Nickel-complex-catalyzed reactions allyl intermediates

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. 

Before entering into a detailed discussion of the nickel-catalyzed reactions of olefins, the convention we will use to describe the stereochemistry of the intermediate 77-allyl complexes should be mentioned. As an example, consider the bis(Tr-allyl) C8 chain postulated as an intermediate in the... 

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

The paUadium-catalyzed reactions occurred with the simple combination of [Pd(allyl) Cl]j and added phosphine, or with Pd(PPh3) and an acid co-catalyst. The scope of these reactions encompassed the additions of arylamines. As shown in Equation 16.81, reaction of various arylamines with cyclohexadiene occurred with high enantioselectivity using Trost s ligand. This ligand is discussed in more detail in Chapter 20. These reactions, and nickel-catalyzed reactions, occur by nucleophilic attack of amines on ir-allyl intermediates generated by protonation of diene complexes or insertion of dienes into palladium hydrides. 

An early example of the application of this oxidative addition reaction to a catalytic process is the DuPont s production of adiponitrile, which includes a nickel-catalyzed isomerization reaction of allylic cyanides (62 -> 63 in Scheme 6.11) [33]. Thorough mechanistic studies identified a jt-allyl nickel cyanide complex formed via oxidative addition of a C-CN bond as the intermediate for this isomerization reaction [34]. 

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