Cyclopropane halocyclopropane

Bromo- and iodocyclopropanes cannot be prepared by the direct halogenation of cyclopropanes. Substituted chloro- and bromocyclopropanes have been synthesized by the photochemical decomposition of a-halodiazomethanes in the presence of olefins iodocyclopropanes have been prepared from the reaction of an olefin, iodoform and potassium f-butoxide followed by the reduction of diiodocyclopropane formed with tri-w-butyl tin hydride. The method described employs a readily available light source and common laboratory equipment, and is relatively safe to carry out. The method is adaptable for the preparation of bromo- and chlorocyclopropanes as well by using bromodiiodomethane or chlorodiiodomethane instead of iodoform. If the olefin used will give two isomeric halocyclopropanes, the isomers are usually separable by chromatography. ... 

The majority of yc/n-dichIorocycIopropane substrates examined in this study provided the desired a-chlorocyclopentenones as a result of sequential electrocyclic ring opening and Nazarov cyclization. In general, those substrates lacking additional substitution on the cyclopropane moiety provided products 75 selectively as a result of regioselective elimination to deliver the more electron-rich olefin. The mechanism for this transformation is believed to involve disrotatory halocyclopropane ring opening... 

Problems with subsequent reaction of a 1-halocyclopropene may be avoided by reduction of the dihalocyclopropane to a monohalocyclopropane prior to dehydrohalogenation. Thus 3,3-dimethylcyclopropene may be obtained in multi-gram quantities by treatment of l-bromo-3,3-dimethylcyclopropane with potassium t-butoxide in DMSO at —78 °C19). Dehydrohalogenation of a series of related 3,3-dialkyl substituted monochloro- or monobromo-cyclopropanes leads to moderate yields of cyclopropenes (11, R = alkyl, alkenyl, aryl, CN, Cl, Rl = H, Ph, t-Bu, R2 = H)20) Indeed, dehydrobromination of (12) leads to either mono- or di-cyclopropenes 2l>. Reaction of a dihalocyclopropane with an alkyl lithium at low temperature followed by carboxylation of the derived 1-lithio-l-halocyclopropane provides a convenient source of 1-halocyclopropane carboxylates dehydrohalogenation leads to cyclo-... 

Organic synthesis via cyclopropanes principles and applications 413 3. Metalation of halocyclopropanes... 

Cyclopropyl halides are versatile starting materials for the preparation of a wide range of cyelopropanes with an additional substituent, which is attached to the ring by a C-C bond. The majority of the reactions has been carried out with monohalocyclopropanes, but there is also a significant body of work dealing with conversions of 1,1-dihalocyclopropanes to 1-substituted 1-halocyclopropanes. Almost without exception the transformations involve ionic intermediates which in most cases include a nucleophilic cyclopropane species. 

A transformation will be regarded as a thermolysis if the halocyclopropane is heated either neat, in the gas phase or in an inert solvent. This solvent should neither serve as a nucleophile HY, nor have a high dielectric constant (to favor cation formation). If the solvent does have one or both of the aforementioned properties, the transformation will be regarded as a solvolysis. Although all these methods in principle obey the same general rules vide supra), their synthetic results are clearly distinct. Conditions in thermolysis are harsh. Since there is no other nucleophile present, the allylic cation can only be trapped by the halide ion that has left the cyclopropane. At the elevated temperature of thermolysis, the resulting allylic halides... 

Halocyclopropanations are given in Section 74F (Cyclopropanations, including Halocy clopropanations). 

Cyclopropanation has also been observed on irradiation of CHjBtj in the presence of alkenes, although in low yield. However, irradiation of diiodo- or dibromohalomethanes in the presence of alkenes affords halocyclopropanes in good yield, as exemplihed by formation of the 7-halonorcaranes 229. ... 

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