Benzyl bromide, reductive coupling

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

A mixture of isomers was, however, obtained if the a,a -dibromoxylene was not symmetrical. This difficulty was overcome by using a reductive coupling of a substituted 3,4-dihydroisoquinoline (71) as its salt with an o-methoxycarbonyl-substituted benzyl bromide (73)129 [Eq. (60)]. 

Reductive coupling benzyl ketones.1 Benzyl bromides and acyl chlorides undergo reductive coupling when treated with zinc (2 equiv.) and catalytic amounts of Pd[P(Cr,H5)3]4 or Cl2Pd[P(C6H5)3]2 to form benzyl ketones. The highest yields are obtained with aryl acyl chlorides. ArCHzZnBr is considered to be an intermediate. 

Shimakoshi, H., W. Ninomiya, and Y. Hisaeda (2001). Reductive coupling of benzyl bromide catalysed by a novel dicobalt complex having two salen units. J. Chem. Soc. Dalton Trans. 13, 1971-1974. 

The electrochemical reductive coupling reaction of various aikenes with benzyl bromides can also been achieved in the absence of supporting electrolyte using the microflow cell (Figure 7.20) [lOlj. When the inter-electrode gap is 160 pm, the desired cross coupling product is obtained effectively, whereas a significant amount of homocoupling product is obtained when the gap is 320 pm. 

The reductive coupling reaction of benzyl chloride with benzoyl chloride proceeded even at room temperature however, improved results were obtained under refluxing glyme (at 85°C). The choice of nickel halide that was reduced was important. Metallic nickel prepared from nickel iodide, bromide, and chloride gave benzyl phenyl ketone in 73, 42, and 11% yields, respectively. Thus, the reaction of benzyl halides with acyl halides using metallic nickel derived from nickel iodide was carried out under refluxing glyme, and the results are summarized in Table 7.8. 

Scheme 3.30 Reductive coupling of vinyl bromides with benzyl chlorides.

There are several nonpyrolytic methods of obtaining spherical surfaces. These include titanium-, vanadium-, and nickel-mediated reductive coupling of benzyl and benzylidene bromides, intramolecular carbenoid coupling of dibromomethyl groups, and Pd-catalyzed intramolecular arylation with aryl halides [291]. 

Figure 8.25 Products of the reaction between benzyl bromide and acrylonitrile (add = addition olig = oligomerization c = radical coupling red = reduction).

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

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