Computational Comparative Mechanistic Study of C-E (E=C,N,O,S) Coupling Reactions through CO Activation Mediated by Uranium(III) Complexes.

A DFT mechanistic study is undertaken on the functionalization of CO to form C-C, C-N, C-S, and C-O bonds promoted by trivalent uranium complexes (Tp*) UR [Tp*=hydrotris(3,5-dimethylpyrazolyl)-borate ligand, R= -C≡CPh (Cpda-CC), -C≡CSiMe (Cpda-CSi), -NHPh (Cpda-N), -SPh (Cpda-S), and -OPh (Cpda-O)]. These model systems are similar in view of their two-step reaction mechanisms, that is, the insertion of CO into the U-E (E=C, N, O, S) bond to form a [U-κ -O C] intermediate, followed by the reorientation of the carboxylate group to coordinate with the U atom in the κ manner (Cpdb-X, X=CC, CSi, N, S, O). However, the free energy barriers to the rate-determining steps are substantially different, increasing in the order Cpda-S<Cpda-CC<Cpda-Csi<Cpda-N<Cpda-O, which suggests that the bond property of the U-E bonds and the nucleophilicity of the R groups govern the reactivity of the trivalent U complexes. The insertion product may then be silylated in the presence of trimethylsilyl iodide. Two potential mechanisms have been investigated with the -O CR group attacking the Si atom from the side cis (frontside) or trans (backside) to the I atom. The backside pathway was found to be more feasible in view of the free energy barriers and thermicity.