Density functional theory has been applied to study the influences of alpha-substituents in aliphatic aldehydes on the enantioselectivities of the (S)-proline-catalyzed direct aldol reactions. Reaction scenarios of three kinds of aliphatic aldehydes were investigated. Four transition states associated with the stereocontrolling step of each reaction have been obtained. They are corresponding to the syn and anti arrangements of enamine intermediates and the si and re attacks to the carbonyl group of an aldehyde. The solvent effect of DMSO was investigated using self-consistent reaction field method based on the polarizable continuum model. The computed energies of transition states explain the origin of the catalysis and enantioselectivities for these (S)-proline-catalyzed aldol reactions and reveal the influences of alpha-substituents in aliphatic aldehydes on the enantioselectivities of these reactions.
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