Rearrangement nonclassical carbocations

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions... 

The main peculiarities of the addition of electrophilic reagents to norbornene are intensive skeletal rearrangements, formation of cis-adducts, exo-addition of both an electrophile and a nucleophile. The unusual process of cis-exo-addition was explained by nonclassical carbocations or rapid equilibrium of classical ions steric... 

Contents 1 Nonclassical carbocations/by V. A Barkhash — 2 Rearrangements of carbocationsby 1,2-shifts/by V G. Shubin —3 Arenium lons/by V A Koptyug 1. Carbonium ions. 1 Barkhash, V A (Vladimir Alexandrovich). 1933- 11 Shubin, V G (Vyacheslav Gennadievich), 1936- 111. Koptyug, V A IV. Series Topics m current chemistry, 116 etc... 

These results were interpreted as implying that the reaction of the exo-substrate occurred solely via a nonclassical carbocation, while the endo-substrate reacted by initial formation of a classical carbenium ion, which then rearranged to the nonclassical carbocation, but not before a small amount had reacted with solvent (attack being sterically directed to the exo-face). 

There is evidence that a bonds can also participate in the formation of nonclassical carbocation intermediates in solvolysis reactions. In the Wag-ner-Meerwein rearrangement, camphene hydrochloride (45) rearranges to isobornyl chloride (46). It is possible to write a mechanism in which... 

A gold(I)-catalysed homo-Rautenstrauch rearrangement of 1-cyclopropyl propar-gylic esters to cyclohexenones is considered evidence for the gold-stabiUzed nonclassical carbocation character of intermediates. A mechanistic study revealed 0 partial chirality transfer from optically active propargyl acetates (Scheme 125). 

Classical and nonclassical carbocations. Anchimeric assistance. Wagner-Meer-wein rearrangements. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

The theory of nonclassical ions offers an explanation of many unique chemical, stereochemical and kinetic peculiarities of bicyclic compounds. It has expanded our knowledge on chemical bonds in carbocations by introducing electron-deficient bonds (as in boron hydrides). It has accounted for many rearrangements of stable cations. As a side result our knowledge has been extended about ionization process in a solution, as well as about stereochemical methods. 

As noted earlier, in exceptionally weakly nucleophilic media the NMR method is used to observe directly many nonclassical ions — intermediates postulated in explaining unusual rates, products and stereochemistry of the above solvolysis reactions. This enables research under stable-ion conditions may result in dis-coverii new, earlier unknown kinds of carbocation rearrangements illustrated by the 7-norbomenyl and 7-norbomadienyl cations. 

A major opponent of this nonclassical ion intermediate was Brown,who published dissenting views throughout the latter half of the twentieth century. He insisted on the existence of a rapid equilibrium between the two classical carbocation forms facilitated via Wagner-Meerwein rearrangement. Exo- and endo-T tQ ratios were attributed to steric effects, as strain caused e (3to-isomers to exhibit more hindrance to ionization. Finally, Brown criticized the bridged intermediate model for not providing sufficient electrons for all bonds. ... 

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