INVESTIGATIONS OF THE MAGNITUDE OF STERIC AND ALPHA-DEUTERIUM KINETIC ISOTOPE EFFECTS IN A CARBON CARBON BOND-FORMING REACTION OF A PERMETHYLSCANDOCENE COMPLEX

No measurable isotope effect is observed in the reaction of (Cp star-d15)2Sc-CH3 (1a, Cp star = eta-5-C5(CH3)5) with CD3C = CCH3 to yield a mixture of (Cp star-d15)2Sc-C(CD3)C = C(CH3)2 (2a) and (Cp star-d15)2Sc-C(CH3)C = C(CH3)(CD3) (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 CH3C = CCH3 at approximately the same rate as does (Cp star-d15)2Sc-CD3 (1b), producing a mixture of (Cp star-d15)2Sc-C(CH3)C = C(CH3)2 (2c) and (Cp star-d15)2Sc-C(CH3)C = C(CD3)(CH3) (2d) (k2c/k2d = 1.02 +/- 0.07). The implication from the latter finding is that a [Sc-(eta-2-CH2-H)] alpha agostic interaction is likely not present in the transition state for 2-butyne insertion into the scandium methyl bond of 1.