Bromine addition with anti stereoselectivity

The addition of bromine and chlorine to a cycloalkene gives a trans dihalocycloal-kane. For example, the addition of bromine to cyclohexene gives trawfrl,2-dibromocyclo-hexane the cis isomer is not formed. Thus, the addition of a halogen to a cycloalkene is stereoselective. A stereoselective reaction is a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. We say that addition of bromine to an alkene occurs with anti stereoselectivity. 

Addition of Bromine with Anti Stereoselectivity (Section 6.3D)... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Treatment of the vinylborane with bromine and base leads to vinyl bromides. The reaction occurs with net anti addition, and the stereoselectivity is explained on the basis of anti addition of bromine followed by a second anti elimination of bromide and boron. 

The possible formation of a delocalised benzyl type carbocation (16) results in much lower (70%) ANTI stereoselectivity than with trans 2-butene (5 =100% ANTI stereoselectivity, p. 180), where no such delocalisation is possible. It is also found that increasing the polarity, and ion-solvating ability, of the solvent also stabilises the carbocation, relative to the bromium ion, intermediate with consequent decrease in ANTI stereoselectivity. Thus addition of bromine to 1,2-diphenylethene (stilbene) was found to proceed 90-100% ANTI in solvents of low dielectric constant, but =50% ANTI only in a solvent with e = 35. 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

There is much evidence that when the attack is by Br+ (or a carrier of it), the bromonium ion 2 is often an intermediate and the addition is anti. As long ago as 1911, McKenzie and Fischer independently showed that treatment of maleic acid with bromine gave the dl pair of 2,3-dibromosuccinic acid, while fumaric acid (the trans isomer) gave the meso compound.5 Many similar experiments have been performed since with similar results. For triple bonds, stereoselective anti addition was shown even earlier. Bromination of dicarboxyacetylene gave 70% of the trans isomer.6... 

Room-temperature ionic liquids may be used as green recyclable alternatives to chlorinated solvents for stereoselective halogenation.577 The bromination of alkenes and alkynes in [bmim][Br] is a/m -stereospecific, whereas that of 1,3-dienes gives selectively the 1,4-addition products. The reactions of arylacetylenes, however, are not selective when carried out in [bmim][PF6]. Tetraethylammonium trichloride, a stable crystalline solid may be used in the chlorination of alkenes and alkynes to afford the products with exclusive anti stereoselectivity.578 It has... 

Bromination.2 This bromine-crown ether complex, like dioxane-bromine (5, 58), can brominate alkenes, but the stereoselectivity is greater than that with free bromine and is less sensitive to solvent effects. Thus, bromination of trans-ifi-methylstyrene with DBC Br2 occurs exclusively by anti-addition and bromination of dr-/J-methylstyrene occurs by anti-addition to the extent of 95-100%. The bromine complex of polydibenzo-18-crown-63 is a particularly useful reagent because it can be packed as a slurry in a chromatography column. The alkene is then placed on the column and eluted with CC14. 

However, a number of examples have been found where addition of bromine is not stereospecificaUy anti. For example, the addition of Br2 to cis- and trans-1-phenylpropenes in CCI4 was nonstereospecific. Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific. Likewise in the case of triple bonds, stereoselective anti addition was found in bromi-... 

Bromine addition in [BMIM][PF6] and [BMIM][BF4] is a stereospecific anti-addition process with dialkyl substituted alkenes, alkyl substituted alkynes and trans-stilbenes, whereas ds-stilbenes and aryl alkynes give mixtures of syn- and anti-addition products, although in the case of dx-diaryl substituted olefins the anti-stereoselectivity is generally higher than in chlorinated solvents. In the case of diaryl substituted olefins, such as stilbenes, it has been shown that stereoselectivity in molecular solvents depends primarily on two factors (i) the nature of the intermediates and (ii) the lifetime of the ionic intermediates [53]. Bridged bromiranium... 

Pyridine Complexes. The addition of a halogen to pyridine affords a molecular complex The bromine complex Is capable of the halogen— atlon of olefins as reported by Lloyd and Durocher and Zupan et al (61-63). Anti stereoselectivity Is observed and a nucleophilic solvent can replace the second halogen, thus Implicating a bromonlum Ion. A similar complex with polyvinyl pyrldlne-N-oxlde acts In a similar fashion. The polymer-bound pyrldlnlum hydrobromide perbrom-Ide has also been formed by Frechet, Farrall and Nuyens. It reacts with olefins In the same fashion but has also been shown to halogenate ketones (64). 

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. 

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

It is found in practice with (5), and with other simple acyclic alkenes, that the addition is almost completely stereoselective, i.e. 100% ANTI addition. This result also is incompatible with a one-step pathway, as the atoms in a bromine molecule are too close to each other to be able to add, simultaneously, ANTI. 

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. 

Hydrogen halides also add to alkynes. The addition of HBr to alkjmes can be difficult to interpret because (as with alkenes) both ionic and free radical mechanisms may occur, and the free radical process can be difficult to suppress. Reaction of HBr with propjme (63) in the liquid phase at —78°C led to the formation of (Z)-l-bromopropene (64, equation 9.63), indicating stereoselective anti addition. When the reaction was carried out at room temperature, however, a mixture of Z (64) and (65) isomers was obtained (equation 9.64). The results suggested that addition of a bromine atom to propyne produces the vinyl radical 66, which abstracts a hydrogen from HBr to produce 64 at -78°C but which can isomerize (with Eg > 17kcal/mol) to the radical 67 at room temperature. 

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