SYNTHESIS, STRUCTURE, AND REACTIVITY OF AN ANTIAROMATIC, 2,5-DICARBOXY-STABILIZED 1,4-DIHYDROPYRAZINE

3-(Chloromethyl)-5,6-dihydro-5,5-dimethyl-1,4-oxazin-2-one (4) undergoes self-condensation in the presence of diisopropylethylamine to yield 4a,8a-diaza-2,6-dioxa-3,4,7,8-tetrahydro-4,4,8,8-tetramethylanthracene-1,5-dione (DDTTA). DDTTA is an example of a modestly stable, almost flat 1,4-dihydropyrazine. It is green in color, and the long-wavelength visible absorption band shows significant solvatochromism. DDTTA gives reversible one-electron-oxidation waves in methylene chloride at -0.33 and 0.61 V vs ferrocene/ferrocenium (0.07 and 1.01 V vs NHE, respectively) and reacts with Fe(1,10-phenanthroline)3(3+) to yield DDT7A+ (5) characterized by ESR spectroscopy. The radical cation is also produced in purple-black crystals by cocrystallization with tetracyanoquinodimethane (TCNQ). The black material has the composition DDTTA+-TCNQ)2-, and in solution it is unstable to molecular oxygen. Air-stable radical cation 5 is produced by air oxidation of DDTTA in trifluoroacetic acid. The resulting trifluoroacetate crystallizes as pale blue parallelepipeds with the composition DDTTA+CF3CO2-(CF3CO2H)2; the radical cations are arranged in sheets with short C-H...O intermolecular contacts as established by X-ray analysis. DDTTA reacts with molecular oxygen in acetic acid to yield the unstable dioxetane 7, 4a,8a-diaza-2,6-dioxa-9,9a-epidioxy-3,4,7,8,9,9a-hexahydro-4,4,8,8-tetramethylanthracene-1,5-dione, and in acetonitrile to yield the aldehyde 9, 5,6-dihydro-2-oxo-5,5-dimethyl-1,4-oxazine-3-carboxaldehyde. Mechanisms for the air oxidations are proposed in Scheme I. [2 + 21-Cycloaddition of DDTTA occurs with 1-phenyl-1,3,4-triazoline-2,5-dione (PTAD) to give the diazetidine 17. Catalytic hydrogenation of DDTTA gives 4a,8a-diaza-2,6-dioxa-3,4,7,8,9,9a-hexahydro-4,4,8,8-tetramethylanthracene-1,5-dione (13), which air oxidizes to a mixture of DDTTA and 4a,8a-diaza-2,6-dioxa-3,4,7,8,9,9a-hexahydro-9a-hydroxy-4,4,8,8-tetramethylanthracene-1,5-dione (14) via the relatively persistent, purple radical cation 16. DDTTA represents a new electron donor with three stable redox states and, consequently, has potential as a component in formation of organic electrically conducting materials.