Homocoupling reactions electrophiles

With allyl chlorides, trapping of the product with an electrophile in situ (which can even be a reactive ketone such as cyclohexanone) prevents homocoupling reactions 32... 

Polar substituents can be handled without protection because nonpolar radicals are involved as reactive intermediates. This saves steps for protection and deprotection that are necessary when substrates with such substituents are submitted to reactions where strong bases, nucleophiles and electrophiles are involved. Together with the availability of a wide variety of carboxylic acids, this method of homocoupling is a unique and attractive method for the construction of symmetrical compounds. A great number of homocoupling reactions have been tabulated in refs. [2, 25, 26]. Table 2 and molecular structures 8-17 show some selected examples. 

Aryl triflates are also useful as electrophiles in the reaction of arylzinc compounds [48]. Pd(0) and Ni(0)catalysed cross-coupling and homocoupling reactions of aryl triflates were reviewed recently [49]. Aryl fluorosulfonates gave biaryls in this reaction too. 

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

Numerous examples of unwanted or deliberate Pd-mediated homocoupling of organometallic reagents have been reported (Scheme 8.14) [38, 122, 123]. Suitable oxidants include a-halo ketones [116, 124, 125] (which can, therefore, not be used as electrophilic component in cross-coupling reactions), oxygen [120, 124, 126], 1,2-diiodoethene [127], 2,3-dibromopropionic acid esters [119], and CuCl2 [128]. 

Synthetic reactions involving activation of alkenyl- and arylsilanes by copper salts have also been reported. A copper-mediated system is effective in the cross-coupling reaction of aryl- and heteroarylsilanes with aryl halides under Pd-free conditions (Equation (13)).66 Alkenyl- and arylsilanes as well as alkynylsilanes undergo homocoupling in the presence of Cul and air.65,65a Functionalized alkenyl- and arylsilanes bearing a coordination site in the vicinity of silicon (e.g., 7) are easily activated by copper salts even at room temperature (Scheme 5).67,67l 67e 68 q-jle organocoppers such as 8 are useful for carbon-carbon bond-forming reaction with carbon electrophiles. 

Finally, any cross-coupling procedures should, in principle, be applicable to the synthesis of homodimers. In addition, other Pd-catalyzed protocols specifically aimed at the synthesis of homodimers have also been developed. Most of them involve Pd-catalyzed dimerization of organometals or organic electrophiles, rather than the reaction of an organometal with an organic electrophile. Some of these homocoupling procedures can be applied to the synthesis of cycUc CToss-dimers. These reactions are discussed in Sect. 

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 organic electrophiles proceeded under a number of different reaction conditions with various reductants (Table 13). From a practical point of view, the most commonly used ones were amines or potassium carbonate, or their combinations with other reductants, such as isopropanol. These reductants are suitable for the homo-coupUng of a variety of alkenyl and aryl halides. 

For skeletal bond-forming reactions in the Lewis-acid realm, the reactive electrophile, an a-synthon, is frequently not used in a stoichiometric fashion, but is generated in substoichiometric amounts in situ in the presence of the donor partner. This can be achieved via in situ umpolung from a d -synthon to an a -synthon, which appears to be a rather roundabout way to reach a natural synthon. The in situ technique, though, has an advantage, because the presence of the less readily oxidized donor partner suppresses any undesired homocoupling (cf. Scheme 2.16) in the oxidation of the starting d -synthon (Scheme 2.21). 

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