Rates and equilibriums for hydroxide ion addition to quinolinium and isoquinolinium cations

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pK,, values have been measured for cation-pseudobase equilibration by 4-X-2-methylisoquinolinium cations (1) (X = Br. CONH,, COC6Hs, CN, NO,) at 25"C, ionic strength0.1. ThesepK,, valuesare wellcorrelated by Hammett equationsusingeithero or o-prrra substituent constants. The best correlation gives: pKRT = -8.8 (k 0.3) c r , + 16.5 (f 0.2) (r = 0.998). The value pK,+ = 16.29 measured by Cook r t a / . (Tetrahedron, 32, 1773(Tetrahedron, 32, (1976) for the 2-methylisoquinolinium cation in dimethyl sulfoxide -water solutions is in reasonable agreement with this correlation equation. For the 2-methyl-5-nitrophthalazinium cation, pk',,, = 7.87, and pKRo_ = 12.10 for alkoxide ion formation by the pseudobase of this cation.The pH dependence of the pseudo first-order rate constants (k,,,) for cation-pseudobase equilibration has been measured for 1:X = CONH,, COC6H,, CN and for the 2-methylphthalazinium cation (3) and its 5-NO, derivative (4). For each of these cations, k, =koH[-OH] + kHZo and k, = k,[H+] + k, and the par-ameters koH kHgo. k,. and k, have been evaluated. Fol-l : X = CONH, and CN and 3, k,, is consistent with a correlation line between log k, , and pK,+ established for other isoquinolinium cations (J. Am. Chem. Soc. 99, 1189Soc. 99, (1977). For 1:X = COC,H,, koH is seven-fold smaller, and for 4. k,, is five-fold greater than predicted by this correlation line. (Tetrahedron, 32, 1773(Tetrahedron, 32, (1976)pourle

Section: Kinetics Of Catiotz-pseudobase Equilibrationsupporting

confidence: 78%

“…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.…”

Section: Kinetics Of Catiotz-pseudobase Equilibrationsupporting

confidence: 59%

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

Bunting¹,

Chew²,

Sindhuatmadja³

1981

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“…These estimates are somewhat larger than the experimental values of 3.0 and 2.7. These experimental isotope effects are also much smaller than found for the I-benzyl-l,4-dihydronicotinamide reduction of the 2-benzyl-5-nitroisoquinolinium cation (k2H/k2D = 6.6 (2)) which has pKR+ = 1 1.3 (23) and is reduced at a similar rate (k2 = 0.4 M-' s-' (2)) to the 1-(4-cyanobenzyl)nicotinonitrile cation (k2 = 0.246 M-' s-' (Table 1)). …”

Section: Discussionmentioning

confidence: 63%

Kinetics of the reduction of nicotinonitrile cations by 1,4-dihydronicotinamides

Bunting¹,

Brewer²

1985

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. Can. J. Chem. 63, 1245Chem. 63, (1985. The rates of reduction of a series of 1-(Z-benzyl)nicotinonitrile cations by a series of I-(X-benzyl)-l,4-dihydronicotinamides have been studied at 25°C in 20% CH3CN -80% HzO (pH 7.0 (5 mM phosphate), ionic strength 1.0 (KCI)). Spectral studies indicate the formation of 1,4-dihydronicotinonitrile products, without the formation of the isomeric 1,2-dihydro-or 1,6-dihydro-nicotinamide intermediates. Second-order rate constants (kz) for these reductions are closely correlated with the Hammett u constants for X and Z. Thus, for X = H, log kz = 0.630, -1.05, while for Z = 4-CN, log kz = -0 . 6 4~~ -0.65. The close correspondence between these p, and p, values indicates that charge neutralization on the nicotinonitrile cation exactly balances charge generation on the nicotinamide cation product in the rate-determining transition state. Thus the migrating hydrogen species is electrically neutral in the rate-determining transition state, which contrasts with the hydridic transition states previously reported in the reduction of isoquinolinium cations by 1,4-dihydronicotinamides. When 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide is used as the reductant, primary kinetic isotope effects of 3.0 and 2.7 are observed for the reduction of the I-methylnicotinonitrile and 1-(4-cyanobenzy1)-nicotinonitrile cations, respectively. These data are evaluated in terms of the various mechanistic possibilities for hydride transfer. 1245 (1985).Operant 25"C, dans des solutions a 20% CH,CN -80% HzO, a un pH de 7,O (5 mM de phosphate) et a une force ionique de I , O (KC]), on a dtterminC les vitesses de rkduction d'une skrie de cations (Z-benzyl)-l nicotinonitrile par une serie de (X-benzy1)-l dihydro-1,4 nicotinamides. Des Ctudes spectrales indiquent qu'il y a formation de produits dihydro-1,4 nicotinonitrile, sans formation des intermediaires isomkres dihydro-l,2 ou dihydro-1,6 nicotinamides. Il existe une correlation entre les constantes de vitesse du deuxikme ordre (k,) de ces reductions et les constantes u de Hammett des substituants X et Z. Ainsi, pour X = H, log kz = 0 , 6 3 1~~ -1,05, alors que pour Z = CN-4, log k2 = -0,640, -0,65. La bonne correspondance entre ces valeurs de p, et de p, indique que, dans 1'Ctat de transition determinant la vitesse de la rkaction, la neutralisation de la charge du cation nicotinonitrile contrebalance exactement la gCnCration de la charge sur le produit cationique nicotinamide. Dans 1'Ctat de transition qui determine la vitesse de la rkaction, l'hydrogene qui migre est Clectriquement neutre et ceci est en opposition avec les Ctats de transition impliquis dans les reductions par les hydrures dont il Ctait fait mention dans les rapports antCrieurs sur la rCduction de cations isoquinolinium par les dihydro-1,4 nicotinamides. Lorsqu'on utilise la benzyl-1 dideuttro-4,4 dihydro-1,4 nicotinamide comme reducteur, on observe des effets isotopiques primaires de 3,O et 2,7 respectivement pour les riductions des cations mCthyl-1 nicotinonitrile et (cyano-4 benzyl)...

“…lo5 for the aryltropylium cations. Values slightly greater than lo7 have been reported for reactions of some quinolinium cations (48). The variations follow no obvious pattern, and the extremely low value for the tris-(2,4-dimethoxypheny1)methyl cation is not found for the similarly substituted xanthylium cation.…”

Section: T~~-( P -N ( C H~)~) Trityl Cation B~s -(~-N ( C H~)~) Tritymentioning

confidence: 75%

Cation–anion combination reactions. 26. A review

Ritchie¹

1986

197

175

Rate and equilibrium constants for reactions of carbocations, and a few other electrophiles, with anionic and neutral nucleophiles in several solvents are summarized. Equilibrium constants for such reactions are a measure of the carbon basicities of the nucleophiles, and are discussed in the context of differences in proton and carbon basicities. The factors affecting the kinetic reactivities of nucleophiles are not well understood, even though the reactivity patterns are frequently the same toward widely different electrophiles.