Biphenylene reactivity

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. ... 

Blackley548 measured the rates of deuteration of biphenylene, fluorene, tri-phenylene, and phenanthrene relative to o-xylene as 6.15 5.85 1.08 1.32, which is in very good agreement with the values of 8.80 7.00 - 1.14 which may be deduced from the detritiation data in Table 159, obtained using anhydrous trifluoroacetic acid. Aqueous trifluoroacetic acid (with the addition in some cases of benzene to assist solubility) was used by Rice550, who found that triptycene was 0.1 times as reactive per aromatic ring as o-xylene (cf. 0.13 derivable from Table 159) whereas the compound (XXXI) was 0.9 times as reactive as o-xylene. An exactly comparable measure is not available from Table 158, but dihydroanthracene (XXXII), which is similar, was 0.51 times as reactive as o-xylene and... 

Rates of detritiation of the 1 and 2 positions of biphenylene and the 5 position of benzo [bjbiphenylene by lithium cyclohexylamide at 50 °C relative to benzene have been determined as 490, 7.0, and 1,865, respectively and the enhanced reactivity of the position a to the strained 4-membered ring has been attributed to the enhanced electronegativity of the strained bridgehead carbon atom. The same... 

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 formation of reactive carbenes from alkylidene Meldrum s acids has also been observed. Thus pyrolysis of 1-indanylidene Meldrum s acid at 640 °C gave the corresponding carbene which further rearranged to benzofulvene and naphthalene (Scheme 12) <1998JA8315>. Similarly, EVP of 9-fluorenylidene Meldrum s acid at 1100°C provided a mixture of phenanthrene and biphenylene <1996TL6819>. 

Fig. 2. Reactivity of aromatic diisocyanates 0.02 M with 2-ethyihexanol 0.4 M and diethylene glycol adipate polyester in benzene at 28°C. (A) l-Chloro-2,4-phenylene diisocyanate. (B) m-Phenylene diisocyanate. (C) p-Phenylene diisocyanate. (D) 4,4 -Methylene bis(phenyl isocyanate). (E) 2,4-Tolylene diisocyanate. (F) Tolylene diisocyanate (60%, 2,4-isomer, 40% 2,6-isomer). (G) 2,6-Tolylene diisocyanate. (H) 3,3 -Dimethyl-4,4 -biphenylene diisocyanate (0.002 M) in 0.04 M 2-ethylhexanol. (I) 4,4 -Methylene bis(2-methylphenyl isocyanate). (J) 3,3 -Dimethoxy-4,4 -biphenylene diisocyanate. (K) 2,2,5,5 -Tetramethyl-4,4 -biphenylene diisocyanate. (L) 80% 2,4- and 20% 2,6-isomer of tolylene diisocyanate with diethylene glycol adipate polyester (hydroxyl No. 57, acid No. 1.6, and average molecular weight 1900). Reprinted from M. E. Bailey, V. Kirss, and R. G. Spaunburgh, Ind. Eng. Chem. 48, 794 (1956). (Copyright 1956 by the American Chemical Society. Reprinted by permission of the copyright owner.)...

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. 

Pyridine rings that are constrained in a biphenylene system show increased reactivity to aqueous sodium hydroxide. TTius the reaction of 1,8-diazabi-phenylene (19) with 2.5M sodium hydroxide at 150°C gives the pyridyl-pyridone (20) in 96% yield... 

Even as stoichiometric reagents, such nickel complexes as bipy-(COD)Ni have already demonstrated their value in desulfurizing and ring-contracting processes, in hydrodesulfurization, in selective deoxygenation of epoxides, and in the controlled cleavage of allyl ethers. Their ability to form metallocyclopentadienes from biphenylenes or from thiophenes can develop into a valuable alternative route to such reactive metal heterocycles. 

Benzocyclobutene (BCB) Biphenylene (BP) [2.2]Paracyclophane (PCP) More recent additions to the list of strained-ring reactive crosslinking groups are [586] cyclobutaquinoline (CBQ) and benzocyclobutaacenaphthylene (BCBAN) ... 

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]. 

Despite the presence of cyclobutadienoid circuits, the chemical reactivity of 1 (Scheme 4.1, right) reflects considerable aromatic character biphenylene under-... 

Scheme 4.1. General modes of biphenylene preparation (left) and reactivity (right).

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