Bromonium ions, cyclic isolation

The idea that a cyclic bromonium ion was an intermediate was a novel concept when It was first proposed Much additional evidence including the isolation of a sta ble cyclic bromonium ion has been obtained since then to support it Similarly cyclic... 

In attempting to add Br2 to the highly unusual alkene (13), it has proved possible actually to isolate the cyclic bromonium ion (14) ... 

Epoxides, though uncharged, have a formal resemblance to cyclic bromonium ion intermediates (cf. p. 180), but unlike them are stable and may readily be isolated. They do, however, undergo nucleophilic attack under either acid- or base-catalysed conditions to yield the 1,2-diol. In either case attack by the nucleophile on a carbon atom will be on the side opposite to the oxygen bridge in (49) such attack on the epoxide will involve inversion of configuration (cf. p. 94) ... 

Chapter 18 by C. Chiappe focuses on the mechanism of bromination of alkenes, exploring the role of solvent on the formation of cyclic bromonium ion versus P-bromocarbemium ion, as key intermediates. In Chapter 19, H. P. A. Mercier et al. discuss the utility of a novel class of noble-gas onium salts as oxidants for generation and isolation of various trihalomethyl cation salts. 

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

We were aware that Wynberg [45] had isolated an adduct when adamantylide-neadamantane, 41 (Scheme 7d), had been treated with bromine. Brown and coworkers [46], with the help of X-ray crystallography, showed that the adduct was the cyclic bromonium ion 42. 

The unstable cyclic bromonium ion intermediate is an electrophile that subsequently reacts with a nucleophile. The epoxide, however, is stable enough to isolate because none of the ring atoms has a positive charge. 

In contrast, a cyclic bromonium ion is postulated in alkylacetylene bromina-tion with the isolation of tran -dibromides from hex-3-ynes andhex-l-ynes. A kinetic and product study of the reaction of diphenylacetylenes with triphenylaluminium is interpreted as monomeric aluminium attacking the acetylene electrophilically in the rate-determining step (71) c/j-Addition products are obtained. Vinylalanes, formed by addition of aluminium... 

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