Environmentally Benign Organic Synthesis Based on Solvent-free Catalysis

Carbon dioxide (CO2) is not only a greenhouse gas but also a renewable Cl source that is important for organic synthesis. On the other hand, minimizing organic solvent is also an important subject from the viewpoint of fossil fuel, energy, resource, cost, safety, and environment. Here we summarize our achievements in solvent-free catalysis. We developed a series of bifunctional metalloporphyrin catalysts for the synthesis of cyclic carbonates or polycarbonates from epoxides and CO2. As a result of structural optimizations, Mg and Zn porphyrins having quaternary ammonium bromide groups were suitable for the synthesis of cyclic carbonates (TOF = 12,000 46,000 h(-1), TON = 55,000-310,000). In contrast, bifunctional Al porphyrins were highly active for the copolymerization of cyclohexene oxide and CO2 to give polycarbonates. The solvent free reaction with a catalyst loading of 0.0025 mol% under CO2 (2 MPa) was conducted at 120 degrees C for 1 h to give poly (cyclohexene carbonate): TOF = 10,000 h polycarbonate selectivity >99%, PDI = 1.02, and M-n = 68,000. The reaction with a catalyst loading of 0.001 mol% for 24 h gave polymers with a M-n of 281,000, which corresponded to 2,000-mer. The high catalytic activities resulted from the cooperative action of the central metal ion and the quaternary ammonium salts. Chiral catalysts for the kinetic resolution of terminal or internal epoxides with CO2 were also developed. In addition, a BINOL derivative (L) was designed to make dinuclear metal complexes (M2L); unexpectedly, heating a mixture of M and L furnished macrocyclic multinuclear metal complexes such as Zn5L3 in high yields, which showed catalytic activities for the temperature dependent N-formylation/N-methylation of amines with CO2 and hydrosilane. On the other hand, tetrabutylammonium acetate catalyzed the solvent-free N-formylation of amines with CO2 and hydrosilane to give formamides including Weinreb formamide, Me (Me-0)NCHO, which was successively converted into aldehydes by one-pot reactions with Grignard reagents. In addition, the intermolecular or intramolecular asymmetric benzoin reaction was catalyzed by a small amount of N-heterocyclic carbene (NHC) under solvent-free conditions, and the solvent-free intramolecular asymmetric Stetter reaction also proceeded efficiently. In some cases, even solid-to-solid conversions took place.

Automated summary

This study summarizes achievements in solvent-free catalysis, including the development of bifunctional metalloporphyrin catalysts for the synthesis of cyclic carbonates or polycarbonates, and the development of chiral catalysts for the kinetic resolution of epoxides with CO2. The high catalytic activities were attributed to the cooperative action of central metal ions and quaternary ammonium salts.