(520d) Mechanistic Study of Regio-Selective Ring-Opening Reactions of Mono-Substituted Epoxides Using Lewis Acid Catalysts

Polyether polyols are important intermediates in the polyurethane industry and are typically made by reacting mono-substituted epoxides with polyalcoholic initiators in the presence of a catalyst.¹ While conventional catalysts such as potassium hydroxide and double metal cyanide catalysts result in less reactive polyols terminated with secondary hydroxyl groups, a family of Lewis acid catalysts, aryl boranes, has been found to achieve high regio-selectivity towards the desired primary alcohol functionality with minimal side reactions.² In addition, recent experiments revealed significant additive effects on enhancing the regio-selectivity towards primary alcohol products. However, the detailed catalytic mechanism has not been fully understood. In addition, the form of the catalyst is still unclear based on existing experimental methods that are unable to monitor the in situ speciation of this complex reaction system. To explore the mechanism of aryl borane-catalyzed regio-selective ring-opening reactions and elucidate additive effects, an ab initio study was conducted, followed by comprehensive microkinetic modeling. Density functional theory (DFT) was used to investigate the reaction pathways of ring-opening reactions in a model system using tris (pentafluorophenyl) borane as catalyst. Competitive binding of ligands to the catalyst was also studied. We identified several possible catalyst forms that contribute to ring-opening reactions with different regio-selectivities. The role of different additives was identified as serving as a co-catalyst that stabilized the transition state. The contributions of different reaction pathways was decomposed into two parts: the relative reaction rate constants and the availability of different catalytic forms, which resulted from competitive binding. Our microkinetic model captured all possible competitive binding and ring-opening reactions. Model predictions of the overall regio-selectivity were in good accord with experimental results under different reaction conditions. Ultimately, this study facilitates greater understanding of catalyzed regio-selective ring-opening reactions and provides a basis for the design of new catalyst/co-catalyst systems to fine tune the overall regio-selectivity.

(1) Chinn, H. K., A.; Loechner, U. SRI Consulting 2006.

(2) Nakaminami, H.; Sugahara, S.; Yasuhara, S.; Murata, K. U. US20120016049A1: 2012.