Homogeneous catalysis with methane.: A strategy for the hydromethylation of olefins based on the nondegenerate exchange of alkyl groups and σ-bond metathesis at scandium

The scandium alkyl Cp*2ScCH2CMe3 (2) was synthesized by the addition of a pentane solution of LiCH2CMe3 to CP*2ScCl at low temperature. Compound 2 reacts with the C-H bonds of hydrocarbons including methane, benzene, and cyclopropane to yield the corresponding hydrocarbyl complex and CMe4. Kinetic studies revealed that the metalation of methane proceeds exclusively via a second-order pathway described by the rate law: rate = k[2][CH4] (k = 4.1(3) x 10(-4) M(-1)s(-1) at 26 degreesC). The primary inter- and intramolecular kinetic isotope effects (k(H)/k(D) = 10.2 (CH4 vs CD4) and k(H)/k(D) = 5.2(1) (CH2D2), respectively) are consistent with a linear transfer of hydrogen from methane to the neopentyl ligand in the transition state. Activation parameters indicate that the transformation involves a highly ordered transition state (DeltaS(double dagger) = -36(1) eu) and a modest enthalpic barrier (DeltaH(double dagger) = 11.4(1) kcal/mol). High selectivity toward methane activation suggested the participation of this chemistry in a catalytic hydromethylation, which was observed in the slow, CP*2ScMe-catalyzed addition of methane across the double bond of propene to form isobutane.