Density Functional Theory Study on [Ni0(1,10-Phenanthroline)]-Catalyzed Reductive Carboxylation of Alkyl and Aryl Halides with CO2: Effect of the Lewis Acid and β-H Elimination Side Reaction in the Crucial CO2 Insertion Step.

Abstract

The [Ni0(phen)] (1)-catalyzed reductive carboxylation of propyl and phenyl chloride with a CO2 molecule has been compared using the density functional theory method. The reactivity of 1 in the initial oxidative addition with the propyl and phenyl chloride and in the subsequent single electron transfer step to form the NiI intermediates, [NiI(phen)(R'-CH2)], 4, (R' = CH3-CH2-), and [NiI(phen)(C6H5)], 4', is almost the same. However, an apparent reactivity difference is interpreted in the CO2 insertion step, which involves the NiI and the NiII intermediates. In both propyl and phenyl chloride, the NiI-mediated CO2 insertion is kinetically more preferred than that mediated by the analogues NiII intermediates (3 and 3') by +5.0 and +33.4 kcal/mol, respectively. This trend in energetics clearly shows that the CO2 insertion of phenyl chloride exclusively occurs via the NiI-mediated pathway, whereas in propyl chloride, it follows both NiI- and NiII-mediated pathways. Overall, the catalytic efficacy of 1 is found to be higher in phenyl chloride (by +11.3 kcal/mol) than that in propyl chloride. Furthermore, the effect of a plausible β-H elimination side reaction in the CO2 insertion step is modeled for propyl chloride. Herein, the β-H elimination of the NiII propyl species (3) is kinetically more feasible than its CO2 insertion, while the β-H elimination of NiI propyl (4) is rather difficult compared to its CO2 insertion by +16.3 kcal/mol. This strongly supports the suitability of the NiI intermediate in the CO2 insertion step. In addition, the role of the Lewis acid (X) in the CO2 insertion step is tested by incorporating various Lewis acids (X = MgCl2, ZnCl2, AlCl3, and LiCl) in the NiII propyl (7) and NiI propyl (5) intermediates. The Lewis acids effectively facilitate the CO2 insertion step, and the effect due to MgCl2 is found to be more evident. MgCl2 enhances the CO2 insertion of 5 and 7 by 89 and 84%, respectively, and hence, the NiI-mediated CO2 insertion of propyl halide (ΔG⧧ = +1.4 kcal/mol) is now comparable with that of phenyl halide (ΔG⧧ = +0.9 kcal/mol). This suggests that in the presence of Lewis acids, the catalytic efficacy of 1 is enhanced for the reductive carboxylation of propyl halide and exhibits similar reactivity to that of phenyl halide.