Chiral fluorous phosphorus ligands based on the binaphthyl skeleton: synthesis and applications in asymmetric catalysis

Two enantiopure fluorous phosphines have been conveniently synthesized by combining palladium-catalyzed coupling reactions of easily available binaphthyl building-blocks with the introduction of fluorous ponytails onto aromatic compounds via ether bond formation. These new fluorous chiral phosphines have been tested as ligands in metal-catalyzed asymmetric transformations, the best results being obtained in the palladium-catalyzed asymmetric allylic substitution of 1,3-diphenyl-2-propenyl acetate affording the products of up to 87% e.e.     

Chelating N-pyrrolylphosphino-N'-arylaldimine ligands: synthesis, ligand behaviour and applications in catalysis

Two families of variously-substituted N-pyrrolylphosphino-N'-arylaldimine ligands, 2-(aryl-N=CH)C4H3N-PR2 {R=Ph; R=Pri2N}, have been prepared from the corresponding pyrrolylaldimines . The donor characteristics/basicity of P-N-chelating and have been assessed using a combination of 31P{1H} NMR and IR spectroscopies through study of the magnitudes of 1JSeP for the phosphorus(V) selenides and , and measurement of nu(CO) for the complexes [RhCl(CO)(-kappa2-P,N)], respectively. The synthesis of the palladium(II) complexes [PdCl2(-kappa2-P,N)] was readily achieved from reaction of or with [PdCl2(MeCN)2] in CH2Cl2. X-Ray crystallographic studies of and confirm the chelating nature of the P-N ligands, which adopt a distorted 'envelope' conformation, and highlight the potentially significant steric demands of these metal scaffolds. Reaction of equimolar quantities of with [NiBr2(DME)] in MeCN afforded [NiBr2(-kappa2-P,N)], while the same reaction undertaken in CH2Cl2 with gave rise to the homoleptic bis(pyrrolatoimine) derivative [Ni{2-(mes-N=CH)C4H3N}2] in 45% yield, following P-N bond cleavage. Complex was characterised in the solid-state by X-ray crystallography. No identifiable metal-containing complexes could be obtained on reaction of with a variety of sources of Ni(II). The palladium dichloride complexes and proved inactive in combination with MAO or EtAlCl2 for ethylene polymerisation, and with methanesulfonic acid for CO/ethylene co-polymerisation. Contrastingly, the nickel complexes in combination with 4.5 eq. EtAlCl2 catalysed the formation of butenes and hexenes with moderate activity from ethylene at 1 bar.     

Chiral bidentate aminophosphine ligands: synthesis, coordination chemistry and asymmetric catalysis

Chiral aminophosphines Ph2PN(R)(CH2)nN(R)PPh2 1-4 [n= 2, R = CH(CH3)(Ph) 1; n= 3, R = CH(CH2CH3)(Ph) 2, n= 2, R = CH(CH3)(1-naphthyl) 3; n= 2, R = CH(CH3)(C6H11) 4] were synthesized by the reaction of ClPPh2 with the appropriate easily accessible enantiopure amine building blocks. For compounds 1 and 2, the corresponding selenides 5 and 6 were prepared to determine the electronic character of the phosphine moieties. By reaction of 1 with either PdCl2(cod) or PdCl(CH3)(cod) the cis-complexes 7 and 8 were obtained. The molecular structure for complex 7, cis-[PdCl2(1)], was determined by X-ray crystallography. Reaction of PtCl2(cod) with 1 or 2 yielded the corresponding monomeric cis-isomers 9 and 10. The rhodium derivative [RhCl(CO)(1)] (11) was obtained as a mixture of cis and trans-isomers. Preliminary results in the rhodium catalyzed hydroformylation of styrene and vinyl acetate, with ee's up to 51% and high regioselectivities, showed the potential of these chiral aminophosphines for homogeneous catalysis.     

Chiral diphosphines and diphosphinites derived from 2,2-biphosphole: Stereodynamic ligands for asymmetric catalysis

In this account, the recent advances on chiral stereochemically dynamic 2,2’-biphosphole ligands for applications in asymmetric catalysis are reported. In the first part, the synthesis of stereodynamic diphosphines and diphosphinites derived from 2,2’-biphosphole is presented. The second part describes the kinetic dynamic resolution to give diastereo- and enantiopure complexes. Applications in asymmetric allylic substitution, hydroformylation and hydrogenation are presented in the last part.     

Chiral phosphinooxazolines with a pentamethylferrocene backbone--synthesis and use as ligands in asymmetric catalysis

Novel phosphinooxazolines, containing a unit of central and a unit of planar chirality in a matched case combination, have been successfully tested in the Pd-catalyzed asymmetric allylic substitution with cycloalkenyl acetates as substrates.     

Role of ligands in homogeneous catalysis based on transition metals

The role of ligands in homogeneous catalysis by transition metals and the promising lines of research related to simulation of biocatalytic processes are considered.     

Chiral oxazoline-1,3-dithianes: new effective nitrogen–sulfur donating ligands in asymmetric catalysis

A series of new chiral oxazoline-1,3-dithianes has been easily synthesized and used as ligands for asymmetric catalysis. The conjugate addition of Et₂Zn to enones resulted in ees of up to 69%, whereas the Pd-catalyzed allylic alkylation led to the expected products in almost quantitative yields and up to 90% enantioselectivity. The ligand’s conformation has been explored using a combination of X-ray and NMR measurements, indicating the presence of a remarkable anomeric effect, which accounts for the preference of the oxazoline ring for the axial location.     

Chiral diphosphine ligands based on camphor: synthesis and applications in asymmetric hydrogenations

The synthesis of a novel class of atropisomer chiral diphosphine ligands with a bornene framework is described. The new ligands showed in Rh catalyzed asymmetric hydrogenation of α- and β-enamides very high ee’s (more than 99%).     

Transition metal complexes of Schiff base ligand based on 4,6-diacetyl resorcinol

Novel Schiff base ligand based on the condensation of 4,6-diacetyl resorcinol with 2-amino-4-methylthiazole in addition to its metal complexes with Cr (III), Mn (II), Fe (III), Co (II), Ni (II), Cu (II), Zn (II) and Cd (II) ions have been synthesized. The structure, electronic properties, and thermal behaviour of Schiff base and its metal complexes have been studied by elemental analysis, mass, ¹H NMR, IR spectra, thermal analysis, and theoretically by density function theory. The ligand acted as mononegative bidentate (NO) ligand and all complexes showed octahedral geometry except Cu (II) showed tetrahedral geometry as indicated from the spectral and magnetic studies. The Cu (II), Zn (II) and Cd (II) complexes were non electrolytes while the rest of the complexes were electrolytes. The antibacterial plus anticancer activities of the parent Schiff base and its metal complexes were screened. In addition, the molecular docking study was performed to explore the possible ways for binding to Crystal Structure of Human Astrovirus capsid protein (5ibv) receptor.     

Concepts for ligand design in asymmetric catalysis: a study of chiral amino thiol ligands

A series of new chiral sulfur–nitrogen chelate ligands, derived from amino acids, has been synthesised rationally. Fruitless experiments into catalytic asymmetric conjugate additions and desymmetrisation of meso-epoxides led us to analyse our ligands in the catalytic asymmetric addition of diethylzinc to aromatic aldehydes. These latter experiments were successful with chiral benzylic alcohols being obtained in up to 82% enantiomeric excess.     

Novel C-H activation and C-S formation reactions on disulfide and diselenide ligands in dinuclear ruthenium complexes

The C-S bond formation reactions of the transition metal sulfides with organic molecules are collected and reviewed to understand the reactivity of the sulfide ligands supported by the transition metals. As an example of the role of the sulfide, the C-H bond activation is focused and discussed.     

Hybrid bidentate phosphorus ligands in asymmetric catalysis: privileged ligand approach vs. combinatorial strategies

In this perspective the development of chiral phosphorus ligands for asymmetric catalysis is discussed, with a special focus on hybrid bidentate phosphorus ligands, in particular phosphine-phosphoramidites. An attempt is made to compare privileged ligand and combinatorial approaches to ligand development--for which the class of phosphine-phosphoramidite ligands is well suited--highlighting differences, similarities and their complementary use.     

Rhodium hydride formation in the presence of a bulky monophosphite ligand: a spectroscopic and solid-state investigation

A study has been carried out on rhodium catalyst preforming when modified with the bulky tris(2,4-di-tert-butylphenyl) phosphite, P(Obtbp)(3). X-Ray crystal structure determinations of a tropolone-type precursor complex [Rh(TropBr(3))(CO){P(Obtbp)(3)}].P(Obtbp)(3).CH(3)COCH(3)(TropBr(3)= 3,5,7-tribromotropolonate) and the free P(Obtbp)(3) ligand are reported. Systematic in situ IR and NMR studies of the particular rhodium phosphite modified catalyst and its precursors have led to the identification of two distinct rhodium hydride species. A {(1)H,(31)P} HMBC NMR experiment afforded clarity on the (31)P NMR spectra observed under hydroformylation conditions. The species were identified as [HRh(CO)(3){P(Obtbp)(3)}] and [HRh(CO)(2){P(Obtbp)(3)}(2)]. Attention was also given to the rate of catalyst formation when starting from different rhodium precursors.     

Polysiloxane-bound ligand accelerated catalysis: a modular approach to heterogeneous and homogeneous macromolecular asymmetric dihydroxylation ligands

Polysiloxane acts as a modular scaffold for macromolecular reagent development. Two separate components were covalently integrated into one material, one constituent provided reagent functionality, the other modulated solubility. In particular cinchona alkaloid based ligands used in the osmium tetroxide catalyzed asymmetric dihydroxylation (AD) reaction were covalently attached to commercially available polysiloxane. To enhance the reactivity of these polymeric ligands, multifunctional reagents were designed to include both the cinchona alkaloid and an alkoxyethylester solubilizing moiety providing random co-polymers. While the mono-functional materials led to heterogeneous conditions, the bi-functional polymers resulted in homogeneous reaction mixtures. Although both reagent types provided diol products with excellent yield and selectivity (>99% ee in nearly quantitative yield) the homogeneous analog has nearly twice the reactivity. Every repeat unit in the heterogeneous material was functionalized along the polysiloxane backbone while approximately half of these sites contained ligand in the homogeneous version. This approach led to macromolecular catalysts with high loadings of ligand and therefore materials with very low equivalent weights. Although these polymers are highly loaded they do maintain reactivity on a par with their free ligand counterpart. Using straightforward purification techniques (i.e. precipitation, simple filtration, or ultrafiltration) these polymeric ligands were easily separated from diol product and reused multiple times. Polysiloxane is a viable support for the catalysis of AD reactions and may provide a generally useful backbone for other catalytic systems.     

Electrophilic C-H activation at Cp*Ir: ancillary-ligand control of the mechanism of C-H activation

Density functional calculations on the low-temperature cyclometalation of dimethylbenzylamine with [IrCl2Cp*]2/NaOAc have characterized a novel electrophilic activation pathway for C-H bond activation. C-H activation occurs from [Ir(DMBA-H)(kappa2-OAc)Cp*]+, and OAc plays a central role in determining the barrier for reaction. Dissociation of the proximal OAc arm sets up a facile intramolecular deprotonation via a geometrically convenient six-membered transition state. Dissociation of the distal OAc arm, however, leads to a higher energy four-membered (sigma-bond metathesis) transition state, while oxidative addition is even higher in energy. For this Ir3+ system, these three mechanisms appear to lie within a continuum in which the participation of the metal center and an H-accepting ancillary ligand are inversely related. The ability of the ancillary ligand to act as a proton acceptor is the key factor in determining which mechanism pertains.     

The unexpected role of pyridine-2-carboxylic acid in manganese based oxidation catalysis with pyridin-2-yl based ligands

A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H(2)O(2) as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H(2)O(2) and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by (1)H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H(2)O(2) required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.     

Chiral organobases: Properties and applications in asymmetric catalysis

Chiral organobases are efficient catalysts used in asymmetric reactions. This review provided a summary of chiral organobases in the aspects of their properties and applications.     

Novel nitrogen ligands based on imidazole derivatives and their application in asymmetric catalysis

Recently prepared chiral amines have been used in the preparation of novel tridentate ligands based on an imidazole ring with an additional (hetero)ring. The synthesis was carried out by the reaction of chiral amines with suitable aldehydes (2-phenylimidazole-4-carbaldehyde, 2-hydroxybenzaldehyde or pyridine-2-carbaldehyde) under reductive conditions (H₂/Pd or NaBH₄). All ligands prepared showed strong hydrogen bonds in d₆-DMSO solution, which resulted in hindered imidazole tautomerism. The observed hindered tautomerism was studied by ¹H NMR spectroscopy. The structures of the prepared ligands were also confirmed by APCI mass spectroscopy. Both chiral amines and tridentate compounds have been applied as ligands in copper (II)-catalyzed nitroaldol reactions (Henry reaction). Various reaction conditions for the Henry reaction have been studied (influence of temperature, molar ratio, solvent or copper (II) precursors). The compounds prepared with the two imidazole rings showed fast reaction times and a reversal in enantioselectivity compared to other chiral amines.