Interaction of heterocycles and nucleophiles: Sigma complexes formed from methoxide ion and some 5-nitropyrimidines and comparison with those from 2,4,6-trinitroanisole

1969 ◽  
Vol 22 (12) ◽  
pp. 2561 ◽  
Author(s):  
MEC Biffin ◽  
J Miller ◽  
AG Moritz ◽  
DB Paul
Keyword(s):  
Complex Formation ◽  
Aprotic Solvent ◽  
Reaction Coordinate ◽  
Thermodynamic Control ◽  
Sigma Complexes ◽  
Protic Solvents ◽  
Lower Symmetry ◽  
Selective Deuteration ◽  
Sigma Complex ◽  
Methoxide Ion

The sigma complexes formed from the reaction of methoxide ion with 6- nitro-pyrimidine and its monomethoxy derivatives have been examined spectroscopically. Structural ambiguities resulting from the lower symmetry of the nitropyrimidines compared with the trinitrobenzenes have been resolved by selective deuteration. Comparison is made with sigma complex formation from 2,4,6-trinitroanisole and the question of kinetic against thermodynamic control is considered with reference to theoretically derived P.E.-reaction coordinate profiles for protic solvents. The calculations are extended qualitatively to consider reactions in an aprotic solvent.

Interaction of heterocycles and nucleophiles. II. Sigma complexes formed from methoxide ion and Methoxy-3,5-dinitropyridines in dimethyl sulphoxide

1970 ◽  
Vol 23 (5) ◽  
pp. 957 ◽  
Author(s):  
MEC Biffin ◽  
J Miller ◽  
AG Moritz ◽  
DB Paul
Keyword(s):  
Dimethyl Sulphoxide ◽  
No Conversion ◽  
Solvation Effects ◽  
Sigma Complexes ◽  
Sigma Complex ◽  
Methoxide Ion ◽  
Rearrangement Reactions

Contrasting behaviour is observed when 2- and 4-methoxy-3,5-dinitropyridine interact with methoxide ion in dimethyl sulphoxide. The 4-methoxy compound affords both methine and acetal sigma complexes, the latter being thermodynamically more stable. The interconversion is catalysed by methanol. Labelling experiments have established that the sigma complex from the 2-methoxypyridine is formed by addition at C6; no conversion into the acetal could be effected. These observations are rationalized in terms of differential steric and solvation effects. Demethylation and rearrangement reactions of 4-methoxy-3,5-dinitropyridine are reported.

Anion effect on alkali ion-crown complex formation in a moderately concentrated solutions: A sensitive probe of hidden interionic interactions operating in dissociating protic solvents

1990 ◽  
Vol 55 (5) ◽  
pp. 1149-1161
Author(s):  
Jiří Závada ◽  
Václav Pechanec ◽  
Oldřich Kocián
Keyword(s):  
Hydrogen Bond ◽  
Complex Formation ◽  
Charge Density ◽  
Macrocyclic Ligand ◽  
Simple Explanation ◽  
Anion Effect ◽  
Protic Solvents ◽  
The Individual ◽  
Alkali Salts ◽  
Alkali Ion

A powerful anion effect destabilizing alkali ion-crown complex formation has been found to operate in moderately concentrated protic (H2O, CH3OH, C2H5OH) solution, following the order HO- > AcO- > Cl- > Br- > NO3- > I- > NCS-. Evidence is provided that the observed effect does not originate from ion-pairing. A simple explanation is provided in terms of concordant hydrogen bond bridges of exalted stability between the gegenions, M+···OR-H···(OR-H)n···OR-H···A-. It is proposed that encapsulation of alkali ion by the macrocyclic ligand leads to a dissipation of the cation charge density destroying its ability to participate in the hydrogen bond bridge. An opposition against the alkali ion-crown complex formation arises accordingly in the solution in dependence on strength of the hydrogen bridge; for a given cation, the hydrogen bond strength increases with increasing anion charge density from NCS- to HO-(RO-). It is pointed out, at the same time, that the observed anion effect does not correlate with the known values of activity coefficients of the individual alkali salts which are almost insensitive to anion variation under the investigated conditions. As a resolution of the apparent paradoxon it is proposed that, in absence of the macrocyclic ligand, the stabilizing (concordant) bonding between the gegenions is nearly balanced by a destabilizing (discordant) hydrogen bonding between the ions of same charge (co-ions). Intrinsic differences among the individual salts are thus submerged in protic solvents and become apparent only when the concordant bonding is suppressed in the alkali ion-crown complex formation.

scholarly journals KINETIC STUDY ON ANIONIC σ-COMPLEX FORMATION BETWEEN 2,4,6-TRINITROANISOLE AND METHOXIDE ION BY HIGH PRESSURE STOPPED-FLOW METHOD

1979 ◽  
Vol 8 (6) ◽  
pp. 671-674 ◽  
Author(s):  
Noboru Takisawa ◽  
Muneo Sasaki ◽  
Fujitsugu Amita ◽  
Jiro Osugi
Keyword(s):  
High Pressure ◽  
Complex Formation ◽  
Kinetic Study ◽  
Stopped Flow ◽  
Flow Method ◽  
Methoxide Ion

Meisenheimer Complex Formation Between 1,3,5-Trinitrobenzene and Hydroxide Ion. Equilibrium Constants for the Dimethylformamide–Water Solvent System

1972 ◽  
Vol 50 (11) ◽  
pp. 1729-1733 ◽  
Author(s):  
E. A. Symons ◽  
E. Buncel
Keyword(s):  
Complex Formation ◽  
Aqueous Medium ◽  
Equilibrium Constant ◽  
Equilibrium Constants ◽  
Solvent System ◽  
Medium Composition ◽  
Hydroxide Ion ◽  
Spectrophotometric Methods ◽  
Water Solvent ◽  
Sigma Complex

Sigma-complex formation between 1,3,5-trinitrobenzene (TNB) and hydroxide ion has been studied quantitatively as a function of medium composition for part of the dimethylformamide (DMF)–water solvent system by spectrophotometric methods. Only a 1:1 complex is detected under the conditions of measurement, with [TNB] ≥ [OH−]. The equilibrium constant (Keq) for complex formation in 22 mol % DMF has the value 3 × 10−3 l mol−1, compared with 3 l mol−1 in purely aqueous medium. Further increases in Kcq occur as the DMF content of the medium is raised; in 50 mol % DMF Keq ≈ 105, but reliable Keq values could not be obtained in this region of medium composition. The increase in Keq with increasing DMF content is interpreted largely on the basis of hydroxide ion desolvation.

N-Methylpicramide–methoxide ion interactions: proton abstraction versus σ-complex formation

1980 ◽  
Vol 58 (16) ◽  
pp. 1615-1620 ◽  
Author(s):  
Erwin Buncel ◽  
Masashi Hamaguchi ◽  
Albert R. Norris
Keyword(s):  
Complex Formation ◽  
Hydrogen Ion ◽  
Proton Abstraction ◽  
Ring Carbon ◽  
Bearing Ring ◽  
Ion Interactions ◽  
Methoxide Ion ◽  
First Time ◽  
Conjugate Base ◽  
Sole Product

The interactions of N-methylpicramide (NMP) with methoxide ion in dimethylsulfoxide–methanol media and with 1,4-diazabicyclooctane in dimethylsulfoxide have been investigated spectrophotometrically at 25.0 °C. Dimethylsulfoxide has been found to stabilize the conjugate base of NMP, formed via loss of hydrogen ion from the methylamino group, with respect to the anionic σ-complex generated via addition of methoxide ion to a ring carbon position. As a result, methanol–dimethylsulfoxide solutions rich in dimethylsulfoxide (i.e., 95/5, v/v, DMSO/MeOH) contain the conjugate base of NMP as the sole product of the 1:1 interaction of NMP and methoxide ion. At mole ratios of methoxide ion to NMP > 1 a second interaction results, yielding a dianion formed via addition of methoxide ion to one of the H-bearing ring carbon atoms of the conjugate base. Values of absorption maxima and molar absorptivities at the absorption maxima for the conjugate base of NMP and the dianion are reported here for the first time.

Sign in / Sign up

Export Citation Format

Share Document