Cobalt(II)–tetraphenylporphyrin-catalysed carbene transfer from acceptor–acceptor iodonium ylides via N-enolate–carbene radicals

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, centered around monosubstituted carbenes. Here, we reveal the formation of disubstituted cobalt(III)–carbene radicals derived from a cobalt(II)–tetraphenylporphyrin complex and acceptor–acceptor λ3-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application. Iodonium ylides generate biscarbenoid species via reversible ligand modification of the paramagnetic cobalt(II)–tetraphenylporphyrin complex catalyst. Two interconnected catalytic cycles are involved in the overall mechanism, with a monocarbene radical and an N-enolate–carbene radical intermediate at the heart of each respective cycle. Notably, N-enolate formation is not a deactivation pathway but a reversible process, enabling transfer of two carbene moieties from a single N-enolate–carbene radical intermediate. The findings are supported by extensive experimental and computational studies. Although cobalt–carbene radicals have proved to be highly versatile intermediates for homogeneous catalysis, their spectroscopic detection and characterization have been limited. Now, by using hypervalent iodonium ylides, the formation and spectroscopic detection of a biscarbenoid N-enolate–carbene radical—which undergoes a complex catalytic pathway involving reversible N-enolate formation—has been demonstrated.