Origin of Product Selectivity in Yttrium-Catalyzed Benzylic C–H Alkylations of Alkylpyridines with Olefins: A DFT Study

DFT studies have been conducted for the direct benzylic C(sp3)–H alkylation of alkylpyridines with olefins catalyzed by a cationic half-sandwich yttrium alkyl complex. It has been found that, in the case of 2-tert-butyl-6-methylpyridine, the successive insertion of two molecules of ethylene, achieving butylation, was the outcome of kinetics. However, the continuous insertion of the third ethylene for hexylation was unfavorable both kinetically and thermodynamically in comparison with C–H activation to release the butylation product, which is in agreement with experimental results. The energy decomposition analyses disclosed that the steric repulsion between the two tBu groups of pyridyl moieties made the C–H activation of the one-ethylene preinserted intermediate relatively unfavorable. In contrast, in the case of 2,6-lutidine, the resulting monoethylation intermediate via feasible ethylene insertion favorably promotes C–H activation of another molecule of 2,6-lutidine rather than undergoes successive eth...