Cationic Rhodium Complexes Research Articles

A Cationic N-Heterocyclic Carbene Containing an Ammonium Moiety

The dicationic 4-ammonio-substituted imidazolium salt (2·H)(OTf)2 is readily obtained from its 4-amino-substituted imidazolium precursor (1·H)(OTf) by methylation of the exocyclic nitrogen. The cationic NHC (2)(OTf) is generated in situ by deprotonation of (2·H)(OTf)2 with 1 equiv of potassium bis(trimethylsilyl)amide and is trapped with sulfur or selenium. The direct linkage of the cationic ammonium moiety to the carbenic heterocycle through a σ bond has a profound effect on the electronic properties of the NHC by significantly decreasing its σ-donor ability and, to a lesser extent, increasing its π-acidity. These features make the 4-ammonio-substituted NHC 2+ an overall weak electron donor. To evaluate the coordination abilities of carbene 2+, several transition-metal complexes supported by this ligand have been prepared, including the cationic rhodium(I) complexes [RhCl(cod)(2)](OTf) and [RhCl(CO)2(2)](OTf), the cationic palladium(II) complexes [PdCl(η3-allyl)(2)](OTf) and [PdCl2(CH3CN)(2)](OTf), the z...

On‐Water Polymerization of Phenylacetylene Catalyzed by Rh Complexes Bearing Strong π‐Acidic Dibenzo[a,e]cyclooctatetraene Ligand

ABSTRACTA series of new mononuclear neutral and water‐soluble cationic rhodium (Rh) complexes bearing strong π‐acidic dibenzo[a,e]cyclooctatetraene (dbcot) diene ligand have been synthesized and structurally characterized. In the polymerization of phenylacetylene, the dbcot Rh complex exhibits higher catalytic activity than the corresponding cod‐based Rh complex in both of organic solvent and aqueous media, affording the high cis‐transoidal PPAs with up to 99% of cis‐contents, moderate molecular weights, and moderate to broad molecular weight distributions. Moreover, on‐water polymerization of substituted phenylacetylenes is achieved by these complexes under air atmosphere, in which 3‐ to 163‐fold acceleration of the polymerization rate is observed in aqueous polymerization compared to that in organic solvents. The nature of the Rh complex, solvent, polymerization temperature, and substituted group on the phenylacetylene impact on the polymer's yield, stereoselectivity, molecular weight, and molecular weight distribution. In addition, the water‐soluble cationic Rh complexes can be reused for three times. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 716–725

Simultaneous Orthogonal Methods for the Real-Time Analysis of Catalytic Reactions

Continuous monitoring of catalyzed reactions using infrared spectroscopy (IR) measures the transformation of reactant into product, whereas mass spectrometry delineates the dynamics of the catalytically relevant species present at much lower concentrations, a holistic approach that provides mechanistic insight into the reaction components whose abundance spans 5 orders of magnitude. Probing reactions to this depth reveals entities that include precatalysts, resting states, intermediates, and also catalyst impurities and decomposition products. Simple temporal profiles that arise from this analysis aid discrimination between the different types of species, and a hydroacylation reaction catalyzed by a cationic rhodium complex is studied in detail to provide a test case for the utility of this methodology.

Ruthenium(II)‐ or Rhodium(III)‐Catalyzed Grignard‐Type Addition of Indolines and Indoles to Activated Carbonyl Compounds

AbstractThe ruthenium(II)‐ or rhodium(III)‐catalyzed pyrimidinyl‐directed Grignard‐type C−H additions of N‐heterocycles with activated aldehydes and ketones are described. A cationic ruthenium catalyst and sodium acetate additive in dichloroethane as solvent were found to be optimal catalytic system for the construction of C‐7 alkylated indolines. In sharp contrast, a cationic rhodium complex allows the generation of C‐2 alkylated indoles and pyrroles as well as C‐1 alkylated carbazoles. The site‐selective C−H functionalization of these heterocyclic scaffolds could be an important asset towards the development of novel bioactive compounds.magnified image

Oxazolidines as Intermediates in the Asymmetric Synthesis of 3-Substituted and 1,3-Disubstituted Tetrahydroisoquinolines.

A diastereoselective mercury(II)-promoted intramolecular cyclization of unsaturated aldehyde via an oxazolidine to prepare C-3-substituted tetrahydroisoquinoline is disclosed. The C-3 stereogenic center is subsequently exploited to create the C-1 stereocenter by coordination of the nucleophilic reagent to the oxygen atom of oxazolidine. Both cis- and trans-1,3-disubstituted tetrahydroisoquinolines can be readily prepared. In addition, when a cationic rhodium complex was used, intramolecular hydroamination was effected, thus avoiding mercury(II) salts and demercuration. The reaction is general and works well using aliphatic and aromatic aldehydes.

Rhodium Hydrogenation Catalysts Supported in Metal Organic Frameworks: Influence of the Framework on Catalytic Activity and Selectivity

The cationic rhodium complexes (dppe)Rh(COD)BF4 and (MeCN)2Rh(COD)BF4 have been supported in metal–organic frameworks bearing anionic nodes (ZJU-28) and anionic linkers (MIL-101-SO3) via ion exchange. These MOF-supported Rh species serve as recyclable catalysts for the hydrogenation of both the terminal alkene substrate 1-octene and the internal alkene substrate 2,3-dimethylbutene. The nature of the MOF support impacts various aspects of catalysis, including: (i) the rate of 1-octene hydrogenation, (ii) the activity and recyclability of the catalyst in 2,3-dimethylbutene hydrogenation, and (iii) the size selectivity of hydrogenation with alkene substrates appended to calixarenes.

Rhodium‐Catalyzed Hydrocarboxylation of Olefins with Carbon Dioxide

The catalytic hydrocarboxylation of styrenes derivatives and α,β‐unsaturated carbonyl compounds with CO2 (101.3 kPa) in the presence of an air‐stable rhodium catalyst was explored. The combination of [RhCl(cod)]2 (cod = cyclooctadiene) as a catalyst and diethylzinc as a hydride source allowed for effective hydrocarboxylation and provided the corresponding α‐aryl carboxylic acids in moderate to excellent yields. In this catalytic process with carbon dioxide, intervention of the RhI–H species, which could be generated from the RhI catalyst and diethylzinc, was clarified. Significantly, the catalytic asymmetric hydrocarboxylation of α,β‐unsaturated esters with carbon dioxide was also performed by employing a cationic rhodium complex possessing (S)‐(–)‐4,4′‐bi‐1,3‐benzodioxole‐5,5′‐diylbis(diphenylphosphine) [(S)‐SEGPHOS] as a chiral diphosphine ligand. A plausible model for asymmetric induction was proposed by determination of the absolute configuration of the product.

Cationic Iridium and Rhodium Complexes with C–N Chelating Primary Benzylic Amine Ligands as Potent Catalysts for Hydrogenation of Unsaturated Carbon–Nitrogen Bonds

Cationic half-sandwich C–N chelating Ir and Rh complexes [Cp*M(NCCH3){κ2(N,C)-NH2CR2-2-C6H4}]+SbF6– (1a, M = Ir, R = CH3; 1b, M = Ir, R = C6H5; 2a, M = Rh, R = CH3; 2b, M = Rh, R = C6H5) are synthesized by AgSbF6-mediated halide abstraction from neutral azametallacycles derived from tritylamine or cumylamine and are fully characterized by NMR spectroscopy and X-ray crystallography. The treatment of the cationic complex 1b with H2 gas under ambient conditions in the presence of triethylamine in THF-d8 quantitatively yielded hydrido(amine) complex [Cp*Ir(H){κ2(N,C)-NH2C(C6H5)2-2-C6H4}] (4). The C–N chelating Ir complex shows a higher catalytic activity than an N–N chelating complex, [Cp*Ir(NCCH3)(Tscydn)]+SbF6– (3; Tscydn = N-(p-toluenesulfonyl)-1,2-cyclohexanediamime), that has been previously used for the asymmetric hydrogenation of acyclic imines. For example, the cationic Ir complex 1a promoted the hydrogenation of N-(1-phenylethylidene)benzylamine under 30 atm at 30 °C in the presence of excess AgSbF6 ...

Evolution of rhodium(III) and iridium(III) chelates as metallonucleases

Half sandwich cationic mononuclear rhodium(III) and iridium(III) complexes [(η5-C5Me5)M(L)Cl]Cl·2H2O where L=dipyridylamine (dpa) (M=Rh, 1 and Ir, 2) and dipyridylketone (dpk) (M=Rh, 3 and Ir, 4) have been synthesized and characterized. The structure of a representative complex (1) was authenticated by single crystal X-ray diffraction analysis. Complexes 1–4 have been fully characterized by various physicochemical techniques, namely elemental analysis, spectral (IR, 1H, NMR, UV–Vis), electrochemical studies (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)). Notably, the UV absorption spectral titrations of the synthesized complexes with DNA reveal that the complexes bind to calf thymus DNA (CT-DNA) through the intercalation mode. The DNA solution hydrodynamic volume (viscosity) measurements show that the interaction of the compounds with CT-DNA occurs by classical intercalation. A molecular docking study suggested intercalation between the synthesized compounds and nucleotide base pairs. Cyclic voltammetry studies of the complexes indicate irreversible oxidation and reduction potentials. A gel electrophoresis assay demonstrates the ability of the complexes to cleave pUC19 DNA. The antibacterial activities were assayed against selected Gram(−ve) and Gram(+ve) microorganisms. The cytotoxic properties of the metal complexes have been evaluated using a brine shrimp lethality bioassay. The results suggest that the binding affinity of 1–4 lies in the order 1>4>2>3.

NH3 Synthesis in the N2/H2 Reaction System using Cooperative Molecular Tungsten/Rhodium Catalysis in Ionic Hydrogenation: A DFT Study.

The ionic hydrogenation of N2 with H2 to give NH3 is investigated by means of density functional theory (DFT) computations using a cooperatively acting catalyst system. In this system, N2 binds to a neutral tungsten pincer complex of the type [(PNP)W(N2)3] (PNP=pincer ligand) and is reduced to NH3. The protons and hydride centers necessary for the reduction are delivered by heterolytic cleavage of H2 between the N2-tungsten complex and the cationic rhodium complex [Cp*Rh{2-(2-pyridyl)phenyl}(CH3 CN)](+). Successive transfer of protons and hydrides to the bound N2, as well as all Nx Hy units that occur during the reaction, enable the computation of closed catalytic cycles in the gas and in the solvent phase. By optimizing the pincer ligands of the tungsten complex, energy spans as low as 39.3 kcal mol(-1) could be obtained, which is unprecedented in molecular catalysis for the N2/H2 reaction system.

Rhodium-Catalyzed Selective Partial Hydrogenation of Alkynes

The cationic rhodium complex [Rh(PcPr3)2(η6-PhF)]+[B{3,5-(CF3)2C6H3}4]− (PcPr3 = triscyclopropylphosphine, PhF = fluorobenzene) was used as a catalyst for the hydrogenation of the charge-tagged alkyne [Ph3P(CH2)4C2H]+[PF6]−. Pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS) was used to monitor reaction progress. Experiments revealed that the reaction is first order in catalyst and first order in hydrogen, so under conditions of excess hydrogen the reaction is pseudo-zero order. Alkyne hydrogenation was 40 times faster than alkene hydrogenation. The turnover-limiting step is proposed to be oxidative addition of hydrogen to the alkyne (or alkene)-bound complex. Addition of triethylamine caused a dramatic reduction in rate, suggesting a deprotonation pathway was not operative.

ChemInform Abstract: Asymmetric Pauson—Khand‐Type Reactions of 1,6‐Enynes Using Formaldehyde as a Carbonyl Source by Cooperative Dual Rhodium Catalysis.

AbstractThe synthesis of cyclopenta[c]furanones (III) and cyclopenta[c]pyrrolones (V) by the title reaction is accomplished by simultaneous use of neutral and cationic rhodium(I) complexes for efficient cooperative catalysis, which involves neutral rhodium(I)‐catalyzed decarbonylation of formaldehyde (II) and cationic rhodium(I)‐catalyzed cyclocarbonylation of 1,6‐enynes [(I) and (IV)] with the free carbonyl moiety.

Crystal structures of di-chlorido-palladium(II), -platinum(II) and -rhodium(III) complexes containing 8-(di-phenyl-phosphan-yl)quinoline.

The crystal structures of di-chlorido-palladium(II), -platinum(II) and -rhodium(III) complexes containing 8-(di-phenyl-phosphan-yl)quinoline, (SP-4)-[PdCl2(C21H16NP)], (1) [systematic name: di-chlor-ido-(8-di-phenyl-phosphanyl-quinoline)-palladium(II)], (SP-4)-[PtCl2(C21H16NP)]·CH2Cl2, (2) [systematic name: di-chlorido-(8-di-phenyl-phos-phanyl-quinoline)-platinum(II) dichlorometh-ane monosolvate], and (OC-6-32)-[RhCl2(C21H16NP)2]PF6·0.5CH2Cl2·0.5CH3OH, (3) [systematic name: cis-di-chlor-ido-bis-(8-di-phenyl-phosphanyl-quinoline)-rhodium(III) hexa-fluorido-phos-phate di-chloro-methane/-methanol hemisolvate] are reported. In these complexes, the phosphanyl-quinoline acts as a bidentate ligand, forming a planar asymmetrical five-membered chelate ring. The palladium(II) and platinum(II) complex mol-ecules in (1) and (2), respectively, show a typical square-planar coordination geometry and form a dimeric structure through an inter-molecular π-π stacking inter-action between the quinolyl rings. The centroid-centroid distances between the stacked six-membered rings in (1) and (2) are 3.633 (2) and 3.644 (2) Å, respectively. The cationic rhodium(III) complex in (3) has a cis(Cl),cis(P),cis(N) (OC-6-32) configuration of the ligands, in which two kinds of intra-molecular π-π stacking inter-actions are observed: between the quinolyl and phenyl rings and between two phenyl rings, the centroid-centroid distances being 3.458 (2) and 3.717 (2) Å, respectively. The PF6 (-) anion in (3) is rotationally disordered, the site occupancies of each F atom being 0.613 (14) and 0.387 (14). The CH2Cl2 and CH3OH solvent mol-ecules are also disordered and equal site occupancies of 0.5 are assumed.

Mechanistic and stereoselectivity study for the reaction of trifluoropyruvates with arylpropenes catalyzed by a cationic Lewis acid rhodium complex

The mechanism and stereoselectivity of a Lewis acid catalyzed carbonyl–ene reaction of trifluoropyruvates with arylpropenes have been investigated using a DFT method.

Asymmetric Pauson–Khand-type reactions of 1,6-enynes using formaldehyde as a carbonyl source by cooperative dual rhodium catalysis

A dual neutral-cationic rhodium(I)-catalyzed Pauson–Khand-type reaction using formaldehyde as a carbonyl source is described. The simultaneous use of neutral rhodium(I) and cationic rhodium(I) complexes is required for efficient cooperative catalysis, which involves the neutral rhodium(I)-catalyzed decarbonylation of formaldehyde and the cationic rhodium(I)-catalyzed cyclocarbonylation of 1,6-enynes using the resulting carbonyl moiety.

ChemInform Abstract: Rhodium(III)‐Catalyzed Intermolecular Amidation with Azides via C(sp3)—H Functionalization.

The amidation reactions of 8-methylquinolines with azides catalyzed by a cationic rhodium(III) complex proceed efficiently to give quinolin-8-ylmethanamine derivatives in good yields via C(sp3)–H bond activation under external oxidant-free conditions. A catalytically competent five-membered rhodacycle has been isolated and characterized, revealing a key intermediate in the catalytic cycle.

A theoretical study on the mechanisms of intermolecular hydroacylation of aldehyde catalyzed by neutral and cationic rhodium complexes

In this paper, we used density functional theory (DFT) computations to study the mechanisms of the hydroacylation reaction of an aldehyde with an alkene catalyzed by Wilkinsons catalyst and an organic catalyst 2-amino-3-picoline in cationic and neutral systems. An aldehydes hydroacylation includes three stages: the C-H activation to form rhodium hydride (stage I), the alkene insertion into the Rh-H bond to give the Rh-alkyl complex (stage Ⅱ), and the C-C bond formation (stage Ⅲ). Possible pathways for the hydroacylation originated from the trans and cis isomers of the catalytic cycle. In this paper, we discussed the neutral and cationic pathways. The rate-determining step is the C-H activation step in neutral system but the reductive elimination step in the cationic system. Meanwhile, the alkyl group migration-phosphine ligand coordination pathway is more favorable than the phosphine ligand coordination-alkyl group migration pathway in the C-C formation stage. Furthermore, the calculated results imply that an electron-withdrawing group may decrease the energy barrier of the C-H activation in the benzaldehyde hydroacylation.

Rhodium(III)-catalyzed intermolecular amidation with azides via C(sp³)-H functionalization.

The amidation reactions of 8-methylquinolines with azides catalyzed by a cationic rhodium(III) complex proceed efficiently to give quinolin-8-ylmethanamine derivatives in good yields via C(sp(3))-H bond activation under external oxidant-free conditions. A catalytically competent five-membered rhodacycle has been isolated and characterized, revealing a key intermediate in the catalytic cycle.

Chemistry of transition-metal complexes containing functionalized phosphines. Part 2. Cationic rhodium(I) complexes stabilized by hemilabile diallylphosphines

Cationic rhodium(I) complexes of the type [Rh(COD){κ3(P,C,C)-RP(CH2CHCH2)2}][BF4] (R=tBu 3a, Ph 3b, iPr2N 3c) have been synthesized starting from the chloro-bridged dimer [Rh2(μ-Cl)2(COD)2] and fully characterized by 31P{1H}, 1H and 13C{1H} NMR. X-ray diffraction analysis of 3a and 3c showed a square planar geometry around of the rhodium atom, with allyl-phosphine ligands in a chelating fashion due to coordination of only one of the allylic double bonds while the other allyl moiety is located away from the metal center. Crystals of 3c showed the presence of the two enantiomers in the asymmetric unit. In solution, a dynamic equilibrium of exchange between the two allylic double bonds was detected by low temperature NMR analysis. Hemilabile properties of diallylphosphine ligands have been demonstrated by ligand exchange reactions and the reversible displacement of coordinated allylic double bond by acetonitrile have been observed. DFT calculations have been performed to explain the order of reactivity on removing the acetonitrile ligand under high vacuum.

Asymmetric Rhodium(I)-Catalyzed C–C Activations with Zwitterionic Bis-phospholane Ligands

The development of unconventional ligand scaffolds is an important aspect to alter reaction pathways of transition-metal-catalyzed reactions. The nature of the counterion of cationic metal complexes plays an important role in the catalyst reactivity. We herein report a chiral anionic bidentate bis-phosphine ligand based on the popular phospholane scaffold. Subsequently, zwitterionic rhodium(I) complexes with no external counterion were synthesized, and their potential was evaluated in asymmetric carbon carbon bond activation of cyclobutanones. This type of rhodium complex allows, for a significantly lower reaction temperature than analogous cationic rhodium complexes and enables, for the first time, asymmetric transformations with up to 93.5:6.5 enantiomeric ratio.