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-(CH 2 PBu t 2 ) 2 C 6 H 4; solv = MeOH, 1a; Pr n OH, 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. C α and C β of the ethyl group in 2 become equivalent via a stereospecific interchange involving [Pd(L−L)H( η 2 -C 2 H 4 )] + 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 13 C-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.