Anomalous Effects of Para Substituents on the Stability of Sterically Congested Triplet Diarylcarbenes. The First Triplet Carbene Surviving over Minutes under Normal Conditions

摘要

For a long time, carbenes were believed not to be stable enough to be isolated in macroscopic amounts at room temperature. However, the recent syntheses of stable singlet carbenes, i.e., phosphinocarbenes and imidazol-2-ylidenes, have upset this extreme view. Naturally, however, interpretation of these species as free carbene has been a topic of debate, especially concerning the possible contribution of the ylidic character as a result of pπ--pπ delocalization.