Chiral Ligands Derived fromAbrine8. An Experimental and Theoretical Study of Free Ligand Conformational Preferences and the Addition of Diethylzinc to Benzaldehyde

Abstract

Three structurally similar series of 1,2,3,4-tetrahydro-beta-carboline ligands, 4a-d, 6a-d and 7a-d, and two series of chiral oxazolidines, 8a-d and 9a-g, were synthesized and used as chiral catalysts in the addition of diethylzinc to benzaldehyde. The enantioselectivities of the resulting 1-phenyl-1-propanol were obtained in each case, and these ee values were, in most cases, related to the conformational populations of the free ligand as expressed by the calculated differences in the energies of the ligand conformations formed by inversion at nitrogen. This suggested the possible existence of a linear free energy relationship. The effect on enantioselectivity of the carbon chain length of the R group located (1) on the C-3 substituent of 4a-d, 6a-d, and 7a-d or (2) at C-5 in 8a-d and 9a-g was studied in detail. On the basis of the correlations observed and the ligands' structural characterization, a structure was proposed for the transition state during ethyl group transfer when using ligands 8a-d. Furthermore, the change in enantioselectivity was successfully predicted when diastereomeric ligands 11 and 12 were compared in this chiral addition.