Alkenes reductive coupling with allyl halides

Nickel(O) triphenylphosphine species have been used to convert allyl halides to 1,5-hexadiene [332], for the reductive coupling of ethylene with aryl halides to give 1,1-diaryl-ethanes [333], and for the coupling of aryl halides and alkenes to prepare substituted olefins [334], In addition, l,2-bis[(di-2-propylphosphino)benzene]nickel(0) has been used for the reductive coupling of aryl halides [335], and l,2-bis[(diphenylphosphino)ethane]-nickel(O) has been employed to synthesize biphenyl from bromobenzene [336] and to prepare benzoic acid from bromobenzene in the presence of carbon dioxide [337]. 

Table 18 Reductive Coupling of Alkenes With Allyl Halides in Aprotic Medium [190]...

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. 

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

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