The 2-Norbornyl System

The solvolysis rates of the C-5-substituted epimeric 2-norbomyl p-bromobenzene-sulphonates (92) and (93) and their 2-methyl homologues have been investigated in 60% aqueous ethanol and in 97% hexafluoropropan-2-ol at various temperatures. The rate ratio exo-(92) endo-(92) is solvent-dependent, rising to the value 1746in hexafluoro-propan-2-ol at 25 °C, indicating the absence of internal return for exo-(92) and nucleophilic solvent assistance for endo- 92). The rates for the epimeric brosylates (93) are lower, the exo-brosylate being affected more, even in hexafluoropropan-2-ol. The rate ratio reductions are much less pronounced in the tertiary series. The results are as expected for the stabilization of the transition state from exo-(92) by 0-delocalization or some other electronic effect. 

Secondary deuterium kinetic isotope effects obtained for the solvolysis 

Lenoir, W. Roll, E. Weiss, and G. Wenke, Tetrahedron Letters, 1976, 1991. 

The 2-Norbornyl System.—useful short review by Collins has been published on the contribution of ion pairing to memory effects, that is the phenomenon by which apparently similar carbocations formed from different reactants give different ratios of the products. Ion-pair return during the solvolysis of l,2-dimethyl-cxo-2-norbornyl p-nitrobenzoate has been investigated by the elegant, even if well tried, 

The fact that the highly electron-demanding 3,5-bis(trifluoromethyl)phenyl group fails to alter appreciably the exo endo rate ratios for solvolysis of 2-aryl-2-norbornyl p-nitrobenzoates, (79) (80), is taken as evidence for the absence of a-participation as a factor determining the relatively high exo endo rate ratios found for such reactions. Support for this conclusion comes from a comparison of the Hammett-Taft p values for the solvolysis of the 1-arylcyclopentyl p-nitrobenzoate, (79), and (80) which are — 3.82. — 3.75, and — 3.83 respectively. 

The products of the buffered acetolysis of 2-deuterio-eiido-norborn-5-en-2-yl p-bromobenzenesulphonate (91) have been analysed by mass spectrometric, C and H n.m.r. procedures, and the results discussed in a clearly written and interesting paper. The deuterium initially on C-2 had migrated partially on to C-1 (30%) in the deuteriated cxo-norborn-5-en-2-yl acetates so formed, and was equally and uniquely distributed between C-1 and C-6 of the deuteriated nortricyclyl acetate, the major solvolysis product. Within the limits of detection products derived from C( 1 - 7) migration or from fragmentation were not formed. The results are consistent with the formation of the symmetrical nortricyclyl cation (92), or ion pair, from the norborn-5-en-2-yl cation, the isomerization being competitive with the reaction of these intermediates with solvent [n.b. C-1 and C-6 in (92) are equivalent positions that correspond to the C-1 and C-2 positions in the norbomenyl system]. 

The relative rates of acetolysis of 2-cyclopentylethyl, 2-(A -cyclopentenyl)ethyl (93), 2-(3-phenyl-A -cyclopentenyl)ethyl (94), and 2-(3,4-diphenyl-A -cyclopentyl)ethyl (95) p-nitrobenzenesulphonates are found to be 0.011,1.0,1.70, and 0.8 7 respectively. The products isolated from (94) and (95) are 2-phenyl and 1,2-diphenyl norbornene. 

It is clear that the additional conjugating phenyl groups in (94) and (95) do not significantly increase the -participation already apparent in the solvolysis of (93), nor does phenyl substitution appear to introduce appreciable steric interactions with a consequent rate deceleration. The n-route transition state, therefore, is thought to resemble a Tc-complex, but is not necessarily symmetrical. 

---

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. 

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. 

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... 

Space does not permit a detailed discussion of the entire topic. Fortunately, a full discussion has recently appeared11. Consequently, the present review will consider only the 2-norbornyl system. Is it better represented as an equilibrating pair of classical cations equivalent to the Meerwein proposal1, or is it better represented as a a-bridged species equivalent to the Wilson proposal2 ... 

The high exo endo rate ratio exhibited by 2-norbornyl is a remarkable phenomenon, perhaps the least ambiguous characteristic of the 2-norbornyl system on which to base a decision. The precise problem of the 2-norbornyl solvolysis is defined by the energetics of the system as revealed by the energy diagram introduced by Goering and Schewene27 (Fig. 2). 

Fig. 4. Effect of enhanced electron demand in the 2-norbornyl system on exo endo rate ratio...

By contrast, the 2-norbornyl cation is easily formed. Moreover, the 2-norbornyl system can readily undergo elimination. On irradiation in CH3OH or (CH3)5COH, the 2-norbornyl iodides 64a underwent exo,endo equilibration and afforded the 1,2- and 1,3-elimination products 66 and 67, along with the nucleophilic trapping product 68. > The corresponding bromides 64b each afforded the same products, along with small amounts of the radical products 65 and 9, but underwent no detectable interconversion. 

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... 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

In general, it is considered very unlikely that free radicals exist as non-classical structures (Walton, 1987). Kinetic and product studies involving the 2-norbornyl radical revealed the lack of importance of the non-classical species (Bartlett and Pincock, 1962 Martin and De Jongh, 1962 Bartlett and McBride, 1965). More recent ESR studies on the systems [150] and [151], in which the geometry was considered favourable for homoconjugation, again demonstrated the ineffectiveness of non-classical structures for radicals (Walton, 1987). 

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

Since the 2-norbornyl radical [117] and the 5-norbornenyl radical [118] have been thoroughly studied and their behavior in solution is well known, studies of Grignard reagent formation using the 2-norbornyl and norbornenyl systems seemed well worthwhile [116]. Relative to the nature of these radicals, they are planar /r-radicals. 

Proposal No. 3. In the nonclassical ion problem , Professor Paul von R. Schleyer has advanced a new interpretation11. He postulates carbon participation in secondary exo-norbornyl and comparable steric hindrance to ionization in the tertiary e/zdonorbornyl derivatives. However, he argues that such steric hindrance to ionization should not be important in the secondary 2-norbornyl system, since the smaller 2-H should provide far less steric hindrance to the departure of the endo anion than the larger 2-R44. ... 

The rearrangements of the 2-norbornyl cation are unexceptional. All 1,2-alkyl shifts, even those in acyclic systems (Section 7.6.1) conform to essentially the same pattern. Individual differences exist in the rates of alkyl shift, relative to the rates of capture by solvent. In that respect the 2-norbornyl cation is superior to acyclic analogs because it can approach the geometry of corner-protonated nortricyclene with only small distortions of the nuclear positions. (A more sophisticated treatment in terms of relaxation theory has been published548. ) Other bicyclic and polycyclic systems provide similarly favorable conditions for 1,2-alkyl shifts. Some of them will be discussed in the following Sections. 

At -100°C, the 2-norbornyl cation (CyHn ) in the SbF5-SO2ClF-SO2 solvent system showed three peaks at 8 H 4.92 (four protons), 2.82 (one proton), and 1.93 (six protons), indicating that the 2,3-hydrogen shift was fully frozen, whereas the 6,1,2-hydrogen and Wagner-Meerwein shifts (if any) remain rapid on the NMR time scale. 

The 50 MHz NMR spectrum of the 2-norbornyl cation (C7H1T) was obtained in the SbF5-SO2ClF-SO2F2 mixed solvent system at In... 

Another class of cyclopentyl radicals, in which high stereoselectivity in hydrogen-abstraction reactions is observed, are bridged systems. The 2-norbornyl radical, without15-16 and with37 substituents at C-2 has been investigated in detail. In the reduction of 2-norbornylmercury halides or acetates with sodium borodeuteride the intermediate 2-norbornyl radical is trapped from the toco-side predominantly. 

Further Kramer has measured the peak area ratio in the ESCA spectrum of the 2-norbomyl cation (from the figure published by Olah) and found it is not 2 5 (as required by the nonclassical ion model) but 4.5 1 or 6 1, depending on the method of measurement. Olah, however, points out that in the ESCA spectrum the main thing is not the ratio of peak areas which is often distorted due to admixtures coming from the vacuum systems and so can somewhat vary from experiment to experiment, but that of the AEb value. Finally, Kramer assumes the reported spectrum to belong to a neutral compound or a mixture of substances rather than to a stable ion. But according to Olah, the NMR spectra of the solutions under study before and after the recording of the ESCA spectrum are perfectly identical and coincide with that of the 2-norbornyl cation. 

Addition of CF3CHN2 to fluorosulfonic acid at -78 C gives a solution the NMR spectrum of which shows a quartet (Jhf = 6.1 Hz) at 8 6.3 ppm from external TMS. On warming to -20°C, this quartet disappears and is replaced by another one (/hf = 7.5 Hz) at 8 5.50 ppm. 2-t-Butyl-exo-norbornyl p-nitrobenzoate is an extremely reactive compound, undergoing solvolysis 2.8 x 10 times faster than t-butyl p-nitroben-zoate. The endo isomer is about 500 times less reactive. In contrast to the unsubstituted norbornyl system, which gives almost exclusively exo product, both t-butyl isomers give about 5% of the endo product. 

The study of a series of exo- and emfo-2-aryl-2-norbornyl p-nitro-benzoates is said to offer no evidence favoring a participation in the secondary 2-norbornyl system [cf. the p values of (33), (34), and (35)]. However, this treatment suffers from the same handicap as found in the 5-norbornen-2-yl system that is, the amount of anchimeric assistance present in the secondary derivative (if any) is small, and hence all the aryl groups commonly used are apparently capable of leveling the proposed a participation.f Nevertheless, cr contributions can be measured by the... 

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. 

---