Alkyls norbornyls

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

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 various carbenium ions /erf-alkyl, bridgehead-, norbornyl-, allyl-, benzyl- or cyclopropylcarbinyl-cations, which are assumed to be intermediates in these decarboxylations are compiled in ref. [293]. 

Scheme 2-8. Diastereoselective alkylation reactions in the norbornyl ring system.

There are also many examples of alkylation reactions involving the norbornyl ring system in which the enolate can be either endo- or c.vo-cyclic. Both the endo-cyclic (6, 7) and evo-cyclic (8) enolates exhibit high levels of asymmetric induction due to the rigid ring system. Scheme 2-8 presents some examples for alkylation involving the norbornyl ring system.15... 

In order to rationalize the data from a non-classical point of view one would have to propose that the relative stability of the edge- and corner-protonated species is different in norbornyl than in alkyl ions and that they are far apart in energy or that only the latter is an intermediate on the energy surface. There are no obvious reasons to believe that this is the case so that the exchange studies favour the classical ion. 

The behaviour of alkyl cations in SbFj solutions was used to establish the basic information needed to interpret the exchange studies with the norbornyl ion. The studies were conducted with butyl, amyl and hexyl ions but only the results with the C4 system are discussed here. 

Alkylation of 8-azaxanthine derivatives 44 with cyclopentyl, cyclohexyl, or m>-2-norbornyl bromide in DMF in the presence of K2C03 gave a mixture of N-8 45 and N-7 46 regioisomers (94JMC2970) (Scheme 9). 

These are generally limited to what are termed kinetically stabilized alkyls, i.e. those devoid of protons p to the metal (Figure 4.5). These also include norbornyl and adamantyl examples since decomposition via p-metal hydride elimination (see below) would require the installation of an alkenic bond between the a and p carbons of the precursor alkyl. This is precluded for these two alkyls because of the prohibitive strain associated with forming a double bond to a bridgehead atom within a small cage structure (Brendt s rules). The tetrakis(norbornyl) complexes are also remarkable because some uncharacteristic oxidations states can be attained, e.g. Cr(IV), Co(IV) (low-spin e4t2°). The second factor which may confer stability is steric bulk bimolecular decomposition routes are thereby discouraged. 

A number of alkyl and aryl migrations, which may be tenuously regarded as similar to carbonium-ion rearrangements, have been summarized by P. Brown and Djerassi (1967). The close similarity of the mass spectra of norbornyl chlorides have been explained by rearrangement of carbonium ions (Bunton and Del Pesco, 1969). 

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