Cycloaddition of CO 2 to Epoxides Using Bifunctional Triphenylphosphonium Catalysts

ABSTRACT A series of pyridine‐grafted triphenylphosphonium salts [Ph 3 P + R]Cl − (R = benzyl ( 1 ), pyridine‐2‐ylmethyl ( 2 ), pyridine‐3‐ylmethyl ( 3 ), and pyridine‐4‐ylmethyl ( 4 )) were synthesized and evaluated for their catalytic efficiency in the cycloaddition of CO 2 to epoxides under mild and solvent‐free conditions. The para ‐substituted pyridyl catalyst ( 4 ) exhibited the highest performance. High yields (up to 99.9%) were obtained within 3 h (at 100°C and 6 mol% loading under 0.1 MPa CO 2 pressure) for a range of epoxides, including styrene oxide (SO), epichlorohydrin (ECH), epibromohydrin (EBR), 2‐butyloxirane, glycidol, phenyl glycidyl ether, allyl glycidyl ether, cyclohexene oxide and three bulky terpene‐based epoxides: α‐pinene oxide, limonene oxide, and limonene dioxide. DFT calculations on catalyst 4 reveal that the nucleophilic attack by the nitrogen of pyridine lowers the epoxide ring‐opening barrier by about 4.6 kcal mol −1 compared to the halide‐mediated pathway, rationalizing the superior performance of the para ‐substituted analog. As demonstrated by independent synthesis of the protonated and alkylated pyridine analogs, the pyridine ring can be protonated during catalysis, generating a dicationic intermediate that exhibits high activity but low durability. This outcome underscores the critical influence of the pyridine substituent's position on catalytic performance. These findings reveal structure–activity relationships critical for catalyst design and demonstrate the viability of tailored phosphonium salts for scalable and sustainable CO 2 valorization, including for challenging bio‐based feedstocks.