Computational Comparative Mechanistic Study of C−E (E=C,N,O,S) Coupling Reactions through CO2 Activation Mediated by Uranium(III) Complexes.

A DFT mechanistic study is undertaken on the functionalization of CO2 to form C−C, C−N, C−S, and C−O bonds promoted by trivalent uranium complexes (Tp*)2UR [Tp*=hydrotris(3,5‐dimethylpyrazolyl)‐borate ligand, R= −C≡CPh (Cpda‐CC), −C≡CSiMe3 (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 CO2 into the U−E (E=C, N, O, S) bond to form a [U‐κ1‐O2C] intermediate, followed by the reorientation of the carboxylate group to coordinate with the U atom in the κ2 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 −O2CR 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. CO2 activation: The DFT method is used to explore the functionalization of CO2 mediated by trivalent uranium complexes and the silylation of the insertion products. The mechanism of the CO2 insertion reaction is determined by the organic group bonded to the U atom. Two pathways are explored for the consumption of insertion product, and a feasible backside channel is proposed for the O−Si coupling reaction (see scheme). [ABSTRACT FROM AUTHOR]