Nickel catalysts reductive coupling

In connection with nickel-catalysed reductive coupling reactions of dihaloarenes, leading to poly(arylene)s, the copper-catalysed reductive carbenoid coupling reactions involving substituted bis(dichloromethyl)arenes and metals or other reductants should be mentioned. The reductive carbenoid coupling of bis(phenyldichloromethyl)arene with zinc in the presence of Cu-based catalysts is shown below ... 

MCRs Involving Ni-NHC Catalysts Nickel-catalyzed reductive couplings and cyclizations allow access to important organic stractures through MCRs that involve two or more r-components, like alkenes and carbonyl groups [83], In this section, these types of catalytic processes based on Ni-NHC are discussed. 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

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. 

Electrogenerated nickel(I) salen has been employed catalytically for the reduction of benzal chloride [152] and for the reductive coupling of 2-bromo- and 2-iodoethanol to prepare 1,4-butanediol [153]. Electrogenerated nickel(I) cyclams have been used as catalysts for the reductive intramolecular cyclizations of o-haloaryl... 

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

The cross coupling of two aryl halides is achieved by the use of organozinc intermediates. Reduction of one component is carried out in dimethylformamide using a stainless steel cathode and a zinc anode with the nickel catalyst in the pres-... 

Some typical examples of cyclooligomerization catalysts other than nickel are listed in Table 5 and 6. Examination of these reactions indicates that the mechanisms are closely related to each other. They all seem to proceed via allylic intermediates in a stepwise oxidative insertion (addition)/reductive coupling (elimination) fashion while the metal center undergoes changes in the formal oxidation state (viz. Fe"-Fe Ti"-Ti Cr -Cr Co -Co" Mn -Mn Mo -Mo" Ni°-Ni [6b]. 

Couplings. A nickel catalyst prepared from reduction of (dppf)NiCl2 with BuLi has been employed to synthesize aiylalkenes by coupling arylborortic acids with enolphosphates. ... 

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