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- CH2PBu2t)2C6H4 ): Key intermediates in the catalytic methoxycarbonylation of ethene to methylpropanoate

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-(CH2PBu2t)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. [Pd(L-)(CH2CH3)]+, 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.