Carbanions 7-norbornyl

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

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). 

These results suggest that a 1,3-elimination can only occur if, in the carbanion, the leaving group is situated in a semi-V/ arrangement (see (236), p. 330) and not in a semi- J arrangement. This is in marked contrast to the ease of concerted 1,3-elimination from both the endo- and exo-norbornyl tosylates. Obviously more investigations are necessary and it would be of interest to study the behaviour of the above sulphones (238, 239, 241) in which the chlorine is replaced by a tosylate group. 

Ab initio calculations and NMR spectroscopy have been used to probe the structure, aggregation, and reactions of sulfonyl-stabilized allylic norbomenyl and norbornyl carbanions. This has included characterization of the exo direction pyramidalized anionic carbon atoms. Lithiosulfones (10) and (11), which exist in THF at —108.5 °C as an approximately 1 1 mixture of monomeric and dimeric forms, have been found to equilibrate rapidly with the respective conjugate acids, (12) and (13), in both exo and endo forms at 100 °C. According to Li H - and H H -NOE experiments, the monomeric and dimeric species endo-(10) and endo- ll), having endo anions, seem to be preferred in THF solution. Ab initio calculations indicate that the C(2) atoms of endo-(10)(Li+) and endo-(ll)(Li+) are pyramidalized in the exo direction whereas the... 

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