Alkynes reaction with bromine

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

Alkenyl boronic acids synthesized from internal alkynes fail to undergo the reaction with iodine and base. However, alkenylboronic acids and esters obtained from internal alkynes react with bromine in the presence of base to afford in excellent yields the corresponding alkenyl bromides (Eq. 112)174),... 

The analogy with alkene chemistry can be continued in that alkynes react with bromine, chlorine, or iodine, but only one of the two Ji-bonds is used. The reaction is known as dihalogenation of alkynes. When 2-hexyne reacts with chlorine (CI2), the alkyne reacts as a Lewis base and the isolated product is the vinyl dichloride 110 (J5-l,2-dichloro-l-pentene). Formation of this product is explained by an intermediate vinyl-chloronium ion, 109, which is analogous to the halonium ions formed from alkenes in Section 10.4.1. As with alkenes, the chloronium ion reacts with the nucleophihc chloride ion (CL) via anti attack... 

Dihalogenation of alkynes gives a dihalogenated alkene, which is also susceptible to reaction with bromine, chlorine, or iodine. Tetrahalo derivatives are available from dihalogenated alkenes (vinyl dihalides). When 1-pentyne reacts with one molar equivalent of diatomic bromine. 111 is the product. Because alkenes are also subject to reaction with halogens. 111 can react with a second molar equivalent of bromine to give 1,1,2,2-tetrabromopentane, 112. 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Well-known double-addition product 58, obtained by an iron carbonyl-mediated reaction sequence from the corresponding cyclic diazenes and alkynes <1975JOM(101)231>, when treated with bromine was reported to give the... 

The bromination with alkali hypobromite in aqueous solution gives good results with (hetero)arylacetylenes, enynes (RCH=CHOCH) and diynes (RC=CC=CH) all acetylenes that are more acidic than those acetylenes in the aliphadc or cycloaliphatic series with an isolated triple bond. For the conjugated systems the hypobromite method is superior to the reaction of metallated acetylenes with bromine. Various acetylenic alcohols are also brominated smoothly, which can be explained in part by their better solubility in water. Since in the case of primary and secondary ethynyl alcohols, oxidation of the alcohol can occur, the use of an excess of hypobromite should be avoided. The best procedure is drop wise additon of a small shot measure of hypobromite ro a mixture of alcohol and water. If the bromoalkynes to be prepared are not too volatile, small amounts of THF or dioxane may be added to effect a better solubility of the alkyne in the aqueous phase. Addition of a co-solvent may also be desired when the starting compound is a solid (e.g. ethynylcyclohexanol). 

Unlike alkenes, which react reversibly with h, alkynes react in solution irreversibly with I2, forming ( )-l,2-diiodoalkenes.89 The reaction may involve radicals rather than ionic species.90 Ordinary alumina appears to promote the addition, forming electrophilic species.866 The addition of iodine to 1,4-dichloro-but-2-yne is analogous to the addition found with bromine cited above (Scheme 19).28... 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

Some examples of these addition reactions are provided in the following equations. Note that each proceeds with anti addition of the two halogen atoms. In the last example, starting with an alkyne, the two bromines end up trans. 

In an attempt to synthesize (Z)-l,2-dihalo-l-alkenes from bromoboration adducts of 1-alkynes with B-X-9-BBN, all efforts have been unsuccessful. Fortunately, it has found that the haloboration of 1-alkynes with tribromoborane, followed by the reaction with iodine or bromine chloride in the presence of sodium acetate gives the expected (Z)-l,2-dihalo- 1-alkenes stereospecifically (>98%) in good yields (Eq. 122)187>. 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

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