The synthetic and mechanistic aspects of the regioselective hydrogenation of representative mono and polynuclear heteroaromatic nitrogen, sulfur, and oxygen model coal compounds such as 2-methylpyridine (1), N-methylindole (2), benzofuran (3), benzothiophene (4), quinoline (5), 2-methylquinoline (6), 5,6- and 7,8-benzoquinolines (7 and 8), and acridine (9) were studied with a (η5-pentamethylcyclopentadienyl)rhodium tris(acetonitrile) dicationic complex, [Cp*Rh(CH3CN)3]2+, as the catalyst precursor. The order of relative rates as a function of structure was found to be 8 >>> 9 > 5 > 7 > 6 > 4 ≫ 1–3. Competitive hydrogenation experiments of 5 with other model coal compounds, 1–4, 6–9, pyridine (10), and isoquinoline (11) and its regioselective reduction product, 1,2,3,4-tetrahydroquinoline (12), showed the following effects on the initial hydrogenation rate of 5: no effect (2–4), enhancement (8), and inhibition (9–12). In addition, 7,8-benzoquinoline (8) and its reduced product, 1,2,3,4-tetrahydro-7,8-benzoquinoline (13), were found to also enhance the initial hydrogenation rates of 2, 6, and 7; a [Cp*Rh]2+-catalyzed transfer hydrogenation mechanism was invoked as one explanation for this rate enhancement by using 13-d5 in the presence of 5, which showed deuterium atom transfer to provide 12-d3. Replacement of H2 with D2 provided information on several of the mechanistic aspects of these selective hydrogenation reactions and included reversibility in the N═C and C═C bond reductions of six-membered N heteroaromatic compounds (1, 5–9), stereoselective reduction of the C═C bond in the five-membered N, S, and O heteroaromatic ring compounds (2–4), and exchange of aromatic ring and 2-methyl group hydrogens (2–9). The catalytic hydrogenation precursor Cp*Rh complexes for ligands 1 and 5–9 had the known structural formulas [Cp*Rh(η1(N)-ligand)(CH3CN)2]2+, while those for 2–4 are speculated to have [Cp*Rh(η2-ligand)(CH3CN)2]2+ structures, η2 bonding of the C═C bond in the five-membered heteroaromatic ring to Cp*Rh. A partially hydrogenated pyridine intermediate, 1,2,5,6-tetrahydropyridine (14), bonded to [Cp*Rh]2+, [Cp*Rh(η1(N)-1,2,5,6-tetrahydropyridine)(CH3CN)2]2+ (15) was synthesized and reacted with D2 gas to provide further evidence for a possible intermediate in the selective hydrogenation process. Moreover, thermal dehydrogenation of 15 was observed with formation of a [Cp*Rh(η1(N)-pyridine)(CH3CN)2]2+ complex. High-pressure NMR experiments were able to further verify the above-mentioned mechanistic pathways with quinoline (5) as an example, but the identity of intermediate [Cp*RhH-quinoline]2+ complexes was not successful. An overall mechanism for selective N, S, and O heteroaromatic ring hydrogenation will be presented.
ASJC Scopus subject areas
- Colloid and Surface Chemistry