Cyclopropanes bromine addition

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

Bromine addition to cyclopropanes is slow in the absence of light unless the ring is heavily substituted . In the presence of light, reaction is rapid since halogen radical attack on substituted cyclopropanes is fast even at — 78°C. In the presence of light, HBr is produced and cyclopropane reacts more competitively with proton acids than with bromine. Addition reactions to cyclopropane are generally slow in the absence of acid catalysts and therefore the conjugate acid is probably involved in reaction. The more facile reaction of cyclopropane with HBr compared with bromine is in contrast with alkene chemistry where bromine addition is the more rapid ... 

In general, cyclopropane undergoes addition less readily than propylene chlorination, for example, requires a Lewis acid catalyst to polarize the chlorine molecule (compare Sec. 11.11). Yet the reaction with sulfuric acid and other aqueous protic acids takes place considerably faster for cyclopropane than for propylene. (Odder still, treatment with bromine and FeBrj yields a grand mixture of bromo propanes.)... 

When halogenation is carried out in the presence of a nucleophile, interception of cation intermediates occurs and results in the formation of monohalogenated products. This was observed when bromine addition to 4,5-homo-l,7a-diazoindene was carried out in ethanol to give the corresponding (l-bromo-2-ethoxyalkyl)cyclopropane as the main product. " Similar products were obtained on treatment of vinylcyclopropanes with A-bromosuccinimide or N-chlorosuccinimide in water, methanol, and acetic acid, and N-bromoacetamide in water. Monohalides were also obtained when 3-acetoxy-6,6-di-phenylbicyclo[3.1.0]hexene derivatives, e.g. 1, and similar compounds reacted with bromine, A-bromosuccinimide in tetrahydrofuran, and sulfuryl chloride in carbon tetrachloride. ... 

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. 

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... 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

This cyclopropane acid is i. uch lees reactive towards addition reagents than any of the ring compounds thus far dealt with. It is not attacked bji cold saturated aqueous liydrcfcroi.ic acia (although it dissolves in it) and is lijcewise uhreactive towards bromine in... 

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