A Density Functional Theory Investigation of the Cobalt‐Mediated η 5 ‐Pentadienyl/Alkyne [5+2] Cycloaddition Reaction: Mechanistic Insight and Substituent Effects

Alkyl‐substituted η 5 ‐pentadienyl half‐sandwich complexes of cobalt have been reported to undergo [5+2] cycloaddition reactions with alkynes to provide η 2 ,η 3 ‐cycloheptadienyl complexes under kinetic control. DFT studies have been used to elucidate the mechanism of the cyclization reaction as well as that of the subsequent isomerization to the final η 5 ‐cycloheptadienyl product. The initial cyclization is a stepwise process of olefin decoordination/alkyne capture, CC bond formation, olefin arm capture, and a second CC bond formation; the initial decoordination/capture step is rate‐limiting. Once the η 2 ,η 3 ‐cycloheptadienyl complex has been formed, isomerization to η 5 ‐cycloheptadienyl again involves several steps: olefin decoordination, β‐hydride elimination, reinsertion, and olefin coordination; also here the initial decoordination step is rate limiting. Substituents strongly affect the ease of reaction. Pentadienyl substituents in the 1‐ and 5‐positions assist pentadienyl opening and hence accelerate the reaction, while substituents at the 3‐position have a strongly retarding effect on the same step. Substituents at the alkyne (2‐butyne vs. ethyne) result in much faster isomerization due to easier olefin decoordination. Paths involving triplet states do not appear to be competitive.