Electrophilic Aromatic Substitution in Biphenyls

Biphenyl undergoes bromination in the same manner as benzene. Unlike benzene, however, a variety of products ean be imagined. 

Obtain the energy of each cation that might be generated by electrophilic addition of Br to biphenyl (biphenyl+Br+). Which one is most stable Are there others of comparable stability Examine the structure of the most stable cation(s), and draw all of the resonance contributors needed to describe this ion(s). Predict the product(s) of biphenyl bromination. Will the reaction be highly selective, moderately selective or unselective  

Bromination of substituted biphenyls is more complicated since bromination of each ring may lead to different products. Repeat the analysis described above for each of the following reactions. 

Data on the intermediate cations are available 4-aminobiphenyl+Br+, 4-amino-2 -methylbiphenyl+Br+, 4-amino-3 -methylbiphenyl+Br+). Which reaction is most selective Least selective Why Hint Consider the torsion angle about the bond connecting the two phenyl rings. 

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Electrophilic aromatic substitution in biphenyl is best understood by considering one ring as the functional group and the other as a substituent. An aryl substituent is ortho, para-directing. Nitration of biphenyl gives a mixture of o-nitrobiphenyl and p-nitrobiphenyl. 

Benzophenone is much less reactive than biphenyl in electrophilic aromatic substitution... 

With monosubstituted benzenes a mixture of isomeric biphenyls is obtained. The substitution pattern corresponds to that observed in electrophilic aromatic substitution, consistent with a mechanism involving electrophilic attack by PdCl2 shown in Eq. (388). 

Regarding the possible mechanism, notably, toluene, ethylbenzene and terf-butylbenzene are less reactive than benzene, which is not consistent with the expected order for an electrophilic aromatic substitutions, such as that found with the classic Fenton reagent. There are also other differences with respect to the Fenton chemistry. In particular, under biphase conditions the reaction is definitely more selective although comparisons are difficult due to the huge amount of data, sometimes inconsistent, on the Fenton system (for which most of the data have been obtained with the iron used in stoichiometric amounts) it seems that selectivities dose to those observed under biphase conditions are only attained at a conversion around of 1%. Furthermore, in the biphase system, only a negligible amount (<1%) of biphenyl was detected among secondary products, whereas in the classic Fenton oxidation this compound is formed by radical dimerization of hydroxycydohexadienyl radicals in typical yields ranging from 8 to 39%. 

The synthesis of URB597 was extremely challenging, more so for the positions of its substituents than for their structure. Both groups are located in meta positions relative to the bond between the rings. Problem 39 revealed that phenyl rings as substituents are ortho, para directing. Therefore electrophilic aromatic substitution reactions performed on biphenyl cannot serve as an entry to the preparation of this important compound. 

Electrophilic aromatic substitution by N-bromosuccinimide (NBS) of biaryl compound such as 3 -hydroxy-[l,l -biphenyl]-2-carboxylic acid catalysed by tripep-tidic compound (Boc-p-Drnaa-Pip-(D)Val-NMe2) gave atropisomeric tribrominated compound in 90% yield with 84% ee (Scheme 5.26) [75]. 

A simplified energy diagram is presented in Fig. S based on that of Olah and associates [12, 13]. In a similar way R. D. Brown [14] considered that an electrophilic substitution begins by the formation of unstable charge-transfer complexes. R. Taylor [15] observed anomalously high and solvent dependent o/p ratios for the nitration of biphenyl and rationalized it that a tr-complex between NO and biphenyl is formed initially and rearranges to a more stable a-complex at the ortho position of one of the aromatic rings of biphenyl. The final experi-... 

The partial rate factors and the isomer distribution in the amination by di-methylamino radical cation of toluene, isopropylbenzene, -butylbenzene, biphenyl and naphtalene are reported in Table 7. These partial rate factors are far the highest ever observed in homolytic substitutions so that the general character of the homolytic amination allows a more relevant analogy to be drawn with the electrophilic substitutions than with the homol5rtic arylation, the only homol5rtic substitution for which numerous and accurate quantitative data exist in homo-cyclic aromatic series. 

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