Investigations of the magnitude of steric and α deuterium kinetic isotope effects in a carboncarbon bond-forming reaction of a permethylscandocene complex

Abstract

No measurable isotope effect is observed in the reaction of (Cp*- d 15) 2Sc-CH 3 ( 1a, Cp* = η 5-C 5(CH 3) 5) with CD 3CCCH 3 to yield a mixture of (Cp*- d 15) 2Sc-C(CD 3)CC(CH 3) 2 ( 2a) and (Cp*- d 15) 2ScC(CH 3)CC(CH 3)(CD 3) ( 2b) ( 2b : 2a = 1.01±0.02:1). Thus steric repulsions between the 2-butyne methyl group and the scandium-bound methyl group are not sufficiently severe to give rise to a measurable steric deuterium kinetic isotope effect. Similarly, 1a reacts with CH 3CCCH 3 at approximately the same rate as does (Cp*- d 15) 2ScCD 3 ( 1b), producing a mixture of (Cp*- d 15) 2Sc-C(CH 3)CC(CH 3) 2 ( 2c) and (Cp*- d 15) 2ScC(CH 3)CC(CD 3)(CH 3) ( 2d) ( k 2c / k 2d = 1.02±0.07). The implication from the latter finding is that a [Sc(η 2-CH 2H)] α agostic interaction is likely not present in the transition state for 2-butyne insertion into the scandium methyl bond of 1.