SOLVENT AND COUNTERION EFFECT ON THE REACTION OF ARYLMETHANEPHOSPHONATE CARBANION METAL COMPLEXES WITH SCHIFF BASES

Abstract

Abstract The influence of solvent and metal ion on the reaction of diethyl esters of phenyl-, 4-methylphenyl- and 4-chlorophenylmethanephosphonic acid la-c and N-benzylidene aniline in the presence of alkaline amides or butyllithium is studied. A strong effect of the counterion on the stereochemistry of the aldol stage of the reaction is established. Complete stereochemical control of the reaction is observed with esters la-c using LiNH2 as metallating agent in ether at 10°C, 1 RS2 RS (erythro) adducts being formed as the only or main products. In the presence of NaNH2 or KNH2 the reaction is considerably or completely shifted to the more stable 1 RS2SR (threo) isomers. In better coordinating solvents (THF, DMF, HMPT), however, the stereoselectivity of the reaction even in the presence of LiNH2 decreases (THF, DMF) or threo adduct predominates (HMPT). The 4-chloro-phenylmethanephosphonic ester gives a constant ratio diastereomeric mixture with KNH2 and NaNH2 in all mentioned solvents. The stereoselectivit...