Direct Cyclopropanation of α-Cyano β-Aryl Alkanes by Light-Mediated Single Electron Transfer Between Donor-Acceptor Pairs

Cyclopropanes are traditionally prepared by the formal [2+1] addition of carbene or radical based C1 units to alkenes. In contrast, the one-pot intermolecular cyclopropanation of alkanes by redox active C1 units has remained unrealised. Herein, we achieved this process simply by exposing beta-aryl propionitriles and C1 radical precursors (N-oxy esters) to base and blue light. The overall process is redox-neutral and a photocatalyst, whether metal- or organic-based, is not required. Our findings support that single electron transfer (SET) from the alpha-cyano carbanion of the propionitrile to the N-oxy ester is facilitated by blue-light via their electron donor-acceptor (EDA) complex. The alpha-cyano carbon radical thus formed can then lose a beta-proton to form a pi-resonance stabilised radical anion that preferentially couples at the benzylic beta-position with a decarboxylated C1 radical unit. This new transition metal-free chemistry tolerates both electron rich and electron deficient (hetero)aryl systems, even sulfide or alkene functionality, to afford a range of cis-aryl/cyano cyclopropanes bearing congested tetrasubstituted quaternary carbons.

Automated summary

A new transition metal-free synthesis method for cyclopropanes has been achieved, which tolerates various aryl systems, as well as sulfide or alkene functionality, to yield cis-aryl/cyano cyclopropanes bearing congested tetrasubstituted quaternary carbons.