Rh Catalysis Research Articles

Construction of Highly Functionalized Xanthones via Rh-Catalyzed Cascade C-H Activation/O-Annulation.

A facile and efficient strategy for obtaining functionalized and multihydroxylated xanthones via Rh catalysis under redox-neutral conditions is developed. Diverse salicylaldehydes bearing heterocycles, aromatics, and fused aromatics can be rapidly coupled with 1,4-benzoquinones or 1,4-hydroquinones to afford valuable xanthones via cascade C-H/O-H functionalization and annulation. This protocol provides a rapid synthetic approach to obtain biologically active materials through late-stage functionalization and prepares natural products such as subelliptenone, pruniflorone N, and ravenelin.

A Combined Computational and Experimental Study of Rh-Catalyzed C-H Silylation with Silacyclobutanes: Insights Leading to a More Efficient Catalyst System.

The study of new C-H silylation reagents and reactions remains an important topic. We reported that under Rh catalysis, silacyclobutanes (SCBs) for the first time were able to react with C(sp2)-H and C(sp3)-H bonds, however the underlying reasons for such a new reactivity were not understood. Through this combined computational and experimental study on C-H silylation with SCBs, we not only depict a reaction pathway that fully accounts for the reactivity and all the experimental findings but also streamline a more efficient catalyst that significantly improves the reaction rates and yields. Our key findings include: (1) the active catalytic species is a [Rh]-H as opposed to the previously proposed [Rh]-Cl; (2) the [Rh]-H is generated via a reductive elimination/β-hydride (β-H) elimination sequence, as opposed to previously proposed endocyclic β-H elimination; (3) the regio- and enantio-determining steps are identified; (4) and of the same importance, the discretely synthesized [Rh]-H is shown to be a more efficient catalyst. This work suggests that the [Rh]-H/diphosphine system should find further applications in C-H silylations involving SCBs.

Access to the C2 C–H olefination, alkylation and deuteration of indoles by rhodium(iii) catalysis: an opportunity for diverse syntheses

A concise approach for a diversity-oriented synthesis via regioselective C2 C–H olefination, alkylation, and deuteration of indoles by Rh(iii) catalysis is described.

Synthesis of oxazolidinones through ring-opening and annulation of vinylene carbonate with 2-pyrrolyl/indolylanilines under Rh(III) catalysis.

Herein, we have developed a rhodium-catalyzed C-H functionalization and subsequent intramolecular ring-opening/cyclization of vinylene carbonate with 2-pyrrolyl/indolylanilines, which leads to oxazolidinones in moderate to good yields. In this transformation, vinylene carbonate only eliminates one oxygen atom rather than -CO3 or CO2. Furthermore, some control experiments are conducted to elucidate the reaction mechanism.

Regiodivergent Access to 2- or 3-Substituted Indanones: Catalyst-Controlled Carboacylation via C–C Bond Activation

Indanones are ubiquitous in biologically active compounds. Intramolecular hydroacylation of aldehydes and alkenes is an efficient and atom-economic route to indane rings. However, these reactions a...

Hydroformylation for reducing the olefin content in the FCC light gasoline with magnetic rhodium-catalysts

The hydroformylation process was conducted on FCC light gasoline in which the total olefin content was 45.17% to reduce the olefin content, without losing its octane number. Under the catalysis of the popular Rh(acac)(CO)2 (acac = acetylacetonate) together with triphenylphosphine (TPP), the olefins, though most were internal and branched, were largely transformed to aldehydes at mild conditions without additional solvents. The gasoline octane number was also improved obviously, which confirmed that hydroformylation is a feasible route to upgrade the gasoline. In consideration of the separating advantage of magnetic particles, four magnetic catalysts, Fe3O4@Rh, Fe3O4@Rh@SiO2, Fe3O4@SiO2-NH2-Rh and Fe3O4@SiO2-NH2-Rh@SiO2, were prepared and characterized. Their catalytic abilities for hydroformylation of FCC light gasoline were studied based on the optimized reaction conditions with Rh(acac)(CO)2. The results showed that all the magnetic catalysts had a good performance, but those with a second SiO2 layer exhibited a better stability.

Synthesis of Stilbenes by Rhodium-Catalyzed Aerobic Alkenylation of Arenes via C-H Activation.

Arene alkenylation is commonly achieved by late transition metal-mediated C(sp2)-C(sp2) cross-coupling, but this strategy typically requires prefunctionalized substrates (e.g., with halides or pseudohalides) and/or the presence of a directing group on the arene. Transition metal-mediated arene C-H activation and alkenylation offers an alternative method to functionalize arene substrates. Herein, we report a rhodium-catalyzed oxidative arene alkenylation from arenes and styrenes to prepare stilbene and stilbene derivatives. The reaction is successful with several functional groups on both the arene and the olefin including fluoride, chloride, trifluoromethyl, ester, nitro, acetate, cyanide, and ether groups. Reactions of monosubstituted arenes are selective for alkenylation at the meta and para positions, generally with approximately 2:1 selectivity, respectively. Resveratrol and (E)-1,2,3-trimethoxy-5-(4-methoxystyryl)benzene (DMU-212) are synthesized by this single-step approach in high yield. Comparison with palladium catalysis showed that rhodium catalysis is more selective for meta-functionalization for monosubstituted arenes and that the Rh catalysis has better tolerance of halogen groups.

Phthalazinone-Assisted C-H Amidation Using Dioxazolones Under Rh(III) Catalysis.

The preparation of phthalazinone derivatives is pivotal for their utilization as pharmaceutical agents and other entities. Herein, we report the phthalazinone-assisted carbon-nitrogen bond forming reaction using dioxazolones as robust amidation sources under Rh(III) catalysis. The broad functional group tolerance and complete site-selectivity are observed. Notably, a series of transformations of synthesized compounds into biologically relevant N-heterocycles demonstrates the applicability of the developed methodology.

Metal-Catalyzed Cyclotrimerization Reactions of Cyanamides: Synthesis of 2-Aryl-α-carbolines.

The synthesis of annulated 2-aryl-α-carboline heterocycles is described using transition metal catalysis. A linear strategy is described that uses Rh(I) catalysis to form the α-carboline scaffold by [2+2+2] cyclotrimerization. Alternatively, a tandem catalytic approach using a Pd(II) precatalyst afforded the same target molecules by mediating a Sonogashira reaction and a [2+2+2] cyclotrimerization in the same reaction flask. In each case, nine different 2-aryl-α-carbolines have been prepared in high to modest isolated yields.

Weak Coordination Enabled Switchable C4-Alkenylation and Alkylation of Indoles with Allyl Alcohols.

A weak carbonyl coordination facilitated tunable reactivity between alkenylation and alkylation of indoles at the C4 C-H site is presented using readily accessible allylic alcohols in the presence of Rh catalysis by switching the additives or directing group. Exclusive site selectivity, functional group tolerance, and late-stage modifications are the important practical features.

Rh-induced Support Transformation and Rh Incorporation in Titanate Structures and Their Influence on Catalytic Activity

Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO2, and CO, the CO+H2O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied supports for Rh nanoparticles. The present study demonstrates that the nature of the support has a marked influence on the specific activity. For comparison, the catalytic activity of TiO2 P25 is also presented. It is pointed out that a certain amount of Rh can be stabilized as cation (Rh+) in ion-exchange positions (i.e., in atomic scale distribution) of the titanate framework. This ionic form does not exists on TiO2. We pay distinguished attention not only to the electronic interaction between Rh metal and the titania/titanate support, but also to the Rh-induced phase transitions of one-dimensional titanate nanowires (TiONW) and nanotubes (TiONT). Support transformation phenomena can be observed in Rh-loaded titanates. Rh decorated nanowires transform into the TiO2(B) phase, whereas their pristine counterparts recrystallize into anatase. The formation of anatase is dominant during the thermal annealing process in both acid-treated and Rh-decorated nanotubes; Rh catalysis this transformation. We demonstrate that the phase transformations and the formation of Rh nanoclusters and incorporated Rh ions affect the conversion and the selectivity of the reactions. The following initial activity order was found in the CO2 + H2, CO + H2O and C2H5OH decomposition reactions: Rh/TiO2 (Degussa P25) ≥ Rh/TiONW > Rh/TiONT. On the other hand it is remarkable that the hydrogen selectivity in ethanol decomposition was two times higher on Rh/TiONW and Rh/TiO(NT) catalysts than on Rh/TiO2 due to the presence of Rh+ cations incorporated into the framework of the titanate structures.

Rh(III)-Catalyzed Reaction of α-Carbonyl Sulfoxonium Ylides and Alkenes: Synthesis of Indanones via [4 + 1] Cycloaddition.

The synthesis of indanone derivatives by the Rh(III)-catalyzed reaction of α-carbonyl sulfoxonium ylides with activated alkenes is reported. The reaction shows a high tolerance for functional groups and furnishes a variety of substituted indanone derivatives via a formal [4 + 1] cycloaddition. Highly stable sulfoxonium ylides were used as substrates in this C-H functionalization, and their bifunctional character could be effectively exploited using Rh(III) catalysis via sequential double C-C bond formation. Based on mechanistic studies including deuterium-labeling experiments, the reaction is proposed to proceed as follows: Rh(III)-catalyzed C-H oxidative alkenylation via β-hydride elimination, readdition of H-Rh species, a 1,2-carbon shift with the elimination of DMSO, and protonation.

The regioselective annulation of alkylidenecyclopropanes by Rh(iii)-catalyzed C-H/C-C activation to access spirocyclic benzosultams.

Functionalized benzosultams are an essential class of structural motif found in various biologically active molecules. The synthesis of spirocyclic benzosultams from N-sulfonyl ketimine and alkylidenecyclopropanes (ACPs) under the catalysis of Rh(iii) has been developed. This transformation enables the formation of two C-C bonds and a double bond with high E-selectivity through C-H and C-C bond activation. This reaction proceeded smoothly with excellent regioselectivity, high efficiency, and tolerance for various functional groups.

A rhodium(ii) catalysed domino synthesis of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles.

An efficient and convenient protocol for the synthesis of a novel class of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles has been disclosed. The reaction proceeds via denitrogenative aza-vinyl rhodium carbene formation to give a carbonyl ylide, which with indole results in 1,3-dipolar cycloaddition followed by sequential semipinacol rearrangement/ring expansion/oxidation to produce azepino fused diindoles. The reaction shows a broad substrate scope giving up to 81% yield. Furthermore, reversible catalytic hydrogenation and photophysical studies were carried out to demonstrate the application of these molecules.

DFT Study on Photosensitizer-Free Visible-Light-Mediated Au-Catalyzed cis-Difunctionalization of Alkynes: Mechanism and Selectivities as Compared to Rh Catalysis.

Density functional theory (DFT) calculations were performed to investigate the photosensitizer-free visible-light-mediated gold-catalyzed cis-difunctionalization of alkynes with aryl diazonium salts. The detailed reaction mechanism is established, and the observed regio- and chemoselectivities are rationalized. The results are compared to those of the rhodium-catalyzed cis-difunctionalization of alkynes. It is indicated that the excitation of the aryl diazonium salt initiates the photocatalytic cycle, and the following single-electron transfer between the Au(I) catalyst and the excited aryl diazonium salt affords the key aryl radical. Both gold- and rhodium-catalyzed reactions involve two major steps: alkyne insertion into the M-N or M-C bond (M = Au, Rh), and C-C or C-N reductive elimination from the M(III) center. The cis-difunctionalized product can be obtained by the trimethylsilyl (TMS)-substituted alkyne through the gold catalysis or by the Ph-substituted alkyne through the rhodium catalysis. The catalyst-dependent reactivity switch of TMS- and Ph-substituted alkynes is attributed to the catalyst-induced shift of the rate-determining step.

Asymmetric Catalysis in Liquid Confinement: Probing the Performance of Novel Chiral Rhodium-Diene Complexes in Microemulsions and Conventional Solvents.

The role of liquid confinement on the asymmetric Rh catalysis was studied using the 1,2-addition of phenylboroxine (2) to N-tosylimine 1 in the presence of [RhCl(C2 H4 )2 ]2 and chiral diene ligands as benchmark reaction. To get access to Rh complexes of different polarity, enantiomerically pure C2 -symmetric p-substituted 3,6-diphenylbicyclo[3.3.0]octadienes 4 and diastereomerically enriched unsymmetric norbornadienes 5 and 6 carrying either the Evans or the SuperQuat auxiliary were synthesized. A microemulsion containing the equal amounts of H2 O/KOH and toluene/reactants was formulated using the hydrophilic sugar surfactant n-octyl β-d-glucopyranoside (C8 G1 ) to mediate the miscibility between the nonpolar reactants and KOH, needed to activate the Rh-diene complex. Prominent features of this organized reaction medium are its temperature insensitivity as well as the presence of water and toluene-rich compartments with a domain size of 55 Å confirmed by small-angle X-ray scattering (SAXS). Although bicyclooctadiene ligands 4 a,b,e performed equally well under homogeneous and microemulsion conditions, ligands 4 c,d gave a different chemoselectivity. For norbornadienes 5, 6, however, microemulsions markedly improved conversion and enantioselectivity as well as reaction rate, as was confirmed by kinetic studies using ligand 5 b.

Configurational Lability of Imino‐Substituted Ethano Tröger Bases. Insight on the Racemization Mechanism

AbstractPolycyclic indoline‐benzodiazepines are afforded in one step by the reaction of Tröger bases with N‐sulfonyl‐1,2,3‐triazoles under Rh(II) catalysis. After α‐imino carbene formation, the process involves a cascade of [1,2]‐Stevens rearrangement, Friedel‐Crafts, Grob fragmentation, and aminal formation reactions. It is highly diastereoselective (d.r. >49:1, four stereocenters incl. two bridgehead N‐atoms). However and in contrast with other reported carbene additions to these moieties, full racemization occurs when enantiopure Tröger bases are used as substrates. To pinpoint the origin of this unexpected behavior, a key elemental step of the mechanism was evaluated and tested. Interestingly, it is not only the initial ring‐opening but also the latter reversible Mannich reaction of the imino‐substituted ethano Tröger base intermediate that is responsible for the loss of enantiospecificity.

Reactions of 5-Aminoisoxazoles with α-Diazocarbonyl Compounds: Wolff Rearrangement vs N-H Insertion.

A highly chemoselective reaction between 5-aminoisoxazoles and α-diazocarbonyl compounds has been described. Both Wolff rearrangement and N-H insertion products can be obtained selectively by the judicious choice of reaction conditions. In the case of the Wolff rearrangement reactions, the N-isoxazole amides are accessed as the sole products under thermal conditions. On the other hand, α-amino acid derivatives of N-isoxazolescan be obtained through N-H insertion reactions in the presence of catalytic Rh2(Oct)4. Both reactions proceed under mild reaction conditions and feature a broad substrate scope.

New Aspects of Recently Developed Rhodium(N‐Heterocyclic Carbene)‐Catalyzed Organic Transformations

AbstractN‐Heterocyclic carbene (NHC) compounds have attracted significant attention as ligands for transition metal catalysis and organometallic complexes. Over the last two decades, among the various transition metals, ruthenium‐NHC‐type metathesis catalysts and palladium‐NHC‐based cross‐coupling catalysts have been extensively studied and developed. Recently, however, there has been a resurgence in developing NHC complexes of group 9 metals (Co, Rh, and Ir) for their use in organic transformations. This review specifically focuses on the advances in rhodium‐NHC catalysts. From the classical NHC ligands to the most recently developed CAAC (cyclic alkyl amino carbene)‐type ligands, the impact of using NHC ligands for increasing activity and enhancing selectivity across multiple reactions is discussed. We address the recent achievements in Rh(NHC) catalysis, which include (i) reactions involving rhodium hydride species, (ii) cross‐coupling reactions, (iii) bond cleavage reactions, and (iv) reduction reactions. Mechanistic insights based on the active catalytic species and a discussion about harnessing their unique reactivity for the future development of rhodium catalysis are given.magnified image

Harnessing Stereospecific Z-Enamides through Silver-Free Cp*Rh(III) Catalysis by Using Isoxazoles as Masked Electrophiles.

The stereospecific synthesis of Z-enamides is described in this paper. For the first time, isoxazoles have been employed as electrophiles in C-H functionalization to afford thermodynamically less stable Z-enamides utilizing salicylaldehydes in an atom- and step-economic fashion. The stereochemistry of enamides might originate from the relative disposition of atoms present in isoxazole and the intramolecular hydrogen bonding. The reaction showed excellent scope as several structurally and electronically diverse salicylaldehydes and isoxazoles reacted efficiently.