摘要:
Chiral and C2-symmetrical 2,6-bis(4'-R-oxazolin-2'-yl)pyridines (pybox, 1a-e, R = i-Pr, sec-Bu, t-Bu, Et, and Ph) have been newly designed and synthesized from the corresponding optically active beta-amino alcohols and pyridine-2,6-dicarboxylic acid as auxiliaries for metal-catalyzed reactions. We have found that the trivalent rhodium-pybox complexes 2a-e can act as catalysts for asymmetric reduction of ketones with diphenylsilane. The (S,S)-ip-pybox-rhodium complex 2a (1 mol % with respect to the ketone) with the aid of AgBF4 has exhibited an extremely high level of enantioselectivity for the reduction of acetophenone derivatives, above 90% ee on the average. Under the same reaction conditions, we have attained complete selection of the prochiral face of 1-tetralone in 99% ee. Several other ketones also have relatively higher results e.g. 95% ee for ethyl levulinate, 94% ee for 1-acetylnaphthalene, and 63% ee for 2-octanone. We have examined alpha,beta-unsaturated ketones, resulting in an exclusive 1,2-reduction for benzalacetone, beta-ionone, and chalcone but in lower enantioselection. We have also examined the effect of the substituents on the pybox ligands in the reduction of acetophenone and ethyl levulinate. In the mixed-ligand experiments a facile ligand-exchange reaction between the coordinating pybox and the free pybox ligand in the reaction media was observed, resulting in complete linearity of the enantiomeric excess between the product and the catalytic system.