Carbanions. Part III. Pyramidal structure of α-sulphonyl carbanions in the norbornene system. Comparison with α-methoxycarbonyl carbanions

The rates of the base-catalysed H–D exchange in endo- and exo-2-methoxycarbonylnorbornene and in endo- and exo-2-phenylsulphonylnorbornene have been measured. In the first two compounds the exchanges occur α to the methoxycarbonyl group; in the second two compounds α to the phenylsulphonyl group. The rates are slightly lower in the esters than in the sulphones, in accordance with the pKa values of simple carboxylic esters and alkyl sulphones. In the disubstituted norbornenes, methyl cis-endo- and cis-exo-3-phenylsulphonyl-bicyclo[2,2,1]hept-5-ene-2-carboxylates, there is isotopic exchange only at the position α to the methoxycarbonyl group, and inversion of the latter, endo→exo and exo→endo respectively; the exchange and inversion rates are identical. The trans-isomers remain unchanged in the reaction conditions of the cis-isomers. The results have been compared with those obtained in the case of the corresponding disubstituted norbornene acids. They are consistent with a pyramidal structure (sp3-hybridisation), and not with a planar structure (sp2-p-hybridisation) of α-sulphonyl carbanions in the norbornene system.