Base-catalyzed hydrogen-deuterium exchange of N-substituted pyridinium ions. Inductive effects and internal return

Abstract

Rates of hydrogendeuterium exchange of N-substituted pyridinium ions in DzO at 75.0" were obtained by the use of an nmr method. Deprotonation is catalyzed by deuteroxide ion. Catalysis by buffer bases is negligible, Relative rates of exchange for substituents 0-, CH2CsH5, CH3, CsH5, ND2, and OCH, are 1.0, 1.1 X 102, 1.4 x 102, 5.3 x 102, 2.7 x 103, and 1.3 x 106, respectively. Rates show a good correlation with the Taft uI inductive parameter; pr is 15. The substituent rate factor for positively charged nitrogen is estimated to be about 1014-'6. The absence of substantial buffer base catalysis for these reactions giving pyridinium ylide intermediates is consistent with internal return during exchange. Generalizations concerning the structure of carbon acids, general base catalysis, and internal return are given.