Chiral Phosphoramide-Catalyzed Enantioselective Addition of Allylic Trichlorosilanes to Aldehydes. Preparative Studies with Bidentate Phosphorus-Based Amides

On the basis of the mechanistic insight that more than one Lewis basic moiety (phosphoramide) is involved
in the rate- and stereochemistry-determining step of enantioselective allylation, bidentate chiral phosphoramides were developed. Different chiral phosphoramide moieties were connected by tethers of methylene
chains of varying length. The rate and enantioselectivity of allylation with allyltrichlorosilane promoted
by the bidentate phosphoramides was found to be highly dependent on the tether length. A new
phosphoramide based on a 2,2‘-bispyrrolidine skeleton has been designed and afforded good yield, efficient
turnover, and high enantioselectivity in allylation reactions. The synthesis of enantiopure 2,2‘-bispyrrolidine
was easily accomplished on large scale by photodimerization of pyrrolidine followed by resolution with
l(or d)-tartaric acid. The scope of the allylation reaction was examined with variously substituted allylic
trichlorosilanes and unsaturated aldehydes. This method has been applied to the construction of stereogenic,
quaternary centers by the addition of unsymmetrically γ-disubstituted allylic trichlorosilanes.