Biphenylenes, cycloaddition

The unsubstituted benzyne (8.26) from 2-diazoniobenzenecarboxylate can be trapped in the absence of a Bronsted acid as biphenylene, i.e., dimeric benzyne (8.30 Ford, 1971), or by cycloaddition to a 4-71 donor such as furan to give 8.31 (Stiles et al., 1963). 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

When benzyne is generated in the absence of another reactive molecule, it dimerizes to biphenylene.123 124 125 126 In the presence of dienes, benzyne is a very reactive dienophile, and [4 + 2] cycloaddition products are formed. 

The thermal reaction between two molecules of olefin to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the olefins are the same or different, but the reaction is not a general one for olefins.921 Dimerization of like olefins occurs with the following compounds F2C=CX2 (X = F or Cl) and certain other fluorinated alkenes (though not F2C=CH2), allenes (to give derivatives of 97),922 benzynes (to give biphenylene deriv-... 

The [2 + 2 + 2] cydotrimerization of alkynes and its analogous arynes [134] leads directly to aromatic hydrocarbons and can be considered as special cases of [2 + 2 + 2] cycloaddition reactions of unsaturated compounds [135, 136]. Especially the catalyzed cydotrimerization reactions are often highly regio- and stereoselective and have been established as valuable methods for the syntheses of highly substituted benzenes, biphenylenes, triphenylenes, and in the approach towards natural products. 

The enantioselective completely intramolecular [2 + 2 + 2] cycloaddition also furnishes Cz-symmetric axially chiral biaryls. Shibata et al. reported the cationic rhodium(I)/tol-BINAP complex-catalyzed enantioselective [2 + 2 + 2] cycloaddition of symmetric hexaynes 58 to give Cz-symmetric axially chiral biaryls 59 with good yields and ee values (Scheme 9.21) [21]. This is the first example of an axially chiral bis(biphenylene) skeleton. 

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