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

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 alpha-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 degrees C, and with 5 mol% loading. NMR titration confirmed the triple H-bonding of the alpha-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.