Reductants, homocoupling mechanisms

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

Regioselective syntheses of 1,3,5-unsymmetrically substituted benzenes (309) are catalyzed by Pd(dba)2/PPh3 mixed alkyne/diyne reactants give mixtures containing homocoupled and mixed products (24 21 from HC CPh + HC=CC= CC Hn). The probable mechanism involves oxidative addition to the Pd(0) center, insertion of the second diyne into the Pd—H bond, reductive coupling and subsequent jr-complexation of this product to Pd(0), followed by Diels-Alder cycloaddition of the third diyne and elimination of product. 

Scheme 8.10. Possible mechanisms of the Pd-catalyzed reduction and homocoupling of organic halides. X= halide, triflate Y = R2(T, NHR, O. ...

The mechanism of the homocoupling of bromobenzene catalyzed by [NiCl2(dppe)] under electroreductive conditions has been studied. At low bromobenzene concentration the oxidative addition step to a Ni(0)-dppe intermediate is rate-determining, while at high [PhBr] the reductive elimination from a Ni(III) intermediate, [Ph2NiBr(dppe)], becomes rate-limiting (Scheme 33). " ... 

Homocoupling likely occurs by a related catalytic cycle, but less information is known about this process. A nickel(O) or palladium(O) complex undergoes oxidative addition of an aryl halide. One could imagine that a second aryl halide adds to generate an intermediate in the M(IV) oxidation state, but this step is unlikely to occur with either palladium or nickel catalysts. One can also envision a mechanism involving disproportionation of the arylmetal-halide intermediate to form a biaryl complex and a dihalide complex. Reductive elimination would form the biaryl, and reduction of the dihalide with Zn or other terminal reductant would regenerate the catalyst. 

A very minor amount of homocoupling biaryl is derived during the reduction of a palladium(II) or nickel(II) halide complex with aryl-boronic acid (Eq. 23) or by the metathetic reaction shown in Eq. 48. However, a large number of homocoupling products of arylboronic acids are reported in literature. The mechanism proceeding through oxidative addition of the C-B bond to palladium(O) is recently proposed as the route to homocoupling (Eq. 54). The oxidative addition of the C-B... 

Oxidative Dimerization of Arylboronic Acids. Cu(OAc)2 was shown to mediate dimerization of various arylboronic acids forming symmetric biaryls in good yields. The oxidative homocoupling proceeds smoothly at rather elevated temperatures with Cu(OAc)2 present in catalytic or stoichiometric amounts. In an earlier case air was employed as an oxidant. The mechanism presumably involves transmetallation of arylboronic acids by copper followed by dimerization of the organocopper intermediate, followed by reductive elimination to give the product. ... 

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