Meisenheimer complexes protonation

Benzo[6]thiophene, methoxynitro-Meisenheimer complexes, 4, 816 Benzo[6]thiophene, 2-methyl-cycloaddition reactions, 4, 793 protonation, 4, 47 sulfonation, 4, 764 synthesis, 4, 879, 915 Benzo[6]thiophene, 3-methyl-cycloaddition reactions, 4, 793 1-oxide... 

Rieger P-G, V Sinnwell, A Preup, W Francke, H-J Knackmuss (1999) Hydride-Meisenheimer complex formation and protonation as key reactions of 2,4,6-trinitrophenol biodegradation by Rhodococcus erythropolis. J Bacterial 181 1189-1195. 

That the formation of molecular complexes (especially EDA complexes) can catalyse the decomposition of the cr-adduct has been discussed in Section n.E. Another possibility is that the substrate and catalyst (nucleophile or added base) form a complex which is then attacked by a new molecule of the nucleophile in this context catalysis need no longer be associated with proton removal. Thus, Ryzhakov and collaborators183 have recently shown that the N-oxides of 4-chloropyridine and 4-chloroquinoline act as jt-donors toward tetracyanoethylene and that the reactions of these substrates with pyridine and quinoline are strongly catalysed by the jr-acceptor. Similarly, the formation of a Meisenheimer complex between 1,3,5-trinitrobenzene and l,8-diazabicyclo[5,4,0]undec-7-ene in toluene has been assumed to take place via an association complex to explain the observed second-order in tertiary amine184. 

Crampton24 has also demonstrated that for Meisenheimer complex formation, increased crowding at the reaction site caused by change from primary amines to piperidine results in rate reduction of proton transfer from the complex to the amine catalyst, and Hirst199... 

Mechanisms, of proton transfer between oxygen and nitrogen acids and bases in aqueous solutions, 22, 113 Mechanisms, organic reaction, isotopes and, 2, 1 Mechanisms of reaction, in solution, entropies of activation and, 1, 1 Mechanisms of reaction, of /3-lactam antibiotics, 23, 165 Mechanisms of solvolytic reactions, medium effects on the rates and, 14, 10 Mechanistic analysis, perspectives in modern voltammeter basic concepts and, 32, 1 Mechanistic applications of the reactivity-selectivity principle, 14, 69 Mechanistic studies, heat capacities of activation and their use, 5, 121 Mechanistic studies on enzyme-catalyzed phosphoryl transfer, 40, 49 Medium effects on the rates and mechanisms of solvolytic reactions, 14, 1 Meisenheimer complexes, 7, 211... 

When base was added to a solution of N-methyl-N-/l-hydroxyethyl picramide 20 in a polar solvent, the isolable cyclic Meisenheimer complex 21 was formed (Scheme 8). At pH > 12 the conversion is virtually complete. Rapid protonation of 21 causes quantitative formation of picryl ether 22 trapped as an ammonium ion. The corresponding free base was unstable and tended to rearrange in the opposite direction [17]. 

This colored complex was used to develop colorimetric tests for many nitro aromatic compounds. Whether the Meisenheimer complex or 2,4,6-trinitrobenzyl anion (TNT ) resulting from proton abstraction reacted further with TNT (or with excess hydroxide) to form larger species is still not clear, but alkaline hydrolysis of TNT in aqueous media led to large molecular weight compounds (60% > 30 kDa [28] 40% > 1 kDa [30]). 

CH2CHNH2CO2H] The complexes were characterized by i.r., u.v., and n.m.r. spectra. The ground- and excited-state configurations of the electrons of 4-substituted benzothiadiazoles (295 X = S R = H, NH2, or OH) were calculated the 7r-/a-bonds, total energies, and heats of atomization of (295), their protonated forms, and their tautomers were tabulated. The hydroxy-and amino-tautomers are more stable than the 0x0- and imino-tautomers, respectively. Compounds (295) are protonated on N-1. The linear dichroism and m.c.d. spectra of 2,1,3-benzothiadiazole and 2,1,3-benzo-selenadiazole were measured and c.d. spectra reported for the j3-cyclodextrin compound with the heterocycle. The kinetics of formation and equilibrium data have been reported for the Meisenheimer complexes of the benzothia- and benzoselena-diazoles (295 X = S, or Se R = 4-NO2) with MeO" in MeOH/DMSO. ... 

The kinetics and equilibria of the reaction of 4,6-dinitrobenzofuroxan with water (forming 235),494 496and of the cyclization of 7-(hydroxyethoxy)-4-nitrobenzofuroxan with alkali, in water and in dimethyl sulfoxide (Eq. II)497 have received detailed study. Proton NMR spectroscopy has shown that methoxide ions add more quickly to the 5-position of 7-methoxy-4-nitrobenzofuroxan, but the thermodynamic product is the 7,7-dimethoxy adduct (237) (Eq. 12).498 Methoxide adducts are also formed with 4-nitro-benzofuroxan.499 The impetus for much of this work was provided by the suggestion that antitumor and antileukemic properties which have been observed in some benzofuroxans might be connected with their ability to form Meisenheimer complexes of the types illustrated in this section.300,416,500... 

An early example of an ion/molecule reaction carried out in an atmospheric-pressure capillary inlet reactor based on an ESI interface to a quadrupole mass spectrometer involved the study of proton transfer between amines and multiply protonated proteins by Smith (333, 334]. Smith also observed protonated adenosine 5 -monophosphate (AMP) as the product of charge inversion in the reaction of [AMP-H] with multiplyatmospheric pressure 335j. In addition, the formation of TNT Meisenheimer complex (336) and the interaction between piperidine and multiply-charged lysozyme ions generated by electrosonic spray ionization (ESSI) [337] under atmospheric pressure has been reported. Eberlin reactions (i.e., the gas-phase transacetalization reaction) of acylium ions have also been observed to occur at atmospheric pressure under in-source ion/molecule reaction conditions [338]. 

Abstract. Alkoxide ion departure from Meisenheimer complexes of the 1,l-dialkoxy-2,6-dinitro-4-X-cyclohexadienate type is catalyzed by pyridinium ions and by the hydronium ion. Bronsted a values which vary between 0.35 and 0.65 decrease when the alkoxy group becomes more electron withdrawing and increase when the X-substituent is made more electron withdrawing. This is inconsistent with a mechanism where proton transfer is rate limiting but in complete agreement with a concerted mechanism of acid catalysis as shown with the help of More O Ferrall-Jencks energy diagrams. 

There are two steps 1) a nucleophile attacks the ring to generate a Meisenheimer complex, followed by 2) loss of a leaving group to restore aromaticity. But inspect the product very carefully. It contains a phenolic proton, highlighted below ... 

For a strain of Rhodococcus erythropolis, Lenke and Knackmuss (35) described the initial reduction of the aromatic nucleus of picric acid (2,4,6-trinitrophenol) by a hydride ion (H"), yielding a hydride-Meisenheimer complex (Fig. 2). With the addition of a proton, the nitro group was subsequently released as nitrite and 2,4-dinitrophenol was produced. 

It is interesting that in his original report (1902) Meisenheimer described the preparation from 9-nitroanthracene and methoxide of an adduct which he formulated as 45. Confirmation of this structure has come from NMR measurements which show that the resonance of the proton at CIO shifts from — 8-93 to — 4-93 p.p.m. on complex formation consistent with a covalency change at this ring position (Foster et al., 1967). Again the NMR spectra (Foster et al., 1967 Fendler et al., 1968)... 

Abstract - This work is concerned with the origin of effects on rate equilibrium processes when reactions are performed in dipolar aprotic media such as dimethyl sulfoxide and dimethyl-formamide compared to typical hydroxylic media. An example of processes studied in our laboratory is the competition between proton abstraction and a-complex (Meisenheimer adduct) formation in the interaction of nitroaromatic compounds with basic systems (e.g. 1,3-dinitrobenzene in DMF-D20-Na0D). Other reactions studied are the base catalyzed isotopic exchange of D2 in aqueous DMSO mixtures, and also isotopic exchange of fluoroform. It has been found possible in certain cases to dissect the initial state and transition state contributions to the reaction rates from the kinetically measured enthalpies of activation and the thermodynamically evaluated enthalpies of transfer of the reactants. This procedure affords insight into transition state properties and behaviour. 

---