|The electrophilic bromination of olefins is a well-studied reaction that has recently attracted renewed interest. The generally accepted mechanism is a two-step trans addition involving the bromonium ion. At first the reaction proceeds via a 1:1 charge-transfer complex which is transformed slowly to the bromonium ion. Since the bromonium ion prevents rotation of the C-C single bond and the Br- or Nu- ion opens the ring quickly on the opposite, the favored product is the anti product.|
However, because of the generally high reactivity of three-membered bromonium ions towards nucleophilic attack, few information exists about the characteristics of these ions in solution. In order to get sme informations about the lifetime of bromonium ions in protic solvents like MeOH, we used the azide clock methodology:
Successful studies have been conducted using azide and thiol ions as diffusion-controled trapping reagents to estimate the rate constants for the reactions of carbocations with solvent and solvent components as a function of the structures of the carbocation and of the nucleohile. Especially the azide ion has been used extensively as a competing nucleophile in the solvolysis reactions. The critical assumption of the azide capture methodology is that N3- reacts with the bromonium ion at the diffusion limit in MeOH.
It is well-known that the Br- + Br2 = Br3- equilibrium ( KMeOH = 177 L mol-1 ) complicates the analysis of the kinetics of olefin bromination. The equilibrium in MeOH is established extremly rapidly ( kf = 2.5 109 L mol-1 s-1 ) and in general Br2 is a more effectice brominating agent than Br3-. In the present set of experiments, the relative importance of Br2 or Br3- in the addition is uncertain and changes since Br- is continually generated because of solvent or azide capture of the intermediate bromonium ions. Nevertheless, when the reaction is carried out without addition of external bromide ion, Br3- can arise only from Br- formed during the bromine addition to the alkene. Therefore, the occurence of the electrophilic species, Br3-, can be minimized by working at very small alkene concentration.
There exists a linear relationship between the logarithm of the bromination rate constants of the cyclopropylidenes and their first ionization energies:
lg kBr2 = - (6.00 +- 0.15) IE + lg k0
An analogous relationship has previously been observed for acyclic alkenes with linear n-alkyl substituents and interpreted in terms of the absence of any steric control in the bromination rates. When branched substituents are introduced, the double bond was found to react much more slowly than predicted by the upper equation because of a steric inhibition to the bromine approach. Furthermore, the slope of this equation which is still larger than that for unstrained alkenes, indicates a very high sensitivity to the IEs of the ehtlenic ground state, in agreement with a predominat control of the reactivity by the nucleophilicity of the cyclopropylalkenes and not by any steric effect.
C. Schröder, M.F. Ruasse, A. de Meijere, I.T.O.D.Y.S. Paris, 1998