N. G. Faleev

The solvation and nucleophilicity of carbanions : Kinetics and the mechanism of condensation of nitro compounds with acetaldehyde in dimethylsulfoxide

Abstract The kinetics and mechanism of the condensation of nitro compounds with a carbonyl group were investigated in DMSO, the addition of 1,1-dinitroethane to acetaldehyde being used as a model reaction. The acid-catalysed addition of the nitro compound anion to the CO group is the rate-limiting stage of the process. Equilibrium and rate constants for this particular stage were determined for 1,1-dinitroethane and methyl-α-nitropropionate anions which characterise the carbon basicities and carbon nucleophilicities of these anions. The correlation between the basicities and nucleophilicities of the two anions in DMSO is “normal”. The more basic anion reacts faster than the less basic one. It is suggested that the unusual correlation between the basicities and nucleophilicities of nitro compound anions previously observed in water is due to the specific solvation of anions by H-bonds.

Influence of factors of specific solvation on the acidity of nitro compounds

1. The dissociation constants of 1,1-dinitroethane, the methyl ester of nitroacetic acid, and the methyl ester ofα-nitropropionic acid were measured in dimethylformamide and dimethylsulfoxide at 25°. 2. The differentiating action of aprotonic dipolar solvents in comparison with protonic solvents on the acidity of various classes of nitro compounds was demonstrated. 3. The energy of the bathochromic shifts, observed in the UV spectrum of the anion of the nitro compound in the transition from a protonic to an aprotonic solvent, varies in proportion to the basicity of the anion in the aprotonic solvent. 4. The change in the acidity of nitro compounds in the transition from protonic to aprotonic solvent is determined chiefly by the specific solvation of the anions by hydrogen bonds in the protonic solvents.

The leveling influence of dimethyl sulfoxide on the acidity of mononitroalkanes

1. The acidity constants of nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane were determined in dimethylsulfoxide at 20–23°. 2. In protonic solvents the anomalous influence of alkyl groups on the acidity of mononitroalkanes is substantially intensified by the action of solvation effects.

Realization of a β-carbocation center in the amino-acid fragment of the bis(N-salicylideneaminoacidato) cobaltate(III) ion

1. Hydrolysis of the diacyl derivatives of bis(N-salicylidene-α-hydroxyalaninato)cobaltate(III) leads to monomolecular cleavage of the acyloxy group with formation of carbocation center at theα-C atom. 2. The stereospecificity of the attack of water on the carbocation center is largely determined by the complex configuration, the effect of the configuration of the neighboring ligand being negligible, at least to within the limits of experimental error.

Epimerization of the diastereomers of bis-(N-salicylidene-α-hydroxyalaninato)cobalt (III) under the action of bases and the thermodynamic stereoselectivity in these complexes

1. The diastereomers of bis-(N-salicylidene-α-hydroxyalaninato)-cobalt(III) undergo epimerization under the effect of bases. A mechanism has been proposed for this process. 2. The equilibrium relationships for the three diastereomers of bis-(N-salicylidene-α-hydroxyalaninato) cobalt(III) have been determined in water and acetonitrile. In both solvents the isomer containing ligands with opposite configurations is relatively unfavorable. This demonstrates the need to take into account the interaction between the distant asymmetric centers of the ligands.

Oxidative deamination of the alanine ligand by air oxygen in stereochemically inert bis-(N-salicylidenealaninato)cobaltate(III) complexes

The optically active potassium Λ(SS)-bis-(N-salicylidenealaninato)cobaltate(III)(I) is oxidised by air in water at pH 11·0 to a mixture of the Λ(SS)-,Λ(RR)-, and Λ(SR)-diastereoisomers of the α-hydroxy-alaninatocobaltate.

Synthesis, separation, and some chemical properties of diastereomers of bis-(N-salicylidene-α-hydroxyalaninato)cobalt (III)

1. bis-(N-Salicylidene-α-hydroxyalaninato)cobalt (III) was synthesized by the oxidation of bis-(N-salicylidenealaninato)cobalt (III). Three possible diastereomers of this product were isolated. 2. Theα-hydroxy group of bis-(N-salicylidene-α-hydroxyalaninato)cobalt (III) undergoes exchange in dilute acid solutions, which leads to the epimerization of the diastereomers in water, and to a mixture of the diastereomers of bis-(N-salicylidene-α-ethoxyalaninato)cobalt (III) in ethanol. 3. It was shown that theoretically bis-(N-salicylidene-α-hydroxyalaninato)cobalt (III) can be synthesized by the reaction of pyruvate and salicylaldehyde with Co(OH)3 in the presence of excess ammonium salt.

Specific solvation of carbanions of nitro compounds as the cause of their anomalous nucleophilicity in water

1. The kinetics of the reversible addition of the methyl ester ofα-nitropropionic acid (MNP) to acetaldehyde in dimethyl sulfoxide (DMSO) under the action of the components of a triethylammonium buffer solution were studied. 2. The equilibrium constant of the addition of MNP to acetaldehyde in DMSO was determined, and the value previously obtained for this constant in water was refined. 3. The rate constant of the nucleophilic addition of the carbanion of nitropropionic ester to acetaldehyde, catalyzed by the triethylammonium ion, was found. 4. The anomalous nature of the relationship of the basicity and nucleophilicity of the carbanions of the nitro compounds with respect to carbon, observed in water, is a consequence of solvation effects.

General acid catalysis of the addition of the carbanion of 1,1-dinitroethane to acetaldehyde in dimethyl sulfoxide

1. The kinetics and mechanism of the reversible addition of 1,1-dinitroethane to acetaldehyde in dimethyl sulfoxide, catalyzed by bases and acids, were studied. 2. The limiting step of the process is the acid-catalyzed addition of the carbanion of 1,1-dinitroethane to acetaldehyde. 3. The acidity constants of dichloroacetic, monochloroacetic, and succinic acids were determined in dimethyl sulfoxide at 20°C.

Relationship between the reactivity of aliphatic nitro-compounds in protolytic reactions and reactions op nucleophilic addition to the carbonyl group

1. The presence of a correlation between the basicity with respect to hydrogen of carbanions of nitrocompounds and their basicity with respect to carbon in the reaction series of acetaldehyde and formaldehyde was demonstrated. 2. The basicity of carbanions with respect to carbon in the series of acetaldehyde is two orders of magnitude less than the basicity of the same carbanions in the series of formaldehyde. This phenomenon was explained from the standpoint of differences in the steric stress in the molecules of the nitroalcohols formed by condensation of the nitro-compounds with acetaldehyde and formaldehyde. 3. The nucleophilicity of anions of nitro-compounds with respect to carbon varies correspondingly to the nucleophilicity of these anions with respect to hydrogen.