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

Abstract

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 pK_a 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.