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

Abstract

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.