Mechanistic aspects of aminium salt-catalyzed Diels-Alder reactions: The substrate ionization step
摘要:
Substituent effects in the aminium salt catalyzed Diels-Alder reactions of 2,3-dimethyl-1,3-butadiene with a series of meta and para substituted beta-methylstyrenes are used to probe detailed mechanistic aspects of these reactions. Kinetic studies were carried out using two different aminium salt catalysts and also electrochemically, using anodic potentials corresponding to the oxidation potentials of the aminium salts. Substituent effects in the equilibrium oxidations of the styrene substrates to the corresponding cation radicals were also studied, via oxidation potential measurements. The results indicate rate determining one electron oxidation of the sytrenes to their cation radicals via an outer sphere electron transfer.
RadicalcationDiels–Alderreactions by titanium dioxide (TiO2) photocatalysis in lithium perchlorate/nitromethane solution are described. TiO2 photocatalysis promotes reactions between electron-rich dienes and dienophiles, which would otherwise be difficult to accomplish due to electronic mismatching. The reactions are triggered by hole oxidation of the dienophile and are completed by the excited