Pincer‐Ruthenium Catalyzed Oxygen Mediated Dehydrative Etherification of Alcohols and Ortho‐Alkylation of Phenols

Abstract

In the current report, a dehydration strategy has been formulated for the selective etherification of secondary alcohols catalyzed by NNN pincer-ruthenium(II) carbonyl complexes based on bis(imino)pyridine ligands in the presence of molecular oxygen under solvent-free and base-free conditions at 140 °C. Under these conditions, very high yields (up to 92%, ca. 6133 TON) of the corresponding ethers are obtained at a very low catalyst loading (ca. 0.015 mol%). Mechanistic studies provide key evidence for the various events that occur during the catalytic cycle. While iodometric titration gives quantitative proof for the liberation of hydrogen peroxide in catalytic amounts and hence confirms the role of oxygen as a co-catalyst, DFT, HRMS, IR and 13 C NMR analysis are indicative of the non-labile nature of the ancillary CO ligand. On the other hand, DFT, NMR and HRMS studies reveal the hemi-labile nature of the pincer-ligand in the presence of oxygen. EPR analysis provide conclusive evidence for generation of Ru(III) species via single electron transfer (SET) from O 2 . Apparently the Ru(III) species ( Ph2 NN)RuCl 2 (CO)(OH) is the resting-state of the etherification reaction and several of its derivatives have been detected by HRMS analysis. The pincer-ruthenium catalyzed etherification of 1-phenyl ethanol has been successfully extended not only to various aromatic secondary alcohols but also to the cross-etherification and/or oalkylation of 1-phenyl ethanols with a variety of phenols. Readily available oxygen and tiny amounts (0.015 mol%) of pincer-Ru catalyst are the only requirement for this atom-economical and environmentally benign dehydrative etherification reaction which apparently has not been reported thus far.

Reforming of ethanol to hydrogen and acetic acid catalyzed by pincer-ruthenium complexes

“…The complexes RuCl 2 (PPh 3 ) 3 , 4a–f , 5a–f , and 6b–d were prepared according to the literature procedure. 10 y ,19–24…”

“…The complexes RuCl 2 (PPh 3 ) 3 , 4a-f, 5a-f, and 6b-d were prepared according to the literature procedure. 10y, [19][20][21][22][23][24] Physical measurements 1 H, 2 H, 13 C{H} and 31 P were recorded on a Bruker ASCEND 600 operating at 600 MHz for 1 H, 150 MHz for 13 C{H}, and 564 MHz for 31 P or on a Bruker AVANCE 400 operating at 400 MHz for 1 H, 100 MHz for 13 C{H}, 376 MHz for 31 P or on a Bruker AVANCE 500 operating at 500 MHz for 1 H, 125 MHz for 13 C{H}, 470 MHz for 31 P. HRMS measurements were performed using an Agilent Accurate-Mass Q-TOF ESI-MS 6546. GC analyses were performed on an Agilent 7820-GC instrument fitted with an Agilent Front SS7 inlet N2 HP-PLOT Q column (30 m length × 530 μm × 40 μm) using the following method: Agilent 7820-GC back detector: TCD starting temperature: 40 °C; time at starting temp: 0 min; ramp: 40 °C min −1 up to 250 °C with holding time = 5 min; flow rate (carrier): 25 mL min −1 (N 2 ); split ratio: 195; inlet temperature: 40 °C; detector temperature: TCD: 250 °C, FID: 250 °C.…”

“…In the presence of 0.25 mol% B and 1.2 equivalents of NaOH, 99% yield of acetic acid and 95% yield of H 2 were obtained after 18 h. They extended the above studies to the dehydrogenation of various primary aliphatic and aromatic alcohols to corresponding carboxylic acids. 18 Our group has recently reported a series of ruthenium 19,20,21a,22a,23,22d Mn, 22e Co 22c and Ni 22b ) complexes based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl)pyridine ligands which contain weaker σ-donor imines while keeping the central pyridyl N intact in comparison to traditional pincers with moderately good π-accepting phosphine flanking groups. The resulting NNN pincer-metal complexes comprising mainly of ruthenium were found to efficiently catalyze several transformations that include but not limited to Kharasch addition of CCl 4 to styrene, 19 dehydrative etherification of secondary alcohols, 20 N-alkylation of amines, 21 β-alkylation of secondary alcohols, 22a,f glycerol dehydrogenation to lactic acid, 23 Guerbet upgradation of ethanol to n-butanol 22,24 and methanol reforming to formic acid and hydrogen.…”

Pincer–Ruthenium-Catalyzed Reforming of Methanol─Selective High-Yield Production of Formic Acid and Hydrogen

“…The NNN pincer–Ru acetonitrile complexes ( 3a–c ) were synthesized in good yields by the treatment of the corresponding ligands − with [Ru­( p -cymene)­Cl 2 ] 2 in acetonitrile under reflux conditions overnight, followed by washing with diethyl ether (Scheme ). The complexes 3a–c were fully characterized by high-resolution mass spectrometry (HRMS) and infrared (IR), 1 H, and 13 C nuclear magnetic resonance (NMR) studies.…”

“…All catalytic reactions were set up under an argon atmosphere using dried glassware. The ligands 4a–c and complexes 1a–f and 2a–f were prepared according to the literature procedure. − , …”