Synthesis of novel P-ketimine bidentate ferrocenyl ligands with central and planar chirality and comparsion in the catalytic activity between P-ketimine and P-aldimine

A series of novel ferrocenylphosphine-ketimine ligands 6 were prepared by reaction of (R,S_p)-PPFNH₂-R or (S,S_p)-PPFNH₂ with a variety of m-substituted acetophenones. A different catalytic activity was observed between ferrocenylphosphine-ketimine ligands and corresponding aldimine ligands. The efficiency and diastereomeric impact of these ferrocenylphosphine-ketimine ligands in Pd-catalyzed asymmetric allylic alkylation were first investigated, and higher enantioselectivity of over 98% e.e. with 95% yield was obtained by the use of ferrocenylphosphine-ketimine ligands. However, in Rh-catalyzed asymmetric hydrosilylation of aryl ketones, only 42% e.e. was obtained by the use of ferrocenylphosphine-ketimine ligands compared to 90% e.e. with the use of aldimine ligands.     

The synthesis of P-stereogenic MOP analogues and their use in rhodium catalysed asymmetric addition

The synthesis is reported of novel P-stereogenic binaphthyl substituted monophosphines via a short five-step synthesis using a nickel coupling reaction with separation of the borane-protected diastereomeric products. Extensive coordination studies of these ligands with a number of well-known metal precursors were performed to more effectively understand their behaviour during catalysis.These ligands and some previously reported P-stereogenic ligands were tested in the rhodium catalysed asymmetric addition of phenyl boronic acid to napthaldehyde. These studies in asymmetric catalysis were used to compare the chiral induction of ligands that combine both axial and central chirality with ligands lacking P-stereogenicity.     

Phosphinidene generation from phosphorus heterocycles and cages – A theoretical study

The dissociation properties of several phosphorus heterocycles and cages (complexes of phosphinidenes with P-donor ligands) were investigated using DFT methods. Good correlations have been found between the dissociation energies of the complexes with P and N ligands. The comparison with N-donor complexes revealed that the stability of complexes with pyrido-annelation(s) is very similar. The trends in dissociation energies for the benzannelated complexes are identical; however, the complexes with P ligands are much more stable than those with N ligands. The major reason for the different behavior is the pyramidality of the bridgehead P/N atoms in these complexes.     

Chiral amino-urea derivatives of (1R,2R)-1,2-diaminocyclohexane as ligands in the ruthenium catalysed asymmetric reduction of aromatic ketones by hydride transfer

Several new chiral urea and thiourea ligands have been prepared by reaction of (1R,2R)-1,2-diaminocyclohexane with various organic isocyanates and isothiocyanates. These were used as ligands in the ruthenium catalysed enantioselective reduction of aromatic ketones by isopropanol. The reduction proceeded at room temperature using 2 mol% of ruthenium catalyst to give good yields of the (R)-alcohol with enantiomeric excesses of up to 83%. By contrast, the use of bis-urea ligands gave much lower enantioselectivities. Amino-thiourea ligands led to the (S)-alcohol with low enantiomeric excess.     

Palladium catalyzed asymmetric sulfonylation mediated chiral β-hydroxy- and β-(o-diphenylphosphino)benzoyloxy (o-diphenyl phosphino)benzamides

The palladium catalyzed asymmetric allylic sulfonylation reaction has been investigated employing β-hydroxy- and β-(o-diphenylphosphino)benzoyloxy (o-diphenyl phosphino)benzamides as chiral, non-racemic ligands. The bisphosphine β-benzoyloxybenzamide ligands proved to be the best ligands for this process. Competitive transition states for the (1S,2R)-norephedrine derived ligand 14 are compared and a rationale is provided for the observed enantioselectivities.     

Palladium complexes of phosphinamine ligands in the intramolecular asymmetric Heck reaction

The synthesis of two novel cyclisation substrates for the asymmetric intramolecular Heck reaction is reported. Their cyclisation, in addition to a known substrate for cis-decalin formation, were tested with palladium complexes of BINAP and heterobidentate oxazoline-containing ligands. In general BINAP provides a more active catalyst system for the range of substrates tested although excellent enantioselectivities of up to 85% were obtained with the P,N ligands studied. A trend was noted whereby the t-leucine-derived oxazoline ligands were more reactive and enantioselective than the valine-derived analogues. Similarly, the diphenylphosphinoferrocenyloxazoline ligands were more reactive and selective than the corresponding diphenylphosphinophenyloxazoline ligands.     

A correlation study of bisphosphine ligand bite angles with enantioselectivity in Pd-catalyzed asymmetric transformations

Among the bisphosphine ligands, we have previously developed C_n-TunePhos (n = 1-6) as a family of ligands with tunable bite angles. The increase in spacer -CH₂- groups in this family of ligands causes changes in ligand dihedral angle, which in turn causes P-Pd-P bite angle variation. Pd-catalyzed asymmetric alkylations and cycloadditions have been tested with C_n-TunePhos ligands. This study aims at a possible correlation between ligand bite angles with enantioselectivity of the Pd-catalyzed asymmetric products.     

The Fluorescent Properties of Heteroligand Europium Cinnamates

Heteroligand europium cinnamates with nitrogen- and phosphorus-containing neutral ligands were studied by luminescent and X-ray electron spectroscopy. A tendency toward an increase in the electron density on Eu³⁺ was observed when neutral ligands were coordinated by donor nitrogen atoms. The Δν₀, ΔF ₁, and I(F ₀)/I(F ₂) values monotonically decreased as the donor properties of neutral ligands increased because of strengthening of the interaction of Eu³⁺ with the ligands.     

Structural features of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms in trans-positions to oxo ligands: Complexes with hydroxo ligands in trans-positions to O(oxo)

Some structural features of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of hydroxo ligands in trans-positions to multiply bonded oxo ligands are considered. The complexes contain monodentate inorganic or organic ligands or bi- and tetradentate organic ligands in the equatorial plane.     

A practical and efficient synthesis of chiral N,N-disubstituted C2 symmetric diamines derived from (R,R)-1,2-diaminocyclohexane

An improved synthesis of chiral diamine ligands derived from (R,R)-1,2-diaminocyclohexane is described, providing N-substituted diamines. The synthesis of other new ligands based on this methodology is also reported.     

Synthesis,structure, and magnetic properties of magnesium bis-3,6-di-<Emphasis Type="Italic">tert</Emphasis>-butyl-<Emphasis Type="Italic">о</Emphasis>-benzosemiquinonate complexes with N-donor ligands

A series of six-coordinate magnesium bis-3,6-di-tert-butyl-о-benzosemiquinonate complexes with different N-donor ligands (2,2´-dipyridyl, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, pyridine, N-phenyl-1-(4-pyridinyl)methanimine) were synthesized. The molecular and crystal structures of these complexes were determined by X-ray diffraction. In the complexes with bidentate N-donor ligands, the O,O-diolate o-semiquinone ligands are cis to each other, whereas the compounds with monodentate ligands contain organic radical anions in trans positions. The specific features of magnetic exchange coupling between organic radical ligands were studied by temperature-dependent magnetic susceptibility measurements.     

Imidazo[1,5-a]pyridine-3-ylidenes—pyridine derived N-heterocyclic carbene ligands

The ready synthesis of differently substituted 2H-imidazo[1,5-a]pyridin-4-ium bromides is reported. These salts are precursors for a new class of N-heterocyclic carbene ligands. As a consequence of their bicyclic geometry, these ligands are capable of influencing the coordination sphere of a carbene bound metal. The usefulness of these ligands was demonstrated in the palladium-catalyzed Suzuki-Miyaura cross-coupling of sterically hindered aryl chlorides.     

A new class of o-hydroxyaryl-substituted N-heterocyclic carbene ligands and their complexes with palladium

A facile synthesis of o-hydroxyaryl-substituted N-heterocyclic carbene ligands and their complexes with palladium is presented. This kind of salicylaldimine-like NHC ligands expands the class of available NHC ligands for organometallic catalysts.     

Studies on some oxovanadium(IV) complexes of acyl pyrazolone analogues

A series of novel bidentate pyrazolone based Schiff base ligands were synthesized by interaction of 4-benzoyl-3-methyl-1-(4′-methylphenyl)-2-pyrazolin-5-one with various aromatic amines like aniline, o-,m-,p-chloroaniline and o-,m-,p-toluidine in a ethanolic medium. All of these ligands have been characterized on the basis of elemental analysis, IR and ¹H NMR data. The molecular geometries of five of these ligands have been determined by single crystal X-ray study. Crystallographic study reveals that these ligands exist in the amine-one tautomeric form in the solid state. NMR study also suggests the existence of the amine-one form in solution at room temperature. Ab initio calculations for representative ligand HL¹ has been carried out to know the coordination site of the ligand. Novel vanadium Schiff base complexes of these ligands with general formula [OV(L^1–7)₂(H₂O)] have been prepared by interaction of aqueous solution of vanadyl sulfate pentahydrate with DMF solution of the appropriate ligands. The resulting complexes have been characterized on the basis of elemental analysis, vanadium determination, molar conductance and magnetic measurements, thermo gravimetric analysis, infrared and electronic spectral studies. Suitable distorted octahedral structures have been proposed for these complexes.     

The preparation and resolution of 2-(2-pyridyl)- and 2-(2-pyrazinyl)-Quinazolinap and their application in palladium-catalysed allylic substitution

The preparation and resolution of two new axially chiral quinazoline-containing phosphinamine ligands, 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, is described. The ligands were synthesised in good yield over eight steps and included two Pd-catalysed reactions, a Suzuki coupling to form the biaryl linkage and the introduction of the diphenylphosphino group, as the key transformations. The racemic ligands were resolved via the fractional crystallisation of diastereomeric palladacycles derived from (+)-di-μ-chlorobis{(R)-dimethyl[1-(1-naphthyl)ethyl]aminato-C₂,N}dipalladium (II) X-ray crystal structures of the (S,R)-2-pyridyl- and (S,R)-2-pyrazinyl-palladacycles are included. Displacement of the resolving agent by reaction with 1,2-bis(diphenylphosphino)ethane gave enantiopure 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, new atropisomeric phosphinamine ligands for asymmetric catalysis. These ligands were applied in the palladium-catalysed allylic substitution of 1,3-diphenylprop-2-enyl acetate resulting in moderate conversions and enantioselectivities of up to 81%.     

Effects of low-molecular-weight organic ligands and phosphate on adsorption of Pseudomonas putida by clay minerals and iron oxide

Adsorption of Pseudomonas putida on kaolinite, montmorillonite and goethite was studied in the presence of organic ligands and phosphate. Citrate, tartrate, oxalate and phosphate showed inhibitive effect on P. putida adsorption by three minerals in a broad range of anion concentrations. The highest efficiencies of the four ligands in blocking the adsorption of P. putida on goethite, kaolinite and montmorillonite were 58–90%, 35–76% and 20–48%, respectively. The ability of organic ligands in prohibiting the binding of P. putida cells to the minerals followed the sequence of citrate > tartrate > oxalate > acetate. The significant suppressive effects on P. putida adsorption were ascribed to the increased negative charges by adsorbed ligands and the competition of ligands with bacterial surface groups for binding sites. The inhibitive effects on P. putida adsorption by organic ligands were also dependent on the steric hindrance of the molecules. Acetate presented promotive effect on P. putida adsorption by kaolinite and goethite at low anion concentrations. The results obtained in this study suggested that the adsorption of bacteria in soils especially in the rhizosphere can significantly be impacted by various organic and inorganic anions.     

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.     

Synthesis of N,N-bis[2-(2-pyridyl)ethyl]amino steroids and related compounds intended as chiral ligands for copper ions

Compounds with the N,N-bis[2-(2-pyridyl)ethyl]amino structure (RPY2) are useful tridentate ligands for copper(I) ions, which can bind and activate oxygen from the atmosphere. For diastereoselective and enantioselective oxidation reactions, hitherto unknown chiral ligands possessing tripodal structures have been synthesized starting from homochiral steroids. The double Michael addition of primary steroidal amines and aminoalcohols to 2-vinyl pyridine was not very succesful. However, homochiral bidentate ligands with N-[2-(2-pyridyl)ethyl]amino steroid structure could be obtained by this procedure in most cases. New routes (acylation of the bidentate ligands with 2-pyridylacetic acid followed by BH₃·THF reduction, or reductive amination of steroidal ketones, acylation and borane reduction) to the desired tridentate RPY2, also at sterically hindered positions, are described. In the last reaction sequence, ‘mixed’ tridentate ligands can also be obtained. Copper complexation and oxygen activation with these ligands are briefly discussed.     

Synthesis and application of C2-symmetric diamino FERRIPHOS as ligands for enantioselective Rh-catalyzed preparation of chiral α-amino acids

C₂-Symmetrical ferrocenyl diamino diphosphines (diamino FERRIPHOS ligands) proved to be excellent ligands for the rhodium-catalyzed enantioselective reduction of methyl α-acetamidoacrylates. The straightforward synthesis, their air stability and the easy modification of their structure makes these ligands especially interesting for transition metal-catalyzed hydrogenations. α-Amino acids with enantioselectivities greater than 95% (and up to 99.3% ee) were obtained with no need of further recrystallization.     

(S)-Proline-derived new chiral ligands with phosphino,organosulfur or organoselenenyl functionality as an enantiocontrollable coordinating element

The synthesis of (S)-proline-derived chiral ligands bearing phosphino, organosulfur or selenenyl groups and their use as chiral ligands in palladium-catalyzed asymmetric allylic alkylations has been accomplished. In particular, the (S)-proline-derived phosphine ligands bearing alkylsulfenyl groups provided high enantioselectivity, and the degree of asymmetric induction was dependent upon the steric bulk of the alkyl substituents in sulfenyl groups. The stereochemical results are rationalized by a plausible mechanism involving the assistance of chelates formed by the participation of the two preferred heteroatoms involved.