Asymmetric phase transfer catalysts derived from the cinchona alkaloids

Abstract

Since the first reported use of asymmetric phase transfer catalysts (PTC) derived from the cinchona alkaloids in 1989, the present third generation N-9-anthracenylmethyl quaternary salts 1 and 2 have been shown to catalyse a number of reactions, affording optically active products with high ee's. The high enantioselectivity of these salts is due to their well-defined geometries, where three of the four tetrahedron faces around the quaternary nitrogen cation are effectively blocked by the other components of the structure, leaving one relatively open face allowing intimate contact with the counterion of the substrate. This complex can then be attacked enantioselectively. Effective mising of phases during reaction is necessary, but more recently, catalysts 1(b) and 2(b) have been successfully employed with Schwesinger bases, in homogeneous solution, yielding similar ee's.