Carbonium ions cyclopentenyl

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

The conversion of a pentadienyl to a cyclopentenyl cation is an electrocyclic reaction of a system containing loury-electrons delocalized over five atomic orbitals, and should therefore be a csorotatory process. In the example just described, the stereochemistry of the cyclization reaction is masked by subsequent rearrangement of (alkj l groups, which ultimately affords the most stable carbonium ion. The stereochemistry of the cyclization of divinyl ketones under acidic conditions, however, is conrotatory, as predicted ... 

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