Reactive Rhodium Research Articles

Rhodium hydride enabled enantioselective intermolecular C\u2013H silylation to access acyclic stereogenic Si\u2013H

The tremendous success of stereogenic carbon compounds has never ceased to inspire researchers to explore the potentials of stereogenic silicon compounds. Intermolecular C–H silylation thus represents the most versatile and straightforward strategy to construct C–Si bonds, however, its enantioselective variant has been scarcely reported to date. Herein we report a protocol that allows for the enantioselective intermolecular C–H bond silylation, leading to the construction of a wide array of acyclic stereogenic Si–H compounds under simple and mild reaction conditions. Key to the success is (1) a substrate design that prevents the self-reaction of prochiral silane and (2) the employment of a more reactive rhodium hydride ([Rh]-H) catalyst as opposed to the commonly used rhodium chloride ([Rh]-Cl) catalyst. This work unveils opportunities in converting simple arenes into value-added stereogenic silicon compounds.

Direct Low-Temperature Oxidative Conversion of Methane to Acetic Acid on Rhodium-Modified Zeolites

Oxidative conversion of methane to acetic acid at 150°С and a pressure of 6.5 MPa with the assistance of carbon monoxide on rhodium-modified ZSM-5 zeolites with different SiO2/Al2O3 molar ratios was studied. A decrease in SiO2/Al2O3 from 300 to 30 leads to a 2.7-fold increase in the acetic acid yield. Such behavior of the catalytic system correlates with an increase in the acidity, primarily with an increase in the density of Bronsted acid sites on the zeolite surface. Introduction of 0.1–0.5 wt % rhodium into the zeolite increases the acetic acid yield by a factor of more than 3. The highest acetic acid yield (0.53 mmol/gcat) is attained on the zeolite with SiO2/Al2O3 = 33 modified with 0.5 wt % Rh. The methane transformation to acetic acid occurs while involving both the acid sites of the zeolite and the sites formed by reactive rhodium particles.

A convenient method for the generation of {Rh(PNP)}+ and {Rh(PONOP)}+ fragments: reversible formation of vinylidene derivatives.

The substitution reactions of [Rh(COD)2][BArF4] with PNP and PONOP pincer ligands 2,6-bis(di-tert-butylphosphinomethyl)pyridine and 2,6-bis(di-tert-butylphosphinito)pyridine in the weakly coordinating solvent 1,2-F2C6H4 are shown to be an operationally simple method for the generation of reactive formally 14 VE rhodium(i) adducts in solution. Application of this methodology enables synthesis of known adducts of CO, N2, H2, previously unknown water complexes, and novel vinylidene derivatives [Rh(pincer)(CCHR)][BArF4] (R = tBu, 3,5-tBu2C6H3), through reversible reactions with terminal alkynes.

Selective Aliphatic Carbon-Carbon Bond Activation by Rhodium Porphyrin Complexes.

The carbon-carbon bond activation of organic molecules with transition metal complexes is an attractive transformation. These reactions form transition metal-carbon bonded intermediates, which contribute to fundamental understanding in organometallic chemistry. Alternatively, the metal-carbon bond in these intermediates can be further functionalized to construct new carbon-(hetero)atom bonds. This methodology promotes the concept that the carbon-carbon bond acts as a functional group, although carbon-carbon bonds are kinetically inert. In the past few decades, numerous efforts have been made to overcome the chemo-, regio- and, more recently, stereoselectivity obstacles. The synthetic usefulness of the selective carbon-carbon bond activation has been significantly expanded and is becoming increasingly practical: this technique covers a wide range of substrate scopes and transition metals. In the past 16 years, our laboratory has shown that rhodium porphyrin complexes effectively mediate the intermolecular stoichiometric and catalytic activation of both strained and nonstrained aliphatic carbon-carbon bonds. Rhodium(II) porphyrin metalloradicals readily activate the aliphatic carbon(sp3)-carbon(sp3) bond in TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) and its derivatives, nitriles, nonenolizable ketones, esters, and amides to produce rhodium(III) porphyrin alkyls. Recently, the cleavage of carbon-carbon σ-bonds in unfunctionalized and noncoordinating hydrocarbons with rhodium(II) porphyrin metalloradicals has been developed. The absence of carbon-hydrogen bond activation in these systems makes the reaction unique. Furthermore, rhodium(III) porphyrin hydroxide complexes can be generated in situ to selectively activate the carbon(α)-carbon(β) bond in ethers and the carbon(CO)-carbon(α) bond in ketones under mild conditions. The addition of PPh3 promotes the reaction rate and yield of the carbon-carbon bond activation product. Thus, both rhodium(II) porphyrin metalloradical and rhodium(III) porphyrin hydroxide are very reactive to activate the aliphatic carbon-carbon bonds. Recently, we successfully demonstrated the rhodium porphyrin catalyzed reduction or oxidation of aliphatic carbon-carbon bonds using water as the reductant or oxidant, respectively, in the absence of sacrificial reagents and neutral conditions. This Account presents our contribution in this domain. First, we describe the chemistry of equilibria among the reactive rhodium porphyrin complexes in oxidation states from Rh(I) to Rh(III). Then, we present the serendipitous discovery of the carbon-carbon bond activation reaction and subsequent developments in our laboratory. These aliphatic carbon-carbon bond activation reactions can generally be divided into two categories according to the reaction type: (i) homolytic radical substitution of a carbon(sp3)-carbon(sp3) bond with a rhodium(II) porphyrin metalloradical and (ii) σ-bond metathesis of a carbon-carbon bond with a rhodium(III) porphyrin hydroxide. Finally, representative examples of catalytic carbon-carbon bond hydrogenation and oxidation through strategic design are covered. The progress in this area broadens the chemistry of rhodium porphyrin complexes, and these transformations are expected to find applications in organic synthesis.

ChemInform Abstract: Rhodium(II)‐Catalyzed Regioselective Carbenoid Insertion Reaction of Simple Indoles with N‐Sulfonyltriazoles: A Rapid Access to Tryptamine Vinylogues.

AbstractThe rhodium‐catalyzed nitrogen extrusion from triazoles gives reactive rhodium(II) aza‐vinylcarbenes, which react with indoles to form predominantly Z‐isomers of tryptamine vinylogues.

ChemInform Abstract: Diverting Hydrogenations with Wilkinson′s Catalyst Towards Highly Reactive Rhodium(I) Species.

AbstractAn unprecedented, facile, and efficient method to expand the reactivity of the benchmark catalyst, RhCl(PPh3)3, in situ without modifying its inner coordination sphere by simple addition of Barton′s base is presented.

Computational ligand design in enantio- and diastereoselective ynamide [5+2] cycloisomerization.

Transition metals can catalyse the stereoselective synthesis of cyclic organic molecules in a highly atom-efficient process called cycloisomerization. Many diastereoselective (substrate stereocontrol), and enantioselective (catalyst stereocontrol) cycloisomerizations have been developed. However, asymmetric cycloisomerizations where a chiral catalyst specifies the stereochemical outcome of the cyclization of a single enantiomer substrate—regardless of its inherent preference—are unknown. Here we show how a combined theoretical and experimental approach enables the design of a highly reactive rhodium catalyst for the stereoselective cycloisomerization of ynamide-vinylcyclopropanes to [5.3.0]-azabicycles. We first establish highly diastereoselective cycloisomerizations using an achiral catalyst, and then explore phosphoramidite-complexed rhodium catalysts in the enantioselective variant, where theoretical investigations uncover an unexpected reaction pathway in which the electronic structure of the phosphoramidite dramatically influences reaction rate and enantioselectivity. A marked enhancement of both is observed using the optimal theory-designed ligand, which enables double stereodifferentiating cycloisomerizations in both matched and mismatched catalyst–substrate settings.

Diverting Hydrogenations with Wilkinson's Catalyst towards Highly Reactive Rhodium(I) Species.

The addition of Barton's base has a dramatic effect on the classic rhodium(III)-mediated hydrogenations promoted by Wilkinson's catalyst. Following the initial oxidative addition, a barrierless reductive elimination of HCl from the traditional rhodium(III) intermediates instantly produces a rhodium(I) monohydride species, which is remarkably reactive in the hydrogenation of several internal alkynes and functionalized trisubstituted alkenes. The direct formation of this species is unprecedented upon addition of molecular hydrogen and its catalytic potential has been hitherto barely explored.

Diverting Hydrogenations with Wilkinson's Catalyst towards Highly Reactive Rhodium(I) Species

AbstractThe addition of Barton's base has a dramatic effect on the classic rhodium(III)‐mediated hydrogenations promoted by Wilkinson′s catalyst. Following the initial oxidative addition, a barrierless reductive elimination of HCl from the traditional rhodium(III) intermediates instantly produces a rhodium(I) monohydride species, which is remarkably reactive in the hydrogenation of several internal alkynes and functionalized trisubstituted alkenes. The direct formation of this species is unprecedented upon addition of molecular hydrogen and its catalytic potential has been hitherto barely explored.

Rh(III)(Cp*)]-catalyzed cascade arylation and chlorination of α-diazocarbonyl compounds with arylboronic acids and N-chlorosuccinimide for facile synthesis of α-aryl-α-chloro carbonyl compounds.

A Rh(III)-catalyzed cascade arylation and chlorination of α-diazocarbonyl compounds with arylboronic acids and N-chlorosuccinimide was achieved. The reaction exhibits excellent functional group tolerance on the organoboron and the diazo reagents; the functionalized α-aryl-α-chlorocarbonyl compounds were obtained in up to 86% yields. The cascade reaction should involve migratory carbene insertion of arylrhodium(III) to form some reactive rhodium(III)-diketonate complexes. Its subsequent reaction with N-chlorosuccinmide afforded the α-chlorocarbonyl products.

ChemInform Abstract: Rhodium‐Catalyzed Asymmetric Addition of Arylboronic Acids to β‐Nitroolefins: Formal Synthesis of (S)‐SKF 38393.

AbstractAn efficient enantioselective addition of an array of arylboronic acids to various β‐nitrostyrenes catalyzed by a novel and reactive rhodium—diene catalyst is developed, providing β,β‐diarylnitroethanes.

Insertion of Reactive Rhodium Carbenes into Boron–Hydrogen Bonds of Stable N-Heterocyclic Carbene Boranes

Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH3) and diazocarbonyl compounds (N2CRCOR'). Stable α-NHC-boryl carbonyl compounds (NHC-BH2-CHRCOR') are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.

Reduction of Rhodium(III) Porphyrin Hydroxide to Rhodium(II) Porphyrin

Highly reactive rhodium(III) porphyrin hydroxides were formed from the ligand substitution of rhodium porphyrin halides in benzene and were rapidly reduced to rhodium(II) porphyrins and hydrogen peroxide. Thus hydroxide acted as the reducing agent. Oxidative addition of rhodium(II) porphyrin with hydrogen peroxide proceeded rapidly at room temperature to give back rhodium(III) porphyrin hydroxides. Rhodium(II) porphyrins and H2O2 therefore were thermally reversible with rhodium porphyrin hydroxides.

A Highly Reactive Rhodium(I)–Boryl Complex as a Useful Tool for CH Bond Activation and Catalytic CF Bond Borylation

A Highly Reactive Rhodium(I)–Boryl Complex as a Useful Tool for CH Bond Activation and Catalytic CF Bond Borylation

A Highly Reactive Rhodium(I)–Boryl Complex as a Useful Tool for CH Bond Activation and Catalytic CF Bond Borylation

A Highly Reactive Rhodium(I)–Boryl Complex as a Useful Tool for CH Bond Activation and Catalytic CF Bond Borylation

Synthesis and Reactivity of Rhodium(II) N-Triflyl Azavinyl Carbenes

Highly reactive rhodium(II) N-trifluoromethylsulfonyl azavinyl carbenes are formed in situ from NH-1,2,3-triazoles, triflic anhydride, and rhodium carboxylates. They rapidly and selectively react with olefins, providing cyclopropane carboxaldehydes and 2,3-dihydropyrroles in generally excellent yields and high enantio- and diastereoselectivity.

Rhodium(0) Metalloradicals in Binuclear C−H Activation

A reactive rhodium(0) metalloradical capable of binuclear activation of an aromatic C-H bond of PPh(3) is disclosed. Kinetic measurements and density functional theory calculations reveal a binuclear mechanism: two metalloradicals add to a 'double bond' of the aromatic substrate while approaching the rate limiting C-H activation step (TS). Such aromatic C-H bond activation with Rh(0) metalloradicals potentially produces kinetically labile Rh(I)-aryl and Rh(I)-H species, and thus, this could become a viable new approach to hydrocarbon functionalization.

Solar energy conversion using photochemical molecular devices: photocatalytic hydrogen production from water using mixed-metal supramolecular complexes

Photocatalytic generation of hydrogen from water is an integral part of the next generation clean fuel technologies. The conversion of solar energy into useful chemical energy is of great interest in contemporary investigations. The splitting of water is a multi-electron process involving the breaking and making of chemical bonds which requires multi-component photocatalytic systems. Supramolecular complexes [{(TL)2Ru(BL)}2RhX2](Y)5 (where TL = terminal ligand, BL = bridging ligand, X = Cl− or Br−, and Y = PF6− or Br−) have been synthesized and studied for their light absorbing, electrochemical and photocatalytic properties. The supramolecular complexes in this investigation are multi-component systems comprised of two ruthenium based light absorbers connected through bridging ligands to a central rhodium, which acts as an electron collecting center upon excitation. These complexes absorb light throughout the ultraviolet and visible regions of the solar spectrum. The supramolecular complexes possess ruthenium based highest occupied molecular orbitals (HOMO) and a rhodium based lowest unoccupied molecular orbital (LUMO). These molecular devices have been investigated and shown to function as photoinitiated electron collectors at the reactive rhodium metal center, and explored as photocatalysts to generate hydrogen from water in an aqueous solution in the presence of an electron donor.

A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts

The catalytic performance of Pt and Rh catalysts for the selective catalytic reduction (SCR) of NO by propylene in the presence of excess oxygen has been investigated over catalysts supported on six different metal oxide carriers (CeO2, Al2O3, TiO2, YSZ, ZrO2 and W6+-doped TiO2). It has been found that the nature of the dispersed metal affects strongly the light-off temperature of propylene, the maximum NO conversion to reduction products and the selectivity towards nitrogen. For a given support, Pt catalysts are always more active for both NO reduction and propylene oxidation, but are much less selective towards N2, compared to Rh catalysts. Rhodium catalysts are able to selectively reduce NO even in the absence of oxygen in the feed. However, their activity is suppressed with increasing oxygen feed concentration possibly due to the formation of less reactive rhodium oxides. In contrast, oxygen promotes the de-NOx activity of platinum catalysts but decreases selectivity towards nitrogen. Results are explained by considering the effects of the nature of the metallic phase and the support on the elementary steps of the propylene-SCR reaction. It is concluded that the catalytic performance of both metals may be improved by proper selection of the support.

Photochemical hydrogen production from water using the new photocatalyst [{(bpy) 2Ru(dpp)} 2RhBr 2](PF 6) 5

The synthesis, electrochemical, and photochemical properties of the new photocatalyst [{(bpy) 2Ru(dpp)} 2RhBr 2](PF 6) 5 (bpy = 2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) which couples two ruthenium light absorbing metal centers to a reactive rhodium center is reported. The substitution of the bromide in lieu of the chloride in the RhX 3· nH 2O starting material provides the bromo analog in high yield and purity. The basic chemical properties of the bromide complex are very similar to the chloride system [{(bpy) 2Ru(dpp)} 2RhCl 2](PF 6) 5, being an efficient metal-to-ligand charge-transfer (MLCT) light absorber in the visible while possessing a lowest lying metal-to-metal charge-transfer ( 3MMCT) state. In the presence of an electron donor and visible light, [{(bpy) 2Ru(dpp)} 2RhBr 2](PF 6) 5 undergoes excited state intramolecular electron transfer affording photoinitiated electron collection on the reactive rhodium center to generate the Rh I complex, [{(bpy) 2Ru(dpp)} 2Rh I](PF 6) 5. The new complex affords photocatalytic hydrogen production from water with Φ ≈ 0.01, generating higher hydrogen yields than the chloride analog, 10.9 versus 8.2 μmol H 2 over a 4 h photolysis period.