Preparation of functionalized aryl- and heteroarylpyridazines by nickel-catalyzed electrochemical cross-coupling reactions.

A general efficient electrochemical method for the preparation of aryl- and heteroarylpyridazines in a nickel-catalyzed cross-coupling reaction of 3-chloro-6-methoxypyridazine and 3-chloro-6-methylpyridazine with a range of functionalized aryl or heteroaryl halides is reported.

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.

[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.

Room-temperature ionic liquids as new solvents for organic electrosynthesis. The first examples of direct or nickel-catalysed electroreductive coupling involving organic halides.

Direct or Ni-catalysed electroreductive homocouplings of organic halides and couplings of organic halides with activated olefins are efficiently conducted by constant current electrolyses in an undivided cell in room-temperature ionic liquids as the solvent-electrolyte media.

Mechanism of the electrochemical conversion of aryl halides to arylzinc compounds by cobalt catalysis in DMF/pyridine.

The study of the electrochemical behavior of cobalt bromide, CoBr2, in the presence of zinc bromide, ZnBr2, and aryl halides, ArX, in a dimethylformamide (DMF)/pyridine (9:1, v/v) mixture allowed us to complete the study of the mechanism of the electrochemical conversion of aryl halides into arylzinc compounds by using cobalt catalysis. The last step of the catalytic process has been shown to be a transmetalation reaction between the arylcobalt(II) species and zinc ions that regenerates the cobalt(II) catalyst. The effect of zinc bromide on each step of the catalytic cycle has been studied. It is especially shown that the presence of ZnBr2 stabilizes the electrogenerated Co1 but has no effect on the rate constant of the oxidative addition of aryl halides, ArX, to Co1. Rate constants for the disproportionation reaction of Co1 and the oxidative addition have been determined in the presence of ZnBr2 and compared with the values obtained in its absence.

Electrochemical homocoupling of 2-bromomethylpyridines catalyzed by nickel complexes.

2,2'-Bipyridine (bpy) and a series of dimethyl-2,2'-bipyridines were synthesized from 2-bromopyridine and 2-bromomethylpyridines, respectively, using an electrochemical process catalyzed by nickel complexes. The method is simple and efficient, with isolated yields between 58 and 98% according to the structure. We first studied the influence of the presence and the position of the methyl group on the yield, using N,N-dimethylformamide (DMF) or acetonitrile (AN) as the solvent, NiBr(2)bpy as the catalyst, and Zn as the sacrificial anode, in an undivided cell and at ambient temperature. On the basis of a better understanding of the reaction mechanism based on electroanalytical studies, we could improve the dimerization both by substituting the catalyst ligand (bpy) by the reagent itself, i.e., 2-bromomethylpyridine or 2-bromopyridine, and by using Fe instead of Zn as the sacrificial anode.

Nickel-Catalyzed Direct Electrochemical Cross-Coupling between Aryl Halides and Activated Alkyl Halides.

The electrochemical reduction of a mixture of aryl halides and activated alkyl halides in DMF in the presence of catalytic amount of NiBr(2)bipy leads to cross-coupling products in good to high yields. The method applies to the synthesis of alpha-aryl ketones, alpha-aryl esters, and allylated compounds from readily available organic halides. Optimization of the process has been obtained by slowly adding the most reactive organic halide (usually the activated alkyl halide) during the electrolysis which is best conducted at 70 degrees C when aryl bromides are involved.