Mapping the catalytic landscape of triphenylborane (BPh3)-catalyzed CO2-epoxide coupling to carbonates: an in silico approach to solve substrate-dependent selectivity

Abstract

The catalytic coupling of CO₂ and epoxides is a promising approach for carbon valorization, enabling the synthesis of cyclic-carbonates (CCs) and poly-carbonates (PCs). However, controlling product selectivity remains a challenge. Triphenylborane (BPh₃) has emerged as a promising metal-free catalyst, yet the origins of its substrate-dependent selectivity remain unclear. While BPh₃ selectively forms CCs with propylene oxide (PO), it exclusively produces PCs with cyclohexene oxide (CHO), highlighting distinct reactivity. To understand this selectivity, we conducted a comprehensive density functional theory (DFT) study, mapping the catalytic landscape of BPh₃-mediated CO₂-epoxide coupling and comparing it with triethylborane (BEt₃) catalysis, which lacks such selectivity. Our calculations reveal that epoxide ring opening is the rate-determining step, consistent with experimental studies. Additionally, high CO₂ concentrations can form an inactive species that inhibits epoxide activation, explaining the experimentally observed inverse rate dependence on CO₂ concentration. Our distortion/interaction analysis (DIA) and non-covalent interaction (NCI) analysis show that in BPh₃-catalyzed CO₂ and PO coupling, weaker intermolecular interactions in the epoxide addition step disfavour PC formation, favoring CC formation. Conversely, for CO₂ and CHO coupling, the high distortion energy in the ring-closing step makes CC formation unfavourable, leading to PC as the dominant product. In contrast, BEt₃ catalysis stabilizes PC formation across both epoxides, eliminating selectivity. This study sketches the catalytic landscape of BPh₃-catalyzed CO₂-epoxide coupling, revealing how boron substitution governs selectivity and offers insights for designing boron-based catalysts for selective CO₂ utilization.