Biphenylene, dimerization

Biphenylene dimer 68, a building block for another putative carbon allotrope, was prepared [109]. However, the reaction of 68 under the conditions typical for the conversion of biphenylene to tetraphenylene failed to yield the double helical polymer or corresponding oligomer (Fig. 15.25) [107]. 

A. Rajca, A. Safronov, S. Rajca, C. R. Ross, II, J. J. Stezowski, Biphenylene Dimer. Molecular Fragment of a Two-dimensional Carbon Net and Double-stranded Polymer, J. Am. Chem. Soc. 1996, 118, 7272-7279. 

Figure 5.11 Isomeric graphenes I, biphenylene, and biphenylene dimer 61a.

The biphenylene dimer (Figure 5.11), a fragment of carbon allotropes I, was synthesized by Rajca et al. [28]. Its nearly coplanar structure was determined by X-ray crystallographic study of its tetra-ferf-butyl derivative 61a. 

A convergent synthetic route to the highly symmetrical biphenylene dimer 61 was implemented based on a stepwise connection via an aryl-aryl homocoupling reaction. As can be seen in Scheme 5.9, after the lithiation of 1,3-dibromo-2-iodobenzenes 62 in diethyl ether, the resulting aryllithium was oxidized with CuBr2 to give the corresponding 2,6,2, 6 -tetrabromobiphenyls 63. Upon... 

Benzyne is capable of dimerizing, so that in the absence of either a nucleophile or a reactive unsaturated compound, biphenylene is formed. The lifetime of benzyne is estimated to be on the order of a few seconds in solution near room temperature. ... 

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. 

Vapor-phase flash pyrolysis of 4-phenyl-1,2,3-benzotriazine (8, R = Ph) at 420°-450° gives a mixture of biphenylene, 9-phenylacridine, unchanged triazine, and the 2-phenylbenzazete (158). Compound 158, which is stable at -80°, dimerizes when warmed to room temperature and reacts readily with nucleophiles and 1,3-dienes. The thermally more... 

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 dimer of perinaphthalyne, perylene (17), is thermally more stable than binaphthylene (15) and a relatively larger amount is found in the naphthalyne-derived products. Lindow and Friedman (1967) showed that biphenylene breaks readily at the cyclobutadiene bond above 300° and gives tetraphenylene (tetrabenzocyclooctatetraene) and minor amounts of biphenyl. Presumably binaphthylene behaves similarly. A consequence of the tendency for perinaphthalyne to abstract hydrogen is the formation of a higher proportion of biphenyl than in the reactions of 2,3-naphthalyne. 

Although there are numerous examples of dimerizations of organo-metallic reagents by copper(II) salts, one particularly interesting example concerns 2,2 -dilithiobiphenyl 304). Wittig and Klar have proposed several transient copper(II) ate complexes and a stable copper(I) complex to account for the high yields of biphenylene and o-tetraphenyl-ene. Diarylcopper(II) compounds were supposedly isolated, and characterized by elemental analysis, from diarylmercury compounds and copper at 120°C 11, 197) an unlikely reaction. 

Stille et al. [87] introduced biphenylene as reactive end groups capable of being cured at elevated temperatures via an addition mechanism. When heated above 350 °C, biphenylene undergoes a thermal ring opening to form primarily its cyclic dimer, tetrabenzocyclooctatetraene. 

Benzocyclobutene (BCB) pciymerizBs at much lower temperatures of about 250 °C than the closely related biphenylene. The reaction undergoes electrocyclic ring ojKiung to form o-xylylene which polymerizes to produce cyclic dimer andpoly-o-xylylene [92]. 

Dendritic-macrocyclic polyradicals consist of small dendritic branches which may be connected to calix[4]arene macrocyclic cores via 3,4 -biphenylene linkers. Polyradicals 45-50 correspond spin dimers, trimers, and pentamers, as illustrated in Fig. 28. Their connectivity is consistent with ferromagnetic coupling scheme. 

I, was oxidized with lead tetraacetate (LTA), the characteristic Diels-Alder adduct with tetracyclone (TC) could be obtained even when the diene was added following completion of N2 evolution, indicating that the polymer-bound intermediate had survived for tens of seconds. Secondly, while LTA oxidation of the monomeric aminotriazole I-a, affords the corresponding biphenylenes in high yield, the only products formed by I-b, in the absence of TC, were the aryl acetates II and III (combined yield of 80%). Monomeric o-benzyne is known to dimerize at the diffusion limited rate in the gas phase, therefore, formation of the Diels-Alder product following delayed addition of diene requires a substantially slower encounter process for the polymer-bound analog. 

However, one of the most intriguing examples in this field, which also demonstrates the power of diffusion NMR to characterize supramolecular systems obtained by self aggregation, is the assignment of the different species that prevail in solution following the reduction of 2,5,8,ll-tetra-tert-butylcycloocta[l,2,3,4-def-,5,6,7,8]bis-biphenylene (65) to its respective tetraanions [59]. In this sample, different species were observed and only diffusion NMR provided a proof, in conjunction with 2-D NOESY, that the different molecular species are indeed different helically-stacked anionic aggregates of 65. When NMR diffusion measurements were performed on the obtained solution, four different diffusion coefficients were found for the mixture. These coefficients that were assigned to the monomer, dimer, trimer and tetramer of 65 (Fig. 6.24). Based on Eq. (6.16) ... 

Closely related to the dimerization of biphenylene (36) to tetraphen-ylene (37, Scheme XV) is the dimerization of an aryl-substituted cyclobutadiene to octadienes or cyclooctadienes by way of nickel complexes. A useful source of the cyclobutadiene group is its air-stable complex with NiBr2. Reduction of this complex with tert-butyllithium (electron-transfer agent) gives the tetraphenylcyclobutadiene-nickel(0)-triphenylphosphine complex (38), which isomerizes to the nickelole (39). The dimerization of 39 leads to 40, whose protonation yields the octadiene. Alternatively, at higher temperatures, 40 can extrude Ni(0) to produce 41 (26, Scheme XVI). 

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