A DFT Study on the Mechanism of Samarium (II)-catalyzed Cyclopropanation Synthesis with α,β -unsaturated Carboxylic Acids, Alcohols and Amides

The mechanisms and regioselectivity of samarium(II)-catalyzed cyclopropanation synthesis with α,β-unsaturated carboxylic acids, alcohols and amides have been investigated using density functional theory calculations at B3LYP/6-31G(d,p) (LANL2DZ for I and RECPs for Sm) level of theory. Solvent effects on these reactions were explored using calculations that included a polarizable continuum model (PCM) for the solvent (THF). Our calculations suggest that the reaction proceeds through two possible channels: methylene transfer and carbometalation. The results show that the activation energies and mechanisms have larger differences, which have been briefly compared with the different α,β-unsaturated substrates. The reactions for α,β-unsaturated carboxylic acids both the carbometalation pathway and the methylene transfer pathway are feasible, while the carbometalation channel occurs more easily. For α,β-unsaturated alcohols, it is widely favored that the methylene transfer mechanism over carbometalation mechanism, while the energy barriers for α,β- unsaturated amides are relatively higher of 16.4-23.1 kcal/mol. The computational results are in good explanation with the experimental observations of Concellon, et al. on the cyclopropanation reaction of samarium(II)-catalyzed synthesis based on α,β-unsaturated substrates.