Rhodium-catalyzed tandem cyclization: formation of 1H-indenes and 1-alkylideneindans from arylboronate esters in aqueous media

Arylboronate esters bearing a pendant Michael-type acceptor olefin or acetylene linkage undergo transmetalation with a rhodium-based catalytic complex to generate a functionalized organorhodium intermediate which can cyclize onto nonterminal acetylenes in good to excellent yields. The catalytic system involves the use of electron-rich, sterically bulky ligands as tri-tert-butylphosphonium tetrafluoroborate stabilizing the organorhodium intermediates and reduces the incidence of protodeboronation in aqueous media.     

Ketone synthesis by intramolecular acylation of organorhodium(I) with ester

New cyclization reactions forming cyclic ketones were developed wherein an intermediate organorhodium(I) species underwent intramolecular acylation with an ester group. A 2-norbornanone skeleton is constructed in a single operation through successive multiple carbon-carbon bond formation. The reactions ended up with generation of an alkoxyrhodium(I) species to promote the next catalytic cycle.     

Hollow‐Shell‐Structured Nanospheres: A Recoverable Heterogeneous Catalyst for Rhodium‐Catalyzed Tandem Reduction/Lactonization of Ethyl 2‐Acylarylcarboxylates to Chiral Phthalides

Chiral organorhodium‐functionalized hollow‐shell‐structured nanospheres were prepared by immobilization of a chiral N‐sulfonylated diamine‐based organorhodium complex within an ethylene‐bridged organosilicate shell. Structural analysis and characterization reveal its well‐defined single‐site rhodium active center, and transmission electron microscopy images reveal a uniform dispersion of hollow‐shell‐structured nanospheres. As a heterogenous catalyst, it exhibits excellent catalytic activity and enantioselectivity in synthesis of chiral phthalides by a tandem reduction/lactonization of ethyl 2‐acylarylcarboxylates in aqueous medium. The high catalytic performance is attributed to the synergistic effect of the high hydrophobicity and the confined chiral organorhodium catalytic nature. The organorhodium‐functionalized nanospheres could be conveniently recovered and reused at least 10 times without loss of catalytic activity. This feature makes it an attractive catalyst in environmentally friendly organic reactions. The results of this study offer a new approach to immobilize chiral organometal functionalities within the hollow‐shell‐structured nanospheres to prepare materials with high activity in heterogeneous asymmetric catalysis.     

Phenylene‐Coated Magnetic Nanoparticles that Boost Aqueous Asymmetric Transfer Hydrogenation Reactions

Phenylene‐coated organorhodium‐functionalized magnetic nanoparticles are developed through co‐condensation of chiral 4‐(trimethoxysilyl)ethyl)phenylsulfonyl‐1,2‐diphenylethylene‐diamine and 1,4‐bis(triethyoxysilyl)benzene onto Fe₃O₄ followed complexation with [{Cp*RhCl₂}₂]. This magnetic catalyst exhibits excellent catalytic activity and high enantioselectivity in asymmetric transfer hydrogenation in aqueous medium. Such activity is attributed to the high hydrophobicity and the confined nature of the chiral organorhodium catalyst. The magnetic catalyst can be easily recovered by using a small external magnet and it can be reused for at least 10 times without loss of its catalytic activity. This characteristic makes it an attractive catalyst for environmentally friendly organic syntheses.     

A new type of catalytic tandem 1,4-addition-aldol reaction which proceeds through an (oxa-pi-allyl)rhodium intermediate

The reaction of 9-aryl-9-borabicyclo[3.3.1]nonanes (B-Ar-9BBN) with alpha,beta-unsaturated ketones and aldehydes in the presence of 3 mol % [Rh(OMe)(cod)](2) in toluene at 20 degrees C for 2 h gave high yields of the tandem 1,4-addition-aldol reaction products with high syn selectivity. The reaction proceeds through the catalytic cycle consisting of 1,4-addition of an organorhodium species to an alpha,beta-unsaturated ketone and the aldol addition of the resulting (oxa-pi-allyl)rhodium intermediate to an aldehyde.     

Organorhodium‐Functionalized Periodic Mesoporous Organosilica: High Hydrophobicity Promotes Asymmetric Transfer Hydrogenation in Aqueous Medium

Three organosilica‐bridged periodic mesoporous organosilicas were prepared by the immobilization of a chiral N‐sulfonylated diamine‐based organorhodium complex within their silicate network. Structural analysis and characterization confirmed their well‐defined single‐site active rhodium centers, whilst electron microscopy revealed their highly ordered hexagonal mesostructures. Among these three different organosilica‐bridged periodic mesoporous organosilicas, the ethylene‐bridged periodic mesoporous organosilica catalyst exhibited excellent heterogeneous catalytic activity and high enantioselectivity in the aqueous asymmetric transfer hydrogenation of aromatic ketones. This superior catalytic performance was attributed to its salient hydrophobicity, whilst its comparable enantioselectivity relative to the homogeneous catalyst was derived from the confined nature of the chiral organorhodium catalytic sites. Furthermore, this ethylene‐bridged periodic mesoporous organosilica could be conveniently recovered and reused at least 12 times without the loss of its catalytic activity. This feature makes this catalyst attractive for practical organic synthesis in an environmentally friendly manner. This study offers a general way of optimizing the bridged organosilica moiety in periodic mesoporous organosilicas, thereby enhancing its catalytic activity in heterogeneous catalysis.     

Intramolecular nucleophilic addition of an organorhodium(I) to a nitrile

Nucleophilic addition of an organorhodium(I) to a cyano group has been observed for the first time in the rhodium-catalysed reaction of cyano-substituted alkynes with arylboronic acids. The higher reactivity of a cyano group relative to an alkoxycarbonyl group toward an organorhodium(I) species is demonstrated by an intramolecular example.     

Rhodium-catalyzed annulation reactions of 2-cyanophenylboronic acid with alkynes and strained alkenes

[reaction: see text]. A new [3 + 2] annulation reaction was developed in which 2-cyanophenylboronic acid reacted as a three-carbon component with alkynes or alkenes to afford substituted indenones or indanones. The use of an alkynoate even produced benzotropone, a formal [3 + 2 + 2] adduct. The cyclic skeletons were constructed by intramolecular nucleophilic addition of an intermediate organorhodium(I) species to a cyano group.     

A Study of the Stereochemical Course of β‐Oxygen Elimination with a Rhodium(I) Complex

The stereochemical course of β‐oxygen elimination of an organorhodium(I) complex was investigated through the Rh‐catalyzed addition of phenylboronic acid to a chiral propargyl acetate to produce an allene. The degree of chirality transfer suggests that the β‐oxygen elimination takes place in both syn and anti modes.     

Reactivity of the sulfur center in rhodium-bound benzaldehyde thiosemicarbazones towards molecular oxygen. A theoretical investigation

The 4-R-benzaldehyde thiosemicarbazones (L-R) are known to react with [Rh(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) to afford organorhodium complexes (2-R), where the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors with the sulfur atom oxidized by aerial oxygen to sulfone. Two triphenylphosphines and a hydride are also coordinated to the metal center. From the reaction with 4-nitrobenzaldehyde thiosemicarbazone, a second organorhodium complex (1-NO₂) is obtained, in which the sulfur atom is not oxidized. Reaction of the 4-R-benzaldehyde thiosemicarbazones with [Rh(PPh₃)₃Cl] in refluxing ethanol in the absence of NEt₃ affords another group of organorhodium complexes (3-R), in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a chloride. In these 3-R complexes also the sulfur atom is not oxidized. Structures of all the complexes have been optimized by DFT calculations and compared with the already known X-ray crystallographic structures. Also the experimentally observed electronic absorption bands have been assigned to specific transitions based on the TDDFT studies. Molecular electrostatic potential (MESP) topographical analysis performed to find the deepest MESP point on the coordinated sulfur atom (V_min) is used as a probe for assessing the oxidizability of the coordinated sulfur in 1-R and 3-R complexes. Energy differences between the three sets of complexes have been estimated and based on the results obtained, 3-R has been experimentally transformed into 2-R, via formation of 1-R as the intermediate.     

Catalytic Radical Reduction in Aqueous Solution by a Ruthenium Hydride Intermediate

Some manganese complexes can catalyze both antioxidant and pro‐oxidant reactions, whereby the disparate reactivity modes are determined by the catalyst environment and afford distinct therapeutic effects. We recently reported the reduction of radicals in buffered aqueous solution catalyzed by a ruthenium complex with biologically relevant non‐tertiary alcohols as terminal reductants. Mechanistic evidence is presented, indicating that this catalytic radical reduction is achieved by a Ru‐hydride intermediate formed by β‐hydride elimination from a Ru‐alkoxide species. A similar mechanism and Ru‐hydride intermediate was previously reported to kill cancer cells with catalytic pro‐oxidant effects. Therefore, our demonstration of catalytic antioxidant effects by the same type of intermediate reveals new potential therapeutic strategies and applications for catalytic systems that form Ru‐hydride intermediates.     

Direct observation of aldehyde insertion into rhodium-aryl and -alkoxide complexes

Several organorhodium(I) complexes of the general formula (PPh(3))(2)(CO)RhR (R = p-tolyl, o-tolyl, Me) were isolated and were shown to insert aryl aldehydes into the aryl-rhodium(I) bond. Under nonaqueous conditions, these reactions provided ketones in good yield. The stability of the arylrhodium(I) complexes allowed these reactions to be run also in mixtures of THF and water. In this solvent system, diarylmethanols were generated exclusively. Mechanistic studies support the formation of ketone and diarylmethanol by insertion of aldehyde into the rhodium-aryl bond and subsequent beta-hydride elimination or hydrolysis to form diaryl ketone or diarylmethanol products. Kinetic isotope effects and the formation of diarylmethanols in THF/water mixtures are inconsistent with oxidative addition of the acyl carbon-hydrogen bond and reductive elimination to form ketone. Moreover, the intermediate rhodium diarylmethoxide formed from insertion of aldehyde was observed directly during the reaction. Its structure was confirmed by independent synthesis. This complex undergoes beta-hydrogen elimination to form a ketone. This alkoxide also reacts with a second aldehyde to form esters by insertion and subsequent beta-hydrogen elimination. Thus, reactions of arylrhodium complexes with an excess of aldehyde formed esters by a double insertion and beta-hydrogen elimination sequence.     

Rhodium assisted C-H activation of benzaldehyde thiosemicarbazones and their oxidation via activation of molecular oxygen

The benzaldehyde thiosemicarbazones are found to undergo oxidation at the sulfur center upon reaction with [Rh(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3). A group of organorhodium complexes are obtained from such reactions, in which the oxidized thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a hydride. From the reaction with para-nitrobenzaldehyde thiosemicarbazone, a second organometallic complex is obtained, in which the thiosemicarbazone is coordinated to rhodium as a tridentate CNS donor, along with two triphenylphosphines and a hydride. Reaction of the benzaldehyde thiosemicarbazones with [Rh(PPh3)3Cl] in refluxing ethanol in the absence of NEt3 affords another group of organorhodium complexes, in which the thiosemicarbazones are coordinated to rhodium as tridentate CNS donors, along with two triphenylphosphines and a chloride. Structures of representative complexes of each type of complexes have been determined by X-ray crystallography. In all of the complexes, the two PPh3 ligands are trans. All of the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation on the positive side of SCE. Redox responses of the coordinated thiosemicarbazones are also displayed by all of the complexes.     

Reduced graphene oxide as a catalyst for hydrogenation of nitrobenzene at room temperature

Reduced graphene oxide was used as a catalyst for reduction of nitrobenzene at room temperature. High catalytic activity and stability were exhibited in circular experiments. The catalytic procedure was in situ monitored by NMR and N-phenylhydroxylamine was proved to be the intermediate in this catalytic reaction.     

煤催化气化过程中钾的迁移及其对气化反应特性的影响

在固定床反应器中研究了钾在热解和水蒸气气化过程中的变迁,并在TG-DSC上考察了钾系催化剂对煤焦水蒸气气化的催化效果以及随钾化合物形态变化的关系。结果表明,干混法和浸渍法添加碳酸钾对煤焦水蒸气气化的催化效果显著,煤焦的气化反应性随着钾添加量的增加而增大,当催化剂添加到一定量时催化效果陡增,同时神府煤钾的负荷饱和添加量为10%。在煤样热解和气化过程中,钾的化学形态会发生变化,发现并定量了还原态钾中间体的生成。在气化过程中碳酸钾的催化规律和还原态钾中间体的数量之间存在对应关系,当碳转化率为0.2～0.4时,气化速率和还原态钾中间体的数量达到最大值。在700～800℃,钾系催化剂的催化作用和还原态钾中间体的数量之间也存在对应关系,即碳酸钾催化效果较好,氯化钾的催化效果较差,硫酸钾的催化效果随温度的变化明显。     

Can the peroxosuccinate complex in the catalytic cycle of taurine/alpha-ketoglutarate dioxygenase (TauD) act as an alternative oxidant?

Density functional theoretical studies on the catalytic properties of the peroxosuccinate intermediate in the catalytic cycle of taurine/alpha-ketoglutarate dioxygenase suggest that it cannot act as a second oxidant.     

Towards 'bio-based' Nylon: conversion of gamma-valerolactone to methyl pentenoate under catalytic distillation conditions

Methyl pentenoate, a promising Nylon intermediate, is produced in >95% yield via the transesterification of gamma-valerolactone, a bio-based intermediate, under catalytic distillation conditions.     

Local structure of the zeolitic catalytically active site during reaction

The structural changes of the catalytic active site that occur during catalytic reaction in an acidic zeolite are detected. The local structure of the zeolitic Br?nsted active site is a distorted tetrahedrally coordinated aluminum that has three short and one long aluminum-oxygen bond. Using in situ Al K edge X-ray absorption spectroscopy, the adsorption of a reactive intermediate in the oligomerization of ethene changed the local structure of the catalytic active site; the long aluminum oxygen bond is partially relaxed. At increasingly higher temperature, extensive coking of the catalyst frees the Br?nsted acid site from the reactive intermediate, restoring the asymmetric coordination. These measurements show that application of in situ Al K edge spectroscopy provides fundamental insight into the structure of zeolitic catalytically active sites during catalytic action.     

Structure, formation and catalytic studies of a meso-palladioporphyrin intermediate in a Heck reaction

A meso-palladioporphyrin intermediate was isolated from a Heck reaction between an iodoporphyrin and a non-activated olefin using a Pd(PPh3)2Cl2/Et3N system; its structure was characterized by NMR, MS and X-ray crystallography. Studies on its formation indicate that the Pd(II) catalyst was reduced in situ by Et3N with the assistance of water. The catalytic activity of the intermediate was confirmed by stoichiometric and catalytic reactions using a more reactive olefin, ethyl acrylate.     

Establishing the mechanism of Rh-catalysed activation of O2 by H2

Reductive activation of O(2) by H(2) with rhodium terpyridine complexes in H(2)O and CH(3)CN is described and the mechanism is fully elucidated. The rhodium complex extracts electrons from H(2) and reductively activates O(2) to form a peroxo active intermediate. This intermediate is able to oxidise triphenyl phosphine to triphenyl phosphine oxide. A model system constructed in CH(3)CN provides isolable analogues of catalytic intermediates in H(2)O, allowing a detailed look at each step in the catalytic cycle.