Photoaddition of water and methanol to 2,2,4,6-tetramethyl-1,2-dihydroquinoline
摘要:
Products of the steady-state photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) in water, methanol, and water-ethanol were isolated for the first time and identified by H-1 and C-13 NMR spectroscopy, IR spectroscopy, and mass spectrometry. As a result of the photolysis, the molecule of the solvent is added to the double bond of the heterocycle with formation of 4-hydroxy- (2) or 4-methoxy-2,2,4,6-tetramethyl-1,2,3,4-tetrahydroquinoline (3) in water and methanol, respectively. Compounds 2 and 3 are converted gradually into 1 in the dark. The rate of the back reaction depends on the solvent and the concentration of the product. Comparison of the products of the photolysis in methanol and hexane at 45 degrees C and of the azoisobutyronitrile-initiated oxidation of 1 at the same temperature has shown that unlike the photolysis in hexane, aminyl radicals are not precursors of the product of the photolysis in methanol. The reaction proceeds via an excited singlet state.
Steady-state photolysis products of 6- and 8-substituted 2,2,4-trimethyl- 1,2-dihydroquinolines in water and lower alcohols were identified by III NMR. [it the case of electron-donor substituents, the solvent molecule is added to the double bond of the heterocycle affording the corresponding 4-hydroxy- or 4-alkoxytetrahydroquinolines. Nitro-substituted dihydroquinolines are photostable. The addition of EtOH and (PrOH)-O-n occurs only in the presence of water to give a mixture of alkoxy- and hydroxy-adducts. A reaction scheme is suggested.