Norbornyl compounds

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. 

Three major arguments have been presented as evidence of bridging or a-participation during solvolysis of norbornyl compounds. 

However, not everyone was convinced by the existence of the non-classical carbocation. H. C. Brown 1977 pointed out that the norbornyl compounds are compared with cyclopentyl rather than with cyclohexyl analogues, 2.21 (eclipsing strain), and in such a comparison the endo-isomev is abnormally slow, the exo-isomer being only 14 times faster than cyclopentyl analogues. He also pointed out that the formation of racemic product is due to two rapidly equilibrating classical carbocation species (Scheme 2.17). The interconversion of enantiomeric classical carbocation species must be very rapid on the reaction timescale. 

This change in the secondary-tertiary energy difference for simple alkyl derivatives and the corresponding 2-norbornyl compounds, 2.7 kcal mol-1 for the methyl and 1.6 kcal mol-1 for the phenyl derivatives, are far from the value, > 7 kcal mol-1, required by the Goering-Schewene diagram. 

Resume. Alkyl participation in the solvolysis of exo-2-norbornyl derivatives is not unambiguously detected by rate measurements. Neither monocyclic analogs nor the corresponding enr/o-2-norbornyl compounds appear to be good models for comparison. Secondary deuterium kinetic isotope effects, however, point to distinct ionization mechanisms of exo- and -norbornyl derivatives. 

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. 

Formation of racemic product from solvolysis of a 2-norbornyl compound. 

X-ray and calculated (HF/6-31G and MP2/6-31G ) structures for the series of spirocyclic norbornyl compounds 54—56 revealed significant cyclopropyl homoconjugation in 54 and 55 as well as hyperconjugation in all three compounds. Boese et al. interpreted the experimental electron densities and calculated dipole moments as indicating an important contribution from the homobicycloaromatic resonance form 57 in determining the electronic and structural properties of 54. 

The sensitivity to quite minor conformational and configurational changes at sites relatively remote from the central atom (for example, in norbornyl compounds and the various cis and trans propenyl compounds precludes simple correlations... 

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. 

As discussed by Zollinger, 1995 (Sec. 7.5) this hypothesis of a detour around intermediates of very low stability is also useful for the differentiation of classical and nonclassical ion intermediates in nucleophilic substitutions of 2-norbornyl and related compounds. 

Recent examples of these reactions are the preparation of a series of norbornyl-, norbornenyl-, and norbornadienyl-tin compounds (87) 1-AdSnMes, (l-Ad)4Sn and 2-AdSnMes (Ad = adamantyl) (81, 83) Pr8SnCH=CHCH=CHj (83) and (Me3Sn)4C (82). 

The simple methylidyne compound [HC=Co]+ has been formulated as a CoIV species.1102 The norbornyl anion (nor), as the lithium salt, reacts with CoCl2 to produce the brown, paramagnetic tetrahedral Co(nor)4. This can undergo both reduction (to [Co(nor)4] ) and oxidation (with AgBF4) to what appears to be [Co(nor)4]+, a diamagnetic tetrahedral compound. This remains one of the very few established examples of Cov. 

Nitrogen compounds, reduction, 103-104 N-(exo-2-Norbornyl)acetamide, ketone amidation, 130 Nucleophilic species ... 

The deshielding of the vinylic carbon atoms in the norbornyl cations 13, 55-57, 62 and 77 compared to the precursor compounds was taken as evidence for the intramolecular coordination of the C=C double bond to the electron-deficient element atom." ° This high-frequency shift, is largest for the silanorbornyl... 

Geurink and Klumpp measured the protodelithiation enthalpies of 3-lithiopropyl methyl ether, 3-lithiobutyl methyl ether, 5-lithiopentyl methyl ether and 7-5yn-methoxy-2-exo-lithionorbornane in the same study that was discussed in an earlier section for the non-oxygenated compounds n-propyl lithium, n-butyl lithium, 5ec-butyl lithium and 2-norbornyl lithium. The reaction enthalpies for the oxygen-containing lithium species with 5ec-butyl alcohol in benzene were —190 2, —199 4, —190 3 and —199 2 kJmoU, respectively, where all of the lithiated ethers purportedly exist as tetrameric species. 

Quadricyclane is a highly strained and reactive compound. It reacts readily with acetic acid to give a mixture of nortricyclyl acetate and exo-norbornyl acetate and with bromine to yield a mixture of 2,6-dibromonortricyclene and exo-5-anti-l-dihvo-monorbornene.3 Quadricyclane undergoes cycloaddition reactions with a variety of dienophiles to give 1 1 adducts.15 1 2 3 4 5 6 7... 

A/-[ F]Fluoro-A/-alkylsulphonamides react with a variety of carbanions and organometallic compounds [84]. For example, A/-[ F]fluoro-A/-enc/o-norbornyl-paratoluenesulphonamide has been successfully used for the preparation of [ F]fluorobenzene (40 and 61% radiochemical yields from phenylmagnesium chloride and phenyllithium, respectively), 1-f F]fluoronaphthalene (53%, from 1-naphthylmagnesium chloride) and [ F]fluorocyc/ohexane (29%, from cyclo-hexylmagnesium chloride) (Scheme 24). 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation... 

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