Mechanistic study of bismuth-catalyzed direct benzylation of 2,4-pentanediones: the case of BiCl3 and generalization

Abstract

At present, we investigate the mechanisms for bismuth-catalyzed direct benzylation of 2,4-pentanediones using various density functional theories and post-Hartree–Fock ab initio methods. First, we deeply consider the role of BiCl3, where six scenarios are proposed. All of them start with the formation of a weakly bound complex between the BiCl3 catalyst and the alcohol or the dione. The most favorable one corresponds to the direct access to the products via a unique transition state (SN2-type mechanism). We also examined the effects of various nonpolar and polar solvents, which are viewed to slightly affect the energy profiles for SN2 and internal nucleophilic substitution (SNi) types of mechanism, whereas strong perturbations are observed for SN1 mechanism. For instance, the later one becomes the most thermodynamically favorable in DMSO solvent. In addition, several classes of benzyl alcohols and catalysts (BiX3, where X = Cl, Br, I, NO3, and OTf) were considered within the framework of SN2 and SNi mechanisms. The reactivity of these alcohols increases going from primary to secondary to tertiary. These findings are in line with the present available experimental results. Finally, our computations suggest that Bi(NO3)3 could be an excellent catalyst for the title reaction.