TRIAZOLINES .24. PERMANGANATE-CATALYZED LOW-TEMPERATURE THERMOLYSIS OF 5-(4-PYRIDYL) SUBSTITUTED 1,2,3-TRIAZOLINES

Potassium permanganate oxidation of 1-aryl-5-(4-pyridyl)-1,2,3-triazbl ines I in a benzene-water two-phase system using the phase-transfer ca talyst tetrabutylammonium chloride yields the corresponding 1H-1,2,3-t riatoles II. However, when the reaction is run in a single phase in an hydrous benzene alone, the products are not triazoles, but imines III that can normally be obtained only by high-temperature thermolysis of the triazolines. The presence of both potassium permanganate and tetra butylammonium chloride to yield the benzene soluble tetrabutylammonium permanganate ion pair appears essential for imine formation, although only in catalytic amounts. Thus a reaction pathway is proposed, which involves the initial coordination of the pyridyl nitrogen with the ma nganese atom of the permanganate ion leading to IV, followed by loss o f nitrogen and regeneration of the permanganate ion via the dihydropyr idine intermediates, V and VI, to yield the enamine VII, which tautome rizes to the imine III. Supportive evidence for the formation of IV is derived from the failure of 1,5-diaryl- and 1-aryl-5-(3-pyridyl)triaz olines to yield the respective imines; they lack the structural requir ements necessary to comply with the proposed mechanism, and are recove red unchanged. Unlike the high temperature pyrolysis, the permanganate catalyzed low temperature thermolysis reactions provide cleaner produ cts in better yields. Thus, the low temperature thermolysis may afford a route for the synthesis of clean samples of 1-arylimino-1-ethyl-4-p yridines, especially when the triazolines are on hand.