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Abstract:The acidity and ket-no1 tautomerism of a series of symmetrical P-diketones (RCOCH2COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl(4, 3-(N-methy1)pyridinio (e), and 4-(Nrnethy1)pyridinio ( f ) ) and two series of unsymmetrical P-diketones (RCOCH2COCH, (7a-7fi and RCOCH,COC6H5 (8a4f )) have been investigated in aqueous solution at 25°C and ionic strength 0.1. Values of p K 2 were measured spectrophotometrically, and the acidities of the enols ( p m were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolate conjugate bases. These data in tum allowed the calculation of the acidities of the keto tautorners (p@) and the equilibrium constants for enolization (K, = [Enoll/ [Keto]). In general, KE is greater for the symmetrical ketones (1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). K, is much more sensitive to the nature of the R substituent in these three series of P-diketones than in the corresponding series of P-keto esters and amides. Correlations between for 8 and 7 combined with the known acidities of the corresponding P-keto esters and arnides provide the first accurate estimates of the acidities of dirnethyl malonate (pKa = 13.0) and malonarnide (pKa = 12.5) in aqueous solution.Key words: acidity, tautomerisrn, P-diketones. (8a4f)). On a rnesuri les pKaq spectrophotomCtriquement et on a dCterminC les acidids des Cnols ( p e ) par une analyse de la relation entre la dCpendance sur le pH de la catalyse du tampon sur la protonation par catalyse acide gCnCrale des bases conjuguCes Cnolates. Ces donnCes permettent ensuite de calculer les aciditCs des cCto-tautomtres et les constantes d9Cquilibre de 1'Cnolisation (KE = [Bnol:~/[~~tone]). En gCnCral, la valeur de KE est plus grande pour les cCtones symCtriques (1) que pour les cCtones non symttriques correspondantes substituies pardes groupes R (7 et 8). La valeur de KE est beaucoup plus sensible la nature du substituant R dans ces trois sCries de P-dicttones que dans les sCries correspondantes de P-cCtoamides. Des corrClations entre les valeurs des des composCs 7 et 8 cornbinCes avec les aciditCs connues des p-cktoesters et P-cCtoamides correspondants foumissent les prernisres Cvaluations prCcises des aciditks du malonate de dimCthyle (pKa = 13,O) et du malonarnide (pKa = 12,5) en solution aqueuse.

Second-order rate constants have been measured (aqueous solution, I = 1.0, 25 °C) for the hydroxide ion catalyzed elimination reactions of 12 7V-(2-(4-nitrophenyl)ethyl)pyridinium cations (3) bearing a variety of substituents in the pyridine ring. Bronsted plots as a function of the basicity of the pyridine leaving group are concave-down, which is consistent with a change in rate-determining step within an ElcB mechanism. These plots are characterized by ftg = -0.17 for the rate-determining deprotonation for < 6.5, and ftg = -0.39 for the rate-determining expulsion of the pyridine nucleofuge from the carbanionic intermediate for pÁfBH > 6.5. Elimination reactions in basic D20 occur without any significant incorporation of deuterium into the 4-nitrostyrene product, and require the presence of a hydrogen-bonded carbanionic intermediate in which nucleofuge expulsion occurs faster than exchange of hydrogen-bonding water molecules. Rate-determining deprotonation in these elimination reactions occurs 50-fold more slowly than for the corresponding reactions of the 7V-[2-(4-nitrophenyl)ethyl]quinuclidinium cations that have also been reported to have ft, = -0.17, but which do not show an analogous change in the rate-determining step upon variation of the nucleofuge basicity. The analogous elimination of the l-methyl-3-[2-(4-nitrophenyl)ethyl]imidazolium cation occurs a further 30-fold more slowly than that predicted for 3 having a pyridine leaving group of the same basicity as 1-methylimidazole. The ElcB reactions of 3 are similar to the analogous reactions of A'-(2-cyanoethyl)pyridinium cations(1) in displaying a change in the rate-determining step with nucleofuge basicity; however, the ftg values for 1 and 3 are quite different for both and k2/k"t.We have recently reported1 a detailed kinetic study of the hydroxide ion catalyzed elimination of pyridines from 7V-(2cyanoethyl)pyridinium cations (1) in aqueous solution for pyridine leaving groups having pAfBH in the range 1.5-9.7. Br0nsted plots as a function of pAfBH showed a clear change in the rate-determining step within an ElcB mechanism. Thus, ftg = -0.30 for rate-determining deprotonation (i.e. (ElcB)irrev or AxhDHl + DN)2 for pAfBH < 5.8 is transformed into ftg = -0.93 for rate-determining leaving group expulsion from the carbanionic intermediate (i.e. (ElcB)rev or AxhDH + DN<) for pAfBH > 5.8. X ;n-ch2ch2cn * ] < + CH2=CHCN + H20 (1)A closely related reaction, the hydroxide ion catalyzed elimination of quinuclidines from Ar-(2-(4-nitrophenyl)ethyl)quinuclidinium cations (2), showed3 4"5 no such change in the rate-determining step with leaving-group basicity, although other evidence firmly established the ElcB mechanism for these reactions. Rate-determining deprotonation, with ftg = -0.17, was found for quinuclidine leaving groups having pKm covering the range 3-11. X :n-ch2ch2c6h4no2 *oh["OH] + CH2=CHC6H4N02 + H20 (2) appears to implicate the quite different activating groups (cyano

. Can. J. Chem. 70, 1195 (1992). The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,Nf-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25°C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Bronsted-type plots of the second-order rate constants (kc,,) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E l c b reaction mechanisms. For 1, the Bronsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pK,, < 5.16 (slope = -0.30), and a change in rate-determining step to bromide ion departure for pKBH > 5.16 (slope -0.58). For 3, the Bronstedtype plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the Elcb reactions of these dications. However, log koH for 3 is a smooth linear function of the previously reported log koH for the E l c b reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E l c b mechanism for 3. The E l c b reactions of 1 are approximately lo4-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1. JOHN W. BUNTING, ANDREA TOTH et JAMES P. KANTER. Can. J . Chem. 70, 1 195 (1992). Operant en solutions aqueuses basiques (force ionique de 0,1, a 25°C) et faisant appel des mkthodes spectrophotomktriques, on a mesurt les vitesses des reactions d'elimination des cations N-(2-bromoethy1)pyridinium ( I ) et des dications N,N1-Cthylkne bispyridinium (3) qui portent une variete de substituants sur les pyridines de chacune des classes de cations pyridinium et qui conduisent a la formation des cations N-vinylpyridinium correspondants (2). Les cinetiques des reactions sont du premier ordre en composCs 1 (ou 3) et du premier ordre en ion hydroxyde. Pour chacun des produits 1 et 3 , les courbes de type Bronsted des constantes de vitesse du deuxikme ordre (koH) en fonction de la basicite (exprime en pKBH) de la pyridine substituke correspondante ne sont pas lineaires; on peut interpreter ces resultats en fonction de mecanismes reactionnels Elcb. Pour les composes 1, la courbe de type Brgnsted presente deux regions lineaires aconcaves vers le basn distinctes; une deprotonation qui determine la vitesse se produit un pKBH < 5,15 (pente = -0,30) et un changement dans I'ktape qui determine la vitesse de depart de l'ion bromure a un pKBH > 5,16 (pente = -0,58).Pour les composCs 3 impli...

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