H−D Exchange between N-Heterocyclic Compounds and D₂O with a Pd/PVP Colloid Catalyst

Abstract

A polymer-protected palladium colloid (Pd/PVP, PVP = poly-N-vinylpyrrolidone) has been used to catalyze H−D exchange between the D2O solvent and pyridine, substituted pyridines, and related N-heterocycles. Within a few hours under ambient conditions, significant exchange at the positions adjacent (α) to the endocyclic nitrogen was observed for 4-dimethylaminopyridine, 4-methoxypyridine, 4-methylpyridine, pyridine, and quinoline. H−D exchange was observed also for the methyl group in 4-methylpyridine and for the fused-ring H8 position in quinoline. N-Methylimidazole was rapidly and selectively deuterated at the H2 position. Nonselective deuteration was observed at various ring positions (but not the methyl group) for 3-methylpyridine. The presence of a β substituent in 3-cyanopyridine and nicotinic acid hindered H−D exchange at the adjacent α (H2) site, but significant deuteration was observed for the second α position (H6). Portions of the 2- and 4-vinylpyridine samples underwent rapid hydrogenation to 2- and 4-ethylpyridine with residual hydrogen used to activate the catalyst. After an extended period, the 1H NMR spectrum of the remaining 2-vinylpyridine showed evidence for H−D exchange not only at the α position but also at both terminal sites in the vinyl group. These results suggest a variety of possible intermediates formed by interaction of the N-heterocycle with the Pd catalyst surface.