Stereochemistry of addition to the carbonyl group. Part 17. Study of the factors affecting asymmetric induction in condensation reactions of methyl- and phenyl-magnesium bromide with chiral carbonyl compounds

Abstract

The dependence of the stereoselectivity upon the nature of the chiral corbonyl compound [2-methyl-3-phenyl-propanal (1), 2,3,3-trimethylbutanal (2), 4,4-dimethyl-3-phenylpentanal (3), 3-phenylbutanone (4), and 2-phenylpropanal (5)], the solvent, and the concentration of the reactive species for various condensation reactions of phenyl- and methyl-magnesium bromide at 30° is reported. The stereochemical results have been interpreted on the basis of the generalized Curtin–Hammett principle and according to a trigonal-type transition state (Pérez-Ossorio model). No effect of the concentration of the reactive species on the product ratio was observed. An unexpected influence of the solvent was observed in the condensation of (4) with phenylmagnesium bromide. A good linear correlation between stereoselectivity and the E_T parameter of the solvent was obtained. The explanation offered here takes into account the steric selection of the transition states and a further differentiating effect of the solvent polarity on them. A slight influence of solvent polarity on reactions of (5) with Grignard reagents was also noticed.