Homocoupling and Oxidative Substitution Reactions of Aromatic Compounds

7 Homocoupling and Oxidative Substitution Reactions of Aromatic Compounds 

Three oxidative reactions of benzene with Pd(OAc)2 via reactive phenylpalla-dium acetate (397) are known. The insertion of alkenes and /3-H elimination afford arylalkenes 398. This topic is treated in Section 2.2.8. Two other reactions, oxidative homocoupling [157,158] and acetoxyladon [159] are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2-No reaction takes place with PdCl2. The oxidative homocoupling of benzene with Pd(OAc)2 affords biphenyl (399). The scope of the homocoupling reaction has been studied [160,161]. The reaction was applied to commercial production of biphenyltetracarboxylate (400) by coupling of dimethyl phthalate using Pd(OAc)2 and Cu(OAc)2. Addition of phenanthroline is important for the regioselective formation of the most important 3,4,3 4 -isomer [162]. 

Biaryls are prepared also by homocoupling of arylmetal compounds using several oxidants. Arylbronic acids undergo efficient oxidative coupling [167]. 4,4 -Dichlorobiphenyl (407) was obtained in 72 % yield from 4-chlorophenylboronic acid (406) using Cu(N03)2 as an oxidant [168]. 

Acetoxybenzene (412) is prepared by the reaction of benzene with Pd(OAc)2 [158,172]. This reaction is regarded as a potentially useful method for phenol production from benzene, if carried out with only a catalytic amount of Pd(OAc 2. Further reaction of acetoxybenzene produces 1,3- and 1,4-diacetoxybenzenes. 

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