[Co(TPP)]–Catalyzed Carbene Transfer from Acceptor–Acceptor Iodonium Ylides via N-enolate Carbene Radicals

Abstract Square-planar cobalt(II)-systems have emerged as powerful carbene transfer catalysts for the synthesis of numerous (hetero)cyclic compounds via cobalt(III)-carbene radical intermediates. Spectroscopic detection and characterization of reactive carbene radical intermediates is limited to a few scattered experiments, centering around mono-substituted carbenes. Here, we reveal the unique formation of disubstituted cobalt(III)-carbene radicals derived from a cobalt(II)-porphyrin complex and acceptor–acceptor λ3-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application. Particularly noteworthy is the fact that iodonium ylides generate novel bis-carbenoid species via reversible ligand modification of the paramagnetic [Co(TPP)]-catalyst. Two interconnected catalytic cycles are involved in the overall mechanism, with a mono-carbene radical and an unprecedented N-enolate-carbene radical intermediate at the heart of each respective cycle. Notably, N-enolate formation is not a deactivation pathway, and both the N-enolate and carbene radical moieties can be transferred to styrene. The findings are supported by extensive experimental and computational studies.