The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Adam S Veige - One of the best experts on this subject based on the ideXlab platform.
-
tethered tungsten Alkylidenes for the synthesis of cyclic polynorbornene via ring expansion metathesis unprecedented stereoselectivity and trapping of key catalytic intermediates
Journal of the American Chemical Society, 2021Co-Authors: Vineet K Jakhar, Digvijayee Pal, Ion Ghiviriga, Khalil A Abboud, Daniel Lester, Brent S Sumerlin, Adam S VeigeAbstract: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.
-
highly tactic cyclic polynorbornene stereoselective ring expansion metathesis polymerization of norbornene catalyzed by a new tethered tungsten Alkylidene catalyst
Journal of the American Chemical Society, 2016Co-Authors: Stella A. Gonsales, Khalil A Abboud, Brent S Sumerlin, Tomohiro Kubo, Madison K. Flint, Adam S VeigeAbstract: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.
-
Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst
2016Co-Authors: Stella A. Gonsales, Khalil A Abboud, Brent S Sumerlin, Tomohiro Kubo, Madison K. Flint, Adam S VeigeAbstract:The tungsten alkylidyne [tBuOCO]WC(tBu) (THF)2 (1) reacts with CO2, leading to complete cleavage of one CO 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
Richard R. Schrock - One of the best experts on this subject based on the ideXlab platform.
-
syntheses of phosphine free molybdenumoxo Alkylidene complexes through addition of water to alkylidyne complexes
Organometallics, 2020Co-Authors: Feng Zhai, Richard R. Schrock, Amir H. Hoveyda, Peter MullerAbstract:Addition of 1 equiv of water to Mo(CArp)(ORF9)3 (Arp = p-methoxyphenyl; ORF9 = OC(CF3)3) in the presence of 5% NEt3 (vs Mo) in THF led to the formation of Mo(O)(CHArp)(ORF9)2(THF)2 in good yield. Mo(O)(CHArp)(ORF9)2(THF)2 reacts with 2 equiv of LiOHMT (OHMT = O-2,6-mesityl2C6H3) at room temperature to yield Mo(O)(CHArp)(OHMT)2 and with 2 equiv of NaOTPP (OTPP = 2,3,5,6-tetraphenylphenoxide) to yield Mo(O)(CHArp)(OTPP)2. In the presence of TMEDA (2.5 equiv), Mo(CR)(ORF9)3 (R = Arp, mesityl, or t-Bu) reacts with 1 equiv of water to yield Mo(O)(CHR)(ORF9)2(TMEDA) complexes, from which (when R = t-Bu) TMEDA is readily displaced by 2,2′-bipyridyl to give Mo(O)(CH-t-Bu)(ORF9)2(bipy). Mo(O)(CH-t-Bu)(ORF9)2(bipy) was converted into Mo(O)(CH-t-Bu)Cl2(bipy) readily, from which Mo(O)(CH-t-Bu)Cl(OHMT)(3-Brpy) (3-Brpy = 3-bromopyridine) and Mo(O)(CH-t-Bu)Cl(OHIPT)(3-Brpy) (OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3) were prepared. X-ray studies were carried out on Mo(O)(CHArp)(ORF9)2(THF)2, Mo(O)(CH-t-Bu)(ORF9)2(TMEDA), Mo(O)(CHArp)(OTPP)2, Mo(O)(CH-t-Bu)Cl(OHMT)(3-Brpy), and Mo(O)(CH-t-Bu)Cl(OHIPT)(3-Brpy).
-
syntheses of molybdenum oxo Alkylidene complexes through addition of water to an alkylidyne complex
Journal of the American Chemical Society, 2018Co-Authors: Konstantin V. Bukhryakov, Richard R. Schrock, Amir H. Hoveyda, Charlene Tsay, Peter MullerAbstract:Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(μ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the Alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,sy...
-
Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex
2018Co-Authors: Konstantin V. Bukhryakov, Richard R. Schrock, Amir H. Hoveyda, Charlene Tsay, Peter MüllerAbstract:Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(μ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the Alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X-ray structures were obtained for 3(PPhMe2), 4(dme), and 6
-
recent advances in the syntheses and applications of molybdenum and tungsten Alkylidene and alkylidyne catalysts for the metathesis of alkenes and alkynes
ChemInform, 2007Co-Authors: Richard R. Schrock, Constantin CzekeliusAbstract:The last several years have produced some key advances in the area of alkene and alkyne metathesis by high oxidation state Alkylidene and alkylidyne complexes along with new applications in organic and polymer chemistry. In this review we cover some of these developments and applications. The first part of this review concerns developments in catalyst synthesis and new catalysts. The second part concerns notable applications in organic and polymer chemistry. We discuss only high oxidation state Alkylidene and alkylidyne chemistry of relevance to alkene or alkyne metathesis reactions and favor studies in the homogeneous phase.
-
High oxidation state Alkylidene and alkylidyne complexes
Chemical Communications, 2005Co-Authors: Richard R. SchrockAbstract:Electron deficient high oxidation state early transition metal complexes that contain metal–carbon double or triple bonds and bulky supporting ligands have been found to be highly reactive catalysts for the alkene and alkyne metathesis reactions, respectively.
Odile Eisenstein - One of the best experts on this subject based on the ideXlab platform.
-
Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds
Accounts of Chemical Research, 2019Co-Authors: Christopher P. Gordon, Christophe Copéret, Christophe Raynaud, Richard A. Andersen, Odile EisensteinAbstract:Metal-bonded carbon atoms in metal–alkyl, metal–carbene/Alkylidene, and metal–carbyne/alkylidyne species often show significantly more deshielded isotropic chemical shifts than their organic counterparts (alkanes, alkenes, and alkynes). While isotropic chemical shift is universally used to characterize a chemical compound in solution, it is an average value of the three principal components of the chemical shift tensor (δ11 > δ22 > δ33). The tensor components, which are accessible by solid-state NMR spectroscopy, can provide detailed information about the electronic structure (frontier molecular orbitals) at the observed nuclei. This information can be accessed in detail by quantum chemical calculations, most notably by an analysis of the paramagnetic contribution to the NMR shielding tensor. The paramagnetic term mainly results from the coupling of occupied and empty molecular orbitals close in energy—the frontier molecular orbitals—under the effect of the external magnetic field (B0). In organometallic compounds, a large deshielding of the isotropic carbon-13 chemical shift of the metal-bonded carbon atom is commonly related to the coupling between the occupied σM–C orbital and low-lying vacant orbitals of πM═C* character. The deshielding at the α-carbon hence probes the extent of σM–C and πM═C* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Ω = δ11 – δ33) observed in metal Alkylidenes and alkylidynes (200 < δiso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs Alkylidenes due to their low-lying πM═C* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the α-carbon of metal–alkyls display deshielded chemical shifts in specific coordination environments. This deshielding, which is often prominently pronounced for cationic species, indicates the presence of partial π-bond character in the metal–carbon bond, making these bonds topologically equivalent to Alkylidene π-bonds. The π-character in metal–alkyl bonds favors (i) α-H abstraction processes in metal bis-alkyl compounds yielding metal Alkylidenes, (ii) [2 + 2]-retrocyclization of metallacyclobutanes that participate in olefin metathesis, (iii) olefin insertion in cationic metal alkyls thus explaining polymerization activity trends and the importance of α-H agostic interactions, and (iv) C–H bond activation on metal–alkyls via σ-bond metathesis. The presence of π-character in the metal–carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal–alkyls toward olefin insertion and σ-bond metathesis and the fact that σ-bond metathesis, olefin insertion, and olefin metathesis are commonly observed with metal atoms in the same ligand field. Because of the similarities in the frontier molecular orbitals involved in these processes, these reactions can be viewed as isolobal. This explains why certain fragments, such as bent metallocenes (d0 Cp2M) or T-shaped L3M, are ubiquitous in these reactions.
-
Metal alkyls programmed to generate metal Alkylidenes by α-H abstraction: prognosis from NMR chemical shift
Chemical Science, 2018Co-Authors: Christophe Gordon, Keishi Yamamoto, Keith Searles, Satoru Shirase, Richard Andersen, Odile Eisenstein, Christophe CopéretAbstract:Metal Alkylidenes, which are key organometallic intermediates in reactions such as olefination or alkene and alkane metathesis, are typically generated from metal dialkyl compounds [M](CH2R)2 that show distinctively deshielded chemical shifts for their α-carbons. Experimental solid-state NMR measurements combined with DFT/ZORA calculations and a chemical shift tensor analysis reveal that this remarkable deshielding originates from an empty metal d-orbital oriented in the M–Cα–Cα′ plane, interacting with the Cα p-orbital lying in the same plane. This π-type interaction inscribes some Alkylidene character into Cα that favors Alkylidene generation via α-H abstraction. The extent of the deshielding and the anisotropy of the alkyl chemical shift tensors distinguishes [M](CH2R)2 compounds that form Alkylidenes from those that do not, relating the reactivity to molecular orbitals of the respective molecules. The α-carbon chemical shifts and tensor orientations thus predict the reactivity of metal alkyl compounds towards Alkylidene generation.
Christophe Copéret - One of the best experts on this subject based on the ideXlab platform.
-
Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds
Accounts of Chemical Research, 2019Co-Authors: Christopher P. Gordon, Christophe Copéret, Christophe Raynaud, Richard A. Andersen, Odile EisensteinAbstract:Metal-bonded carbon atoms in metal–alkyl, metal–carbene/Alkylidene, and metal–carbyne/alkylidyne species often show significantly more deshielded isotropic chemical shifts than their organic counterparts (alkanes, alkenes, and alkynes). While isotropic chemical shift is universally used to characterize a chemical compound in solution, it is an average value of the three principal components of the chemical shift tensor (δ11 > δ22 > δ33). The tensor components, which are accessible by solid-state NMR spectroscopy, can provide detailed information about the electronic structure (frontier molecular orbitals) at the observed nuclei. This information can be accessed in detail by quantum chemical calculations, most notably by an analysis of the paramagnetic contribution to the NMR shielding tensor. The paramagnetic term mainly results from the coupling of occupied and empty molecular orbitals close in energy—the frontier molecular orbitals—under the effect of the external magnetic field (B0). In organometallic compounds, a large deshielding of the isotropic carbon-13 chemical shift of the metal-bonded carbon atom is commonly related to the coupling between the occupied σM–C orbital and low-lying vacant orbitals of πM═C* character. The deshielding at the α-carbon hence probes the extent of σM–C and πM═C* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Ω = δ11 – δ33) observed in metal Alkylidenes and alkylidynes (200 < δiso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs Alkylidenes due to their low-lying πM═C* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the α-carbon of metal–alkyls display deshielded chemical shifts in specific coordination environments. This deshielding, which is often prominently pronounced for cationic species, indicates the presence of partial π-bond character in the metal–carbon bond, making these bonds topologically equivalent to Alkylidene π-bonds. The π-character in metal–alkyl bonds favors (i) α-H abstraction processes in metal bis-alkyl compounds yielding metal Alkylidenes, (ii) [2 + 2]-retrocyclization of metallacyclobutanes that participate in olefin metathesis, (iii) olefin insertion in cationic metal alkyls thus explaining polymerization activity trends and the importance of α-H agostic interactions, and (iv) C–H bond activation on metal–alkyls via σ-bond metathesis. The presence of π-character in the metal–carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal–alkyls toward olefin insertion and σ-bond metathesis and the fact that σ-bond metathesis, olefin insertion, and olefin metathesis are commonly observed with metal atoms in the same ligand field. Because of the similarities in the frontier molecular orbitals involved in these processes, these reactions can be viewed as isolobal. This explains why certain fragments, such as bent metallocenes (d0 Cp2M) or T-shaped L3M, are ubiquitous in these reactions.
-
Metal alkyls programmed to generate metal Alkylidenes by α-H abstraction: prognosis from NMR chemical shift
Chemical Science, 2018Co-Authors: Christophe Gordon, Keishi Yamamoto, Keith Searles, Satoru Shirase, Richard Andersen, Odile Eisenstein, Christophe CopéretAbstract:Metal Alkylidenes, which are key organometallic intermediates in reactions such as olefination or alkene and alkane metathesis, are typically generated from metal dialkyl compounds [M](CH2R)2 that show distinctively deshielded chemical shifts for their α-carbons. Experimental solid-state NMR measurements combined with DFT/ZORA calculations and a chemical shift tensor analysis reveal that this remarkable deshielding originates from an empty metal d-orbital oriented in the M–Cα–Cα′ plane, interacting with the Cα p-orbital lying in the same plane. This π-type interaction inscribes some Alkylidene character into Cα that favors Alkylidene generation via α-H abstraction. The extent of the deshielding and the anisotropy of the alkyl chemical shift tensors distinguishes [M](CH2R)2 compounds that form Alkylidenes from those that do not, relating the reactivity to molecular orbitals of the respective molecules. The α-carbon chemical shifts and tensor orientations thus predict the reactivity of metal alkyl compounds towards Alkylidene generation.
Daesung Lee - One of the best experts on this subject based on the ideXlab platform.
-
hydrohalogenative aromatization of multiynes promoted by ruthenium Alkylidene complexes
Organic and Biomolecular Chemistry, 2016Co-Authors: Rajdip Karmakar, Sang Young Yun, Kung Pern Wang, Phani Mamidipalli, Daesung LeeAbstract:A new functionalization method of arynes promoted by a novel catalytic role of the Grubbs-type ruthenium Alkylidene complex is described. Through a sequence of aryne formation followed by their halo-functionalization, various bis-1,3-diynes were directly converted to functionalized aryl chlorides, bromides and iodides in good yields in the presence of a catalytic amount of a ruthenium Alkylidene complex and halogenated hydrocarbons such as CH2Cl2, CHCl3, CH2Br2, and CH2I2. The utility of this novel transformation is demonstrated by a formal synthesis of herbindole B.
-
nonmetathetic activity of ruthenium Alkylidene complexes 1 4 hydrovinylative cyclization of multiynes with ethylene
Journal of the American Chemical Society, 2012Co-Authors: Sang Young Yun, Kung Pern Wang, Mansuk Kim, Daesung LeeAbstract:An efficient 1,4-hydrovinylative cyclization reaction of triynes and tetraynes catalyzed by ruthenium Alkylidene complexes under ethylene is described. The regioselectivity of vinyl group incorporation can be controlled by the nature of the substituent on the alkyne, and the Grubbs second-generation catalyst is the most effective among typical ruthenium Alkylidene complexes.