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 lambda(3)-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application. lodonium 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.

Automated summary

Square-planar cobalt(II) systems play a significant role in the synthesis of (hetero) cyclic compounds through the formation of cobalt(III)-carbene radical intermediates. This study reveals the formation of disubstituted cobalt(III)-carbene radicals and their catalytic application using cobalt(II)-tetraphenylporphyrin complex and iodonium ylides as carbene precursors. The findings highlight the importance of N-enolate formation in the reversible transfer of two carbene moieties.