Showing papers on "Metathesis published in 2016"
TL;DR: The recent uptake of molecular metathesis catalysts in specialty-chemicals and pharmaceutical manufacturing is reviewed.
Abstract: The recent uptake of molecular metathesis catalysts in specialty-chemicals and pharmaceutical manufacturing is reviewed.
210 citations
TL;DR: A catalytic carbonyl–olefin ring-closing metathesis reaction that uses iron, an Earth-abundant and environmentally benign transition metal, as a catalyst is demonstrated and is distinguished by its operational simplicity, mild reaction conditions, high functional-group tolerance, and amenability to gram-scale synthesis.
Abstract: The olefin metathesis reaction of two unsaturated substrates is one of the most powerful carbon-carbon-bond-forming reactions in organic chemistry. Specifically, the catalytic olefin metathesis reaction has led to profound developments in the synthesis of molecules relevant to the petroleum, materials, agricultural and pharmaceutical industries. These reactions are characterized by their use of discrete metal alkylidene catalysts that operate via a well-established mechanism. While the corresponding carbonyl-olefin metathesis reaction can also be used to construct carbon-carbon bonds, currently available methods are scarce and severely hampered by either harsh reaction conditions or the required use of stoichiometric transition metals as reagents. To date, no general protocol for catalytic carbonyl-olefin metathesis has been reported. Here we demonstrate a catalytic carbonyl-olefin ring-closing metathesis reaction that uses iron, an Earth-abundant and environmentally benign transition metal, as a catalyst. This transformation accommodates a variety of substrates and is distinguished by its operational simplicity, mild reaction conditions, high functional-group tolerance, and amenability to gram-scale synthesis. We anticipate that these characteristics, coupled with the efficiency of this reaction, will allow for further advances in areas that have historically been enhanced by olefin metathesis.
153 citations
TL;DR: It is shown that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituting Z-alkenyl halides.
Abstract: Olefin metathesis has had a large impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is limited. Here we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of a catalyst that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents, and proceed to high conversion at ambient temperature within four hours. We obtain many alkenyl chlorides, bromides and fluorides in up to 91 per cent yield and complete Z selectivity. This method can be used to synthesize biologically active compounds readily and to perform site- and stereoselective fluorination of complex organic molecules.
130 citations
TL;DR: The first enantioselective total synthesis of the cytotoxic natural product (+)-psiguadial B is reported, using a ring-closing metathesis to build the bridging bicyclo- terpene framework.
Abstract: The first enantioselective total synthesis of the cytotoxic natural product (+)-psiguadial B is reported. Key features of the synthesis include (1) the enantioselective preparation of a key cyclobutane intermediate by a tandem Wolff rearrangement/asymmetric ketene addition, (2) a directed C(sp3)–H alkenylation reaction to strategically forge the C1–C2 bond, and (3) a ring-closing metathesis to build the bridging bicyclo[4.3.1]decane terpene framework.
108 citations
01 Jan 2016
TL;DR: The handbook of metathesis is universally compatible with any devices to read and will help you to enjoy a good book with a cup of coffee in the afternoon instead of coping with some malicious bugs inside their laptop.
Abstract: Thank you for reading handbook of metathesis. As you may know, people have look numerous times for their chosen novels like this handbook of metathesis, but end up in harmful downloads. Rather than enjoying a good book with a cup of coffee in the afternoon, instead they cope with some malicious bugs inside their laptop. handbook of metathesis is available in our book collection an online access to it is set as public so you can get it instantly. Our digital library saves in multiple countries, allowing you to get the most less latency time to download any of our books like this one. Merely said, the handbook of metathesis is universally compatible with any devices to read.
90 citations
01 Mar 2016
TL;DR: In this paper, it was shown that Halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstitized Z-alkenyl halides.
Abstract: Olefin metathesis has had a large impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is limited. Here we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of a catalyst that is generated in situ and used with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents, and proceed to high conversion at ambient temperature within four hours. We obtain many alkenyl chlorides, bromides and fluorides in up to 91 per cent yield and complete Z selectivity. This method can be used to synthesize biologically active compounds readily and to perform site- and stereoselective fluorination of complex organic molecules.
88 citations
TL;DR: The combination of strong σ-donating N-heterocyclic carbene ligands with weak σ -donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts is described.
Abstract: Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ-donating N-heterocyclic carbene ligands with weak σ-donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well-defined silica-supported catalysts, [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(+) ][B(Ar(F) )4 (-) ] [IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(Ar(F) )4 =B(3,5-(CF3 )2 C6 H3 )4 ] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.
86 citations
TL;DR: In this article, a dynamic approach towards rigid aryleneethynylene covalent organic polyhedrons (COPs) was explored and the relationship of the COP structures and the geometry of their building blocks was revealed.
Abstract: A dynamic covalent approach towards rigid aryleneethynylene covalent organic polyhedrons (COPs) was explored. Our study on the relationship of the COP structures and the geometry of their building blocks reveals that the topology of aryleneethynylene COPs strongly depends on the size of the building blocks. A tetramer (D2h symmetric), dimer, or interlocked complex can be formed from monomers with the same face-to-edge angle but in different sizes. As alkyne metathesis is a self-exchange reaction and non-directional, the cyclooligomerization of multi-alkyne monomers involves both intramolecular cyclization and intermolecular metathesis reaction, resulting in complicated thermodynamic process disturbed by kinetic competition. Although a tetrahedron-shaped tetramer (Td symmetric) has comparable thermodynamic stability to a D2h symmetric tetramer, its formation is kinetically disfavored and was not observed experimentally. Aryleneethynylene COPs consist of purely unsaturated carbon backbones and exhibit large internal cavities, which would have interesting applications in host–guest chemistry and development of porous materials.
83 citations
TL;DR: The protocol allows access to a range of carbo-/heterocyclic alkenes with good efficiency and excellent trans diastereoselectivity and presents one of the rare examples of catalytic ring-closing carbonyl-olefin metathesis.
Abstract: Exploiting catalytic carbonyl-olefin metathesis is an ongoing challenge in organic synthesis. Reported herein is an FeCl3 -catalyzed ring-closing carbonyl-olefin metathesis. The protocol allows access to a range of carbo-/heterocyclic alkenes with good efficiency and excellent trans diastereoselectivity. The methodology presents one of the rare examples of catalytic ring-closing carbonyl-olefin metathesis. This process is proposed to take place by FeCl3 -catalyzed oxetane formation followed by retro-ring-opening to deliver metathesis products.
78 citations
TL;DR: A cofactor-free decarboxylase with a ruthenium metathesis catalyst is combined to produce high-value antioxidants from bio-based precursors to unify the high selectivity of the former with the versatility of the latter.
Abstract: The combination of enzymes with traditional chemical catalysts unifies the high selectivity of the former with the versatility of the latter. A major challenge of this approach is the difference in the optimal reaction conditions for each catalyst type. In this work, we combined a cofactor-free decarboxylase with a ruthenium metathesis catalyst to produce high-value antioxidants from bio-based precursors. As suitable ruthenium catalysts did not show satisfactory activity under aqueous conditions, the reaction required the use of an organic solvent, which in turn significantly reduced enzyme activity. Upon encapsulation of the decarboxylase in a cryogel, the decarboxylation could be conducted in an organic solvent, and the recovery of the enzyme after the reaction was facilitated. After an intermediate drying step, the subsequent metathesis in pure organic solvent proved to be straightforward. The synthetic utility of the cascade was demonstrated by the synthesis of the antioxidant 4,4′-dihydroxystilbene in an overall yield of 90 %.
75 citations
TL;DR: The efficient and selective ortho-alkylation of N,N-dimethyl anilines via C–H addition to alkenes has been achieved for the first time using a cationic half-sandwich yttrium catalyst.
Abstract: The efficient and selective ortho-alkylation of N,N-dimethyl anilines via C–H addition to alkenes was achieved for the first time using a cationic half-sandwich yttrium catalyst. This protocol constitutes a straightforward and atom-economical route for the synthesis of a new family of tertiary aniline derivatives with branched alkyl substituents, which are otherwise difficult to obtain. DFT calculation studies suggest that the interaction between the yttrium atom and the NMe2 group plays an important role and the intramolecular C–H activation through a σ-bond metathesis pathway is the rate-determining step, which is consistent with the experimental KIE observations.
TL;DR: In this paper, the authors demonstrate that a simple strategy allows successful immobilization of olefin metathesis catalysts inside metal-organic frameworks (MOFs) by simple absorption from solution.
Abstract: Despite their record-breaking sorption capacities, metal–organic frameworks (MOFs) have rarely been used for the immobilization of homogeneous catalysts by simple absorption from solution. Here we demonstrate that this simple strategy allows successful immobilization of olefin metathesis catalysts inside MOFs. Ruthenium alkylidene complexes bearing ammonium-tagged NHC ligands were successfully supported inside (Al)MIL-101-NH2·HCl. The materials thus obtained are true heterogeneous catalysts, active toward various substrates with TONs up to 8900 (in batch conditions) or 4700 (in continuous flow). Although the catalysts were held inside the MOF by noncovalent forces only, leaching was not observed and heavy metal contamination of the products was found to be below the detection limit of ICP MS (0.02 ppm). The robustness of the catalyst attachment allowed their use in a continuous flow setup.
TL;DR: Complex 2 is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly cis-syndiotactic cyclic polynorbornene.
Abstract: The tungsten alkylidyne [tBuOCO]W≡C(tBu) (THF)2 (1) reacts with CO2, leading to complete cleavage of one C═O bond, followed by migratory insertion to generate the tungsten-oxo alkylidene 2. Complex 2 is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly cis-syndiotactic cyclic polynorbornene.
TL;DR: This behavior sends a warning about the use of phosphine-stabilized metathesis catalysts in donor solvents, or with substrates bearing readily accessible donor sites, even for a readily cyclized substrate.
Abstract: Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl2(NHC)(PCy3)(═CHPh) (Ru-2: NHC = H2IMes, a; IMes, b; H2IPr, c; IPr, d; H2ITol, e) and indenylidene complexes RuCl2(NHC)(PCy3)(═C15H10) (NHC = H2IMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl2(NHC)(═CH2) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl2(NHC)(PCy3)(═CH2) Ru-4 as the catalyst resting state. On treatment with a 10-fold excess of pyridine, Ru-4a and Ru-4b decomposed within minutes in solution at RT, eliminating [MePCy3]Cl A by net loss of three ligands (PCy3, methylidene, and one chloride), and a mesityl proton. In comparison, loss of A from Ru-4a in the absence of a donor requires up to 3 days at 55 °C. The σ-alkyl intermediate RuCl2(13CH2PCy3)(NHC) (py)2 resulting from nucleophilic attack of free PCy3 on the methylidene ligand was undetecta...
TL;DR: In this paper, a simple pretreatment of MoO3/SiO2 under an olefin-containing atmosphere at elevated temperatures leads to a 100-1000-fold increase in the low-temperature propylene metathesis activity.
Abstract: Supported metal oxide based olefin metathesis catalysts are widely used in the chemical industry. In comparison to their organometallic catalyst cousins, the oxide catalysts have much lower activity due to the very small fraction of active sites. We report that a simple pretreatment of MoO3/SiO2 and WO3/SiO2 under an olefin-containing atmosphere at elevated temperatures leads to a 100–1000-fold increase in the low-temperature propylene metathesis activity. The performance of these catalysts is comparable with those of the well-defined organometallic catalysts. Unprecedentedly, the catalyst can be easily regenerated by inert gas purging at elevated temperatures. Furthermore, using UV resonance Raman spectroscopy and electron microscopy, we provide strong evidence that the active sites for MoO3/SiO2 are derived from monomeric Mo(═O)2 dioxo species.
TL;DR: Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology.
Abstract: Tungsten alkylidynes [CF3–ONO]W≡CC(CH3)3(THF)2 (1) and [tBuOCO]W≡CC(CH3)3(THF)2 (3) react with ethylene. Complex 1 reacts reversibly with ethylene to give the metallacyclobutene (2). Complex 3 reacts with ethylene to form the tethered alkylidene (4) featuring a tetraanionic pincer ligand. Complexes 1 and 3 initiate the polymerization of norbornene at room temperature. The polymerization of norbornene by 1 is not stereoselective, whereas 3 generates a highly cis and syndiotactic cyclic polynorbornene. Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology.
TL;DR: The first kinetically controlled, highly trans-selective (>98%) olefin cross-metathesis reaction is demonstrated using Ru-based catalysts using new stereoretentive catalysts synthesized for improved reactivity.
TL;DR: This work reviews the use, from 2003 to 2015, of ring-closing alkene metathesis (RCM) for the generation of dihydrofurans or -pyrans in natural product synthesis.
Abstract: In the past two decades, alkene metathesis has risen in prominence to become a significant synthetic strategy for alkene formation. Many total syntheses of natural products have used this transformation. We review the use, from 2003 to 2015, of ring-closing alkene metathesis (RCM) for the generation of dihydrofurans or -pyrans in natural product synthesis. The strategies used to assemble the RCM precursors and the subsequent use of the newly formed unsaturation will also be highlighted and placed in context.
TL;DR: In this article, a soluble ruthenium olefin metathesis catalyst was proposed to convert linear α-olefins to longer internal linear olefins with high selectivity.
Abstract: The search for a low-cost process for the valorization of linear α-olefins combining high productivity and high selectivity is a longstanding goal for chemists. Herein, we report a soluble ruthenium olefin metathesis catalyst that performs the conversion of linear α-olefins to longer internal linear olefins with high selectivity (>99%) under neat conditions at low loadings (50 ppm) and without the need of expensive additives. This robust catalytic process allowed us to efficiently and selectively re-equilibrate the naphtha fraction (C5–C8) of a Fischer–Tropsch feed derived from non-petroleum resources to a higher-value product range (C9–C14), useful as detergent and plasticizer precursors.
TL;DR: Ring-opening metathesis polymerization (ROMP) of various cyclic olefins especially using three (arylimido)vanadium(V)-alkylidene catalysts using dichlorophenylimido analogues were effective for ROMPs of various norbornene derivatives.
Abstract: Ring-opening metathesis polymerization (ROMP) of various cyclic olefins especially using three (arylimido)vanadium(V)-alkylidene catalysts, V(CHSiMe3)(N-2,6-Cl2C6H3) (OC6F5) (PMe3)2 (3), V(CHSiMe3) (NR)[OC(CF3)3](PMe3)2 [R = Ph (6), 2,6-Cl2C6H3 (7)] have been explored. Complex 3 exhibited the highest catalytic activity (ex. TOF = 603 000 h–1, 168 s–1) for ROMP of norbornene (NBE) among a series of (imido)vanadium(V)-alkylidenes, and the cis-specific living ROMPs of NBE proceeded with remarkable activities (TOF = 125 000–157 000 h–1 at 25 °C) by the fluorinated alkoxo analogues (6, 7). The activities by 6, 7 increased at high temperature (50 and 80 °C) and/or upon addition of PMe3 without decrease in the cis selectivity (98%). The ROMPs in the presence of 1-hexene by 3, 7 proceeded without significant decrease in the activities, and the Mn values could be controlled by degree of chain transfer (cross metathesis). These dichlorophenylimido analogues (3, 7) were effective for ROMPs of various norbornene deri...
TL;DR: A low temperature activation process of well-defined metal oxo surface species using organosilicon reductants that generates a large amount of active species at only 70 °C (0.6 active sites/W) broadens the scope of these catalysts to functionalized substrates.
Abstract: Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C...
TL;DR: The cationic aluminum complex can be easily generated from NacNacAlH2 by hydride abstraction and functions as a catalyst for the hydrosilylation of olefins and alkynes as mentioned in this paper.
Abstract: The cationic aluminum complex [NacNacAlH]+ (2; NacNac = CH{C(Me)N(2,6-Pri2C6H3)}2) can be easily generated from NacNacAlH2 by hydride abstraction and functions as a catalyst for the hydrosilylation of olefins and alkynes. Mechanistic studies suggest that, although olefin insertion into the Al–H bond is very facile, the catalysis does not proceed by an insertion/metathesis mechanism but likely by Lewis acid activation. Stoichiometric reactions of 2 with alkynes furnished unexpected products of C≡C addition across the NacNacAl moiety to give tripodal aluminum cations, which are also potent catalysts for the hydrosilylation of alkynes.
TL;DR: In this paper, the authors report that ultrasound irradiation is suitable for generating clean equilibrium mixtures of disulfides within one hour or one day, depending on the sonication source.
TL;DR: Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy towards surface species in heterogeneous alkene metathesis catalysts, which allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles.
Abstract: Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous alkene metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the alkene metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous metathesis catalysts.
TL;DR: The first total synthesis of the alkaloid (-)-haliclonin A is reported, which relied on a novel organocatalytic asymmetric conjugate addition of nitromethane with 3-alkenyl cyclohex-2-enone to set the stereochemistry of the all-carbon quaternary stereogenic center.
Abstract: The first total synthesis of the alkaloid (−)-haliclonin A is reported. The asymmetric synthesis relied on a novel organocatalytic asymmetric conjugate addition of nitromethane with 3-alkenyl cyclohex-2-enone to set the stereochemistry of the all-carbon quaternary stereogenic center. The synthesis also features a Pd-promoted cyclization to form the 3-azabicyclo[3,3,1]nonane core, a SmI2-mediated intermolecular reductive coupling of enone with aldehyde to form the requisite secondary chiral alcohol, ring-closing alkene and alkyne metathesis reactions to build the two aza-macrocyclic ring systems, and an unprecedented direct transformation of enol into enone.
TL;DR: The second generation Grubbs catalyst, RuCl2(H2IMes)(PCy3) (=CHPh) [GII], is shown to decompose during olefin metathesis to generate Ru nanoparticles (RuNPs) as mentioned in this paper.
Abstract: The second-generation Grubbs catalyst, RuCl2(H2IMes)(PCy3) (=CHPh) [GII; H2IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, Cy=cyclohexyl], is shown to decompose during olefin metathesis to generate Ru nanoparticles (RuNPs). These RuNPs appear to contribute significantly to competing isomerization during metathesis. Larger, partially oxidized RuNPs are also observed in commercial GII, but these exhibit modest isomerization activity. Removal of RuNPs from the precatalyst does not prevent isomerization, because new, more reactive NPs are generated by catalyst decomposition during metathesis.
TL;DR: In this article, the effect of various additives on the success of the polymerization was explored in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP.
Abstract: Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.
TL;DR: Accessing highly electron deficient partially alkylated tungsten hydrides on silica via controlled hydrogenolysis of surface organometallic complex via controlledHydrolysis (Si–O–)W(Me)5.5.
Abstract: The well-defined single-site silica-supported tungsten complex [( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Si-O-)W(Me)5], 1, is an excellent precatalyst for alkane metathesis. The unique structure of 1 allows the synthesis of unprecedented tungsten hydrido methyl surface complexes via a controlled hydrogenolysis. Specifically, in the presence of molecular hydrogen, 1 is quickly transformed at -78 °C into a partially alkylated tungsten hydride, 4, as characterized by 1H solid-state NMR and IR spectroscopies. Species 4, upon warming to 150 °C, displays the highest catalytic activity for propane metathesis yet reported. DFT calculations using model systems support the formation of [( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Si-O-)WH3(Me)2], as the predominant species at -78 °C following several elementary steps of hydrogen addition (by σ-bond metathesis or α-hydrogen transfer). Rearrangement of 4 occuring between -78 °C and room temperature leads to the formation of an unique methylidene tungsten hydride [( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Si-O-)WH3( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CH2)], as determined by solid-state 1H and 13C NMR spectroscopies and supported by DFT. Thus for the first time, a coordination sphere that incorporates both carbene and hydride functionalities has been observed.
TL;DR: In this article, the parent bis-alkyl oxo derivative (SiO)2WOR2] prone to carbene formation was targeted, and grafting of [WO(CH2EMe3)3Cl] (E = C, 1-Np; E = Si, 1Ns) onto silica dehydroxylated at 200 °C was performed.
Abstract: In order to access realistic models to the industrial olefin metathesis catalyst WO3/SiO2, which is the bigrafted tungsten oxo alkylidene species [(≡SiO)2WO(═CHR)], we targeted the parent bis-alkyl oxo derivative [(≡SiO)2WOR2] prone to carbene formation. Thus, grafting of [WO(CH2EMe3)3Cl] (E = C, 1-Np; E = Si, 1-Ns) onto silica dehydroxylated at 200 °C was performed. While 1-Np affords the monopodal species [(≡SiO)WO(CH2CMe3)3] (2-Np), the neosilyl derivative 1-Ns reacts to yield the well-defined bipodal species [(≡SiO)2WO(CH2SiMe3)2] (2-Ns), via consecutive HCl and SiMe4 release. This was demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C, 29Si, and 17O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. The material 2-Ns proved to lead to the most stable and efficient supported tungsten oxo catalyst for propene metathesis under dynamic conditions at 80 °C.
TL;DR: Successful syntheses of nanohoop targets containing benzo[k]tetraphene and dibenzo[c,m]pentaphene moieties not only provide proof of principle that aromatic belts can be derived by this new strategy but also represent some of the largest aromatic belt fragments reported to date.
Abstract: The construction of all sp2-hybridized molecular belts has been an ongoing challenge in the chemistry community for decades. Despite numerous attempts, these double-stranded macrocycles remain outstanding synthetic challenges. Prior approaches have relied on late-state oxidations and/or acid-catalyzed processes that have been incapable of accessing the envisaged targets. Herein, we describe the development of an iterative reductive aromatization/ring-closing metathesis approach. Successful syntheses of nanohoop targets containing benzo[k]tetraphene and dibenzo[c,m]pentaphene moieties not only provide proof of principle that aromatic belts can be derived by this new strategy but also represent some of the largest aromatic belt fragments reported to date.