The reactions of two σ-anionic compounds (Jackson–Meisenheimer adducts) in liquid ammonia studied by<sup>13</sup>C nuclear magnetic resonance spectroscopy

Chudek, J. A.; Foster, R.; Marr, A. W.

doi:10.1039/p29870001341

Cited by 7 publications

(2 citation statements)

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“…A likely interpretation is that 2,4-dinitroaniline is attacked by a solvent molecule to form stable Meisenheimer complexes (Scheme ), which are written as anions rather than zwitterions, as we have shown that aminium ions exist as free bases in liquid ammonia . The formation of Meisenheimer complexes between highly activated benzenes and nucleophiles is well-known, and has been shown spectroscopically, including NMR . The UV−vis absorption of a typical Meisenheimer complex is in the range 450−600 nm, and therefore often color is observed with the formation of the σ-adducts.…”

Section: Resultsmentioning

confidence: 90%

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

Atherton

Page

2011

J. Org. Chem.

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The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pK(a) of the substituted phenols in liquid ammonia, the Brønsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second-order rate constants generate a Brønsted β(nuc) of 0.36 using either the pK(a) of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.

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Section: Resultsmentioning

confidence: 90%

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

Atherton

Page

2011

J. Org. Chem.

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“…The Δδ(H) values lie in the range −0.61, −0.79 ppm, comparing well with the proton shifts observed in the ionization of various 2,4,6-trinitrobenzyl derivatives, including 2,4,6-trinitrotoluene (TNT) (Δδ(H 3,5 ) = −0.74 ppm), , (3) the resonances of the C 2,6 and C 4 carbons, especially the latter, move markedly to high field, whereas those of the C 3,5 carbons move slightly to low field on going from 3a−c to C-3a−c : Δδ(C 2,6 ) ≈ −10; Δδ(C 4 ) ≈ −18; Δδ(C 3,5 ) ≈ +1. Interestingly, the situation is reminiscent of the one reported in 13 C studies of the formation of the related carbanion C-5 of 2,4,6-trinitrophenylacetone as well as of that of picryl σ-adducts of general structure 5 according to eq 9. − In these latter instances, SCFMO calculations have indicated that the σ-complexation results in increases in π-electron density at the 2, 4, and 6-ring positions as well as on the NO 2 substituents and in decreases at the 3- and 5-positions, accounting well for the observed variations in the carbon chemical shifts of the cyclohexadienyl ring: Δδ(C 2,6 ) ≈ −17, Δδ(C 4 ) ≈ −29, Δδ(C 3,5 ) ≈ 2. , In addition, the calculations confirmed the major role of the paraquinoid structure 6B in the delocalization of the negative charge of the adducts…”

Section: Discussionmentioning

confidence: 71%

Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

et al. 1996

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The kinetics of the reversible deprotonation of 4-X-substituted picrylacetophenones 3a-c (X ) NO 2 , H, MeO) by a variety of bases have been measured in 50% H 2 O-50%Me 2 SO (v/v) at 25°C. Comparison of Bronsted B values for the ionization of each carbon acid by phenoxide and carboxylate bases and R CH values for deprotonation of 3a-c by individual buffers bases indicates that the reaction proceeds through strongly imbalanced transition states. The intrinsic reactivities of 3a-c, as determined from the Bronsted plots for phenoxide ion reactions, are typical for the formation of resonance-stabilized polynitrobenzyl-type carbanions, but the intrinsic rate constant k 0 decreases regularly on going from the less acidic p-methoxyacetophenone derivative 3c to the more acidic p-nitroacetophenone derivative 3a. This trend is attributed to the fact that the contribution of the benzoyl moiety to the resonance stabilization of the resulting carbanion C-3 is completely negligible for the p-methoxy compound but not for the p-nitro compound. An extensive 1 H and 13 C NMR study of the ionization of 3a-c confirms this proposal, with steric hindrance to rotation of the picryl ring around the C R -C ipso linkage being observed at the probe temperature in the p-methoxysubstituted carbanion C-3c, at -40°C for the unsubstituted carbanion C-3b, but not at all for the p-nitro carbanion C-3a. A major finding, however, is that the three carbanions undergo protonation at the p-nitro group of the picryl ring to form nitronic acids in acidic media. This behavior clearly shows that charge delocalization through the 2,4,6-trinitrophenyl moiety is predominant in the three systems, including C-3a.

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Structure and Reactivity of Anionic α‐Complexes

Terrier

2013

Modern Nucleophilic Aromatic Substitution

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The reactions of two σ-anionic compounds (Jackson–Meisenheimer adducts) in liquid ammonia studied by¹³C nuclear magnetic resonance spectroscopy

Cited by 7 publications

References 4 publications

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

Structure and Reactivity of Anionic α‐Complexes

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The reactions of two σ-anionic compounds (Jackson–Meisenheimer adducts) in liquid ammonia studied by13C nuclear magnetic resonance spectroscopy

Cited by 7 publications

References 4 publications

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

Structure and Reactivity of Anionic α‐Complexes

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The reactions of two σ-anionic compounds (Jackson–Meisenheimer adducts) in liquid ammonia studied by¹³C nuclear magnetic resonance spectroscopy