Alkenes bromohydrins

Bromohydrins - alkenes. Bromohydrins are reduced generally in satisfactory yields to alkenes by this low-valent titanium species. The reaction is not stereoselective. ... 

Begin by asking the question What kind of compound is the target molecule and what methods can I use to prepare that kind of compound The desired product has a bromine and a hydroxyl on adjacent carbons it is a vicinal bromohydrin The only method we have learned so far for the preparation of vicinal bromohydrms involves the reaction of alkenes with Bi2 m water Thus a reasonable last step is... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid-catalyzed dehydration (Section 5.9) or from alkyl halides by dehydrohalogenation (Section 5.14). Because our designated starting material is ten-butyl alcohol, we can combine its dehydration with bromohydrin formation to give the conect sequence of steps ... 

Overall, the stereospecificity of this method is the same as that observed in per-oxy acid oxidation of alkenes. Substituents that are cis to each other in the alkene remain cis in the epoxide. This is because formation of the bromohydrin involves anti addition, and the ensuing intranolecular- nucleophilic substitution reaction takes place with inversion of configuration at the carbon that bear s the halide leaving group. 

Mechanism of bromohydrin formation by reaction of an alkene with Br2 in the presence of water. Water acts as a nucleophile to react with the intermediate bromonium ion. 

In practice, few alkenes are soluble in water, and bromohydrin formation is often carried out in a solvent such as aqueous dimethyl sulfoxide, CH3SOCFI3 (DMSO), using a reagent called W-brornosuccinimide ( rBS) as a source of Br2. NBS is a stable, easily handled compound that slowly decomposes in water to yield Br2 at a controlled rate. Bromine itself can also be used... 

Bromohydrin (Section 7.3) A 1,2-disubstituted bromo-alcohol obtained by addition of I lOBr to an alkene. 

The present procedure affords a simple and general method for preparing bromohydrins from alkenes which avoids the heterogeneous solvent systems often used in such reactions. Labeling experiments have demonstrated that the oxygen from the dimethyl sulfoxide appears in the hydroxyl group of the bromohy-drin. Therefore the role of the water is to hydrolyze the intermediate /3-bromodimethylsulfoxonium ion. 

When electron-withdrawing groups are attached to the double bond, the reaction is strongly inhibited and may fail completely. In such cases, the bromide anion, produced by the reaction of dimethyl sulfoxide with N-bromosuccinimide, competes with the dimethyl sulfoxide for the bromonium (or bromo carbonium) ion, an intermediate of the reaction. Thus, dibromide may accompany recovered alkene or any bromohydrin formed. Similarly, exogenous anions often compete with dimethyl sulfoxide for the cation. ... 

Bromohydrins from Alkenes with N-Bromosuccinimide in Aqueous... 

In 1975, van der Baan and Bickelhaupt reported the synthesis of imide 37 from pyridone 34 as an approach to the hetisine alkaloids, using an intramolecular alkylation as the key step (Scheme 1.3) [23]. Beginning with pyridone 34, alkylation with sodium hydride/allyl bromide followed by a thermal [3,3] Claisen rearrangement gave alkene 35. Next, formation of the bromohydrin with A -bi omosuccinimide and subsequent protection of the resulting alcohol as the tetrahydropyranyl (THP) ether produced bromide 36, which was then cyclized in an intramolecular fashion to give tricylic 37. 

Brown s result was supported by later experiments in which bromonium ions were generated by bubbling gaseous hydrobromic acid through a solution of bromohydrins in halogenated solvents. Under these conditions, bromine is eliminated as it is formed, so that the resulting alkene is observed directly (Scheme 15). This method has been applied to the bromohydrins derived from cis- and trans-stilbenes (Scheme 16) and from 5//-dibenzo[a,d]-cycloheptene and -azepine systems ([29a] and [29b] respectively Scheme 17), in which steric constraints should favour elimination (path a) as against substitution (path b). 

Laboratory scale bromination of alkenes in homogeneous solution using an undivided cell is adaptable to the formation of epoxides, bromohydrins or dibromides depending on the conditions [64]. Epoxides are generated using an initially neutral solution and a low concentration of bromide ions. The reaction sequence is similar to that of Scheme 2.3. Formation of bromohydrins requires dilute hydrobromic acid as the supporting electrolyte. Dibromides are obtained using a concentrated solution of sodium bromide as electrolyte. 

Conversion of bromohydrins to alkenes. The 14,15-epoxide (1) of arachidonic acid methyl ester has been converted into the 11,12-epoxide (5) in two steps. The first is conversion of 1 into a mixture of isomeric bromohydrins (2 and 3) with KBr. The mixture is then epoxidized under the conditions of Sharplcss (5, 75 76) to give, after chromatographic purification, 4, the epoxide of 2. Conversion of 4 into 5 presented a problem, but was eventually achieved by treatment of the w c-bromo Inflate with P[N(CHj)2]j as a Br acceptor. 

A mixture of products, that contains regioisomeric bromohydrin acetates, is obtained20 by the bromine/acetic acid oxidation of an alkene having a tertiary perfluoroalkyl group in the allylic position. It is interesting that a different composition of the product mixture 61-63 is obtained depending on the addition of bromine or of the alkene to the reaction mixture.20... 

Perfluoroalkyl)ethenes generally react with halogens in the presence of mcrcury(II) salts. Among this type of alkene. 3,3,3-trifluoroprop-l-ene is oxidized to the bromohydrin derivative at room temperature with 62% yield (Table 6).100 Bromohydrin acetates 64 are also formed by the oxidation of the vinyl group in (pcrfluoroalkyl)ethenes with bromine in acetic acid.101... 

Bromination of (Z)-2-methyl-3-alkenal acetal 98 with NBS in DMSO under irradiation affords selectively bromohydrin 99. The reaction is assumed to proceed via a bromo-radical (generated by irradiation of NBS) that reacts with DMSO to give a yellow intermediate (possibly BrO—SMe2) which may further react either with olefin to give the adduct 99 or with water, furnishing hypobromous acid185. 

Coughlin P, Roberts S, Rush C, Willetts A (1993) Biotransformation of Alkenes by Haloperoxidases Regiospecific Bromohydrin Formation from Cinnamyl Substrates. Biotechnol Lett 15 907... 

From the structural formula of the desired product, we see that it is a vicinal bromohydrin. Vicinal bromohydrins are made from alkenes by reaction with bromine in water. 

In aqueous solution bromine reacts with alkenes to give bromohydrins. Bromine is the electrophile in this reaction and adds to the carbon that has the greater number of attached hydrogens. 

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