Electroreductive coupling

Electroreductive coupling of ben2otrifluorides with sacrificial aluminum or magnesium anodes in the presence of acetone, carbon dioxide, or /V, /V-dimethylformamide provides a novel route to ArCF2-derivatives (310). 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

The complex [Ni(bpy)2]2+ catalyzes the electroreductive coupling of organic halides and carbon monoxide into ketones under a CO atmosphere,226 or in the presence of a metal carbonyl,227 especially iron pentacarbonyl. Unsymmetrical ketones have been obtained from mixtures of two different organic halides.228 CO is very reactive towards reduced Ni° species to form the stable [Ni°(bpy)(CO)2]° complex, which probably evolves to a transient arylnickel [Nin(bpy)(R)(CO)X]° complex in the presence of both ArX and [Ni°(bpy)]° species.229,230... 

Table 2.9 Electroreductive coupling of ketones with nitrones...

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

Organic electroreductive coupling reactions using transition-metal complexes as catalysts have been widely investigated. Reviews on the subject have been published [89, 90]. The method involving the most common transition-metal complexes (nickel, cobalt, palladium) appears to be a useful tool to synthetize heterocycles from organic halides via radical intermediates. Nickel catalyst precursors are nickel(II) salts that are cathodically reduced either to nickel(I) or to nickel(O) and cobalt catalyst... 

Scheme 63 Electroreductive coupling of aryl halides to biaryls.

Tab. 15 Electroreductive coupling of organic halides by using Ni complexes as mediators [268]... 

Tab. 16 Nickel-catalyzed electroreductive coupling of a-chloroesters with aryl and vinyl halides [294]...

The Pd(0)-catalyzed electroreductive coupling of aryl halides (303) is a currently relevant topic. In the electroreduction of aryl halides (307) the replacement of the halogen atom by hydrogen predominantly takes place giving (306). Difficulties are encountered, however, when aryl-aryl coupling products (305) via (304) are wanted (Scheme 116). An efficient electroreductive coupling of aryl bromides (307) (X = Br) and iodides (307) (X = I) into biaryls (310) has been shown to occur in a DMF/Et4NOTs/(Pb cathode) system in the presence of Pd(0) and/or Pd(II) catalysts (Scheme 117) [440]. 

The electroreductive coupling of the hindered aromatic ketones (367) has been achieved in a DMF-BU4NBr-(Hg) system by the aid of CrCh or MnCl2 as the electrocatalyst (Scheme 134) [484, 485]. The reductive coupling proceeds at a less negative potential [ 1/2 —1.44 —1.53 V (SCE)] than the reduction of the ketone (—1.63 —2.01 V). In some cases, Mn electrocatalysts favor the reduction to the carbinol (369), whereas a Cr catalyst promotes the formation of the pinacol (368). 

Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts... 

Tabk 1. Ni- or Co-catalyzed electroreductive coupling of alkyl halides... 

The electroreductive coupling of organic dihalides is an interesting approach to synthesize polymers. 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 2. Reductive electropolymerisation of aryl dihalides using nickel catalysts... 

Apart from the electrocarbonylation reactions of organic halides described in Sect. 6, other Ni-catalyzed reactions leading to ketones have been reported. Thus the electroreductive coupling between acylchlorides and alkyl halides, catalyzed by NiBr2bpy leads to unsymmetrical ketones [129]. Recently acylchlorides have been converted to symmetrical ketones in an undivided cell fitted with a nickel or stainless steel anode. In this reaction the active metallic species... 

Nedelec, J.-Y., J. Perichon, and Troupel, M. Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts. 185, 141-174 (1997). 

Shono and Kise have investigated the electroreductive coupling reaction of y- and -cyanoketones, yielding bicyclic a-hydroxy ketones 218 and their dehyd-roxylated equivalents 221 [198], Optimized yields are obtained when the electroreduction is carried out in i-propanol at a controlled potential of — 2.8 V using a divided cell equipped with a ceramic diaphragm and an Sn or Ag cathode. The product ratio is controlled by the reaction temperature. When the reaction is carried out at 25°C, almost exclusively the a-hydroxy ketone 218 is obtained, whereas at 65°C the obviously thermally dehydroxylated ketone 221 is the predominant product (Scheme 42). Furthermore, this methodology has been... 

N d lec J-Y,P richon J,Troupel M (1997) Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts. 185 141-174 Nicotra F (1997) Synthesis of C-Glycosides of Biological Interest. 187 55-83 Nishimura J,see Inokuma S (1994) 172 87-118 Nolte RJM,see Sijbesma RP (1995) 175 25-56 Nordahl A,see Carlson R (1993) 166 1-64... 

TABLE 1. Electroreductive coupling between CF3Br and aldehydes... 

It has been reported that the electroreductive coupling between aryl halides and 2-chloro- or 2-bromopyridines in the presence of NiBr2(bpy) as the catalyst led to the cross-coupling product in better yields (55-80%) (equation 52)49. 

The method may be used for the synthesis of phthalide alkaloids as cordrastine by electroreductive coupling of 2-methyl-3,4-dihydro-6,7-di-me-thoxyisoquinolinium iodide130 with 3-bromomoconine. A further variation... 

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