Benzoquinoline, hydrogenation

The reaction network for 5,6-benzoquinoline [101] has been proposed in a more detailed level than that of acridine. In this network, conversely to acridine network, only one primary hydrogenation product, l,2,3,4-tetrahydro-5,6-benzoquinoline, was identified, and in contrast to the quinoline case however, no aniline derivatives were detected. 

Scheme 52 explains the [(Cp )Rh(MeCN)3]2+-assisted regioselective hydrogenation of pyridines, benzoquinolines, acridines as well as indoles and benzothiophene.258 The relative hydrogenation rates were attributed to both electronic and steric effects, the rate generally decreasing with increasing basicity and steric hindrance at the nitrogen atom. 

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

The substitution of two or three hydrogen atoms of methane by six-membered nitrogen heterocycles, like pyridine, quinoline, or benzoquinoline, leads to a class of compounds which we call quinolylmethanes, as its representatives are in structure as well as in their chemical and physical properties similar to di-(2-quinolyl)methane, the compound prepared first.1 Substitution of the central —CH2— (or =CH—) bridge by —NH— (or =N—) leads to the corresponding quinolylamines. Quinolylmethanes and quinolylamines are the fundamental substances of many cyanine dyestuffs. 

Verily that the liqitid-phase reaction of 5,6-benzoquinoline with hydrogen is pseudo-first-order. [2nd Ed.P4-7]... 

Several homogeneous systems have been reported to catalyze the hydrogenation of polynuclear N-heterocycles efficiently the regioselective reduction of the N-containing ring in quinoline, isoquinoline, indole, benzoquinolines, acridine and other related molecules can be achieved with relative ease and under moderate conditions of temperature and pressure. A number of metals are known to be active (Cr, Mo, W, Mn, Fe, Ru, Os, Co, Rh, Ir) but most of the published work has concentrated on ruthenium and rhodium precursors [95-100] as shown in Table 3.3. 

The hydrogenation of other polynuclear N-heterocycles was also reported by Rosales group, by use of the same Ru(II) catalyst precursor operating at 125° C and 3 atm. H2. The reactivity order acridine > quinoline 5,6-benzoquinoline > 7,8-benzoquinoline > indole > isoquinolinc was in line with other trends previously observed, reflecting steric and electronic effects. A kinetic analysis of the hydrogenation of acridine reveals a different, second order, rate law -d[A]/dr = consistent... 

Other examples of C-H H-M short contacts [44] were found in iridium complexes, and the competition between agostic interactions and hydrogen bonds was tested by QMMM calculations for several R groups hanging from an orthometallated benzoquinoline ligand. The agostic structure is favored for t-Bu, but the hydrogen bond is preferred by isopropyl [44b]. 

The molecular hydrogen complex CirH (H2 ) (PPh >2 (where C gHgN=metallated 7,8-benzoquinoline) has been synthesised. Structural data on a number of iridium complexes such as [IrH2(PPhg)2b] (L=8-methylquinoline, caffeine), containing a... 

Ishii and coworkers reported that the JV-heterocyclization of naphthylamines with diols can be achieved with an iridium catalyst. In a typical example, the reaction of 1-naphthylamine with 1,3-propanediol was carried out with a catalytic amount of IrClg (5 mol%), r c-2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) (10 mol%), and the corresponding 7,8-benzoquinoline was obtained in 96% yield (Scheme 11.11) [159]. The proposed reaction mechanism indicates that the imine intermediate is formed by the reaction of the amine and aldehyde by Ir-catalyzed dehydrogenation. Subsequent hydrogenation by the in situ generated Ir hydride leads to an aminoalcohol followed by cyclization to the desired quinoline products. 

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