Brønsted Acid-Catalyzed Direct Substitution of 2-Ethoxytetrahydrofuran with Trifluoroborate Salts

Abstract

Metal-free transformations of organotrifluoroborates are advantageous since they avoid the use of frequently expensive and sensitive transition metals. Lewis acid-catalyzed reactions involving potassium trifluoroborate salts have emerged as an alternative to metal-catalyzed protocols. However, the drawbacks to these methods are that they rely on the generation of unstable boron dihalide species, thereby resulting in low functional group tolerance. Recently, we discovered that in the presence of a Brønsted acid, trifluoroborate salts react rapidly with in situ generated oxocarbenium ions. Here, we report Brønsted acid-catalyzed direct substitution of 2-ethoxytetrahydrofuran using potassium trifluoroborate salts. The reaction occurs when tetrafluoroboric acid is used as a catalyst to afford functionalized furans in moderate to excellent yields. A variety of alkenyl-and alkynyltrifluoroborate salts readily participate in this transformation.

Selective Functionalization of Achmatowicz Rearrangement Products by Reactions with Potassium Organotrifluoroborates under Transition-Metal-Free Conditions

“…We started our investigations by considering the selective replacement of the γC–O bond in O -unprotected 2 with a γC–C­(sp 2 ) bond (Scheme ) toward the synthesis of 10 . The first challenge of this transformation was finding reaction conditions devoid of competence with conjugate addition to the α,β-unsaturated ketone moiety of the AR starting materials 2 , which would lead to 12 .…”

“…The reactions of potassium organotrifluoroborates 11 with the AR products 2 can be understood (Scheme ) by considering the direct conjugate addition to the enone moiety versus addition to the oxonium I that results from dehydroxylation …”