Bromination/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,... 

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. 

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). 

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

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 ... 

Iron(If) bromide, FeBr2 (ferrous bromide), is a pale yellow/brown hygroscopic solid soluble in water and donor organic solvents. It may be prepared from iron and bromine at 200 °C, or iron and HBr in methanol, hydrobromination of iron(III) oxide at 200-350 °C, or dehydration of [FeBr2(H20)4]. The anhydrous compound is readily purified from FeBr3 by sublimation under nitrogen or in vacuo. It... 

We also prepared deuterated butylcyclooctatetraenes by brom-ination of butylcyclooctatetraene followed by dehydrobromination, metallation with butyllithium and quenching with D2O. Location of the deuterium in the product is, however, not straightforward. Pacquette has studied the bromination of methylcyclooctatetraene and has identified different bromination products on different occasions (58,59). It appears from his work that bromination-de-hydrobromination of methylcyclooctatetraene can lead to all four possible methylbromocyclooctatetraenes. 

Step CF2 CF2 O CF2 CF2 NF + Av CF2 CF2 0 CF2 CF2-N. -f F-). Pyrolysis (700 °Q of the cyclobutyl derivative (32) provides perfluoro-(iV-vinyl-morpholine) (35) (86%), photochemical bromination and hydrobromination of which yields the ethyl derivatives (33) and (34), respectively. Reinvestigation of the thmmal decomposition of perfluoro-iV-fluoromorpholine in platinum at 600 °C has shown that the product comprises perfluoro-fiV-methyloxa2 lidine) (37) (17%),... 

Hydrobromination of the same substrate in the a-cyclodextrin complex gave the monobromide with the opposite configuration at 46 % e.e.. This clearly shows that ethyl trans-cinnamate forms complexes with a- and 3-cyclodextrins such that the anti-addition of hydrogen bromide occurs with high but different enantioselectivities in the two cases to yield monobromide derivatives of opposite chiralities. A detailed mechanism, however, could not be described at the present time for the observed asymmetric induction in the hydrobromination and bromination of the a-cyclodextrin complex, because no crystalline or molecular structures were determined for the ester included in a-cyclodextrin. 

A convenient synthesis of glaziovine proceeds from the dihydroproaporphine amuronine which is available by the Bernauer tricyclic ketone approach. Thus when amuronine was refluxed with 20-36% hydrochloric acid, O-demethylation occurred preferentially at C-1 to yield 11,12-dihydroglaziovine which was 0-acetylated, and then converted to glaziovine through bromination and de-hydrobromination. It should be mentioned here that preferential 0-demethyla-tion cannot be applied directly to a proaporphine since under such strongly... 

Bromo-l-alkenes can be prepared regioselectively and in high yield by direct hydrobromination of terminal acetylenes with tetraethylammonium hydrogen dibromide in dichloromethane. The reagent, which is prepared in situ from tetraethylammonium bromide and hydrogen bromide, also hydro-brominates acetylenic alcohols and ethers. 

Brown, H. C., Lane, C. F. 1970. Base-induced reaction of organoboranes with bromine—K convenient procedure for anti-Markovnikov hydrobromination of terminal olefins via hydroboration-bromination. J. Am. Chem. Soc. 92 6660-6661. 

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