Dimerization reactions arene coupling

Electron-rich olefins with substituents Y = phenyl, vinyl, amino, or alkoxy can be coupled by anodic oxidation to tail-tail dimers being either deprotonated to dienes and/or substituted a to Y, depending on Y and the reaction conditions (Eq. 6). Alkyl substituted arenes can be dehydrodimer-ized to diphenyls or diphenylmethanes depending on the kind of substitution (Eq. 7). 

Biaryls have also been prepared by coupling support-bound aryl halides with aryl-zinc compounds (Figure 5.20) or with aryl(fluoro)silanes [203]. As with Suzuki or Stille couplings, these reactions also require transition metal catalysis. An additional strategy for coupling arenes on solid phase is the oxidative dimerization of phenols (Figure 5.20). 

Palladium-catalyzed dimerization of 2-arylpyridines by employing oxone as the terminal oxidant has been reported [75], The reaction was shown to proceed via a Pd(II)-Pd(TV) pathway. A method for cross-coupling of simple arenes with 2-arylpyridine derivatives by using catalytic palladium acetate in the presence of two equivalents of silver carbonate requires use of a large excess of the less reactive arene [76],... 

The first cross-dehydrogenative intermolecular arylation of a heteroarene with an arene was reported by Fagnou in 2007. N-acetyl-lH-indoles were coupled with simple arenes and selective C3-arylation was obtained (88 89) in the presence of Pd(TFA)2 as catalyst in combination with superstoichiometric Cu(OAc)2 as terminal oxidant (Scheme 40) (2007SCI1172). The N-acetyl group proved to be crucial as no reaction product was achieved with IH-indoles, furthermore N-methyl-lH-indoles gave only self-dimerized products. 

The reaction mechanism of oxidative polymerization of aniline has been a big controversy (Fig. 7). The dimerization step is generally proposed as (i), in which aniline is one-electron-oxidized to a cation radical, followed by coupling of two molecules of the cation radical to a dimer. The subsequent steps of chain extension are under discussion routes involving coupling of cation radicals such as (ii)-(iv) (137-140) and routes via electrophilic attack of a two-electron-oxidized quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The addition of electron-rich arenes does not inhibit the polymerization, and therefore the route through the nitrenium ion (vi) seems to be rejected (137). 

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