Hydrogen bromide with cyclopropanes

Jfor and Jrev are useful in describing individual mechanistic steps, as in the reaction of cyclopropane with hydrogen bromide - 276+277- 278- 279 (Figure 8.20, p. 26). 

There are various methods for the synthesis of cyclopropyltriphenylphosphonium bromides, the precursor of choice for the preparation of cyclopropylidenetriphenylphosphoranes. Interestingly, attempts to generate the former salt from cyclopropane derivatives, e.g. 1-bromocy-clopropane, have been unrewarding. One exception is the reaction of cyclopropyllithium with tetraphenylphosphonium bromide followed by elimination of benzene from the initial adduct. The intermediate phosphorane 5 was converted into phosphonium salt 13 by reaction with hydrogen bromide. 

Hydrogen bromide underwent rapid addition to methylcyclopropane in the presence of bromine to give predominantly 2-bromobutane (la, 50% yield). A small amount (10%) of disubstituted butanes 3a and 4a was also isolated. A 100% excess of bromine over the cyclopropane was added in order to convert alkene products into dibromides. The reaction was carried out in the dark to avoid radical reactions of bromine. A similar result was obtained with ethylcy-clopropane. cis- and /ran5-l,2-Dimethylcyclopropane gave a mixture of four isomeric bro-mopentanes in 76% and 89% yield, respectively, on treatment with hydrogen bromide/bromine at -78°C. 3... 

The reactions of alkylcyclopropanes with bromine and with hydrogen bromide have been investigated in detail. Methylcyclopropane (4, R = Me) scarcely underwent addition of bromine in the cold when light and catalysts were excluded. Similar observations were made with other alkyl-substituted cyclopropanes 4 such as ethylcyclopropane, butylcyclopropane, cis-and trans-1,2-dimethylcyclopropane. ... 

The cleavage with hydrogen bromide has been applied to the ring opening of a number of cyclopropanes incorporated into a polycyclic system. Treatment of tricyclo[3.3.0.0 ]octan-3-one (36) with hydrogen bromide in dichloromethane gave two bromo-substituted bicycles 37 and 38 as the result of the cleavage of the two different activated cyclopropane bonds. ... 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

The most general entry into functionalized sulfur-containing cyclopropanes is the addition reactions of cyclopropanones and thiols (see Section 5.2.3.5.4.). Thus, reaction of cyclo-propanone with thiols in dichloromethane affords l-(alkylsulfanyl)cyclopropanols 39 which are readily transformed into the corresponding cyclopropyl bromides 40 and cyclopropyl chlorides 41 upon reaction with anhydrous hydrogen bromide and hydrogen chloride, respectively. ... 

The majority of the substrates have been converted to alk-l-enylcyclopropanes by abstraction of a hydrogen atom a or to the cyclopropane ring accompanied, more or less simultaneously, by expulsion of a leaving group (Y ) or a to the ring. The majority of reactions have been carried out under basic conditions with chloride, bromide, 1 25-1737 jodide, hydroxide, ... 

A study with the cyclopropane incorporated into a polycyclic system, e.g. 9, showed that the addition of hydrogen bromide across one of the cyclopropane bonds occurred with retention of configuration at the electrophilic bridgehead carbon atom. ... 

In all of these studies with polycyclic cyclopropane systems, the proton underwent addition to the methylene group. However, there are reports on the addition reaction of hydrogen bromide in which the proton attacked the bridgehead carbon and the bromide finally became the substituent at the methyl group, e.g. formation of 10 and 11. °... 

Cyclopropane-1,1-dicarboxylic acid (30a) reacted with hydrobromic acid to (2-bro-moethyl)malonic acid (31a) . In a similar reaction, ethyl 1-acetylcyclopropane-l-carboxylate (31b) ( C enriched) was converted to 5-bromopentan-2-one upon treatment with hydrobromic acid and decarboxylation." In 2-benzoyl-3-phenylcyclopropane-1,1-dicarboxylic acid (31c), two different activating functions are present and can influence the addition of hydrogen bromide. In fact, products arising from the cleavage of either bond that link the phenyl-substituted carbon were isolated. Both primary products had lost hydrogen bromide and carbon dioxide. 

The Hunsdiecker process has the disadvantage of requiring dry silver salts. As a consequence, other methods have been sought, and it has been found that some carboxyhc acids undergo oxidative decarboxylation on formation of mercury (Hg), lead (Pb), or copper (Cu) salts. For example, cyclopropane carboxyhc acid, on treatment with bromine (Br2) in the presence of red mercury(II) oxide (HgO), yields hydrogen bromide (HBr), carbon dioxide (CO2), and bromcxyclopropane (cyclopropyl bromide) as shown in Equation 9.84. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Perbenzoic acid fcrf-butjl ester (51) is the source — under copper I) bromide catalysis —of a benzoate anion (52) and radical 53. Radical 53 subsequently abstracts a hydrogen atom selective ) from the 11 -position of 55 in a homolytic bond cleavage to give a buta diene system with opening of the cyclopropane ring.Jn... 

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

Whereas photosensitized bromination of cyclopropane affords 1,3-dibromo-propane, bromination with NBS gives cyclopropyl bromide. Thus, whereas Br reacts exclusively by attack on carbon to give ring-opening, the succinimidyl radical abstracts hydrogen to give a cycloprop radical, which rapidly combines with a bromine atom. ... 

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