Steric activation in prototropic reactions of pyrazine derivatives. I. Polar and conformational effects

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We have used iodometry to study the enolization of acetone in water catalyzed by a series of general acids, comprised of hydrochloric acid, methanesulfonic acid, 24 aliphatic monocarboxylic acids, nine aromatic monocarboxylic acids, eight aliphatic dicarboxylic acids, and 20 monoanions of dicarboxylic acids. The log k–pK profile for unbuffered solutions of strong and moderately strong acids shows a maximum near pk ≈ 0. The Brønsted α value for a set of eight aliphatic monocarboxylic acids in which effects of bulk, charge, and polarizability are at a minimum is 0.56. Steric effects, probably augmented by polarizability effects in some cases, cause positive deviations from the Brønsted line drawn with respect to these standard acids. Anionic carboxylic acids are also more reactive than would be predicted from their equilibrium acid strengths, whereas cationic acids tend to be less reactive. Using D2O as solvent has only a small effect on the rate of carboxylic acid catalysis. Using acetone-d6 gives values of kH/kD in the range of 7.0–8.0 at 25 °C, values consistent with proton or deuteron being transferred between two bases of comparable strength, the carboxylate anion and the enol form of acetone. Keywords: general acid catalysis, enolization, Brønsted relation, steric effects, deuterium isotope effects.

Section: Steric and Polarizability Effectsmentioning

confidence: 94%

General acid catalysis in the enolization of acetone

Shelly¹,

Venimadhavan²,

Nagarajan³

et al. 1989

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“…The measurements of the rate of exchange for 1,2,3-trimethylpyrazinium iodide in D20 were made using nmr spectroscopy, essentially as previously described (1,4). For each compound two to four kinetic runs were performed at a given pH; in most cases the reproducibility of the derived second-order rate constants was -+ 10%.…”

Section: Methodsmentioning

confidence: 99%

“…We report herein the results of a study of the reaction of carboxylate ions, anilines, and pyridines with 1,2,3-trimethylpyrazinium ion (1).…”

Section: Ch3 Pmentioning

confidence: 99%

See 1 more Smart Citation

Steric activation in prototropic reactions of pyrazine derivatives. II. The Brønsted relation

Nagarajan¹,

Lee²,

Perkins³

et al. 1986

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The rate of deprotonation of the 2-methyl group in 1,2,3-trimethylpyrazinium ion by carboxylate, aniline, and pyridine bases has been measured in D20. Carboxylate ions containing bulky groups near the reaction centre, e.g. ortho benzoates, react faster than predicted by the Bransted equation that correlates the reactions of unhindered carboxylates. Anilines and pyridines, on the other hand, show conventional steric effects. A tentative explanation for the activation engendered by groups adjacent to the carboxylate centre is based on the known effect that high concentrations of organic electrolytes have on the strengths of carboxylic acids but not of amines. Since ortho carboxylate ions have their relative basicities increased by an alkyl-rich environment, it is argued that the reactive methyl groups of the substrate might provide such an interaction in the transition state.

“…We have shown previously that proton removal from activated methyl groups in a number of cyclic conjugated systems is catalyzed by general acids and bases and exhibits steric activation. Activation by adjacent alkyl groups in the substrate has been demonstrated in many cases (1)(2)(3)(4)(5) and in two instances (involving general base catalysis) by adjacent groups in the base (6,7). The latter reactions are the deprotonations of 1,2,3-trimethylpyrazinium ion and methylcreatininium ion by carboxylate ions (eqs.…”

Section: Introductionmentioning

confidence: 99%

Arylphosphonic acids. II. General acid and general base catalysis of acetone enolization

Shelly¹,

Nagarajan²,

Stewart³

1987

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We have measured the rate constants for the enolization of acetone catalyzed by 29 arylphosphonate dianions (ArP03'-) and by 20 arylphosphonic acids (ArP03H2). An excellent Bronsted correlation is found for the former reaction, with most ortho substituted compounds falling on the line drawn for the rneta and para compounds (P = 0.72). The largest deviation is found for o-iodo, whose small positive deviation is ascribed to a polarizability effect in the transition state. The arylphosphonic acids give a fairly good Broiisted plot ( a = 0.37) but here the ortho substituents tend to react slightly faster than would be expected on the basis of their equilibrium acid strengths. Catalysis by the monoanion ArP03H-is difficult to detect; such ions appear to be acting as general acids, not general bases, and do not appear to act as bifunctional catalysts; it is shown that protonating acetone is 3.4 X lo4 times as effective as deprotonating ArP03H-. The Bronsted coefficients (P) for the rate-controlling steps for the enolization of acetone by the principal routes are shown to be inversely related to the magnitude of the rate constants. KEVIN P. SHELLY, K. NAGARAJAN et Ross STEWART. Can. J. Chem. 65, 1734(1987.On a mesurC les constantes de vitesse pour l'knolisation de l'acCtone, catalysCe par 29 dianions arylphosphonates (~r P 0~~-) et par 20 acides arylphosphoniques (ArP03H2). On a obtenu une excellent correlation de Bronsted pour la premikre rkaction; la plupart des composCs substitues en ortho se retrouvent sur la courbe obtenue a I'aide des composCs meta et para (P = 0,72).On a trouvC que le derive o-iodo conduit a la plus grande deviation et on attribue sa petite deviation positive un effet de polarisabilite dans 1'Ctat de transition. Les acides arylphosphoniques ont conduit a une courbe de Bronsted assez bonne ( a = 0,37); toutefois, dans ce cas, les vitesses de reaction des composCs portant des substituants en position ortho sont generalement plus grandes que celles que l'on pourrait attendre en se basant sur les forces de leur Cquilibre acide. I1 est difficile de dCtecter la presence de catalyse par le monoanion ArP03H-; ces ions semblent agir par catalyse acide gCnCrale, non par catalyse basique generale, et ils ne semblent pas agir comme catalyseurs bifonctionnels; on dCmontre que la protonation de l'acttone est 3,4 X lo4 fois plus efficace que la deprotonation de ArP03H-. On demontre qu'il existe une correlation inverse entre l'amplitude des constantes de vitesse et les coefficients de Bronsted (P) des Ctapes contr6lant la vitesse d'tnolisation de l'acetone par le principales voies.[Traduit par la revue]