Aminosulf(ox)ides as ligands for iridium(I)-catalyzed asymmetric transfer hydrogenation

A new class of efficient catalysts was developed for the asymmetric transfe r hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates ( 6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduc tion of ketones. Both formic acid and a-propanol proved to be suitable as h ydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provide d a diastereomeric ligand family containing a chiral sulfur atom. The two c hiral centers of these ligands showed a clear effect of chiral cooperativit y. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcoho ls and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the r eaction was favorably affected by selecting the most appropriate hydrogen d onor. Enantioselectivities of up to 97% were reached in the reduction of ar yl-alkyl ketones.