Chiral diphosphorus ligands are widely used in asymmetric homogeneous catalysis . Many of these ligands feature stereogenic carbon atoms (e.g. Chiraphos, based on a chiral 2,3-dimethylpentane backbone), some form of planar chirality (e.g. Josiphos) or chiral atropisomerism, as present in e.g. binaphthyl-based scaffolds, with BINAP as a well-known example . Diphosphine ligands bearing P-stereogenic centers have also been widely explored [3-6]. It is assumed that the close proximity of the P-stereogenic atom to the catalytically active metal centre offers high potential for asymmetric induction . Several methods for the synthesis of the P-stereogenic phosphine ligands have been developed, including resolution of racemates, synthesis by asymmetric catalysis and also stereoselective synthesis [6,8]. However, the synthesis of P- stereogenic phosphines remains challenging, typically involves multiple steps, generating products that generally display at least some degree of oxidation-sensitivity and also because the free P-stereogenic phosphines are potentially prone to racemisation at phosphorus.
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This work was supported by an EASTCHEM fellowship (to C.F.C.) and by the European Union, through a Marie Curie Excellence Grant MEXT-2004-014320. A.G.J. thanks the UK Catalysis Hub for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC (portfolio grants EP/K014706/1, EP/K014668/1, EP/K014854/1 and EP/K014714/1) and the EPSRC for EPSRC Critical mass grant ‘Clean catalysis for sustainable development’ (EP/J018139/1). J.I.v.d.V. thanks the ERC for a Starting Grant 2790097. F.J.L.H. thanks the European Union (Marie Curie ITN SusPhos, Grant Agreement No. 317404) for financial support. We thank Stephen Boyer (London Metropolitan University) for elemental analysis and the EPSRC UK National Mass Spectrometry Facility (NMSF), Swansea for MS analysis. We are grateful to members of COST action CM0802 (PhoSciNet) for scientific discussions.
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In summary we have demonstrated a novel modular solid- phase synthetic procedure for libraries of supported diphosphine ligands on polystyrene resin. Using this facile and efficient method, incorporating only a simple work-up after each step, the supported diphosphines were obtained in high purity. A new synthetic protocol has been devel- oped for the synthesis of supported secondary phosphine 6 incorporating a bulky t-butyl group into the ligand struc- ture. Subsequently, immobilised bidentate phosphine ligands 9c–g were successfully screened in the Rh-cat- alyzed asymmetric hydrogenation of several benchmark substrates. The ligands displayed low to high activity and moderate selectivities, demonstrating that small changes in ligand structure can have a profound effect on the actual catalysis. The importance of trial-and-error in ligand dis- covery is demonstrated by these results and therefore the necessity of the development of facile combinatorial methods towards large ligand libraries. The extension to larger structural diversity will enable to combine a wider screening of the substrate scope with the anticipated good recycling performance. We are currently extending both the polystyrene supported diphosphine library and the substrate scope. Also, we are investigating the possibilities towards polystyrene supported P-stereogenic ligands and towards supported phosphine-phosphite/phosphinite ligands.
One of the main purposes of contemporary organic synthesis is the preparation of compounds in optically pure form. Among the methods developed to achieve this goal, the transition metal-catalyzed reactions using chiral ligands now play undoubtedly the most important role, especially for large-scale industrial applications. 1 In 2001, the pioneers of the metal-catalyzed asymmetric synthesis Knowles 2 , Noyori 3 and Sharpless were awarded the Nobel Prize. This emphasized the exceptional industrial significance of this branch of modern organic chemistry. The number of new metal-catalyzed asymmetric reactions and discovered ligands has been increasing from year to year. The chiral bis-phosphine ligands are the most important class of ligands for the transition metal catalysis. The first important ligand of this type, DIPAMP, discovered by Knowles and Horner, 4 enabled the first industrial asymmetric synthesis of an amino acid L-DOPA, an extremely important pharmacological agent in the treatment of the Parkinson’s disease. 5 The preparation of chiral diphosphine ligands which are rather complex molecules, bearing multiple chiral centers, is often expensive and lengthy. Therefore, the search for new promising structures of this class as well as the novel methods for their synthesis is an important and challenging task for organic chemists.
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The hydrogenation experiments were performed in a stainless steel autoclave charged with an insert suitable for 10 reaction vessels including Teflon mini stirring bars. In a typical experiment, a reaction vessel was charged with a deprotected resin-bound diphosphine (5 mg, approximately 3.0 μmol) and a solution of [Rh(COD) 2 ]BF 4 (3.0 μmol) in CH 2 Cl 2 (1 mL) and the heterogeneous mixture was allowed to
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Treatment of Na 2 PdCl 4 or K 2 PtCl 4 with the ligands LH (LH = AH or BH) gave complexes of the type [MX 2 (LH) 2 ] (M = Pd trans or Pt cis). The ligands LH behave as monodentate neutral ligands coordinated through the nitrogen atom of the pyridine ring for AH or the NH group attached to the methylene group in BH ligand, Treatment with the sodium salts of the ligands NaL gave the chelated complexes [ML 2 ]. The anionic ligands L behave as bidentate chelates bonded to metal through the nitrogen atom of the pyridine ring and the thiourea sulfur atom.
be rationalized in terms of H-bonding, non-bonding repulsive interactions between the chelating ligand and nucleobase substituents, and the electronic properties of the various nucleobase coordination sites. 30 The chelating ligands in the present work also have a significant effect on the selectivity of nucleobase binding. Complex 1 containing N,N-chelating 2,2′-bipyridine reacted only with 9-EtG, whilst complex 2 containing C,N-chelating 2-phenylpyridine formed both 9-EtG and 9-MeA adducts, preferentially binding to 9-EtG when in competition, which may result from the steric hindrance of the NH 2 group at the 6-position of the adenine ring. Some Pt II antitumor
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electronic properties of the ligands employed revealed that oxidation of the Au(I) center was favoured with small and more electron donating NHC ligands, providing important information about the ligand requirements to favour the oxidative addition of Au(I) species to Au(III) with hypervalent iodine oxidants. This study allowed for the design of a new system where oxidation of the metal centre appears to be more facile. Complex [Au(BMIM) 2 ][BF 4 ] was synthesized in multi-gram scale via a one-
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coordinated to Ru(1). There is an η 2 -quadruply bridging carbonyl ligand which is coordinated to three metal atoms Ir(1), Ru(2) and Ru(3) by its carbon atom. The oxygen atom is coordinated only to Ru(1). Compound 3.2 contains a total of 76 valence electrons which is precisely the number expected for a spiked-tetrahedral cluster of five metal atoms. Compound 3.3 contains only three metal atoms, one of Ir and two Ru. They are arranged in a triangle. There is one hydrido ligand that bridges the Ir(1) – Ru(1) bond. The phosphine ligand is coordinated to the iridium atom. There are eight terminally coordinated carbonyl ligands and there is one CO, C(1) – O(1), ligand that bridges the Ir(1) – Ru(1) bond. Overall, compound 3.3 contains a total of 46 valence electrons and it is thus electron deficient by the amount of two electrons. DFT molecular orbital calculations were performed by using the PBEsol functional in the ADF program library. Compound 3.4 contains four metal atoms, one of Ir and three of Ru. They are arranged in the form of a butterfly tetrahedron. There are two P(t-Bu) 3 ligands, one on Ir(1) and the
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The use of different phenolic substituents (methyl, butyl and pentyl) will provide a library of compounds suitable for metal catalytic investigations in polymerization reactions. Attachment of synthesized ligands to zinc, calcium, tin and palladium metals would offer new research opportunities in asymmetric synthesis and metal catalyzed ring-opening polymerization of lactones. There is great interest in investigating the effect of one ste- reogenic center in conjunction with phenolic bulky substituent on catalytic selectivity. The [ON] ligands are ex- pected to be bidentate with the possibility of having a tridentate coordination via the phenyl pendant arms while the [ONO] ligand has an additional coordinating oxygen atom.
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The IR spectrum of the complexes thereby excluding the involvement of any other goup in coordination and suggesting bidentate nature of the ligands NBALHC and NBALTSC respectively. The presence of coordinated water molecules is indicated by the appearance of new bands at 3400- 3200cm -1 and 840-825cm -1 due to stretching mode
Among the most difficult challenges in moving to first row transition metal catalyst systems lies in the divergent reactivity displayed by these metals relative to the second and third row metals. The first row metals have a greater propensity towards single electron reactivity often involving radical intermediates, while the 2 nd - and 3 rd -row metals are more prone to two-electron reactivity. 2-5 Recent developments in ligand design have been able to overcome this problem of divergent reactivity in several cases. Some researchers have used ligands which employ redox-active moieties in the ligand framework. Others have incorporated Lewis-acidic or –basic functionality into the ligand in order to facilitate the desired 2-electron reactivity. 6-17
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The popularity of metal-organic polyhedral cages as a major research field has two distinct underpinnings. 1,2 The first is structural: such cages represent some of the most structurally elaborate and attractive edifices available using self-assembly methods, and our emerging understanding of the underlying principles of symmetry and structural control allow preparation of species that were, until recently, unimaginable. 1 The second is functional, and largely driven by host-guest chemistry. The central cavities of many coordination cages can accommodate guest species in an environment different from that in bulk solution, leading to potential applications in areas from transport across cell membranes to catalysis of reactions inside cage cavities. 2
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Synthesis of the alkyl ether terphenyl ligands was accomplished via the procedure shown in Scheme 2.1. A substituted phenol can be treated with a 2:1 mixture of sodium iodide and sodium chlorite in the presence of acid to form the desired diiodophenol. The diiodophenol can be alkylated with a variety of alkyl halides in acetone utilizing potassium carbonate as a base to form the diiodoalkoxybenzene. Treatment of the diiodo precursor with 2-bromophenyl boronic acid under Suzuki coupling conditions yields the dibromide terphenyl ether backbone. A lithium halogen exchange followed by treatment with diisopropylchlorophosphine leads to the formation of the desired diphosphine terphenyl ether ligand. This synthesis is highly modular and several ligand variants have been synthesized. The functional group in 4’-position of the phenol can easily be changed by varying the starting phenol, which allows for variation of the electronics of the central arene. Scheme 2.5 Diphosphine Terphenyl Methyl Ether Synthesis
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are bridged by three -coordinated molecules of dibenzyl- ideneacetone (dba) (Ukai et al., 1974), have been extensively used as precursors to prepare a large variety of palladium complexes. The dba fragment is labile and can readily be exchanged for other ligands, while the palladium(0) centre can undergo oxidative addition. The title complex, (II), was obtained as the unexpected major product from the reaction of o-BrC 6 H 4 B(pinacol), (I), with [Pd 2 (dba) 3 ]dba, in an
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Data in Fig.1 and Table 2 show that all the complexes show catalytic activity of enzyme in the degradation of methyl orange. There exist postponing stage and linear increasing stage. The complexes 2a is the worst, and the complex 8a is the best. The catalysis mechanism of biomimetric catalysis, the prepared complexes, which were used as catalysis in the reaction of the methyl orange degradation, was deduced (Fig.2).
analogue, there is only a small diﬀerence in energy between com- plexes of type D / E and H . Furthermore, our data suggest that the solvent may play a key role in determining the nuclearity of gold- containing complexes and this should also be considered as an important factor in catalysis.
The isolation of iminium 169 under conditions similar to those used in catalysis raised the question of whether a similar ring-opening process was taking place during catalytic reactions and, if so, were any newly formed compounds also catalytically active? We set out to investigate this possibility with asymmetric catalyst (5R)-163 (Figure 5.5). As (5R)-163 contains an imido group, rather than the amidocarbamate functionality present in racemic catalyst 114, it was envisaged that methanolysis would take place in both an endo- (Step A) and exocyclic (Step C) manner. Step A resembles the methanolysis observed with racemic catalyst 114 to give the acyclic hydrazide 170. 170 also contains an amide bond which might be susceptible to further attack by methanol, as shown in Step B. This would give rise to CBz-proline methyl ester 171 and hydrazide (R)-172. Steps C and D represent the reverse of this process, wherein the exocyclic methanolysis takes place first to generate (R)-84 which would then be opened to hydrazide (R)-172 after a second methanol attack.
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The field of self-assembly through dynamic coordination chemistry is dominated by the use of bridging nitrogen- or oxygen-donor ligands, 1 but there is increasing interest in the use of bridging phosphorus-donor ligands for the self-assembly of molecular materials. 2,3 One strategy that has been used to increase the dimensionality of molecular materials, or to tailor their ability to act as selective hosts in host-guest chemistry, is to incorporate hydrogen bonding groups, especially amide groups, into the nitrogen- or oxygen-donor ligands. It is then possible to enable self-assembly by a combination of dynamic coordination chemistry and hydrogen bonding. 4,5 However, this strategy has rarely been used with ligands containing both diphosphine and amide functionality. 6 Phosphine-amide ligands are known and they have been studied primarily for applications in catalysis and inorganic medicine. 7-9 This chapter reports a new diphosphine ligand, N,N’-bis(2-diphenylphosphinoethyl)terephthalamide, dppeta, and its complexes with silver(I) and gold(I), all of which have sheet structures arising, at least in part, by intermolecular hydrogen bonding.
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Until recently there have been no truly efficient chiral catalysts for regio- and enantioselective hydroxy- and methoxy-carbonylation of styrene derivatives, perhaps partly due to the regioselectivity and activity problems that bidentate phosphine based catalysts have. Therefore, the need of new Pd-diphosphine based catalyst systems are of great interest, which not only gives unprecedented selectivity and activity but also provides enantioselectivity. Due to the intensive studies of different working groups, we focused on the preparation of bulky chiral diphosphine palladium systems, to combine regio- and stereo-selectivity. We envisaged that desirable properties for a new system could be a chiral backbone, larger bite angle and (for activity) possibly steric bulk at the phosphorus atom, since these characteristics have been reported of having a positive influence on the B/L ratio.
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