As nonclassical ions

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. 

Scheme 7.47. A pathway using classical ions and the Wagner-Meerwein rearrangement as well as nonclassical ions to depict the formation of pinene hydrochloride (exo-2-chloro-2,6,6-trimethylbicyclo[3.3.1]heptane) from a-pinene (2,6,6-trimethylbicyclo[3.1.1]-2-heptene) and 3-pinene (2-methylene-6,6-dimethylbicyclo[3.3.1]heptane) and the subsequent interconversion of pinene hydrochloride (eA o-2-chloro-2,6,6-trimethylbicyclo[3.3.1]heptane) and bornyl chloride (enrio-2-chloro-l,7,7-trimethylbicyclo[2.2.1]heptane).

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. 

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 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. 

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. 

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. 

The unsubstituted phenonium ion, as well as other phenonium ions substituted with electron-donating groups, have been recently observed as stable ions in superacid medium.34 That the structure is actually 18 and not an unsym-metrically bridged ion (19) nor a nonclassical ion (20) (see Section 6.2) in which there are three-center bonds was shown by the nmr evidence. The ring carbon that is bonded to the aliphatic carbons was established by 13C shifts to be tetrahedral in nature and 13C and proton chemical shifts in the ring were similar to those of cations shown to have Structure 21. 

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41... 

Fig. 7.9 The classical/nonclassical 2-norbornyl cation problem. Grey a pair of rapidly equilibrating classical cations with a nonclassical, bridged transition structure black the nonclassical cation as the minimum cation. The fully MP2/6-31G(d) optimised 2-norbornyl cations are depicted the nonclassical ion is 13.6 kcal mofi1 more stable at this and comparable levels of theory.

Superelectrophilic onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

There have been a very large number of investigations of carbocationic reactions of cyclobutyl, cyclopropylcarbinyl and allylcarbinyl derivatives under so-called stable-ion as well as solvolytic conditions. Bartlett (1965) has stated Among nonclassical ions the ratio of conceptual difficulty to molecular weight reaches a maximum with the cyclopropylcarbinyl-cyclobutyl system . The term nonclassical was first used by Roberts and Mazur (1951) to describe the nature of tricyclobutonium ion [32] suggested to be involved in reactions of cyclopropylcarbinyl derivatives. Later Roberts and coworkers (Mazur et al., 1959) favoured a set of rapidly equilibrating nonclassical bicyclo-butonium ions [34] instead of a single non-classical species. Essentially all experimental evidence on indicates that the species is a nonclassical... 

To test these alternative hypotheses, a tremendous amount of work has been done, by Brown and by others. For example, camphene hydrochloride is known to undergo ethanolysis 6000 times as fast as rer/-butyl chloride, and this had been attributed to anchimeric assistance with formation of a bridged ion. Brown pointed out that the wrong standard for comparison had been chosen. He showed that a number of substituted (3°) cyclopentyl chlorides (examine the structure of camphene hydrochloride closely) also react much faster than rerr-butyl chloride. He attributed these fast reactions—including that of camphene hydrochloride—to relief of steric strain. On ionization, chloride ion is lost and the methyl group on the ap hybridized carbon moves into the plane of the ring four non-bonded interactions thus disappear, two for chlorine and two for methyl. For certain systems at least, it became clear that one need not invoke a nonclassical ion to account for the facts. 

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation10. The bonding concept required to define nonclassical ions is simply to consider them as penta- (or higher coordi-... 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions 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. 

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