HOBr, with alkenes

HOCl and HOBr react with alkenes to give halohydrins. 

Chlorine, bromine, and iodine react with alkenes to give the three-membered ring halonium ion, which reacts with the halide nucleophile to give trans dichlorides, dibromides, or diiodides. Alkynes react to give vinylhalonium ions that lead to vinyl dihalides 14,15,16,17,18,19, 20, 21, 22, 64, 73, 74, 75, 82, 83, 98,108. HOCl and HOBr react with alkenes to give halohydrins 23, 24, 75, 82, 83. 

Scheme 6.22. A representation of the path by which hypohalous acids in general and hypo-bromous acid (HOBr) in particular reacts with alkenes in general and cyclopentene (cyclo-CsHs) in particular. Note that the addition is antarafadal.

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

A key step in a synthesis140 of 2-deoxy-L-en/thro-pentose was the addition of the elements of HOBr across the carbon-carbon double bond of an alkene on treatment with N-bromosuccinimide and water. 

Hypochlorous acid and hypobromous acid react with acyclic alkenes to give Mar-kovnikov products. In striking contrast, exclusive anti-Markovnikov orientation was observed in the transformation of methylenecycloalkanes with HOBr, and mixtures of chlorohydrins were formed with HOC1 146... 

Even the weak nucleophile CF3SO2O can participate in the second step The addition of CI2 or Br2 to alkenes in the presence of this ion resulted in the formation of some p-haloalkyl triflates. There is evidence that the mechanism with CI2 and H2O is different from that with HOCl. HOCl and HOBr can be added to triple bonds to give dihalo carbonyl compounds —CX2—CO—. 

The reaction of chloro- or bromohydrins with bases provides an economical route for the preparation of epoxides. Halohydrins are readily accessible by treatment of an alkene with either hypochlorous acid (Clj -1- H2O —> HOCl), hypochlorite bleach solution (NaOCl), or hypobromous acid (NBS -1- HjO HOBr). These reactions involve the... 

Reaction of NBS or NCS with a minimal amount of H2O in DMSO is a convenient method for the preparation of HOBr or HOCl, respectively.254 Subsequent reaction with an alkene leads to a chlorohydrin or... 

This section will begin with a simple method for producing epoxides that uses the reaction of alkenes and hypohalous acids (generated by the reactions CI2 + H2O HOCl or Br2 + H2O HOBr) to give the trans-(or anti-) halohydrin as the major product. An example is the conversion of cyclohexene to bromohydrin 149 (sec. 2.10.C). Subsequent treatment with a base such as sodium hydride generates the alkoxide (150), which is anti- to the adjacent bromine, which anti- orientation leads to displacement of halide... 

Electrophilic Addition.—The hydroxylation of alkenes, including several steroids, with osmium tetroxide has been reviewed. Addition of HOBr to the 19-functionalized-5a-cholest-6-enes (13) gave mixtures of the corresponding bromohydrins (14) and the 6,19-epoxides (15) whereas the analogous B-homo-compounds (16) gave only the 6,19-epoxides (17). Similar effects were noted for the related HBr- and HC104-catalysed opening of the 6a,7a-epoxides and it was observed that the predominant attack by the 19-0 atom [5(0)" attack] in the B-homo-series lay in the possibility of its linear approach with the C-6—Br or C-6—O bond. The reactions of chromyl chloride and chromyl fluoride with steroidal alkenes and dienes have been reported " and it was observed that the... 

It is known that dissolving chlorine in water leads to a solution that contains hypochlorous acid (HOCl) and bromine dissolved in water contains hypobromous acid (HOBr). In one experiment, 1-pentene is mixed with chlorine and water (HOCl in aqueous media) and the major product is l-chloro-2-pentanol (48), in 43% isolated yield. ° In the previous section, chlorine reacted with 1-pentene in a nonaqueous solvent such as carbon tetrachloride to give a dichloride. To ascertain why this reaction is different, the first useful observation is that HOCl is in solution rather than Cl-Cl. The polarization of HOCl is HO -Cl, where chlorine is the electrophilic atom. The 7t-bond of an alkene should react with the positive chlorine atom, and cleavage of the Cl-H bond will give hydroxide ion, which is a... 

Addition of HOCI and HOBr is regioselective (halogen adds to the less substituted carbon atom) and anti stereoselective. Both the regioselectivity and anti stereoselectivity are illustrated by the addition of HOBr to 1-methylcyclopentene. Bromine and the hydroxyl group add anti to each other with Br bonding to the less substituted carbon and OH bonding to the more substituted carbon. Dimethyl sulfoxide (DMSO) is used as a cosolvent with water to enhance the solubility of the alkene. 

We saw in the previous section that when Bt2 reacts with an alkene, the cyclic bromonium ion intermediate reacts with the only nucleophile present, Br ion. If the reaction is carried out in the presence of an additional nucleophile, however, the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to a different product. In the presence of water, for instance, water competes with Br ion as nucleophile and reacts with the bromonium ion intermediate to yield a bromohydrin. The net effect is addition of HOBr to the alkene by the pathway shown in Figure 8.2. 

---