CHIRAL LEWIS BASES ACTIVATION OF TRICHLOROSILYL DERIVATIVES

The design and synthesis of new chiral Lewis bases is a field of extraordinary activity. Among the different classes of Lewis Bases that are able to promote stereoselective reactions, we have studied three class of compounds: phosphoramides, phosphoramidates and phosphine oxides. These compounds, in presence of trichlorosilyl derivatives, generate hypervalent cationic species able to promote stereoselective C-H bond formations (by reaction with HSiCl3) and stereoselective C-C bond formations as allylation reactions of aldehydes and aldol condensations in presence of SiCl4. Phosphoramides and phosphoramidates were employed as catalysts in the enantioselective addition of allyltrichloro silane to aldehydes and in the reduction of keto-imines promoted by trichlorosilane; (S)-tetramethyl-bithiophene phosphine oxide [(S)-TetraMe-BITIOPO] instead, has been employed as catalysts in organic reactions involving the formation of a chiral Lewis acid starting from SiCl4. Important results have been obtained in the direct aldol reaction of ketones and activated thioesters to aromatic aldehydes. β-hydroxy-ketones have been obtained with good anti diastereoselecitivity (up to 86%) and excellent enantioselectivity for the major anti isomer (up to 95%). By the same procedure β-hydroxy trifluoroethylthioester can also be obtained with 80% yield, 98:2 syn/anti diastereoisomeric ratio and 89% of enantiomeric excess for the major stereoisomer.