SECONDARY BETA-DEUTERIUM KINETIC ISOTOPE EFFECTS IN REACTIONS OF GRIGNARD-REAGENTS

Kinetic isotope effects (KIEs) for addition reactions of beta-deuteriated Grignard reagents have been determined by isotopic separation of reaction products by capillary GC. Separation of products from the reactions studied required a minimum of three substitutions of deuterium for hydrogen. The method was used for the addition of beta-perdeuteriated ethyl, isopropyl, and tert-butyl-magnesium halide to benzophenone, 2-octanone, 4,4-dimethyl-1-phenyl-1-penten-3-one, and 1,3-diphenyl-2-propen-1-one. The observed effects were small, within +/- 5 %, apparently because a steric and a hyperconjugative effect were opposing each other. A relatively large KIE in the reaction of isopropylmagnesium bromide with benzophenone is interpretated as evidence of assistance from the beta-hydrogen in a six-center transition state in the initial homolysis.