Dötz Benzannulation Reactions: Heteroatom and Substituent Effects in Chromium Fischer Carbene Complexes

We have carried out a theoretical investigation of the Dötz reaction between acetylene and a series of chromium Fischer‐type carbenes [(CO)₅Cr?C(X)R] with different representative substituents (R=CH?CH₂, Ph) and heteroatom ligands (X=OH, NH₂, OCH₃, N(CH₃)₂) by using density functional theory with the B3LYP functional. We have studied the Dötz and chromahexatriene mechanisms of benzannulation and also the reaction mechanism leading to cyclopentannulation. For the benzannulation, it was found that the most likely mechanism in the case of vinylcarbenes is the chromahexatriene route, whereas for phenylcarbenes, the Dötz route via a ketene intermediate is clearly the most favorable. The reactions leading to the cyclopentannulated and benzannulated products are more exothermic with vinylcarbenes than with phenylcarbenes and also more exothermic with alkoxycarbenes than with aminocarbenes. The relative stability of the cyclopentannulated products as compared with the benzannulated products increases for bulkier X substituents and on going from alkoxy‐ to aminocarbenes. The kinetic data concurs with the experimental product distribution found for vinylcarbenes, by which mainly benzannulated products are obtained, and dimethylaminophenylcarbenes, which lead exclusively to cyclopentannulated adducts.