Benzocyclopropenones

Although benzocyclopropenes 164 have been isolated126 and were found to be moderately stable, derivatives of the more strained but electronically more stabilized benzotriafulvene system 165 have not yet been synthesized. However, benzocyclopropenones have been shown to be reactive intermediates in several reactions. 

Thus oxidation of both 6- and 7-substituted 3-amino-benzo-l,2,3-triazin-4-ones with lead tetraacetate in methanol produced a mixture of p- and m-substituted benzoates, clearly indicating that a symmetrical intermediate, i.e. benzocyclopropenone (166) was formed and underwent ring opening by attack of solvent121 ... 

Only the appearance of the rearranged product is significant for benzocyclopropenone intermediacy, since the indazolone 168 was detected and trapped by tetra-cyclone via 170 in the oxidation of 167, which was shown to yield only unrearranged ester 169 by nucleophilic cleavage by the solvent thus 168 cannot be a precursor of benzocyclopropenone 171. 

The intervention of an intermediate with the symmetry of a benzocyclopropenone (166, R = Cl) is also demanded by the formation of methyl p-chlorobenzoate in the photolysis of the lithio derivative 173 of the chlorosubstituted 3-p-tolylsulpho-nylamino-benzo-l,2,3-triazin-4-one128) ... 

A benzocyclopropenone intermediate 177 may account for the formation of terphenic acid 178 on hydrolytic decomposition of benzocyclopropenium cation i 761303). Likewise, the intermediacy of benzocyclopropenone (166, R = H) is claimed from spectroscopic evidence in the low-temperature photolysis of benzocyclobutene dione leading to benzyne130b). 

Benzocyclopropenone (219) or, more generally, oxocycloproparenes have so far not been isolated under synthetic conditions. Although 1,1-dihalogenocyclo-... 

When 3-diazobenzofuranone (226) is irradiated in an argon matrix at 8 K, a ketene 227 results as the primary product. This loses CO, and the resulting carbene 228 ring contracts to a species which exhibits an infrared (IR) frequency at 1838 cm", which was tentatively assigned to intermediate 219. Upon further irradiation, benzyne (3) is formed. The benzocyclopropenone 219 is also generated upon irradiation of benzocyclobutanedione (229) in an Ar matrix at 8 K. " An analogous sequence occurs upon irradiation of 230 at 16 K, and leads initially to cyclo-propaacenaphthenone 231, which decarbonylates to acenaphthyne (232). ... 

The discovery of alkylidenecycloproparenes as a class of isolable compounds is the most significant event in recent cycloproparene chemistry. Methylidenecyclo-propabenzene (234) combines the structural features of benzocyclopropene (1), methylenecyclopropene (233), and trimethylenecyclopropane (235) in one and the same structure. Although the parent 234 has so far not been isolated, many substituted and annulated derivatives have been synthesized and charaeterized. Their unexpected stability contrasts sharply with that of the benzocyclopropenones 219, which are only observable as transient reaction intermediates. Reviews on the chemistry of alkylidenecycloproparenes are available. ... 

Oxidation of l-aminonaphtho[l,8-de]triazine (95) with lead tetraacetate has been shown by trapping experiments to result in formation of 1,8-dehydronaphthalene, again almost certainly via initial formation of the nitrene followed by loss of two molecules of nitrogen. " The same meta aryne is formed during the thermolysis of the lithium salt of l-p-toluenesulfonylaminonaphtho[l,8-de]triazine (119), in a formal oxidation process, while benzocyclopropenone (118) has similarly been generated by photolysis of the lithium salt 120. °... 

Adding legs or walls to the two-dimensional cyclic cavity leads to the formation of three-dimensional cavities. Further addition of a cap to the cavity creates an enclosed cavity space. Such spaces are called molecular capsules, and the trapped guest is shielded from the outer environment. If unstable species are trapped in the molecular capsule, their lifetimes can be extended and their properties are easily measured. An example of an unstable species that is stabilized inside a molecular capsule is shown in Fig. 2.22. Photoirradiation of the benzocyclobutendiol in the molecular capsule at -196 °C converts it to benzyne via benzocyclopropenone. Although benzyne is usually quite unstable, benzyne trapped in the molecular capsule can be characterized with H-NMR and i C-NMR at -75°C. 

Benzocyclopropenone (3a) was formed as a reactive intermediate on photochemical decomposition of the lithium salt of 3-tosylamino-l,2,3-bcnzotriazin-4(3//)-one in methanol 3-chlorobenzocyclopropenone (3b) was formed analogously. Further reaction of the ketone 3a with methanol produced methyl benzoate as the main product. Additional evidence for the occurrence of benzocyclopropenone as reactive intermediate resulted from the oxidation of... 

The diazenes formed from derivatives of l,2,3-benzotriazin-4-one have the interesting property of extruding one or two molecules of nitrogen, whereby the resulting fragments apparently collapse to benzocyclopropenone before being trapped by methanol (Scheme 70).412,413... 

Oxidation of 3-amino-l,2,3-benzotriazin-4(3//)-ones 6 with lead(IV) acetate proceeded by two simultaneous independent routes, which involved the loss of one mole of nitrogen to form indazolones 8 or loss of two moles of nitrogen to form benzocyclopropenones 9200- 252.253 cp Houben-Weyl, Vol. E 17d, p 2899ff). Concerted fragmentation of the intermediate nitrene 7 has been proposed to explain these results. The suggested mechanism is supported by 15N-label-ing experiments and MO calculations. 

Triazines are oxidized by peroxy acids yielding 1- or 3-oxides. 3-Amino-l,2,3-benzotriazin-4(3//)-ones 5 are dehydrogenated at the amino group by lead tetraacetate. The intermediate nitrenes 6 are stabilized by eliminating a molecule of N2 yielding 3//-indazolones 7. Alternatively, by loss of two molecules of N2, they form benzocyclopropenones 8 which are confirmed by trapping [13] ... 

Photolysis of hemicarceplex 4029 yielded hemicarceplex 4072. Benzocyclopropenone 72 had previously been studied in solution below -78 °C, where it rapidly hydrolyzed in the presence of moisture. However, protected inside the hemicarcerand, it was stable at room temperature in water-saturated CDCI3. The proton chemical shifts of incarcerated 72 suggest co-alignment of its C2 axis with the C4 axis of 4 (Figure 9.21) such that the reactive C=0 resides in the shielded region of the inner phase, which explains the high stabihty of the incarcerated guest. 

No benzocyclopropenone has been isolated but they appear to be formed as intermediates in the decomposition of benzotriazin-4-ones, in photolysis of 3-p-tolylsulphaminobenzotriazinone [160] or by oxidation of 3-aminobenzotriazinone by lead tetracetate [161]. It is likely that a nitrene is formed initially and collapses, with loss of nitrogen, to form the benzocyclopropenone. Isolated... 

Benzocyclopropenium salts, 162 Benzocyclopropenones, 180 Benzofulvalenes, 404 Benzotropolones,. 120 Benzotropones, 106 Benzotropylium salts, 84 Bicyclic compounds, 350 Bicyclic ions, 390 Bi cyclo[6.2.Ojdecapentaenes,... 

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