Oxidants homocoupling mechanisms

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

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. ... 

Scheme 6 Proposed mechanism for the oxidative homocoupling of alkynes catalyzed by [y-H2SiWio036Cu2(N3)2] [126]. POM frameworks are omitted for clarity...

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. 

Because all currently known mechanisms of oxidative acetylenic homocouplings are very specific to single reaction conditions, e.g. pH or oxidation state of the used copper salt, this section summarizes the most reasonable mechanistic ideas proposed for the commonly utilized coupling procedures. 

It must be emphasized that current mechanistic understanding of copper-mediated oxidative acetylenic couplings is unsatisfactory. Several studies have shown the strong dependency of the mechanism on the experimental setup, suggesting highly complex coherences and interactions. Nevertheless, the mechanistic idea of Bohlmann et al. described above still provides the most accepted picture for Glaser-type oxidative acetylenic homocouplings. 

The mechanism of the homocoupling of dienes is one of the representative reactions proceeding through a n-allylruthenium intermediate. Indeed, a bis 7r-allylruthenium complex was produced by oxidative cyclization of two dienes and the coupling of the terminal carbon atoms led to a cationic (diene) (allyl)hydridoruthenium species. 

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). " ... 

In comparison with the homocoupling of organic electrophiles, Pd-catalyzed homocoupling of organometals requires the presence of an oxidant A generally accepted reaction mechanism is presented in Scheme The homocoupling of organometals... 

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. 

CO insertion prior to the transmetallation step. The mechanism of nickel-catalyzed coupling reactions is less established. Early studies indicated that homocoupling processes occur by oxidative addition through radical intermediates and possible intermediacy of Ni(I) and Ni(III) complexes. The copper-catalyzed cross-coupling reactions likely occur by transmetallation prior to oxidiative addition of the aryl halide. Iron-catalyzed reactions likely occur by low-valent, even sub-valent, species. 

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... 

It should be noted that most presentations of the Glaser and related acetylene homocouplings show a simpler mechanism involving base-catalyzed formation of a copper(I) acetylide 8, oxidation to copper(ll) acetylide 9, and homocoupling of the resultant acetylenic radical 10 to afford... 

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