Comparison of the Effect of a Nucleophile on Epoxide Ring Opening Catalyzed by Potassium tert-Butoxide or Tris(pentafluorophenyl)borane

Epoxide ring opening by alcohols generates molecules that are used in a wide range of applications, including as polyols that are components of polyurethanes. The effects of nucleophile structure on regioselectivity and initial reaction rates were compared for base-catalyzed and coordination-catalyzed 1,2-epoxyoctane ring opening by 1-propanol (1PrOH) and tert-butanol (tBuOH) using potassium tert-butoxide and tris(pentafluorophenyl)borane as case studies. Experimentally measured initial rates of the tris(pentafluorophenyl)borane-catalyzed system were deconvoluted using a microkinetic model built upon a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) reaction scheme we previously developed. Benchmark, base-catalyzed systems consistently showed slow rates and low selectivities to the primary alcohol regioisomer (P1), especially for the bulkier nucleophile. In contrast, aryl boranes achieved high P1 selectivity and relatively fast epoxide ring-opening rates. Interestingly, low water levels led to tBuOH being the faster nucleophile, while at high water levels, 1PrOH had faster rates. Analyses of reactive flux and catalyst speciation using the model showed that the unexpectedly high relative rate of tBuOH at low water levels was due to higher concentrations of catalytically active species from faster pathways and lower flux through the slow, low-selectivity alcohol-mediated pathway for tBuOH.