Cyclic bromonium species

Yates and Wright (1967) have produced one possible criterion for the intervention of a cyclic bromonium species. They obtained p — 2 for 10... 

The first possible reason is that the electrons in the alkynes are held more closely to the nucleus than is the case in alkenes, as evidenced by the difference in acidity of hydrogens that are attached to each type of carbon. The second possible reason is that in the reaction pathway of the alkyne, there is a possible hybridisation of the bromine atom in the cyclic bromonium species that gives rise to an anti-aromatic species and which is hence unstable. This would in turn result in a higher activation energy for this reaction pathway. However, probably the most important reason is that the cyclic bromonium ion that contains a carbon/carbon double bond is far more strained than the saturated version, and hence is more unstable, and so less readily formed. 

This neighbouring group participation by bromine (cf. p. 93) does not of course prove that addition to alkenes proceeds via cyclic bromonium ions, but it does mean that such species are no longer merely ad hoc assumptions, and to that extent are correspondingly more plausible as intermediates. 

Jung et al. s synthesis of aplysiapyranoid A 58, a cytotoxic monoterpene, also demonstrated another successful substrate-controlled route to the halogenated natural cyclic ether (Scheme 43.6). 2,4,4,6-Tetrabromocyclohexadie-none (TBCO) 55 triggered the cyclization of hydroxyl alkene 54, which after biomimetic bromoetherification, provided bromoether 57. The intermediacy of bromonium species 56, in which a stericaUy less demanding chloroal-kenyl substituent is positioned axially, is postulated to be responsible for the stereochemical outcome in this bromina-tive cyclization. 

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. 

In the present reaction, both the bromine and the (E)-stilbene are achiral. However, the bromonium ion that is produced is chiral. In this ion, the bromine atom bridges both carbon atoms of the original carbon-carbon double bond to form a three-membered ring intermediate. The generation of a cyclic species has a profound effect on the stereochemistry of the second step of the bromine addition. 

Contrast the bromonium ion of Figure 21.33 with the species formed by intramolecular displacement of tosylate using the n bond. In the bromonium ion, there are sufficient electrons to form the new o bond in the three-membered ring. In the all-carbon species there are not enough electrons. In this cyclic ion, two... 

This can be explained by the formation of a bromonium ion, 11.2, in which the bromine atom bridges the two carbons of the alkene. The opening of the cyclic species by the bromide... 

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