Nonclassical structure, 2-norbornyl cation

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.

In further theoretical studies, Schleyer and coworkers found" that the nonclassical 2-norbornyl cation (126) is more stable by 13.6kcalmoE than the 127 classical structure, and 126 is the only stable form in the gas phase and in solution, and the classical form (127) is unlikely to be involved in solvolysis reactions [MP4(fc)SDQ/6-31G /MP2(full)/6-31G -p ZPVE level]. They also performed a rigorous ab initio calculation and defined the classical 127 structure as an artifact and found that it is a transition state in the rearrangement of 126 into the bridged 2-norpinyl cation. The computed MP2-GIAO 0 chemical shifts for 126 are close to the experimental values, whereas those calculated by Schindler (IGLO) for 127 deviate considerably. The comparison of the chemical shifts for the two structures with the experimental data proves once again the symmetrical nonclassical structure. 

It would be very interesting for the problem of the existence of the nonclassical 2-norbornyl cation to determine the crystal structure of one of its salts. Due to the generally low stability of such salts, it was not possible to analyze a salt of the unsubstituted 2-norbornyl cation. Laube (1987) was, however, successful in obtaining a crystal structure of the l,2,4,7-aAZ//-tetramethyl-2-norbornyl cation (as salt with Sb2Fn, 7.102). 

The IR spectrum published by Koch et al. (1990) was recorded using Vancik and Sunko s technique (1989) in which 2-chloronorbornane and SbF are deposited simultaneously from the gas phase on a Csl window at 77 K. During a slow temperature increase to 150 K, formation of the carbocation could be observed by the change in the IR spectrum. Koch et al. (1990) compared the frequencies and intensities of 21 absorption bands observed in the range 878-3110 cm with results that were calculated on the basis of a structure of the nonclassical 2-norbornyl cation obtained by Koch et al. (1989) using the 6-31G basis set and dynamic electron correlation on the MP2 level. The calculated IR bands correlated very well with the experimental frequencies and intensities. The calculated structure of this cation was. 

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... 

Theoretical quantum mechanical calculations903-908 have also been performed on the 2-norbornyl cation at various levels. These calculations reveal a significant preference for the o-delocalized nonclassical structure. An extensive calculation by Schaefer and co-workers906 using full geometry optimization for symmetrically and... 

J. D. Roberts,seems to have first used the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylcarbinyl cation. See J. D. Roberts and R. H. Mazur, J. Am. Chem. Soc. 73, 3542 (1951). Winstein referred to the nonclassical structure of norbornyl, cholesteryl, and 3-phenyl-2-butyl cations. See S. Winstein and D. 

The norbornyl cation is at the heart of the nonclassical ion problem . The argument over the stable cation concerns whether it is a rapidly equilibrating pair of classical ions or rather a symmetrical ion (Brown, 1977). The isotopic perturbation studies by Saunders and Kates (1980) show that the postulated rapid Wagner-Meerwein rearrangement (4) is not consistent with the results but is a static structure like [4],... 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

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. 

Thus, in the case of the 2-norbornyl cation, there seems to be at least an unaccounted 7.5 kcal mol extra stability. The extra stability can be easily rationalized if the cation has the 126 nonclassical structure. 

Unequivocal evidence for the existence of the 7-norbornyl cation, however, was subsequently provided by Sunko and coworkers by using the cryogenic matrix isolation technique. They codeposited 7-chloronorbornane 146 and a large excess of SbFj at -263°C then slowly warmed it to -173°C. The IR spectrum was in very good agreement with the calculated frequencies [MP4(sdq,fc)/6-31G //MP2(full)6-31G + ZPE level] for the 7-norbornyl cation 80 (Scheme 5.11). Further warming gave the 2-norbornyl cation 126 at -73°C. Tlie possibility of the formation of a complex of 146 with Sbf. was ruled out when similar experiments with both 146 and 147 resulted in identical spectra. Recent ab initio calculations support the nonclassical bent structure of the cation. 

This reaction has been the center of a controversy concerning the structure of norbornyl cations such as 74 and 75. Norbornyl cations have been described in terms of the resonance structures shown in 77, or as the nonclassical ion 78.33 This view was questioned, primarily by Brown,who did not believe that the evidence proved the existence of a bridged intermediate.34 Brown suggested that the facile rearrangements are explain-... 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

Brown has questioned whether the spatial structure of the solvolysis products of 2-exo-norbornyl brosylate is a proof of the intermediate nonclassical ion He believes the classical 2-norbornyl cation to yield, when attacked by the nucleophile, only exo compounds for two resons ... 

The conclusion that the 2-norbornyl cation is a nonclassical carbocation has been strengthened by the experimental determination of its infrared spectmm when the cation is generated in a cryogenic Sbp5 matrix. The experimental IR spectra agree with those calculated for a nonclassical structure. 

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