Electrophilic Additions toanti-Sesquinorbornene. Br₂in Methanol and the Addition of Acetic Acid

Abstract

The kinetics and products of addition of Br2 in MeOH to anti-sesquinorbornene (1) have been studied at 25 °C in the presence of varying [LiBr]. The products consist of ∼10% cis dibromide and 90% cis methoxy bromide, the amounts of these being independent of the [Br-]. The kinetics reveal that the reaction proceeds via the addition of Br2, and that Br3 - is not a brominating agent for this olefin. Acetic acid is shown to add rapidly to the double bond of 1 in a spontaneous reaction and via an acid-catalyzed process in the presence of HClO4. For the spontaneous reaction a solvent deuterium kinetic isotope effect (DKIE) of 3.3 is observed. Proton inventory plots of k obs vs atom fraction of deuterium in the medium indicate that the DKIE arises from a transition state for addition involving two or more protons undergoing changes in bonding. The HClO4-catalyzed addition exhibits a large DKIE of 5.4 and is attributable to a simultaneous attack of HClO4 or ClO4 - H2OAc+ and HOAc on the double bond. LiClO4 also catalyzes the addition of solvent through a process exhibiting a DKIE of ∼8. Analysis of the various equilibria involved indicates that HClO4 is produced in appreciable amounts when LiClO4 is dissolved in HOAc. Analysis of the kinetic dependence of the rate on [LiClO4] indicates that the active species produced may be the ion aggregate Li+/ClO4 -/H2OAc+.

Investigation of the Early Steps in Electrophilic Bromination through the Study of the Reaction with Sterically Encumbered Olefins

“…-10 10 M -1 s -1 ), and the observed lack of such, a lifetime of 10 -11 s for the ionic forms was suggested. 36 This value is slightly less than the experimentally determined lifetimes of 10 -9 -10 -10 s for the bromonium ions of cyclohexene, cyclopentene, tetramethylethylene, and styrene in methanol.…”

Acid-Catalyzed Hydration of anti-Sesquinorbornene

“…In an earlier report, we have indicated that the β-bromocarbocation of 1 must have a lifetime of less than 10 -11 s in methanol. This was based on our inability to detect changes in the dibromide/methoxy bromide product ratio (∼10/90) as a function of added LiBr up to a concentration of 0.3 M. Although the solvent system here is different, it might be expected that the lifetime of a protonated 1 ( 1- H + ), in an aqueous medium, might be similar or slightly less than this.…”

“…Ethylene glycol dimethyl ether (DME, Aldrich, 99.9%, HPLC grade) was used with no further purification. LiClO 4 was purified as described previously …”

“…The reactivity appears to result from a large ground state strain around the double bond which is relieved by pyramidalization of one or both of the central carbons. 6d In a recent study 7 we have shown that 1 reacts rapidly with acetic acid to generate the monoacetate derivative. The reaction exhibits a primary solvent Dkie of k HOAc /k DOAc ) 3.3, and proton inventory analysis 8 indicated that the mechanism for the addition involved two or more protons in flight or undergoing significant change in their overall bonding in the transition state.…”

“…anti -Sesquinorbornene ( 1 ) is an unusual olefin for study of the mechanism of electrophilic additions because its structure precludes any trans solvent-assisted additions since the structure will only tolerate cis adducts. , This, and the fact that it is highly reactive toward electrophiles and has a convenient chromophore at 230−250 nm in the UV spectrum, makes it convenient to study without recourse to the more drastic conditions of high acidity usually employed to study the hydration of less reactive alkenes. The reactivity appears to result from a large ground state strain around the double bond which is relieved by pyramidalization of one or both of the central carbons 6d.…”