Homogeneous catalytic hydrogenation. 4. Regioselective reduction of polynuclear heteroaromatic compounds catalyzed by hydridochlorotris(triphenylphosphine)ruthenium(II)

Article abstract of DOI:10.1021/om00129a009

The selective reduction of polynuclear heteroaromatic nitrogen compounds such as quinoline, 1, 5,6-benzoquinoline, 2, 7,8-benzoquinoline, 3, acridine, 4, phenanthridine, 5, and indole, 7, and in one case the sulfur heterocycle benzothiophene, 6, with hydridochlorotris(triphenylphosphine)ruthenium(II) as catalyst, under rather mild hydrogenation conditions, provided in each case the corresponding saturated nitrogen-or sulfur-containing ring compound with reasonable conversion rates and total yields. In addition, it was found that several compounds would inhibit the reduction of 1, to 1,2,3,4-tetrahydroquinoline, 8, including 8 itself, as well as 2, 7, 9 (2-methylpyridine), 10 (3-methylpyridine), and 11 (carbazole). Compounds 3 and 6 did not inhibit the rate of reduction. These inhibitions are seen as effects of competition for binding to the catalyst metal center, this competitive binding being affected by steric constraints and electronic effects such as basicity of the substrates. The substitution of deuterium gas for hydrogen gas in the reduction of 1 provided information on the reversibility of the hydrogenation step and implies a cyclometalation reaction is occurring leading to exchange of the 8-position of the product. Additional deuterium experiments starting with 8 instead of 1 indicate that the catalyst can partially dehydrogenate 8 to form an imine intermediate leading to deuterium exchange at the 2-position.