Biphenylene bond alternation

Perchlorotriphenylene has been reported in the literature and said to show severe molecular twisting, however, recent work suggests this molecule has not actually been isolated with perchlorofluorene-9-spirocyclohexa-2, 5 -diene being formed instead. The X-ray stmcture of the linear [3]phenylene (benzo[3,4]cyclobuta-[l,2-b]biphenylene, 44) has been obtained, and it shows a relatively large degree of bond alternation while the center distorts to a cyclic bis-allyl frame. ... 

An alternative strategy for promoting Diels-Alder reaction by proton involves the activation of dienophile by hydrogen bonding [93]. Biphenylene diol 143 (Scheme 4.26) forms doubly hydrogen-bonded complexes with a,j]-unsaturated carbonyl compounds, which strongly accelerate the Diels-Alder... 

The following notation will be used throughout this review the number of carbon atoms, hence also of 71-electrons, will be denoted by n = D/2, the number of double bonds by D, the number of rings by R, and the numbers of Kekule structures by K. For benzenoid catafusenes, = 4R + 2, and their formulas are C4R+2H2R+6, or alternatively C2r 4(CH)2r+6, which immediately shows the number (2R - 4) of internal or quaternary carbon atoms participating in the fusion of rings. While clearly all benzenoids are alternant hydrocarbons, not all alternants are benzenoid (e. g. biphenylene). 

The angular mode of fusion in 15 further reduces the diatropism of the center (NICS = —3.3 3 = 6.18 ppm), in conjunction with decreased paratropism of the cyclobutadienes (NICS = 3.1). As a consequence, the termini are more diatropic than in biphenylene (NICS = —9.5 vs. 8.0) [120]. In the remainder of the angular series, the arguments advanced previously for the rationalization of the trends in bond-localizations are clearly augmented by the magnetic data. Thus, 19, as an example, shows the alternation of diatropism of the six-membered rings NICS =... 

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