Rhodium-Stabilized Diarylcarbenes Behaving as Donor/Acceptor Carbenes

Rhodium-stabilized diarylcarbenes typically generated from diaryldiazomethanes have been generally classified as donor/donor carbenes. This combined computational and experimental study demonstrates that diarylcarbenes display reactivity characteristics that are more reminiscent of donor/acceptor carbenes. When the reactions are carried out with the chiral dirhodium catalysts Rh-2(S-PTAD)(4) and Rh-2(S-NTTL)(4), highly enantioselective and diastereoselective cyclopropanations can be achieved, forming 1,1,2-triarylcyclopropanes. The reason for this behavior is because the two rings are unable to align in the plane of the rhodium carbene at the same time. The aryl ring aligned in the plane of the carbene behaves as a donor group, whereas the aryl ring aligned orthogonally behaves as an acceptor group.