Fig. 4: Reaction mechanism.
From: Palladium catalyzed radical relay for the oxidative cross-coupling of quinolines
a Intermolecular competition experiments between substituted quinolines showing the effects of electron-withdrawing or donating substituents on the Pd catalyzed direct methoxylation of quinolines. b Kinetic isotope effects measured using deuterated substrates under (i) intermolecular competition and (ii) independent experiments. c Radical trapping experiments and EPR spectra. (i) TEMPO, BQ and BHT were used as radical scavengers to quench the reaction. (ii) Excluding the complete poisoning effect of the radical scavenger of TEMPO on the Pd nanocatalyst in a reaction which is apparently not a radical-involving reaction (hydrogenation of 2-methyl-3-butyn-2-ol). (iii–v) EPR spectra of radical anions when the radical spin trapping reagent DMPO was added to (iii) the reaction system, (iv) in the absence of methanol, and (v) in the absence of PhI(OAc)2. d DFT calculations. (i) Generation of methoxyl radicals by Pd-catalyzed radical relay reaction over an electron-deficient Pd(111) surface. (ii) Reaction paths and energy landscape of the direct methoxylation of 8-MeQ over Pd(111) and an electron-deficient Pd(111) surface.
