Download or read book Structure property Insights Into Molybdenum Imido Alkylidene N heterocyclic Carbene Complexes for Olefin Metathesis written by Mohasin Momin and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Synthesis of Molybdenum and Tungsten Oxo and Imido Alkylidene NHC Complexes and Their Use in Stereoselective Ring Opening Metathesis Polymerization written by Mathis Benedikter and published by Cuvillier Verlag. This book was released on 2021-04-08 with total page 306 pages. Available in PDF, EPUB and Kindle. Book excerpt: Im Rahmen der Dissertation wurden unterschiedliche Aspekte der Olefinmetathese mit Molybdän- und Wolframbasierten Katalysatoren untersucht. Zunächst wurde die Eignung von Molybdän Imido Alkyliden N-heterocyclischen Carben (NHC) Komplexen als Initiatoren für die ringöffnende Metathese-Polymerisation (ROMP) erforscht. Durch Einsatz von chiralen, enantiomerenreinen Norbornenderivaten als Monomer konnte gezeigt werden, dass mit diesen Komplexen selektiv trans-isotaktische Polymere hergestellt werden können. Die beobachtete Selektivität ist dabei stark abhängig von der Ligandensphäre. Des Weiteren konnte vollständig hydriertes, syndiotaktisches Polydicyclopentadien hergestellt und erstmals mittels Schmelzspinnen zu Fasern versponnen werden. Ein weiterer Schwerpunkt der Dissertation lag auf der Entwicklung neuer Katalysatoren für die Olefinmetathese. So wurde eine neue Syntheseroute zur Herstellung kationischer Wolfram Imido Alkyliden NHC Komplexen entwickelt. Durch Anpassung der Ligandensphäre konnten luftstabile kationische Molybdän und Wolfram Imido Alkyliden NHC Komplexe hergestellt werden, die hohe Produktivitäten in der Olefinmetathese von Substraten mit verschiedenen sauerstoff- und schwefelhaltigen funktionellen Gruppen zeigen. Schließlich konnte der erste Molybdän Oxo Alkyliden NHC Komplex hergestellt und charakterisiert werden.

Download or read book Mechanistic Insights Into Late Transition Metal Catalyzed Olefin Hydrosilylation and Synthesis of New N Heterocyclic Carbene and Dinuclear Complexes written by Korbinian Riener and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Synthesis Structure and Applications of Metallacyclic Alkylidene Complexes of Molybdenum and Tungsten written by Kapil Shyam Lokare and published by . This book was released on 2009 with total page 308 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book High oxidation state Molybdenum and Tungsten Monoalkoxide Pyrrolide Alkylidenes as Catalysts for Olefin Metathesis written by Erik Matthew Townsend and published by . This book was released on 2014 with total page 195 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chapter 1 describes work toward solid-supported W olefin metathesis catalysts. Attempts to tether derivatives of the known Z-selective catalyst W(NAr)(C3H6)(pyr)(OHIPT) (Ar = 2,6- diisopropylphenyl, pyr = pyrrolide; HIPT = 2,6-bis-(2,4,6-triisopropylphenyl)phenyl) to a modified silica surface by covalent linkages are unsuccessful due to destructive interactions between W precursors and silica. W(NAr)(C3H6)(pyr)(OHIPT) and W(NAr)(CHCMe2Ph)(pyr)(OHIPT-NMe2) (HIPT-NMe 2 = 2,6-bis-(2,4,6-triisopropylphenyl)-4- dimethylaminophenyl) are adsorbed onto calcined alumina. W(NAr)(C 3H6 )(pyr)(OHIPT) is destroyed upon binding to alumina, while W(NAr)(CHCMe 2Ph)(pyr)(OHIPT-NMe 2) appears to bind through a non-destructive interaction between the dimethylamino group and an acidic surface site. The heterogeneous catalysts perform non-stereoselective metathesis of terminal olefins, and W(NAr)(CHCMe2Ph)(pyr)(OHIPT-NMe2) can be washed off the surface with polar solvent and perform solution-phase Z-selective metathesis. Chapter 2 details selective metathesis homocoupling of 1,3-dienes with Mo and W monoalkoxide pyrrolide (MAP) catalysts. A catalytically relevant vinylalkylidene complex, Mo(NAr)(CHCHCH(CH3)2)(Me2pyr)(OHMT) (HMT = 2,6-bis(2,4,6-trimethylphenyl)phenyl; Me2pyr = 2,5-dimethylpyrrolide), is isolated. A series of Mo and W MAP catalysts is synthesized and tested for activity, stereoselectivity, and chemoselectivity in 1,3-diene metathesis homocoupling. Catalysts containing the OHIPT ligand display excellent selectivity in general, and W catalysts are less active but more selective than their Mo counterparts. Chapter 3 recounts the synthesis and characterization of several heteroatom-substituted alkylidene complexes with the formula Mo(NAr)(CHER)(Me2pyr)(OTPP) (TPP = 2,3,5,6- tetraphenylphenyl; ER = OPr, N-pyrrolidinonyl, N-carbazolyl, pinacolborato, trimethylsilyl, SPh, or PPh2). Synthesis proceeds via alkylidene exchange between Mo(NAr)(CHR)(Me2pyr)(OTPP) (R = H, CMe2Ph) and a CH2CHER precursor. Each complex behaves similarly to known MAP complexes in olefin metathesis processes; the electronic identity of ER has little effect on catalytic properties. Distinctive features of alkylidene isomerism and catalyst resting state are examined. Chapter 4 contains synthetic and catalytic studies of thiolate-containing Mo and W imido alkylidene complexes. The species M(NAr)(CHCMe 2Ph)(pyr)(SHMT) (M = Mo or W), Mo(NAr)(CHCMe2Ph)(Me2pyr)(STPP), and Mo(NAr)(CHCMe2Ph)(STPP)2 are synthesized by substitution of the appropriate thiol or thiolate ligands for pyrrolide or triflate ligands in metal precursors. These complexes show similar structural and spectral characteristics to alkoxidecontaining species. The thiolate complexes and their alkoxide analogues are compared for activity and selectivity in metathesis homocoupling and ring-opening metathesis polymerization processes. In general, thiolate catalysts are slower and less selective than alkoxide catalysts.

Download or read book Carbene Chemistry written by Guy Bertrand and published by CRC Press. This book was released on 2002-05-14 with total page 278 pages. Available in PDF, EPUB and Kindle. Book excerpt: Highlights recent discoveries in the development of rapid kinetic techniques that allow for direct visualization and state-of-the-art computational methods.

Download or read book Molybdenum and Tungsen Alkylidene Species for Catalytic Enantio Z and E selective Olefin Metathesis Reactions written by Smaranda Constanţa Marinescu and published by . This book was released on 2011 with total page 204 pages. Available in PDF, EPUB and Kindle. Book excerpt: CHAPTER1 A general introduction to olefin metathesis is given. Highlights include a detailed discussion of group VI imido alkylidene catalysts. CHAPTER 2 Several bispyrrolide species Mo(NAr)(CHCMe 2Ph)(pyr)2 (Ar = 2,6-i-Pr2C6H3, pyr = 2,3,4,5- tetramethylpyrrolide, 2,5-diisopropylpyrrolide, or 2,5-diphenylpyrrolide) have been synthesized and characterized. X-ray structural studies of these species display one r 1-pyrrolide ring and one 5-p1y rrolide ring. Monohexafluoro-t-butoxide pyrrolide (MAP) species can be prepared, either through addition of one equiv of Me(CF 3)2COH to a bispyrrolide or through reactions between the lithium pyrrolide and the bishexafluoro-t-butoxide. Trimethylphosphine adducts of MAP hexafluoro-t-butoxide species, Mo(NAr)(CHCMe 2Ph)(pyr)[OC(CF 3)2Me](PMe3), have been prepared. An X-ray structural study of one of these phosphine adducts was found to have PMe3 bound approximately trans to the pyrrolide. This adduct serves as a model for the structure of the initial olefin adduct in olefin metathesis. CHAPTER 3 The two diastereomers of Mo(NAr)(CHCMe2Ph)(2,5-dimethylpyrrolide)(OBitet) ((SMRJ)-1 and (RMR])-1, respectively, where OBitet is an enantiomerically pure (R) phenoxide and Ar = 2,6- diisopropylphenyl), form adducts with PMe3. One of these ((RmR)-1(PMe3)) has been isolated. An X-ray structure reveals that PMe3 has added trans to the pyrrolide; it is a model for where an olefin would attack the metal. Trimethylphosphine will catalyze slow interconversion of (SMRI)- 1 and (RMRJ)-1 via formation of weak PMe3 adducts, which undergo a series of Berry pseudorotations or (equivalent) turnstile rearrangements. The interconversion of diastereomers in the presence of trimethylphosphine was investigated by a variety of kinetic studies, variable temperature NMR spectroscopic studies, and labeling studies. CHAPTER 4 Addition of ethylene to Mo(NAr)(CHCMe 2Ph)(OBitet)(2,5-Me2Pyr) led to the trigonal bipyramidal metallacyclobutane complex, Mo(NAr)(C 3H6)(OBitet)(2,5-Me 2Pyr), in which the imido and aryloxide ligands occupy axial positions. NMR studies of Mo(NAr)(C 3H6)(OBitet)(2,5-Me 2Pyr) showed that the metallacyclobutane - species is in equilibrium with ethylene/methylidene intermediates before losing ethylene to yield the respective methylidene complexes. Detailed NMR studies of Mo(NAr)(C3H6)(OBitet)(Me 2Pyr) were carried out and compared with previous studies of W(NAr)(C 3H6)(OBitet)(Me 2Pyr). .It could be shown that Mo(NAr)(C 3H6)(OBitet)(Me 2Pyr) forms an ethylene/methylidene intermediate at 20 0C at a rate that is 4500 times faster than the rate at which W(NAr)(C 3H6)(OBitet)(Me 2Pyr) forms an ethylene/methylidene intermediate. It is proposed that the stability of methylidene complexes coupled with their high reactivity account for the high efficiency of many olefin metathesis processes that employ MonoAryloxidePyrrolide (MAP) catalysts. CHAPTER 5 MonoAryloxide-Pyrrolide (MAP) olefin metathesis catalysts of molybdenum that contain a chiral bitetralin-based aryloxide ligand are efficient for ethenolysis of methyl oleate, cyclooctene, and cyclopentene. Ethenolysis of 5000 equivalents of methyl oleate produced 1- decene (1D) and methyl-9-decenoate (M9D) with a selectivity of >99%, yields up to 95%, and a TON (turnover number) of 4750 in 15 hours. Tungstacyclobutane catalysts gave yields approximately half those of molybdenum catalysts, either at room temperature or at 50 0C, although selectivity was still >99%. Ethenolysis of 30000 equiv of cyclooctene to 1,9-decadiene could be carried out with a TON of 22500 at 20 atm (75% yield), while ethenolysis of 10000 equiv of cyclopentene to 1,6-heptadiene could be carried out with a TON of 5800 at 20 atm (58% yield). Some MonoAryloxide-Pyrrolide (MAP) olefin metathesis catalysts of molybdenum that are Z selective for the homocoupling of terminal olefins can be employed for the selective ethenolysis of Z internal olefins in the presence of E internal olefins in minutes at 22 0C. Therefore it is possible to take an E:Z mixture to a pure E product by selectively destroying the Z component and removing the resulting low molecular weight ethenolysis products. Exclusively E olefins can be obtained from terminal olefins in a two step process: the first step consists of a nonselective homocoupling to give approximately a 4:1 E:Z; while the second step consists of Zselective ethenolysis of the olefinic mixture to generate pure E-olefin. Several functional groups can be tolerated, such as ethers and esters. CHAPTER 6 3,5-Dimethylphenylimido complexes of tungsten can be prepared using procedures analogous to those employed for other tungsten catalysts, as can bispyrrolide species, and MonoAryloxide- Pyrrolide (MAP) species. X-ray structural studies of metallacylcobutane MAP species show them to have the expected TBP geometry with the imido and aryloxide ligands in apical positions. Homocoupling of 1-hexene, 1-octene, and methyl-10-undecenoate are achieved in 45- 89% yield and a Z-selectivity of >99% with W(NAr")(C 3H6)(pyr)(OHIPT) (Ar" = 3,5-Me 2C6H3; HIPT = 2,6-(2,4,6-(i-Pr) 3C6H2)2C6H3) as a catalyst. Homocoupling of terminal olefins in the presence of E olefins elsewhere in the molecule was achieved with excellent selectivity. CHAPTER 7 A monotriflate species, Mo(NAd)(CHCMe 2Ph)(OHIPT)(OTt) (Ad = 1-Adamantyl), is obtained by salt metathesis of bistriflate species and one equivalent of lithium alkoxide. Addition of PMe3 to the monotriflate species led to the formation of a phosphine adduct. An X-ray structural study revealed a square pyramidal coordination environment, with the alkylidene in the apical position and the phophine trans to the triflate ligand. The triflate can be exchanged with a variety of anionic ligands, such as 2-Mespyrrolide and t-butoxide. These species have been characterized by X-ray crystallography and they reveal the expected tetrahedral geometry. CHAPTER 8 Exposure of diethylether solution of Mo(NAr)(CHCMe 2Ph)(Me2Pyr)(OSiPh3) (1) to one atmosphere of ethylene for one hour led to the formation of the ethylene complex Mo(NAr)(CH 2CH 2)(Me 2Pyr)(OSiPh 3) (2). Addition of one equivalent of triphenylsilanol to a solution of 2 gives Mo(NAr)(CH 2CH2)(OSiPh 3)2 (3) readily. Mo(NAr)(CHCMe 2Ph)(OTf)2(dme) reacts slowly with ethylene (60 psi) in toluene at 80 'C to give cis and trans isomers of Mo(NAr)(CH 2CH 2)(OTf)2(dme) (4a) in the ratio of -2(cis):1. Addition of lithium 2,5- dimethylpyrrolide to 4a under 1 atm of ethylene produces Mo(NAr)(CH 2CH 2)(h-Me2Pyr)(h 5- Me2Pyr) (5). Neat styrene reacts with 2 and 3 to generate the styrene complexes, Mo(NAr)(CH 2CHPh)(Me2Pyr)(OSiPh 3) (6) and Mo(NAr)(CH 2CHPh)(OSiPh3)2 (7), respectively. Similarly, the trans-3-hexene complex, Mo(NAr)(trans-3-hexene)(OSiPh 3)2 (8a), can be prepared from 3 and neat trans-3-hexene. When 3 is exposed to 1 atm of ethylene, the molybdacyclopentane species, Mo(NAr)(C 4Hs)(OSiPh3)2 (9), is generated. X-ray structural studies were carried out on 2, 5, 7, 8a, and 9. All evidence suggests that alkene exchange at the Mo(IV) center is facile, followed by cis,trans isomerization and isomerization via double bond migration. In addition, trace amounts of alkylidene complexes are formed that result in slow metathesis reactions of free olefins to give (e.g.) a distribution of all possible linear olefins from an initial olefin and its double bond isomers. APPENDIX A Monopyrrolide monothiolate species of type Mo(NAr)(CHR)(2,5-Me 2NC4H2)(SR') (Ar = 2,6-i- Pr2C6H3; R = CMe3, CMe2Ph; R'= 2,6-Me 2C6H3, C6F5) have been synthesized by protonolysis of Mo(NAr)(CHR)(2,5-Me 2NC4H2)2 with one equivalent of R'SH. Addition of one equiv of 2,6- Me2C6H3SH to Mo(NAr)(CHCMe 2Ph)[OC(CF3)2Me] 2 led to the formation of Mo(NAr)(CHCMe 2Ph)(2,6-Me2C6H3S)[OCMe(CF 3)2] (3) in good yield. Using the same method, Mo(NAr)(CHCMe 3)(SCMe 3)[OC(CF 3)2Me] (4) was synthesized. A ligand scrambling effect was observed by 1H NMR spectroscopy leading to the formation of bisalkoxide and bisthiolate species. The bisalkoxide species, Mo(NAr)(CHCMe 2Ph)(OBitet) 2, was synthesized by salt metathesis of Mo(NAr)(CHCMe 2Ph)(OTf) 2(dme) and two equivalents of BitetONa. An X-ray structural study of this compound shows an anti configuration of the alkylidene.

Download or read book Investigations of Sterically Demanding Ligands in Molybdenum and Tungsten Monopyrrolide Monoalkoxide Catalysts for Olefin Metathesis written by Laura Claire Heidkamp Gerber and published by . This book was released on 2013 with total page 217 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chapter 2 investigates the mechanism of the temperature-controlled polymerization of 3- methyl-3-phenylcyclopropene (MPCP) by Mo(NAr)(CHCMe 2Ph)(Pyr)(OTPP) (Ar = 2,6- diisopropylphenyl, Pyr = pyrrolide, OTPP = 2,3,5,6-tetraphenylphenoxide). Cissyndiotactic poly(MPCP) is obtained at -78 °C, while atactic poly(MPCP) is obtained at ambient temperature. The syn initiator (syn refers to the isomer in which the substituent on the alkylidene points towards the imido ligand and anti where the substituent points away) reacts with MPCP to form an anti first-insertion product at low temperatures, which continues to propagate to give cis,syndiotactic polymer. At higher temperatures, the anti alkylidenes that form initially upon reaction with MPCP rotate thermally to syn alkylidenes on a similar timescale as polymer propagation, giving rise to an irregular polymer structure. In this system cis,syndiotactic polymer is obtained through propagation of anti alkylidene species. Chapters 3 - 5 detail the synthesis and reactivity of compounds containing a 2,6- dimesitylphenylimido (NAr*) ligand in order to provide a better understanding of the role of steric hindrance in olefin metathesis catalysts. A new synthetic route to imido alkylidene complexes of Mo and W, which proceeds through mixed-imido compounds containing both NAr* and NtBu ligands, was developed to incorporate the NAr* ligand. Alkylidene formation is accomplished by the addition of 3 equivalents of pyridine*HCl to Mo(NAr*)(NBu)(CH 2CMe2Ph)2 or the addition of 1 equivalent of pyridine followed by 3 equivalents of HCl solution to W(NAr*)(N'Bu)(CH 2CMe2Ph)2 to provide M(NAr*)(CHCMe 2Ph)Cl 2(py) (py = pyridine). Monoalkoxide monochloride, bispyrrolide, and monoalkoxide monopyrrolide (MAP) compounds are isolated upon substitution of the chloride ligands. Reaction of W MAP complexes (W(NAr*)(CHCMe 2Ph)(Me2Pyr)(OR)) with ethylene allows for the isolation of unsubstituted metallacycle complexes W(N Ar*)(C 3H6)(Me 2Pyr)(OR) (R = CMe(CF 3)2, 2,6-Me2C6H3, and SiPh 3). By application of vacuum to solutions of unsubstituted metallacyclebutane species, methylidene complexes W(NAr*)(CH 2)(Me2Pyr)(OR) (R = tBu, 2,6-Me2C6H3, and SiPh 3) are isolated. Addition of one equivalent of 2,3- dicarbomethoxynorbornadiene to methylidene species allows for the observation of firstinsertion products by NMR spectroscopy. Investigations of NAr* MAP compounds as catalysts for olefin metathesis reactions show that they are active catalysts, but not E or Z selective for ring-opening metathesis polymerization the homocoupling of 1-octene or 1,3-dienes. Methylidene species W(NAr*)(CH 2)(Me2Pyr)(OR) (R = 2,6-Me 2C6H3 or SiPh3) catalyze the ring-opening metathesis or substituted norbornenes and norbornadienes with ethylene.

Download or read book Longer lived Olefin Metathesis Catalysts Based on Molybdenum and Ruthenium written by Joseph Yoon and published by . This book was released on 2020 with total page 91 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has seen considerable growth in the recent past. Some of the earliest milestones in the field include the synthesis of well-defined catalysts based on molybdenum, tungsten, and ruthenium. The efficiencies of these catalysts, however, are limited by their decomposition. Efforts have been made to increase the lifetime of these catalysts by changing the ligand sphere, to stabilize catalytic intermediates. Examples include the employment of the N-heterocyclic carbene (NHC) and the chelating (o-isopropoxy)benzylidene ligand seen in the second-generation Grubbs and Hoveyda catalysts. Processes that utilize the olefin metathesis processes, like those in the petroleum industry and large-scale production of chemicals, are bound by the need for high catalyst loadings which translate to high costs. The work herein presents the pursuit of longer-lived olefin metathesis catalysts based on molybdenum and ruthenium. The first goal of this thesis project was to develop a stable molybdenum-based olefin metathesis catalyst supported by a tridentate PONOP ligand and a chelating (o- x methoxy)benzylidene ligand. Previous attempts in our lab employed nonchelating alkylidene initiators - yielding no success in isolation. The rationale behind this design was that a chelating ether moiety will stabilize the molybdenum-center enough to be isolable. Attempts to isolate the chelating molybdenum-alkylidene species were also unsuccessful. Instead, we probed the in-situ ROMP of norbornene using iPrPONOP MoCl3 as a precatalyst and (2-methoxybenzyl)magnesium chloride as a cocatalyst. This cocatalyst did not lend any improvements to the simpler nonchelating Grignard cocatalysts. The synthesis of a novel dialkyl zirconocene complex is also reported. The second and more heavily pursued endeavor was the development of longer-lived ruthenium olefin metathesis catalysts. Specifically, we aimed at improving the second-generation Hoveyda catalyst with the use of a hemilabile tridentate NHC ligand. Two novel catalysts bearing NHC ligands with a hemilabile ethoxy-pyridyl arm were synthesized along with their unique organic frameworks. The catalyst containing the 2,6-diisopropylphenyl group (C1-Me) was investigated more comprehensively because it was more readily prepared. This complex was characterized by high thermal stability under metathesis conditions and remarkable TONs in the self-metathesis of 1-decene. In our efforts to prepare C1-Me without utilizing a Grubbs I intermediate, a new complex (6) bearing our NHC ligand was isolated and characterized by 1H NMR and single crystal x-ray diffraction spectroscopy. The reaction of C1-Me with ethylene did not produce the desired C1-Me-methylidene variant - however, the same reaction with propylene gave C1-Me-ethylidene with relative ease. Analyzing the active catalytic species under the metathesis of 1-decene revealed that the resting state of the catalyst is not the expected methylidene, but rather the longer chain nonylidene. xi Initiation studies were conducted to compare the rates of initiation for catalyst C1-Me and the nonmethylated C1-H. First, the rate of metathesis was followed in the irreversible reaction with ethyl vinyl ether. Second, ligand exchange equilibrium experiments were carried out to compare the dissociation constants for the pyridyl moieties in both catalysts. The outcome of these studies revealed that catalyst C1-Me, with a methyl group in the phenoxide ring, exhibits a 10-fold increase in initiation versus the nonmethylated C1-H catalyst. The NHC ligand scaffold reported in this work may assist in the development of other inorganic and organometallic catalytic systems, as many rely on the use of ancillary ligands for support. Furthermore, fixing a hemilabile ethoxy-pyridyl arm onto already robust systems, such as ruthenium catalysts bearing a cyclic alkyl amino carbene ligand, may offer even greater catalytic turnover numbers (TONs).

Download or read book Synthetic Investigations of Molybdenum Pyrrolide and Related Complexes written by Keith Michael Wampler and published by . This book was released on 2010 with total page 260 pages. Available in PDF, EPUB and Kindle. Book excerpt: CHAPTER 1: A general introduction to olefin metathesis is given. Highlights include a historical perspective of the development of olefin metathesis and a detailed discussion of group VI imido alkylidene catalysts. CHAPTER 2: Monosiloxide and disiloxide complexes have been prepared through the addition of silanols to Mo(NR)(CHCMe 2Ph)(pyrrolyl) 2 species (R = 1 -adamantyl (Ad) or 2,6-i-Pr2C6H3 (Ar)). The silanols employed include (t-Bu)3SiOH (Hsilox), (i-Pr)3SiOH, (Me3Si)3SiOH, (t-Bu-0)3SiOH, Me2(t-Bu)SiOH, and Ph 3SiOH. The mono(silox) complex, Mo(NAr)(CHCMe 2Ph)(silox)(pyrrolyl) .(2a), could be isolated, while Mo(NAd)(CHCMe2Ph)(silox)(pyrrolyl) was observed in situ but could not be crystallized. Reaction of Mo(NAr)(CHCMe 2Ph)(OTf) 2(DME) with (silox)Li(THF) resulted in the formation of Mo(NAr)(CHCMe2Ph)(silox)(OTf) (3). Disiloxides that could be crystallized include Mo(NAd)(CHCMe 2Ph)(Silox)2 (1b), Mo(NAd)(CHCMe2Ph)[OSi(SiMe3)3]2 (5), Mo(NAd)(CHCMe 2Ph)[OSi(O-t-Bu) 3]2 (6), and Mo(NAr)(CHCMe 2Ph)[OSiMe2(t-Bu)] 2 (7); other disiloxide examples could be observed in situ, but could not be crystallized. Compound 2a reacts readily with (CF 3)Me2COH, (CF3)2MeCOH, (CF 3)2CHOH, ArOH, C6F5OH, ( - )-menthol, and ( - )-borneol to give compounds of the type Mo(NAr)(CHCMe 2Ph)(silox)(OR) (4a-g) in situ. No reaction was observed upon heating of lb under 5 atm of ethylene at 120 *C in toluene-d8 ; only at 240 'C in o-dichlorobenzene-d4 did lb react with ethylene to yield CH2=CHCMe2Ph, but the Mo-containing product could not be identified. Compound 2a reacts with ethylene at 120 'C to give Mo(NAr)(CH2)(silox)(pyr), while 4a-e react with ethylene at -60 'C; methylene species could be observed in several cases but could not be isolated. X-ray studies were carried out for lb and 2a. CHAPTER 3: Molybdenum imido alkylidene complexes which may be used as precursors for the in situ generation of molybdenum olefin metathesis catalysts are presented. Reaction of Mo(NR)(CHCMe 2Ph)(OTf)2(DME) (R = 1-adamantyl (Ad) or 2,6-i-Pr 2C6H3 (Ar)) with two equivalents of Li(ind) (ind = indolide) results in the formation of Mo(NR)(CHCMe 2Ph)(ind)2 (R = Ar, 1; Ad, 2). Unlike other molybdenum complexes of nitrogen containing heterocyclic ligands, 1 and 2 react productively with olefins. 1 and 2 react with alcohols to give previously characterized bisalkoxide olefin metathesis catalysts. Reaction of Li(3,5-R 2-pyrazolide) (R = t- Bu or Ph, R2pz) with Mo(NAr)(CHCMe 2Ph)(OTf) 2(DME) yields Mo(NAr)(CHCMe 2Ph)(3,5- R2pz)2 (R = t-Bu, 5; Ph, 6) in good yields. These complexes react with alcohols or the surface silanols of silica, to yield respectively bisalkoxy and surface monosiloxy olefin metathesis catalysts. The benzyl complexes Mo(NR)(CHCMe 2Ph)(CH2Ph)2 (R = Ar, 7; Ad, 8; Ar" = 9) have been prepared and structurally characterized. These complexes react with alcohols and phenols to give either monobenzyl monoalkoxide(aryloxide) species or trialkyl alkoxide(aryloxide) complexes. Additionally, several species that were found to not be precursors for the in situ generation of olefin metathesis catalysts are discussed. CHAPTER 4: Three substituted tris(pyrrolyl-a-methyl)amines (H3[Aryl 3TPA]) (Aryl = 2,4,6-C 6H2Me3 (Mes), la; 2,4,6-C 6H2(i-Pr)3 (Trip), 1b; 3,5-C 6H3(CF3)2 (ArF), 1c) have been prepared. An X-ray study of [Trip 3TPA]MoCl (2) shows it to be a distorted trigonal bipyramidal species in which the 2,4,6-triisopropylphenyl substituents surround and protect the apical chloride. Reaction of MoN(NMe 2)3 with H3[ArF3TPA] yields MoN(NMe2)-K3_[ArF3TPA] (3) in which only two of the ligand arms have metalated. The x-ray crystal structure revealed that the un-metalated pyrrole arm has a hydrogen bonding interaction with nitride ligand. Similarly, reaction of Mo(NMe2)4 with H3[ArF3TPA] yields Mo(NMe2)2-K3C[ArF3TPA] (3). Reaction of M(NMe 2)4 (M = Zr or Hf) with H3[ArF3TPA] results in the full metalation of the ligand to yield M(NMe2)(HNMe 2)[ArF 3TPA] (M = Zr, 5; Hf, 6), in which an equivalent of dimethylamine remains in the coordination sphere. CHAPTER 5: The monomeric, homoleptic molybdenum(III) complex molybdenum tris(2,5-dimethylpyrrolide) (1) has been prepared. Reduction with KC8 in THF yields the molybdenum(II) complex potassium [molybdenum tris(2,5-dimethylpyrrolide)] (2), while protonation with [H(OEt 2)2][BArF4] or [HNMe2Ph][B(C6F5)4] yields cationic species that contains an 9-3Hpyrrole ligand (3a and 3b). All of the complexes have been structurally characterized. The paramagnetic species have been characterized by EPR and CV. Additionally, a review of group VI pyrrolide complexes is given. APPENDIX A: The preparation and reactivity of polystyrene-supported molybdenum and tungsten imido alkylidene monoaryloxide monopyrrolide catalysts is presented. The reactivity and selectively of these complexes in the homodimerization of terminal olefins was found to be similar to their homogenous analogues. APPENDIX B: The synthesis and characterization of W(O)(CHCMe3)(Me2Pyr)2(PMe2Ph) (1), W(CCMe3)(OTf) 3(DME) (2), and [Li(OEt2)2][MoCl2(C4H3N-CH(=NAr)]) 2] (3) is described.

Download or read book High Oxidation State Molybdenum and Tungsten Imido Alkylidene and Metallacycle Chemistry written by W. C. Peter Tsang and published by . This book was released on 2004 with total page 526 pages. Available in PDF, EPUB and Kindle. Book excerpt: (Cont.) unsubstituted tungstacyclobutane complexes (82), ethylene complexes (84), tungstacyclopentane complexes (86), and a heterochiral methylene dimer (85a). The tungstacyclopentane complexes catalyzed slow dimerization of ethylene to 1-butene. The observation of the methylene dimer provides the first direct evidence of a bimolecular decomposition pathway for methylene complexes. Chapter 3 Racemic and enantiomerically pure molybdenum alkylimido alkylidene complexes, Mo(NAd)(CHCMe2Ph)(Biphen) (19d, Ad = 1-adamantyl) and Mo(NAd)(CHCMe2Ph)[Trip]-(THF) (20d) were prepared and structurally characterized. Complex 19d was observed almost exclusively as a syn alkylidene isomer, in contrast with 20d which was observed almost exclusively as an anti-THF adduct. Complexes 19d and 20d are the only reported chiral alkylimido alkylidene complexes for enantioselective olefin metathesis reactions. Complex 19d is the first crystallographically characterized four-coordinate adamantylimido alkylidene complex in its base-free form. It offers unique reactivity and selectivity profiles in tandem AROM/RCM and AROM/CM reactions. Complex 19d is compatible with a variety of common functional groups, including boron-containing reagents. Van't Hoff analyses suggest that the bias toward syn-19d isomer is entropy-driven. Chapter 4: Solvent- and base-free molybdenum methylene complexes, Mo(NAr)(Biphen)(CH2) (114a, Ar = 2,6-i-Pr2C6H3) and Mo(NAd)(Biphen)(CH2) (114d, Ad = 1-adamantyl) ...

Download or read book Catalysts for Olefin Metathesis written by Tina Maria Trnka and published by . This book was released on 2003 with total page 246 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Molybdenum Alkylidene Complexes written by Tatiana Pilyugina and published by . This book was released on 2007 with total page 302 pages. Available in PDF, EPUB and Kindle. Book excerpt: (Cont.) Chapter 3. Reactions of Mo Bispyrrolide Complexes with Enantiomerically Pure Diols: In Situ Catalyst Generation and Studies of Olefin Metathesis Reactions In the Fume Hood. Reactions of bispyrrolide molybdenum complexes Mo(NAd)(CHCMe2Ph)(pyr)2 and Mo(N-2-6-i-Pr2C6H3)(CHCMe2Ph)(pyr)2 with (R)-BiphenH2, and (R)-Benz2BitetH2 were examined (pyr = C4H4N, Benz2BitetH2 = 3,3'-dibenzhydryl-5,5',6,6',7,7',8,8'-octahydro-1,1'- binaphthyl-2,2'-diol). The resulting in situ generated catalysts were studied in three olefin metathesis reactions. These systems were found to be as active and enantioselective as the analogous isolated complexes. When the stock solutions of Mo(NAd)(CHCMe2Ph)(pyr)2, Mo(N- 2,6-i-Pr2C6H3)(CHCMe2Ph)(pyr)2, (R)-BiphenH2, and (R)-Benz2BitetH2 were stored in the fume hood over a period of one month, the in situ prepared catalysts were determined to be nearly identical in terms of their catalytic properties to the catalysts generated in situ in the glovebox.

Download or read book Novel Olefin Metathesis Catalysts Bearing a Hemi labile NHC Ligand written by Nicolas Cena and published by . This book was released on 2019 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has grown appreciatively in recent decades. Elucidation of the mechanism and a deeper understanding of the key intermediates have enabled chemists to design catalysts, which exhibit greater activity, stability, and selectivity towards a variety of substrates. However, the economic impact of performing this reaction on the industrial scale is often governed by the high cost of the catalysts in comparison to modest turnover numbers (TON). The lifetime of an active catalyst species depends on the stability of intermediates in the catalytic cycle, and this ultimately determines the TON. One of the remaining goals in the field is to further stabilize the intermediates of the catalytic cycle in order to prolong the catalyst lifetimes and increase turnover numbers (TON). The ruthenium catalytic species cycle through several electron deficient intermediates, which lead towards decomposition pathways. These decomposition pathways are directed towards reactions that provide the metal with additional electron density. The primary focus of this thesis project was to develop a new generation of olefin metathesis catalysts employing a tridentate N-heterocyclic carbene (NHC) ligand bearing a hemi labile pyridine arm in the ortho position of the aromatic ring. The rationale behind incorporating these functional groups was to stabilize the reactive intermediates of the catalytic cycle via electron donation from the ether O → Ru and the pyridine N → Ru. These ligands increase the likelihood of stabilization of the metal center by chelation of electron-donating substituents from the O and N, thus adding electron density back into the electron-deficient metal center. The ligand not only donates electron density back to the metal but also shields the sterically open position trans to the NHC. The hypothesis was that stabilizing these reactive intermediates would prolong the lifetime of the active catalyst and thus increase TON and allow for a much lower catalyst loading for industrial applications, thus vastly impacting the economical aspect of olefin metathesis processes. A novel set of two catalysts bearing tridentate NHC ligands with hemi labile pyridine arms were synthesized. The ligands differed in one aromatic ring containing either a 2,6 diisopropyl phenyl (DIPP) or mesityl (Mes) moiety. The result of the x-ray crystallographic analysis revealed the NHC ligand coordinated in the proposed tridentate meridional fashion around the central Ru atom. This coordination was proposed in order to affect a hinge-like mechanism in which the pyridine arm's hemi-labile nature would be in close proximity to the electron deficient metal center, so that it could bind reversibly in order to satiate the metal's desire for electron density while still allowing reactivity upon dissociation. NMR spectroscopy revealed information about the proposed structure in solution and revealed that the ligand was bound in solution to the metal center in one orientation, owing to the coordination of the O and N to the metal center. Catalyst decomposition studies were performed using the methylidene variant of the catalysts at elevated temperatures under inert conditions as well as under an atmosphere of ethylene gas. The purpose of intentionally decomposing the catalyst was to generate the electron deficient Ru center and probe the stabilizing effects of the pyridine arm coordination. These reactive intermediates are electronically and sterically more similar to the 14-electron active catalyst than the 16-electron pre-catalyst, giving insight into how the catalysts behave in solution upon metathesis active conditions. Decomposition products of the DIPP variant were analyzed by NMR and x-ray crystallography, giving insight into possible decomposition pathways for these novel catalysts. The catalysts were screened for metathesis activity in ring-closing metathesis (RCM) and ring opening metathesis polymerization (ROMP). Both showed noticeable differences from previous generations of Grubbs and Hoveyda-Grubbs catalysts in overall efficacy. The prolonged lifetimes of these new catalysts were competitive with commercially available catalysts in terms of lifetime and TON. Though slightly lower in TON, these catalysts lasted much longer in solution at elevated temperatures than their predecessors, thus indicating much more stabilized reactive intermediates. The data gathered from decomposition studies and metathesis activity along with NMR and x-ray crystallography allowed for potential structure-activity relationships and mechanistic pathways to be proposed. The insight into olefin metathesis catalyst structure, performance, and design provided by these studies may assist in future endeavors in the field of olefin metathesis catalyst design and employment.

Download or read book Ligand Variation in Molybdenum Imido Alkylidene Complexes written by Alejandro Gaston Lichtscheidl and published by . This book was released on 2012 with total page 222 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chapter 1. A general introduction is given. Chapter 2. The biscarboxylate species, Mo(NR)(CHCMe 2Ph)(O 2CPh3)2 (R = 2,6-i-Pr2C6H3, 2,6- Me2C6H3, 2-t-BuC 6H4, or 1 -adamantyl) are compared to newly synthesized bis(terphenylcarboxylate) species, Mo(NR)(CHCMe 2Ph)(O 2CTer)2 (Ter = 2,6-diphenyl-4- methylphenyl or 2,6-diphenyl-4-methoxyphenyl). Preparation of bis(terphenylcarboxylate) species was accomplished through protonolysis of Mo(NR)(CHCMe2R')(Me2Pyr)2 with two equivalents of TerCO2H and one of them was characterized through X-ray crystallography. Photolysis experiments of many of the biscarboxylate complexes led to rate constants for the converstion of anti to syn species, which are much slower than bisalkoxide species. Trimethylphosphine adducts of selected triphenylacetate complexes have been isolated and studied in solution. Protonolysis of Mo(NAr)(CHCMe 2R')(Me 2Pyr)2 (Ar = 2,6-i-Pr 2C6H3) with one equivalent of TerCO2H led to the isolation of a handful of monocarboxylate species, Mo(NAr)(CHCMe 2Ph)(O 2CAr')(Me2Pyr). An X-ray structure of one of them was also characterized. Several of the bis(triphenylacetate) complexes and all of the monocarboxylates are active initiators for the regioselective polymerization of diethyl dipropargylmalonate (DEPDM). In the case of the latter compounds, activity towards olefins is also observed and briefly mentioned.

Download or read book Metathesis Polymerization written by Michael R. Buchmeiser and published by Springer. This book was released on 2010-02-12 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: 1 T.W. Baughman, K.B. Wagner: Recent Advances in ADMET Polymerization.- 2 G. Trimmel, S. Riegler, G. Fuchs, C. Slugovc, F. Stelzer: Liquid Crystalline Polymers by Metathesis Polymerization.- 3 M.R. Buchmeiser: Regioselective Polymerization of 1-Alkynes and Steroselective Cyclopolymerization of alpha, omega -Heptadiynes.

Download or read book Stereoselective Alkene Synthesis written by Jianbo Wang and published by Springer. This book was released on 2012-08-13 with total page 283 pages. Available in PDF, EPUB and Kindle. Book excerpt: Stereoselective Synthesis of Tetrasubstituted Alkenes via Torquoselectivity-Controlled Olefination of Carbonyl Compounds with Ynolates, by Mitsuru Shindo and Kenji Matsumoto.- Stereoselective Synthesis of Z-Alkenes, by Woon-Yew Siau, Yao Zhang and Yu Zhao.- Stereoselective Synthesis of Mono-fluoroalkenes, by Shoji Hara.- Recent Advances in Stereoselective Synthesis of 1,3-Dienes, by Michael De Paolis, Isabelle Chataigner and Jacques Maddaluno.- Selective Olefination of Carbonyl Compounds via Metal-Catalyzed Carbene Transfer from Diazo Reagents, by Yang Hu and X. Peter Zhang.- Selective Alkene Metathesis in the Total Synthesis of Complex Natural Product, by Xiaoguang Lei and Houhua Li.- Olefination Reactions of Phosphorus-Stabilized Carbon Nucleophiles, by Yonghong Gu and Shi-Kai Tian.- Alkene Synthesis Through Transition Metal-Catalyzed Cross-Coupling of N-Tosylhydrazones, by Yan Zhang and Jianbo Wang.