A Density Functional Theory Investigation of the Cobalt‐Mediated η⁵‐Pentadienyl/Alkyne [5+2] Cycloaddition Reaction: Mechanistic Insight and Substituent Effects

Abstract

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.

Mechanistic Studies and Expansion of the Substrate Scope of Direct Enantioselective Alkynylation of α-Ketiminoesters Catalyzed by Adaptable (Phebox)Rhodium(III) Complexes

“…The observed inverse kinetic isotope effects could be due to hybridization-induced changes of the terminal carbon of the alkyne 3 from sp to sp 2 in the transition state . On the basis of the fact that hybridization of the carbon on alkynes changes from sp to sp 2 upon coordination to transition metal complexes, coordination of alkyne 3 to (amido)­rhodium­(III) complex 6 , rather than deprotonation of the terminal alkyne 3 , would be the turnover-limiting step in the overall catalytic cycle …”

Cobalt-Mediated η5-Pentadienyl/Alkyne [5 + 2] Cycloaddition Reactions: Substitution Effects, Bicyclic Synthesis, and Photochemical η4-Cycloheptadiene Demetalation

“…To achieve reasonable reaction rates, the system is heated to 40 °C; under these conditions, formation of the intermediate η 2 ,η 3 -cycloheptadienyl complex is not observed and the reaction proceeds directly to the η 5 -dimethylcycloheptadienyl complexes 3 . The lower barrier to isomerization is attributed to weaker Co–alkene bonding arising from steric repulsion between the alkene methyl groups and the Cp* ancillary ligand, leading to facile alkene dissociation ( 2 → 5 in Scheme ) . Only cycloadduct 2e , obtained from 1,5-dimethylpentadienyl complex 1e (Table , entry 11), is kinetically stable under the reaction conditionsthe η 2 ,η 3 → η 5 isomerization is again blocked by the syn -methyl substituents.…”

“…The η 2 ,η 3 -cycloheptadienyl complex 2e decomposes without isomerization at elevated temperature, and the complex slowly decomposes in solution at room temperature. Interestingly, the cycloadditions of pentadienyl complexes 1c , e with 2-butyne proceed at room temperature to yield the η 5 -cycloheptadienyl complexes 3g (entry 7), characterized by X-ray diffraction (Figure ), and η 2 ,η 3 -cycloheptadienyl complex 2f (entry 11), respectively, suggesting a much higher degree of reactivity for terminally substituted pentadienyl systems …”

“…For Ph-substituted 7c , the ratio of η 2 ,η 3 to η 5 isomers is 7:1 (entry 6); in contrast, the Cp* analogue 2c shows significant isomerization even at room temperature in CH 2 Cl 2 . DFT studies have suggested the lower barrier to isomerization in the Cp* case is the result of increased steric repulsion between the alkene substituents and the methyl groups of the Cp* ancillary ligand, leading to facile dissociation of the alkene prior to β-hydride elimination ( 2 → 5 in Scheme ) …”

“…The reduced cycloaddition reactivity of the Cp vs the Cp* pentadienyl system is likely a result of differing steric effects. Computations have indicated that steric repulsion between the Cp* ancillary ligand and terminal pentadienyl substituents, manifested as a decrease in the dihedral angle between the pentadienyl planes, activates the system for cycloaddition (vide supra) . In the Cp system, the steric effects of the ancillary ligand are significantly reduced, likely leading to a larger dihedral angle and attenuated reactivity.…”