Tertiary 2-norbornyl

At the present time, many supporters of the nonclassical position, such as Professor Paul von R. Schleyer, now accept the classical formulation of tertiary 2-norbornyl cations1 such as 2,3,3-trimethyl-2-norbornyl cation (5) and the... 

The marked similarity in the Goering-Schewene diagrams for secondary 2-norbornyl (Fig. 2) and tertiary 2-norbornyl (Fig. 3) is persuasive of the interpretation that similar physical origins must be involved in both systems. However, when the question was posed, Is it reasonable to propose two very different explanations for phenomena which appear so similar , Professor Paul von R. Schleyer answered, Yes, it certainly is. Indeed, over the years a number of workers have taken this position that the origins of the high exo endo rate ratios in secondary and tertiary 2-norbomyl derivatives are different. It is appropriate then to consider the various proposals. 

On this basis, the high exo endo rate ratio in secondary 2-norbornyl was attributed to an enhanced exo rate, resulting from carbon participation, with a normal endo rate, whereas the high exo endo rate ratio in tertiary 2-norbornyl was attributed to an enhanced exo rate, resulting from relief of steric strain, with a normal endo rate. 

In summary, the interpretation of exo endo ratios is obscured by the inherent preference of the norbornane skeleton for exo attack. The stereochemistry of tertiary 2-norbornyl cations appears to be governed by steric factors. There is substantial evidence, however, that bridging contributes to the selectivity of secondary norbornyl cations. 

These conclusions are supported by data on the tertiary 2-norbornyl esters (723) 510 and (724)S11) in which the C(3)-D2 effect is nearly independent of configuration. The kH/kD of (724) is much smaller than that of (723), due to charge delocalization into the phenyl ring. 6,6-Dideuteration has a negligible effect on the solvolysis rate of 1,2-dimethyl-2-norbornyl p-nitrobenzoate (725)S12 Although the interpretation of secondary deuterium isotope effects is often difficult, comparison of the various norbornyl derivatives discussed here has uniformly been claimed to support alkyl participation. 

The exo endo rate ratio can be correctly predicted without due regard for anchimeric assistance this points to its absence. Hence the initial product to ionize these systems is the classical tertiary 2-norbornyl cation. This conclusion, however, does not preclude the possibility of the subsequent rearrangement of the classical cation into a nonclassical one, nor does it give any evident of the relative stability of classical and nonclassical tertiary 2-horbomyl cations. 

Both the secondary and the tertiary 2-norbornyl ions are stabilized by thea-partici-pation of the C bond, the only difference being in the degree of this participation as the measure of the latter Sorensen used the strength of the C —C bond (Fig. 8). 

The tabulation shows that the t-butyl system is a reasonable model for some equilibrating ions. It fails badly, however, when applied to the norbornyl compounds. The isopropyl system is a poor model for sec-butyl and cyclopentyl ions and is a very poor model for the norbornyl cation. The failure of the models to provide reasonable estimates of the shifts in the tertiary norbornyl cations which are undergoing either VVagner-Meerwein shifts or hydride migration makes it clear that the experimentad shifts in the secondary system cannot be used as structural proofs. Rather they should be regarded as fascinating results to be rationadized in terms of the structure, whatever it may be. 

These observations clearly demonstrate the dominance of steric effects in tertiary norbornyl derivatives but do not necessarily apply to secondary systems. The different response to em-dimethyl substitution in (701)90 and (702)494 as compared with (699) and (700) is a disturbing feature. 

Thus, in the absence of anchimeric assistance tertiary norbornyl compounds would be expected to solvolyze ca. 7 times as fast as the corresponding tertiary cyclopentyl analogues. This is in good agreement with experiments and does confirm the absence of anchimeric assistance due to the participation of the —C bonding pair of electrons in the ionization process. 

Table 5. Exo Endo Rate Ratio for the Solvolysis of Secondary and Tertiary Norbornyl Derivativeis...

In summary, it should be emphasized that an intimate understanding of the solvolyses of all the systems under discussion, and especially the secondary derivatives of some of the tertiaries listed in Table 1, is not yet available. When a break in the Hammett plot occurs [as with (17)] there is a transition from a to a mechanism. When no such break occurs over a certain range of aryl derivatives the mechanism may be either or key and one concludes that all such cases in Table 1 are of the k type. If the tertiary norbornyl systems (33) and (34) are of the k type, it is doubtful that their behavior has any relevance to the behavior of the complex secondary 2-norbornyl systems. 

Subsequent studies of other tertiary norbornyl cations have indicated that they are essentially classical structures. 

The value for K was determined to be 10 litre mole at 20°C, which is of the same order of magnitude as those for tertiary alkyl cations ((0-07 — 2) X 10 litre mole . Section II) and dramatically different from those for secondary alkyl cations (about 10 ° litre mole , calculated from Figs. 2 and 3). These data show that the 2-norbomyl ion is only 1-6 kcal mole less stabilized than, for example, the tertiary butyl cation and about 8 kcal mole" more stabilized than secondary alkyl cations. Another thermodynamic argument for the high stability of the 2-norbornyl ion in solution is found in the work of Amett and Larsen (1968)... 

The spectra of other norbornyl cations have also been investigated at low temperatures. Spectra of the tertiary 2-methyl- and 2-ethylnorborny cations show less delocalization,153 and the 2-phenylnorbornyl cation (54) is essentially classical,154 as are the 2-methoxy-155 and 2-chloronorbornyl cations.156 We may recall (p. 170) that methoxy and halo groups also... 

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)]. 

Bicyclo[2.2.1]heptane (norbomane) and bicyclo[2.2.2]octane, when treated with nitronium tetrafluoroborate in nitrile-free nitroethane, unexpectedly gave no nitro products. Instead, only bicyclo[2.2.1]heptane-2-one and bicyclo[2.2.2]octan-l-ol were isolated, respectively.500 Observation of bicyclo[2.2.1]heptane-2-yl nitrite as an intermediate and additional information led to the suggestion of the mechanism depicted in Scheme 5.48. In the transformation of norbomane the first intermediates are the 2-norbornyl cation 126 formed by hydride abstraction and nonclassical cation 127 formed through insertion of N02+ into the secondary C—H bond. In the case of bicyclo [2.2.2]octane, the oxidation of bridgehead tertiary C—H bond takes place and no further transformation can occur under the reaction conditions. Again these electrophilic oxygenation reactions testify to the ambident character of the nitronium ion. 

The Mo-catalyzed AROM/CM may be performed on highly functionalized norbornyl substrates (e.g., 71 and 72 in Scheme 16) and those that bear tertiary ether sites (e.g., 73-75, Scheme 16). Although initial studies indicate that the relative orientation of the heteroatom substituent versus the reacting olefin can have a significant influence on reaction efficiency, the products shown in Scheme 16 represent versatile synthetic intermediates accessed in the optically pure form by Mo-catalyzed AROM/CM. 

One of the crucial points in the debate about the norbornyl cation has been the extra stability gained from the presumed o-bridging in comprison with a classical carbocation. This stabilization was recently determined calorimetric-ally from the heats of isomerization of the 4-methyl-2-norbornyl cation [212] to the tertiary 2-methyl-2-norbornyl cation [213] in SbFj-SOgClF by Arnett et al. (1980), thereby avoiding the large initial state contributions that follow the use of different neutral precursors. As a classical reference system the... 

The energy separation between secondary and tertiary aliphatic cations in solution is ca 10-15 kcal mol- . In the norbornyl system where cr-bridging stabilizes the secondary ions more than their 2-methyl counterparts this difference is much lower (5.5-7.5 kcal mol- ) as verified by several unrelated experimental techniques. Schmitz and Sorensen (1980) measured this energy difference in [234] using free energy of activation data from nmr observations. 

The solvolyses of either exo- or cn-2-aryl-2-norbornyl-OPNBs gave good Brown correlations with nearly identical p values, -3.83 for exo- and -3.75 for endo-isomQvs, which are identical to that of 1-arylcyclopentyl-OPNB (Takeuchi and Brown, 1968 Brown and Takeuchi, 1968). Electron demand at the electron-deficient centre of the two systems should be the same, and obviously cr-participation cannot be a significant factor in the predominant exo-substitution in these derivatives. In the solvolysis of 2-aryl-2-norbornenyl-OPNB [8], there is also no difference in the p values between exo- and endo-isomevs which are -4.21 and -4.17, respectively (Brown and Peters, 1975). Similarly, in 2-arylbenzonorbornen-2-yl OPNB [9] (Brown et ai, 1969) and its 6-methoxy derivatives (Brown and Liu, 1969), the increasing electron demand cannot be detected at all. It is therefore remarkable that the solvolysis of cxo-2-aryl-fenchyl-OPNB [10] gives an rvalue close to 1.0, and nevertheless, a distinctly lower p value of —2.27, the lowest extreme of p values for tertiary dialkylbenzyl solvolyses (Fujio et al., unpublished). 

Coates and Fretz have investigated some substituent effects on their 9-penta-cyclononyl system and found results which support a trishomocyclopropenium ion intermediate. Solvolysis of either the tertiary or secondary methyl-substituted p-nitrobenzoates 115 or 116 in 65 % acetone at 100 °C furnished the same 80 20 mixture of tertiary and secondary alcohols. Similarly, both phenyl-substituted p-nitrobenzoates gave a 94 6 mixture of tertiary to secondary alcohols. Also, the methyl and phenyl substituent effects on 115 are very much lower than those observed for the corresponding 7-norbornyl system (10 and 10, respectively) indicating a stabilized, delocalized intermediate for the reaction of 115. 

Proposal No. 1. It was originally proposed back in 1966 that steric effects in the tertiary 2-methyl-2-norbornyl derivatives would be very large, with the steric requirements of the methyl substituent far exceeding those for the acyloxy group. 

Proposal No. 2. It was next proposed that the solvolysis of entfo-norbomyl tosylate is enhanced by large solvent participation comparable in magnitude to carbon participation in the exo isomer. Thus, one of the two interpretations considered, referring to the 2-Me/2-H reactivity ratios, was that, These = 10s values can be rationalized by the postulation of anchimeric assistance in the exo and solvent assistance in the endo secondary cases 40. This position was adopted and fully discussed by J. M. Harris and S. P. McManus in their interesting attempt to extrapolate from tertiary to secondary 2-norbornyl rates41. ... 

However, both J. M. Harris and we, in independent studies, have now concluded that solvent participation is not a significant factor in the solvolysis of endo-norbornyl tosylate in solvents of moderate or low nucleophilicities42,43. Consequently, we can no longer account for the similarity in the behavior of secondary and tertiary 2-nor-bornyl derivatives in terms of the fortuitous presence of comparable solvent participation in enoto-norbornyl and carbon participation in exonorbornyl, both of which vanish in the tertiaries, resulting in essentially constant tertiary/secondary rate ratios40,41. ... 

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