Pincer-Cobalt-Catalyzed Guerbet-Type β-Alkylation of Alcohols in Air under Microwave Conditions

Abstract

Pincer complexes of the type (R2NNN)CoCl2 (R = iPr, tBu, Cy, Ph, p-F-C6H4) derived from the environmentally benign, earth-abundant, and inexpensive base metal cobalt have been synthesized that exist in equilibrium with [(R2NNN)2Co]CoCl4. Among these complexes, (iPr2NNN)CoCl2 has been found to be highly efficient for the catalytic β-alkylation of alcohols in air at 140 °C. While the pincer-Co (0.0025 mol %)-catalyzed β-alkylation in the presence of NaOtBu (2.5 mol %) took 24 h (85%, 34000 TON at 1417 TO/h) under conventional heating (140 °C), the corresponding reaction under microwave conditions (140 °C at 75 W) was complete in only 2 h with comparable yields (83%, 33200 TON), albeit with a better TOF (16600 TO/h). In contrast, CoCl2 (0.0025 mol %) under otherwise identical conditions leads to lower yields under both conventional (66%, 26400 TON at 1100 TO/h) and microwave heating (61%, 24400 TON at 12200 TO/h). The catalytic system has been successfully implemented (39 examples) to accomplish the β-alkylation of secondary alcohols with several primary alcohols. Well-defined molecular Co(II) species are involved during catalysis, as inferred from HRMS and hot-filtration experiments. EPR studies and magnetic moment measurements using the Evans method demonstrate that Co remains in its +2 state and in an octahedral geometry throughout the reaction. Pivotal evidence for the involvement of β-hydride elimination leading to acetophenone in the RDS of the pincer-Co catalyzed β-alkylation is obtained from competitive deuterium labeling experiments, which are indicative of a KIE of 6.14. The current report on base-metal catalysis in air under microwave conditions that leads to high yields of β-alkylation with unprecedented turnovers (33200 TON at 16600 TO/h) in a very short time (2 h) offers exciting atom-economical and greener possibilities for these class of synthetically valuable Guerbet-type reactions.