Methoxybiphenyls, radical cations

Kinetic analysis of the coupling of 4-methoxybiphenyl radical cation"... 

This discussion of electrodimerization will be concluded with another radical cation example. The anodic coupling of 4-methoxybiphenyl has been obseryed to give good yields of the dimer, 4,4" -dimethoxyquaterphenyl [16]... 

The reaction of n radical cations with n nucleophiles usually leads to C-C bond formation, a reaction that can be very fast (cf. pericyclic reactions also), as in the oxidative dimerization of triphenylamine k = 1-10 x 10 M s ) [293], Hence, such a reaction mechanism can even operate in anodic oxidations (4-methoxybiphenyl [294], tetrahydrocarbazole [295], 4,4 -dimethoxystilbene [296] and 9-methoxyanthracene) [297], where the radical cation concentration is very high. 

DCV kinetic data for the coupling reactions of the 4-methoxybiphenyl cation radical in acetonitrile ... 

In acetonitrile, methoxybiphenyls are coupled in low yield, but almost quantitatively in dichloromethane-trifluoroacetic acid [135,136]. Yields are low in the first solvent because the cyclized products are more easily oxidized than their precursors and the oxidation products are not stable. In the presence of TEA, the cation radicals derived from the oxidation of cyclized XXII are stable and after reduction with zinc dust, for example, cyclized XXII may be isolated in high yield. 

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