Selective partial reduction of quinolines: Hydrosilylation vs. transfer hydrogenation

Two mild and versatile catalytic routes give regioselective hydrogenation of the heterocyclic ring of quinoline derivatives avoiding the high pressures of hydrogen required in the conventional hydrogenation route. Hydrosilylation using H3SiPh and catalyzed by [Rh(nbd)(PPh3)2]PF6 at room temperature gives dihydroquinoline, a product not obtainable via direct hydrogenation. Hydrosilylation of the Cdouble bond; length as m-dashN bond of PhCHdouble bond; length as m-dashNPh is also observed under these conditions while PhCHdouble bond; length as m-dashCHPh is unreactive. Initial in situ disproportionation of phenylsilane to H2SiPh2 and SiH4, catalyzed by the same catalyst, was required for substrate reduction, as SiH4 proved to be the active reductant. No N-silyl intermediates were ever observed, hydrolysis presumably occurring in situ. This disproportionation reaction is of potential use in gaining access to silane (SiH4), a material otherwise not readily available. In a separate approach, transfer hydrogenation from isopropanol using [Ir(cod)(NHC)PPh3]BF4 (NHC = 1-neopentyl-4-n-butyl triazole-5-ylidene) as catalyst exclusively produces the tetrahydro product.