Debromination reactions reaction rate

The kinetic study suggests a first-order dependence in both telluride and vic-dibromide for the debromination reaction. The rate law can be expressed as ... 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

Accordingly to Eq. 29.1, any changes made in the concentration or reactivity of the reagents will affect the reaction rate. In consequence, more electron-rich tellu-rides (also more nucleophilic) will accelerate the debromination. This experimental result could be justified by both mechanisms. On the other hand, either Mechanism 1 or Mechanism 2 propose the formation of a polar intermediate and hence, both should be influenced by a change in solvent polarity. In fact, debromination rates are faster in acetonitrile than in chloroform. 

Apparently, in both mechanisms the steric factors are important to explain the relative reaction rates observed for the different substrates. At this point we should study in detail the remaining data, starting with the differences observed in die rates of debromination of l,2-dibromo-2-methyl-l-phenylpropane 11 and 1,2-dibromodecane 12. 

Let us consider whether Mechanism 2 could justify the differences in reaction rates between 11 and 12. We should remember that the stability of a bromonium ion not only depends on steric factors but, like carbocations, they are very sensitive to the presence of substituents able to stabilize the positive charge (i.e. a phenyl group). This could explain why l,2-dibromo-2-methyl-l-phenylpropane 11 reacts 330 times faster than 1,2-dibromodecane 12. The bromonium ion intermediates obtained from 11 and 12 are represented in Scheme 29.9. Although intermediate 15 is more crowded than intermediate 16, the presence of a phenyl group should make it far more stable. In consequence, the debromination of 12, has to be considerably slower. 

As we have seen, the study of the stereochemistry of the debromination reaction is the key choosing between the two mechanistic pathways. Both proposals could justify the kinetic data (rate law, nucleophilicity of the telluride, effects of solvent polarity) however, only Mechanism 2 could satisfactorily explain the stereoselectivity in all cases. The intermediacy of a bromonium ion and the role of the telluride as a scavenger of the Br seem to be the best option with all the data in hand. 

Rates of debromination of bromonitro-thiophenes and -selenophenes with sodium thio-phenoxide and sodium selenophenoxide have been studied. Selenophene compounds were about four times more reactive than the corresponding thiophene derivatives. The rate ratio was not significantly different whether attack was occurring at the a- or /3-position. As in benzenoid chemistry, numerous nucleophilic displacement reactions are found to be copper catalyzed. Illustrative of these reactions is the displacement of bromide from 3-bromothiophene-2-carboxylic acid and 3-bromothiophene-4-carboxylic acid by active methylene compounds (e.g. AcCH2C02Et) in the presence of copper and sodium ethoxide (Scheme 77) (75JCS(P1)1390). 

H from C0, the commonest probably being 1,2-dehalogenations and, in particular, 1,2-debromination. This can be induced by a number of different species including iodide ion, I , metals such as zinc, and some metal ions, e.g. Fe2. The reaction with I in acetone is found to follow the rate law (after allowance has been made for the I complexed by the I2 produced in the reaction),... 

Nucleophilic substitution reactions in the selenophene series have attracted some interest. Debromination of bromonitro compounds [(50, X = S, Se) and (53, X = S, Se)] with sodium thiophenoxide and sodium selenophen-oxide72 was studied. Selenophene compounds were four times more reactive than the thiophene derivatives. The position of attack, a or /), had very little influence on the rate ratio. The kinetics of the side-chain nucleophilic reactions of selenophene derivatives, shown in Scheme 4, has been reported.7 3... 

The analogous transients of acetophenone and fluorenone have been observed (Adams et al., 1964,1967b) as well as that formed from thiobar-bituric acid (Gordon, 1964). The negative-ion derivatives of acrylamide, methacrylamide and acrylonitrile may be considered as further examples of (R1.CO.R2)- (Dainton, 1967). The electron transfer from (CH3. CO. CH3) to acetophenone has been found to proceed at a rate of 8 x 108 m" sec-1 (Adams et al., 1967a). The same species has been found to react efficiently with bromoacetate and bromopropionate ions, and this reaction, which is probably an electron-transfer process, results in a quantitative debromination of the bromaliphatic compounds (Anbar and Neta, 1967c). The electron adduct of benzoquinone was found to react very rapidly with water to form the semiquinone (Adams and Michael, 1967). 

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