Arylboronic transmetallation with copper

The reaction of arylboronic acids with copper(II) acetate is speculated to form an arylcopper species which can add in a 1,2-fashion to an azodicarboxylate. Subsequent transmetallation with the arylboronic acid and hydrolysis would give the desired product selectively (Scheme 4.21). Indeed, it was found that several copper(I) and copper(II) salts would facilitate this reaction. While Cul (55%), CuCl (73%) and Cu(OTf)2 (86%) could be used-in contrast to the reaction depicted in Scheme 4.19-Cu(OAc)2 (88%) proved to be superior [58]. Additional azodicarboxylate derivatives function equally well under the optimized reaction conditions. Non-symmetrical azodicarboxylates are also useful substrates and can be arylated in a reproducibly regioselective manner in excellent to quantitative yields at ambient temperature. In this case, methanol was the preferred reaction solvent [59]. 

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 4.26 shows the development of two general reactions based on copper(I) thiophene-2-carboxylate (CuTC) or Cu(OAc)2 for the formal arylation of imtnes with arylboronic acids [62]. The O-acetyl or O-pentafluorophenyl oximes react with a wide range of electron-rich, electron-poor and electron-neutral boronic acids, and even ortho-substituted substrates react well. The mechanism is proposed to proceed, as shown in Scheme 4.27, via an initial oxidative addition of the copper(I) species, either CuTC or through reduction of Cu(OAc)2 by the boronic acid, to the ketoxime O-carboxylate. This is followed by transmetallation and reductive elimination to generate the final product and regenerate the catalytically active copper(I) species. 

There has been a wide-ranging review of transmetalation reactions of arenes concentrating on the use of boron-containing compounds. The reaction of arylboronate esters and related compounds with alkyl halides is catalysed by copper(I) formation of an aryl—copper intermediate followed by an 5N2-type reaction with the alkyl electrophile is likely. Palladium complexed with a diimine is an excellent catalyst in the phenylation reaction of Michael acceptors with phenylboronic acid so as to yield products such as (16). The nickel-catalysed reaction of phenylboronic acid with styryl epoxides has been shown to yield a-substituted alcohols such as (17). 

The suggested mechanism (eq 23) of the V-arylation starts with formation of a copper (II)-amine complex. Transmetallation by the arylboronic acid followed by reductive elimination finally affords the V-aryl product. ... 

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