COBALT-CATALYZED ACCEPTORLESS DEHYDROGENATIVE HOMOCOUPLING OF PRIMARY ALCOHOLS TO ESTERS

Abstract

Esters are an important class of organic compounds widely used by human beings in their daily life. Currently existing industrial methods for esterification involve harmful starting compounds, toxic byproducts and high manufacturing costs. In this work, tripodal tetradentate iPrPPPNHPyMe cobalt complex is synthesized for use as a catalyst for the acceptorless dehydrogenative homocoupling of primary aliphatic and aromatic alcohols to esters. This method is economical, oxidant free and environmentally benign. KOtBu is found as the best co-catalyst for the acceptorless dehydrogenative homocoupling. Primary aromatic alcohols with electron releasing or withdrawing groups at the ortho position are converted to esters with higher yields. Primary aliphatic alcohols also showed outstanding reactivity but slightly higher temperature and catalyst/co-catalyst loading are required. Lactones are also obtained in good yield from diol substrates. The mechanistic study suggests a two step reaction pathway. In the first step, cobalt complex catalyzes dehydrogenation of alcohol to intermediate aldehyde. In the second step, co-catalyst KOtBu mediates Tishchenko-type condensation of intermediate aldehydes to esters.