Reaction mechanism homocoupling

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. 

Figure 4.9 The proposed mechanism of catalysis of Au NPs-rGO composites towards Ullmann homocoupling reaction of aryl bromide. [Reproduced with permission from Ref. [54], Copyright 2014, RSC Publication, RSC Advances, 2014, VoL 4, pp. 5243-5247.]...

Neither the synthetic scope nor mechanism was significantly investigated until 1935, when Baddeley and co-workers provided new insights into the Scholl reaction. Their work provided the first mechanistic interpretation, suggesting that the homocoupling reaction of naphthalene proceeds via an arenium cation intermediate. Based on experiments in... 

Figure 1. Mechanism of Homocoupling Reaction of Organic Electrophiles in the Presence of Excess Zinc.

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. 

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. 

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. 

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

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