Cyclopropanes bromination

NBS can also be used to brominate alkanes. For example, cyclopropane, cyclopentane, and cyclohexane give the corresponding bromides when irradiated in a solution of NBS in dichloromethane. Under these conditions, the succinimidyl radical appears to be involved as the hydrogen-abstracting intermediate ... 

Cyclopropanes undergo a ready reaction with bromine to give 1,3-dibromopropane, and can in turn be formed from 1,3-dibromopropane by the Wurtz reaction. A variation... 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

Four-membered rings also exhibit angle strain, but much less, and are less easily opened. Cyclobutane is riiore resistant than cyclopropane to bromination, and though it can be hydrogenated to butane, more strenuous conditions are required. Nevertheless, pyrolysis at 420°C gives two molecules of ethylene. As mentioned earlier (page 177), cyclobutane is not planar. 

Only one set of data for the addition of bromine to substituted cyclopropanes has been reported (223). In this set, the substituents are of the type XCHj, and therefore the data were correlated by Charton with the constants. Once again, this area requires extensive investigation. 

One such agent is prepared by NBS and peroxide bromination of ethyl 4-chiorophenylacetate (108) to give 109. This is converted by sodium hydride to the benzylic carbene, which is inserted into the double bond of ethyl acrylate to give cis-cyclopropane 110. Partial saponification cleaves the less hindered ester moiety to give 111. This is next converted to the alkoxyimide (112) on reaction with diethyl carbonate and diammonium phosphate. Stronger base (NaOEt)... 

The deshielding effects of chlorine and bromine appear to be similar, with the chlorine having a greater deshielding influence in the methane examples above but a smaller influence in the cyclopropane example in Scheme 3.11. 

In a rather remarkable reaction, methylene groups activated by two electron-withdrawing substituents react with non-activated alkenes under soliddiquid phase-transfer conditions in the presence of a molar equivalent of iodine to yield cyclopropane derivatives (Scheme 6.29) [62, 63], The reaction fails, when the catalyst is omitted or if iodine is replaced by bromine or chlorine. The intermediate iodomethylene systems are unstable in the absence of the reactive alkene and dimerize to produce, for example, ethane-1,1,2,2-tetracarboxylie esters and ethene-1,1,2,2-tetracarboxy lie esters. 

Conant and Lutz is a two-step process involving initial di-bromination of the diketone followed by ring closure with zinc and sodium iodide. The overall yield of traws-l,2-dibenzoyl-cyclopropane is approximately 15%. The submitters have extended the described method to the preparation of other frans-1,2-diaroylcyclopropanes, namely acidic hydrogens on both a-carbons. A probable mechanism for this transformation is as follows ... 

Some variations of the method have been used to prepare cyclopropyl and cyclobutyl halides. Simultaneous addition of bromine and 3-bromocyclobutanecarboxylic acid to the suspension of mercuric oxide gives 1,3-dibromocyclobutane in good yield.7 Similarly, cyclopropanecarboxylic acid gives bromo-cyclopropane,9 and 3-(bromomethyl)cyclobutanecarboxylic acid gives 3-(bromomethyl)cyclobutyl bromide.10 In the latter reaction, it was found desirable to remove the water from the reaction as it is formed in order to obtain high yields. Another variation is the addition of a mixture of the acid and mercuric oxide to excess bromine in bromotrichloromethane.6... 

Despite these significant differences in 13C chemical shifts, it may be problematic assigning the configuration of a given cyclopropane derivative if its diastereomeric counterpart is not available. Therefore, in order to predict 13C chemical shifts, increment rules have been derived which include cross-terms for pairwise interaction of bromine, methyl and ethynyl substituents425. This approach, however, requires an investigation of a large number of model compounds if further substituents are involved. 

B3LYP/6-31G computations. Thus, 3 mol of 1-pentene was cyclopropanated to 89-93% conversion with CCI2 (generated by the phase-transfer-catalyzed reaction of CHCI3 and NaOH). The unreacted 1-pentene was isolated and brominated, and the resulting dibromide was analyzed by and nuclear magentic resonance (NMR) spectroscopy. The C and H isotopic abundances were compared to those in dibromide derived from a sample of the initial 1-pentene, leading to the isotope effects shown for a typical experiment in structure 18 (Fig. 7.10). 

Although cyclopropane and cyclobutane have similar strain energies, they differ markedly in their reactivity toward electrophiles. Thus, whereas cyclopropane reacts readily with bromine to give 1,3-dibromopropane, cyclobutane does not react with bromine. 

The two species of 202 amu are taken to be the initially formed Franck-Condon structure and the parent species giving coherent resonance motion loss of a bromine atom gives BrCH2CH2CH2, which is detected after ionization by the probe pulse as C3H5, at 41 amu. The decay of BrCH2CH2CH2 leads to cyclopropane, a product not ionized by the probe pulse, and hence not seen through mass spectrometry. 

Since the carbon-carbon bonds in strained cyclopropane rings have large s-character, cyclopropyl bromides underwent smooth bromine-magnesium exchange by the action of lithium tributylmagnesate (equation 27)". 

Analogous ionic ring-openings have been described for polycyclic gem-dibromo-cyclopropanes. For example, when solvolyzed in 50% aqueous acetone in the presence of triethylamine, the already mentioned dibromocarbene adduct of benzvalene loses its endo-bromine substituent and opens in a disrotatory fashion to a tricycloheptenyl cation which may be intercepted by water to provide the bromoalcohol shown [175]. 

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