REACTIONS OF CHARGED SUBSTRATES .6. THE METHOXYMETHYL CARBENIUM ION PROBLEM .1. A SEMIEMPIRICAL STUDY OF THE KINETIC AND THERMODYNAMIC STABILITIES OF LINEAR AND CYCLIC OXOCARBENIUM AND THIOCARBENIUM IONS GENERATED FROM PYRIDINIUM AND DIMETHYLANILINIUM IONS

AM1-calculated energy profiles for dissociation of (methoxymethyl)pyri dinium and dimethylanilinium ion substrates show that the methoxymethy l carbenium ion is not sufficiently stable to exist as an intermediate on the reaction coordinate for this model reaction. [(Tkiomethoxy)met hyl]pyridinium ion, however, has a distinct transition state because o f the stability of the resulting ion-neutral complex. The complete pot ential energy surfaces for water displacement on the methoxymethyl sub strate with either pyridine or dimethylaniline as the leaving group sh ow distinct transition states and very flat surfaces for the ion-neutr al complexes in which interaction of the carbenium ion with both leavi ng group and nucleophile is stabilizing. Secondary systems studied, in cluding linear methoxy and thiomethoxy substrates, 5- and 6-membered c yclic oxo and thio substrates, and ribosyl-, xylopyranosyl-, and gluco pyranosylpyridinium ions yield ion-neutral complexes with sufficient i ntrinsic stability to exist as intermediates. Comparison with solution data, primarily activation entropy and Bronsted coefficients, suggest s that the sugar oxocarbenium ions, either as distinct, solvent-equili brated intermediates or elements of ion-neutral complexes, are formed by unimolecular dissociation of the respective substrates in solution.