Reduction organohalides

Complex (16), which has a similar structure to Co11 salen, catalyzes the electrocarboxylation of arylmethyl chlorides.274 The enhancement of the catalytic life of (16) as compared to Co-salen may be due to the absence of imino bond in its ligand. The catalytic reduction of halogenated compounds has also been attempted at poly[Mn(salen)]-coated electrodes (M = Ni,253 Co275), which might have potential use for determination of organohalide pollutants.275... 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

As the atomic number of M increases, the possibility of reducing R3MH rises sharply as well. For example, the reduction of organohalides by trialkylsilanes (M = Si) requires the use of catalysts (AICI3, Ni, Pt and others), but the heavier R3MH (M = Ge, Sn, Pb) react in the absence of catalysts. The reactivity of the M—H bond in R3MH towards elemental chalcogens increases as the atomic number of M increases. 

Mechanism The reaction proceeds first by the oxidative addition of organohalide to the Pd(0) complex to give a palladium(II) intermediate as in the case of Stille coupling. The Pd(II) complex then undergoes transmetallation with the base-activated boronic acid to give complex B. This is followed by reductive elimination to form the active Pd(0) species, HX and the cross-coupled product (Scheme 5.17). 

The blossoming area of the suitability of the hypophosphorous acid and its derivatives as hydrogen donors comes from the crucial re-examination by Barton and Jaszberenyi s group of some very old radical reductions involving hypophosphorous acid.9 These workers seminal contribution to the use of water as a solvent for water-soluble organohalides to be reduced cleanly and... 

PhotoadditioH of organohalides to alkenes. CuBr PBu is an effective catalyst for the regioselective photoinduced addition to electron-deficient alkenes. On reduction with zinc dust the overall result is the same as the conjugate addition using an organocuprate. However, in many cases this method is more convenient. 

Carbonylation of organohalides is achievable by using CHCI3 and KOH, whereas reductive alkoxycarbonylation requires also SnCla. 

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