title: Lewis base activation of lewis acids. Addition of silyl ketene acetals to aldehydes.
abstract: The weak Lewis acid silicon tetrachloride can be activated by catalytic amounts of the chiral bisphosphoramide (R,R)-3 to form a highly reactive, chiral trichlorosilyl cation which is an extremely effective promoter of aldol addition reactions between aldehydes and silyl ketene acetals. The tert-butyldimethylsilyl ketene acetal of methyl acetate adds nearly instantaneously to aromatic and olefinic aldehydes as well as aliphatic aldehydes (albeit more slowly) with excellent enantioselectivity. The homologous tert-butyldimethylsilyl ketene acetal of tert-butyl propanoate adds with nearly exclusive anti diastereoselectivity to a similar range of aldehydes also with excellent enantioselectivity. The origin of the slower reaction rate with aliphatic aldehydes is revealed to be the formation of chlorosilyl ether adducts.
authors.affiliation: Roger Adams Laboratory, Department of Chemistry, University of Illinois, Urbana 61801, USA. denmark@scs.uiuc.edu
authors.forename: Scott E
authors.initials: SE
authors.lastname: Denmark
authors.forename: Thomas
authors.initials: T
authors.lastname: Wynn
authors.forename: Gregory L
authors.initials: GL
authors.lastname: Beutner