Quantum Chemistry Studies On The Stereoselectivity Of Diffierent Aldehydes In The Direct Aldol Reactions

Ab initio (HF) and density functional theory (B3LYP) in G98 were employed to study the following four direct aldol reactions between acetone and isobutyraldehyde or 3,3-dimethylbutyric aldehyde or benzaldehyde or 4-bromo-phenyl aldehyde catalyzed by (S)-proline .The relative stabilities of R and S transition states formed in the stereocontrolling step reasonably explain the stereoselectivity and overwhelming handedness of the final products of the aldol reactions.For the stereocontrolling step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of enamine relative to the carboxylic acid group of (S)-proline and the two diastereoisomeric approach modes to the re and si of the carbonyl group of the aldehyde were studied. The compted results support and reinforce the List et. al proposed enamine mechanism.By compared the (S)-proline-catalyzed aldol reaction acetone and isobutyraldehyde with acetone and 3,3-dimethylbutyric aldehyde,It could reaveal the experiment fact that in the fatty aldehydes more the　s?ubstituents better the stereoselectivity .By compared the (S)-proline-catalyzed aldol reaction acetone and benzaldehyde with acetone and 4-bromo-phenyl aldehyde,It could reaveal the experiment fact that added the absorbing electron bases into 4-place of the aromatic aldehydes could improve the stereoselectivity to the aldol reactions .