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Química Organometálica e Catálise · Catalytic cycles

Metátese de olefinas e polimerização por coordenação

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O que você vai aprender

  • Draw the Chauvin mechanism of olefin metathesis as a [2+2] cycloaddition to a metallacyclobutane followed by cycloreversion, and use it to predict RCM, cross-metathesis, enyne and ROMP products.
  • Compare Grubbs ruthenium and Schrock molybdenum catalysts by electron count, oxidation state, initiation and functional-group tolerance.
  • Explain how strain, ethylene loss, concentration and statistics decide the outcome and the E/Z geometry of metathesis reactions, and calculate molar masses of living ROMP polymers.
  • Draw Cossee–Arlman migratory insertion at a d⁰ metal, explain how MAO activates a metallocene and how catalyst symmetry sets polypropylene tacticity.
  • Draw β-hydride elimination, β-hydride transfer to monomer and hydrogenolysis as chain-transfer steps and use their rates to predict chain length, end groups and dispersity.

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The Chauvin mechanism

Olefin metathesis swaps the alkylidene halves of two alkenes: RCH=CHR + R′CH=CHR′ ⇌ 2 RCH=CHR′. The catalyst is a metal alkylidene, M=CHR. Its M=C π bond and the alkene C=C π bond combine in a [2+2] cycloaddition to give a four-membered metallacyclobutane (shown). The ring then splits the other way, a retro-[2+2] cycloreversion, releasing a new alkene and a new alkylidene. A direct thermal [2+2] between two C=C bonds is symmetry-forbidden; the d orbital in the M=C π bond makes the metal version easy. Every step is reversible, and the metal alkylidene carries one carbon fragment from one alkene to the next. Labelling proves this chain process: ring-closing a 1:1 mixture of 1,7-octadiene and its CD₂-terminated isomer gives C₂H₄, C₂H₂D₂ and C₂D₄ in a statistical 1:2:1 ratio. In the drawings the metal is shown bare, without its ligands, as a bookkeeping atom.

Grubbs and Schrock catalysts

First-generation Grubbs catalyst, RuCl₂(PCy₃)₂(=CHPh), is a 16-electron Ru(II) complex when the alkylidene is counted as a neutral 2-electron carbene (neutral method: 8 + 2 × 1 + 2 × 2 + 2 = 16). It initiates by losing one PCy₃ to give a 14-electron species that binds the alkene, so added PCy₃ slows it. The ruthenacyclobutane is formally Ru(IV) and still 14 electrons. The second-generation catalyst replaces one PCy₃ with an N-heterocyclic carbene (H₂IMes). It loses PCy₃ more slowly, so it initiates more slowly, but it propagates more efficiently because PCy₃ rebinding to its 14-electron intermediate competes less well with alkene binding. Third-generation catalysts carry labile pyridines and initiate very fast. Schrock catalysts, Mo(=NAr)(=CHCMe₂Ph)(OR)₂, are Mo(VI) d⁰ alkylidenes: more active, but oxophilic and sensitive to air, water, alcohols and aldehydes. Selectivity for alkenes over polar groups rises from titanium to molybdenum to ruthenium; ruthenium catalysts tolerate water, alcohols and acids.

Ring-closing, cross and enyne metathesis

In ring-closing metathesis (RCM) the two terminal alkenes of a diene join and the terminal CH₂ groups leave as ethylene. Escape of the gas, and one molecule becoming two, drive the reaction. Count ring size from one internal alkene carbon to the other: diethyl diallylmalonate gives a cyclopentene (shown) and loses 28 g/mol, an atom economy of 212.2/240.3 ≈ 88%. Rings up to about seven atoms can only hold a Z alkene. Large rings need high dilution (about 1–5 mM) so that ring closure beats intermolecular metathesis, which would give oligomers or, with a neat diene under vacuum, an ADMET polymer. In cross-metathesis two terminal alkenes of equal reactivity give a statistical 1:2:1 mixture of AA, AB and BB, only 50% cross product; pairing a reactive terminal alkene with an acrylate, which hardly homodimerises, gives the cross product selectively and mostly as the E isomer. A 1,6-enyne undergoes ring-closing enyne metathesis to a 1-vinylcyclopentene, a 1,3-diene.

ROMP: strain pays for polymerization

Ring-opening metathesis polymerization (ROMP) opens a cyclic alkene and adds it to the growing alkylidene chain end; ring strain is the driving force. Approximate strain energies are cyclohexene ≈ 10 kJ/mol (no ROMP), cis-cyclooctene ≈ 31, norbornene (shown) ≈ 114 and cyclobutene ≈ 128 kJ/mol. ROMP with fast-initiating ruthenium catalysts is living: every catalyst starts one chain, chains grow at the same rate and do not terminate, so DP = ([M]₀/[Ru]₀) × conversion, Mn = DP × monomer molar mass, and Đ is close to 1. Initiation must be faster than propagation for a narrow distribution. Ethyl vinyl ether ends the polymerization by converting the alkylidene into an inactive Fischer carbene. The unhindered backbone alkenes of polycyclooctene undergo secondary metathesis (backbiting and chain exchange), which broadens Đ towards 2 and releases cyclic oligomers; the hindered backbone alkenes of polynorbornene resist it.

Ziegler–Natta and metallocene catalysts

Heterogeneous Ziegler–Natta catalysts combine TiCl₄ on MgCl₂ with AlEt₃, which alkylates and reduces titanium to make the Ti–C bond. Homogeneous metallocenes such as Cp₂ZrCl₂ are activated by methylaluminoxane (MAO), which methylates zirconium and then removes a methyl anion to give the cation Cp₂ZrMe⁺: Zr(IV), d⁰, 14 electrons (4 + 2 × 5 + 1 − 1), paired with a weakly coordinating aluminate. In the Cossee–Arlman mechanism the alkene binds at the vacant site and the alkyl migrates onto it through a four-centre transition state: a 1,2-migratory insertion that lengthens the chain by two carbons and opens a new vacant site where the alkyl was. A d⁰ metal cannot back-donate, so the alkene is weakly held and inserts quickly. Propene and 1-hexene insert 1,2, with the metal on the CH₂ end; each inserted 1-hexene leaves a butyl branch, the basis of linear low-density polyethylene. A C₂-symmetric ansa-metallocene presents the same enantioface at both sites and gives isotactic polypropylene; a Cs-symmetric catalyst has mirror-image sites and gives syndiotactic polypropylene; unbridged Cp₂ZrCl₂ gives atactic polymer.

Chain transfer and termination

A coordination polymer chain stops growing on one metal when it is transferred, and the metal usually starts a new chain. β-Hydride elimination gives M–H and a vinyl-terminated chain (a vinylidene end after a 1,2-inserted propene). β-Hydride transfer to a coordinated monomer releases the same unsaturated end but hands the hydrogen straight to the monomer; because it and propagation are both first order in monomer, it fixes the molar mass regardless of monomer pressure. Hydrogen cleaves the M–C bond by σ-bond metathesis, since a d⁰ metal cannot undergo oxidative addition; it releases a saturated chain and M–H, so H₂ is used to lower the molar mass. Late metals such as nickel undergo chain transfer easily and give oligomers unless bulky α-diimine ligands block the axial sites. The number-average degree of polymerization equals the propagation rate divided by the sum of the transfer rates. One kind of site gives the most-probable distribution, Đ ≈ 2; heterogeneous catalysts have many kinds of site and give broader distributions.

What to carry forward

Metathesis is a reversible [2+2]/retro-[2+2] cycle through a metallacyclobutane: ethylene loss drives RCM, strain drives ROMP, statistics limit simple cross-metathesis, and the late, tolerant ruthenium catalysts trade some activity for robustness. Coordination polymerization is repeated 1,2-migratory insertion at a cationic d⁰ metal: catalyst symmetry sets tacticity, and the ratio of insertion to β-hydride and hydrogenolysis transfer sets chain length and end groups.

Problemas de prática (40)

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Desenhe o mecanismo 10

Chain transfer and termination

  • Chain release by β-hydride eliminationDraw β-hydride elimination from a titanium polymeryl cation that releases a vinyl-terminated chain, and the ethylene insertion into Ti–H that starts the next chain.
  • Chain transfer by β-hydride transfer to monomerDraw the concerted β-hydride transfer from a titanium polymeryl chain to a coordinated ethylene and use the rate laws to tell it apart from β-hydride elimination.
  • Chain transfer to hydrogen by σ-bond metathesisDraw the four-centre σ-bond metathesis between a titanium polymeryl cation and H₂ that releases a saturated chain, explain why a d⁰ metal cannot use oxidative addition, and draw the restart of chain growth from Ti–H.

Ring-closing and cross-metathesis

  • Cross-metathesis of 1-hexene with methyl acrylateDraw the two metathesis events of a cross-metathesis cycle between a terminal alkene and methyl acrylate, and explain why the product is the E enoate with ethylene as the only by-product.
  • Ring-closing enyne metathesis to a 1-vinylcyclopenteneDraw the ruthenium cycle of a ring-closing enyne metathesis: alkene metathesis, intramolecular [2+2] to a ruthenacyclobutene, ring opening to a vinyl alkylidene and metathesis with ethylene that releases a 1,3-diene.
  • Ring-closing metathesis of diethyl diallylmalonateDraw the full ruthenium catalytic cycle that closes diethyl diallylmalonate to a cyclopentene: two [2+2] cycloadditions and two cycloreversions, releasing ethylene and regenerating the methylidene.

Chauvin mechanism and catalysts

  • Initiation of a Grubbs catalyst by ethyleneDraw the [2+2] cycloaddition of a ruthenium benzylidene with ethylene to a ruthenacyclobutane and the cycloreversion that gives the ruthenium methylidene and styrene.

Ziegler–Natta and metallocene catalysts

  • 1-Hexene insertion makes a butyl branchDraw the 1,2-insertion of 1-hexene into a titanium–alkyl bond followed by an ethylene insertion, and show how the comonomer becomes a butyl branch on a polyethylene backbone.
  • Cossee–Arlman insertion of ethylene at a titanocene cationDraw two successive 1,2-migratory insertions of ethylene into the Ti–C bond of a cationic d⁰ titanocene and explain why each insertion leaves a vacant site for the next monomer.

Ring-opening metathesis polymerization

  • Ring-opening metathesis polymerization of norborneneDraw initiation and one propagation step of the ROMP of norbornene and show how ring opening turns each bicyclic monomer into a cyclopentane-1,3-diyl unit joined by C=C links.

Preveja o produto 6

Ring-closing and cross-metathesis

  • Metathesis of a neat α,ω-dienePredict the outcome of acyclic diene metathesis of a neat terminal diene under vacuum.
  • Ring size from an unsymmetrical dieneCount the ring atoms formed when an unsymmetrical allyl homoallyl ether undergoes ring-closing metathesis.
  • RCM of N,N-diallyl tosylamidePredict the ring formed by ring-closing metathesis of an N,N-diallyl sulfonamide by counting ring atoms between the internal alkene carbons.

Ziegler–Natta and metallocene catalysts

  • Tacticity from a C₂-symmetric metallocenePredict the tacticity of polypropylene from the symmetry of a chiral ansa-metallocene catalyst.

Chain transfer and termination

  • End group of a polypropylene chainPredict the unsaturated end group formed when a 1,2-inserted polypropylene chain is released by β-hydride elimination.

Ring-opening metathesis polymerization

  • Which cycloalkene will not ROMP?Predict which cycloalkene fails to give a polymer by ROMP from its ring strain.

Classificar 3

Chauvin mechanism and catalysts

  • The key intermediate of olefin metathesisIdentify the metallacyclobutane as the intermediate of the Chauvin mechanism and reject the pairwise alternatives.

Ziegler–Natta and metallocene catalysts

  • What MAO does to a zirconoceneIdentify how methylaluminoxane converts a metallocene dichloride into the active cationic alkyl catalyst.
  • The role of the aluminium alkyl in Ziegler–Natta catalysisClassify the function of the trialkylaluminium cocatalyst in a heterogeneous Ziegler–Natta catalyst.

Comparar 3

Chauvin mechanism and catalysts

  • Alkene selectivity of metathesis catalystsRank titanium, molybdenum and ruthenium alkylidenes by their selectivity for alkenes over polar functional groups.
  • Why the second-generation Grubbs catalyst is more activeExplain the higher activity of the NHC-ligated Grubbs catalyst despite its slower phosphine dissociation.

Ring-opening metathesis polymerization

  • Rank monomers by ROMP driving forceOrder cycloalkenes by the enthalpic driving force for ring-opening metathesis polymerization using their ring strain.

Calcular 5

Chain transfer and termination

  • Chain length from competing ratesCalculate the number-average degree of polymerization of a coordination polymer from the propagation rate and two chain-transfer rates.

Chauvin mechanism and catalysts

  • Electron count of the first-generation Grubbs catalystCount the valence electrons of RuCl₂(PCy₃)₂(=CHPh) and relate the count to its initiation by phosphine loss.

Ring-opening metathesis polymerization

  • Molar mass of a living polynorborneneCalculate the number-average molar mass of a living ROMP polymer from the monomer-to-catalyst ratio and conversion.

Ring-closing and cross-metathesis

  • Statistical outcome of cross-metathesisCalculate the statistical fraction of cross product when two terminal alkenes of equal reactivity are cross-metathesized.

Ziegler–Natta and metallocene catalysts

  • Electron count of the active zirconocene cationCount the valence electrons of the cationic metallocene alkyl that propagates olefin polymerization.

Encontre o erro 1

  • Find the flaw in a cyclobutane metathesis mechanismDiagnose why a metal-templated alkene–alkene [2+2] cannot be the metathesis mechanism and what the metal alkylidene changes.

Interprete as evidências 3

Chauvin mechanism and catalysts

  • Labelled ethylene from ring-closing metathesisUse a deuterium-crossover experiment to distinguish the chain alkylidene mechanism from pairwise alkene exchange.

Ring-opening metathesis polymerization

  • Why polycyclooctene has a broad distributionInterpret the broad dispersity and cyclic oligomers from ROMP of a low-strain monomer as evidence of secondary metathesis.

Chain transfer and termination

  • Reading dispersity: single-site versus multi-siteInterpret the dispersities of polyethylenes from metallocene and heterogeneous Ziegler–Natta catalysts in terms of the kinds of active sites.

Trabalhe de trás para frente 1

  • Which diene closes to this tetrahydropyridine?Work backwards from a cyclic alkene to the diene that ring-closing metathesis would close to it.

Planeje uma síntese 1

  • Rescuing a macrocyclic RCMChoose conditions that favour ring-closing metathesis to a macrocycle over intermolecular oligomerization.

Explique por quê 7

Chauvin mechanism and catalysts

  • Why extra phosphine slows a Grubbs catalystExplain the inhibition of a first-generation Grubbs catalyst by added phosphine from its dissociative initiation.

Ziegler–Natta and metallocene catalysts

  • How bulky diimines turn a nickel oligomerization catalyst into a polymerization catalystExplain how axial steric shielding by α-diimine ligands suppresses chain transfer at late-metal polymerization catalysts.
  • Syndiotactic polypropylene from a Cs-symmetric catalystExplain how chain-migratory insertion at a Cs-symmetric metallocene produces syndiotactic polypropylene.
  • What a comonomer does to polyethyleneExplain how 1,2-insertion of an α-olefin comonomer creates short branches that lower polyethylene density.

Ring-opening metathesis polymerization

  • Why fast initiation gives narrow polymersExplain how the ratio of initiation to propagation rate controls the dispersity of a living ROMP polymer.

Chain transfer and termination

  • Why hydrogen lowers polyethylene molar massExplain how H₂ acts as a chain-transfer agent in coordination polymerization.

Ring-closing and cross-metathesis

  • What drives ring-closing metathesis?Explain why ring-closing metathesis of a terminal diene goes to completion although every metathesis step is reversible.

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