Electrophilic substitution biphenylene

The bis(trimethylsilyl) derivative is conveniently obtained and is useful for subsequent conversion into biphenylene itself, by the action of trifluoroacetic acid, and into various substituted biphenylenes by means of electrophilic substitution reactions [146]. 

Elect Pophilio Substitution. Electrophilic substitution of biphenylene takes place at the 2-position in some instances reaction is accompanied by ring-opening of the four-membered ring to provide benzocyclooctatetraene derivatives. 

Presence of a deactivating group such as acetyl or nitro at the 2-position directs further electrophilic substitution into the other six-membered ring, mainly at the 6-position, but also to a lesser extent at the 7-position. Thus dinitration of biphenylene gives rise to 2,6-dinitrobiphenylene [130], and 2-acetylbiphenylene is sulphonated at the 6-position [155] and on acetylation provides principally 2,6-diacetylbiphenylene together with a very small amount of the 2,7-isomer [156]. Similarly 2-benzoylbiphenylene is nitrated... 

Bicyclo[6.3.0]undecapentaenium salts, 390 Biphenylene, 221 preparation, 222 reactions, 224 acylation, 226 with carbenes, 228 electrophilic substitution, 224 halogenation, 225 homolytic substitution, 228 lithiation, 227 mercuriation, 226 nitration, 224 with nucleophiles, 227 oxidation, 224 photolysis, 228 reduction, 224 thermolysis, 228 Biphenylenes, benzo, 230 Biphenylene dianion, 229 Biphenylene dication, 229 Biphenylenes, heterocyclic analogues, 233... 

In the..case of biphenylenes having activating substituents at the 2-position further electrophilic substitution takes place at the 3--position, as predicted by early molecular orbital calculations [158]. Tims, for example, 2-amino- and 2-hydroxy-biphenylene couple with benzene diazonium salts at the 3-position [159,160], and numerous other examples involving 2-acetamido- [161-163], 2-acetoxy- [163], 2-hydroxy- [163], 2-methoxy- [163,164], 2-methyl- [155] and 2-ethyl-[165] biphenylenes are known. Where the substituent is less strongly activating, as in the case of 2-ethylbiphenylene, some substitution occurs alternatively at the 6,7-positions [165], Friedel-Crafts benzoylation of 2-bromobiphenylene gave rise to a 7-benzoyl-2-bromo-biphenylene [157]. 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

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