Enantioselective Addition of Dialkylzinc Reagents to N-(Diphenylphosphinoyl) Imines Promoted by 2-Azanorbornylmethanols.
A set of new beta-amino alcohols 2, with the 2-azanorbornyl framework, has been prepared and evaluated as promoters for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl) imines 1. By variation of the substitution pattern in the ligand, ee's up to 92% could be obtained. Although a stoichiometric amount of the ligand was used, about 90% of it could be recovered during the workup. Amino alcohol 2b, that gave the best enantioselectivities in the stoichiometric reaction, was also applied in a catalytic process, and ee's up to 85% were achieved using 0.25 equiv of the ligand, which is the highest ee obtained so far using that catalytic amount of the ligand. Addition products 3 could be converted into the free amines 4 without racemization by acidic hydrolysis. The utility of ligands 2 as catalysts in the addition of diethylzinc to benzaldehyde has also been investigated, and ee's up to 75% were achieved.
Guijarro
David
D
Department of Organic Chemistry, Uppsala University, Box 531, S-751 21 Uppsala, Sweden.
Pinho
Pedro
P
Andersson
Pher G.
PG