Reduction of quinolines to 1,2,3,4-tetrahydroquinolines with hydrosilane/ethanol catalyzed by TiO2-supported gold nanoparticles under solvent free conditions

Abstract

Gold nanoparticles supported on TiO2 (1 mol%) catalyze the reduction of a series of functionalized quinolines into 1,2,3,4-tetrahydroquinolines using hydrosilanes/ethanol (hydride/proton) as the reductant system. A typical reaction requires 4 molar equivalents of phenyldimethylsilane (reductant of choice), 4 molar equivalents of ethanol as a reagent and heating to 70 °C under solvent free conditions. The isolated yields are moderate to excellent and in certain cases the reaction rate is exceedingly fast. Mechanistic analysis revealed the stereoselective addition of two hydrides (from hydrosilane) on positions C2 and C4 of the quinoline ring and two protons (from ethanol) on positions C3 and the nitrogen atom.