Mechanistic investigation of the catalyzed vapor-phase formation of pyridine and quinoline bases using 13CH2O, 13CH3OH, and deuterium-labeled aldehydes

Solid-acid catalyzed vapor-phase reactions forming pyridine and beta-picoline, alpha- and gamma-picoline, and 8-methylquinoline (8-MeQ) and 3,8-dimethylquinoline (3,8-DMQ) were examined using labeled feeds (C-13, H-2) combined with NMR spectroscopy. In the presence of (CH2O)-C-13, amorphous silica-alumina (S-A) converted CH3CHO-CH2O-NH3 feed (480 degrees C) to pyridine containing C-13-4 (major) and C-13-2 (minor). Co-product beta-picoline contained C-13-4 and (CH3)-C-13 (major) and C-13-2 and C-13-6 (minor). Replacing (CH2O)-C-13 by (CH3OH)-C-13 led to subtle changes in the levels of C-13 incorporation due to an induction period during which methanol converts to formaldehyde equivalent. Pyridine and beta-picoline formation require a common propenimine intermediate that either condenses with aldehyde/imine species (major route) or undergoes Michael additions (minor route). Use of (CH3OH)-C-13 and either ZnO/S-A or H+-MFI led to similar patterns of C-13 incorporation, albeit at lower levels than S-A. Reaction of acrolein-D2O-ND3 (S-A, 480 degrees C) gave predominantly 3,5-dideuteriopyridine, 3-methyl-5-deuteriopyridine, and 3-deuteriomethyl-5-deuteriopyridine from imines (CH2CDCHND, CD3CHND, and CH2ND) formed in situ. A slower Bronsted acid-catalyzed reaction of pyridine bases with D2O incorporates deuterium at C-2,6 ring positions. The CH3CHO-D2O-NH3 reaction (S-A, 440 degrees C) showed acetaldehyde's carbonyl carbon locates at C-2, C-4, and C-6 positions in both alpha- and gamma-picoline. Addition of either (CH2O)-C-13 or (CH3OH)-C-13 to CH3CHO-(CHO)-O-2-ortho-toluidine feed (S-A, 470 degrees C) gave 8-MeQ containing C-13-2 (major) and C-13-4 (minor). Byproduct 3,8-DMQ contained C-13-2 and 3-(CH3)-C-13 (major) and C-13-4 (minor). Subtle changes in the C-13 incorporation patterns are once more due to an induction period for (CH3OH)-C-13 dehydrogenation. The predominant reaction pathway to 8-MeQ and 3,8-DMQ involves N-o-tolyl-methanimine. (c) 2004 Elsevier B.V. All rights reserved.