Halogen-substituted cyclopropenes

Examination of the IR spectra of halogen-substituted cyclopropenes data in Figure 4,... 

The reaction of l-chloro-3,3-dimethyl-2-phenylcyclopropene with lithium metal resulted in lithium-halogen exchange and the derived cyclopropene 1 was trapped by a range of electrophiles giving substituted cyclopropenes 2. 

Tetrachlorocyclopropene in the presence of Lewis acids or the trichlorocyclopropenyl cation are important starting materials in cyclopropenone syntheses In this method the cyclopropene or salt is reacted with benzene or derivatives of benzene bearing functional groups such as alkyl, alkoxy, hydroxy, or halogen to yield diaryl-substituted cations. Upon hydrolysis these cations yield the cyclopropenone. In some cases the monoaryl cation (32) can be obtained and converted to the aryl chlorocyclopropenone 33. Alternatively, the monoaryl cation can be reacted with a second aromatic species to give cyclopropenones with different aryl groups (equation 30). 

Many cyclopropyl chlorides and bromides have been converted to alkoxycyclopropanes by treatment with a strong base, in most cases potassium rerf-butoxide, in an appropriate organic solvent (Table 13). Under such conditions, hydrogen halide elimination takes place, yielding strained cyclopropene intermediates, which are trapped by nucleophilic attack of the alkoxide. Overall, a simple substitution occurs when a bond is formed between the alkoxide group and the carbon atom to which the halide was attached. This is the case when l-chloro-5-methyl-exo-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (1) was reacted with potassium /ert-butoxide l-/er/-butoxy-5-methyl-ent/o-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2) was isolated in 94% yield.If a C-O bond is established at the other olefinic carbon atom, a C H bond is concomitantly formed at the carbon atom, to which the halide was attached. The result is a double substitution which is discussed elsewhere (see Section 5.2.1.3 ). When the substrate contains more than one halogen atom, several elimination reactions usually take place. Thus, treatment of 1 -bromo-2-chloro-2-methylcyclopropane (3) with an excess of potassium /er/-butoxide gave l-ter/-butoxy-2-methylenecyclopropane (4) in 30% yield. 

When halocyclopropanes react with an alkali metal alkoxide in an organic solvent, elimination-addition reactions take place and result in formation of alkoxycyclopropanes. In most cases the overall reaction can be regarded as a simple substitution of a halogen atom by an alkoxy group (see Section 5.2.1.1.11.2.2.), but in some cases a double substitution is observed. The latter reaction occurred when 6,6-dichloro-3-thiabicyclo[3.1.0]hexane was treated with potassium fcr/-butoxide and gave l-tcrt-butoxy-e r/o-6-chloro-3-thiabicyclo[3.1.0]hexane (1) in 68% yield. The reaction is believed to take place via a strained cyclopropene intermediate, which is trapped by nucleophilic attack of the ferr-butoxide or by reaction with furan if present.Other 1,1-dichlorocyclopropanes react analogously under similar conditions, 22-724 jjyj jien an excess of base is used, a second elimination-addition... 

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