Dehydrogenative Heck Reaction of Furans and Thiophenes with Styrenes under Mild Conditions and Influence of the Oxidizing Agent on the Reaction Rate

Abstract

Transition-metal-catalyzed reactions dealing with CH activation is a challenging task in organic chemistry. [1] The Pdcatalyzed couplings of arenes with alkenes through CH activation, also called intermolecular dehydrogenative Heck reactions (DHRs), have gained interest these past years. [2] Despite noticeable progresses, the substrate scope is often limited to olefins bearing an electron-withdrawing group as the coupling partner, which limits the applications of these reactions. We have recently reported an efficient method for the coupling of furans with styrenes, which allowed the use of chlorinated and fluorinated substrates. [3] However, brominated substrates were reluctant to react. These compounds would allow the synthesis of valuable products for further functionalization through other cross-coupling reactions. Glorius et al. have recently shown that brominated compounds can be formed with no proto-debromination using Rh catalysts; however few examples have been reported and the scope of substrates remains to be developed. [4] A recent example has been reported by Zhang et al. using palladium catalysts, but the method requires the presence of a removable pyridylsulfinyl group on the arenes. [5] Among the arenes used in DHRs, furans and thiophenes have received little attention. The former are acid sensitive and the latter require elevated temperatures, [6] due to their higher aromatic resonance energy and stability. [7] We report herein the DHRs, under mild conditions, of furans and thiophenes with styrenes, including brominated substrates. The influence and the role of the oxidizing agent on the activity of the catalyst are also discussed. We noticed an induction period during our previous study