Facile one-pot fabrication of a silica gel-supported chiral phase-transfer catalyst—N-(2-cyanobenzyl)-O(9)-allyl-cinchonidinium salt

Abstract

A novel type of silica gel-supported cinchona alkaloid-based quaternary ammonium salt was prepared by available one-pot synthesis for the first time through the free radical addition of the sulfhydryl group of 3-mercaptopropyltrimethoxysilane to an exocyclic carbon–carbon double bond in N-(2-cyanobenzyl)-O(9)-allyl-cinchonidinium bromide and subsequent hydrolysis of trimethoxysilane. In the α-alkylation of N-(diphenylmethylene)glycine tert-butyl ester with alkyl halides, it was found that the various substituted benzyl bromides, both with electron-withdrawing (–CF₃ and –F) and electron-donating (–CH₃) substituents, afforded the corresponding α-alkylation products with moderate to excellent enantioselectivities (76.0–96.9% ee) in high yields (80–96%). However, allyl bromides gave poor yields (10–50%) and enantioselectivities (52.0–67.1% ee). After completion of the α-alkylation reaction, the silica gel-supported chiral phase-transfer catalyst was readily recovered in quantitative yield by filtration and reused for five consecutive runs without significant loss in the catalytic performance.