Osmium(II) CNN Pincer Complexes as Efficient Catalysts for Both Asymmetric Transfer and H₂ Hydrogenation of Ketones

Abstract

Catalytic asymmetric hydrogenation (HY) and transfer hydrogenation (TH) of carbonyl compounds are usually performed with Ru, Rh, and Ir complexes, [1] and much less attention has been devoted to other transition-metal basedsystems. Although osmium complexes have occasionally been employed in the HY [2] and TH [3] of carbonyl compounds, we have recently shown that [OsCl 2 P 2 (Pyme)][4a] [P 2 = diphosphane; Pyme = 1-(pyridin-2-yl)methanamine)] complexes are extremely active catalysts in the TH of ketones, with reaction rates comparable to those of their Ru analogues.[4b] Owing to the stronger bonding of Os compared to Ru, robust and thermally stable chiral complexes can be obtained using a suitable set of ligands, which is a crucial breakthrough in achieving catalysts that display high productivity.[5] Since the CNN pincer complexes [RuCl(CNN)P 2 ][6] obtained from 1-(6-arylpyridin-2-yl)methanamines (HCNN) are among the most active catalysts for the TH of carbonyl compounds, we decided to investigate the chemistry of osmium complexes with CNN ligands and report herein the isolation of the first CNN pincer osmium derivatives [OsCl(CNN)P 2 ]. These complexes are extremely active and productive catalysts for both the TH and HY of ketones (turnover frequency (TOF) of up to 1.3 10 6 h À1 ). Moreover, asymmetric TH [7] and HY [1] have been achieved (ee values of up to 98 %) with a remarkably low loading (0.005-0.002 mol %) of the chiral version of these pincer complexes. Finally, the reversible nature of the ketone insertion into the Os À H bond suggests that Os-hydride and Os-alkoxide species are involved in TH and HY. Treatment of [OsCl 2 (PPh 3 ) 3 ] with 1 a (1.2 equiv) in 2-propanol in the presence of the weak base NEt 3 at reflux for 2 h affords the CNN pincer complex 2 [8] in 94 % yield (Scheme 1). The 31 P NMR spectrum of 2 shows two doublets at d = 7.1 and À1.6 ppm with a 2 J P,P coupling constant of 15.7 Hz, which is typical for a cis OsP 2 arrangement. The singlet at d = 7.95 ppm in the 1 H NMR spectrum is due to the CH proton close to the orthometalated carbon. Complex 3[8] is easily obtained in high yield upon treatment of [OsCl 2 (PPh 3 ) 3 ] with Ph 2 P(CH 2 ) 4 PPh 2 (dppb) and then with 1 a in the presence of NEt 3 (Scheme 1). The NCH 2 group gives a 13 C NMR signal at d = 54.8 ppm ( 3 J C,P = 2.6 Hz), whereas the orthometalated carbon appears at d = 161.5 ppm ( 2 J C,P = 9.0 and 3.9 Hz). The chiral pincer complex 4 was prepared in a similar manner to 3 but with dppb in combination with 1 b (85 % yield), whereas 5 and 6 (55 and 47 % yield, respectively) were obtained from (S,R)-Josiphos with 1 a and 1 c, respectively (Scheme 1).[8] 31 P NMR spectroscopic control experiments revealed the formation of different isomers in the reaction mixtures; the predominant stereoisomer was isolated from the reaction mixtures.The osmium derivatives 2-6 were found to be highly active catalysts for the TH of different ketones 7 (0.1m) in basic 2-propanol (Scheme 2). Thus, fast reduction of acetophenone (7 a) into 1-pheny...

Pincer and Diamine Ru and Os Diphosphane Complexes as Efficient Catalysts for the Dehydrogenation of Alcohols to Ketones

“…[1e] Recently, the Ru and Os phosphane complexes cis-[MCl 2 (PP)A C H T U N G T R E N N U N G (ampy)] (M = Ru, [13] Os; [14] PP = diphosphane, ampy = 2-(aminomethyl)pyridine) were found to be highly efficient catalysts for the (asymmetric) transfer hydrogenation and hydrogenation of ketones. Furthermore, the related pincer complexes [MClA C H T U N G T R E N N U N G (CNN)(PP)] (M = Ru, [15] Os [16] ), prepared by using 6-(4'-methylphenyl)-2-pyridylmethylamine (HCNN), catalyze the reduction of carbonyl compounds with high productivity when using 0.05-0.001 mol % of catalyst and a high rate (turnover frequency (TOF) up to Abstract: The ruthenium and osmium complexes…”

Chiral Bis(oxazolinyl)phenyl RuII Catalysts for Highly Enantioselective Cyclopropanation

“…In this context, introduction of a chiral moiety as part of the ligand framework to create transition-metal catalysts for asymmetric reactions has been investigated. Recently, several highly reactive Ru and Os complexes with chiral CNN-type ligands [2] and Fe and Co complexes with chiral bis(pyridylimino)isoindole ligands [3] were reported as active chiral catalysts.During our investigation of transition-metal catalysts with chiral meridional NCN ligands, we reported the bis(oxazolinyl)phenyl (phebox) ligand as one of a larger family of ligands including bis(oxazolinyl)pyridine (pybox).[4] Although the meridional geometry of the phebox ligand is similar to that of the pybox ligand, the metal complex with the phebox ligand possesses a metal-carbon s bond that may induce different reactivities. We anticipated that the simple change from a benzene substituent to a pyridine ring as the ligand backbone would lead to a change in the catalytic performance of the complex.…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…These have been demonstrated to be powerful catalysts for the enantioselective reduction of ketones and imines. Recently, Baratta and co-authors [11] found that 2-(aminomethyl) pyridine (ampy) accelerated the transfer hydrogenation (TH) of ketones [12][13][14] in Ru(II) and Os(II) complex catalysts. Very recently, Morris group [15,16] reported that chiral NNPP ligands and their iron(II) complexes were effective in the ATH of ketones under mild conditions, representing a rare case of using inexpensive iron as the active metal.…”