Homogeneous Catalytic Hydrogenation. 2. Selective Reduction of Polynuclear Heteroaromatic Compounds Catalyzed by Chlorotris(triphenylphosphine)rhodium(I)

Article abstract of DOI:10.1021/jo00197a035

The selective reduction of polynuclear heteroaromatic nitrogen compounds such as quinoline, 1, 5,6-benzoquinoline, 2, 7,8-benzoquinoline, 3, acridine, 4, phenanthridine, and in one case, a sulfur heterocyclic compound, benzothiophene, 6, with chlorotris(triphenylphosphine)rhodium(I), (Ph3P)3RhCl, provided under rather mild hydrogenation conditions the corresponding saturated nitrogen and sulfur heterocyclic analogues of the above-mentioned compounds in reasonable conversion rates and total percent yields.In addition, compounds that inhibit the initial rate of hydrogenation of 1 in the conversion to 1,2,3,4-tetrahydroquinoline, 10, include pyridine, 7, 3-methylpyridine, 8, and 10 itself.These results are indicative of electronic effects in these competitive hydrogenation reactions, while 2-methylpyridine, 9, slightly reduces the rate of hydrogenation of 1, implicating a steric effect at the metal center.It was also observed that substrate 6, indole, 11, pyrrole, 12, carbazole, 13, thiophene, 14, dibenzothiophene, 15, and p-cresol, 16, enhanced the initial rate of hydrogenation of 1 to 10 by an average factor of >1.5.The substitution of deuterium gas for hydrogen gas in the reduction of 1 provided information on the reversibility of the hydrogenation step, stereoselectivity in the reduction of the 3,4-double bond, and the implication of cyclometalation reactions which caused the exchange of H for D at the 8-position and possibly the 2-position.Similar deuteration data with compound 5 strengthened the concept of dehydrogenation in the hydrogenation step and in fact provided independent evidence for the facile dehydrogenation of 1,9,9,10-tetradeuterio-9,10-dihydrophenanthridine, 19, catalyzed by (Ph3P)3RhCl. 1H NMR and IR experiments also verify some of the postulated mechanistic aspects of these selective hydrogenation reactions.