Rate and equilibrium constants of transfer of the dimethylcarbamoyl group between substituted pyridines and their N-oxides

Abstract

Transfer of the dimethylcarbamoyl group from N-acyloxypyridinium salts to pyridines and from N-acylpyridinium salts to pyridine N-oxides was studied in acetonitrile. Equations relating the reaction rates and equilibria in the N3O and O3N acyl transfer series to the basicity of the nucleophile and leaving group were obtained. The reactions all are one-stage and occur by the forced concerted SN2 mechanism. The intersecting state model was used to obtain a modified Marcus equation that accounts for the asymmetry of the transition state with respect to reagents and products and allows uncontradictory analysis of the reaction mechanism.Previously we obtained kinetic and equilbrium data for reaction (1) of transfer of the dimethylcarbamoyl group from N-acylpyridinium salts to pyridines [1] and from N-acyloxypyridinium salts to pyridine N-The reaction mechanism was found out, and the reaction rate and equilibrium constants in these symmetrical reaction series were shown to adhere the Brønsted relation with b 0.5. At the same time, we found that the quality of reactivity predictions is much higher provided characteristics of identical reaction and the Marcus equation are used [2].In the present work we turned to unsymmetrical reactions (1) in which the carbamoyl group is transferred from N-acylpyridinium salts to peridine Noxides and, vice versa, from N-acyloxypyridinium salts to pyridine and substituted pyridines.In CH 3 CN we studied reactions of acylonium salts with nucleophiles (X = BPh 4 and Nu i , Nu j = Nu 1 3 Nu 20 ).The rate and equilibrium constants of reaction (1) in acetonitrile at 298 K are given in the table. The progress of reactions nos. 135,8,9, 14317, 20323, 36, 38, 40, and 53 was followed by UV spectroscopy at an optimal wavelength under pseudo-first-order reaction conditions at a tenfold or greater excess excess of acylonium salt or nucleophile. Reactions (41)3(44) were studied in a similar way by Fourier transform IR spectroscopy (FT-IR), following the decrease or increase of the intensity of absorption of acylonium salts in reactants and products, respectively.The corresponding n C=O values were as follows, cm !1 : 1797 (i = 5), 1789 (i = 6) 1787 (i = 7), and 1738 (i = 12). The rates of fast reactions nos. 6, 7, 18, 19, 24, 25, 31, 32, and 47352 were measured by the stoppedflow method. The second-order rate constants k 2 were calculated by Eq. (2) relating the apparent rate constants k ap (336 values) to the concentration of excess reagent by the least-squares method (k rev is the rate constant of the reverse reaction).(2) Treatment of kinetic data by Eq.(2) showed that k 2 >> k rev , whereas k rev~0 . Therefore, k rev values were not used in assessing the rate of the reverse reaction. The concentration of acylonium salts in acetonitrile in the kinetic experiments with IR monitoring was up to 10 !2 M and in the other cases it was not higher then 5 0 10 !3 M. Previously we showed [3] that under these conditions the dissociation degree of ionic dissociation of acylonium tetrafluoroborates in aceton...

Structure and characteristics of the ionic forms of heteroarenium salts in organic solvents (review)

“…In a series of kinetic investigations of the reactions of pyridinium salts under the influence of arylamines only the order of the dissociation constants was used; the range of the constants made it possible to consider that the salts were almost completely dissociated under the conditions of the kinetic experiment [43,[78][79][80][81][82][83]; however, their values were not used in the kinetic calculations in all cases, and the role of the other ionic associates under the working conditions was disregarded.…”

“…In [79][80][81][82][83][84][85] the transfer of acyl groups (the rate and equilibrium constants) from the respective N-and O-acylonium salts to the N-oxides of pyridines, highly basic pyridines, and N-methylimidazole in acetonitrile solutions (298 K) was studied: At low concentrations no higher than 5·10 −3 M and with the corresponding dissociation constants (in the order of 10 −2 mol/l) it was assumed that the degree of dissociation is α ≥ 0.7 and that only solvated cations of the pyridinium derivatives take part in the processes. Under such conditions the reaction takes place in one stage by a concerted S N 2 mechanism [80,81,83].…”

“…Under such conditions the reaction takes place in one stage by a concerted S N 2 mechanism [80,81,83].…”

Bifunctional sulfur-silicon podands as new nucleophilic ionophores in acyl transfer reactions. Influence of monovalent cations on the reaction kinetics

“…For the last few years, this group has conducted research on the mechanism of the acyl transfer reaction [17][18][19][20][21][22][23][24][25]. By systematically investigating the formation of the guest-host type between multiple nucleophiles and different ions, a particularly strong effect of alkali metal ions on the kinetics of the acyl group transfer reaction was observed [26].…”

Rate and Equilibrium Constants of Benzoyl Group Transfer between Pyridine N-Oxides

“…This fact is rationalized as formation on the reaction coordinate of the corresponding acylonium salt [3]. We formerly investigated the structure of some benzoylonium salts [4,5] and estimated the kinetic and thermodynamic characteristics of formation reactions for a series of N-benzoyloxypyridinium and N-benzoylpyridinium salts [5,6]. The present study concerns the reactivity of these compounds in the exchange processes of the type (1).…”