Hepta-l,3,5-triene

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 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 ... 

The influence of solvent and base concentration on the product yield is significant for this series of cycloproparenes. In the case of 1/f-cyclopropabenzene, it was necessary to perform the reaction in dimethyl sulfoxide with a large excess of base in order to drive the reaction to completion, and only traces of product were obtained in tetrahydrofuran. For l//-cyclo-propa[6]naphthalene (15), on the other hand, the reaction was much more successful in tet-rahydrofuran. The yield of 15 in tetrahydrofuran with 4 molar equivalents of potassium tert-butoxide was only 11%, while 16 molar equivalents of base gave a 38% yield.The yields of 3-bromo- and 3-chlorobicyclo[4.1.0]hepta-l,3,5-triene were also higher in tetrahydrofuran than in dimethyl sulfoxide. ... 

Thus, 7,7-difluorobicyclo[4.1.0]hepta-l,3,5-triene (l,l-difluoro-l//-cyclopropabenzene, 2) was first made by the addition of l,2-dibromo-3,3-difluorocyclopropene to buta-1,3-diene, followed by double dehydrobromination with potassium hydroxide in triethylene glycol at 80 °C and low pressure. Reaction of the adduct of the 1,2-dichloro analog 1 (X = Cl) similarly gave the difluorocyclopropabenzene (2). In subsequent work, potassium /er/-butoxide was used as base and the reaction has been carried out in tetrahydrofuran at low temperatures, which gives a better yield of product. ... 

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... 

Dichloro-2.5-diphenylbicyclo[4.1.0]hepta-l,3,5-triene (625mg, 2 mmol) was dissolved in THE (50 mL) under argon, then 1.6 M BuLi in hexane (0.65 mL, 1.04 mmol) was added at - 78 "C. After 15 min, the solution was allowed to thaw the intense red precipitate was filtered under suction through a porcelain frit and washed with EtjO yield 110 mg (23%) mp 245 C (dec.) deep-red needles after recrystallization from 1 -bromonaphthalcne. 

Pyrolysis in vacuo without a solvent has been successfully used in recent years for aryldiazomethanes (Creary, 1990 Meese, 1985) and for cycloalkyldiazomethanes (Chari et al., 1982). Heating the sodium salt in an apolar solvent (e.g., hexane) can also be recommended (e.g., for l-diazo-4,4-dimethyl-l,4-dihydronaphthalene, Mathur et al., 1985, or for diazoindenes, Kapur et al., 1988). Under extremely careful exclusion of moisture it is possible to obtain 3-diazo-2-oxobicyclo[2.2.1]heptane (2.86) (Yates and Kronis, 1984) in good yield. Yet, diazobicycloalkenes without an a-keto group, e.g., 4-diazobicyclo[3.2.1]oct-2-ene (2.87) and -octa-2,6-diene (2.88) are formed only in modest yields (Murahashi et al., 1982), as well as 7-diazocyclo-hepta-l,3,5-triene (2.89 Kuzaj et al., 1986). 

For the following compounds Chem. Abstr. incomprehensibly uses the von Baeyer names bicyclo[4.2.0]octa-l,3,5,7-tetraene and lif-bicyclo[4.1.0] hepta-l,3,5-triene that totally disregard the aromatic character of these compounds. 

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