Hydrobromination

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

Commercial Olefin Reactions. Some of the more common transformations involving a-olefins ia iadustrial processes iaclude the oxo reaction (hydroformylation), oligomerization and polymerization, alkylation reactions, hydrobromination, sulfation and sulfonation, and oxidation. 

The starting amino acid for nylon-11 is produced from methyl ricinoleate [141 -24-2] which is obtained from castor oil (qv). The methyl ricinoleate is pyrolized to methyl 10-undecylenate [25339-67-7] and heptanal [111-71-7]. The unsaturated ester is hydroly2ed and then converted to the amino acid by hydrobromination, followed by ammoniation and acidification. The CO-amino acid product is a soft paste containing water, which is dried in the first step of the polymeri2ation process. 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

EXERCISE 11.53 In the following hydrobromination reaction, determine whether or not you should use peroxides ... 

The reactions were cleaner than those with cis,cis-1,5-cyclooctadiene, and purification of the intermediate bis-mercurated peroxide was unnecessary. Indeed, it was a simple matter to isolate 52 (28 %) and 53 (38 %) from crude products obtained using a sample of 1,4-cyclooctadiene containing 30% of cis,cis-1,5-cyclooctadiene, which is readily accessible via hydrobromination-dehydrobromination of the 1,5-diene. Two of the diastereoisomers of 53 were separately isolated by HPLC and the most... 

H. Frenkel-Mullerad and D. Avnir, The Chemical Reactivity of Sol-Gel Materials Hydrobromination of Ormosils, Chem. Mater., 2000, 12, 3754. 

Dibromoethane is a halogenated aliphatic hydrocarbon produced when gaseous ethylene comes in contact with bromine. The mixing of ethylene and bromine is accomplished in a variety of ways. One of the more common manufacturing processes involves a liquid-phase bromination of ethylene at 35°-85°C. After the bromination of ethylene, the mixture is neutralized to free acid and then purified by distillation. Other methods of 1,2-dibromoethane formation include the hydrobromination of acetylene and a reaction of 1,2-dibromoethane with water (Fishbein 1980 HSDB 1989). 

Phenylbromoethyne has been prepared by base-catalyzed de-hydrobromination of 1,1- or 1,2-dibromostyrene by the thermal... 

On the other hand, the fact that the methyl resonances appear as sharp singlets at 1.53 ppm for the hydrochlorinated product and at 1.69 ppm for the hydrobrominated product indicates the exclusiveness of Markownikoff s rule for the addition of either hydrogen halide to the repeating units of 1,4-polyisoprene. 

Scheme 43. Bromination and hydrobromination of bicyclopropylidene (1) and spirocyclopro-panated bicyclopropylidenes 55,56,62 [33,64,119,134]...

Mixtures of isomeric allylic chlorides are formed when 1,1-disubstituted allenes react with HC1.1,3-Disubstituted allenes yield products of both central and terminal carbon attacks. In contrast, selective transformations occur with HBr. The gas-phase, photocatalytic addition of HBr gives selectively vinylic bromides129 [Eq. (6.20)], while hydrobromination in solution phase yields allylic bromides130 [Eq. (6.21)] ... 

Similar differences may be observed in hydrobromination of tetrasubstituted allenes. Ring size and reaction conditions determine the outcome of hydrobromination of cyclic allenes.92... 

In gas-phase hydrobromination, where a radical mechanism is operative, the bromine atom always adds to the central carbon atom of the allenic system. As a result, vinylic bromides are formed through the stable allylic radical. In the solution phase under ionic addition conditions, either the vinylic or the allylic cation may be the intermediate, resulting in nonselective hydrobromination. Allylic rearrangement or free-radical processes may also affect product distributions. 

In the particular case of the hydrobromination of VEST, the polymer was first dissolved into chloroform. Gazeou s bromhydric acid was introduced at -18°C. Complete reaction (3-5 days at 8°C) resulted in the formation of brominated polyethylene ... 

Like alkenes, the double bonds of ,/3-unsaturated acids can be brominated, hydroxylated, hydrated, and hydrobrominated, although the reactions often are relatively slow. In the addition of unsymmetrical reagents the direction of addition is opposite to that observed for alkenes (anti-Markownikoff). Thus propenoic (acrylic) acid adds hydrogen bromide and water to form 3-bromo-and 3-hydroxypropanoic acids ... 

On occasions rearrangements have been observed during hydrobromination (equations 72 and 73).3I.48,115... 

The hydrobromination of a number of halogenated alkenes has been examined. Allyl bromide reacts with HBr in the absence of peroxides, or upon addition of antioxidants121 or FeCb,61 or in acetic acid,122 to produce primarily 1,2-dibromopropane (equation 74). Allyl chloride behaves similarly.123... 

A number of functional groups including sulfones, ethers and even alcohols (occasionally) can be accommodated during hydrobromination. Functional groups in close proximity to the C—C double bond can have an important directing effect (equations 88 and 89).133,134... 

The hydrobromination of functionally substituted allenes has received little attention. Vinylic allenes generate mixtures of bromides,76 while tetrafluoroallene affords the allylic bromide,83 and allenic acids produce 3-bromo-3-alkenoic acids (equation 109). 37,138... 

The hydrobromination of arylalkynes in acetic acid produces good yields of vinylic bromides in which syn addition is favored over anti addition by -3 to 1 (equation 112)-90 6 - 62... 

---