text{red}}\) ) for 2a studied in this work are reported as +0.94 V [5b] and −0.94 V [23] (versus SCE). This suggests that PQ •− cannot reduce 2a to realize the photocatalytic cycle, thus ruling out the SET pathway in this protocol (see Scheme S13 for details). Scheme 3 Mechanistic study. Conclusions In conclusion, an unprecedented photocatalytic d -HAT protocol of the Ge–H bond was developed, wherein the photoexcited 9,10-phenanthraquinone ( PC HAT 9 ) efficiently initiates the triphenylgermane to generate germanium-centered radicals, marking a departure from the conventional reliance on the combined use of PC SET with hydrogen abstractors. By employing hypervalent iodine reagents as SOMOphiles, a streamlined germylation is achieved through a sequence involving germyl radical addiction/β-cleavage of carboxyl radical, leading to the successful synthesis of ethynyl-, vinyl-, nitrile-, phenyl-functionalized germanes. By leveraging innovative mechanisms, this research not only advances the methodology for generating germanium radicals from Ge–H bonds via d -HAT process but also unlocks access to a diverse array of structurally unique and pharmaceutically potential organogermanium compounds that are difficult to synthesize by routine techniques. Experimental General procedure for photocatalytic germylation. In a 25 mL Schlenk tube, a mixture of 1 (0.1 mmol), 2 (0.2 mmol, 2.0 equiv), and PC HAT 9 (5 mol%) in 1 mL DMSO was allowed to stir with irradiation of 10 W 430 nm blue LED under N 2 at rt for 12 h. The reaction was monitored by TLC. The reaction extracted with ethyl acetate (3 × 5 mL) and saline solution. The combined organic layer was dried over Na 2 SO 4 and concentrated. The residue was purified by chromatography on silica gel using petroleum ether/ethyl acetate as eluent to afford the desired product 3–45. Supporting Information The supporting information for this article is available on the WWW under https://doi.org/10.1002/cjoc.202400xxx. Acknowledgement This work was supported by the National Key R&D Program of China (2021YFB4001100, 2021YFB4001101), the National Natural Science Foundation of China (22071222 and 22171249), the Science & Technology Innovation Talents in Universities of Henan Province (23HASTIT003), the Key Research Projects of Universities in Henan Province (23A150054), Natural Science Foundation of Henan Province (232300421363, 242300420185), the Key Scientific and Technological Project of Henan Province (232102230140), the Doctor Foundation of Henan University of Engineering (D2022004). References 23. (a) Scappucci, G.；Kloeffel, C.；Zwanenburg, F. A.；Loss, D.；Myronov, M.；Zhang, J.-J.；De Franceschi, S.；Katsaros, G.；Veldhorst, M. The germanium quantum information route. Nat. Rev. Mater. 2021 , 6 , 926-943; (b) Vaughn, D. D., II；Schaak, R. E. 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Manuscript received: XXXX, 2024 Manuscript revised: XXXX, 2024 Manuscript accepted: XXXX, 2024 Version of record online: XXXX, 2024 Entry for the Table of Contents Photocatalytic Germylation via Direct Hydrogen Atom Transfer Wenshan Wang, Yan Liu, Gonghong Qiu, Igor B. Krylov, Alexander O. Terent’ev, Shuxia Cao, Kai Sun, Xiaolan Chen, Lingbo Qu, and Bing Yu Chin. J. Chem. 2024 , 42 , XXX—XXX. DOI: 10.1002/cjoc.202400XXX An innovative visible-light-driven direct hydrogen atom transfer (d-HAT) of Ge–H bond was reported. The power of this strategy is demonstrated for the access to structurally unique organogermanium compounds that are difficult to synthesize by routine methods. Supplementary Material File (image10.emf) Download 24.77 MB File (image4.emf) Download 448.83 KB File (image9.emf) Download 9.04 MB Information & Authors Information Version history V1 Version 1 12 February 2025 Peer review timeline Published Chinese Journal of Chemistry Version of Record 18 Apr 2025 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection Chinese Journal of Chemistry Keywords direct hydrogen atom transfer germyl radical germylation photocatalysis Authors Affiliations Wenshan Wang Zhengzhou University College of Chemistry View all articles by this author Yan Liu Zhengzhou University College of Chemistry View all articles by this author Gonghong Qiu Zhengzhou University College of Chemistry View all articles by this author Igor B. Krylov FGBUN Institut organiceskoj himii imeni N D Zelinskogo Rossijskoj akademii nauk View all articles by this author Alexander O. Terent’ev FGBUN Institut organiceskoj himii imeni N D Zelinskogo Rossijskoj akademii nauk View all articles by this author Shuxia Cao Zhengzhou University College of Chemistry View all articles by this author Kai Sun 0000-0003-2135-0838 Zhengzhou University College of Chemistry View all articles by this author Xiaolan Chen Zhengzhou University College of Chemistry View all articles by this author Lingbo Qu Zhengzhou University College of Chemistry View all articles by this author Bing Yu 0000-0002-2423-1212 [email protected] Zhengzhou University College of Chemistry View all articles by this author Metrics & Citations Metrics Article Usage 436 views 292 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Wenshan Wang, Yan Liu, Gonghong Qiu, et al. Photocatalytic Germylation via Direct Hydrogen Atom Transfer. Authorea . 12 February 2025. 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