On the Mechanism of Acrylate and Propionate Silyl Esters Synthesis by Ruthenium-Catalyzed Coupling of CO2 with C2H4 in the Presence of Hydrosilanes: Combined Experimental and Computational Investigations

The upcycling of CO2 to value-added chemicals using catalytic approaches constitutes a challenge in the topical area of sustainable development and use of renewable resources. We report herein on a Ru(II)-catalyzed reductive carboxylation of ethylene in the presence of hydrosilane (Et3SiH) affording acrylate and propionate silyl esters. Upon using high-throughput screening (HTS) and batch-reactor techniques, some promising catalyst systems incorporating 1,4-bis(dicyclohexylphosphino)butane (DCPB) as ligand, namely monohydrido-chloro complex Ru(H)(Cl)(CO)(DCPB)(PPh3) (Ru-2) or dihydride complex Ru(H)2(CO)(DCPB)(PPh3) (Ru-5), were identified among a few others. Detailed mechanistic studies involving the isolation of a Ru(II)-acrylate intermediate and computational investigations unveiled the possible operational mechanism leading to the construction of the acrylate platform from CO2 and ethylene. The selectivity toward the desired silyl esters is affected by side-processes (mainly CO2 hydrosilylation, ethylene hydrosilylation and dehydrogenative coupling of ethylene with hydrosilane) and could be improved by varying the substrates’ concentrations (CO2/ethylene/hydrosilane ratios), while the acrylate vs. propionate selectivity depends on the processes producing in situ H2, which is responsible for the reduction of the acrylate C=C double bond. In particular, a marked role of water on the selectivity was rationalized as a potential H2 source when used in combination with hydrosilane. A better selectivity towards the production of triethylsilyl acrylate could be achieved using dihydride complex Ru-5 as discrete precatalyst (up to 47% vs. 15% with Ru-1/DCPB), in line with the mechanistic studies.