Propylene Carbonate Formation from Propylene Oxide and CO2 Catalyzed by Bifunctional β/γ-Hybrid Pentamers: A DFT Mechanistic Study

Abstract

We investigated the mechanism of propylene carbonate (PC) formation from propylene oxide (PO) and CO2 catalyzed by bifunctional β/γ-hybrid pentamers (BGHP1C and BGHP2C) using DFT calculations with PO as the solvent. The catalysts were derived from helical pentamers BGHP1 and BGHP2 by introducing adjacent pyrrolidine and −CH2OH groups at the first and fourth residues, respectively. Preferred catalyst conformations were identified through conformational searches and DFT optimization, while all intermediates and transition states were validated by intrinsic reaction coordinate calculations followed by DFT reoptimization. BGHP2C exhibited the highest catalytic efficiency, with a turnover frequency of 8.4 × 10–2 h–1, approximately 290 and 9 times greater than those of BGHP1C and the pyrrolidine/MeOH system, respectively. It also showed the smallest energetic span, 23.7 kcal mol–1, corresponding to the activation free energy of the cyclization step. Despite greater backbone and side-chain flexibility, BGHP2C undergoes smaller conformational energy changes, indicating enhanced energetic stability. This flexibility promotes substrate accommodation and stronger catalyst–substrate interactions, leading to a lower Gibbs free-energy profile and superior catalytic performance.