Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines

Abstract

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.

Effect of microheterogeneity of CF3CH2OH-H2O mixed solvent on reactions of carbocations from 1,2,2,3-tetramethyl-1,2-dihydroquinoline

“…[16] As was found before for the carbocations from DHQ, the activation energy of the combination with alcohols is lower than that with water by 10-15 kJ mol -1 and monotonously changes with solvent composition. [9,10,14] The minimum in the activation energy in the case of the carbocation from 1 in the TFE-H 2 O mixture 3 : 7 (19 kJ mol -1 against 25 kJ mol -1 in pure TFE and 41 kJ mol -1 in H 2 O (Fig. 1)) indicates also the change in the nucleophile at this solvent composition.…”

“…The absorption maximum of the carbocations generated from DHQ ranges from 470 to 490 nm. [9][10][11] The concentration of DHQ was varied in the range (1.…”

“…[9] Samples were irradiated in a quartz cell with an optical path length of 20 cm by a xenon lamp with pulse energy of 150 J. The cell was provided with a jacket, which allowed to carry out experiments at different temperatures.…”

“…[9][10][11][12][13][14] The rate constants of the decay for the carbocations from DHQ in mixed alcohol-alcohol or alcohol-water binary solvents depend nonlinearly on the solvent composition that was accounted for by the joint effect of the solvent nucleophilicity and basicity on the reaction rate. [14] In binary mixtures of MeOH with EtOH, Pr n OH and Pr i OH, the dependence of the rate constant of the carbocation decay on the solvent composition has a maximum.…”

Primary Photophysical and Photochemical Processes in the Photolysis of 1,2-Dihydroquinolines in Different Solvents

“…The reaction occurs from the excited singlet state. The process is accompanied by a drastic increase in the quantum yield of the reaction with a decrease in the quantum yield of fluorescence [3,6]. A carbocation formed in the reaction of proton transfer from the solvent molecule to the substrate in the excited singlet state to the C(3) atom of the heterocycle is the primary transient species observed in the flash photolysis of tert -DHQ.…”