Nitration of Aromatic and Heteroaromatic Compounds by Dinitrogen Pentaoxide.

Abstract

Nitration of benzene and monosubstituted benzenes in liquid SO, by dinitrogen pentaoxide at -11 degrees C gave the corresponding nitroarenes with substitution patterns similar to those obtained by nitrations with HNO3-H2SO4. For acetophenone an o/m ratio of 0.94 was obtained. The yields were dependent on the substituents. With a 1:1 ratio of arene:N2O5 the yields varied from 73% for toluene to 0.4% for nitrobenzene as substrates. From competition experiments and the nitration of bibenzyl it was concluded that the reaction was faster than the macroscopic rate of mixing. The qualitative order of reactivity for PhX was X = OCH3 > CH3 > H > Cl > CH3CO > NO2. Nitration with N2O5 in liquid CO2 gave similar results. Nitration of pyrimidine, pyrrole, imidazole and indole with N2O5-SO2 gave no nitrated products. With thiophene, 2- (34%) and 3-nitrothiophene (5%) together with 2,4-(16%) and 2,5-dinitrothiophene (8%) were obtained. With pyridine, mono- and di-methylpyridines, quinoline, isoquinoline and 4-phenylpyridine nitration of the pyridine ring was obtained. The yields varied from ca. 70% to 16%, except for 3,5-, 2,5- and 2,6-dimethylpyridine for which only traces of nitro-dimethylpyridines were obtained. The reaction with the pyridines appears to be intramolecular both in the SO2 phase and in the water phase used for quenching the reaction. The reaction was proposed to proceed by a complex formed in liquid SO2: PyrH + N2O5 + SO2 --> [PyrH (.) SO2 (.) N2O5] -->/H2O 3-NO(-)Pyr + HNO3