Density functional computations of enantioselective alkynylation of aldehydes catalyzed by oxazaborolines. Part 2. Structures of transition states and the mechanism of enantioselective reduction

Quantum chemical computations of the enantioselective alkynylation of aldehyde with alkynylborane catalyzed by chiral oxazaborolidine are performed by means of the density functional theory method. All the transition states for the alkynylation are optimized completely at the B3LYP/6-31G(d) level and the mechanism of the enantioselective alkynylation is studied. As demonstrated, the enantioselective alkynylation of aldehyde is exothermic and includes the formation of the catalyst-alkynylborane adduct, the formation of the catalyst-alkynylborane-aldehyde adduct, the transfer of alkynyl to the carbonyl carbon of the aldehyde moiety, and the decomposition of the catalyst-alkoxyborane adduct leading to the regeneration of the catalyst. There are four reduction paths for the alkynylation reduction. The transfer of the alkynyl moiety is the controlling-velocity step for this reduction and the alkynylation results mainly in R-chiral alcohols that are in correspondence to the experiment.