Selective partial reduction of quinolines: Hydrosilylation vs. transfer hydrogenation

• Published in: Journal of organometallic chemistry Vol. 693; no. 10; pp. 1815 - 1821
• Main Authors: Voutchkova, Adelina M., Gnanamgari, Dinakar, Jakobsche, Charles E., Butler, Chase, Miller, Scott J., Parr, Jonathan, Crabtree, Robert H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2008
• Subjects: 1,2-Dihydroquinoline; Iridium; Phosphine; Rhodium; Silane disproportionation; 1,2-Dihydroquinoline; Iridium; Silane disproportionation; Rhodium; Phosphine
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/j.jorganchem.2008.02.004

Two mild catalytic routes for regioselective hydrogenation of the heterocyclic ring of quinoline derivatives are presented. The first route occurs through hydrosilylation using H 3SiPh and is catalyzed by [Rh(nbd)(PPh 3) 2]PF 6 to give 1,2-dihydroquinoline, while the second uses transfer hydrogenation with [Ir(cod)(NHC)PPh 3]BF 4 as catalyst and exclusively produces 1,2,3,4-tetrahydroquinoline. 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 H 3SiPh and catalyzed by [Rh(nbd)(PPh 3) 2]PF 6 at room temperature gives dihydroquinoline, a product not obtainable via direct hydrogenation. Hydrosilylation of the C N bond of PhCH NPh is also observed under these conditions while PhCH CHPh is unreactive. Initial in situ disproportionation of phenylsilane to H 2SiPh 2 and SiH 4, catalyzed by the same catalyst, was required for substrate reduction, as SiH 4 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 (SiH 4), a material otherwise not readily available. In a separate approach, transfer hydrogenation from isopropanol using [Ir(cod)(NHC)PPh 3]BF 4 (NHC = 1-neopentyl-4- n-butyl triazole-5-ylidene) as catalyst exclusively produces the tetrahydro product.