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

作者：Ylijoki Kai E O; Budzelaar Peter H M^*; Stryker Jeffrey M

来源：Chemistry - A European Journal, 2012, 18(32): 9894-9900.

DOI：10.1002/chem.201200319

• coordination modes
• cyclopentadienyl ligands
• density functional calculations
• ligand effects
• substituent effects

Alkyl-substituted ?5-pentadienyl half-sandwich complexes of cobalt have been reported to undergo [5+2] cycloaddition reactions with alkynes to provide ?2,?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 ?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 ?2,?3-cycloheptadienyl complex has been formed, isomerization to ?5-cycloheptadienyl again involves several steps: olefin decoordination, beta-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.

• 出版日期2012-8