Pierluigi Rigo

Osmium(II) CNN pincer complexes as efficient catalysts for both asymmetric transfer and H2 Hydrogenation of Ketones

We describe the isolation of the first CNN pincer osmium complexes [OsX(CNN)P 2 ] to display high catalytic activity and productivity in the reduction of ketones by either TH or HY (TOFand TON up to 10 6 h 1 and 10 5 , respectively). A high enantioselectivity (up to 98% ee ) is possible in both reactions with a remarkably low catalyst loading (0.005–0.002 mol%). To the best of our knowledge, this is the first example of an osmium complex that catalyzes the asymmetric HY of ketones. Evidence for an Os-OR vs. Os-H equilibrium suggests that both these species are involved in the catalytic pathways. Mechanistic studies as well as the preparation of new CNN pincer osmium catalysts are currently underway.

Highly Productive CNN Pincer Ruthenium Catalysts for the Asymmetric Reduction of Alkyl Aryl Ketones

Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] (2-7; Josiphos = 1-[1-(dicyclohexylphosphano)ethyl]-2-(diarylphosphano)ferrocene) have been prepared by treating [RuCl(2)(PPh(3))(3)] with (S,R)-Josiphos diphosphanes and 1-substituted-1-(6-arylpyridin-2-yl)methanamines (HCNN; substituent = H (1 a), Me (1 b), and tBu (1 c)) with NEt(3). By using 1 b and 1 c as a racemic mixture, complexes 4-7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2-propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 10(5)-10(6) h(-1) (60 and 82 degrees C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H(2) (5 atm) at 0.01 mol % relative to the complex with a TOF of approximately 10(4) h(-1) at 40 degrees C.

Osmium Pyme Complexes for Fast Hydrogenation and Asymmetric Transfer Hydrogenation of Ketones

The osmium compound trans,cis-[OsCl2(PPh3)2(Pyme)] (1) (Pyme=1-(pyridin-2-yl)methanamine), obtained from [OsCl2(PPh3)3] and Pyme, thermally isomerizes to cis,cis-[OsCl2(PPh3)(2)(Pyme)] (2) in mesitylene at 150 degrees C. Reaction of [OsCl2(PPh3)3] with Ph2P(CH2)(4)PPh2 (dppb) and Pyme in mesitylene (150 degrees C, 4 h) leads to a mixture of trans-[OsCl2(dppb)(Pyme)] (3) and cis-[OsCl2(dppb)(Pyme)] (4) in about an 1:3 molar ratio. The complex trans-[OsCl2(dppb)(Pyet)] (5) (Pyet=2-(pyridin-2-yl)ethanamine) is formed by reaction of [OsCl2(PPh3)3] with dppb and Pyet in toluene at reflux. Compounds 1, 2, 5 and the mixture of isomers 3/4 efficiently catalyze the transfer hydrogenation (TH) of different ketones in refluxing 2-propanol and in the presence of NaOiPr (2.0 mol %). Interestingly, 3/4 has been proven to reduce different ketones (even bulky) by means of TH with a remarkably high turnover frequency (TOF up to 5.7 x 10(5) h(-1)) and at very low loading (0.05-0.001 mol %). The system 3/4 also efficiently catalyzes the hydrogenation of many ketones (H2, 5.0 atm) in ethanol with KOtBu (2.0 mol %) at 70 degrees C (TOF up to 1.5 x 10(4) h(-1)). The in-situ-generated catalysts prepared by the reaction of [OsCl2(PPh3)3] with Josiphos diphosphanes and (+/-)-1-alkyl-substituted Pyme ligands, promote the enantioselective TH of different ketones with 91-96 % ee (ee=enantiomeric excess) and with a TOF of up to 1.9 x 10(4) h(-1) at 60 degrees C.

Convenient syntheses of novel ruthenium catalysts bearing N-heterocyclic carbenes

Abstract The 16-electron ruthenium(II) complexes Cp*Ru[ C(R)N(H)CC(H)N (R)]Cl (Cp*=η5-C5Me5; R=Cy (ICy), 1a; Mes (IMes), 1b) containing N-heterocyclic carbenes are easily accessible in quantitative yields from [Cp*Ru(OMe)]2 (Me=CH3) and the corresponding 1,3-diorganylimidazolium chloride by methanol elimination. Compounds 1a–b can also be prepared in 75–80% yield by treating the commercially available polymeric ruthenium(III) compound [Cp*RuCl2]n with the free 1,3-diorganylimidazolin-2-ylidenes in 1 to 1.5 molar amounts. 1a reacts with CO, PPh3, pyridine and ethyl diazoacetate (EDA) affording the 18-electron derivatives Cp*Ru(ICy)(L)Cl (L=CO, 2; PPh3, 3; py, 4; CHCO2Et, 5). The mixed dicarbene complex 5 is the first isolable ruthenium cyclopentadienyl species bearing a CHCO2Et moiety. Compounds 1a–b catalyze the carboncarbon coupling of terminal alkynes HCCR (R=Ph, SiMe3, tBu, p-Tol) under mild conditions, with the selectivity strongly depending on the substituent R.

Cationic complexes of rhodium(I) as catalysts in the homogeneous O2-oxidation of terminal alkenes to methyl ketones

Abstract [Rh(LL) 2 ] + , [Rh(LL)(diene)] + and [Rh(LL)S 2 ] + complexes are effective as catalysts for the oxidation by dioxygen of terminal olefins to methyl ketones. The complexes act as monooxygenases, the second oxygen atom being transferred to the alcohol solvent.

Functionalised cis-Alkenes from the Stereoselective Decomposition of Diazo Compounds, Catalysed by [RuCl(η5-C5H5)(PPh3)2]

A catalytic amount of [RuCl(η5-C5H5)(PPh3)2] (1) (0.1 mol-%) at 60 °C stereoselectively decomposes α-diazo carbonyl compounds N2CHCOR [R = EtO, Me, Et, nPr, iPr, Ph, (CH2)10Me, and (CH2)14Me] affording quantitatively RCOCH=CHCOR carbene dimers [R = EtO (10), Me (11), Et (12), nPr (13), iPr (14), Ph (15), (CH2)10Me (16), and (CH2)14Me (17)]. The cis isomer is formed in 95−99% purity, depending on the R group. Under the same experimental conditions, N2CHCOR1 and N2CHCOR2 react in equimolar amounts to give mixtures of unsymmetrical cis-R1COCH=CHCOR2 (18−32), and symmetrical cis-R1COCH=CHCOR1 and cis-R2COCH=CHCOR2 alkenes. The unsymmetrical cis-alkenes 23−32 are formed in about 50% yield from the reaction between two α-diazo ketones. The yield of the cis-R1COCH=CHCOR2 compounds 18−22 increases to ca. 60% when a mixture of α-diazo ketone and N2CHCOOEt is catalytically decomposed. Higher conversions into the unsymmetrical products have been obtained by treating N2CHCOR and N2CHSiMe3, from which cis-RCOCH=CHSiMe3 derivatives 34−41 are formed in 83−91% yield. The catalytic decomposition of α,ω-bis(diazo) ketones N2CHCO(CH2)nCOCHN2 (n = 4, 8, or 10) has also been investigated, and found to afford cyclic cis-alkenes arising from both intra- [n = 4 (45), 8 (46), and 10 (47)] and intermolecular carbene-carbene coupling processes.

Efficient chemoenzymatic synthesis of chiral pincer ligands.

Chiral, nonracemic pincer ligands based on the 6-phenyl-2-aminomethylpyridine and 2-aminomethylbenzo[h]quinoline scaffolds were obtained by a chemoenzymatic approach starting from 2-pyridyl and 2-benzoquinolyl ethanone. In the enantiodifferentiating step, secondary alcohols of opposite absolute configuration were obtained by a bakers yeast reduction of the ketones and by lipase-mediated dynamic kinetic resolution of the racemic alcohols. Their transformation into homochiral 1-methyl-1-heteroarylethanamines occurred without loss of optical purity, giving access to pincer ligands used in enantioselective catalysis.

Synthesis, NMR characterisation and crystal structure of the silver(I) annular complex [Ag2(μ-ppye)2] [PF6]2 · 2 (CH3)2CO (ppye =1-(diphenylphosphino)-2-(2-pyridyl)ethane)

Abstract The silver(I) complex [Ag 2 (μ-ppye) 2 ][PF 6 ] 2 ·2(CH 3 ) 2 CO (ppye=1-(diphenylphosphino)-2-(2-pyridyl)ethane) has been prepared and its crystal structure determined by single-crystal X-ray diffraction. The dication [Ag 2 (μ-ppye) 2 ] 2+ exhibits a twelve-membered annular core structure with ppye ligands bridging head-to-tail the silver ions, which lie in a distorted linear environment. Each Ag(I) centre shows also a weak electrostatic interaction with the oxygen atom of an acetone molecule (Ag ⋯ O=2.86(1) A), which is probably responsible for the deviation from linearity of the PAgN angle (165.8(1)°). The size of the cavity can be estimated on the basis of the Ag ⋯ Ag distance, 5.077(1) A. Crystallographic data: monoclinic, space group P 2 1 / n , a =13.330(1), b=13.469(1), c=14.102(2) A, β=103.02(5)°, U=2466.7(6) A 3 , Z=2, R=0.071. The dynamic behaviour of the complex in acetone solution has been investigated by means of variable-temperature multinuclear ( 1 H, 13 C and 31 P) NMR spectroscopy.

Rhodium(I) and iridium(I) complexes with 1,2-bis(dicyclohexylphosphino) ethane ligands and their reactions with carbon monoxide

The ditertiary-phosphine 1,2-bis(dicyclohexylphosphino)ethane (dcpe) has been shown to form the mononuclear, square-planar rhodium(I) and iridium(I) complexes [M(COD)(dcpe)]BPh4 (COD = 1,5-cyclooctadiene), [M(dcpe)2]BPh4, and [MCl(CO)(dcpe)], which have been characterized by elemental analyses, infrared, and 1H and 31P{1H} NMR spectroscopy. Their behavior toward carbon monoxide in dichloromethane solution at different temperatures was examined, and the nature of the products formed established by IR and 31P{1H} NMR spectroscopy. Some of the products, including [M(CO)2(dcpe)]BPh4 and [Ir(COD)(CO)(dcpe)]BPh4, were isolated in the solid state and fully characterized. The catalytic activity of the rhodium derivatives in the decarbonylation of benzaldehyde has been studied, and compared with that of the analogous complexes containing phenyl-substituted diphosphines.

Synthesis and NMR studies of palladium(II) and platinum(II) complexes with hybrid bidentate ligands Ph2P(CH2)2SR

Abstract The reactions of a series of hybrid, potentially bidentate ligands Ph 2 P(CH 2 ) 2 SR (P-SR; R = Me, Et, Ph) with palladium(II) and platinum(II) salts are described, together with the syntheses and characterization of a variety of cationic and neutral square- planar complexes of the types [MCl 2 ( P - S R)], cis - or trans -[MX 2 ( P -SR) 2 ] (X = Cl, CN), [MCl( P - S R)( P - SR)] + and [M( P - S R) 2 ] 2+ , in which the P-SR molecules can act either as chelating ( P - S R) or monodentate, P-bound ligands ( P -SR). In solution, the various species are interconverted among each other by equilibria which show a marked dependence on solvent polarity, presence of excess ligand, temperature, nature of X and R groups.