Bromohydrin from alkenes

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. 

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

Although the combination of Br2 and H2O effectively forms bromohydrins from alkenes, other reagents can also be used. Bromohydrins are also formed with A-bromosuccinimide (abbreviated as NBS) in aqueous DMSO [(013)28 = 0]. In H2O, NBS decomposes to form Br2, which then goes on to form a bromohydrin by the same reaction mechanism. 

BROMOHYDRINS FROM ALKENES AND N-BROMO SUCCINIMIDE IN DIMETHYL SULFOXIDE erythro -2-BROMO-l,2-DIPHENYLETHANOL... 

The serendipitous event that inspired the development of NPGs (see Sect. 2.1) was the attempt to prepare a bromohydrin from alkene 4 (Scheme 1) [6, 19]. With the benefit of hindsight, we now realize that this objective was predestined to fail, because the first-formed cyclic bromonium ion, e.g., 15 (Scheme 4), would undergo facile R05 interaction [26] to give a furanylium ion, 16, and thence the oxocarbe-nium ion 17. The latter would be scavenged by water to give 18 (R =H), the product of oxidative hydrolysis. 

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 ... 

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. 

Epoxides give rise to many 1,2-difunctionalised compounds such as 48 with control over stereochemistry. Reactions of the epoxide 49 from 44 give the anti stereochemistry in 48 in contrast to the syn stereochemistry in 43. Other compounds made from alkenes include 1,2-bromides and bromohydrins from reaction with bromine alone or bromine and water. 

Fig. 3.43. Stereo- and regioselective formation of a bromohydrin from a 1,2-disub-stituted alkene.

Syn hydroxylation from the more hindered face of a ir-system can be effected using Woodward s pro-cedure in which an alkene is treated with iodine-silver acetate in acetic acid containing water. Variants of this method avoid the use of silver salts.A versatile pocedure by which syn hydroxyla-tion can be performed on either the more hindered or less hindered face of an alkene relies on stereoselective formation of the appropriate traru-bromohydrin from the alkene. ... 

Direct synthesis of a-bromo ketones from alkenes is carried out by use of sodium bromite (NaBr02). The reaction proceeds via the bromohydrin (48) as intermediate (equation 19). 

Epoxides may be prepared from alkenes by the action of a peroxy acid such as m-chloroperbenzoic acid (Scheme 2.20a) or via the formation of a bromohydrin or iodohydrin and the treatment of this with base (Scheme 2.20b). Since the initial electrophile, the bromine or the iodine, is displaced in the second step when the epoxide is formed, the stereochemistry of this epoxidation is likely to differ from that of the reaction with peroxy acid. 

The treatment of bromohydrins (41), which can be readily prepared from alkenes (40), with potassium selenocyanate in DMF produces the /i-hydroxyselenocyanates (42) in high yields. When... 

One method for the preparation of epoxides from alkenes involves (1) treating the alkene with chlorine or bromine in water to form a chlorohydrin (or bromohydrin) followed by (2) treating the halohydrin with a base to bring about intramolecular displacement of Cl . These steps convert propene first to l-chloro-2-propanol and then to methyloxirane (propylene oxide). 

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