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Synthesis of Stereoregular Polymers through Ring-Opening Metathesis Polymerization

Author: Richard R. Schrock (1295601)
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ConspectusSome of the most readily available and inexpensive monomers for ring-opening metathesis polymerization (ROMP) are norbornenes or substituted norbornadienes. Polymers made from them have tacticities (the stereochemical relationship between monomer units in the polymer chain) that remain after the CC bonds in the polymer backbone are hydrogenated. Formation of polymers with exclusively a single structure (one tacticity) was rare until approximately 20 years ago, when well-defined ROMP catalysts based on molybdenum imido alkylidene complexes that contain a chiral biphenolate or binaphtholate ligand were shown to yield cis,isotactic-poly(2,3-dicarbomethoxynorbornadiene) and related polymers through addition of the monomer to the same side of the MC bond in each step. Over the past few years, molybdenum and tungsten monoaryloxide pyrrolide (MAP) imido alkylidene initiators have been found to produce cis,syndiotactic polynorbornenes and substituted norbornadienes through addition of the monomer to one side of the MC bond in one step followed by addition to the other side of the MC bond in the next step. This “stereogenic metal control” is possible as a consequence of the fact that the configuration of the stereogenic metal center switches with each step in the polymerization. Stereogenic metal control also allows syndiotactic polymers to be prepared from racemic monomers in which enantiomers of the monomer are incorporated alternately into the main chain. Because pure trans polymers have not yet been prepared through some predictable mechanism of stereochemical control, it seems unlikely that all four basic polymer structures from a single given monomer can be prepared simply by choosing the right initiator. However, because tactic, and relatively oxygen-stable, hydrogenated polymers are often a desirable goal, the ability to form pure cis,isotactic polymers (through enantiomorphic site control) and cis,syndiotactic polymers (through stereogenic metal control) is sufficient for preparing hydrogenated polymers with a single structure. It is hoped that the principles of forming polymers that have a single structure through ring-opening metathesis polymerization will be general for a relatively large number of monomers and that some important problems in ROMP polymer chemistry can benefit from knowledge of polymer structure at a molecular level. With an increase in knowledge concerning the mechanistic details of polymerization by well-defined initiators, more elaborate ROMP polymers and copolymers with stereoregular structures may be possible

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Synthesis of Tungsten Oxo Alkylidene Complexes

Author: Dmitry V. Peryshkov (1865707)
Richard R. Schrock (1295601)
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Reaction of W(O)2(CH2-t-Bu)2(bipy) with a mixture of ZnCl2(dioxane), PMe2Ph, and trimethylsilyl chloride in toluene at 100 °C produced the known tungsten oxo alkylidene complex W(O)(CH-t-Bu)Cl2(PMe2Ph)2 (1a) in 45% isolated yield. The neophylidene analogue W(O)(CHCMe2Ph)Cl2(PMe2Ph)2 was prepared similarly in 39% yield. The reaction between 1a and LiOR (LiOR = LiOHIPT, LiOHMT) in benzene at 22 °C led to formation of the off-white W(O)(CH-t-Bu)Cl(OR)(PMe2Ph) complexes 4a (OR = OHMT = 2,6-dimesitylphenoxide) and 4b (OR = OHIPT = 2,6-(2,4,6-triisopropylphenyl)2phenoxide). Compound 4a serves as a starting material for the synthesis of W(O)(CH-t-Bu)(OHMT)(2,6-diphenylpyrrolide) (6), W(O)(CH-t-Bu)[N(C6F5)2](OHMT)(PMe2Ph) (7), W(O)(CH-t-Bu)[OSi(t-Bu)3](OHMT) (8), and W(O)(CH-t-Bu)(OHMT)2 (10). The reaction between 8 and ethylene was found to yield the square-pyramidal metallacyclobutane complex W(O)(C3H6)[OSi(t-Bu)3](OHMT) (9), while the reaction between 10 and ethylene was found to yield the square-pyramidal metallacyclobutane complex W(O)(C3H6)(OHMT)2 (11). Compound 11 loses ethylene to yield isolable W(O)(CH2)(OHMT)2 (12). X-ray structures were determined for 6, 7, 9, and 12

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Synthesis of Tungsten Oxo Alkylidene Complexes

Author: Dmitry V. Peryshkov (1865707)
Richard R. Schrock (1295601)
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Synthesis of Methylidene Complexes that Contain a 2,6-Dimesitylphenylimido Ligand and Ethenolysis of 2,3-Dicarbomethoxynorbornadiene

Author: Laura C. H. Gerber (1440232)
Richard R. Schrock (1295601)
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Monoalkoxide pyrrolide (MAP) complexes that contain a 2,6-dimesitylphenylimido (NAr*) ligand react with ethylene to yield unsubstituted metallacyclobutanes that are in equilibrium with methylidene complexes, W(NAr*)(CH2)(Me2Pyr)(OR) (R = t-Bu, OCMe(CF3)2, SiPh3, or 2,6-Me2C6H3). Polymerization of 2,3-dicarbomethoxynorbornadiene (DCMNBD) with MCHCMe2Ph (M = Mo or W) initiators is slow as a consequence of a slow propagation step. However, W(NAr*)(CH2)(Me2Pyr)(OR) (R = SiPh3 or 2,6-dimethylphenyl) complexes react readily with 1 equiv of DCMNBD to give a monoinsertion product. The facile reaction between the monoinsertion product and ethylene then allows these complexes to be catalyts for the ring-opening cross-metathesis (ethenolysis) of DCMNBD and DCMNBE (2,3-dicarbomethoxynorbornene) with minimal formation of polymer

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Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a tert-Butylimido Ligand

Author: Hyangsoo Jeong (1502629)
Peter Müller (1295607)
Richard R. Schrock (1295601)
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A variety of molybdenum or tungsten complexes that contain a tert-butylimido ligand have been prepared. For example, the o-methoxybenzylidene complex W(N-t-Bu)(CH-o-MeOC6H4)(Cl)2(py) was prepared through addition of pyridinium chloride to W(N-t-Bu)2(CH2-o-MeOC6H4)2, while Mo(N-t-Bu)(CH-o-MeOC6H4)(ORF)2(t-BuNH2) complexes (ORF = OC6F5 or OC(CF3)3) were prepared through addition of two equivalents of RFOH to Mo(N-t-Bu)2(CH2-o-MeOC6H4)2. An X-ray crystallographic study of Mo(N-t-Bu)(CH-o-MeOC6H4)[OC(CF3)3]2(t-BuNH2) showed that the methoxy oxygen is bound to the metal and that two protons on the tert-butylamine ligand are only a short distance away from one of the CF3 groups on one of the perfluoro-tert-butoxide ligands (H···F = 2.456(17) and 2.467(17) Å). Other synthesized tungsten tert-butylimido complexes include W(N-t-Bu)(CH-o-MeOC6H4)(pyr)2(2,2′-bipyridine) (pyr = pyrrolide), W(N-t-Bu)(CH-o-MeOC6H4)(pyr)(OHMT) (OHMT = O-2,6-(mesityl)2C6H3), W(N-t-Bu)(CH-t-Bu)(OHMT)(Cl)(py) (py = pyridine), W(N-t-Bu)(CH-t-Bu)(OHMT)(Cl), W(N-t-Bu)(CH-t-Bu)(pyr)(ODFT)(py), W(N-t-Bu)(CH-t-Bu)(OHMT)2, and W(N-t-Bu)(CH-t-Bu)(ODFT)2 (ODFT = O-2,6-(C6F5)2C6H3). Interestingly, W(N-t-Bu)(CH-t-Bu)(OHMT)2 does not react with ethylene or 2,3-dicarbomethoxynorbornadiene. Removal of pyridine from W(N-t-Bu)(CH-t-Bu)(BiphenCF3)(pyridine) (BiphenCF3 = 3,3′-di-tert-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) with B(C6F5)3 led to formation of a five-coordinate 14e neopentyl complex as a consequence of CH activation in one of the methyl groups in one tert-butyl group of the BiphenCF3 ligand, as was proven in an X-ray study. An attempted synthesis of W(N-t-Bu)(CH-t-Bu)(BiphenMe) (BiphenMe = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) led to formation of a 1:1 mixture of W(N-t-Bu)(CH-t-Bu)(BiphenMe) and a neopentyl complex analogous to the one characterized through an X-ray study. The metallacyclobutane complexes W(N-t-Bu)(C3H6)(pyrrolide)(ODFT) and W(N-t-Bu)(C3H6)(ODFT)2 were prepared in reactions involving W(N-t-Bu)(CH-t-Bu)(pyr)2(bipy), ZnCl2(dioxane), and one or two equivalents of DFTOH, respectively, under 1 atm of ethylene

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Formation of Alternating trans-A-alt-B Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Author: Hyangsoo Jeong (1502629)
Jeremy M. John (1502626)
Richard R. Schrock (1295601)
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Ring-opening metathesis polymerization (ROMP) is used to prepare trans-poly(A-alt-B) polymers from a 1:1 mixture of A and B where A is a cyclic olefin such as cyclooctene (A1) or cycloheptene (A2) and B is a large norbornadiene or norbornene derivative such as 2,3-dicarbomethoxy-7-isopropylidenenorbornadiene (B1) or dimethylspirobicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate-7,1′-cyclopropane (B2). The most successful initiators that were examined are of the type Mo(NR)(CHCMe2Ph)[OCMe(CF3)2]2 (R = 2,6-Me2C6H3 (1) or 2,6-i-Pr2C6H3 (2)). The trans configuration of the AB linkages is proposed to result from the steric demand of B. Both anti-MB and syn-MB alkylidenes are observed during the copolymerization, where B was last inserted into a MoC bond, although anti-MB dominates as the reaction proceeds. anti-MB is lower in energy than syn-MB, does not react readily with either A or B, and interconverts slowly with syn-MB through rotation about the MoC bond. Syn-MB does not readily react with B, but it does react slowly with A (rate constant ∼1 M–1 s–1) to give anti-MA and one trans-AB linkage. anti-MA then reacts with B (rate constant ∼300 M–1 s–1 or larger) to give syn-MB and the second trans-AB linkage. The reaction has been modeled using experimental data in order to obtain the estimated rate constants above. The reaction between anti-MA and A is proposed to give rise to AA linkages, but AA dyads can amount to <5%. Several other A and B monomers, initiators, and conditions were explored

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Correction to “Z‑Selective Metathesis Homocoupling of 1,3-Dienes by Molybdenum and Tungsten Monoaryloxide Pyrrolide (MAP) Complexes”

Author: Amir H. Hoveyda (1306548)
Erik M. Townsend (1338657)
Richard R. Schrock (1295601)
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Correction to “Z‑Selective Metathesis Homocoupling of 1,3-Dienes by Molybdenum and Tungsten Monoaryloxide Pyrrolide (MAP) Complexes

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Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

Author: Laura C. H. Gerber (1440232)
Peter Müller (1295607)
Richard R. Schrock (1295601)
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Molybdenum and tungsten bispyrrolide alkylidene complexes that contain a 2,6-dimesitylphenylimido (NAr*) ligand have been prepared, in which the pyrrolide is the parent pyrrolide or 2,5-dimethylpyrrolide. Monoalkoxide pyrrolide (MAP) complexes were prepared through addition of 1 equiv of an alcohol to the bispyrrolide complexes. MAP compounds that contain the parent pyrrolide (NC4H4–) are pyridine adducts, while those that contain 2,5-dimethylpyrrolide are pyridine free. Molybdenum and tungsten MAP 2,5-dimethylpyrrolide complexes that contain O-t-Bu, OCMe(CF3)2, or O-2,6-Me2C6H3 ligands were found to have approximately equal amounts of syn and anti alkylidene isomers, which allowed a study of the interconversion of the two employing 1H–1H EXSY methods. The Keq values ([syn]/[anti]) are all 2–3 orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of interconversion of syn and anti isomers were found to be 1–2 orders of magnitude faster for W MAP complexes than for Mo MAP complexes

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Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

Author: Laura C. H. Gerber (1440232)
Peter Müller (1295607)
Richard R. Schrock (1295601)
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Tungsten Oxo Alkylidene Complexes as Initiators for the Stereoregular Polymerization of 2,3-Dicarbomethoxynorbornadiene

Author: Jonathan C. Axtell (1611628)
Richard R. Schrock (1295601)
William P. Forrest (1338660)
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We have employed 2,3-dicarbomethoxynorbornadiene (DCMNBD) as a monomer to explore new tungsten oxo alkylidene complexes as initiators for stereoregular ROMP (ring-opening metathesis polymerization). The initiators include MAP (monoaryloxide pyrrolide) oxo alkylidene complexes with the general formula W(O)(CHCMe2Ph)(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide, OAr = an aryloxide) and W(O)(CHCMe2Ph)(OR)2 (OR = an aryloxide or OC(CF3)3), or PPh2Me or CH3CN adducts thereof. We have found that MAP initiators yield cis,syndiotactic-poly(DCMNBD) as a consequence of stereogenic metal control. In contrast, W(O)(CHCMe2Ph)(OR)2(L) initiators (where L = PPh2Me or acetonitrile) are strongly biased toward formation of cis,isotactic structures, while W(O)(CHCMe2Ph)(OR)2 initiators are strongly biased toward formation of cis,syndiotactic structures. Addition of B(C6F5)3 to W(O)(CHCMe2Ph)(Me2Pyr)(OR) species leads to a dramatic increase in the rate of polymerization and to an increase in the cis,syndiotacticity of the polymer (if not already high), while addition of B(C6F5)3 to W(O)(CHCMe2Ph)(OR)2 initiators leads to a dramatic increase in the rate of polymerization and to the formation of highly cis,syndiotactic polymers. All evidence supports the proposal that 16e W(O)(CHCMe2Ph)(OR)2(L) complexes can operate either through loss of L to yield 14e W(O)(CHCMe2Ph)(OR)2 species (which yield largely cis,syndiotactic-poly(DCMNBD)) or by directly reacting with DCMNBD to yield an 18e intermediate and largely cis,isotactic-poly(DCMNBD). All polymerizations by W(O)(CHCMe2Ph)(OR)2(L) and W(O)(CHCMe2Ph)(OR)2 initiators are proposed to operate through some version of chain end control

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Synthesis of Stereoregular Polymers through Ring-Opening Metathesis Polymerization

Synthesis of Tungsten Oxo Alkylidene Complexes

Synthesis of Tungsten Oxo Alkylidene Complexes

Synthesis of Methylidene Complexes that Contain a 2,6-Dimesitylphenylimido Ligand and Ethenolysis of 2,3-Dicarbomethoxynorbornadiene

Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a <i>tert</i>-Butylimido Ligand

Formation of Alternating <i>trans</i>-<b>A</b>-<i>alt</i>-<b>B</b> Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Correction to “<i>Z</i>‑Selective Metathesis Homocoupling of 1,3-Dienes by Molybdenum and Tungsten Monoaryloxide Pyrrolide (MAP) Complexes”

Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP) Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido Ligand

Tungsten Oxo Alkylidene Complexes as Initiators for the Stereoregular Polymerization of 2,3-Dicarbomethoxynorbornadiene