Unconventional sulfonium C–H H-bond donor catalyst in fixation of carbon dioxide

Abstract

H-bond donor (HBD) catalysis is essential in the cycloaddition of CO₂ with epoxide (CCE) where a strong HBD is prevalent; however, current C–H H-bond donors are extremely weak and scarcely viable in catalysis. We reveal sulfonium featuring multiple α-C–H bonds as a new type of efficient HBD catalyst in the CCE reaction. Two series of cyclic tertiary sulfoniums were designed and evaluated in the cycloaddition of CO₂ to epoxide. An optimal S-alkyl tetrahydrothiophenium bromide (THT3-Br) catalyst converted epoxides to the corresponding cyclic carbonates with high conversion (>90%) and near quantitative selectivity (99%) in 6 h under 1 MPa pressure of CO₂, at 100 °C, and with 5 mol% loading. NMR titration confirmed the triple H-bonding of the α-C–H of sulfonium to epoxide, which activated the substrate oxirane and facilitated bromide attack on the methylene carbon. The first observation of sulfonium in catalytic CCE reactions suggested a wider application of C–H H-bonding catalysis in carbon dioxide fixation.