Alessandro Del Zotto

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.

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.

2-(Diphenylphosphino)pyridine (dppy) as monodentate or bridging ligand in homoleptic silver(I) complexes. Crystal structures of [Ag(η1-dppy)4][ClO4] and [Ag2(η1-dppy)(μ-dppy)2][ClO4]2

Abstract The silver(I) homoleptic complexes [Ag2(dppy)n](ClO4)2 n = 2 for 1 or 3 for 2) and [Ag(dppy)n]ClO (n = 3 for 3 or 4 for 4) [dppy = 2-(diphenylphosphino) pyridine] have been isolated in the solid state and characterised in solution by means of variable-temperature 31P NMR spectroscopy. Both dinuclear species 1 and 2 possess an 8-membered annular core structure with two ligands bridging head-to-tail the metal ions, while in the mononuclear derivatives all dppy ligands act as P-bound monodentate. Treatment of 1 with increasing amounts of dppy revealed (by 31P NMR spectroscopy) that complexes 2, 3 and 4 are formed in a stepwise manner. Surprisingly, no stable single complex is formed when the dppy/Ag(I) ratio is 2. A mixture of complexes 2, 3 and 4, along with unidentified products, is conversely present in solution. The solid state structures of complexes 2 and 4 have been determined by single-crystal X-ray diffraction. In 2 the Ag(I) centres posses different environments, one metal ion being two-coordinated (NP donor set) and the other being three-coordinated (NP2 donor set). The cations of 4 present the metal bound to four dppy ligands in a slightly distorted tetrahedral environment. Of the two crystallographically independent cations, one showsthe axial Ag-P bond lenght shorter (2.566(8) A) than the value for the off-axis Ag bonds (2.624(5) A).

Reactions of Diazo Compounds with Alkenes Catalysed by [RuCl(cod)(Cp)]: Effect of the Substituents in the Formation of Cyclopropanation or Metathesis Products

The reaction of diazo compounds with alkenes catalysed by complex [RuCl(cod)(Cp)] (cod = 1,5-cyclooctadiene, Cp = cyclopentadienyl) has been studied. The catalytic cycle involves in the first step the decomposition of the diazo derivative to afford the reactive [RuCl(Cp){=C(R(1))R(2)}] intermediate and a mechanism is proposed for this step based on a kinetic study of the simple coupling reaction of ethyl diazoacetate. The evolution of the Ru-carbene intermediate in the presence of alkenes depends on the nature of the substituents at both the diazo N(2)=C(R(1))R(2) (R(1), R(2) = Ph, H; Ph, CO(2)Me; Ph, Ph; C(R(1))R(2) = fluorene) and the olefin substrates R(3)(H)C=C(H)R(4) (R(3), R(4) = CO(2)Et, CO(2)Et; Ph, Ph; Ph, Me; Ph, H; Me, Br; Me, CN; Ph, CN; H, CN; CN, CN). A remarkable reactivity of the complex was recorded, especially towards unstable aryldiazo compounds and electron-poor olefins. The results obtained indicate that either cyclopropanation or metathesis products can be formed: the first products are favoured by the presence of a cyano substituent at the double bond and the second ones by a phenyl.

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.

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a Platinum(II) Complex

Complexes of the type cis-[PtPh2(CO)(η1-P−N)] and cis-[PtPh2(CO)(η1-P−S)], where bidentate phosphorus−nitrogen and phosphorus−sulfur ligands are bound to the metal centre in a monodentate fashion [P−N = Ph2PC5H4N (Ph2PPy), Ph2P(CH2)2C5H4N (ppye), Ph2P(o-C6H4)NMe2 (PNMe2), Ph2P(CH2)nNMe2 (n = 2, 3, i.e., peNMe2 and ppNMe2) and P−S = Ph2P(CH2)2SEt (P−SEt), Ph2P(CH2)nSPh (n = 1, 2, i.e., P−CH2SPh and P−SPh)], were prepared in situ by reaction of the hybrid ligands with cis-[PtPh2(CO)(SEt2)]. In each case, the first observed process was the fast substitution of diethyl sulfide by the phosphanyl group leading to the monosubstituted ring-open cis-[PtPh2(CO)(η1-P−X)] (X = N or S) complexes, which were characterised in solution by 1H and 31P{1H} NMR spectroscopy. These initially formed species undergo a slow ring closure process with extrusion of carbon monoxide and formation of the chelate [PtPh2(P−X)] products, except in the case of the short-bite Ph2PPy and P−CH2SPh ligands and of ppNMe2, where ring closure was not observed. The chelate complexes were isolated as solids from the reaction of the ligands with cis-[PtPh2(Me2SO)2]. A single-crystal X-ray diffractometric study of cis-[PtPh2(P−SEt)] (18) was performed. The crystal packing showed linear chains originated by weak intermolecular Pt···H−C hydrogen bonding interactions. The chelation kinetics of P−X in the cis-[PtPh2(CO)(η1-P−X)] complexes have been monitored in [D]chloroform by 1H and 31P{1H} NMR. The rates of ring closure were found to be strongly dependent on the nature (S or N) and steric hindrance of the chelating end of the monocoordinated bidentate P−X ligand, and on the size of the ring formed. In contrast, ring size plays a negligible role, if any, in the dechelation reactions of cis-[PtPh2(S−S)] [S−S = 1,2-bis(phenylthio)ethane, dpte and 1,3-bis(phenylthio)propane, dptp] using diphosphanes (dppm and dppp) as reagents. These kinetic data, together with those of previous work, give useful insight into the factors controlling cyclisation reactions and the stability of the rings in square planar platinum(II) complexes.

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.

Pd Metal Catalysts for Cross-Couplings and Related Reactions in the 21st Century: A Critical Review

Cross-couplings and related reactions are a class of highly efficient synthetic protocols that are generally promoted by molecular Pd species as catalysts. However, catalysts based on more or less highly dispersed Pd metal have been also employed for this purpose, and their use, which was largely limited to the Heck reaction until the turn of the century, has been extended in recent years to most reactions of this class. This review provides a critical overview on these recent applications of Pd metal catalysts. Particular attention is devoted to the discussion of the mechanistic pathways that have been proposed to explain the catalytic role of Pd metal. Furthermore, the most outstanding Pd metal based catalytic systems that have emerged are illustrated, together with the development of novel approaches to boost the reactivity of Pd metal. A section summarizing the current industrial applications of Pd metal catalyzed reactions of this kind concludes the review.

Generation and Rearrangements of Ylides from Tertiary Amines and α-Diazo Ketones − Very High Catalytic Activity of [RuCl(η5-C5H5)(PPh3)2]

The reaction of N,N-dimethyl-2-propen-1-amine, N,N-dimethylbenzylamine and N,N-dimethyl-2-propyn-1-amine with the α-diazo ketones N2CHCOR [R = Me (2a), Et (2b), nPr (2c), iPr (2d), (CH2)10Me (2e), (CH2)14Me (2f) or Ph (2g)] in a 1:1 molar ratio, catalysed by the complex [RuCl(η5-C5H5)(PPh3)2] (1) (1% mol), have been investigated. Noticeably, the corresponding α-amino ketones 3a−g, 4a−g and 5a−g are readily and quantitatively formed by rearrangement of transient nitrogen ylides. Compounds 3−5, most of which have not yet been reported, have been isolated and fully characterised by IR, 1H and 13C NMR spectroscopy and GC-MS. It has been proved that complex 1 is an excellent and specific catalyst for the generation of nitrogen ylides from diazo carbonyls.