Mixed Effect of the Supporting Electrolyte and the Zinc Anode in the Electrochemical Homocoupling of 2-Bromopyridines Catalyzed by Nickel Complexes in an Undivided Cell

Abstract

[reactions: see text] Nickel-catalyzed electroreductive homocoupling of 2-bromomethylpyridines and 2-bromopyridine has been investigated in an undivided cell in the presence of a zinc sacrificial anode. A series of reactions were performed with various types and concentrations of supporting electrolyte. It was observed that a key step in this process is the formation of an arylzinc through a nickel-zinc transmetalation. This intermediate can be transformed back to the reactive arylnickel species to afford the homocoupling as the final product. The back process from the arylzinc intermediate is, however, suppressed in the presence of high concentration (0.2 M) of tetraalkylammonium salts. On the contrary, with NaI, the formation of the dimer is not prevented, whatever the NaI concentration.

Contribution of Electrochemistry to Organometallic Catalysis

“…The electrolyses are performed at room temperature in an undivided cell equipped with a sacrificial Mg anode. ,, The electrolyses have also been performed in protic solvents, alcohols (Troupel et al) or water (Troupel et al), or in ionic liquids (Fe anode, Barhdadi et al, Jouikov et al). The homocoupling has been extended to substituted-2-bromopyridines (Zn or Fe anode) by Navarro et al , …”

“…147,162,163 The electrolyses have also been performed in protic solvents, alcohols (Troupel et al) 164 or water (Troupel et al), 165 or in ionic liquids (Fe anode, Barhdadi et al, 166 Jouikov et al 167 ). The homocoupling has been extended to substituted-2-bromopyridines (Zn or Fe anode) by Navarro et al 168,169 The mechanism investigated by Devaud et al may proceed by two paths. 170 A mechanism involving paths A + C in Scheme 72 is reminiscent of that established for NiCl 2 (dppe) catalyst, that is, oxidative addition of the electrogenerated Ni 0 (bpy) to ArX.…”

Bimetallic gold–palladium alloy nanoclusters: an effective catalyst for Ullmann coupling of chloropyridines under ambient conditions

“…6,7 As an alternative to copper catalysts, several homogeneous nickel-catalyzed reductive homocoupling reactions of haloarenes are reported. 4,6,[12][13][14][15][16][17][18] The primary benefit of nickel-mediated coupling of aryl halides is the use of readily available and inexpensive aryl chlorides and aryl bromides as starting materials under mild conditions. [12][13][14][15] The ligated Ni(II) is in general required for the reaction to progress and a co-reductant such as zinc or electrochemical reduction is indispensable for regenerating the active catalyst.…”

“…[12][13][14][15] The ligated Ni(II) is in general required for the reaction to progress and a co-reductant such as zinc or electrochemical reduction is indispensable for regenerating the active catalyst. 2,3,[16][17][18] On the other hand, the search for new reusable catalytic systems to replace the existing homogeneous ones is one of the major issues. In this respect, ligandless metal nanoclusters as catalysts have emerged as some of the most promising systems.…”

A Heterogeneous‐Catalyst‐Based, Microwave‐Assisted Protocol for the Synthesis of 2,2′‐Bipyridines

“…Current protocols for the preparation of 2,2′‐bipyridines involve lithiation–addition,7, 8 radical‐anion coupling,9 Stille,10, 11 Suzuki,12 Kumada,13, 14 and Negishi15–19 reactions. All these methods have limitations,15 and most employ homocoupling20–27 methodologies. Some cross‐couplings have been reported, but all involve extremely extended reaction times 15–19.…”