Reactions of Arylcarbenes with Lewis Acids

The reactions of the three triplet ground state arylcarbenes diphenylcarbene 1 , fluorenylidene 2 , and dibenzocycloheptadienylidene 3 with the Lewis acids H 2 O, ICF 3 , and BF 3 were studied under the conditions of matrix isolation. H 2 O was selected as typical hydrogen bond donor, ICF 3 as halogen bond donor, and BF 3 as strong Lewis acid. H 2 O forms hydrogen‐bonded complexes of the singlet carbenes with 1 and 2 , but not with 3 . This is rationalized by the larger singlet–triplet gap of 3 , which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding. With ICF 3 , both 1 and 3 form halogen‐bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF 3 , all three carbenes form zwitterionic Lewis acid/base complexes.