Quaternary Nitrogen Heterocycles. II. Kinetics of Reversible Pseudobase Formation from N-Methyl Heterocyclic Cations

Abstract

The kinetics of the formation and decomposition of the pseudobases from the 2-methyl-4-nitroisoquinolinium, 10-methylacridinium, and 10-methyl-9-phenylacridinium ions have been studied. The pH-rate profiles of these reactions indicate that for each of these ions, pseudobase formation may kinetically involve either attack of a water molecule or of hydroxide ion on the heterocyclic cation depending upon the pH of the reaction. Pseudobase decomposition to the cation may occur through either the neutral or protonated pseudobase species or their kinetic equivalents. The temperature dependences of the equilibrium and kinetics are reported for each ion, and deuterium isotope effects for the reactions of the 2-methyl-4-nitroisoquinolinium ion have been measured. Possible mechanisms for the reactions are discussed on the basis of the observed activation parameters and isotope effects and are compared with related reactions.L.es cinetiques de formation et de decomposition des pseudobases a partir des ions du methyl-2 nitro-4 isoquinolinium, methyl-10 acridinium et methyl-10 phknyl-9 acridinium ont etC Ctudiees. Les profils de vitesse en fonction du pH de ces reactions indiquent que pour chacun de ces ions, la formation de la pseudobase fait intervenir cinktiquement, soit l'attaque d'une molCcule d'eau soit un ion hydroxyde sur le cation hCtCrocyclique selon le pH de la reaction. La dCcomposition de la pseudobase en cation pourrait se produire soit par I'intermidiaire de la pseudobase sous forme neutre ou protonee ou encore leurs equivalents cinetiques. Les Cquilibres et les cinitiques en fonction de la temperature ont CtC rapportes pour chaque ion; les effets isotopiques du deuterium pour les rCactions de I'ion methyl-2 nitro-4 isoquinolinium ont ete mesurts. Les mecanismes possibles de ces reactions ont ete discutCs i partir des parametres d'activation observes et des effets isotopiques, puis compares aux reactions apparentees.[Traduit par le journal]Can. J . Chem., 51. 1965 (1973) Recently, we reported (1) the structures of the pseudobases (QOH) formed by the addition of hydroxide ion to various aromatic N-methyl heterocyclic cations (Q'), and also equilibrium constants for pseudobase formation (eq. 2; K, is the ionic product of water). We have nowcarried out a comprehensive study of the kinetics of pseudobase formation and decomposition for three such quaternary heterocyclic cations: the 10-methylacridinium2 (1; R = H), 10-methyl-9-phenylacridinium (1; R = Ph), and 2-methyl-4-nitroisoquinolinium (3) ions. The pH-rate profiles and temperature dependences 'For Part I, see ref.1. 'We previously referred to this cation as the 9-methylacridinium ion (1). In the IUPAC systematic nomenclature this is the 10-methylacridinium ion (2).of these reactions have been determined and have been used to interpret the reaction mechanisms. In addition, hydrogen isotope effects have been measured for the reactions of 3 and 4 and preliminary measurements of the catalytic effects of buffer species have been made. We have been unable, at t...

Substituent effects upon cation–pseudobase equilibration for isoquinolinium and phthalazinium cations

“…1 and 2. In all cases k,,, vs. pH passes through a minimum as observed in previous studies (10,19). Figures 1 and 2 As discussed previously (lo), k,,, may be converted into separate rate constants for pseudobase formation (k3 from the cation and pseudobase decomposition (k,) to the cation.…”

Kinetic and thermodynamic control of pseudobase formation from C-3 substituted 1-methylquinolinium cations

“…(15), and in this case eq. [8] predicts koH = 5000 M -' s-I, which is almost exactly tenfold greater than the experimentally observed value of 550 M -' s-' (15).…”

“…and decomposition (k,) were evaluated from the observed pseudo-first-order rate constants for cation-pseudobase equilibration by the standard algebraic methods described previously (10,15).…”

“…At pH < 10 the absorbance change becomes too small to allow accurate evaluation of these rate constants, while for pH > 11 this reaction becomes too rapid to allow investigation by the stopped-flow technique. These rate constants represent the rate of equilibration of 5 and 6, and may be used to evaluate the individual forward (kr for formation of 6) and reverse (kd for reversion of 6 to 5) rate constants in the usual way (15). In this pH range, kr proved to be linear in [-OH], allowing the secondorder rate constant for hydroxide ion attack at C-2 of 5: W = CN to be evaluated as 7.8 (k0.7) X loS M-' s-I, while kd = 98 + 8 s-I was pH independent.…”

Kinetics and mechanism of disproportionation and ferricyanide oxidation of the 10-methylacridinium cation in aqueous base

“…The only related reaction that we are aware of that allows a similar comparison of reactivity is the loss of hydroxide ion from the pseudobases to regenerate these heteroaromatic cations. In this case the 2-benzyl-5-nitroisoquinoline system is over 2000-fold more reactive than the 10-methylacridine species (10,17). On this basis it seems unlikely that both the above pseudobase alkoxide ion oxidations involve hydride abstraction by ferricyanide ion, although it is difficult to assess the possible role of steric differences in these reactions.…”