Stilbene, bromine addition

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. ... 

In halogenated solvents the results indicate that return can occur, even for the uncongested stilbenes. Unfortunately, its importance, as measured by the k i/kN ratio (Fig. 10), cannot be estimated. It must be noted that Bellucci s experiments prove only that return is possible, but do not demonstrate conclusively that it occurs in bromination, since reversibility is controlled by the relative energy levels of TS and TSN which can be affected by the reaction conditions. Now, these conditions are not the same for nucleophilic substitution on bromohydrins and for bromine addition in particular, the counter-ions, Br and Br3 respectively, can alter the lifetime of the intermediate and thus control its partitioning between return and nucleophilic attack. 

In agreement with other processes occurring through carbocations, elimination and transposition reactions can compete with the addition process which also depend on solvent. Finally, solvent properties can affect the stereochemical course of the addition reactions for example, the stereoselectivity of bromine addition to cis- and /r[Pg.392]

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

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... 

Table 5-23. Stereoselectivity of electrophilic bromine addition to cis- and tranr-stilbene, carried out at 0 °C in the dark [79, 386] cf. Eqs. (5-29) and (5-140).

For electrophilic additions of halogens to alkenes, not only is the reaction rate strongly solvent-dependent [79-81] [cf. Eq. (5-29) in Section 5.3.2), but the stereochemical course may also be affected by the polarity of the medium [79, 386-388], For example, the stereoselectivity of bromine addition to cis- and trans -stilbene according to Eq. (5-140) has been found to be solvent-dependent, as shown in Table 5-23 [79, 386],... 

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... 

Most studies of bromine addition to alkenes have presumed that the intermediate proceeds to product and does not revert to alkene and bromine. Brown and co-workers determined that bromonium ions generated from the solvolysis of the fra s-2-bromo-l-brosylates of wclohexene or cyclopentene could react with added Br to produce Br2- Furthermore, erythro-2-hTomo-l, 2-diphenylethanol was foimd to react with anhydrous HBr (in 1,2-dichloroethane or chloroform) to produce both frans-stilbene and meso-l,2-dibromo-l,2-diphenylethane. The reaction of the erythro diastereomer can be explained by a mechanism involving anchimeric assistance in departure of water, which leads to a bromonium ion that reverts to the stilbene, as shown in Figure 9.10. ... 

Proposed mechanism for bromine addition to stilbenes. (Note the process leading to cis-trans isomerization.)... 

In the present reaction, both the bromine and the (E)-stilbene are achiral. However, the bromonium ion that is produced is chiral. In this ion, the bromine atom bridges both carbon atoms of the original carbon-carbon double bond to form a three-membered ring intermediate. The generation of a cyclic species has a profound effect on the stereochemistry of the second step of the bromine addition. 

Stilbene decolorises bromine only on heating. Proceed as above, but keep the stilbene solution hot during the addition of the CCI4 solution of bromine. Stilbene dibromide has m.p. 237 . 

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). 

The product obtained from this type of decarboxylation is reported to contain only about 5% of /ra s-stilbene.5 A sample made according to the above directions can be treated with bromine in carbon tetrachloride at room temperature in the dark to give an 80-85% yield of the d/-dibromide which arises from trans addition to cw-stilbene. The meso-dibromide, which is very soluble and easily separated, is obtained only to the extent of 10% or less. Part of this latter product may arise from the action of bromine atoms on cw-stilbene rather than from trans addition to tfnms-stilbene. The cu-stilbene prepared by this method is readily and completely soluble in cold absolute ethanol. It freezes solid at about —5°. Its ultraviolet absorption coefficient (8) is 1.10 X 104 at 274 mil and 8.7 X 103 at 294 mp, quite different from h-aws-stilbene. 

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. 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

Finally, as shown in Table 13, p for an aromatic ring is also strongly dependent on the other substituents at the double bond it varies from —1.6 to — 5.5 on going from a-methoxystyrenes to stilbenes. This variation, which is related to the well-known non-additivity of multiple substituent effects, and contrasts with what is observed for alkene bromination, is discussed in the next paragraph, devoted to substituent interaction and selectivity relationships in bromination. 

It is well known that the kinetic effects of several substituents on one or two aromatic rings are not additive. This is exemplified in the bromination of 1,1-diphenylethylenes, stilbenes and a-methylstilbenes. The presence of a substituent, particularly one capable of electron donation by resonance, on the aromatic ring so alters the charge distribution at the transition state that the second substituent in the other ring then interacts with a charge different from that which would prevail if the substituent were alone. This is expressed... 

The bromination of /razw-stilbenes in methanol, trifluoroethanol and in acetic acid leads almost exclusively to the erythro adducts via a 100% anti addition, regardless the substituents and the solvent. In contrast, the stereochemistry of the reaction of c/s-stilbenes exhibits a considerable dependence on the substituents and on the solvents the reaction of p-methoxystilbenes is always stereo-convergent, that of p,p -bis(trifluoromethyl)stilbenes is stereo-specific in all investigated solvents, whereas unsubstituted stilbenes can produce variable stereochemical outcomes ranging from stereo-specific to stereo-convergent in going from methanol to trifluoroethanol as solvent. 

Furthermore, the conclusion that the formation of the meso dibromide upon gas-solid addition of bromine to czs-stilbene is due to cis-trans isomerization prior to or during addition [54,56] cannot be accepted without further proof, as there is also the possibility for cis addition [58-61]. 

The claims of exclusive formation of rac-stilbene dichloride upon gas-solid addition of chlorine to frans-stilbene (103) [71] and of meso-stilbene dibromide in the gas-solid addition of bromine to trans- or czs-stilbene [54] could not be verified. Scheme 12 shows the results of more detailed studies indicating the mesolrac ratios on the solid-state chlorination and bromination of trflns-stilbene (103) and some variations when the crystal size was changed [58, 60-61]. There is a risk of partial transient liquefaction if the chlorine is added too rapidly, due to initially heavy reaction. But even at the start with a stoichiometric amount of chlorine at 0.1 bar and 0 °C, a persistent product layer forms on the unground crystal powder of 103 that cannot be disintegrated by the ultrasound of a cleaning bath at 20 °C for 60 h (only 7% conversion with mesolrac ratio of 11 89 under these conditions) [22]. It is therefore unavoidable to mill the crystals of 103 to sizes <1 pm in order to overcome these rare diffi-... 

The kinetics and the products of bromination of several substituted stilbenes with Bu4N+Br3 have been investigated in aprotic solvents at different temperatures and concentrations. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products the reaction followed a second-order rate law and inverse kinetic isotope effect Ap/Ap = 0.85 ( 0.05) was... 

While bromination of m-stilbene (72e) in CHCI3 gives c/,/-di bromide at higher Br2 concentration, preferential formation of meso-dibromide has been observed at lower concentrations131. Moreover, at low concentrations, the addition is accompanied by a cis-trans isomerization of the unreacted olefin (72e —> 73e). This behaviour can be rationalized by assuming the reversibility of the formation of the bromonium ions and by their isomerization131. 

Judged by the magnitude of , these eliminations have product-like transition states. With respect to stilbene chemistry the pattern of Fig. 27 is rather general. That is, the relative reactivity of the two stilbenes is often small, e.g. within a factor of two in some (not all) additions of bromine (Buckles et al., 1967), 2,4-dinitrosulfenyl chloride (Slobodkin and Kharasch, 1960) or trichloromethyl (Cadogan and Inward, 1962). 

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