Characterization and Dynamics of [Pd(L−L)H(solv)]⁺, [Pd(L−L)(CH₂CH₃)]⁺, and [Pd(L−L)(C(O)Et)(THF)]⁺ (L−L = 1,2-(CH₂PBu^t₂)₂C₆H₄): Key Intermediates in the Catalytic Methoxycarbonylation of Ethene to Methylpropanoate

Abstract

A detailed spectroscopic study has allowed the solution structure and dynamic properties of all the intermediates in the Pd-catalyzed methoxycarbonylation of ethene to be established. [Pd(L−L)H(solv)]+ 1 (L−L = 1,2-(CH2PBut2)2C6H4; solv = MeOH, 1a; PrnOH, 1b; THF, 1c; EtCN, 1d) is static, and the two inequivalent P atoms do not become equivalent through solvent exchange over all the temperatures studied. 2, contains a strong β-agostic C−H interaction which is remarkably stable and is not displaced even in strongly coordinating solvents such as EtCN. Cα and Cβ of the ethyl group in 2 become equivalent via a stereospecific interchange involving [Pd(L−L)H(η2-C2H4)]+ without making the two P atoms equivalent; at higher temperatures these two inequivalent P atoms do become equivalent probably via a T-shaped intermediate. For [Pd(L−L)(C(O)Et)(solv)]+, 6, there is no β-agostic C−H interaction and multiple 13C-labeling of the C(O)Et group shows that the inequivalent P atoms become equivalent via movement of the intact C(O)Et group. The crystal structure of the related complex [Pd(L−L)(C(O)Et)Cl] cocrystallized with dibenzylacetone has been determined.