FORMATION OF PYRIDINIUM YLIDES AND CONDENSATION WITH ALDEHYDES

Deuterium-exchange studies of N-methylpyridinium salts indicate that the 2 and 6 ring protons exchange more rapidly than the N-methyl protons under basic conditions which give no detectable exchange of the 3, 4, and 5 ring protons. However, under the same conditions condensation of these salts with aldehydes occurs exclusively at the N-methyl position. Discussion of these results is given in terms of formation of intermediate ylides 1 and 2. Formation of either ylide 1 or 2 by decarboxylative methods in the presence of aldehydes leads to direct condensation to give N-(2-hydroxy-2-arylethyl)pyridinium and 2-(α-hydroxybenzyl)-N-methylpyridinium salts, respectively. These and other data are discussed and the conclusion drawn that the rate ratio of condensation to reprotonation is greater for ylide 1 than for ylide 2. © 1969, American Chemical Society. All rights reserved.