When the reaction between aniline 1b and nitropyridine 2a was monitored by in situ IR spectroscopy under an N2 atmosphere, the instantaneous disappearance of the N=O stretch of 2a (1538 and 1360 cm -1 ) upon addition of LiHMDS coincided with the formation of a signal belonging to an intermediate (1390 cm -1 ) that decayed upon exposure to air. In situ IR and 1 H NMR experiments 10 support our tentative assignment of this intermediate as the lithium dianion salt of the hydrated nitrosopyridine ArN(OLi)2, (structure II in Scheme 4), or a variation thereof that can lose water to give the nitrosopyridine. To probe the possibility of the nitro(hetero)arene serving as the oxidant, an analogous experiment was performed wherein excess nitropyridine starting material 2a was added after the addition of base (Supporting Information). We observed no disappearance of the intermediate and no formation of the product following this procedure. These results suggest that atmospheric oxygen acts as the external oxidant to convert the intermediate to the observed ONSH nitropyridine products. Nitrosoarenes are known to undergo oxidation to the corresponding nitroarenes under aerobic conditions at high pH, which lends support to our observations of nitro(hetero)arene formation. Scheme 4. DFT mechanism for the ONSH reaction between lithiated aniline 1b•Li(THF)2 and nitropyridine 2a.