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

Abstract

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) × 10^-4 M^-1s^-1 at 26 °C). 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 (ΔS^‡ = −36(1) eu) and a modest enthalpic barrier (ΔH^‡ = 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.