Reactivity of the Bis(dihydrogen) Complex [RuH₂(η²-H₂)₂(PCy₃)₂] toward N-Heteroaromatic Compounds. Regioselective Hydrogenation of Acridine to 1,2,3,4,5,6,7,8-Octahydroacridine

Abstract

The reaction of pyridine (Py), pyrrole (Pyr), or acridine with the bis(dihydrogen) complex [RuH2(η 2-H2)2(PCy3)2] (1) produces compounds containing heteroaromatic ([RuH2(η 2-H2)(η 1(N)-C5H5N)(PCy3)2] (2), [RuH(η 5-C4H4N)(PCy3)2]·Pyr (3)) or aromatic rings ([RuH2(η 4-C13H9N)(PCy3)2] (5)) coordinated in η 1(N) (2), η 5 (N,C) (3), or η 4(C,C) (5) modes for Py, Pyr, and acridine, respectively. Complex 3 has been characterized by X-ray crystallography. Its protonation by HBF4 affords the cationic dihydride complex [RuH2(η 5-C4H4N)(PCy3)2][BF4] (4). The coordinated Py ligand in 2 and acridine in 5 can readily be displaced by dihydrogen, with regeneration of 1. Regioselective hydrogenation of representative polynuclear heteroaromatic nitrogen compounds is achieved in the presence of 1 under mild reaction conditions (80 °C, 3 bar of H2). Quinoline (Q) and isoquinoline (iQ) are hydrogenated to 5,6,7,8-tetrahydro derivatives, while acridine is quickly reduced to 1,2,3,4-tetrahydroacridine followed by much slower saturation to 1,2,3,4,5,6,7,8-octahydroacridine (8H-Acr), the nitrogen-containing aromatic ring remaining intact. 8H-Acr has been isolated in analytically pure form and characterized by 1H and 13C NMR as well as by X-ray crystallography. 5 is also active for catalytic acridine hydrogenation and can be regarded as an intermediate in the catalytic cycle. Saturation of a five-membered indole ring proceeds much slower than hydrogenation of six-membered aromatic rings in Q and iQ. Pyridine, pyrrole, and 7,8-benzoquinoline are not hydrogenated under the applied reaction conditions, as a result of the formation of new stable complexes.

Hydrogenation of Arenes and Heteroaromatics

“…In an attempt to correlate the catalytic performance of comparable precursors with the nature of the metal center, Sánchez-Delgado and Gonzáles have investigated the selective hydrogenation of quinoline to THQ (150 8C, 30 bar H 2 , toluene) by RuCl 2 [77,78], and RuH 2 (g 2 -H 2 ) 2 (PCy 3 ) 2 (TOF quinoline/5,6,7,8-THQ = 2; TOF indole/indoline < 1) [79]. The latter complex also led to saturation of the aromatic ring, which has been proposed to involve the coordination of the substrate through the aromatic ring, in a manner similar to that reported for g 4 -arene complexes (see Scheme 16.4).…”

Kinetics and Mechanisms of Homogeneous Catalytic Reactions. Part 10. Regioselective Hydrogenation of Quinoline Catalyzed by the Systems M2Cl2(COE)4/2 Triphos [M = Rh, Ir; COE = Cyclooctene; Triphos = 1,1,1-Tris(diphenylphosphinomethyl)Ethane]

“…[7,8] [10,11] and the systems RuCl 2 (NCMe) 4 /Tp and M 2 Cl 2 (COE) 4 /Tp (Tp = tris(pyrazolyl)borate ligands) [12]; in all of cases only the reduction of the heterocyclic ring was observed, obtaining 1,2,3,4-tetrahydroquinoline (THQ) as the product. However, Borowski et al [13] reported the regioselective hydrogenation of the nonheterocyclic ring of Q by using RuH 2 (g 2 -H 2 ) 2 (PCy 3 ) 2 as the precatalyst.…”

Regioselective Reduction of Quinolines Catalyzed by Rhodium and Iridium Complexes with mono-, di-, and tri-dentated Phosphine Ligands

“…In all these cases only the reduction of the heterocyclic ring was observed. Recently, Borowski et al [9] reported the regioselective hydrogenation of the non-heterocyclic rings of quinoline, isoquinoline and acridine by using [RuH 2 (g 2 )H 2 ) 2 (PCy 3 ) 2 ] (Cy = cyclohexyl) as the precatalyst. So far the used precatalysts are based on monodentade phosphines, although systems containing tris(pyrazolyl)borate ligands have been reported in the literature [10].…”