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1H nuclear magnetic resonance study of methoxide addition to pyrylium and thiopyrylium cations; heteroatom and substituent effects
No full textMethoxide addition to pyrylium and thiopyrylium salts with various α-substituents (Ph or But) and γ-substituents (H, Me, But, Et3C, Ph, or MeO) in methanol at -40 and 25 °C has been studied by 1H n.m.r. The composition of the product mixtures, kinetically controlled at -40 °C and thermodynamically controlled at 25 °C, gives information on the factors affecting positional selectivity and the relative thermodynamic stability of the addition products (the 2H- and 4H-adduct). The methyl-substituted substrates also undergo deprotonation, to yield the corresponding anhydro-bases. Analysis of heteroatom and substituent effects on the reaction course emphasizes the role of polar and steric interactions, and sheds light on the structures of the transition states
Rates and equilibria of the reaction of 2,4,6-triphenylthiopyrylium ion with piperidine and morpholine in Me2SO. An unusual proton transfer to a nitrogen base
No full textThe complete set of kinetic and equilibrium constants of the reaction of 2,4,6-triphenylthiopyrylium ion (1) with piperidine and morpholine has been obtained in Me2SO at 25°C. The reaction involves the formation of both the corresponding 2H- and 4H-thiopyrans, which equilibrate to form only the more stable 2H adduct. The kinetic data are consistent with a two-step process wherein the proton transfer from the protonated 2H- and 4H-thiopyran intermediates to the amine is the rate-controlling step. The thermodynamically favored proton transfer to the neutral adducts by the solvated proton shows a rate below the diffusion limit, whereas the observed Brønsted coefficient would indicate a diffusion-controlled process. This behavior is discussed in terms of the Eigen mechanism. The factors affecting the nucleophilic addition are discussed, and a comparison is made with the previously reported reaction of 1 with primary amines. © 1984 American Chemical Society
Steric effects on rates and equilibria of a cation-anion combination reaction: The methoxide attachment to 4-substituted 2,6-di-tert-butylpyrylium cations
No full textThe kinetic and equilibrium constants for the reaction of 2,6-di-tert-butyl-4-R-pyrylium cations (R = H, Me, t-Bu, Et3C, Ph) with methoxide ion, to yield the corresponding 2H and 4H adducts, have been determined in MeOH at 25°C. The reaction involves the kinetically controlled formation of the 4H adduct only when R = H or Me, whereas in the other cases a mixture of both the ZH and 4H adducts is formed. The 2H adducts are the thermodynamically favored products, though in the case of the methyl-substituted cation a comparable amount of the anhydro base is also formed. The rate constants for the formation of the 4H adducts follow a regular trend showing a low sensitivity to steric effects, whereas the corresponding equilibrium constants are not affected by steric interactions until a certain value of the steric hindrance of the γ-substituent is reached. Above this value steric effects are greater on equilibria than on rates. These observations are interpreted in terms of an ion pair-like transition state in which the nucleophile specifically interacts with the electrophilic center. © 1988 American Chemical Society
Effects of α and γ substituents on the relative reactivity of pyrylium and thiopyrylium cations
No full textThe rates of methoxide attachment to the γ position of nine pyrylium cations, symmetrically substituted at the α position with Ph and Bu t, and at the γ position with H, Me, But, Ph, and OMe, have been compared with those of the corresponding thiopyrylium cations. The results suggest that the effect of the α substituent depends on the nature of the ring heteroatom. Steric inhibition to solvation of the ring heteroatom is taken into account to explain such features. By contrast the effect of the γ substituent, with the exception of the OMe group, is independent of the ring heteroatom. It is suggested that such invariance is only apparent due to the equality of the steric effect in the two series which tends to conceal small differences in sensitivity to electronic effects
Quantitative comparison of the heteroatom effects in the methoxide attachment to pyrylium and thiopyrylium cations. Thermodynamics of the isomerization of pyrans and thiopyrans
No full textThe complete set of kinetic and equilibrium constants for the methoxide attachment to a series of 2,6-di-tert-butyl-4-arylpyrylium cations (aryl = XC6H4 with X = p-NO2, m-Cl, p-Cl, H, p-Me, p-OMe, p-NMe2) has been obtained in MeOH at 25°C. These data complement those previously obtained by studying the methoxide attachment to the corresponding thiopyrylium cations. In both series the reaction involves the kinetically controlled formation of both the corresponding 2H and 4H adducts which equilibrate to form only the thermodynamically more stable 2H adduct. The observed kinetic patterns show that the rate-determining step is the combination of the nucleophile with the cation to give the adducts. Moreover, the experimental data indicate that the Leffler-Hammond postulate cannot give information on the position of the transition state along the reaction coordinate. Both kinetic and equilibrium constants for the formation of the 2H and 4H adducts are correlated with the σ+ constants. The obtained ρ values show, for the pyrylium series, a greater sensitivity to the substituent effects with respect to the corresponding thiopyrylium series. From the equilibrium data we estimate that, in contrast with quantum mechanical calculations, the unsubstituted 2H-pyran is at least 4.6 kcal/mol more stable than the corresponding 4H isomer. © 1986 American Chemical Society
The problem of regioselectivity in nucleophilic additions to pyridinium and related cations - role of generalized anomeric effect
No full textA MNDO and AM1 study of the isomerization of pyrans, thiopyrans, dihydropyridines, and their methoxy derivatives was carried out so as to clarify the nature of the effects causing the observed thermodynamic regioselectivity in nucleophilic addition to pyridinium, pyrylium and thiopyrylium cations. The results show that, in contrast to 2H-thiopyrans, 2H-pyrans and 1,2-dihydropyridines are significantly stabilized by a generalized anomeric effect when the group bonded to the 2 position is OMe. It is pointed out that in the case of reversible additions, a clear distinction should be made between the kinetic and the thermodynamic regioselectivity because, in contrast with current usage, the latter cannot be interpreted in terms of frontier orbital theory but by the presence or absence of a generalized anomeric effect. It is also pointed out that predictions of kinetic regioselectivity based on frontier orbital theory are not supported by literature data. The kinetic regioselectivity seems to be governed by the relative electron density at the carbon under attack, independent of the hard or soft character of the nucleophile
The First Homolytic Substitution of Pyrylium Salts. C-4 Methylation of 2,6- Disubstituted Pyrylium Cations
No full text2,6-Diphenylpyrylium and 2,6-di-t-butylpyrylium cations react with methyl radical generated by ButooH-Fe+2 in acidic aqueous acetonitrile, to yield substitution exclusively at C-4 positio
Improved Preparation of Thiopyrylium Salts. Application to the Synthesis of 2,4,6-Tri-t-butyl- and 2,6-Di-t-butyl-4-aryl- thiopyrylium Perchlorates
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