Reduction of carbonyl compounds via hydrosilylation: V. Synthesis of optically active allylic alcohols via regioselective asymmetric hydrosilylation

Regioselective asymmetric reduction of prochiral α,β-unsaturated ketones to optically active allylic alcohols was performed via hydrosilylation catalyzed by a rhodium(I) complex with (+)-BMPP, (+)-DIOP and (?)-DIOP as chiral ligands. The allylic alcohols with optical purity up to 69% e.e. were obtained in good yields. The extent of asymmetric induction was found to depend on the stereo-electronic matching of the chiral ligand, ketone and hydrosilane employed. In the asymmetric reduction of (R)-carvone, leading to carveol, the extent of asymmetric induction was found to depend markedly on the ligand/rhodium ratio. Either trans-(5R,1S)-carveol or cis-(5R,1R)-carveol was obtained with good stereoselectivity by using (?)-DIOP or (+)-DIOP as chiral ligand, and it turned out that the chiral center present in carvone had only a slight influence on the asymmetric induction by the chiral catalysts.     

Nickel complexes with cyclic ligands containing P and N atoms as coordination sites: novel biomimetic catalysts for hydrogen oxidation

Nickel complexes with new cyclic ligands containing phosphorus and nitrogen atoms as coordination sites are novel efficient catalysts for hydrogen oxidation. A systematic study of their electrochemical properties made it possible to classify the nickel systems in question into four groups according to the sequence of electron transfer processes in the reduction (M^II-M^I-M⁰) and to the nature of solvents and counterions. Regularities of catalytic transformations involving nickel complexes with P,N-cyclic ligands in the H₂ oxidation reaction in the coordination sphere of the catalyst and a correlation between the structure of the complex and its redox properties were established. The most efficient catalysts contain phenyl and 2-pyridyl substituents at the phosphorus atom and benzyl or 2-pyridyl substituents at the nitrogen atom.     

Cationic rhodium hydrogenation catalysts containing chelating diphosphine ligands: effect of chelate ring size

Earlier studies on the [(1,2-bis(diphenylphosphino)ethane)rhodium]^p+-catalyzed hydrogenation of 1-hexene and methyl-(Z)-α-acetamidocinnamate have been extended to catalysts containing larger chelating diphosphine ligands, i.e., Ph₂P(CH₂)_nPPh₂, where n = 3, 4 and 5. Comparisons include measurements of equilibrium constants for the binding of the olefinic substrates to the catalysts and of the catalytic hydrogenenation rates. Some related measurements also are reported for the corresponding catalyst systems containing the chiral ligand, 4R,5R-bis(diphenylphosphinomethyl)-2,2,-dimethyldioxalane (DIOP) and non-chelating PPh₃ ligands.     

Zur synthese von tetramesitylmolybdän-verbindungen

Hydrogen transfer from isopropanol to various ketones such as cyclohexanone, 4-t-butylcyclohexanone and acetophenone are catalyzed by cationic rhodium(I) complexes of the type [Rh(Diene)L₂]⁺ (Diene = 1,5-cyclooctadiene (COD) or norbornadiene (NBD); L₂ or L = mono- or bi-dentate phosphine ligands). The results indicate higher activities for complexes containing chelating ligands.     

Regulation of Catalytic Activity by Phosphine Ligand Design

Abstract

Starting with the catalytic complex [Rh(PPh₃)₃Cl], the influence of variation of phosphine ligand properties on the activity of rhodium phosphine complexes as catalysts for the hydrogenation of olefins was systematically studied. The following catalyst modifications were examined (a) varying the basicity of the triarylphosphine ligands, (b) replacing Cl^? by a non-coordinating anion (BF₄ ^?) to make the catalyst cationic, (c) substituting a chelating diphosphine for the monophosphine ligands to ensure cis-coordination, and (d) varying the chain length of the diphosphine ligand to vary the chelate ring size and flexibility. By systematic manipulation of these parameters, enhancements of catalytic activity by factors in excess of 10⁴ were achieved.     

New chiral diphosphoramidite rhodium(I) complexes for asymmetric hydrogenation

Chiral 1,5‐cyclooctadiene rhodium(I) cationic complexes with C₂‐symmetric chelate diphosphoramidite ligands containing (R,R)‐1,2‐diaminocyclohexane as the backbone and two atropoisomeric biaryl units were easily synthesized and fully characterized by multinuclear one‐ and two‐dimensional NMR spectroscopy and elemental analysis. These complexes were used as catalysts in the asymmetric hydrogenation of dimethyl itaconate, methyl 2‐acetamidoacrylate and (Z)‐methyl‐2‐acetamido‐3‐phenylacrylate. The rhodium complexes derived from diphosphoramidite ligands that contain two (R) or (S) BINOL (2,2′‐dihydroxy‐1,1′‐binaphthyl) units proved to be efficient catalysts, giving complete conversion and very good enantioselectivity (up to 88% ee). An uncommon positive H₂ pressure effect on the enantioselectivity was observed in the hydrogenation of dimethyl itaconate catalyzed by Rh‐complex with diphosphoramidite ligand that contains two (S)‐binaphthol moieties. Copyright © 2014 John Wiley & Sons, Ltd.     

Transformations of the chiral diphosphine rhodium catalyst [(1,5-COD)Rh(—)R,R-DIOP]+CF3SO3– under conditions of hydrogenation

Transformation products of the cationic rhodium complex [(1,5-COD)Rh(—)R,R-DIOP]⁺CF₃SO₃ ^– (1) (COD is cycloocta-1,5-diene and DIOP is (±)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane), which were obtained in its reactions with molecular hydrogen, base (NEt₃), and solvents in the absence of a substrate, were investigated by ¹H and ³¹P NMR spectroscopy. The solvate complexes [(Solv)₂Rh(—)R,R-DIOP]⁺CF₃SO₃ ^–, which were generated from complex 1 in its reaction with molecular hydrogen, underwent destruction of the diphosphine ligand with elimination of benzene and were subjected to oxidation by traces of moisture and oxygen to form the DIOP dioxide complex with Rh^I. In the absence of hydrogen, complex 1 in solutions produced the diphosphine dioxide rhodium(i) complex and mono- and binuclear rhodium(i) solvate complexes. The scheme of deactivation of the complex in the absence of the substrate was proposed. The catalytic activity of the solvate complexes [(ArH)Rh(—)R,R-DIOP]⁺CF₃SO₃ ^–, which contain benzene, p-xylene, and mesitylene in the coordination sphere, was studied in hydrogenation of Z--acetamidocinnamic acid.     

Development of an Improved Rhodium Catalyst for Z‐Selective Anti‐Markovnikov Addition of Carboxylic Acids to Terminal Alkynes

To develop more active catalysts for the rhodium‐catalyzed addition of carboxylic acids to terminal alkynes furnishing anti‐Markovnikov Z enol esters, a thorough study of the rhodium complexes involved was performed. A number of rhodium complexes were characterized by NMR, ESI‐MS, and X‐ray analysis and applied as catalysts for the title reaction. The systematic investigations revealed that the presence of chloride ions decreased the catalyst activity. Conversely, generating and applying a mixture of two rhodium species, namely, [Rh(DPPMP)₂][H(benzoate)₂] (DPPMP=diphenylphosphinomethylpyridine) and [{Rh(COD)(μ₂‐benzoate)}₂], provided a significantly more active catalyst. Furthermore, the addition of a catalytic amount of base (Cs₂CO₃) had an additional accelerating effect. This higher catalyst activity allowed the reaction time to be reduced from 16 to 1–4 h while maintaining high selectivity. Studies on the substrate scope revealed that the new catalysts have greater functional‐group compatibility.     

Hydroformylation of alkenes with paraformaldehyde catalyzed by rhodium-phosphine complexes

The hydroformylation of medium-chain C6 olefins and of allyl alcohol was achieved with paraformaldehyde in dioxane solution using rhodium catalysts with mono-, bi-, and tri-dentate phosphine ligands. The highest activities with n/i ratios around 2, were obtained for a system derived from [Rh(dppe)₂]⁺, prepared in situ by reaction of Rh(acac)(CO)₂ with 2 eq of dppe.     

Rhodium(I) complexes with 1′-(diphenylphosphino)ferrocenecarboxylic acid as active and recyclable catalysts for 1-hexene hydroformylation

The rhodium complex trans-[Rh(CO)(Hdpf-κP)(dpf-κ²O,P)] (1), (Hdpf = 1′-(diphenylphosphino)ferrocenecarboxylic acid) was used as an efficient and recyclable catalyst for 1-hexene hydroformylation producing ca. 80% of aldehydes at 10 atm CO/H₂ and 80 °C. After the reaction, unchanged complex 1 was separated from the reaction mixture and used again three times with the same catalytic activity. The effect of modifying ligands, phosphines and phosphites, on the reactivity of 1 was investigated. The active catalytic systems containing 1 or trans-[Rh(CO)(L)(dpf-κ²O,P)] (2) were formed in situ from acetylacetonato rhodium(I) precursors [Rh(CO)₂(acac)] (3) or [RhL(CO)(acac)] (4) and Hdpf or Medpf (L = phosphine, Medpf = methyl ester of Hdpf).     

Chiral C1-symmetric 2,2′:6′,2′′-terpyridine ligands: Synthesis,characterization, complexation with copper(II), rhodium(III) and ruthenium(II) ions and use of the complexes in catalytic cyclopropanation of styrene

Three new optically pure C₁-terpyridine ligands (L1–3) were prepared and the copper(II) complexes, of formula [Cu(L)Cl₂], the rhodium(III) complexes, of formula [Rh(L)Cl₃], and the ruthenium(II) complexes, of formula cis- or trans-[Ru(L)(X)Cl₂] (X = DMSO or CO), were synthesized. Structures of a chiral C₁-ligand, a copper complex, a rhodium complex and a ruthenium DMSO complex were analysed using X-ray crystal structure analysis. The copper, rhodium and ruthenium complexes were shown to be precursors of catalysts for cyclopropanation. Reaction of [Cu(L)Cl₂], [Rh(L)Cl₃] or cis- or trans-[Ru(L)(X)Cl₂] with AgOTf converted the complex to catalyst, which in the case of trans-[Ru(L)(CO)Cl₂] gave enantioselectivities of up to 67% ee for the cis-isomers of styrene cyclopropanes with t-butyl diazoacetate. Comparisons with C₂-analog of copper, rhodium and ruthenium catalysts were made.     

Asymmetric hydrogenation of methyl (<Emphasis Type="Italic">Z</Emphasis>)-2-acetamido-3-(3,4-dimethoxyphenyl)acrylate catalyzed by Rh complexes with available amidophosphite ligands

A convenient express procedure for the preparation of methyl (Z)-2-acetamido-3-(3,4-dimethoxyphenyl)acrylate was developed. Asymmetric hydrogenation of this substrate in the presence of rhodium catalysts involving synthetically available amidophosphite ligands was carried out, which is characterized by high enantioselectivity (to 99.5% ee) and complete conversion. An approach to the selective formation of cationic complexes containing two ligands of different nature in one coordination sphere of rhodium was suggested.     

Polymer-bound bulky-phosphite modified rhodium hydroformylation catalysts

Attachment of phosphites to styrene copolymers is described which are used as rhodium hydroformylation catalysts. The influence of the chain loading on the activity and complex formation of three types of copolymer-bound rhodium hydroformylation catalysts in comparison with their low molecular weight analogues has been studied. The catalytic activity of the polystyrene-bound system with the most bulky phosphite, the first system studied, is identical to that of the low molecular weight analogue. The catalysts show a high activity towards the hydroformylation of the otherwise unreactive cyclooctene. It was found that only one phosphite is coordinated to the rhodium complex in its active form. An equilibrium between this complex and an inactive complex without phosphite ligands prohibits its use in continuous flow reactors. Secondly, as polymer support a perfectly random copolymer of styrene and less bulky 3,3′,5,5′-tetra-tert-butylbiphenyl-2,2′-diyl p-vinylphenylphosphite was used. The chain loading α of this copolymer with phosphite ligands has a large influence on the complex formation of the catalyst. With high chain loadings moderately active bis-phosphite catalysts are formed. Low chain loadings give active, easily accessible, monophosphite complexes. The active species in the hydroformylation of sterically hindered alkenes is a mono-phosphite rhodium complex. The activity of the copolymer-bound catalyst towards the hydroformylation of cyclooctene is found to be as high as the activity of its low molecular weight analogue. For styrene, this polymer catalyst yields a slower catalyst than the low-molecular weight analogue. The third part demonstrates that silica-grafted polymer-bound phosphite modified rhodium complexes can be used in continuous flow reactors. The hydroformylation of styrene was carried out at moderate pressure (pCO/H₂ = 3 MPa) and temperature (T = 100°C), yielding constant conversions over a period of at least ten days. These positive results were obtained in benzene as a solvent and for a ligand to rhodium ratio of only four.     

Structural evolution of a recoverable rhodium hydrogenation catalyst

A recoverable, water soluble, hydrogenation catalyst was synthesized by reacting poly-N-isopropylacrylamide containing a terminal amino group (H₂N-CH₂CH₂-S-pNIPAAm) with [Rh(CO)₂Cl]₂ in organic solvents to form the square planar rhodium complex (Rh(CO)₂Cl(H₂N-CH₂CH₂-S-pNIPAAm)). The catalyst-ligand structure was characterized using in situ multinuclear NMR, XAFS and IR spectroscopic methods. Model complexes containing glycine (H₂NCH₂COOH), cysteamine (H₂NCH₂CH₂SH) and methionine methyl ester (H₂NCH(CH₂CH₂SCH₃)COOCH₃) ligands were studied to aid in the interpretation of the coordination sphere of the rhodium catalyst. The spectroscopic data revealed a switch in ligation from the amine bound (Rh-NH₂-CH₂CH₂-S-pNIPAAm) to the thioether bound (Rh-S(-CH₂CH₂NH₂)(-pNIPAAm)) rhodium when the complex was dissolved in water. The evolution of the structure of the rhodium complex dissolved in water was followed by XAFS. The structure changed from the expected monomeric complex to form a rhodium cluster of up to four rhodium atoms containing one SRR′ ligand and one CO ligand per rhodium center. No metallic rhodium was observed during this transformation. The rhodium-rhodium interactions were disrupted when an alkene (3-butenol) was added to the aqueous solution. The kinetics of the hydrogenation reaction were measured using a novel high-pressure flow-through NMR system and the catalyst was found to have a TOF of 3000/Rh/h at 25 °C for the hydrogenation of 3-butenol in water.     

Monohydride-Dichloro Rhodium(III) Complexes with Chiral Diphosphine Ligands as Catalysts for Asymmetric Hydrogenation of Olefinic Substrates

We report full details of the synthesis and characterization of monohydride-dichloro rhodium(III) complexes bearing chiral diphosphine ligands, such as (S)-BINAP, (S)-DM-SEGPHOS, and (S)-DTBM-SEGPHOS, producing cationic triply chloride bridged dinuclear rhodium(III) complexes ( 1 a : (S)-BINAP; 1 b : (S)-DM-SEGPHOS) and a neutral mononuclear monohydride-dichloro rhodium(III) complex ( 1 c : (S)-DTBM-SEGPHOS) in high yield and high purity. Their solid state structure and solution behavior were determined by crystallographic studies as well as full spectral data, including DOSY NMR spectroscopy. Among these three complexes, 1 c has a rigid pocket surrounded by two chloride atoms bound to the rhodium atom together with one tBu group of (S)-DTBM-SEGPHOS for fitting to simple olefins without any coordinating functional groups. Complex 1 c exhibited superior catalytic activity and enantioselectivity for asymmetric hydrogenation of exo-olefins and olefinic substrates. The catalytic activity of 1 c was compared with that of well-demonstrated dihydride species derived in situ from rhodium(I) precursors such as [Rh(cod)Cl]₂ and [Rh(cod)₂]⁺[BF₄]⁻ upon mixing with (S)-DTBM-SEGPHOS under dihydrogen.     

Structural Characterization of Alumina‐Supported Rh Catalysts: Effects of Ceriation and Zirconiation by using Metal–Organic Precursors

The effects of the addition of ceria and zirconia on the structural properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ‐Al₂O₃) are studied. Ceria and zirconia are deposited by using two preparation methods. Method I involves the deposition of ceria on γ‐Al₂O₃ from Ce(acac)₃, and the rhodium metal is subsequently added, whereas method II is based on a controlled surface reaction technique, that is, the decomposition of metal–organic M(acac)_x (in which M=Ce, x=3 and M=Zr, x=4) on Rh/γ‐Al₂O₃. The structures of the prepared catalyst materials are characterized ex situ by using N₂ physisorption, transmission electron microscopy, high‐angle annular dark‐field scanning transmission election microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy (XPS), and X‐ray absorption fine structure spectroscopy (XAFS). All supported rhodium systems readily oxidize in air at room temperature. By using ceriated and zirconiated precursors, a larger rhodium‐based metallic core fraction is obtained in comparison to the undoped rhodium catalysts, suggesting that ceria and zirconia protect the rhodium particles against extensive oxidation. XPS results indicate that after the calcination and reduction treatments, a small amount of chlorine is retained on the support of all rhodium catalysts. EXAFS analysis shows significant Rh? Cl interactions for Rh/Al₂O₃ and Rh/CeO_x/Al₂O₃ (method I) catalysts. After reaction with H₂/He in situ, for series of samples with 1.6 wt % Rh, the EXAFS first shell analysis affords a mean size of approximately 30 atoms. A broader spread is evident with a 4 wt % rhodium loading (ca. 30–110 atoms), with the incorporation of zirconium providing the largest particle sizes.     

Polymerization of 3‐ethynylthiophene with homogeneous and heterogeneous Rh catalysts

3‐Ethynylthiophene (3ETh) was polymerized with Rh(I) complexes: [Rh(cod)acac], [Rh(nbd)acac], [Rh(cod)Cl]₂, and [Rh(nbd)Cl]₂ (cod is η²:η²‐cycloocta‐1,5‐diene and nbd η²:η²‐norborna‐2,5‐diene), used as homogeneous catalysts and with the last two complexes anchored on mesoporous polybenzimidazole (PBI) beads: [Rh(cod)Cl]₂/PBI and [Rh(nbd)Cl]₂/PBI used as heterogeneous catalysts. All tested catalyst systems give high‐cis poly(3ETh). In situ NMR study of homogeneous polymerizations induced with [Rh(cod)acac] and [Rh(nbd)acac] complexes has revealed: (i) a transformation of acac ligands into free acetylacetone (Hacac) occurring since the early stage of polymerization, which suggests that this reaction is part of the initiation, (ii) that the initiation is rather slow in both of these polymerization systems, and (iii) a release of cod ligand from [Rh(cod)acac] complex but no release of nbd ligand from [Rh(nbd)acac] complex during the polymerization. The stability of diene ligand binding to Rh‐atom in [Rh(diene)acac] catalysts remarkably affects only the molecular weight but not the yield of poly(3ETh). The heterogeneous catalyst systems also provide high‐cis poly(3ETh), which is of very low contamination with catalyst residues since a leaching of anchored Rh complexes is negligible. The course of heterogeneous polymerizations is somewhat affected by limitations arising from the diffusion of monomer inside catalyst beads. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2776–2787, 2008     

Ionic diamine rhodium(I) complexes--highly active catalysts for the hydroformylation of olefins

Rhodium(I) complexes composed of an anionic rhodium centre containing chloride ligands, and a cationic rhodium centre coordinated by a diamine ligand, were synthesized and characterized. These complexes are able to catalyze the hydroformylation reaction under mild reaction conditions in excellent activity and regioselectivity, and in the absence of a phosphorus ligand.     

An evaluation of silica-supported catalysts for the reaction between ethanol and polymethylhydrogensiloxane

Catalysis of the reaction between Me₃SiO[Si(H)Me)O]_nSiMe₃ (n ≈ 50) and ethanol by silica-supported rhodium and iridium catalysts has been investigated. Donor groups in the anchored ligands were PPh₂, S^?, or C₅H₄^?. The rhodium-PPh₂ system showed marked inhibition by dihydrogen. The supported iridium catalysts all showed high activity which declined rapidly during successive cycles of re-use, but the iridium-PPh₂ catalyst was the least affected. In every case, the separated liquid products showed activity as homogeneous catalysts, indicating that leaching of the metal from the support was occurring. That ligand was also being leached was shown by labelling with tritium. The results demonstrate the necessity to test supported catalysts through more than one cycle.     

Supraphos: A supramolecular strategy to prepare bidentate ligands

Herein, we report a new strategy for the preparation of chelating bidentate ligands, which involves the mixing of two mondentate ligands functionalized with complementary binding sites. The assembly process is based on selective metal-ligand interactions employing phosphite zinc(II) porphyrins 1-6 and the nitrogen-containing phosphorus ligands b-i (Scheme 1). Only 14 monodentate ligands were utilized to generate a library of 48 palladium catalysts based on supraphos-type bidentate ligands. The characterization of rhodium complexes based on representative Supramolecular bidentate ligands and the comparison of their performance in the hydroformylation of styrene will be presented. The current library of catalysts was tested in the asymmetric palladium-catalyzed alkylation of rac-1,3-diphenyl-2-propenyl acetate, which resulted in a large variety in the observed enantioselectivity for the different catalysts. Importantly, small variations in the supraphos building blocks, lead to large differences in the enantioselectivity imposed by the catalyst, the most selective catalyst producing 97% ee.