1-Phenylpropenes, bromination

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine because it is less polarizable and more resistant to becoming positively charged. Comparison of the data for bromination and chlorination of E- and Z-l-phenylpropene confirms this trend (see Table 6.2). Although anti addition is dominant in bromination, syn addition is slightly preferred... 

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. ... 

Certain convenient brominating agents, such as pyridine hydrobromide perbro-mide (PyHBr3) and tetramethylammonium tribromide, may be used to transform alkenes to vicinal dibromides. They often give better yields than does liquid bromine, but may react by different mechanisms. Isomeric 1-phenylpropenes, for instance, react nonstereoselectively with bromine, but exhibit near-exclusive anti addition with the other reagents 244... 

Experiments have shown that the bromination of cis- and frans-phenylpropenes in CC14 is non-selective.134 For stilbene, trims addition occurs in solvents having low dielectric constants, but the reaction loses stereoselectivity if e rises above 35.135... 

Figure 1.30 illustrates that the allyl radical intermediate of several Wohl-Ziegler brominations can be accessed from isomeric alkene substrates as starting materials. This is worth considering when one substrate is more easily accessible or cheaper than its isomer. The price of allyl benzene, for example, is just a fraction of what has to be paid for 1 -phenylpropene and would thus be preferred for the synthesis shown in Figure 1.30. 

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-... 

The effect of the benzene ring on orientation can be well illustrated by a single example, addition of HBr to 1-phenylpropene. In the absence of peroxides, bromine becomes attached to the carbon adjacent to the ring in the presence of peroxides, bromine becomes attached to the carbon once removed from the ring. According to the mechanisms proposed for these two reactions, these products are formed as follows ... 

The mechanism of bromination is discussed more fully in Section 5.3, but the fundamental cause of the stereospecificity is the involvement of the positively charged bromonium ion intermediate. The bromonium ion is opened by an anti approach of the bromide, leading to net anti addition. Entry 1 in Scheme 2.7 illustrates this behavior. Stereoisomeric products are obtained from the E- and Z-isomers, both as the result of anti addition. Stereospecificity is diminished or lost when the bromonium ion is not the only intermediate in the reaction. Entry 2 in Scheme 2.7 shows this behavior for cw-stilbene in nitromethane, where most of the product is the result of syn addition. The addition is anti in less polar solvents such as cyclohexane or carbon tetrachloride. The loss of anti stereospecificity is the result of a change in mechanism. The polar solvent permits formation of a carbocation intermediate. If the bromonium ion can open to a carbocation, a mixture of syn and anti products is formed. In the stilbene case, the more stable anti product is formed. Some loss of stereospecificity is also observed with 1-phenylpropene, where the phenyl group provides stabilization of an open carbocation intermediate. Part of the product from both isomers is the result of syn addition. 

In the absence of Br and at low concentrations of Br2, the kinetic expression is first order in alkyne and first order in bromine. A p value of —5.17 for bromine addition to substituted derivatives of 56 is consistent with the intermediacy of a vinyl cation-bromide ion pair that can combine to form both cis and trans dibromide adducts or can react with solvent to form cis- and traMS l-acetow-2-bromo-l-phenylpropene, as shown in equations 9.58 through 9.60. - ... 

What about diatomic fluorine, F-F For many years, elemental fluorine was thought to be too reactive and too dangerous for reaction with alkenes. To void such problems, fluorine is typically mixed with an inert gas such as nitrogen or argon. Diluted in this manner, fluorine does react with alkenes, but the yields are often poor and, in some cases, solvents for the alkene, such as methanol, participate in the reaction. 1-Phenylpropene (PhCH=CH2), for example, reacted with fluorine in methanol to give 51% of the corresponding difluoride, along with 49% of 2-fluoro-l-methoxy-l-phenylpropane. The problems associated with fluorine lead to a simpliflcation. In this chapter, alkene reactions are reported only for chlorine, bromine, or iodine but not fluorine. 

When trans-1-phenylpropene is treated with bromine, some syn addition is observed. Explain why the presence of a phenyl group causes a loss of stereospecificity. 

The mechanism for the addition of hydrogen bromide to 1-phenylpropene in the presence of peroxides is a chain mechanism analogous to the one we discussed when we described anti-Markovnikov addition in Section 10.9. The step that determines the orientation of the reaction is the first chain-propagating step. Bromine attacks the second carbon atom of the chain because by doing so the reaction produces a more stable benzylic radical. Had the bromine atom attacked the double bond in the opposite way, a less stable secondary radical would have been formed. 

Pyridine bromide is a useful reagent for halogenation of alkenes and ketones, but both pyridine and bromine are noxious reagents, and the solid pyridinium perbromide complex is dangerous to handle. Poly(vinylpyridine perbromide) (21) quantitatively produces a-bromo ketones from cyclohexanone and propiophenone and is easy to handle. Bromination of (Z)-stilbene and ( )- and (Z)-1-phenylpropene gave 98-100% anti addition, as in equation (2), while bromine in solution yielded dibromophenylpropanes with 25-92% anti addition, depending upon the solvent. 

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