Carbocations nonclassical ions

Schleyer, Olah, and co-workers.55 In this method, the sums of all 13C chemical shifts of carbocations with their respective hydrocarbon precursors are compared. A trivalent carbocation has a sum of chemical shifts of at least 350 ppm higher than the sum for the corresponding neutral hydrocarbon. This difference can be rationalized by partly attributing it to the hybridization change to sp2 and to the deshielding influence of an unit positive charge in the trivalent carbocation. Since higher coordinate carbocations (nonclassical ions) have penta- and hexa-coordinate centers, the sum of their chemical shifts relative to their neutral hydrocarbons is much smaller, often by less than 200 ppm. 

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. 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

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. 

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... 

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. 

Cram s original studies287 established, based on kinetic and stereochemical evidence, the bridged ion nature of (3-phenylethyl cations in solvolytic systems. Spectroscopic studies (particularly1H and 13C NMR)288-291 of a series of stable long-lived ions proved the symmetrically bridged structure and at the same time showed that these ions do not contain a pentacoordinate carbocation center (thus are not nonclassical ions ). They are spiro[2.5]octadienyl cations 111 (spirocyclopropylbenzenium ions)—in... 

Schleyer et al. (1980) use another type of criterion for discrimination between classical and nonclassical carbocations. C-nmr chemical shift sums of carbocations and their respective hydrocarbon precursors are compared. A classical carbocation has a C-nmr chemical shift sum of at least 350 ppm higher than the sum for the corresponding hydrocarbon. This difference can be attributed partly ca 124 ppm) to the hybridization change, an increase of one sp -centre, and partly to the deshielding influence of the positive charge. Since nonclassical ions prefer bridged pentaco-ordinated structures to sp -hybridization their chemical shift differences, relative the hydrocarbons, are much smaller, often less than 200 ppm. 

Saunders and coworkers have developed a powerful tool which makes it possible to discriminate between nonclassical ions and rapidly equilibrating ions. It seems to have the capacity to resolve much of the controversy about the structures of nonclassical carbocations. The method is based upon a combination of equilibrium isotope effects and nmr spectroscopy and is also used for accurate determination of different types of equilibrium isotope effects. Below the essentials and applications of this new tool will be reviewed. 

Applying the additivity of chemical shift analysis to the 2-norbornyl cation also supports the nonclassical bridged nature of the ion. The chemical shift difference of 168 ppm between 2-norbomyl cation (C7H11+) and its parent hydrocarbon norbornane 129 is characteristic of the <200 ppm difference observed between a nonclassical ion and its parent hydrocarbon. In contrast, an ordinary classical trivalent carbocation such as the cyclopentyl cation (75) reveals a chemical shift difference of >360 ppm (between the ion and the parent hydrocarbon, cyclopentane). This is consistent with the 350ppm difference characteristic of classical carbocations and their precursor hydrocarbons. 

The next focus of this section is deamination of exo- and eA2rfo-2-norbornylamine because these reactions belong to the group of aliphatic nucleophilic substitutions in bicyclo[2.2.1]heptyl systems, for which Winstein and Trifan (1949, 1952) postulated a new type of carbocation intermediate, which was later called a nonclassical ion (see Bartlett, 1965, page V Roberts, 1990, p. 67). We will, therefore, first briefly discuss nonclassical carbocations in general and then deamination in bicyclo[2.2.1]heptyl systems. 

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. 

If we admit the intramolecular interaction of the developing carbocation centre with remote a- and n-bonds often forming the same nonclassical ion (see above) it is but a step to recognize a similarity between the mechanisms of the intermolecular interactions of both 7t- anda-bonds with the external electrophile forming intermediate structures with 2-electron 3-centre bonding arrangement (27t3C). 

The anchimeric assistance is a sufficient, but not a nece ary sign of formation of the nonclassical ion. Anchimeric assistance is only observe) if it is essential for the transition state of the rate-determining step for solvolysis. Absence of anchimeric assistance does not preclude formation of the less stable nonclassical ion at stages succeeding the primary formation of the classical carbocation at the limiting stage... 

Another variant to identify the classical or the nonclassical nature of carbocations by NMR data has been suggested by Olah, Schleyer et al. They calculate the difference between the sum of the chemical shifts of all the carbons of an ion and that of all the carbons of the corresponding hydrocarbon formed by adding a hydride-ion. For static and rapidly equilibrated classical carbocations the difference is usually 350 ppm. For nonclassical ions this value is by hundreds of ppm less thus for the 2-norbornyl ion A6 is +175 ppm, for the 7-norbomenyl one —1 ppm etc. 

Referring carbocations to classical or nonclassical on the basis of the value of AS ( C) it is also useful to compare these values for the secondary ion in question and for the corresponding methyl-substituted tertiary ion. The comparison of classical secondary and tertiary ions shows small differences (for isopropyl and tert-butyl the difference is —4 ppm, for cyclopentyl and methylcyclopentyl it is —10 ppm), for nonclassical ions these differences are far larger 2-norbornyl — 2-methylnorbomyl, — 129 2-bicyclo[2,l,l]hexyl — 2-methyl-2-bicyclo[2,l,l]hexyl, —97 ppm etc. 

From the above data Brown assumed the intermrfiate carbocation not to be the nonclassical ion 19 but to have the structure of the classical tritgrclic carbocation 219 in the rapid equilibration with its epimer. As noted above, the solvolysis of tosylate 209 under weak-alkaline conditions yields only alcohol 210. 

The configuration of the intermediate halogenonium ion determines unequivocally the direction of its isomerization into nonclassical carbocations — homo-allylic or homobenzylic ions. These are attacked by the nucleophile strictly stereo-specifically contrary to the steric factors but in agreement with the steroelectronic requirements of nonclassical ions. 

For a more convincing choice between structurally different carbocations study was made of the epimeric monofunctional dejivatives of polyfluorobenzocyclenes in which the splitting of the X-group must be followed by the formation of either different nonclassical ions S47 and 348 or the same classical ion 349 ... 

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. 

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