Well-Defined Robust Imidazole-Based Metal–Ligand Cooperative Ru(II)-<i>para</i>-Cymene for Transfer Hydrogenation of Furanic Aldehydes Using Renewable Alcohols
Rahul Daga Patil, Sanjay Pratihar
Abstract
The present study addresses the challenges in catalytic transfer hydrogenation using methanol/ethanol for converting biomass-derived furanic aldehydes to furfuryl alcohols. The introduction of air-stable Ru(II)- para -cymene catalysts with imidazole-based metal–ligand cooperativity represents a significant advancement. Spectroscopic, in situ monitoring, labeling, and DFT investigations reveal mechanistic details, highlighting the role of Ru–H generation through the dehydrogenation of Ru(II)-alkoxide intermediates. Hydride and proton transfers are facilitated by the interconvertible coordination mode of imidazole with Ru(II)–para-cymene, which is crucial for maintaining the catalyst’s efficiency and selectivity. Notably, the p K a of the N–H in coordinated imidazole significantly influences the reactivity, following a specific order depending on the attached heterocycle: imidazole > pyridine > thiazole. This order correlates well with the computed activation barrier for Ru–H generation. The catalyst exhibits ease of synthesis, stability in air and moisture, use of renewable hydrogen sources, excellent selectivity for aldehydes, applicability to various furanic aldehydes, and potential for large-scale processes. These features collectively contribute to the economic and sustainable nature of both the catalyst and the protocol, providing a valuable contribution to the field of catalytic transfer hydrogenation.