Molecular modeling of salt (lithium chloride) effects on the enantioselectivity of diethylzinc addition to benzaldehyde in the presence of chiral beta-amino alcohols.
Beta-amino alcohols (S,S,S)-1 and (R,R,S)-1, derived from cyclohexene oxide and containing alpha-phenylethyl auxiliaries, were examined as chiral promoters in the addition of diethylzinc to benzaldehyde. In agreement with literature precedent, the N-alpha-phenylethyl chiral auxiliary had no significant impact on enantioinduction, which is determined by the configuration of the framework's C(OH), with unlike induction. Contrary to some literature reports, stereoinduction by lithium salt derivatives of (S,S,S)-1 and (R,R,S)-1 was lower than that obtained with the free amino alcohol. Remarkable lithium chloride salt effects were observed in the reaction. In particular, an opposite chiral induction was found with (S,S,S)-1-Li(2) as ligand and in the presence of "inert" salt. N-Alkylated derivatives (S,S,S)-3-7 proved to be more efficient ligands, providing higher yields and enantioselectivities in the formation of carbinols (R)- or (S)-2. BP86/DN**//PM3 theoretical calculations proved remarkably successful in reproducing the experimental observations and permitted expansion of Noyori's catalytic cycle [J. Am. Chem. Soc. 1995, 117, 6327] to understand the relevant N-substitution and medium salt effects that determine the enantioselection in this catalytic asymmetric reaction.
Sosa-Rivadeneyra
Martha
M
Centro de Investigación de la Facultad de Ciencias Químicas, Universidad Autónoma de Puebla, 72570 Puebla, México.
Muñoz-Muñiz
Omar
O
Anaya de Parrodi
Cecilia
C
Quintero
Leticia
L
Juaristi
Eusebio
E