Biphenyl, derivatives direct substitution

Direct Substitution in Biphenyl. Direct nitration, chlorination, and bromination of biphenyl can. be used to prepare certain derivatives.87 Direct nitration is probably the most satisfactory method of preparing large quantities of 2- and 4-nitrobiphenyl. In certain substitution reactions it is sometimes difficult to separate the mixture of... 

As already mentioned, the C-H/C-X coupling between simple arenes and aryl halides to form biphenyl derivatives is difficult in terms of reactivity and regiose-lectivity without directing group. In 2004, Fujita and Yamaguchi [28] reported the iridium-catalyzed direct arylation of benzene with aryl iodides. For example, when the reaction was carried out in the presence of the [Cp IrHCl]2 catalyst (5mol%) and KOt-Bu (3.3 equiv.) at 80°C, 4-methoxybiphenyl 34 was obtained in 66% yield. However, when substituted arenes were used instead of benzene, several... 

Palladium-catalysed directed C-H oxidation with (diacetoxy)iodobenzene of a series of meta -substituted aryl pyridine and aryl amide derivatives resulted in the formation of the corresponding acetoxy compounds. The reactions generally proceed with high levels of regioselectivity for functionalization of the less sterically hindered ortho-C-H bond.144 The mechanism shown in Scheme 4 has been proposed for the oxidation of 2,6-dimethylphenol with (diacetoxyiodo)benzene for the formation of 3,5,3, 5 -tetramethyl-biphenyl-4,4 -diol, via C-C coupling.145... 

Thiophene is not reducible by direct electroreduction, but by indirect reduction (Chapter 29) in DMF using biphenyl radical anion as electron transfer agent it is possible to reduce thiophene to 2,5-dihydrothiophene and tetrahydrothiophene in high yield [192] thiophenes substituted at C-2 with carboxyl, however, may be reduced by direct electrolysis to the 2,5-dihydro derivatives [193]. 

Finally, the importance of GC/MS techniques for the analysis of hydroxyaromatic compounds generated during microsomal hydroxylation of benzene derivatives is mentioned here. Using various partially ring-deuteriated substituted benzenes, including biphenyl, evidence for direct aromatic hydroxylation was gained from the careful mass spectrometric tracing of the fate of the label in the various silyl-derivatized hydroxylation products . 

Chesta et al. (1986, 1988) have shown that the photolysis of N,N-dimethylaniline can be coupled with uptake of the electron (or with direct electron transfer from the excited aniline singlet or triplet) by chlorinated benzenes and fragmentation of the resulting radical anion to chloride ion. Hydrogen-substituted benzene derivatives appear to be the other products. Bunce and Gallagher (1982) have shown that a similar aniline-sensitized dechlorination occurs with polychlorinated biphenyls. 

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