Biphenylene derivatives

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-... 

R1 and R2 = Si(CH3)3, but is only moderate when R1 and/or R2 = H,C5H, C6H5, or COOCH3. Even so, this reaction provides access to novel biphenylene derivatives. 

The availability of benzocyclobutenes from cocycloadditions of 1,5-hexadiynes and hindered alkynes has been elaborated into an iterative synthesis of a variety of novel polycyclic biphenylene derivatives. 

This has met with success in a number of areas related to the low temperature observation of the parent species itself, the synthesis of relatively stable substituted derivatives, and heterocyclic and annelated analogs, e.g., benzocyclobutadiene and biphenylene derivatives. These cyclobutadienoids , each class of which is of significant importance in its own right, have interesting physical and chemical characteristics due to incorporation of a potentially delocalized, antiaromatic four-membered ring. Recent progress in this area will be the topic of this report. 

In the following instances, dibromobenzocyclobutadiene, generated in situ, reacted with thiophene dioxides to give the initial adducts 58, which in turn were debrominated to give the biphenylene derivatives 59 in modest yields (Scheme 34) [161]. 

Noncoplanar aromatic monomers, such as 2,2 -disubstituted biphenylene derivatives and 4,4 -functionalized 1,1 -binaphthyl derivatives (Fig. 14) have been used as comonomers in para-linked aromatic polyesters with remarkable effects on the phase transition temperatures, the crystallinity, and the solubility. The incorporation of these noncoplanar monomers will not initially reduce the chain stiffness. The phenyl rings are forced by the 2,2 -substitution into a noncoplanar conformation which strongly decreases the intermolecular inter-... 

However spectroscopic evidence shows that the 6-tt-electron systems are not entirely independent. Thus there is u.v. absorption at much longer wavelengths than in the case of biphenyl a split peak at 320-400 nm appears to be characteristic o biphenylene derivatives [125, 129, 130]. H and C-n.m.r. spectra [131,132] are best explained in terms of some paramagnetic ring current in the four-membered ring. 

The use of biphenylene monomers for incorporation into a polymer main chain requires the synthesis of a di functional biphenylene in monomer-grade purity. Because these synthetic reactions generally require more than one step, and generally do not proceed in high yields, the synthesis of simpler, mono func t iona 1 biphenylene derivatives for use in crosslinking reactions was desirable. In this case the high purity of the biphenylene derivative is not necessary. 

Similarly, the biphenylene derivative was prepared by starting with 4,4 -dib-romodiphenyl. The X-ray diffraction of these two polymers proved them to be amorphous [235]. Conversely, other work [236] reported the mdting point of polyCp-phenylene ditelluride) to be 158 °C with decomposition around 390 °C. Both phenyl and biphenylene diteUuride polymers exhibited photoconductivity upon irradiation by near-infrared light at around 1000 nm. The electrical conductivities of these polymers increased from insulators (10 Scm ) to... 

Ring Opening of the Strained Four-Membered Ring of Biphenylene Derivatives... 

The late 1950s saw several expanded biphenylene topologies. Some of the more notable include Cava s 1,2-benzobiphenylene 78 (Fig. 1.6a) obtained through a highly reactive benzocyclobutadiene [68] as well as his production of 1.2,7.8-dibenzobiphenylene 79, a structure similar to [4]helicene [69]. One of the more spectacular biphenylene derivatives was Nenitzescu and coworkers bulky 1,2,5,8-tetraphenyl-2.3,6.7-dibenzobiphenylene 80 produced by Diels-Alder chemistry in 1962 (Scheme 1.19) [70]. 

Figure 66. Selection of biphenylene derivatives representing alternant non-benzenoid hydrocarbons. (RE values are shown in Table 25.)...

Table 36. REPE and the Percentage Aromatic Character for a Selection of Biphenylene Derivatives (Shown in Figure 66)...

Figure 87. Plot of REPE values against the aromaticity index A for the biphenylene derivatives shown in Figure 66 (listed in Table 36).

The approach based on discrimination of Kekule valence structures of different degree of freedom and selection of only those Kekule valences structures that have the maximum innate degree of freedom automatically extends the idea of Clar structures from benzenoid hydrocarbons to non-benzenoid hydrocarbons, and even fullerenes. In Table 48 we show RE values calculated using only Kekule valence structures of maximal degree of freedom for a selection of biphenylene derivatives. When we restrict the contributions to molecular RE only to Kekule valence structures of maximal degree of freedom, we obtain higher RE values than previously calculated. That the corrected RE will increase can be easily understood, because in summing the contributions from different Kekule valence structures, we have discarded the contributions from Kekule valence structures that make substantially smaller contributions... 

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