Meisenheimer complexes bonded at carbon and at oxygen

Abstract

The carbon-bonded gas-phase Meisenheimer complex of 2,4,6-trinitrotoluene (TNT) and the nitromethyl carbanion CH 2 NO 2 Ϫ (m/z 60) is generated for the first time by chemical ionization using nitromethane as the reagent gas. Collision-induced dissociation (CID) of the Meisenheimer complex furnishes deprotonated TNT, a result of the higher gas-phase acidity of TNT than nitromethane. The formation of Meisenheimer complexes with CH 2 NO 2 Ϫ in the gas phase is selective to highly electron-deficient compounds such as dinitrobenzene and trinitrobenzene and does not occur with organic molecules with lower electron-affinity such as methanol, methylamine, propionaldehyde, acetone, ethyl acetate, chloroform, toluene, m-methoxytoluene, and even nitrobenzene and p-fluoronitrobenzene. As such, the reaction allows selective detection of TNT in mixtures. Meisenheimer complexes between CH 2 NO 2 Ϫ and the three dinitrobenzene isomers display distinctive fragmentations. The oxygen-bonded -complex of TNT with the deprotonated hemiacetal anion CH 3 OCH 2 O Ϫ (m/z 61), represents a different type of Meisenheimer complex. It displays characteristic fragmentation involving loss of HNO 2 upon CID. The combination of a selective ion/molecule reaction (Meisenheimer complex formation) followed by a characteristic CID process provides a second novel and highly selective approach to the detection of TNT and closely related compounds in mixtures. The assay is readily implemented using neutral loss scans in a triple quadrupole mass spectrometer. Gas-phase reactions of denitrosylated TNT with benzaldehyde produce the corresponding dihydrofuran in an aldol condensation, a result that parallels the corresponding condensed-phase reaction. (J Am Soc Mass Spectrom 2004, 15, 998 -1004

Using tandem mass spectrometry to predict chemical transformations of 2-pyrimidinyloxy-N-arylbenzyl amine derivatives in solution

“…A gas-phase anionic type Smiles rearrangement mechanism is proposed to explain the formation of the ion at m/z 349. The mechanism is shown in Scheme 7 [33,34]. The in-source CID spectrum of deprotonated 3 is shown in Figure 6b.…”

“…The Smiles rearrangement reaction is known mainly through a base-catalyzed rearrangement mechanism, which involves a Meisenheimer complex [33][34][35][36][37][38]. The goal of our calculations for this interesting gas-phase anionic type Smiles rearrangement is to show the possible existence of some transition states and reaction intermediates and their relative energies.…”

Gas Phase Reactions of 1,3,5-Triazine: Proton Transfer, Hydride Transfer, and Anionic σ-Adduct Formation

“…The dominance of the proton transfer channel when the reaction is highly exothermic is consistent with other gas phase work. For example, Briscese and Riveros [24,26] observed exclusive proton transfer and complete inhibition of the S N Ar reaction in their flourobenzene systems when proton transfer was about 9 kcal mol -1 exothermic. It is therefore intriguing that the Meisenheimer complex was observable in our studies of NH 2 − + 1,3,5-triazine where the acidity difference is 17 kcal mol -1 ; this result suggests inhibition from conversion barriers and/or a more stable Meisenheimer complex.…”

“…[26][27][28][29]. Of particular interest to our current work is the presence of a hydride transfer channel with 1,3,5-triazine to form a hydride-Meisenheimer complex.…”

“…While hydrogen bound complexes may be present at a wide range of relative acidities, the proton transfer channel does not become active until about the same relative acidity range as that of nucleophilic substitution. For highly basic nucleophiles, the proton transfer reaction is significantly favored over the substitution reaction [24,26].…”