Structure of the 2-Norbomyl Cation

The structure of the 2-norbomyl cation 5 is indirectly proved by the solvolytic cyclization of 2-(3-cyclopentenyl)ethanol. Thus the ar tolysis rate of nosylate 36 exceeds that of a saturated analogue by a factor of 95 This fact has been interpreted in favour of direct participation of a double bond in the substitution of an arylsulphonate anion. As shown by Sargent and Bartlett the substitution of... 

In recent years Olah and co-workers have research the structure of the 2-norbomyl cation under so-called long-life conditions in exceptional non-nucleophilic, strong-acid media (SbFj—SOj, FSOjH—SbFj — SO FCl etc.)- In these investigations wide use was made of the H and NMR, laser Raman and ESCA spectra... 

As with other carbocations, NMR spectroscopy has been utilized to determine the structure of the 2-norbomyl cation. The NMR spectrum at -70 °C showed... 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy based on the difference of the total chemical shift of a carbocation and the corresponding alkane. Differences in total chemical shift of 350 ppm or more suggest classical carbocations, while differences... 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy that is based on the difference between the total chemical shift of a carbocation and that of the corresponding alkane. Differences in total chemical shift of 350 ppm or more are associated with classical carbocations, while differences of less than 200 ppm are thought to indicate nonclassical, bridged carbonium ions. For example, the sum of the total NMR chemical shift of propane is 47 ppm, while the sum for the 2-propyl cation is 423 ppm. The difference, 376 ppm, indicates that the 2-propyl cation is a classical ion. For the 2-norbomyl system the total of the shifts is 408 ppm, while the total for norbomane is 233 ppm. The difference, 175 ppm, was taken as evidence for a nonclassical structure. ... 

Olah and co-workers represented the nonclassical structure as a corner-protonated nortricyclane (62) the symmetry is better seen when the ion is drawn as in 63. Almost all the positive charge resides on C-1 and C-2 and very little on the bridging carbon C-6. Other evidence for the nonclassical nature of the 2-norbomyl cation in stable solutions comes from heat of reaction measurements that show that the 2-norbomyl cation is more stable (by 6-10 kcal mol or 25 0 kJ mol than would be expected without the bridging.Studies of ir spectra of the 2-norbomyl cation in the gas phase also show the nonclassical stmcture. Ab initio calculations show that the nonclassical stracture corresponds to an energy minimum. ... 

In the 1980s and NMR spectra of the 2-norbomyl cation at substantially higher fields, that is, 395 MHz H and 50 MHz C, have been obtained at similar low temperatures. The 395 MHz ll NMR spectrum of structure of CyHii is fully resolved compared with that previously reported at 100 MHz (Fig. 5.6). 

Diese discussed results, together with theoretical calculations where the classical form of 2-norbomyl cation is not even an energy minimum, clearly proves the symmetrically (or very close to symmetrical) bridged structure of the 2-norbornyl cation (126) involving hypercoordinate carbons. 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

Comparison of Cl and C6 l3C chemical shifts showed that the ff-participation from the 2-norbomyl ring is significantly reduced in the 2-methyl analogue, whereas in the cyclopropyl and phenyl analogues it has essentially vanished. The STO-3G calculated structures show that the spirocyclopropyl participation is mainly from the exo-C—C bond. The l3C NMR studies of these cations adequately accounted for the vanishingly low values of solvolytic keJkenio rate constants, and show that 3-spirocyclopropyl groups effectively compete with the Cl—C6 ff-bond participation in the 2-norbomyl cation framework. 

On the basis of all the data obtained Olah has ascribed to the 2-norbomyl cation the structure of a comer-protonated nonclassical cation. The bond nature can be best described with a two-electron three-centre molecular orbital (2e3C) ... 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

The conclusion that the 2-norbomyl cation is a nonclassical carbocation was strengthened by the experimental determination of its infrared spectrum when the cation was generated in a cryogenic SbFs matrix. The experimental IR spectra agreed with those calculated for a nonclassical structure. ... 

The protracted and controversial discussions on the structure and dynamics of the 2-norbornyl cation have been recently summarised (Walling, 1983 Olah et al 1983 Brown, 1983 Grob, 1983). The question has been whether the 2-norbornyl cation has a symmetrically bridged hypercoordinated structure [99] with a pentacoordinated carbon atom or whether it is a rapidly equilibrating pair of trivalent cations [100], In the context of this review only the evidence from isotope effects on the nmr-spectra of the 2-norbomyl and related cations is discussed. 

After years of discussion, the nonclassical nature of the 2-norbomyl and 7-norbomenyl cations 9 and 15 seems to be beyond doubt and flieir structure and charge delocalization is discussed in almost every organic reaction mechanisms textbook (Figure 9.2). 

The X-ray structure of a number of alkoxycarbenium ions has been determined.66 An interesting example is 2-methoxy-l,7,7,-trimethylbicyclo[2.2.1]hept-2-ylium tetrafluroborate 326.630 It is a substituted 2-norbomyl cation and, indeed, the C(2)-C(l)-C(6) bond angle (98.8°) and the C(l)-C(6) bond distance (1.603 A) indicate G-bond charge delocalization, that is, the contribution of the 326b resonance form. 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

---