Chiral-phosphoramide-catalyzed enantioselective allylation reactions: Mechanistic, structural and preparative studies.

摘要

The addition of allylic trichlorosilanes to aldehydes was promoted by chiral phosphoramides to give chiral homoallylic alcohols. From a combination of non-linear effect and kinetics studies, the reaction was found to be second order in phosphoramide. The presence of two phosphoramides in the stereochemically determining transition structure was further documented by the dependence of enantioselectivity on the tether length in a series of bisphosphoramides. A new phosphoramide based on a 2,2′-bispyrrolidine skeleton has been designed and provided good yield, efficient turnover and high enantioselectivity. The scope of the reaction was examined with various substituted allylic trichlorosilanes and unsaturated aldehydes.; Solution NMR spectroscopic and single crystal X-ray crystallographic studies on the bisphosphoramide•SnCl₄ complexes provided an understanding on the structure-selectivity correlation. The formation of cis-configured, octahedral 1/1 bisphosphoramide•SnCl₄ complexes was supported by both crystallographic and solution NMR studies. Single crystal X-ray analysis of bisphosphoramides•SnCl₄ complexes also provided detailed information on the stereochemical environment experienced in the allylation reactions.; The method of generating chiral Lewis acids with a combination of SiCl ₄ and chiral phosphoramides has been applied in the formal hetero-Diels-Alder reaction and modest to high selectivities have been obtained.