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 H₂ O, ICF₃ , and BF₃ were studied under the conditions of matrix isolation. H₂ O was selected as typical hydrogen bond donor, ICF₃ as halogen bond donor, and BF₃ as strong Lewis acid. H₂ 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 ICF₃ , 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 BF₃ , all three carbenes form zwitterionic Lewis acid/base complexes.