Pincer‐Ruthenium Catalyzed Oxygen Mediated Dehydrative Etherification of Alcohols and Ortho‐Alkylation of Phenols

Abstract

AbstractIn the current report, a dehydration strategy has been formulated for the selective etherification of secondary alcohols catalyzed by NNN pincer‐ruthenium(II) carbonyl complexes based on bis(imino)pyridine ligands in the presence of molecular oxygen under solvent‐free and base‐free conditions at 140 °C. Under these conditions, very high yields (up to 92%, ca. 6133 TON) of the corresponding ethers are obtained at a very low catalyst loading (ca. 0.015 mol%). Mechanistic studies provide key evidence for the various events that occur during the catalytic cycle. While iodometric titration gives quantitative proof for the liberation of hydrogen peroxide in catalytic amounts and hence confirms the role of oxygen as a co‐catalyst, DFT, HRMS, IR and 13C NMR analysis are indicative of the non‐labile nature of the ancillary CO ligand. On the other hand, DFT, NMR and HRMS studies reveal the hemi‐labile nature of the pincer‐ligand in the presence of oxygen. EPR analysis provide conclusive evidence for generation of Ru(III) species via single electron transfer (SET) from O2. Apparently the Ru(III) species (Ph2NN)RuCl2(CO)(OH) is the resting‐state of the etherification reaction and several of its derivatives have been detected by HRMS analysis. The pincer‐ruthenium catalyzed etherification of 1‐phenyl ethanol has been successfully extended not only to various aromatic secondary alcohols but also to the cross‐etherification and/or o‐alkylation of 1‐phenyl ethanols with a variety of phenols. Readily available oxygen and tiny amounts (0.015 mol%) of pincer‐Ru catalyst are the only requirement for this atom‐economical and environmentally benign dehydrative etherification reaction which apparently has not been reported thus far.magnified image