Bromine alkynes

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,... 

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6 14) alkenes especially terminal alkenes can serve as starting mate rials for the preparation of alkynes as shown m the following example... 

Alkynes react with chlorine and bromine to yield tetrahaloalkanes Two molecules of the halogen add to the triple bond... 

Halogenation (Section 9 13) Addition of 1 mole of chlorine or bromine to an alkyne yields a trans dihaloalkene Atetrahalide is formed on addition of a second equivalent of the halogen... 

The classification of hydrocarbons as aliphatic or aromatic took place m the 1860s when It was already apparent that there was something special about benzene toluene and their derivatives Their molecular formulas (benzene is CgHg toluene is C7Hj ) indicate that like alkenes and alkynes they are unsaturated and should undergo addition reac tions Under conditions m which bromine for example reacts rapidly with alkenes and alkynes however benzene proved to be inert Benzene does react with Bi2 m the pres ence of iron(III) bromide as a catalyst but even then addition isn t observed Substitu tion occurs instead ... 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

The rates of bromination of a number of alkynes have been measured under conditions that permit comparison with the corresponding alkenes. The rate of bromina-hon of styrene exceeds that of phenylacetylene by about For internal alkyne-... 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

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. 

The rates of bromination of dialkylacetylenes are roughly 100 times greater than for the corresponding monosubstituted alkynes. For hydration, however, the rates of reaction are less than 10 times greater for disubstituted derivatives. Account for this observation by comparison of the mechanisms for bromination and hydration. 

Thermodynamics and kinetics need not go hand in hand. Consider all possible products resulting from addition of one equivalent of bromine to phenylacetylene (phenylacetylene+Br2) and to styrene (styrene+Br2). Calculate the heat of reaction for each addition. (The energy of Br2 is given at right.) Is addition to the alkyne or to the alkene more favorable ... 

Bromine and chlorine also add to alkynes to give addition products, and trans stereochemistry again results. 

Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

A somewhat unusual sequence to generate azepanones 80 involved the intramolecular addition of hydroxylamines to alkynes 76 to form cyclic nitrones 77. A vinyl magnesium bromide addition at low temperatures and a reduction with TiCls followed by N-Boc protection led to the azepane 78. Double bond bromination and subsequent RUO4 oxidation gave the lactam 79. Several further steps allowed the generation of the lactam structure 80 proposed for d,/-aca-cialactam, but the spectral data of the synthetic material differed from that of the natural product (Scheme 16)] [23 a, b]. 

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... 

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