Theoretical investigation of enolborane addition to α-heteroatom-substituted aldehydes.: Relevance of the cornforth and polar Felkin-Anh models for asymmetric induction

The addition of enolborane nucleophiles to chiral alpha-heteroatom-substituted aldehydes (CH3CH(X)CHO, X = F, Cl, OMe, SMe, NMe2, and PMe2) was investigated using density functional theory by means of B3LYP/6-31G(d) calculations, with particular emphasis on determining the relevance of the polar Felkin-Anh and Comforth models for asymmetric induction in these reactions. The relative energy of the polar Felkin-Anh and Comforth transition-state structures is found to depend on the nature of the alpha-heteroatom substituent, with electronegative substituents (F, OMe, Cl) favoring Cornforth structures, while less electronegative substituents (PMe2, SMe, NMe2) favor polar Felkin-Anh structures. These transition-state preferences are correlated with the relative energy of the corresponding rotamer of the uncomplexed reactant aldehyde, indicating that the transition states are particularly sensitive to the conformation of the aldehyde. The proposed Nu -> sigma*(C-X) interaction that forms the basis of the polar Felkin-Anh model appears to be insignificant in reactions with enolborane nucleophiles. The calculated transition-state structures for the addition of E- and Z-enolborane nucleophiles to 2-methoxypropanal predict a diasterecifacial selectivity that is in good agreement with the experimentally determined values.