The synthesis and characterization of Ru complexes of the type [L2X2RuCHR] (L=PPh3, PCy3, N‐heterocyclic carbenes or their derivates; X=Cl) grafted on mesoporous SBA‐15 material are described. In this heterogeneous catalyst the Ru complexes are anchored in the pore channels of mesoporous silica material SBA‐15 to prevent the decomposition of the catalytic species. Compared with formal organic‐supported heterogeneous catalysts, this heterogeneous catalyst shows relatively high catalytic activity in olefin metathesis reactions and can be used several times without any decrease in catalytic activities. 相似文献
A ruthenium complex bearing an “anti‐Bredt” N‐heterocyclic carbene was synthesized, characterized and evaluated as a catalyst for olefin metathesis. Good conversions were observed at room temperature for the formation of di‐ and tri‐substituted olefins by ring‐closing metathesis. It also allowed for the ring‐opening metathesis polymerization of cyclooctadiene, as well as for the cross‐metathesis of cis‐1,4‐diacetoxy‐2‐butene with allylbenzene, with enhanced Z/E kinetic selectivity over classical NHC‐based catalysts.
The alkene is a central functional group in organic synthesis. While myriad reliable methods exist to access this moiety from other functionalities, acceptorless dehydrogenation, or the direct synthesis of alkenes from alkanes with hydrogen gas as the sole byproduct, remains a challenging, albeit highly desirable, transformation. This essay provides an account of our recent efforts toward accessing this difficult reaction class, with particular attention paid to the diverse precedents that informed our explorations. This report highlights the benefits of maintaining a broad range of interests, and we hope that it illustrates the vast connectivity between chemical disciplines. 相似文献
A tremendous number of solvents, either as liquids or vapors, contaminate the environment on a daily basis worldwide. Olefin metathesis, which has been widely used as high-yielding protocols for ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), and isomerization reactions, is typically performed in toxic and volatile solvents such as dichloromethane. In this study, the results of our systematic experiments with the Grubbs G1, G2, and Hoveyda-Grubbs HG2 catalysts proved that benzotrifluoride (BTF) can replace dichloromethane (DCM) in these reactions, providing high yields and similar or even higher reaction rates in certain cases. The ROMP of norbornene resulted not only in high yields but also in polynorbornenes with a high molecular weight at low catalyst loadings. Ring-closing metathesis (RCM) experiments proved that, with the exception of the G1 catalyst, RCM occurs with similar high efficiencies in BTF as in DCM. It was found that isomerization of (Z)-but-2-ene-1,4-diyl diacetate with the G2 and HG2 catalysts proceeds at significantly higher initial rates in BTF than in DCM, leading to rapid isomerization with high yields in a short time. Overall, BTF is a suitable solvent for olefin metathesis, such as polymer syntheses by ROMP and the ring-closing and isomerization reactions. 相似文献
A systematic study of a ring‐closing metathesis towards a tetrasubstituted double bond as part of a seven‐membered ring in a 5.7.5‐tricyclic guaianolide system is described. By combining two techniques, namely sparging an inert gas through the solution together with dielectric heating via microwave irradiation a high‐yielding ring‐closing metathesis reaction in this particularly challenging case was achieved. The results obtained compare favorably with conventional heating conditions or with microwave irradiation in a closed system. The key aspects seem to be that rapid microwave irradiation diminishes catalyst decay by allowing the required high reaction temperature to be reached quickly and homogeneously and thereby providing enough energy for a successful metathesis reaction, while inert gas sparging is purging off evolving ethylene to shift the equilibrium to the product. 相似文献
Stereodefined cyclohexene and cyclopentene derivatives were prepared by the coupling of allylic alcohols and other allylic precursors with unsaturated aldehydes. These reactions are based on a multicatalytic one‐pot approach involving palladium pincer complex‐catalyzed boronation, allylation and ring‐closing metathesis reactions. This reaction sequence can be performed in an operationally simple procedure affording the cycloalkene products in high overall yields and excellent regio‐ and stereoselectivities. The presented procedure has a broad synthetic scope and high functional group tolerance, which allows the synthesis of bicyclic lactone and spirane skeletons and various substitution patterns including hydroxy, silyl, vinyl, allyl, and sulfonyl groups. The studied catalytic one‐pot reactions involve up to three individual processes performed by up to four acid‐ and transition metal‐catalyzed events. 相似文献
Iridium‐catalyzed asymmetric etherifications of allylic carbonates with 2‐vinylphenols and 2‐allylphenols were realized. With a catalyst generated from 2 mol% of [Ir(cod)Cl]2 (cod=cycloocta‐1,5‐diene) and 4 mol% of the phosphoramidite ligand L2 , the etherification products were obtained in excellent ees and then subjected to the ring‐closing metathesis reaction providing an efficient synthesis of enantioenriched 2H‐chromene and 2,5‐dihydrobenzo[b]oxepine derivatives. 相似文献
An improved immobilised Grubbs 2nd generation catalyst and its application in flow‐through devices, shown for on‐column reaction gas chromatography (ocRGC), has been studied. The coupling of a reaction capillary and a separation column in GC/MS allows direct reaction monitoring and analysis of conversion as well as reaction kinetics. The presented permanently bonded N‐heterocyclic carbene ligand shows a great stability and activity in ring closing metathesis reactions. A salt‐free approach was used to generate the carbene ligand, which can be directly monitored by mass spectrometry. The very flexible design of the immobilised ligand system in reaction channels and capillaries of flow through systems allows the preparation of various catalysts using a broad variety of metal precursors. This strategy of immobilised catalytically active complexes offers a wide range of on‐column reactions combinable with fast reaction screening by high throughput experimentation.
The second generation Hoveyda–Grubbs metathesis catalyst was successfully immobilized on silica in pellet and powder form following a practical and fast synthesis procedure. The activity of the solid system is truly heterogeneous, efficient in various metathesis reaction types and stable for at least 4000 TON. Ru contamination of the products was very low (ppb levels). The successful use of the robust system has been demonstrated in a continuous reactor set‐up. 相似文献
A convenient and efficient application of heterogeneous poly(4‐vinylpyridine), poly(4‐vinylpyridine N‐oxide), and polystyrene/methylrhenium trioxide systems for the selective oxidation of tocopherols and tocopherol derivatives to the corresponding ortho‐ and para‐tocopherylquinones is described. Environment friendly, easily available, and low‐cost hydrogen peroxide (H2O2) was used as the oxygen atom donor. The antiviral activity of the newly synthesized tocopherylquinones and their parent tocopherols against influenza A virus is also reported. On the basis of the biological assay, the activity of tocopherols against influenza virus is higher than that showed by the corresponding tocopherylquinones, thus suggesting, for the first time, a drawback effect of the oxidative metabolism on the antiviral activity of these compounds. 相似文献
The synthesis of silica‐ and monolith‐supported Grubbs–Herrmann‐type catalysts is described. Two polymerizable, carboxylate‐containing ligands, exo, exo‐7‐oxanorborn‐2‐ene‐5,6‐dicarboxylic anhydride and 7‐oxanorborn‐2‐ene‐5‐carboxylic acid were surface‐immobilized onto silica‐ and ring‐opening metathesis (ROMP‐) derived monolithic supports using “grafting‐from” techniques. The “1st generation Grubbs catalyst”, RuCl2(CHPh)(PCy3)2, was used for these purposes. In addition, a poly(norborn‐2‐ene‐b‐exo, exo‐norborn‐2‐ene‐5,6‐dicarboxylic anhydride)‐coated silica 60 was prepared. The polymer supported anhydride and carboxylate groups were converted into the corresponding mono‐ and disilver salts, respectively, and reacted with the Grubbs–Herrmann catalyst RuCl2(CHPh)(IMesH2)(PCy3) [IMesH2=1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene]. Heterogenization was accomplished by exchange of one chlorine ligand with the polymeric, immobilized silver carboxylates to yield monolith‐supported catalysts 4, 5 , and 6 as well as silica‐supported systems 7, 8 and 9 . The actual composition of these heterogenized catalysts was proven by the synthesis of a homogeneous analogue, RuCl[7‐oxanorbornan‐2‐(COOAg)‐3‐COO](CHPh)(IMesH2)(PCy3) ( 3 ). All homogeneous and heterogeneous catalysts were used in ring‐closing metathesis (RCM) of diethyl diallylmalonate, 1,7‐octadiene, diallyldiphenylsilane, methyl trans‐3‐pentenoate, diallyl ether, N,N‐diallyltrifluoracetamide and t‐butyl N,N‐diallylcarbamate allowing turnover numbers (TON's) close to 1000. In a flow‐through set‐up, an auxiliary effect of pendant silver carboxylates was observed with catalyst 5 , where the silver moiety functions as a (reversible) phosphine scavenger that both accelerates initiation and stabilizes the catalyst by preventing phosphine elution. Detailed catalytic studies were carried out with the monolith‐supported systems 4 and 6 in order to investigate the effects of temperature and chain‐transfer agents (CTA's) such as cis‐1,4‐diacetoxybut‐2‐ene. In all RCM experiments Ru‐leaching was low, resulting in a Ru‐content of the RCM products ≤3.5 μg/g (3.5 ppm). 相似文献
A nanocrystalline magnesium oxide‐stabilized molybdenum(VI) complex catalyzed the oxidation of primary and secondary alcohols to carbonyl compounds in excellent yields using molecular oxygen as stoichiometric oxidant. The nanomaterials with their three‐dimensional structure and defined size and shape act as suitable supports for metal complexes. The catalyst can be reused for four runs without any significant loss of activity. 相似文献
A simple alumina‐supported palladium catalyst prepared by an adsorption method is highly efficient and recyclable in the solvent‐free oxidation of alcohols with molecular oxygen. The adsorption method results in high dispersion of palladium probably as mononuclear or oligonuclear species on alumina surface. These palladium species are transformed to small Pd nanoparticles (ca. 5 nm), which are probably the true active species, during the course of alcohol oxidation. 相似文献
A ferrocene‐tagged ruthenium carbene 15 that can be reversibly immobilized in an ionic liquid (IL) via the controlled oxidation and reduction of a ferrocene tag was prepared. This offers a new strategy which uses redox chemistry to control immobilization and to recycle both the catalyst and the IL. In this experiment, 11 recycles were performed for the ring‐closing metathesis (RCM) of a substrate using 16 as the catalyst in an ionic liquid (IL). More importantly, after the reaction was completed, the ruthenium catalyst was easily separated from the supporting IL by just adding decamethylferrocene (DMFc) to reduce the cationic ferrocene and then extracting it with benzene. Thus, this recycle system offers an easy way to recycle both the ruthenium catalyst and the IL. 相似文献
Protein compatible . Olefin metathesis has emerged as a viable strategy for site‐selective protein modification. This minireview traces its development from early peptide models and metathesis in water to its ultimate application to protein substrates. Prospects in chemistry and biology are also discussed.