Showing papers on "Metathesis published in 2021"
TL;DR: In this paper, the first azolate Pd-MOF, BUT-33(Pd), obtained via a deuterated solvent-assisted metal metathesis, retains the underlying sodalite network and mesoporosity of the template BUT-31(Ni) and shows excellent chemical stability (resistance to an 8 M NaOH aqueous solution).
Abstract: Constructing stable palladium(II)-based metal-organic frameworks (MOFs) would unlock more opportunities for MOF chemistry, particularly toward applications in catalysis. However, their availability is limited by synthetic challenges due to the inertness of the Pd-ligand coordination bond, as well as the strong tendency of the Pd(II) source to be reduced under typical solvothermal conditions. Under the guidance of reticular chemistry, herein, we present the first example of an azolate Pd-MOF, BUT-33(Pd), obtained via a deuterated solvent-assisted metal metathesis. BUT-33(Pd) retains the underlying sodalite network and mesoporosity of the template BUT-33(Ni) and shows excellent chemical stability (resistance to an 8 M NaOH aqueous solution). With rich Pd(II) sites in the atomically precise distribution, it also demonstrates good performances as a heterogeneous Pd(II) catalyst in a wide application scope, including Suzuki/Heck coupling reactions and photocatalytic CO2 reduction to CH4. This work highlights a feasible approach to reticularly construct noble metal based MOFs via metal metathesis, in which various merits, including high chemical stability, large pores, and tunable functions, have been integrated for addressing challenging tasks.
48 citations
TL;DR: In this paper, a method to synthesize degradable polymers by copolymerizing 2,3-dihydrofuran with NBEs is presented.
Abstract: Norbornene derivatives (NBEs) are common monomers for living ring-opening metathesis polymerization and yield polymers with low dispersities and diverse functionalities. However, the all-carbon backbone of poly-NBEs is non-degradable. Here we report a method to synthesize degradable polymers by copolymerizing 2,3-dihydrofuran with NBEs. 2,3-Dihydrofuran rapidly reacts with Grubbs catalyst to form a thermodynamically stable Ru Fischer carbene-the only detectable active Ru species during copolymerization-and the addition of NBEs becomes rate determining. This reactivity attenuates the NBE homoaddition and allows uniform incorporation of acid-degradable enol ether linkages throughout the copolymers, which enables complete polymer degradation while maintaining the favourable characteristics of living ring-opening metathesis polymerization. Copolymerization of 2,3-dihydrofuran with NBEs gives low dispersity polymers with tunable solubility, glass transition temperature and mechanical properties. These polymers can be fully degraded into small molecule or oligomeric species under mildly acidic conditions. This method can be readily adapted to traditional ring-opening metathesis polymerization of widely used NBEs to synthesize easily degradable polymers with tunable properties for various applications and for environmental sustainability.
41 citations
TL;DR: A comprehensive review of carbonyl-olefin metathesis reactions with stepwise, stoichiometric, or catalytic approaches can be found in this paper, where a comprehensive overview of currently available methods is provided starting with Paterno-Buchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid -cat
Abstract: This Review describes the development of strategies for carbonyl-olefin metathesis reactions relying on stepwise, stoichiometric, or catalytic approaches. A comprehensive overview of currently available methods is provided starting with Paterno-Buchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid-catalyzed strategies relying on the formation of intermediate oxetanes, and protocols based on initial carbon-carbon bond formation between carbonyls and alkenes and subsequent Grob-fragmentations. The Review concludes with an overview of applications of these currently available methods for carbonyl-olefin metathesis in complex molecule synthesis. Over the past eight years, the field of carbonyl-olefin metathesis has grown significantly and expanded from stoichiometric reaction protocols to efficient catalytic strategies for ring-closing, ring-opening, and cross carbonyl-olefin metathesis. The aim of this Review is to capture the status quo of the field and is expected to contribute to further advancements in carbonyl-olefin metathesis in the coming years.
41 citations
TL;DR: In this paper, the authors describe a functional group metathesis between aryl nitriles and aryyl thioethers, where the catalytic system nickel/dcype is essential to achieve this fully reversible transformation in good to excellent yields.
Abstract: We describe a new functional group metathesis between aryl nitriles and aryl thioethers. The catalytic system nickel/dcype is essential to achieve this fully reversible transformation in good to excellent yields. Furthermore, the cyanide- and thiol-free reaction shows high functional group tolerance and great efficiency for the late-stage derivatization of commercial molecules. Finally, synthetic applications demonstrate its versatility and utility in multistep synthesis.
38 citations
TL;DR: This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of the last comprehensive reviews in this field.
Abstract: Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5–7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure–activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.
38 citations
TL;DR: In the Review, the origin of the EWG‐activation concept and selected applications of the resulting catalysts in target‐oriented synthesis, medicinal chemistry, as well as in the preparation of fine‐chemicals and in materials chemistry is discussed.
Abstract: Advanced applications of the Nobel Prize winning olefin metathesis reaction require user-friendly and highly universal catalysts. From many successful metathesis catalysts, which belong to the two distinct classes of Schrock and Grubbs-type catalysts, the subclass of chelating-benzylidene ruthenium complexes (so-called Hoveyda-Grubbs catalysts) additionally activated by electron-withdrawing groups (EWGs) provides a highly tunable platform. In the Review, the origin of the EWG-activation concept and selected applications of the resulting catalysts in target-oriented synthesis, medicinal chemistry, as well as in the preparation of fine-chemicals and in materials chemistry is discussed. Based on the examples, some suggestions for end-users regarding minimization of catalyst loading, selectivity control, and general optimization of the olefin metathesis reaction are provided.
36 citations
TL;DR: In this paper, a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox Bi(I), was reported, where a main group center performs three elementary organometallic steps in a low-valent redox manifold.
Abstract: Herein, we report a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox-Bi(I). Mechanistic studies on the elementary steps support a Bi(I)/Bi(III) redox cycle that comprises C(sp2)-F oxidative addition, F/H ligand metathesis, and C(sp2)-H reductive elimination. Isolation and characterization of a cationic Phebox-Bi(III)(4-tetrafluoropyridyl) triflate manifests the feasible oxidative addition of Phebox-Bi(I) into the C(sp2)-F bond. Spectroscopic evidence was provided for the formation of a transient Phebox-Bi(III)(4-tetrafluoropyridyl) hydride during catalysis, which decomposes at low temperature to afford the corresponding C(sp2)-H bond while regenerating the propagating Phebox-Bi(I). This protocol represents a distinct catalytic example where a main-group center performs three elementary organometallic steps in a low-valent redox manifold.
34 citations
TL;DR: In this article, the main olefin metathesis polymerization strategies that have been used to access degradable polymers, including acyclic diene, entropy-driven and enthalpy-driven ring-opening polymerization, as well as (iv) cascade enyne metathetic polymerization.
32 citations
TL;DR: In this paper, the first instance of the incorporation of acylsilanes into a polymer backbone was reported, and the degradation process supported the intermediacy of an α-siloxy carbene, formed via a 1,2-photo Brook rearrangement, which underwent insertion into water followed by cleavage of the resulting hemiacetal.
Abstract: Materials capable of degradation upon exposure to light hold promise in a diverse range of applications including biomedical devices and smart coatings. Despite the rapid access to macromolecules with diverse compositions and architectures enabled by ring-opening metathesis polymerization (ROMP), a general strategy to introduce facile photodegradability into these polymers is lacking. Here, we report copolymers synthesized via ROMP that can be degraded by cleaving the backbone in both solution and solid states under irradiation with a 52 W, 390 nm Kessil LED to generate heterotelechelic low-molecular-weight fragments. To the best of our knowledge, this work represents the first instance of the incorporation of acylsilanes into a polymer backbone. Mechanistic investigation of the degradation process supports the intermediacy of an α-siloxy carbene, formed via a 1,2-photo Brook rearrangement, which undergoes insertion into water followed by cleavage of the resulting hemiacetal.
26 citations
TL;DR: In this article, the synthesis of long-chain aliphatic polyesters by polycondensation of 1,ω-dicarboxylic acids and the diols, de-decreasing of the dics, and acyclic diene metathesis (ADMET) polymerization of nonconjugated 1-diene (and the tandem ADMET polymerization and hydrogenation system for synthesis of saturated polyester) have been reviewed.
Abstract: Recent development in efficient synthesis of (unsaturated) 1,ω-fatty acid diesters (and the diols), which can be used as monomers for synthesis of long chain aliphatic polyesters, especially by Pd-catalyzed isomerization alkoxycarbonylation, Ru-catalyzed olefin metathesis (self-metathesis and cross metathesis), and the tandem olefin metathesis and the ester hydrogenation from plant oils (unsaturated fatty acid esters such as methyl oleate, methyl linoleate, etc.) have been summarized. Recent development for synthesis of (semicrystalline) long chain aliphatic polyesters by polycondensation of 1,ω-dicarboxylic acids and the diols, dehydrogenation polycondensation of the diols or carbonylation polycondensation of the ω-alken-1-ol, and acyclic diene metathesis (ADMET) polymerization of nonconjugated 1,ω-diene (and the tandem ADMET polymerization and hydrogenation system for synthesis of saturated polyester) have also been reviewed. Depolymerization of these long chain aliphatic polyesters was also demonstrated (by olefin metathesis, treatment with alcohols, etc.). Catalyst development for these reactions (more active, under mild conditions) plays a key role in efficient synthesis.
26 citations
TL;DR: In this paper, an effective strategy was described to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type Ruthenium alkylidene catalysts.
Abstract: Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.
TL;DR: Two-coordinate homoleptic iron complex Fe(HMTO) 2 capable of performing ring-opening metathesis polymerization of norbornene to produce highly stereoregular polynorbornene (99% cis, syndiotactic) is presented.
Abstract: Developing well-defined iron-based catalysts for olefin metathesis would be a breakthrough achievement in the field not only to replace existing catalysts by inexpensive metals but also to attain a new reactivity taking advantage of the unique electronic structure of the base metals. Here, we present a two-coordinate homoleptic iron complex, Fe(HMTO)2 [HMTO=O-2,6-(2,4,6-Me3 C6 H2 )2 C6 H3 ], that is capable of performing ring-opening metathesis polymerization of norbornene to produce highly stereoregular polynorbornene (99 % cis, syndiotactic). The use of heteroleptic Fe(HMTO)(RO) [RO=(CH3 )2 CF3 CO, CH3 (CF3 )2 CO, or Ph(CF3 )2 CO] prepared in situ significantly increases the polymerization rate while preserving selectivity. The resulting polymers were characterized by 1 H and 13 C NMR spectroscopy and gel-permeation chromatography.
TL;DR: In this paper, the Grela catalyst RuCl2(H2IMes) was shown to be water-tolerant and showed that even low concentrations of water cause catalyst decomposition, severely degrading yields.
Abstract: Ruthenium catalysts for olefin metathesis are widely viewed as water-tolerant. Evidence is presented, however, that even low concentrations of water cause catalyst decomposition, severely degrading yields. Of 11 catalysts studied, fast-initiating examples (e.g., the Grela catalyst RuCl2(H2IMes)(=CHC6H4-2-O i Pr-5-NO2) were most affected. Maximum water tolerance was exhibited by slowly initiating iodide and cyclic (alkyl)(amino)carbene (CAAC) derivatives. Computational investigations indicated that hydrogen bonding of water to substrate can also play a role, by retarding cyclization relative to decomposition. These results have important implications for olefin metathesis in organic media, where water is a ubiquitous contaminant, and for aqueous metathesis, which currently requires superstoichiometric "catalyst" for demanding reactions.
TL;DR: Perutz and Sabo-Etienne as discussed by the authors defined a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product.
Abstract: In 2007 two of us defined the σ-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew. Chem. Int. Ed. 2007 , 46 , 2578-2592), i.e. the σ-CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal-element bonds through partner-interchange of σ-bond complexes. The key concept that defines a σ-CAM process is a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ-bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ-CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional s-bond ligands, agostic complexes, sp 2 -carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal-element bonds.
TL;DR: In this paper, an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand has been described.
Abstract: This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand. Treating the tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 (1) with isocyanates (RNCO; R = tBu, Cy, and Ph) leads to cycloaddition occurring exclusively at the C═N bond to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery of [(O2CtBuC═)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(tBuOCO)W(≡NR)(tBuCCO)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = tBu (11-tBu) and is isolated and characterized by single-crystal X-ray diffraction. Finally, adding to the short list of catalysts capable of ring expansion metathesis polymerization (REMP), complexes 6-NR and 11-tBu are active for the stereoselective synthesis of cyclic polynorbornene.
TL;DR: The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N–H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.
Abstract: Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon in their ability to operate via ‘non-innocent’ bond activation processes. Herein we describe our initial results towards the development of scalable acyclic chelating germylene ligands (viz.1a/b), and their utilization in the stabilization of Ni0 complexes (viz.4a/b), which can readily and reversibly undergo metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N–H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.
TL;DR: For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing ADMET oligomerization.
Abstract: For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido ...
TL;DR: In this article, a transition-metal-free metathesis of purpose-built unsymmetrically substituted digermenes is reported. But the use of tethered auxiliary donors (dimethylaniline groups) in substituents B ensures intramolecular donor-acceptor stabilization of the transient germylene fragments, the intermediacy of which is proven by trapping experiments.
Abstract: The metathesis of carbon–carbon double bonds—the ‘reshuffling’ of their constituting carbene fragments—is a tremendously important preparative tool in industry and academia. Metathesis of heavier alkene homologues is restricted to occasional unproductive examples in phosphorus chemistry and cross-metathesis to mixed heavier alkynes. We now report the thermally induced, transition-metal-free metathesis of purpose-built unsymmetrically substituted digermenes. The A2Ge=GeAB starting materials are thus converted to symmetrically substituted derivatives of the A2Ge=GeA2 and ABGe=GeAB types. The use of tethered auxiliary donors (dimethylaniline groups) in substituents B ensures intramolecular donor–acceptor stabilization of the transient germylene fragments, the intermediacy of which is proven by trapping experiments. Density functional theory calculations shed light on the thermodynamic driving force of the metathesis and validate the crucial role of the tethered donor. With an analogously equipped bridged tetragermadiene precursor (A2Ge=GeB-X-BGe=GeA2), heavier acyclic diene metathesis polymerization occurs, in analogy to the widespread acyclic diene metathesis (ADMET) polymerization in the carbon case, yielding a polydigermene. The metathesis of carbon–carbon double bonds is an important tool in organic synthesis and now a similar reshuffling has been carried out with heavier alkene analogues featuring unsymmetrically substituted Ge=Ge double bonds. This reaction enables the synthesis of symmetric molecular digermenes as well as a polymer based on Ge=Ge repeat units.
TL;DR: In this paper, the authors summarized the historical development of latent sulfur-chelated ruthenium precatalysts from the Lemcoff group's perspective, showing that they appear in the more stable cis-dichloro configuration, which is latent towards olefin metathesis reactions.
Abstract: This Account summarizes the historical development of latent sulfur-chelated ruthenium precatalysts from the Lemcoff group’s perspective. The most unique feature of this family of complexes is that they appear in the more stable cis-dichloro configuration, which is latent towards olefin metathesis reactions. Activation of the precatalyst, brought about by isomerization from the cis-dihalo to the trans-dihalo forms, can be achieved either by thermal or light stimuli. Modifications of the ligand sphere bestows unique properties upon the catalysts, which have been used in diverse applications, from 3D printing of metathesis polymers to orthogonally divergent synthetic pathways. 1 Introduction 2 Effect of Sulfur Substituents 3 Effect of Benzylidene Ligands 4 Effect of the NHC Ligands 5 Effect of the Anionic Ligands 6 Conclusions
TL;DR: In this article, the authors describe a distinct class of ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbene (uNHC) ligands, from its historical beginning to the present state of the art.
Abstract: This review describes a distinct class of ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbene (uNHC) ligands, from its historical beginning to the present state of the art. Thanks to advantageous traits, such as pronounced thermodynamic stability, chemical latency, outstanding selectivity, and compatibility with green solvents, these catalysts led to good results in a number of specialized metathesis transformations. Therefore, while being a niche, the uNHC complexes can potentially be implemented in a number of industrial processes, such as valorization of Fischer-Tropsch olefin fractions, ethenolysis of renewable products, and modern pharmaceutical production.
TL;DR: In this paper, the authors introduced the application of designed Hoveyda-Grubbs-type complexes for bio-relevant studies, including the construction of hybrid olefin metathesis biocatalysts and the development of in-vivo OE reactions.
Abstract: Hoveyda–Grubbs-type complexes, ruthenium catalysts for olefin metathesis, have gained increased interest as a research target in the interdisciplinary research fields of chemistry and biology because of their high functional group selectivity in olefin metathesis reactions and stabilities in aqueous media. This review article introduces the application of designed Hoveyda–Grubbs-type complexes for bio-relevant studies including the construction of hybrid olefin metathesis biocatalysts and the development of in-vivo olefin metathesis reactions. As a noticeable issue in the employment of Hoveyda–Grubbs-type complexes in aqueous media, the influence of water on the catalytic activities of the complexes and strategies to overcome the problems resulting from the water effects are also discussed. In connection to the structural effects of protein structures on the reactivities of Hoveyda–Grubbs-type complexes included in the protein, the regulation of metathesis activities through second-coordination sphere effect is presented, demonstrating that the reactivities of Hoveyda–Grubbs-type complexes are controllable by the structural modification of the complexes at outer-sphere parts. Finally, as a new-type reaction based on the ruthenium-olefin specific interaction, a recent finding on the ruthenium complex transfer reaction between Hoveyda–Grubbs-type complexes and biomolecules is introduced.
TL;DR: The first total synthesis of samroiyotmycin A (1), a C2-symmetric 20-membered anti-malarial macrodiolide isolated from Streptomyces sp. as discussed by the authors, was achieved using the latest generation of molybdenum alkylidynes endowed with a tripodal silanolate ligand framework.
Abstract: We report the first total synthesis of samroiyotmycin A (1), a C2-symmetric 20-membered anti-malarial macrodiolide isolated from Streptomyces sp. The convergent synthetic strategy orchestrates bisalkyne fragment-assembly using an unprecedented Schӧllkopf-type condensation on a substituted β-lactone and an ambitious late-stage one-pot alkyne cross metathesis - ring-closing metathesis (ACM-RCAM) reaction. The demanding alkyne metathesis sequence is achieved using the latest generation of molybdenum alkylidynes endowed with a tripodal silanolate ligand framework. Subsequent conversion to the required E-alkenes uses contemporary hydrometallation chemistry catalysed by tetrameric cluster [{Cp*RuCl}4].
TL;DR: The first asymmetric total synthesis of (+)-mannolide C has been accomplished by lipase-mediated resolution, Ru-complex-catalyzed double ring-closing metathesis (RCM) reactions, Ni(II)-calyzed diastereoselective Michael addition, and Mn(III)-catalyze allylic oxidation as the key transformations as mentioned in this paper.
Abstract: (+)-Mannolide C is a complex hexacyclic C 20 cephalotane-type diterpenoid featuring a highly strained 7/6/6/5 tetracyclic core embedded with eight consecutive stereocenters and two bridging lactones. The first asymmetric total synthesis of (+)-mannolide C has been accomplished by lipase-mediated resolution, Ru-complex-catalyzed double ring-closing metathesis (RCM) reactions, Ni(II)-catalyzed diastereoselective Michael addition, and Mn(III)-catalyzed allylic oxidation as the key transformations.
TL;DR: In this article, the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis, is summarized.
Abstract: Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known.
TL;DR: A series of supported ReOx catalysts were synthesized by incipient-wetness impregnation of perrhenic acid onto one component (Al2O3 and SiO2) and surface modified mixed-oxide supports (SiO2/Al 2O3, the authors ).
Abstract: A series of supported ReOx catalysts were synthesized by incipient-wetness impregnation of perrhenic acid onto one component (Al2O3 and SiO2) and surface-modified mixed-oxide supports (SiO2/Al2O3,
TL;DR: A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF3 -I, was synthesized and characterized in this article.
Abstract: A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF3 -I, was synthesized and characterized. This latent precatalyst provides a distinct activity and selectivity profiles for olefin metathesis reactions depending on the substrate. For example, 1,3-divinyl-hexahydropentalene derivatives were efficiently obtained by ring-opening metathesis (ROM) of dicyclopentadiene (DCPD). Ru-S-CF3 -I also presented a much more effective photoisomerization process from the inactive cis-diiodo to the active trans-diiodo configuration after exposure to 510 nm (green light), allowing for a wide scope of photoinduced olefin metathesis reactions. DFT calculations suggest a faster formation and enhanced stability of the active trans-diiodo species of Ru-S-CF3 -I compared with Ru-S-Ph-I, explaining its higher reactivity. In addition, the photochemical release of chloride anions by irradiation of Cl-BODIPY in the presence of DCPD derivatives with diiodo Ru benzylidenes, led to in situ generation of chloride complexes, which quickly produced the corresponding cross-linked polymers. Thus, novel selective pathways that use visible light to guide olefin metathesis based synthetic sequences is presented.
TL;DR: In this article, it was shown that triflic acid, TfOH, instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive.
Abstract: Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Bronsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Bronsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive. TfOH's conjugate base is also found to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction. The mechanism explains available experimental information, including the skipped diene species that appear transiently before product formation. The present study clarifies the mechanism for activation of intramolecular carbonyl-olefin substrates by Bronsted acids and provides important insights that will help develop this exciting class of catalysts.