New Benzo[h]quinoline‐Based Ligands and their Pincer Ru and Os Complexes for Efficient Catalytic Transfer Hydrogenation of Carbonyl Compounds

Abstract

New benzo[h]quinoline ligands (HCN'N) containing a CHRNH2 (R=H (a), Me (b), tBu (c)) function in the 2-position were prepared starting from benzo[h]quinoline N-oxide (in the case of ligand a) and 2-chlorobenzo[h]quinoline (for ligands b and c). These compounds were used to prepare ruthenium and osmium complexes, which are excellent catalysts for the transfer hydrogenation (TH) of ketones. The reaction of a with [RuCl2(PPh3)3] in 2-propanol at reflux afforded the terdentate CN'N complex [RuCl(CN'N)(PPh3)2] (1), whereas the complexes [RuCl(CN'N)(dppb)] (2-4; dppb=Ph2P(CH2)4PPh2) were obtained from [RuCl2(PPh3)(dppb)] with a-c, respectively. Employment of (R,S)-Josiphos, (S,R)-Josiphos*, (S,S)-Skewphos, and (S)-MeO-Biphep in combination with [RuCl2(PPh3)3] and ligand a gave the chiral derivatives [RuCl(CN'N)(PP)] (5-8). The osmium complex [OsCl(CN'N)(dppb)] (12) was prepared by treatment of [OsCl2(PPh3)3] with dppb and ligand a. Reaction of the chloride 2 and 12 with NaOiPr in 2-propanol/toluene afforded the hydride complexes [MH(CN'N)(dppb)] (M=Ru 10, Os 14), through elimination of acetone from [M(OiPr)(CN'N)(dppb)] (M=Ru 9, Os 13). The species 9 and 13 easily reacted with 4,4'-difluorobenzophenone, via 10 and 14, respectively, affording the corresponding isolable alkoxides [M(OR)(CN'N)(dppb)] (M=Ru 11, Os 15). The complexes [MX(CN'N)(P2)] (1-15) (M=Ru, Os; X=Cl, H, OR; P=PPh3 and P2=diphosphane) are efficient catalysts for the TH of carbonyl compounds with 2-propanol in the presence of NaOiPr (2 mol %). Turnover frequency (TOF) values up to 1.8x10(6) h(-1) have been achieved using 0.02-0.001 mol % of catalyst. Much the same activity has been observed for the Ru--Cl, --H, --OR, and the Os--Cl derivatives, whereas the Os--H and Os--OR derivatives display significantly lower activity on account of their high oxygen sensitivity. The chiral Ru complexes 5-8 catalyze the asymmetric TH of methyl-aryl ketones with TOF approximately 10(5) h(-1) at 60 degrees C, up to 97 % enatiomeric excess (ee) and remarkably high productivity (0.005 mol % catalyst loading). High catalytic activity (TOF up to 2.2x10(5) h(-1)) and enantioselectivity (up to 98 % ee) have also been achieved with the in-situ-generated catalysts prepared from [MCl2(PPh3)3], (S,R)-Josiphos or (S,R)-Josiphos*, and the benzo[h]quinoline ligands a-c.

Synthesis of 2-acetylbenzo[h]- and 3-acetylbenzo[f]quinolines by the reaction of benzo[h]- and benzo[f]quinolines with C2H5OH and CCl4 catalyzed by Cu-containing catalysts

“…Two-proton singlet at δ = 8.24 ppm was assigned to degenerate protons Н(3) and Н(4) in the ring A, which is consistent with the published data. 8,9 The HSQC and HMBC experimental data fully confirmed the structure 1. Thus, the H (4) 1 H-1 -13 C-3 ( 13 C-4a).…”

“…In the COSY spectrum of compound 1 one could observe the four-spin system (d, δ = 7.93; t, δ =7.77; t, δ = 7.80; d, δ = 9.35 ppm) associated with proton interactions in the ring C and assigned to four protons H (7,8,9,10). The spectrum also demonstrates the two-spin system associated with proton interactions in the ring B and assigned to Н(5) and Н(6).…”

“…They are used in the synthesis of antimalarial agents, azahelicenes having unique optical and spectral properties, sensitizers, and ligands for coordination chemistry. [1][2][3][4][5][6][7][8] The known methods for the synthesis of acetylbenzoquinolines are complicated and comprise several steps. For example, 2-acetylbenzo[h]quinoline 1 was synthesized in a 77% yield in two steps from 2-bromobenzo[h]quinoline by metallation with n-butyllithium followed by acylation of 2-lithiobenzo[h]quinoline with N,N-dimethylacetamide at -78 о С.…”

Facile synthesis, characterization and DFT calculations of 2-acetyl pyridine derivatives

“…asymmetric synthesis 19 atom transfer radical polymerization 20 oligomerization 21 polymerization 22 and transfer hydrogenation of carbonyl compounds. 23 In this work an easy and facile substitution on -CH 3 group of the 2-acetylpyridine was carried out. The reaction was found to be equally effective with aryl and alkyl groups, formation of ether as byproduct or other side reactions were not observed.…”

Palladium-catalyzed asymmetric hydrogenation of simple ketones activated by Brønsted acids

“…1 A series of efficient systems have been successfully developed for the asymmetric hydrogenation of ketones with chiral Ru, 2 Rh, 3 Ir, 4 Os, 5 1, entry 1). Gratifyingly, the catalytic activity was improved dramatically in the presence of 10 mol % Brønsted acid, benzoic acid 3a, full conversion and 59% enantioselectivity was obtained (Table 1, entry 2).…”