Norbornyl systems

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

Consequently, these must be the properties of the norbornyl system, rather than a consequence of cr-participation under solvolytic conditions. 

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

A vast amount of work has been done143 on solvolysis of the 2-norbornyl system in an effort to determine whether the 1,6 bond participates and whether 49 is an intermediate. Most,144 although not all,145 chemists now accept the intermediacy of 49. 

As Sargent has pointed out, experiments and examination of molecular models both indicate that this system should offer more extreme steric hindrance to endo ionization than the norbornyl system does. One kcal mole-1, then, is an upper limit for the increase in nonbonded interaction experienced by the leaving group in going from the ground to the transition state in endo-2-nor-bornyl tosylate.107 But 1 kcal mole-1 cannot be responsible for the exo/endo ionization rate ratio of 1550 reported by Winstein and Trifan, a ratio since corroborated by hundreds of other studies. 

Norbornyl systems were used to study the /i-effect in the synperiplanar geometry42. The solvolyses of the mesylates (X = MeSOaO) of ewrfo-3-(trimethylsilyl)-ewrfo-2-norbomeol 12 and ewrfo-2-norborneol 13 were shown to take place via carbocationic mechanisms, and the trimethylsilyl-substituted system showed a rate acceleration of 105. 

Kirmse and Sollenbohmer69 studied the trifluoroethanolysis of the norbomyl p-nitrobenzoate 57, and observed the formation of the intermediate 58, followed by a 6,2 migration of silicon and Wagner-Meerwein shifts to give a mixture of products. The solvolysis of the norbornyl system 57 is accelerated by a factor of 3 x 104 compared to unsubstituted 2-norbornyl p-nitrobenzoate. 

When the glycopyranosyl radical adopts a B2 5 conformation then the preference for axial coupling products is readily understood on the basis of well-known steric interactions. Approach of a reagent from the P-face is significantly sterically hindered, particularly by the substituent at C(3), whereas attack from the a-face is less so. If ease of approach to position-2 of the norbornyl system [145] is taken as a model for ease of approach to C(l) of a pyranose ring in the B2 5 conformation, then the favoured approach to the carbohydrate corresponds to the favoured exo-approach to the norbornyl system. If the glycopyranosyl radical adopts the 4H3 conformation, then axial reaction is favoured by the least motion effects set out in Scheme 13. 

On balance, the solvolysis results just described indicate that there is little reason to require the norbornyl ion reached under solvolytic conditions to be non-classical. Before we turn to other aspects of the problem, it is interesting briefly to consider the results of deamination studies in the norbornyl system. 

Brown et al. (1977) applied the tool of increasing electron demand to this bicyclic system with the solvolysis of a series of 2-aryl-2-bicyclo[2.1.1]hexyl p-nitrobenzoates in 80% aqueous acetone. The excellent log k rj a+ relationship, gave a p+ of —4.31, a value more negative than for the related 2-aryl-2-e rfo-norbornyl system where a-participation is believed to be absent. Also no support for u-participation in secondary bicyclo[2.l.l]hexyl derivatives could be found. 

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. 

Further evidence for the S l mechanism is that reactions run under S l conditions fail or proceed very slowly at the bridgehead positions of [2.2.1] (norbornyl) systems" (e.g., 1-chloroapocamphane, 8). If S l reactions require carbocations... 

The Carbon-Carbon Single Bond as a Neighboring Group a. The 2-Norbornyl System. In the investigations to determine whether a C—C CT bond can act as a neighboring group, by far the greatest attention... 

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. 

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