Homocouplings mechanism

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. ...

Scheme 8.13. Possible mechanism of the homocoupling of the organometallic component of Pd-mediated cross-couplings [119,121].

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. 

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. 

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 of aryl iodide in the presence of Cu (Ullmann reaction) proceeds via a radical mechanism (Scheme 4.44). 

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... 

Pd-catalyzed homocoupling of terminal alkynes proceeds in the presence of a catalytic amount of Cu(l) salts and amines. During this process a terminal alkyne reacts with copper(I) salt in the presence of an amine to give an alkynyl copper compound that reacts with the Pd catalyst. Therefore, the homocoupling of terminal alkynes falls into the category of homocoupling of organometals and follows the same reaction mechanism. 

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