NNN Pincer-Manganese-Catalyzed Guerbet-Type β-Alkylation of Alcohols under Microwave Irradiation

Abstract

A series of manganese carbonyl complexes based on bis(imino)pyridine ligands of the type R2NNN (R = Cy, iPr, Ph, p-FC6H4) were synthesized. While the complex appears to be an NN bidentate-Mn(I) triscarbonyl species in the solid state, in solution, there is a gradual loss of carbon monoxide leading first to an NNN pincer-Mn(I) biscarbonyl complex, which finally disproportionates to an NNN pincer-Mn(II) dibromide and an NNN pincer-Mn(0) complex. Evidence for this transformation has been obtained from single-crystal X-ray diffraction, IR, EPR, and HRMS analysis. The loss of carbon monoxide is apparently faster with a cyclohexyl-substituted complex than with others. The considered complexes (0.05 mol %) were utilized for catalyzing the β-alkylation of secondary alcohols at 140 °C in air under conventional heating (12 h), as well as under microwave heating (2 h) in the presence of 2.5 mol % of NaOtBu. Among the considered complexes, the catalyst (p-FC6H4NNN)Mn(CO)2Br was found to be highly efficient. While under conventional heating, a maximum of 92% (1840 TON at 153 TOs/h) of the β-alkylated product was obtained; microwave heating provided the β-alkylated product in 85% yield (1700 TON at 850 TOs/h) in just 2 h. The catalytic system has been successfully implemented to alkylate several other secondary alcohols, as well (35 examples). IR, EPR, and HRMS analyses indicate the intermediates in the catalytic cycle to be based both on NNN pincer-Mn(I) biscarbonyl species and on an NNN pincer-Mn(II)/Mn(0) couple. Deuterium labeling studies point to the involvement of C-H bond activation in the rate-determining step (RDS) with a kinetic isotope effect (KIE) of 9.0.