On the Mechanism of the Dimethyldioxirane Oxidation of σHAdducts (Meisenheimer Complexes) Generated from Nitroarenes and Carbanions

2000 ◽  
Vol 65 (4) ◽  
pp. 1099-1101 ◽  
Author(s):  
Waldemar Adam ◽  
Mieczyslaw Makosza ◽  
Cong-Gui Zhao ◽  
Marek Surowiec
Keyword(s):  
Meisenheimer Complexes

Electrochemical Preparation and Characterization of Meisenheimer Complexes of Trinitrofluorenone with Pyrrole, Indole, and Carbazole

2002 ◽  
Vol 14 (3) ◽  
pp. 1067-1074 ◽  
Author(s):  
Nobuo Ozawa ◽  
Hiroko Seki ◽  
Takashi Kitamura ◽  
Hiroshi Kokado ◽  
Tsutomu Ishikawa ◽  
...  
Keyword(s):  
Electrochemical Preparation ◽  
Meisenheimer Complexes

Reactions of the ambident super-electrophile series: the 2-(nitroaryl)-4,6-dinitrobenzotriazole 1-oxides with isopropoxide ion. Pathways of decomposition of the anionic σ-adducts

1994 ◽  
Vol 72 (1) ◽  
pp. 218-226 ◽  
Author(s):  
Julian M. Dust ◽  
Erwin Buncel
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Magnetic Resonance Spectroscopy ◽  
Nuclear Magnetic Resonance Spectroscopy ◽  
Adduct Formation ◽  
Resonance Spectroscopy ◽  
Energy Profiles ◽  
Meisenheimer Complexes ◽  
Bond Scission ◽  
Relative Selectivity

To elucidate the reactivity of super-electrophiles such as 4,6-dinitrobenzofuroxan as compared to normal electrophiles such as 1,3,5-trinitrobenzene, reaction of isopropoxide ion (iPrO−) with a series of ambident super-electrophiles was studied by 400 MHz 1H nuclear magnetic resonance spectroscopy. The 2-(nitroaryl)-4,6-dinitrobenzotriazole 1-oxides, 1–3, possess both a super-electrophilic (C-7) site and a normal electrophilic (C-1′) site. Nucleophiles can demonstrate selectivity for attack at C-7, which leads to formation of persistent anionic σ-adducts (Meisenheimer complexes), as compared to C-1′, which leads to N-2:C-1′ bond scission. The most reactive substrate, 2-(2′,4′,6′-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide (Pi-DNBT, 1) was found to be the least selective substrate in C-7 adduct formation, while 2-(2′,4′-dinitrophenyl)- and 2-(4′-nitrophenyl)-4,6-dinitrobenzotriazole 1-oxides (DNP-DNBT, 2, and NP-DNBT, 3, respectively) showed increasing selectivity towards iPrO−, in turn. These results are discussed on the basis of overall selectivity for C-7 adduct formation and the relative selectivity of iPrO− as compared to methoxide and tert-butoxide ions. The conclusions are illustrated using comparative energy profiles. In terms of pathways for decomposition of the adducts, the C-7 adducts decompose via dissociation back to substrate and nucleophile and, thence, through C-1′ adduct formation to the scission products. However, for 1, the C-7 adduct 1a has now been found to decompose to 7-isopropyl-2-picryldinitrobenzotriazole, 1c. The possible mechanism of this formal internal redox will be discussed.

AM1 studies on the stabilities of anionic σ-complex regioisomers: thermodynamics of regioselectivity in the reaction of methide, methoxide, and hydroxide anions with electron-deficient aromatics

1994 ◽  
Vol 72 (7) ◽  
pp. 1709-1721 ◽  
Author(s):  
Erwin Buncel ◽  
Richard M. Tarkka ◽  
Julian M. Dust
Keyword(s):  
Gas Phase ◽  
Global Minimum ◽  
Condensed Phases ◽  
Am1 Calculations ◽  
Calorimetric Data ◽  
Electron Pairs ◽  
X Ray ◽  
Carbon Nucleophiles ◽  
Meisenheimer Complexes ◽  
Thermochemical Calculation

Heats of formation (ΔHf) for a series of aromatics that are progressively more electron deficient (benzene, 6; nitrobenzene, 7; 4-fluoronitrobenzene, 8; 1,3-dinitrobenzene, 9; 2,4,6-trinitroanisole, 2; and 1,3,5-trinitrobenzene, 1) were determined by semiempirical AM1 calculations. As a probe of the factors that govern the regioselectivity exhibited in the formation of anionic σ-adducts (Meisenheimer complexes), experimental gas-phase ΔHf values for the prototypical oxygen and carbon nucleophiles (hydroxide, methoxide, and methide anions) were used in a thermochemical calculation along with the calculated ΔHf of the electrophiles and the adducts to determine the heats of complexation (ΔHc). The present results show that for the series of nitroaryl electrophiles, 7, 9, and 1, hydroxide and methide anions exhibit the same regioselectivity based on thermodynamics of Meisenheimer complex formation. Specifically, Meisenheimer complexes derived from attack at a position para to at least one nitro group (designated MC-4) are formed with the greatest exothermicity (ΔHc). Exothermicity of complexation increases for both hydroxide and methide adduct formation as the number of nitro groups in the electrophile is increased, from 7 to 9 and to 1, but formation of the methide adducts occurs uniformly with greater exothermicity than that of hydroxide adducts. These results are considered in light of solution calorimetric data that quantify adduct stability in condensed phases. Surprisingly, it is found that regioselectivity inverts for CH3−as compared to OH−and CH3O−in complexation with 2,4,6-trinitroanisole, 2. Thus, while methoxide and hydroxide form adducts at C-1 of TNA with higher exothermicity than at C-3, methide preferentially forms an adduct at C-3 according to the same enthalpy criterion. These results arise from the degree of stereoelectronic stabilization that may be imparted to the respective Meisenheimer complexes formed from ipso attack, that is, the adducts (MC-1) that are geminally disubstituted with electronegative heteroatom groups. For the methoxide MC-1 of TNA, 2, full stereoelectronic stabilization is provided by n–σ* donation from nonbonding electron pairs of the acetal-like methoxyl moieties to suitable C—O acceptor bonds. However, the methide moiety of the comparable MC-1 of TNA cannot partake in such an interaction and, so, with methide, MC-3 formation is preferred over MC-1. Further evidence is provided by consideration of the two energy minima obtained from optimization of the geometry of the oxygen-centred adducts formed by attack of methoxide at C-1 of TNA, 2. In the presence of a point charge that simulates an ion-paired cation, an "M-shaped" conformer is favoured for MC-1, while in the absence of a counterion the "S-shaped" conformer is favoured. Without a complexing counterion M and S conformers are both local minima, while the "S" conformer constitutes the global minimum. The AM1 optimized structure for the "M" conformer compares favourably to published X-ray data. The greater exothermicity of formation of the "S" conformer in the absence of the counterion is indicative of stereoelectronic stabilization of the O-adduct. The geometry is rationalized as a result of minimizing steric repulsion and maximizing the n-σ* stabilization of the C-1 adduct.

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