Computational Examination of the Mechanism of Alkene Epoxidation Catalyzed by Gallium(III) Complexes with N‑Donor Ligands

2014-01-06T00:00:00Z (GMT) by Michael L. McKee Christian R. Goldsmith
The ability of gallium­(III) complexes to catalyze the epoxidation of alkenes by peracetic acid has been examined with density functional theory calculations. According to the calculations, the chloride anions of the precatalyst [Ga­(phen)<sub>2</sub>Cl<sub>2</sub>]<sup>+</sup> (phen = 1,10-phenanthroline) can be displaced by either acetic or peracetic acid through dissociative ligand exchange pathways; both acetic and peracetic acid deprotonate upon binding to the formally tricationic metal center. Because of the high basicity of peracetate relative to that of chloride, only the acetate for chloride exchange occurs spontaneously, providing a rationale for the preponderance of gallium acetate adducts observed in the reaction mixtures. With respect to the mechanism of olefin epoxidation, the computational results suggest that the peracetic acid is most efficiently activated for redox activity when it binds to the metal center in a κ<sup>2</sup> fashion, with the carbonyl oxygen atom serving as the second point of attachment. The phen ligands’ coordination to the gallium is essential for the catalysis, and the lowest energy pathways for alkene oxidation proceed through hexacoordinate Ga­(III) species with four Ga–N bonds. A natural bond order analysis confirms the electrophilic nature of the metal-containing oxidant.