Nickel- and Palladium-Catalyzed Homocoupling of Aryl Triflates. Scope, Limitation, and Mechanistic Aspects

Abstract

Whereas the direct reduction of aryl triflates affords mainly phenols and some arenes, the presence of a catalytic amount of palladium or nickel results in the formation of biaryls. The homocoupling is performed in the presence of an electron source, either a cathode or zinc powder. A judicious choice of the metal (nickel or palladium), the ligand (monodentate or bidentate phosphine), and the reduction process (electrochemical or chemical) allows the synthesis of functional symmetrical biaryls. Nickel and palladium complexes ligated by bidentate ligands such as NiCl(2)(dppf) and Pd(OAc)(2) + 1 BINAP are very efficient for the homocoupling of 1-naphthyl triflate, since the dimer was obtained in almost quantitative yield. However, the homocoupling is sensitive to steric hindrance, excluding for the moment the synthesis of atropisomers. The homocoupling proceeds via an activation of the C-O bond of the aryl triflate by a palladium(0) (or a nickel(0)) complex, providing an intermediate arylpalladium(II) (or nickel(II)) complex that after activation by electron transfer affords a new complex able to undergo a second oxidative addition with the aryl triflates.

Mechanisms of the Mizoroki–Heck Reaction

“…The mechanism depicted in Scheme 1.22 is also valid for Mizoroki-Heck reactions performed with aryl triflates, since ArPd(OAc)L 2 complexes are formed in the oxidative addition (Scheme 1.17b) [37]. This mechanism is also applicable when the catalytic precursor is not Pd(OAc) 2 [18].…”

“…4 gives in DMF the cationic complex trans-ArPd(DMF)L 2 + as characterized by conductivity measurements (Scheme 1.17a) [35,36]. When the same reaction is performed from Pd 0 (PPh 3 ) 2 (OAc) − generated from Pd(OAc) 2 and 3 equiv PPh 3 , the neutral complex trans-ArPd(OAc)L 2 is formed in equilibrium with the cationic complex (Scheme 1.17b) [37]. This again emphasizes the important role of acetate ions delivered by the precursor Pd(OAc) 2 .…”

One-Pot Homo- and Cross-Coupling Reactions of Arenediazonium Tosylate Salts for the Synthesis of Biaryls and Polyaryls

“…5 Traditionally, the Ullmann reaction 6 is one of the most important methods for the synthesis of polyaryls, but its requirement of high temperature has motivated the development of a number of milder Pd and Ni mediated procedures [7][8][9][10][11][12][13][14] starting from aryl halides and aryl sulphonates. In addition, symmetrical biaryls are also accessible through the Pd catalyzed homo-coupling of arylboronic acids.…”

Nickel catalyzed one pot synthesis of biaryls under air at room temperature

“…Significantly, lower cost and excellent redox properties of nickel, in part, given the utility of its heavy metal congener, palladium, making this objective economically as well as synthetically alluring. 14 In our previous work, we reported ruthenium(III) catalyst containing dmit (2-thioxo-1,3-dithiole-4,5-dithiolate) and triphenylphosphine ligands for the homocoupling reaction. 15 In continuation with our interest to afford symmetrical biaryls, in this paper, the ligand and metal precursors are varied and their influence on the coupling reaction is being studied.…”