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
Rh2(S-PTAD)4 and Rh2(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.