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 °C and the activation energies E₁ ≈ 0 and E₂ (31.4 ± 3.1) kJ mol⁻¹ were determined. In MeOH acidified with acetic acid only the second transient (λ_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-S_E2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (λ_max = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 × 10⁻³ mol dm⁻³. In the case of compound 2, the formation of a carbocation with λ_max = 490 nm 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 × 10⁶ mol⁻¹ dm³ s⁻¹. The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.