Biphenylene, formation

Following the discovery of benzyne and biphenylene formation by pyrolysis of phthalic anhydride (Fields and Meyerson, 1965), two separate groups announced isolation of substituted biphenylenes by pyrolysis of substituted phthalic anhydrides. Brown et al. (1966) obtained chloro-and methyl-substituted biphenylenes by pyrolysis at 700-720° and 0-1 to 1 5 torr. The yields were (Brown et al., 1967) ... 

The pyrolysis of a number of compounds at temperatures around 600— 800° and at pressures of the order of 10 2 mm. has been shown to give rise to benzyne. These compounds include for example indanetrione 29>, and phthalic anhydride 30 38>. The dimerisation of benzyne to yield biphenylene has been used preparatively 31 33>, an(j the pyrolysis of tetrafluorophthalic anhydride 34>, and tetrachlorophthalic anhydride 3i-33) gave the corresponding octahalobiphenylenes. In the case of the pyrolysis of tetrachlorophthalic anhydride some hexachlorobenzene is also formed, while the pyrolysis of tetrabromophthalic anhydride results in the formation of hexabromobenzene but no octabromobiphenylene. The disproportionation of tetrabromobenzyne to form carbon and bromine is a function of the high temperature involved and, as we shall see later, both tetrabromo- and tetraiodo-benzyne behave normally in solution. 

Oxidation of 1-aminobenzotriazole (283) among other products results in the formation of bistriazolo compound 284. This reaction involves a benzene intermediate and illustrates an example wherein IBD behaves differently than lead tetra-acetate, which in this case gives biphenylene [80JCR(S)303] (Scheme 71). 

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>. 

Loss of ethylene and subsequent formation of the stable 3//-pyrrolinine cation dominates the mass spectrum of 2,3,4,5-tetrahydro-l//-l-benzazepine <75OMS(10)992). Biphenylene (m/e 152), acridinium cation (m/e 179) and fluorene cation (m/e 165) are the major fragments in the mass spectra of dibenz[T>,/]azepines (74CJRV101). 

Chlorexolone as diuretic, 1, 174 Chlorides synthesis, 1, 448 Chlorin, 4, 370 metal chelates, 4, 391 Chlorin, dihydroxy-, 4, 393 Chlorin e6, 4, 404 trimethyl ester, 4, 398 synthesis, 4, 416 Chlorination pyridazines, 3, 20, 21 Chlorine trifluoride bonding, 1, 564 Chlorin-phlorin, 4, 398 Chlorins, 4, 378 absorption spectra, 4, 389 formation, 4, 394 molecular structure, 4, 385 oxidation, 4, 395 Chlorisondamine chloride as hypertensive agent, 1, 176 Chlormethiazoles metabolism, 1, 235 Chlormethiuron against ectoparasites veterinary use, 1, 217 Chlormezanone as antidepressant, 1, 169 Chlorocruoroheme, 4, 380 Chlorofucin conformation, 7, 703 Chloronium iodide, biphenylene-reactions, 1, 566... 

Bryce-Smith and Gilbert have shown that toluene, t-butylbenzene, chlorobenzene, o- andp-xylene, and biphenyl all undergo photoaddition to maleic anhydride, yielding 1 2 adducts [33], All the substituents decrease the rate of formation of adducts. Benzonitrile, nitrobenzene, phenol, methyl benzoate, durene, hexamethylbenzene, naphthalene, and biphenylene fail to undergo the addition of these, benzonitrile, nitrobenzene, methyl benzoate, and biphenylene do not form charge-transfer complexes. Bradshaw [34] found that various alkylben-zenes give 1 2 adducts with maleic anhydride upon photosensitization with acetophenone. 

By contrast, the formation of arynes in the vapor phase has received scant attention. Wittig (1961) pyrolyzed bisiodophenylmercury (2) and phthaloyl peroxide (3) in argon at 600° and 5 torr and obtained 54% and 27% yields, respectively, of biphenylene (4) ... 

Fisher and Lossing (1963) decomposed o-diiodobenzene at 960° and 10 3 torr in a reactor coupled to a mass spectrometer and found a product of mass 76, whose vertical ionization potential, 9 75 v.,was appreciably higher than calculated for open-chain C6H4 isomers and strongly indicated formation of benzyne. This assignment was supported by the formation of biphenylene, mass 152. 

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. 

The formation of benzyne from chlorobenzene at 690° is further substantiated by the presence of a small amount, 0 5 on the same scale, of biphenylene among the products. The high-temperature behavior of chlorobenzene thus parallels somewhat its behavior at ordinary tempera-... 

The reaction of 2,2 -dilithiobiphenyl, or the analogous Grignard reagent, with excess copper(II) chloride gave a mixture of products from which biphenylene and o-tetraphenylene were isolated 235, 304). Other products were tetra- and octaphenyls. The reaction was explained in terms of the formation of a copper(II) ate complex (IX) which, under the influence of further Cu " " ions, oxidatively couples to give a 60%... 

Even severely strained ring systems such as the bridged biphenylene 9 [20c] and the helical chiral [2.2]metacyclophane 10 [22,23] could be synthesized by formation of C —S bonds. 

According to Scheme 27, the rate of formation of ferricenium ions is given by Eq. 28, where the catalytic rate constant (/ cat) corresponds to 2A et- The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] (DPA = bisporphyrin with anthrocene spacer, DPB = bisporphyrin with biphenylene spacer, DPD = bisporphpyrin with dibenzofuron spacer) also catalyze the reduction of O2 by Fe(CsH4Me)2, but the amount of [Fe(C5H4Me)2] formed is <4 equiv of O2 (168). Thus, the clean four-electron reduction of O2 by [Fe-(C5H4Me)2]... 

Strained carbon-carbon bonds can undergo reaction with electron-rich complexes such as Pd(PEt3 (3. Biphenylene reacts to form two new C-C bonds with Pd(PEt3)3 in a reaction that is undoubtedly aided by the formation of two strong Pd-C bonds as well as by the strain of the bond being broken (equation 17). ... 

Metallacycles (see Metallacycle) are readily prepared via C-C bond activation, particularly in a strained ring. Insertion of Pt(0) into biphenylene leads to the formation of biphenyl complex. Following this strategy, the reaction of semibuckminsterfullerene (C30H12) with Pt(0) yields a buckybowl complex (Scheme 21). Similar ring expansion... 

In contrast, when a larger tetracationic cyclophane ring was employed in another set of experiments, the formation of new [3]-catenanes was found to be more facile. ° The expanded ring was obtained by replacing the phenylene linker units in the normal tetracationic cyclophane with biphenylene units. This larger ring was found to form readily around two threaded crowns when the latter are present in the acetonitrile reaction mixture in several-fold excess. Both bis-p-phenylene-34-crown-lO and l,5-dinaphtho-38-crown-10 were successfully employed in separate reactions to produce the corresponding [3]-catenanes in 20 and 31% yields, respectively. 

Similarly, biphenylene (63) was obtained in ca. 7% yield, from treatment of the thermally unstable tetrayne 8a with potassium /-butoxide, probably by transannular bond formation between the triple bonds closely placed in the intermediate 62 . ... 

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. 

Comparison of the calculated63 and experimentally derived74 heats of formation of biphenylene, suggest a destabilization of 74.2 kcal in 56. This has usually been associated with the strain energy contained in the four-membered ring. To what extent the cyclobutadienoid character of this ring contributes to the observed... 

Jones et al. recently found that Pt(PEt3)3 and Pd(PEt3)3 are also capable of catalyzing the formation of tetraphenylene from biphenylene (16 turnovers/day with Pd(PEt3)3 at 120°C) [102]. A detailed mechanistic study was performed with platinum. The catalytic cycle which involves sequential oxidative addition of two molecules of biphenylene onto Pt(0) and Pt(II) is proposed on the basis of kinetic analysis and identification of the intermediate Pt(II) and Pt(IV) complexes 81-83. 

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