Transfer Hydrogenation of CO2 and CO2 Derivatives using Alcohols as Hydride Sources: Boosting an H2-Free Alternative Strategy

Numerous strategies have been developed for the reduction of highly challenging CO2 gas and its conversion into useful feedstock chemicals. Among all of the developed protocols, the traditional approach where H-2 gas is used as a reductant has been dominantly exploited. During the past decade, enormous efforts have been made in tackling the challenge by keeping sustainability as a major goal. As an alternative option, the adoption of a "transfer hydrogenation" strategy has received attention for the CO2 reduction process. The utilization of biomass-derived alcohols as hydride donors promises to make the process viable and advantageous over the hydrogenation process. The survival of homogeneous transition-metal-based catalysts used in these processes under the harsh reaction conditions (elevated temperature and highly basic reaction medium) is a considerable challenge. Hence, the development of efficient and robust homogeneous catalysts for the CO2-transfer hydrogenation process is highly important. In this Perspective, we highlight the overall evolution of the transfer hydrogenation strategy for the reduction of CO2 gas (and its derivatives) to hydrogen-rich useful products achieved during the past decade. The role of tuning the ligand backbone to make the process kinetically more favorable is discussed in detail. The available reports in the field emphasized the advantages of using biomass-derived alcohols as hydride donors in place of nonrenewable H-2 gas. Potential benefits and opportunities of the CO2-transfer hydrogenation process over the traditional hydrogenation are critically presented to encourage further intense research in the field.