Norbornyl systems, and

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. 

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. 

Gassman and Fentiman (1970) used the Brown equation to investigate participation by a remote ir-bond in solvolysis reactions. They compared substituent effects in the solvolyses of 7-aryl-7-norbornyl [6], and syn-l-aryl-anri-7-norbornenyl [7] systems. The saturated bicyclic system [6] gave a linear... 

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. 

A number of papers concerned with the decomposition behaviour of organic halogen compounds in the mass spectrometer and related systems is to be found in the journal Organic Mass Spectrometry. For example, in the case of fluoro-compounds, McCarthy has discussed mass spectral correlations for fluorinated alkanes studies by other workers have included those on aromatic fluoro-com-pounds , fluorine-containing dimethyl esters , and tetrafluoroethanes . In the same journal, studies on other halogen compounds include dichlorocyclo-propanes , norbornyl chlorides and bromides , j8-phenylethyl bromides , and chloro-substituted benzynes . 

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. 

The norbornyl system is a [2.2.1] bicyclo system, and so the S number is five. Bridgehead double bonds only form if the S number is seven or greater. There is an additional requirement, in that the double bond may exist only in a ring that contains eight or more atoms, i.e. a derivative of trans-cyclooctene or larger. 

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). 

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. ... 

Another line of evidence indicating that bridging is important in solvolysis comes from substituent effects for groups placed at C(4), C(5), C(6), and C(7) in the norbornyl system. The solvolysis rate is most strongly affected by C(6) substituents and the exo isomer is more sensitive to these substituents than the endo isomer. This implies that the TS for solvolysis is especially sensitive to C(6) substituents, as would be expected if the C(l)-C(6) bond participates in solvolysis. 

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