Direct Brønsted acid-catalyzed functionalization of benzhydryl alcohols and 2-ethoxytetrahydrofuran using potassium trifluoroborate salts

Abstract

Metal-free transformations of organotrifluoroborates are advantageous since they avoid using frequently expensive and sensitive transition metals. Lewis acid-catalyzed reactions involving organotrifluoroborates have emerged as an alternative to metal-catalyzed protocols. However, these methods rely on generating unstable boron dihalide species thereby resulting in low functional group tolerance. A Brønsted acid-catalyzed carbon-carbon bond forming methodology involving alkenyl- and alkynyltrifluoroborates and in situ generated carbocations has been developed. In the presence of HBF4, we have shown that organotrifluoroborates react with benzhydryl alcohols to afford alkenes and alkynes in good to excellent yields. This protocol features excellent atom economy since alcohols and organotrifluoroborates react in a 1:1 ratio. Functional group tolerance superior to Lewis acid- and metal-catalyzed approaches was demonstrated. Furthermore, we were able to extend this method to 2-ethoxytetrahydrofuran which underwent direct substitution to afford functionalized furans in moderate to excellent yields. A variety of alkenyl- and alkynyltrifluoroborates readily participated in this transformation.