Protonation Effects on Homogeneous Phosphonium Catalysts for CO2 Cycloaddition under Mild Atmospheric Conditions

Triphenylphosphonium salts incorporating tetrazolylmethyl (1) and pyrimidinylmethyl (2) substituents were synthesized via nucleophilic substitution of the corresponding chloromethyl-heterocycles with triphenylphosphine, followed by selective protonation of 2 to yield pyrimidinium 3. These catalysts were systematically evaluated for the cycloaddition of CO2 to epoxides under mild, solvent- and co-catalyst-free conditions (0.1 MPa atmospheric pressure, 80–120 °C). Protonated catalyst 3 demonstrated superior performance, delivering quantitative conversion (96% yield) of styrene oxide to the corresponding cyclic carbonate within 6 h at 2 mol% loading. In situ FT-IR monitoring provided mechanistic insights, revealing that protonation accelerates initial ring-opening rates, and confers stability by mitigating phosphine oxide (P=O) degradation pathways prevalent in 1 (complete conversion to P=O). A factorial design of experiments (DoE) delineated parameter effects, temperature exerted dominant influence, followed by loading and time, establishing optimized conditions (120 °C, 2 mol%, 6 h) with factorial convergence at 96–98% yields, indicative of operational robustness for scale-up. Substrate scope validation encompassed structurally diverse epoxides, achieving 98–99% yields for halogenated, unsaturated, sterically encumbered, aliphatic, and aromatic derivatives, confirming high functional group tolerance without elevated pressures or additives. These economical metal-free catalysts versus conventional organometallics establish critical protonation–stability relationships, providing a scalable platform for sustainable CO2 valorization into commodity cyclic carbonates from renewable feedstocks.