Bromonium ion from adamantylideneadamantane

Fig. 5.2. X-ray crystal structure of the bromonium ion from adamantylideneadamantane. Reproduced from J. Am. Chem. Soc., 107, 4504 (1985), by permission of the American Chemical Society.

Reversibility of bromonium ion formation has been observed directly in the regeneration of adamantylideneadamantane from the bromonium ion salt (Figure 9.12). Further evidence for reversibility is the observation that the bromonium ion from adamantylideneadamantane can transfer Br to cyclohexene in CH2CI2 solution. Since a sterically hindered bromonium ion can transfer Br, it seems reasonable that bromonium ions that are not sterically hindered should also be capable of transferring Br. Therefore, reversibility of bromonium ion formation could be a general process. Reversibility may... 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

Data on molecular structure of bromonium ions are sometimes extrapolated from that of the tribromide-adamantylideneadamantane bromonium ion pair [6] (Slebocka-Tilk et ai, 1985), the only stable ionic bromination intermediate that can be isolated and whose crystal structure has been determined. Since the first observation by Strating et al. (1969), it has been established that bromine addition to adamantylideneadamantane [5] in... 

A preliminary indication that bromonium ions could be formed reversibly was provided by the reaction of adamantylideneadamantane (p. 249) leading to a highly stable bromonium-tribromide ion pair that readily releases bromine and the initial alkene (Strating et al, 1969). However, the first evidence for possible return came from the acetolysis of 2-bromocyclohexyl-brosylate in the presence of bromide ions. It was shown (Brown et al, 1984) that the cyclohexylbromonium ion intermediate is able to release bromine. The drastic reaction conditions (high temperature, long duration and high bromide concentrations) cast some doubt on the generality of this observation. 

The rate of the return from bromonium ion to CT complex depends on the nature of the solvent and of the substituents bonded to olefin81,84. By using adamantylideneadamantane 33, halonium ions 34 are formed as stable (and isolatable) salts85, because of steric hindrance of the nucleophilic counter ion. Consequently, the products of addition, such as the dibromides 29, cannot be obtained. 

In the same way, the thiiranium ion (34, X = S—R) has been isolated from a mixture of adamantylideneadamantane and methane sulphenyl chloride86. The stability of 34, X = Br, which is the only known stable bromonium ion, and the fact that 34 cannot form the saturated products of the bromination reactions of olefins enable the investigation87 (by a dynamic 1H-NMR technique) of equilibrium 5, which proceeds via the dissociation of the 1 1 CT complex between 33 and bromine. 

The bridging bromine prevents rotation about the remaining bond, and back-side nucleophilic opening of the ring by bromide ion would lead to the observed anti addition. Direct evidence for the existence of bromonium ions has been obtained from NMR measurements. A bromonium ion salt (with BrJ as the counterion) has been isolated from the reaction of bromine with the very hindered alkene adamantylideneadamantane. ... 

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