Rhodium N-heterocyclic Carbene Research Articles

N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: dynamics and catalysis

Novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me, Bz, Tr, tBu]: determination of the rotation barriers about the Rh-carbene and catalytic activity in the hydrosilylation of terminal alkynes.

Highly selective directed arylation reactions via back-to-back dehydrogenative C–H borylation/arylation reactions

Dimeric rhodium N-heterocyclic carbene complexes are demonstrated to be effective catalyst precursors for directed C-H borylation reactions at room temperature. The reactions are highly selective for mono-borylation and can be combined with a one-pot Suzuki-Miyaura coupling to give C-H arylation products with exclusive selectivity for mono-arylation without the requirement for steric blocking groups.

Radioiodinated and astatinated NHC rhodium complexes: Synthesis

IntroductionThe clinical development of radioimmunotherapy with astatine-211 is limited by the lack of a stable radiolabeling method for antibody fragments. An astatinated N-heterocyclic carbene (NHC) Rhodium complex was assessed for the improvement of radiolabeling methodologies with astatine. MethodsWet harvested astatine-211 in diisopropyl ether was used. Astatine was first reduced with cysteine then was reacted with a chlorinated Rh-NHC precursor to allow the formation of the astatinated analogue. Reaction conditions have been optimized. Astatine and iodine reactivity were also compared. Serum stability of the astatinated complex has been evaluated. ResultsQuantitative formation of astatide was observed when cysteine amounts higher than 46.2 nmol/μl of astatine solution were added. Nucleophilic substitution kinetics showed that high radiolabeling yields were obtained within 15 min at 60°C (88%) or within 5 min at 100°C (95%). Chromatographic characteristics of this new astatinated compound have been correlated with the cold iodinated analog ones. The radioiodinated complex was also synthesized from the same precursor (5 min. at 100°C, up to 85%) using [125I]NaI as a radiotracer. In vitro stability of the astatinated complex was controlled after 15 h incubation in human serum at 4°C and 37°C. No degradation was observed, indicating the good chemical and enzymatic stability. ConclusionThe astatinated complex was obtained in good yield and exhibited good chemical and enzymatic stability. These preliminary results demonstrate the interest of this new radiolabeling methodology, and further functionalizations should open new possibilities in astatine chemistry. Advances in knowledge and implications for patient careAlthough there are many steps and pitfalls before clinical use for a new prosthetic group from the family of NHC complexes, this work may open a new path for astatine-211 targeting.

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh(μ-OH)(NHC)(η2-olefin)]2 (coe, IPr (3), IMes (4); ethylene, IPr (5)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex 3 has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including RhI-thiolato species Rh(SCH2Ph)(IPr)(η2-coe)(py) (6) and Rh(SCH2Ph)(IPr)(η2-HC≡CCH2Ph)(py) (7), and the RhIII-hydride-dithiolato derivative RhH(SCH2Ph)2(IPr)(py) (8) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity.

Back Cover: Synthesis of Poly(silyl ether)s by Rhodium(I)–NHC Catalyzed Hydrosilylation: Homogeneous versus Heterogeneous Catalysis (ChemCatChem 5/2013)

Rhodium-catalyzed polymerization goes heavyweightThe cover picture shows a neutral rhodium(I) N-heterocyclic carbene hydrosilylation catalyst immobilized on the surface of MCM-41. In their Full Paper on p. 1133 ff., F. J. Fernández-Alvarez, L. A. Oro et al. describe how these rhodium-containing materials catalyze the preparation of poly(silyl ether)s by hydrosilylation. These results show that the immobilized catalysts provide polymers with higher average molecular weight than those obtained by using the corresponding homogenous systems, which could be attributed to a confinement effect exerted by the channels of MCM-41.

Unsaturated NHC Complexes Immobilized by the Backbone: Synthesis and Application

AbstractThe synthesis and an exemplary catalytic application of a SBA‐15‐supported rhodium–N‐heterocyclic carbene (NHC) complex is reported. In contrast to the conventional method of immobilization of unsaturated NHC complexes, the rhodium–NHC compound described here is connected to the SBA‐15 surface by a linker originating from the backbone of the unsaturated NHC ligand. The unique characteristics of the new immobilization mode enable the supported NHC complexes to maintain two unchanged “wing‐tip” ligands. This method of immobilization of unsaturated NHC complexes provides a new way to maintain the original configuration of homogeneous NHC complexes in the heterogenized catalytic system. The immobilized rhodium–NHC catalyst is used in a hydrogenation reaction with styrene as substrate.

Dioxygen adducts of rhodium N-heterocyclic carbene complexes

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.

Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes by Rhodium N-Heterocyclic Carbene Catalysts

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(η(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step.

Rhodium(I) N-Heterocyclic Carbene Complexes as Catalysts for Hydroformylation of Olefins: An Overview

The present state of art on the application of carbene-rhodium complexes as stable catalysts for hydroformylation reaction is described. A critical analysis of the influence of pressure and temperature on the hydroformylation of non-modified rhodium complexes and those ones described in the presence of carbenes is also done. Finally, a set of experiments using NHCs prepared by us at different reactions conditions is presented and discussed. Keywords: Aliphatic olefins, hydroformylation, rhodium, N-heterocyclic carbene ligands, Rhodium-NHC Complexes, Rhodium-Carbonyl Complexes, Influence of pressure, styrene, Influence of Temperature

A cationic rhodium(I) N-heterocyclic carbene complex isolated as an aqua adduct

The title complex, aqua­[1,3-bis­(2,6-diiso­propyl­phen­yl)imid­az­ol-2-yl­idene](η4-cyclo­octa-1,5-diene)rhodium(I) tetra­fluor­ido­borate, [Rh(C8H12)(C27H36N2)(H2O)]BF4, exihibits a square-planar geometry around the Rh(I) atom, formed by a bidentate cyclo­octa-1,5-diene (cod) ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF4 − anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via inter­actions between the aqua ligand H atoms and the BF4 − F atoms.

Synthesis and characterization of κ-2- bis- N-heterocyclic carbene rhodium(I) catalysts: Application in enantioselective arylboronic acid addition to cyclohex-2-enones

This manuscript describes the synthesis and characterization of new derivatives of di- N-heterocyclic carbene (diNHC) ligands derived from trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diylmethanediyl (DEAM) and trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diyl (DEA). Synthesized as the diazolium salts, the new ligands are employed, in conjunction with known derivatives, to examine specific properties than influence the chiral induction in rhodium-catalyzed arylboronic acid addition to cyclohex-2-enones. Three properties of the diNHC ligands are modified and include: 1) the size and composition of the N-heterocyclic substituent (Me, i Pr, Bn, CHPh 2, o-MeBn, and R-CHMePh), 2) the type of N-heterocycle (benzimidazole versus imidazole), and 3) the size of the chiral pocket (DEAM versus DEA). Results from the catalytic studies indicate the DEAM ligand, which contains an extra CH 2 group compared to DEA, is too flexible to induce enantiomeric excess. The DEA derivatives containing an imidazole or benzimidazole provide distinctly different results despite relatively small differences between the heterocyclic carbene donors. An unexpected source of chiral induction is rationalized using results from catalytic data (% e.e.) and complemented by X-ray structural data and DFT calculations.

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( η 4-1,5-cyclooctadiene)(1,3-di-[(1 R,2 R,3 R,5 S)-2,6,6-trimethylbicyclo[3.1.1]hept-3-yl] imidazolin-2-ylidene)rhodium(I) ( 5a), chloro( η 4-1,5-cyclooctadiene)(1,3-di-[(1 R,2 S,5 R)-2-isopropyl-5-menthylcyclohex-1-yl]imidazol-2-ylidene)rhodium(I) ( 5b) and chloro( η 4-1,5-cyclooctadiene)(1,3-di-[(2 R,4 S,5 S)-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 C 2-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.

Synthesis and Properties of Chelating N-Heterocyclic Carbene Rhodium(I) Complexes: Synthetic Experiments in Current Organometallic Chemistry

The preparation and characterization of two air-stable Rh(I) complexes bearing a chelating N-heterocyclic carbene (NHC) ligand is described. The synthesis involves the preparation of a Ag(I)−NHC complex and its use as carbene transfer agent to a Rh(I) precursor. The so obtained complex can be further reacted with carbon monoxide to give the corresponding carbonyl derivative. The students will be able to identify the products using standard characterization techniques such as NMR and IR spectroscopy. The required equipment is typically available in most of the standard laboratories for undergraduate students.

N-Heterocyclic Carbene–Rhodium(I) Complexes Derived from Proline for the 1,4-Conjugate Addition of Arylboronic Acids to Enones in Neat Water

N-Heterocyclic carbene–rhodium(I) complexes derived from N-benzyl substituted proline have been successfully synthesized and were found to be efficient catalysts for the 1,4-conjugate addition of arylboronic acids to enones in neat water at 40 °C. Under the optimal reaction conditions, all reactions gave the addition products in good to high yields.

A simple procedure for the polymer-supported N-heterocyclic carbene–rhodium complex via click chemistry: a recyclable catalyst for the addition of arylboronic acids to aldehydes

A novel polymer-supported carbene-rhodium complex was prepared using a simple procedure via click chemistry. This polymer-supported N-heterocyclic carbene-rhodium complex was characterized and used as a catalyst for the addition of arylboronic acids to aldehydes in good to excellent yields.

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( η 4-1,5-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 BMimPF 6 could be reused without noticeable loss of catalytic activity and selectivity.

A Rhodium IBiox[(−)-menthyl] Complex as a Highly Selective Catalyst for the Asymmetric Hydroarylation of Azabicyles: An Alternative Route to Epibatidine

The synthesis and characterization of a new chiral rhodium N-heterocyclic carbene complex, Rh(IBiox[(-)-menthyl](CO)(2)Cl, is reported. In addition, the very high enantioselectivity exhibited by this complex, as a catalyst for the asymmetric hydroarylation of azabicycles, is demonstrated and applied to the synthesis of epibatidine.

Rhodium N-Heterocyclic Carbene Complexes as Effective Catalysts for [2+2+2]-Cycloaddition Reactions

Rhodium complexes of N-heterocyclic carbenes (NHC) [RhCl(cod)(NHC)], [cod = 1,5-cyclooctadiene, NHC = 1,3-diisopropylimidazolin-2-ylidene, and NHC = 1,3-dimesitylimidazolin-2-ylidene] are tested for intra- and partially intramolecular [2+2+2]-cycloaddition reactions demonstrating their effectiveness in this kind of process. This is the first use of a rhodium-NHC complex in a [2+2+2]-cycloaddition reaction of three alkynes. ESI-MS has been used to detect oxidative addition intermediates using rhodium NHC as the catalytic system.

The synthesis and X-ray structure of a chiral rhodium–NHC complex: Implications for the use of NHCs in asymmetric hydroformylation catalysis

An optically active N-heterocyclic carbene (NHC) has been metallated with [RhCl(COD)] 2 to give a rhodium(I) monocarbene substituted complex. This complex has been fully characterized by 1H and 13C NMR spectroscopy and by single-crystal X-ray analysis. Preliminary tests of hydroformylation of styrene catalyzed with this compound have shown that branched to linear aldehyde ratio was rather low 0.67 (40%) and that the enantiomeric excess measured on the corresponding alcohols was only 10%. In the presence of an excess of triphenylphosphine, the ratio of branched to linear aldehyde increased markedly (91%) whereas the enantioselectivity of the reaction remained almost unchanged (12.5%).

Supported N-heterocyclic carbene rhodium complexes as highly selective hydroformylation catalysts

New supported rhodium(I) catalyst precursors were obtained in reaction of [Rh(OMe)(cod)] 2 with polymeric monoliths containing methyl-imidazolium moieties and applied in 1-hexene hydroformylation at 10 atm of CO/H 2. Under solventless conditions with a small amount of P(OPh) 3 as a modifying ligand, [P(OPh) 3]/[Rh] = 2, ca. 80% of aldehydes with a high n/iso ratio of ca. 6 was obtained in eight successive runs. When P(OCH 2CF 3) 3 was used instead of P(OPh) 3, n/iso ratios up to 49.8 were obtained, however, the systems were active only in two subsequent reactions. With PPh 3 as a modifying ligand, the yield of aldehydes decreased to ca. 20% already in the third run and n/iso was ca. 3.