Hydroformylation activity of multinuclear rhodium complexes coordinated to dendritic iminopyridyl and iminophosphine scaffolds

The synthesis of poly(propyleneimine)-iminopyridyl and iminophosphine rhodium(I) metallodendrimers, with rhodium coordinated to monodentate (N-donor) and chelating, heterobidentate (P,N) moieties respectively located on the periphery, has been accomplished in order to evaluate their potential as hydroformylation catalysts. Related mononuclear complexes were obtained in a similar manner to model the multinuclear complexes. The multinuclear rhodium(I) complexes were found to be effective catalyst precursors in the hydroformylation of 1-octene, achieving higher conversions, faster reaction rates and slightly enhanced catalytic activity when compared with analogous mononuclear rhodium complexes. Hydroformylation reactions using the tetra- and octanuclear rhodium complexes generally show a chemoselective formation of aldehydes, together with a small amount of isomerisation products.     

Molekülsruktur und abs. konfiguration von (+)-p-menthadien-(1,8)-on-(2)-rhodium(I)-h5-cyclopentadienid, (+)-(4S-Carvon) RhI(C5H5)

The title compound crystallizes in the orthorhombic space group P2₁2₁2₁ with 4 molecules in the unit cell (cell dimensions: a 9.778(2), b 10.639(2) and c 12.423(4) Å). The structure was solved by means of the heavy atom method. The rhodium atom is linked to both olefinic double bonds. The terpene carbonyl group does not participate in coordination to rhodium. Unlike the endocyclic olefinic group, which is approximately perpendicular to the coordination plane of rhodium, the exocyclic Cz.sbnd;C double bound shows a considerable deviation from this arrangement. The π-complexation of carvone with rhodium proceeds diastereospecifically. The absolute configuration of (+)-carvone is 4S in agreement with the assignment derived by indirect chemical correlation.     

Equilibration of ortho- and para-hydrogen by homogeneous hydrogenation catalysts in solution; A test for the reversibility of hydrogen addition using Raman spectroscopy

para-Enriched hydrogen is converted into the ortho-para equilibrium mixture by tris(triphenylphosphine)rhodium chloride in toluene solution. The rotational Raman effect can be used to show that this equilibration occurs during the course of hydrogenation of cyclohexene by the above catalyst. The methanol solvate formed by reduction of 1,4-bis(diphenylphosphino)butanebicyclo[2.2.1] heptadienerhodium tetrafluoroborate also catalyses the ortho-para equilibration of hydrogen, in common with related cationic rhodium complexes. No equilibration occurs, however, during the course of hydrogenation of (Z)-α-benzamidocinnamic acid by cationic chelate rhodium complexes, including that derived from the chiral ligand (R,R)-1,2-bis(o-methoxyphenylphenyl)ethane (DIPAMP), thus demonstrating that addition of hydrogen to rhodium is irreversible.     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

Synthesis and characterization of asymmetric NHC complexes

New chiral and non-chiral rhodium(I)–NHC complexes were synthesized. The first attempt by deprotonation of an imidazolinium salt with KOtBu and reaction with [Rh(COD)Cl]₂ leads to the corresponding rhodium(I) complex. Due to the basic conditions during the reaction a loss of chirality occurs. An alternative transmetallation reaction with a silver(I)–NHC complex yields the desired rhodium(I)–NHC complex under retention of chirality. Both Rh complexes were fully characterized by analytical methods.     

Carbosilane dendrimers peripherally functionalized with P-stereogenic diphosphine ligands and related rhodium complexes

The first carbosilane dendrimer functionalized with P-stereogenic diphosphine ligands was prepared along with its cationic rhodium derivative. A mononuclear rhodium model compound was also synthesized. Both species were used as catalysts in the hydrogenation of dimethylitaconate and the results compared with those obtained with the related rhodium-containing P-stereogenic monophosphine dendrimers.     

Phosphinerhodium complexes as homogeneous catalysts : VIII. Enantioselective hydrogenation of ketones: evidence for several mechanisms with catalysts containing diop

Optical yields obtained in the hydrogenation of acetophenone with cationic and in situ rhodium complex catalysts depend on the P/Rh ratio and on the ionic or non-ionic character of the active species. The enantioselectivity of the in situ catalyst containing (+)-DIOP is reversed by addition of achiral tri-n-alkyl-phosphines. On the basis of these observations and the amount of H₂ consumed in preforming the catalysts, several different mechanisms are suggested: for example: cycles involving cationic rhodium complexes containing two (or three) phosphorus ligands and cycles involving non-ionic rhodium complexes with two phosphorus ligands in cis or trans positions. In the in situ catalyst with a Rh/(+)-DIOP/P-n-Bu₃  1/1/1 ratio (+)-DIOP functions as a monodentate ligand.     

Rhodium-catalyzed addition of aryldifluoromethylsilanes to N-sulfonylaldimines

The addition of aryldifluoromethylsilanes to N-sulfonylaldimines was found to be catalyzed by a rhodium complex, [Rh(cod)(MeCN)₂]BF₄, in the presence of potassium fluoride to give the corresponding arylated N-sulfonylamines in good yield. The reaction mechanism would involve the generation of a fluoride-coordinated arylsilicate and the transmetalation between the arylsilicate and the rhodium complex to give the arylrhodium species as a key intermediate.     

Synthesis of phthalan and phenethylamine derivatives via addition of alcohols to rhodium(II)-azavinyl carbenoids

1-Sulfonyl-1,2,3-triazoles undergo inter- and intramolecular 1,3-OH insertion with rhodium(II)-azavinyl carbenoid intermediates upon treatment with a rhodium(II) catalyst. Products of this transformation contain a synthetically versatile N-(2-alkoxyvinyl)sulfonamide, enabling divergent reactivity toward several N-protected phenethylamine derivatives under various conditions. Notably, products with a phthalan framework can be accessed directly from 4-aryl-1-sulfonyl-1,2,3-triazoles bearing a pendant alcohol.     

Chiral and diastereoisomeric rhodium(I) coordination compounds with carvone as bidentate ligand

The chiral dienone p-mentha-6,8-dien-2-one (carvone) coordinates readily with rhodium(I) to form complexes of pronounced stability. Novel diastereoisomeric planar coordination compounds of rhodium(I) containing two chiral bidentate ligands have been prepared for the first time. The synthesis of the compounds is simple, and the yields are high.     

Synthesis of rhodium N-heterocyclic carbene complexes and their catalytic activity in the hydrosilylation of alkenes in ionic liquid medium

Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis(η⁴-1,5-cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Additionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF₆ could be reused without noticeable loss of catalytic activity and selectivity.     

Rhodium-catalysed synthesis of fused pyrimidine derivatives employing N-sulfonyl-1,2,3-triazoles as a 1-aza-[4C] synthon

A new synthetic application of N-sulfonyl-1,2,3-triazoles acting as a 1-aza-[4C] synthon via the 1,2-shift reaction of an α-imine rhodium carbene was developed for the synthesis of fused pyrimidine derivatives. The high reactivity of the strained three-membered 2H-azirine ring facilitated the unusual cyclization of electron-deficient dienes with electron-deficient dienophiles. The compatibility was good with common functionalities tolerated. Excellent chemoselectivity was observed, and no reactions occurred between the rhodium carbene and 2H-azirine. The products could be converted into seven-membered multi-functionalized 1H-1,4-diazepine derivatives, illustrating the potential application of the protocol in medium-sized N-heterocycle synthesis.     

Differential catalytic determination of iridium(IV) and rhodium(III) based on the oxidation of N-methyldiphenylamine-4-sulfonic acid

The kinetics of the oxidation of N-methyldiphenylamine-4-sulfonic acid with periodate ions was studied in weakly acidic solutions in the presence of iridium(IV), rhodium(III), and their mixtures. Oxidation rate constants were determined in the presence of individual catalysts and their mixtures. The synergetic effect of iridium(IV) and rhodium(III) on the rate of the indicator reaction was estimated; the range of catalyst ratios for the simultaneous determination of analytes was determined. The effect of some factors (oxidant nature and concentration, temperature, the ionic strength of solution, and interfering ions) on the rate of the indicator reaction in the presence of iridium(IV) and rhodium(III) mixtures was assessed. A procedure for the differential catalytic determination of iridium(IV) and rhodium(III) was proposed and tested in the analysis of artificial mixtures and a platinum concentrate of complex composition (KP-5).     

Asymmetric 1,4-addition of alkenylzirconium reagents to α,β-unsaturated ketones catalyzed by chiral rhodium complexes

Highly enantioselective 1,4-addition of alkenylzirconocene chlorides to α,β-enones was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN)₂]BF₄ and (S)-BINAP. The reaction can be applied to either cyclic or acyclic enones and the optical yield was up to 99% ee. The reaction mechanism would involve the transmetalation between the alkenylzirconocene chloride and the rhodium complex to give the alkenylrhodium species as a key intermediate.     

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.     

Investigations into the catalytic activity of rhodium(III) in red-ox reactions by capillary zone electrophoresis

Speciation of rhodium(III) in different acidic media has been studied by capillary zone electrophoresis (CZE). Depending on the nature of the acid, rhodium was shown to occur in the form of positive, neutral and/or negatively charged complexes. The relationship between the distribution of rhodium forms and its catalytic action on the oxidation of N-methyldiphenylamine-4-sulfonic acid by periodate ions has been investigated. It was found that only positively charged complexes of rhodium, such as those dominating in perchloric acid solutions, catalyzed a given reaction to form a colored oxidation product. The rate of the catalyzed reaction was optimized with respect to the pH, reagent and oxidant concentration levels, ionic strength, concentration of the catalyst, as well as the presence of interfering ions. The developed kinetic spectrophotometric method features rather high sensitivity (limit of determination 10 μg l⁻¹) and tolerance for most platinum metals and was applied to a complex industrial sample of a platinum concentrate.     

Catalytic asymmetric hydrogenation of indoles using a rhodium complex with a chiral bisphosphine ligand PhTRAP

Highly enantioselective hydrogenation of N-protected indoles was successfully developed by use of the rhodium catalyst generated in situ from [Rh(nbd)₂]SbF₆ and the chiral bisphosphine PhTRAP, which can form a trans-chelate complex with a transition metal atom. The PhTRAP–rhodium catalyst required a base (e.g., Cs₂CO₃) for the achievement of high enantioselectivity. Various 2-substituted N-acetylindoles were converted into the corresponding chiral indolines with up to 95% ee. The hydrogenations of 3-substituted N-tosylindoles yielded indolines possessing a stereogenic center at the 3-position with high enantiomeric excesses (up to 98% ee).     

Synthesis and characterization of chiral mono N-heterocyclic carbene-substituted rhodium complexes and their catalytic properties in hydrosilylation reactions

Different chiral mono-substituted N-heterocyclic carbene complexes of rhodium were prepared, starting from [Rh(COD)Cl]₂ (COD = cyclooctadiene) by addition of free N-heterocyclic carbenes (NHC), or an in-situ deprotonation of the corresponding iminium salt. All new complexes were characterized by spectroscopy methods. In addition, the structures of chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(1R,2R,3R,5S)-2,6,6-trimethylbicyclo[3.1.1]hept-3-yl] imidazolin-2-ylidene)rhodium(I) (5a), chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(1R,2S,5R)-2-isopropyl-5-menthylcyclohex-1-yl]imidazol-2-ylidene)rhodium(I) (5b) and chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(2R,4S,5S)-2-methyl-4-phenyl-1,3-dioxacyclohex-5-yl]imidazolin-2-ylidene)rhodium(I) (5i) were analyzed by DFT-calculations. The enantioselective hydrosilylation of acetophenone, ethylpyruvate and n-propylpyruvate with diphenylsilane and hydrolysis was carried out with chiral C₂-symmetrical mono-substituted N-heterocyclic carbene rhodium complexes giving for the first time an enantioselective excess of up to 74% ee in the case of the n-propylpyruvate.     

Mono- and di-nuclear rhodium complexes of meso- and dl-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane. Stereochemical control of reactivity and complexation geometry

Thermolysis of meso- and dl-1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (200°C, 10 min) followed by fractional crystallisation from ethanol/dichloromethane gives two sharp-melting diastereomers. The higher melting compound, herein shown to be the meso-isomer, reacts with 1,5-cyclooctadiene-2,4-pentanedionatorhodium and HBF₄ to give the dinuclear rhodium complex (3). This underwent hydrogenation slowly in methanol solution with deposition of rhodium metal and formation of a mononuclear complex (5) with four coordinated phosphorus nuclei, also obtained by independent synthesis. This proved to be highly susceptible to oxidation, forming a dioxygen complex (6) with P(1) and P(3) mutually trans. The lower melting dl-isomer likewise formed a dinuclear rhodium complex (4) on reaction with 1,5-cyclooctadiene-2,4-pentanedionatorhodium and HBF₄. This reacted more rapidly than complex 3 with hydrogen forming a mononuclear dihydride (7) and metallic rhodium. In the presence of cyclohexene, a tetracoordinate phosphinerhodium complex (9) was formed. This reacted with oxygen to give dioxygen complex (10), although here P(1) and P(4) are mutually trans, and with carbon monoxide to give a five-coordinate monocarbonyl (11).The corresponding dirhodium bis-cyclooctadiene complex of 1,1,4,8,11,11-hexaphenyl-1,4,8,11-tetraphosphaundecane (13) (a single diastereomer of unknown stereochemistry), reacted with hydrogen in methanol to form a dinuclear solvate without reductive degradation.