7-Norbomyl cation nonclassical structure

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) ... 

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. ... 

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

The theoretical energies indicate that the 6-sila-2-norbomyl cation 1 might be prepared from cyclopentenyl precursors (n route. Fig. 18). The nonclassical structure is favored thermodynamically by ca 36 kcal mol" (MP2-FC/6-31G ) over 2. 

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 1970, Olah reported that he had prepared a stable norbomyl cation in SbF5r4S02. From its pmr, cmr, and Raman spectra, he concluded that it has, inde, the nonclassical structure with delocalization of a electrons. The 2-phenyl-norbomyi cation, on the other hand, has the classical structure this l nzylic... 

J. D. Roberts was the first to use the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylmethyl cation. Winstein referred to the nonclassical structure of norbomyl, cholesteryl, and... 

Adamantonium Ions Mota and coworkers have made a theoretical study for adamantonium ions [MP2(full)/6-31G level]. lliree isomeric structures (81-83) and two van der Waals complexes (1-adamantyl cation + H2 and 2-adamantyl cation + IQ were found.The C-H bond lengths in the 3c-2c interactions in ions 81 and 82 are 1.276 and 1.266 A, and 1.266 and 1.280 A, respectively. In the C-adamantonium ion (83) the C-H bond distances are nonequivalent (1.191 and 1.294A) and the C-C bond distance is 2.348A. Interestingly, the 2-adamantyl cation + H2 complex shows a nonclassical bonding nature similar to the 2-norbomyl cation. The most stable carbocationic structure is 83, but it is less stable by 9.30 kcal mol than the 1 -adamantyl cation -1- H2 complex. 

The PMR spectra give no possibility to chose between the nonclassical ion 5 and the rapid equilibrium of a pair of enantiomeric classical ions 6. The ion structure was therefore studied by Raman laser spectra, because the rates of vibrational transitions are far higher than those of Wagner-Meerwein rearrangements. The spectrum of the norbomyl cation proved to be very similar to that of nortricyclejie and quite different from that of norbomane. For rapidly equilibrated classical ions 6, the RS spectrum would be similar to that of norbornyl derivatives. Besides, the rapid equilibration, according to Olah, would lead to the doublet splitting of the bands of some skeletal deformational vibrations which is not observed. 

The calculation of the averaged chemical shift for the cyclopentyl cation agrees well with a value observed in experiment. However, the same models when used to calculate the averaged shift of C, C and C in the classical norbomyl cation (—125 + 2 150/3) yield the value 58 ppm quite different from the observed one (+101.8 ppm). To obtain a calculated averaged shift close to the experimental value for the classical ion model one should assume either the chemical shift of the charged C atom to equal —6 ppm or those of either adjacent C atom to equal +215 ppm. Both variants (or any other value combinations) are absolutely unacceptable hence the spectrum supports quite a different structure and a different charge distribution, for instance, like that in the nonclassical ion 5. 

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... 

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. ... 

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. ... 

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. ... 

Electrophilic attack on C-C bonds is less common, and most studies have been concerned with strained C-C bonds. These studies have established that cleavage of strained C-C bonds is a major reaction of such systems with electrophiles. The existence of the nonclassical pentacoordinate ion in SbFs-SOa requires that it be more stable than the classical ion. It is reasonable that the nonclassical pentacoordinate carbocation structure of the norbomyl cation could be more stable than the classical tricoordinate carbocation structure. The classical structure incorporates an electrophilic carbon at C(2) in a favorable geometric relationship for interaction with the strained C(l)-C(6) bond. 

The arguments discussed to this point, both for and against the nonclassical structure, rest on indirect evidence derived from interpretation of the kinetic and stereochemical characteristics of the substitution reactions. When techniques for direct observation of carbocations became available, the norbomyl cation was subjected to intense study from this perspective. 

---