Hydrogenations of quinoline

New Ways for the Full Hydrogenation of Quinoline in Mild Conditions... 

Figure 1 Proposed reaction pathways for the hydrogenation of quinoline (a = 1,2-dihydroquinoline, b = 1,4-dihydroquinoline, c = 3,4-dihydroquinoline).

Table 1 Amines co-fed in the hydrogenation of quinoline, their quadrant classification (16) and pKa values (15) n.a. = not available.

Table 2 Catalytic activity for the hydrogenation of quinoline in the presence of aliphatic or aromatic amines (0.3 g of the 5 wt% Rh/Al203-1 catalyst T = 373 K PH2 = 2.0 MPa reaction time (where not indicated) = 6 h 41 mmoles of quinoline (82 in the test in absence of amine) and 41 mmoles of amine in 100.0...

Figure 3 Activities in the hydrogenation of quinoline for the commercial catalysts (a) Rh/Al203-1 and (b) Rh/Al203-2 (0.3 g of catalyst T = 373 K PH2 = 2.0 MPa 82 mmoles of quinoline in 100.0 mL of 2-propanol).

Hydrogenation of Quinolines Under Water Gas Shift Conditions and Oxidation of 1,2,3,4-Tetrahydro-quinolines to Hydroxamic Acids 6-Methoxy-1,2,3,4-tetrahydroquinoline and 1-Hydroxy-6-methoxy-3,4-dihydroquinolin-2(1 H)-one. 

The asymmetric hydrogenation of quinoline continues to be of interest. Li et al. reported the asymmetric hydrogenation of a variety of 2-substituted-quinolines to the corresponding tetrahydroquinolines using an Ir-catalyst with a BINOL-derived diphosphonite ligand... 

Using Ir/MeO-Biphep/l2 catalyst system, a variety of substituted quinoline derivatives were hydrogenated in 95% yield and up to 96% ee. This method provided an efficient accesss to three naturally occurring alkaloids (Scheme 17).328 Ferrocene N, P ligand 108 is also effective for the asymmetric hydrogenation of quinolines with up to 92% ee.188a... 

Deuterium gas experiments, continuous NMR and GC/MS analysis, in situ high-pressure NMR spectra and the isolation of some intermediates provided Fish with sufficient information to propose the mechanism shown in Scheme 16.16 for the hydrogenation of quinoline to THQ, catalyzed by [Rh(NCMe)3Cp ]2+ (40°C, 33 bar H2, CH2C12) [55]. 

Under biphasic conditions, the zwitterionic Rh1 complex Rh(cod)(sulphos) proved to be very efficient for the hydrogenation of quinoline to THQ (TOF = 20 at 160°C, 30 bar H2, water/n-heptane) [8c]. 

The supported complex [Rh(cod)(POLYDIPHOS)]PF6, obtained by stirring a CH2C12 solution of [RhCl(cod)]2 and Bu4NPF6 in the presence of a diphenyl-phosphinopropane-like ligand tethered to a cross-linked styrene/divinylbenzene matrix (POLYDIPHOS), forms an effective catalyst for the hydrogenation of quinoline (Fig. 16.8) [84]. Under relatively mild experimental conditions (80 °C, 30 bar H2), quinoline was mainly converted to THQ, though appreciable formation of both 5,6,7,8-THQ and decahydroquinoline also occurred (Scheme 16.20). 

Fig. 16.8 Schematic of a diphosphine rhodium complex covalently tethered to a cross-linked styrene/divinylbenzene matrix, used for the hydrogenation of quinoline.

Fig. 9 Summary of the best results obtained in the Ir-catalyzed hydrogenation of quinolines using...

Chan and coworkers developed a new diphosphine 9, related to MeO-BiPhep 5 (Fig. 10) [21]. [lr(p-Cl)(COD)]2/9/l2 catalytic system provided similar enantios-electivities than [lr(p-Cl)(COD)]2/5/l2 in the Ir-catalyzed hydrogenation of quinolines but higher enantioselectivitites in the reduction of 2-methyl-quinoxaline and 2,3,3-trimethylindolenine (Fig. 10). 

Asymmetric Synthesis Based on Hydrogen Transfer 113 Table 5.2 Transfer hydrogenation of quinolines catalyzed by [Cp lrCl2]2 (1) ... 

Quinoline homologs and derivatives, including those with double bonds in the side chains, were reduced selectively by catalytic hydrogenation over platinum oxide (side chain double bonds), and to dihydro- and tetrahydro-quinolines by sodium in butanol, by zinc and formic acid, and by triethylam-monium formate [319, 472]. Catalytic hydrogenation of quinoline and its derivatives has been thoroughly reviewed [439]. 

Mechanistically, the Brpnsted acid-catalyzed cascade hydrogenation of quinolines presumably proceeds via the formation of quinolinium ion 56 and subsequent 1,4-hydride addition (step 1) to afford enamine 57. Protonation (step 2) of the latter (57) followed by 1,2-hydride addition (step 3) to the intermediate iminium ion 58 yields tetrahydroquinolines 59 (Scheme 21). In the case of 2-substituted precursors enantioselectivity is induced by an asymmetric hydride transfer (step 3), whereas for 3-substituted ones asymmetric induction is achieved by an enantioselective proton transfer (step 2). 

Practically pure cw-decahydroquinoline (43) (containing 3% trans isomer) can be obtained by prolonged hydrogenation of quinoline in concentrated hydrochloric acid over platinum black (72JCS(P2)615). Hydrogenation over platinum in aqueous acetic acid gives a preponderance (80%) of the frans-decahydroquinoline (44 Scheme 31). 

Herein, we report an extension of this methodology and report the metal-free hydrogenation of quinolines using 1 mol % of diphenyl phosphate (DPP) and the asymmetric variant of this procedure using 1 mol % of a chiral BlNOL-phosphate 1 as catalyst (Figure 4.2). 

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