Reactivity of Homoallylic Substituted Adamantylideneadamantanes with Bromine. Substituent Effects on the Stability of the Ionic and Nonionic Intermediates

Abstract

Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species.

Influence of alkene structure on the stability of alkene–Br2complexes: Effect of chlorine substitution

“…It is noteworthy that in preceding calculations on the p complexes between bromine and substituted adamantylideneadamantanes, bearing an electron withdrawing group at the homoallylic position, the B3LYP level was able to reproduce the experimental trend of the formation constants but generally the absolute values of the complex stabilization energies were very low, significantly lower than the experimental ones. 10 Higher values, in agreement with the experimental data, were obtained by MP2 level calculation which in this case followed the same trend of B3LYP calculations. This latter method was therefore considered 10 more adequate for the study of the bromine olefin complexes.…”

“…This behaviour confirmed the 1:1 stoichiometry of olefin-Br 2 species excluding, with all examined chloroethylenes, the formation of a bromiranium-Br 3 À ion pair having 1:2 stoichiometry. The presence of an equilibrium between the olefin-Br 2 complex and the corresponding bromiranium tribromide, characterized instead the solutions of adamantylideneadamantane, 13 and of several adamantylideneadamantane homollylic derivatives 10 and bromine. In the case of olefins 1-4 the substituent electron withdrawing effect seems to prevent the formation of the corresponding ionic intermediate and the interaction of chloroethylenes with Br 2 is limited to p complex formation.…”

“…9 Furthermore, on the basis of the data arising from an experimental and theoretical investigation on homoallylic substituted adamantylideneadamantanes it has been shown that the formation constant of the p-complexes depends strongly on the substituent's nature, being mainly affected by dipole-dipole or induced dipole-dipole interactions. 10 In order to obtain further information about the nature of the intermolecular interaction in the Br 2 -alkene complexes, we have extended our studies to four deactivated ethylenes bearing chlorine atoms at the double bond, 1,1-dichloroethylene, trans-1,2-dichloroethylene, trichloroethylene, tetrachloroethylene (1-4), which are not able to give the corresponding addition products by reaction with Br 2 under ionic conditions, by measuring the corresponding equilibrium constants and thermodynamic parameters of CT-complexes by means of UV-visible spectroscopic techniques.…”

What is the Nature of the First‐Formed Intermediates in the Electrophilic Halogenation of Alkenes, Alkynes, and Allenes?

“…The double bond of crotonic acid has a low K f value of 0.08 in DCE at 25 °C, probably due to the negative inductive effect of the carboxyl groups 29. Recently the reaction of 4‐equatorial homoallylderivatives of adamantylideneadamantane ( 15 : X=F, Cl, Br, OH, OEt, OAc) with bromine were studied 30…”

“…Although the OEt group is chemically the most similar to OH, the inductive effect on the π complex is very similar to that of the halogen substituent. This has been explained by considering the different geometries of the π complexes of the OEt and OH derivatives; these are affected by the different steric requirements of the two groups 30…”