Stereo- and Regioselective Addition of Arene to Alkyne Using Abnormal NHC Based Palladium Catalysts: Elucidating the Role of Trifluoroacetic Acid in Fujiwara Process

Abstract

Hydroarylation of alkyne was reported by Fujiwara nearly two decades back. Interestingly, this reaction does not proceed in the absence of trifluoroacetic acid; however, the exact role of TFA has not been unambiguously established, in particular with the support of X-ray crystallography by isolating the TFA-involved catalytically active species. In this work, abnormal N-heterocyclic carbene (aNHC) based Pd catalysts have been used for the efficient hydroarylation of aromatic C–H bonds leading to new C–C bond formation through regio- and stereoselective addition to alkynes. The addition reaction has been realized by a catalytic amount of Pd (II) compound (0.5 mol %) in trifluoroacetic acid (TFA) under ambient conditions. Various arenes undergo transhydroarylation selectively across the triple bond (containing functional groups CO2Me, CO2Et, and CO2H), affording the kinetically controlled cis adduct predominantly in good yields. A simple reaction condition through an intermolecular reaction has been outlined under ambient conditions for the synthesis of coumarin derivatives, which are considered as an important class of bioactive compounds. It was noted that the reaction does not proceed in the absence of TFA. Hence, the major emphasis was given to understand the role of TFA in such hydroarylation reactions. A catalytically active reaction intermediate, [aNHCPd(CF3COO)]2, containing trifluoroacetate anion was characterized as the first solid-state evidence by single-crystal X-ray crystallography, which helps to understand the exact role of TFA in such a reaction. A detailed mechanistic understanding of this fascinating catalytic process has been proposed by tandem experimental and computational experiments.