104086-47-7Relevant academic research and scientific papers
Steric Effects on Rates and Equilibria of a Cation-Anion Combination Reaction: The Methoxide Attachment to 4-Substituted 2,6-Di-tert-butylpyrylium Cations
Doddi, Giancarlo,Ercolani, Gianfranco
, p. 1729 - 1733 (2007/10/02)
The 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 deg 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 2H 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.
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.
