Combining Meyer-Schuster Rearrangement with Aldol and Mannich Reactions: Theoretical Study of the Intermediate Interception Strategy

Interception of the transient allenyl enolate intermediate of the vanadium-catalyzed Meyer-Schuster rearrangement with aldehydes and imines has been studied computationally using density functional theory. Mechanistic details of the catalytic cycles for each of the reaction variants are established. In particular, it is shown that the active form of I the catalyst contains two triphenylsiloxy ligands, the transesterification of vanadate occurs via sigma-bond metathesis, and vanadium enolate is directly involved in the key C-C bond formation. The calculations also provide support for the dissociative course of the key 1,3-shift step. The stereochemistry of the reaction is thoroughly investigated, and the obtained energy barriers reproduce and rationalize the experimentally observed (Z)-, (E)-selectivity. The calculated free energy profiles are analyzed in terms of efficiency of the intermediate enolate interception. It is shown that the investigated reactions represent borderline cases, in which the intermediate trapping is only slightly favored over the undesired isomerization pathway.