Adamantylideneadamantanes bromination

The second step of the bromination reaction in aprotic chlorinated solvents consists of the ionization of the CTC s, and leads to bromonium or bromocarbonium tribromide ion pairs. A direct evidence for the formation of bromonium-tribromide pairs is the isolation and X-ray structural characterization of the adamantylideneadamantane-bromonium tribromide species, obtained by Brown (ref. 13). 

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... 

The rate of the product-forming step is generally considered to be very fast, since it is an anion-cation reaction. However, bulky substituents can also slow this last step by making nucleophilic attack on the bromonium ion difficult. The most famous example of steric inhibition of product formation is the bromination (29) of adamantylideneadamantane in carbon tetrachloride (Strating et al, 1969). Bromine adds to this alkene, i.e. the electrophile can... 

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. 

What is the precise mechanistic process by which bromine (and by extension chlorine) adds to alkenes Studies designed to probe this question are generally difficult to conduct due to the very fast product-forming steps. By applying stopped-flow spectrokinetics to sterically crowded alkenes such as adamantylideneadamantane (Ad2>, where steric hindrance retards the final addition step by hampering rearside nucleophilic attack of the counterion at the bromonium carbons of the intermediate ion pairs, it... 

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. 

Kinetic studies92 of the bromination of cyclohexene, alken-l-ols 41 and 4-pentenoic acid, by the bromonium ion of adamantylideneadamantane bear out the findings shown in Scheme 16. 

Slebocka-Tilk H, Ball RG, Brown RS (1985) The question of reversible formation of bromonium ions during the course of electrophilic bromination of olefins. 2. The crystal and molecular structure of the bromonium ion adamantylideneadamantane. J Am Chem Soc 107 4504-4508... 

Bellucci and coworkers reported evidence for the interaction of adamantyl-ideneadamantane and bromine to form complexes with different stoichiometries, having alkene Br2 ratios of 2 1, 1 1, 1 2, and 1 3. Conductivity experiments suggested that the 1 2 and 1 3 complexes are tribromide and pentabromide salts, respectively, of the adamantylideneadamantane bromo-... 

We drew the conclusion that, at 0.2 mM, the concentration is too dilute for the Br transfer depicted in panel D to occur. Each alkene was therefore reacting independently, and without the intmsion of nondegenerate Br" transfer, the reactimis occurring according to their rate constants [41, 42]. As noted above. Brown [46, 48] had previously observed cyclic bromonium ion transfer with highly hindered alkenes, notably adamantylideneadamantane[s]. But Fig. 3 makes it evident that unhindered alkenes are also candidates, and thus bromonium ion transfer appears to be a hitherto unrecognized phenomenon in the bromination of alkenes [47,49]. 

PROBLEM 10.6 One example in which the bromonium ion is stable to further addition of nucleophiles is the reaction of adamantylideneadamantane with bromine (see below). Explain why this cyclic ion might be stable. 

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. ... 

The ion of this type was spectrally proved in the case of the reaction of adamantylideneadamantane with Bt2. The intermediate formation of the cation in olefin halogenation is also evidenced by the fact that halide ions accelerate this reaction. In acetic acid olefin bromination in the presence of Bf or Cf occurs with the rate... 

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