Adamantylideneadamantanes

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

The highly hindered alkene adamantylideneadamantane forms a bromonium ion which crystallizes as a tribromide salt. An X-ray crystal structure (Fig. 6.1) has confirmed the cyclic nature of the bromonium ion species. This particular bromonium ion does not react further because of extreme steric hindrance to back-side proach by bromide ion. 

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... 

We passed then to a particular olefin, adamantylideneadamantane, whose reaction with Br2 had been shown to stop at the stage of bromonium ion formation because of steric hindrance to backside nucleophilic attack. An UV-Vis spectrophotometric study (ref. 10) has shown that the complicated equilibrium reported in Scheme 4 is immediately established on mixing the olefin and Br2 in DCE. Equilibrium (1) could be isolated by working at low Br2 and ten to hundred fold higher olefin concentrations. A Scott plot followed by a NLLSQ refinement of the data gave a Kf = 2.89 x 10 (4.0) M-l. It is worth noting that conductimetric measurements showed the non-ionic nature of the 1 1 adduct, consistent with a CTC intermediate, but not with a bromonium-bromide species. 

In similarly substituted olefins Kf is strongly influenced by steric effects, as shown by the comparison of tetraisobutylethylene with adamantylideneadamantane and (i,/-D3-trishomocubylidene-D3-trishomocubane. In particular, the comparison between cyclohexene and the two tetrasubstituted cage olefins indicates that Kf increases at least by a factor of 103 on passing from a 1,2 disubstituted to a tetrasubstituted olefin. This dependence is likely to be similar in other solvents, because solvent effects on Kf are modest. 

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). 

Quantitative information about the equilibrium between olefin and Br2 on the one hand and CTC s and bromonium ion species on the other (Scheme 4) has been obtained by the already mentioned UV-Vis spectrophotometric study of the adamantylideneadamantane Br2 system (ref. 10). The spectrophotometric UV-Vis data of a large set of solutions of different reagents concentrations have been used to dissect, using a program based on NLLSQ fitting procedures, the complex spectra in those of the single species present at the equilibrium, as shown in Figure 5. 

Fig. 7. Energy profile for the interconversion of complexes of adamantylideneadamantane with Br2...

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

Fig. 7 Molecular structure of adamantylideneadamantane bromonium, as its salt with Br5 counterion (Rosokha et al. unpublished results)...

These authors have applied their method to measure the formation constant of the CTCs arising from adamantylideneadamantane (Bellucci et al, 1989) and tetraisobutylethylene in several solvents (Brown et al, 1990). The results, together with previous less reliable data, are shown in Table 2. 

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. 

The kinetics of the reaction of bis(iym-collidine)bromonium triflate (17) with adamantylideneadamantane (12), pent-4-en-l-ol (20), and cyclohexene (22) have been investigated in 1,2-dichlorethane at 25 °C under a variety of conditions (Scheme 2). The rates of all the reactions proved to be depressed by added collidine, indicating that the first step for all is a reversible dissociation of (17) into free collidine and a reactive intermediate (18), which is then captured by the alkene. The product of the reaction of (12) with (18) is complex (19), while that of reaction of (20) is... 

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... 

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. 

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. 

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