Homocoupling organometallics

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

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]. 

In spite of the considerable difference in the rates of the reactions of organometallic reagents with 2-pyridyl and 2-quinolinylsulfoxides, a cross-coupling reaction was successfully performed with different combination of mixtures of sulfoxides [(155) + (156)]. This reaction is not selective and affords mixtures of cross-coupling products (157) and homocoupling products (158) and (159). 

Anodic Homocoupling of Anions, at-Complexes, Organometallics and Phenolates... 

The first method involves oxidative homocoupling of bis(terminal alkynyl) complexes in the presence of a catalytic amount of cuprous halides and O2 as the oxidizing agent (Scheme 16, Equation (58)). The use of this catalyst system in organic synthesis is extensive and is better known as Hay s coupling reaction. Extension of this methodology to organometallic synthesis was demonstrated by the conversion of... 

The last class of organometallic compounds that undergo Pd-catalyzed homocouphng are organomercury compounds. An early example of homocoupling of ahcenyhnercury... 

The oxidative homocoupling reaction of organometallic reagents provides a simple method for synthesizing symmetrical biaryls. However, the reaction has not been extensively studied because the dimerization of aryl halides, rather than arylmetal reagents, is synthetically a straightforward route to such biaryls. 

A number of methods based on the use of organometallic reagents have been devised. The methods can be crudely divided into homocoupling and heterocoupling methods, depending on whether the monomers are identical or not (Fig. 8). 

Homocoupling of organometallic generated in situ by the reaction of alkyl, aryl of vinyl halides with lithium in THF takes place on sonication (Scheme 19). No reaction takes place in absence of ultrasound. 

Intermediates, ArNix and Ar2Ni (Ar = C6F5), were isolated as their phosphine complexes in the homocoupling reaction of aromatic halides mediated by metallic nickel [99] see also Matsumoto, H. Inaba, S. Rieke, R.D./. Org. Chem. 1983, 48, 840 see also Kavahunas, A.V. Taylor, A. Rieke, R.D. Organometallics 1983, 2, 377. 

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