And nonclassical ions

Figure 9.2. Carbon Is photoelectron spectrum Is core-hole-state spectra for the 2-norbornyl cation of tert-butyl cation and Clark s simulated spectra for the classical and nonclassical ions.

As discussed by Zollinger, 1995 (Sec. 7.5) this hypothesis of a detour around intermediates of very low stability is also useful for the differentiation of classical and nonclassical ion intermediates in nucleophilic substitutions of 2-norbornyl and related compounds. 

Figure 5.10. Carbon li core-hole state spectra of the 2-norbornyl cation and simulated spectra for classical and nonclassical ions according to Clark et al ...

Apart from the two extreme categories of classical and nonclassical ions there is a large group of ions with intermediate types of structure. Thus, for a 2-methylnorbomyl ion which was earlier, on the basis of the NMR spectrum, assigned the structure of a partially cr-delocalized ion the value of A8 is 304ppm less than the analogous value for the typically tertiary ions — methylcyclopentyl (374 ppm), methylcyclohexyl (362 ppm), 2-methyl-2-bicyclo[2,2,2]octyl (385 ppm) and 2-methyl-2-bicyclo[3,2,l)octyl (355 ppm). 

The Brown-Winstein nonclassical ion controversy can be summed up as differing explanations of the same experimental facts (which were obtained repeatedly and have not been questioned) of the observed significantly higher rate of the hydrolysis of the 1-exo over the 2-endo-norbornyl esters. As suggested by Winstein, the reason for this is participation of the Ci-Q single bond leading to delocalization in the bridged nonclassical ion. In contrast. Brown maintained that the... 

Nonclassical ions, a term first used by John Roberts (an outstanding Caltech chemist and pioneer in the field), were defined by Paul Bartlett of Harvard as containing too few electrons to allow a pair for each bond i.e., they must contain delocalized (T-electrons. This is where the question stood in the early 1960s. The structure of the intermediate 2-norbornyl ion could only be suggested indirectly from rate (kinetic) data and observation of stereochemistry no direct observation or structural study was possible at the time. 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

In the nonclassical ion controversy discussed in Chapter 9, there was never any question on either side of the debate about the validity of the observed data, only about their interpretation. Had any of the experimental data been questioned or found to be incorrect, this would have been soon found out because so many people repeated and rechecked the data. This is the strength of science (in contrast to politics, economics, etc.), i.e., that we deal with reproducible experimental observation and data. Nevertheless, interpretation can still result in heated discussions or controversies, but science eventually will sort these out based on new results and data. 

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. 

Attack ty acetate at C-1 of C-2 would be equally likely and would result in equal amounts of the enantiomeric acetates. The acetate ester would be exo because reaction must occur from the direction opposite the bridging interaction. The nonclassical ion can be formed directly only from the exo-brosylate because it has the proper anti relationship between the C(l)—C(6) bond and the leaving group. The bridged ion can be formed from the endo-brosylate only after an unassisted ionization. This would explain the rate difference between the exo and endo isomers. 

The nonclassical-ion concept proved to be an intriguing one, and many tests for the... 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

The concepts of ct participation and the nonclassical ion 52 have been challenged by Brown, who suggested that the two results can also be explained by postulating that 48 solvolyzes without partieipation of the 1,6 bond to give the classical ion 53, which is in rapid equilibrium with 54. This... 

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

The elasticity can be related to very different contributions to the energy of the interface. It includes classical and nonclassical (exchange, correlation) electrostatic interactions in ion-electron systems, entropic effects, Lennard-Jones and van der Waals-type interactions between solvent molecules and electrode, etc. Therefore, use of the macroscopic term should not hide its relation to microscopic reality. On the other hand, microscopic behavior could be much richer than the predictions of such simplified electroelastic models. Some of these differences will be discussed below. 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

The expressions nonclassical and hypervalent ion have also been used by some authors to describe distonic ions, but these are incorrect and thus should no longer be used. The term ylidion is limited to species where charge and radical are at adjacent positions. Thus, to describe the distance between charge and radical site, the terms a- (1,2-) distonic ion, p- (1,3-) distonic ion, y- (1,4-) distonic ion, and so forth are now in use [42,43]... 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

In the early days of stable ion chemistry, the experimental measurements of parameters such as NMR chemical shifts and IR frequencies were mainly descriptive, with the structures of the carbocations being inferred from such measurements. While in cases such as the tert-butyl cation there could be no doubt of the namre of the intermediate, in many cases, such as the 2-butyl cation and the nonclassical ions, ambiguity existed. A major advance in reliably resolving such uncertainties... 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

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. 

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