Comparative Reactivity of Different Types of Stable Cyclic and Acyclic Mono- and Diamino Carbenes with Simple Organic Substrates

A series
of stable carbenes, featuring a broad range of electronic properties,
were reacted with simple organic substrates. The N,N-dimesityl imida­zolyl­idene (NHC) does
not react with isocyanides, whereas anti-Bredt di­(amino)­carbene (pyr-NHC),
cyclic (alkyl)­(amino)­carbene (CAAC), acyclic di­(amino)­carbene (ADAC),
and acyclic (alkyl)­(amino)­carbene (AAAC) give rise to the corresponding
ketenimines. NHCs are known to promote the benzoin condensation, and
we found that the CAAC, pyr-NHC, and ADAC react with benzaldehyde
to give the ketone tautomer of the Breslow intermediate, whereas the
AAAC first gives the corresponding epoxide and ultimately the Breslow
intermediate, which can be isolated. Addition of excess benzaldehyde
to the latter does not lead to benzoin but to a stable 1,3-dioxolane.
Depending on the electronic properties of carbenes, different products
are also obtained with methyl acrylate as a substrate. The critical
role of the carbene electrophilicity on the outcome of reactions is
discussed.