15904-74-2Relevant academic research and scientific papers
Effects of α and γ Substituents on the Relative Reactivity of Pyrylium and Thiopyrylium Cations
Doddi, Giancarlo,Ercolani, Gianfranco
, p. 1393 - 1396 (2007/10/02)
The rates of methoxide attachment to the γ position of nine pyrylium cations, symmetrically substituted at the α position with Ph and But, 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.
1H Nuclear Magnetic Resonance Study of Methoxide Addition to Pyrylium and Thiopyrylium Cations; Heteroatom and Substituent Effects
Doddi, Giancarlo,Ercolani, Gianfranco
, p. 271 - 276 (2007/10/02)
Methoxide addition to pyrylium and thiopyrylium salts with various α-substituents (Ph or tBu) and γ-substituents (H, Me, tBu, Et3C, Ph or MeO) in methanol at -40 and 25 deg C has been studied by 1H n.m.r.The composition of the product mixtures, kinetically controlled at -40 deg C and thermodynamically controlled at 25 deg 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 rean course emphasizes the role of polar and steric interactions, and sheds light on the structures of the transition states.
