Bicyclo hepta-2,4-6 -triene

Benzoxepin, 55, 89 Benzoyl chloride, 55, 123 BICYCLO[4 1 0] HEPTA-1,3,5-TRIENE... 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

Thus, heptafulvalene (522) was isolated in 33 and 65% yield after thermolysis of 517 in diglyme and its photolysis in THF, respectively [193]. An almost quantitative yield of 522 resulted when a mixture of 1-, 2- and 3-chloro-l,3,5-cycloheptatriene (518a) was treated with KOtBu in THF [206]. Even on variation ofthe concentration of the starting material and the temperature of the reaction, 522 turned out to be the exclusive product [207]. Also, the treatment of (trimethylsilyl)tropylium tetrafluoro-borate (519) with tetrabutylammonium fluoride [208] and the gas-phase pyrolysis of 7-acetoxynorbornadiene and 7-acetoxy-l,3,5-cycloheptatriene [209] afforded high yields of 522. Further, 522 was observed on FVT of N-nitroso-N-(7-norbornadienyl)-urea at 350 °C, which is believed to be converted into 7-diazonorbornadiene initially. Its decomposition should proceed via 7-norbornadienylidene to bicyclo[3.2.0]hepta-l(2),3,6,-triene (514) (Scheme 6.103) and then on to 5 [210]. The intermediacy of 514 is also suspected in the formation of 522 from 7-acetoxynorbornadiene. 

Another paper64, which also contains a literature survey about the problem discussed, describes the rearrangements of C7H6 systems which can be generated by thermolysis of phenyldiazomethane. By using spectral methods and chemical reactions, the formation of bicyclo[4.1.0]hepta-2,4,6-triene (139), cycloheptatrienylidene (137a) and bicyclo[3.2.0] hepta-l,3,6-triene (135) was excluded, and evidence for the formation of intermediate cycloheptatetraene (137b) (see equation 45) was furnished. 

Computational studies of the electrocyclic ring closures of cycloheptatetraene to bicyclo[3.2.0]hepta-l,3,6-triene and bicyclo[3.2.0]hepta-2,3,6-triene have been reported,292 and the reaction of P-chloroiminophosphine with 1 -(dialkylaminojalkynes has been shown to yield 1,2-azaphosphetines via the (E)-1 -aza-2-phosphabuta-1,3-dienes.293... 

The systematic nomenclature for the cycloproparenes is confused because the fusion rule (lUPAC Rule A 21.3) requires that at least two rings of five or more members be present before the prefix cyclopropa may be used. Thus while l//-cyclopropa[a]- and -[ Jnaphthalene are correct for 10 and 11, respectively, li/-cyclopropabenzene is incorrect for 1. The Chemical Abstracts service and lUPAC are unanimous in naming 1 as bicyclo-[4.1.0]hepta-l,3,5-triene la. Thus if the parent member is strictly named, not only does it differ from that of its higher homologues, but also it could be taken to imply a bond localized structure. Throughout this chapter parent 1 and its derivatives 5-9 are referred to as cyclopropabenzenes and numbered as shown for structure 1. 

The generation of strained bicyclo[4.1.0]hepta-l(7),2,4-trienes by the dehydrohalogenation route is now well established Although these molecules are highly reactive their presence has been established from nucleophilic addition reactions and by ringopening to the corresponding aryl carbene (equation 18). The preparation of a number of... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

When diazoacetonitrile was photolyzed in refluxing benzene, bicyclo[4.1.0]hepta-2,4-diene-7-carbonitrile (5) was formed, which rapidly tautomerized to cyclohepta-l,3,5-triene-7-carbo-... 

The double elimination of 1,1-dichlorocyclopropanes with potassium tert-butoxide to give cycloproparenes is an important variant of the reaction discussed in Section 5.2.2.1.2.3. When a six-membered ring fused to the cyclopropane contains a double bond, the elimination is accompanied by isomerization of the new double bonds to give an aromatic ring (Table 7). The reaction is illustrated by the synthesis of bicyclo[4.1.0]hepta-l,3,5-triene (l//-cyclo-propabenzene, 3) in two steps from cyclohexa-1,4-diene (1), the first step being dichlorocarbene addition to the diene to give 7,7-dichlorobicyclo[4.1.0]hept-3-ene... 

Bicyclo[4.1.0]hepta-l,3,5-triene (lf/-Cyclopropabenzene, 3) Typical Procedure ... 

Various bi- and oligocyclic ring systems, such as norcaradiene (1 bicyclo[4.1.0]hepta-2,4-diene), bicyclo[6.1.0]nona-2,4,6-triene (2), bullvalene (3 tricyclo[3.3.2.0 ]deca-3,6,9-triene), semibul-Ivalene (4 tricyclo[3.3.0" ]octa-2,7-diene), barbaralone (5 tricyclo[3.3.1.0 ]nona-3,6-dien-10-one) and many other systems containing divinylcyclopropane subunits, exhibit interesting transition metal mediated reaction pathways leading to new carbon skeletons. ... 

Ring contraction of appropriately substituted 1,6-di- or tetrahalocycloheptatrienes or -cyclo-heptanones provides access to the bicyclo[4.1.0]heptene system. Thus l-bromo-6-chlorocyclo-hepta-l,3,5-triene reacted with butyllithium directly to alford benzocyclopropene (la) in 50% yield, presumably via a norcaradiene intermediate. The 2-methyl derivative lb was... 

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