Heterolysis carbocation formation

For a long time, it was considered that the formation of a bromonium ion from olefin and bromine is irreversible, i.e. the product-forming step, a cation-anion reaction, is very fast compared with the preceding ionization step. There was no means of checking this assumption since the usual methods—kinetic effects of salts with common and non-common ions—used in reversible carbocation-forming heterolysis (Raber et al., 1974) could not be applied in bromination, where the presence of bromide ions leads to a reacting species, the electrophilic tribromide ion. Unusual bromide ion effects in the bromination of tri-t-butylethylene (Dubois and Loizos, 1972) and a-acetoxycholestene (Calvet et al, 1983) have been interpreted in terms of return, but cannot be considered as conclusive. 

Both If i 0i) s ted and Lewis acids are effective in coordinating with the hydroxyl oxygen to induce heterolysis of the C-O bond and cause formation of the necessary carbocation intermediate. The reactions are frequently conducted... 

The rate of an SnI reaction depends on the rate of formation of the carbocation (the product of the rate-determining step) via heterolysis of the C-X bond. 

S—C Bond heterolysis dominates the photochemical reactivity of the keto tosylate 263247. Irradiation in benzene gives p-toluenesulphonic acid in 74% yield. The other products formed from this reaction are the ketones 264 and 265. The reaction is presumed to proceed via the intermediate carbocation 266 formed by S—C bond fission. The tosylate 267 is more able to undergo intramolecular addition due to the electron-donating methoxy group and gives 268 and 269 in 24% and 23% yields. An analogous mechanism is involved in the conversion of the sulphonate 270 on irradiation in benzene into the two products 271 and 272 in a ratio of 4 1249. The formation of the major product 271 presumably involves the heterolytic fission of an O—C bond to afford a cation which... 

Miller and coworkers treated 1-chloro- and l-bromO 2-phenylethyne with antimony pentafluoride in liquid sulphur dioxide benzene was added as a carbocation scavenger. Instead of carbon-halogen heterolysis they unexpectedly observed formation of 2-halo-3-phenylbenzothiophene 5 -oxide (41). On closer examination and depending upon substituent X various other products were observed, e.g. 1-halo-2,2-diphenylvinylsulphinic acids (42). The initial step in the formation of these products is probably electrophilic addition of sulphur dioxide, assisted by the Lewis acid. Trapping of the carbon cation by benzene and elimination of antimony pentafluoride explained the various products. 

In polar solvents, a-halomethyl aromatics give rise to photochemical reactions that can be explained by both radical and ionic mechanisms. Equation 12.77 shows the results for irradiation of 1-chloromethylnaphtha-lene (119) in methanol. The most direct pathway for formation of the methyl ether 120 is heterolytic dissociation of the C-Cl bond to give a chloride ion and a 1-naphthylmethyl carbocation, the latter then undergoing nucleophilic addition by the solvent. Indeed, naphthylphenylmethyl carbocations were detected spectroscopically following laser flash photolysis of (naphthylphenylmethyl)triphenylphosphonium chlorides. On the other hand, products 121, 122, and 123 appear to be formed via the 1-naphthylmethyl radical. Therefore, an alternative source of the carbocation leading to 120 could be electron transfer from the 1-naphthylmethyl radical instead of direct photochemical heterolysis of 119.215-216 jaj-g g p. 

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