Microwave‐Assisted Transfer Hydrogenation of Ketones by Ru(xantphos) Arene Complexes

Abstract

A Ru‐catalyzed hydrogen transfer from 2‐propanol to acetophenone in the presence of the strong base KOtBu afforded 1‐phenyl alcohol in >99 % yield within 5 min under microwave irradiation; no reaction occurred in the absence of the base. A series of catalysts was then prepared in situ from [RuCl2‐η6‐p‐cymene]2 and xantphos family ligands, which were characterized by having natural bite angles ranging from 102.2 to 137.9°. The Ru(xantphos)arene catalyst was used for various substrates to give yields and turnover frequencies between 48 and 99 % and between ≥1431 and 2982 h−1, respectively, for acetophenone derivatives and >99 % and ≥2982 h−1 for sterically bulky ketones (benzophenone, 9H‐fluoren‐9‐one, and α‐tetralone).

Half‐Sandwich Osmium(II) Complexes with Bidentate N,N‐Chelating Ligands and Their Use in the Transfer Hydrogenation of Ketones

“…The bands at around 400 nm can be assigned to a metal–ligand charge‐transfer transition (MLCT) (dπ–π* bands), as a result of the low‐spin d 6 configuration of the Os II complexes, which provide filled orbitals with correct symmetry that interact with the low‐lying π* orbitals of the ligand. The bands of medium and high intensity are assigned to the ligand‐centered π–π* and n–π* transitions, respectively, in agreement with observations on complexes with similar N,N′ bidentate ligands , …”

“…The formation of complexes 1 – 4 was also supported by observing clear shifts of the proton CH=N and 13 C NMR signals of the imine between the complex and free ligands. Thus, the proton signal of the imine in the complex shifted downfield relative to that of the free ligand, owing to the imine protons being deshielded because the nitrogen donates a lone pair of electrons to the osmium , . Also, the 13 C NMR data supported that the complexes had formed in that the carbon peak of the imine shifted by approximately 6–10 ppm downfield for the complexes 1 – 4 relative to the free ligand.…”

“…The IR spectra of the complexes 5 – 12 exhibit bands resulting from the chelated N,N‐donor bidentate ligands in the 1626 to 1384 cm –1 region, corresponding to the different stretching frequencies of the C=C and C=N bonds of these complexes. Also, the infrared spectra of 1 – 12 showed a strong band between 836 and 847 cm –1 , owing to the stretching frequency ν(P–F) of the PF 6 counter ion, of the complexes. The ESI‐MS of complexes 1 – 12 revealed only one m/z peak, which matched the expected molecular ions [ M + ].…”

“…Research has shown that the type of arene, the nature of the chelating ligands and leaving groups in these compounds can significantly influence their chemical and biological activity and they show structure–activity relationships . In this study, in continuation of our study on catalytic applications of half‐sandwich compounds with bidentate ligands, we describe the preparation of new half‐sandwich osmium(II) complexes with nitrogen‐based bidentate ligands and then use of these complexes in the transfer hydrogenation of ketones.…”

Synthesis and Characterization of Half‐Sandwich Ruthenium Complexes Containing Aromatic Sulfonamides Bearing Pyr­id­inyl Rings: Catalysts for Transfer Hydrogenation of Acetophenone Derivatives

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Keywords: Transfer hydrogenation / Sulfonamides / Ruthenium / N ligands benzenesulfonamides 1-6 were successfully synthesized by the reaction of 8-aminoquinoline and various benzenesulfonyl chlorides. [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Herein, a series of half-sandwich Ru II complexes containing sulfonamide ligands were synthesized and characterized by various spectroscopic techniques. The synthesized compounds were characterized by NMR and FTIR spectroscopy and elemental analysis, and compounds 8 and 9 were further ana-

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3224 lyzed by X-ray diffraction.

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