Interaction of a Weakly Acidic Dinitroaromatic with Alkylamines: Avoiding the Meisenheimer Trap

Abstract

Polynitroaromatics are well-known to form anionic σ-complexes (Meisenheimer complexes). The formation of such complexes was assumed in the past to explain the blue color of solutions of 2,4-dinitrotoluene (DNT) and amines. However, this work shows that caution is warranted to avoid the hasty misidentification of Meisenheimer complexes. (1)H NMR spectra exhibit no significant shifts in the positions of the DNT protons, indicating that the majority of DNT species in solutions of DNT and amines retain their aromaticity. Density functional calculations on DNT-ethylamine complexes suggest that Meisenheimer complexes are sufficiently high in free energy so that they make up only a very small fraction of the full equilibrium population. While principal component analysis of the UV/vis spectra of the DNT-amine solutions reveals that only one absorbing species of significant concentration is formed, quantitative fits of Job's plots show that 1:1 association of DNT with the amines alone cannot explain the visible absorption spectra. Instead, the Job's plots can be accurately interpreted by deprotonation of DNT, with the amines acting as bases. The deprotonation equilibria lie far on the side of the unreacted DNT, preventing the detection by NMR of the deprotonated minority species that gives the solutions their strong blue color. The analysis of systems with DNT and n-butylamine, diethylamine, triethylamine, or benzylamine provides a consistent pK(a) of DNT in dimethyl sulfoxide of 15.3 ± 0.2.

A Strategy Enabling Enantioselective Direct Conjugate Addition of Inert Aryl Methane Nucleophiles to Enals with a Chiral Amine Catalyst under Mild Conditions

“…The formation of such complexes would prevent the use of an amine as the catalyst for reactions involving nitroaromatic compounds. However, a recent study suggests that only a small number of the complexes is formed 12. The relative ease of deprotonation of the methyl group and low tendency to interact with amine nucleophiles (catalysts) gave us confidence in the exploration of this class of nucleophiles, which has, surprisingly, not been employed previously in synthesis.…”

“…Furthermore, this study provides additional experimental evidence supporting the recent investigation by Bühlmann and co‐workers, which demonstrated that 2,4‐dinitrotoluene “forms Meisenheimer complexes with hydroxide or basic amines in DMSO, contradicting assumptions previously made in the literature. Instead, deprotonated DNT was observed to exhibit intense absorption in the visible region, explaining the deep blue color of solutions of 2,4‐dinitrotoluene and alkylamines” (DNT=2,4‐dinitrotoluene) 12. Moreover, as suggested in their conclusion “Finally, a reevaluation of the reactivity of 2,4‐dinitrotoluene with special attention to deprotonation and the interaction of this nitroaromatic with nucleophiles may lead to a better understanding of the decomposition of this dinitroaromatic pollutant in the environment.” We also believe that the design strategy described in our studies will offer an effective alternative approach to the development of new reagents for organic synthesis to solve unmet synthetic challenges.…”

Nucleophilic Aromatic Substitution

“…A kinetic study 161 of the cyclization process to give the dianionic adduct (70) indicates that either proton abstraction or ring closure may be rate limiting depending on the nature of the substituent R. There have been two reports of Diels−Alder addition reactions between superelectrophiles and dienes. 165 Two methods for the detection of 2,4,6-trinitrotoluene (TNT) using fluorescence techniques rely on the formation of its complexes with amines. 162 DFT has been used to help the understanding of the reactivity of nitrobenzofuroxans in these reactions.…”

Communication: Vibrational study of a benzyl carbanion: Deprotonated 2,4-dinitrotoluene

“…6 Interestingly, an intense blue color developing in solutions of 2,4-dinitrotoluene (DNT) and alkylamines has been recently reinterpreted as due to deprotonation at the methyl group of DNT, rather than to the formation of an anionic σ -complex, in agreement with nuclear magnetic resonance (NMR) spectra showing that DNT aromaticity is retained upon addition of the amine. 10 The surprisingly scant propensity of DNT to form Meisenheimer complexes with alkylamines in basic solution, along with an unfavourable deprotonation equilibrium that hinders the detection of the anion [DNT-H] − , formed in only minor fraction, by NMR spectroscopy, prompted us to turn to IRMPD spectroscopy in the mid-IR region to elucidate the characteristics of this elusive species. This sensitive structural probe exploits the coupling of ion trap mass spectrometry with the high fluence of a tunable IR-free electron laser (FEL).…”

“…However, the remarkable electronwithdrawing power of the two nitro groups imparts acidic properties to the methyl protons (pKa = 15.3 in dimethyl sulfoxide). 10 In order to gain a structural assay of [DNT-H] − ions by IRMPD spectroscopy, the prominent ion at m/z 181 was mass-selected and exposed to the CLIO beam in the 950-1800 cm −1 region. Exemplary mass spectra were recorded following mass selection of the parent ions and storing them in the trap either with or without irradiation by IR photons at 1180 cm −1 as shown in Figure 1S of the supplementary material.…”