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OTfꢀ ions. The solid portion was insoluble in CD3CN but dissolved in
acetone-d6 (evolution of a yellow gas was noted while adding acetone-d6).
Its 1H [1.25 (t, 3H, J = 7.25 Hz), 3.04ꢀ3.10 (m, 2H), 6.98 (brs, 3H)] and
19F [ꢀ79.38 (s)] NMR spectra were compatible with [EtNH3][OTf] salt
formed via metathesis.
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(11) In an effort to shed light on the nature of intermediates in EAN/
TFAA and EAN/Tf2O nitrating systems, we employed natural abun-
dance 15N NMR. Using neat MeNO2 as an external reference, the 15
N
NMR spectrum of neat EAN exhibited two resonances at ꢀ346.8 (for
+
NO3ꢀ) and ꢀ4.3 ppm (for EtNH3 ). Addition of 1 equiv of TFAA to 2.5
equiv of EAN gave a homogeneous solution that exhibited three 15N
signals at ꢀ346.7, ꢀ39.6, and ꢀ8.7 ppm. The 15N signal observed at
ꢀ39.6 (∼340 ppm from ammonia) is indicative of a covalent CF3COO-
NO2 species (see ref 12). The reported value for a nitronium species is
considerably more shielded (∼250 ppm from ammonia). The slight 15
N
upfield shift of the EtNH3+ signal likely stems from in situ metathesis,
forming mixed counterions (NO3ꢀ and OTfꢀ). The EAN/Tf2O system
behaved differently: similar addition of Tf2O to ice-cooled EAN gave a
precipitate after mixing. The 15N NMR of the liquid portion (in CD3CN)
exhibited two resonances at ꢀ346.4 and ꢀ8.9 ppm, similar to those
detected in the EAN/TFAA system, but an additional signal attributable
to TfONO2 or NO2+ was not detectable at natural abundance. The 1H
NMR [1.24 (t, 3H, J = 7.25 Hz), 3.03ꢀ3.11 (m, 2H), 7.27 (brs, 3H)] and
+
the 19F NMR [ꢀ79.33 ppm (s)] corroborated the presence of EtNH3 /
8094
dx.doi.org/10.1021/jo201374a |J. Org. Chem. 2011, 76, 8088–8094