Ligand-free Guerbet-type reactions in air catalyzed by in situ formed complexes of base metal salt cobaltous chloride

Abstract

Inexpensive, earth-abundant and environmentally benign cobaltous chloride efficiently accomplishes the catalytic β-alkylation of alcohols in air at 140 °C. At higher loadings of cobaltous chloride (1 mol%) in the presence of 2.5 mol% NaO^tBu, there is a rapid formation of heterogeneous Co nanoparticles (NPs) which are apparently sensitive to air and result in poor yields (ca. 25%) of β-alkylated products. In contrast, performing the reaction in an argon atmosphere under otherwise identical conditions leads to higher yields (ca. 44%). The heterogenization and eventual loss of activity in air could be delayed by operating at a lower (0.01 mol%) CoCl₂ loading in the presence of 2.5 mol% NaO^tBu at 140 °C. Under these conditions, the catalytic β-alkylation of alcohols proceeded with high yields (up to 89%) and unprecedented turnovers (ca. 8900). Mechanistic studies are indicative of the involvement of catalysts based on in situ generated molecular Co complexes of alcohols. Labelling studies provide key evidence for the involvement of C–H activation in the cobaltous chloride catalyzed β-alkylation with a KIE of 1.61. Kinetic studies indicate linear dependence of the rate on the concentration of cobaltous chloride and sodium t-butoxide along with a non-linear dependence on the concentration of 1-phenyl ethanol and benzyl alcohol.