Corner-protonated

In order to rationalize the data from a non-classical point of view one would have to propose that the relative stability of the edge- and corner-protonated species is different in norbornyl than in alkyl ions and that they are far apart in energy or that only the latter is an intermediate on the energy surface. There are no obvious reasons to believe that this is the case so that the exchange studies favour the classical ion. 

If the intermediate were corner-protonated, however, and undergoing exchange with the acid, it would be most reasonable to expect all the protons at the comer to exchange with the acid before ring cleavage occurred. Thus, corner protonated methylcyclopropane would be expected to exchange three protons with the acid if it exchanged any. 

Structures of protonated cyclobutanes have been studied in the same fashion (see Figure 9 B). In the corner-protonated cyclobutane, the structure corresponds essentially to a methyl cation interacting with a trimethylene diyl, and is much less favorable than that for cyclopropane. Similarly, for the edge-protonated ion, the proton must come much closer to the carbons to form a bond than for cyclopropane, and as a result, cyclobutane is much less basic. 

Olah and co-workers represented the nonclassical structure as a corner-protonated nor-tricyclane (52) the symmetry is better seen when the ion is drawn as in 53. Almost all the... 

Stable than cation 35,2 kcalmoT less stable than corner protonated cyclopropene (39) and 1 kcal mol less stable than the perpendicular allyl cation (36). It has been speculated that 34 is the transition state for the stereomutation of the planar allyl cation This is confirmed by MP2/6-31G(d,p) calculations ", which show that the cyclopropyl cation is located at a first-order transition state possessing one imaginary frequency of symmetry that describes a disrotatory movement of the CH2 groups in line with a stereomutation process of the allyl cation. Alternatively, stereomutation can also follow a stepwise route via perpendicular allyl cations 36 that are somewhat more stable than 34 (Table 26) and therefore lead to lower stereomutation barriers. ... 

The 1,2-methide shift as in the 2,3,3-trimethyl-2-butyl cation [5] takes place as shown in (6) with a very small barrier, AG = 3,5 0.1 kcal mol- (Saunders and Kates, 1978), via a corner protonated transition state or intermediate [6] ... 

Theoretical calculations of the ab initio type by Radom et a/. (1971, 1972) and at semi-empirical level by Bodor and Dewar (1971 Bodor et al., 1972) did not give consistent results. Recent ab initio calculations including electron correlation by Lischka and Kohler (1978) are inconsistent with earlier ab initio work. Their calculations have confirmed the stability of the 2-propyl cation and the instability of face-protonated cyclopropane [40]. Edge-protonated cyclopropane [41] is found to be a saddle point on the potential energy surface of lower energy than the corner protonated species [42]. 

This ion appears to have a classical structure, which is in equilibrium with a bridged ion under conditions of low nucleophilicity, but undergoes extremely fast 1,2-hydride shifts which have not been frozen out. It also shows complete hydrogen and carbon degeneracies which most probably involve corner protonated cyclopropane intermediates. At higher temperatures it rearranges to t-butyl cation. 

Saunders and Rosenfeld (1969) extended their H-nmr investigation to temperatures above 100°C and discovered another, slower process which exchanges the two methylene protons with the nine methyl protons, resulting in coalescence of these bands above 130°C. The band shape analysis gave an activation energy of 18.8 1 kcal mol" for this new process. Since any mechanism involving primary alkyl cations is expected to have a barrier of ca 30 kcal mol" (the enthalpy difference between tertiary and primary carbo-cations), the formation of a methyl-bridged (corner protonated cyclopropane)... 

These results by Olah and coworkers that the norbornyl cation adopts a corner protonated nortricyclane structure [4] in superacid media have recently got strong support. Saunders isotopic perturbation technique (Section 4) has been applied to this ion (Saunders and Kates, 1980). They prepared the dideuterio derivative [210] of [4] under stable ion conditions and... 

The results of Baird and Aboderin cannot be accommodated by assuming that corner protonated cyclopropanes are the only reactive intermediate. If they were, 1- and 2-deuteriopropanol would of necessity be formed in equal amounts (except for a small isotope effect) no matter what the acid concentration (Eq. (24)). 

However, recent studies on long-lived carbocations in super acid solutions are beginning to give additional data. Thus Olah 6) has obtained evidence, by the use of a number of spectroscopic techniques, that the norbomyl cation is best represented as a corner protonated cyclopropane 31). 

Can all of the present results be accommodated without the need of postulating an edge-protonated cyclopropane ring The tracer work of Baird Lee 32) and Deno can be fitted to a scheme in which corner protonation and direct opening with 1,3-hydride shifts are competitive, but an edge protonated cyclopropane ring accommodates the data in a much simpler fashion ). An edge protonated cyclopropane... 

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. ... 

Mechanism a involves a corner-protonated cyclopropane (28) we have already seen examples of such ions in the 2-norbornyl and 7-norbornenyl cations (pp. 453, 460). Mechanism b involves an edge-protonated cyclopropane (29). Mechanism c... 

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