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Química Organometálica e Catálise · Catalytic cycles

Catálise homogênea industrial

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O que você vai aprender

  • Walk through the catalytic cycles of alkene and ketone hydrogenation (Wilkinson, Knowles, Noyori), hydroformylation (Co, Rh) and methanol carbonylation (Monsanto, Cativa), tracking the metal's oxidation state and electron count.
  • Draw nucleophilic attack on a coordinated alkene or CO ligand, as in the Wacker oxidation, vinyl acetate synthesis and the water–gas shift, and explain how the metal is reoxidised or regenerated.
  • Predict the products and regiochemistry of hydroformylation, Wacker oxidation, hydrocyanation, hydrosilylation and ethylene oligomerisation from the elementary steps involved.
  • Compare industrial catalysts and processes, including Haber–Bosch and Fischer–Tropsch, by activity, selectivity, conditions, atom economy and turnover.

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Hydrogenation: Wilkinson, Knowles and Noyori

Wilkinson's catalyst, RhCl(PPh₃)₃ (Rh(I), d⁸, 16 electrons), loses a phosphine, adds H₂ to become a Rh(III) dihydride, binds the alkene, inserts it into Rh–H and releases the alkane by C–H reductive elimination. Both hydrogens reach the same face of the alkene (syn addition). Unhindered alkenes react fastest: monosubstituted and cis-disubstituted alkenes are reduced quickly, trisubstituted ones much more slowly and tetrasubstituted ones hardly at all. Ethylene is the exception: it binds rhodium so strongly that it blocks the cycle. In the Knowles L-DOPA process, a cationic rhodium complex of the chiral diphosphine DiPAMP hydrogenates an enamide. Here the substrate binds first, chelating through its C=C and its amide oxygen; H₂ then adds, the hydride goes to the β-carbon, and C–H reductive elimination at the α-carbon sets the stereocentre. With the matching DiPAMP enantiomer, the model enamide gives N-acetyl-(S)-phenylalanine methyl ester (shown). Surprisingly, the minor, less stable catalyst–substrate diastereomer reacts with H₂ far faster and so gives the major enantiomer. Noyori's Ru(II) diphosphine–diamine catalysts hydrogenate ketones without the ketone binding to the metal. A Ru–H hydride and an N–H proton are delivered together to the C=O group, so C=C bonds in the same molecule survive.

Hydroformylation with cobalt and rhodium

Hydroformylation adds H and CHO across a C=C bond. Propene, CO and H₂ give butanal (linear, n) and 2-methylpropanal (branched, iso), both shown. The first catalyst, HCo(CO)₄ (Co(I), d⁸, 18 electrons), must lose a CO to give 16-electron HCo(CO)₃ before the alkene can bind. It needs about 140–180 °C and 200–300 bar, the high CO pressure keeping the carbonyl hydride from decomposing, and gives n : iso about 4 : 1 for propene. Cobalt hydrides also isomerise alkenes by β-hydride elimination and re-insertion, so internal alkenes still give much linear aldehyde. Rhodium with excess PPh₃, HRh(CO)(PPh₃)₃, runs at about 100 °C and 10–20 bar, is roughly a thousand times more active, and gives n : iso about 10 : 1 because the bulky phosphines favour rhodium on the terminal carbon. The cycle is the same for both metals: ligand loss, alkene insertion into M–H, CO binding, migratory insertion of CO into the M–alkyl bond, H₂ oxidative addition and C–H reductive elimination of the aldehyde.

Acetic acid: Monsanto and Cativa

Methanol and CO give acetic acid with 100% atom economy. Iodide shuttles the methyl group: HI converts methanol into CH₃I, and cis-[Rh(CO)₂I₂]⁻ (Rh(I), d⁸, 16 electrons) adds CH₃I by an SN2-type oxidative addition to give [CH₃Rh(CO)₂I₃]⁻ (Rh(III), 18 electrons). The methyl then migrates to a cis CO, CO binds, and reductive elimination releases acetyl iodide, which water hydrolyses to acetic acid and HI. In the rhodium (Monsanto) process the oxidative addition is rate-determining: the rate is first order in rhodium and in CH₃I and does not depend on the CO pressure. The iridium (Cativa) process uses [Ir(CO)₂I₂]⁻. Its oxidative addition of CH₃I is more than a hundred times faster, so the slow step moves to methyl migration. In anionic [CH₃Ir(CO)₂I₃]⁻ the migration is slow; losing an iodide, helped by promoters such as ruthenium carbonyl iodides that take up I⁻, and binding a third CO gives neutral CH₃Ir(CO)₃I₂, in which migration is much faster. Iridium also stays active at lower water concentration, which cuts the water–gas shift side reaction and the cost of drying the product.

Wacker oxidation: nucleophiles attack coordinated alkenes

Palladium(II) withdraws electron density from a coordinated alkene and makes it electrophilic. In the Wacker process ethylene binds to [PdCl₄]²⁻ in water. Water attacks one alkene carbon while palladium bonds to the other (hydroxypalladation); β-hydride elimination and re-insertion then move palladium onto the oxygen-bearing carbon, and loss of a proton and of Pd(0) gives acetaldehyde. All four hydrogens of the acetaldehyde come from ethylene: in D₂O no deuterium enters the product, so free vinyl alcohol is not an intermediate. Two CuCl₂ reoxidise Pd(0) to Pd(II), and O₂ reoxidises the CuCl, so the net reaction is C₂H₄ + ½ O₂ → CH₃CHO. With higher alkenes (the Tsuji–Wacker oxidation) water attacks the more substituted carbon, and a terminal alkene gives a methyl ketone. Acetate in place of water gives vinyl acetate, because the β-hydride elimination now forms an enol ester, which has no O–H to lose.

Adding H–CN and H–Si across alkenes

Nickel(0) aryl phosphite complexes add HCN oxidatively to give H–Ni(II)–CN. Butadiene inserts into Ni–H to give a methylallyl (crotyl) nickel cyanide, and C–CN reductive elimination at either allyl terminus gives linear 3-pentenenitrile or branched 2-methyl-3-butenenitrile. The branched nitrile is isomerised to the linear one, the linear one is isomerised to 4-pentenenitrile, and a second hydrocyanation, promoted by a Lewis acid that binds the nitrogen of the nickel-bound cyanide, adds HCN anti-Markovnikov to give adiponitrile (shown), the precursor of hexane-1,6-diamine for nylon-6,6. Platinum catalysts (Speier's H₂PtCl₆, Karstedt's Pt(0) complex) add Si–H across alkenes by the Chalk–Harrod cycle: Si–H oxidative addition to Pt(0), insertion of the alkene into Pt–H with platinum on the terminal carbon, and Si–C reductive elimination. The silicon therefore ends up on the terminal carbon (anti-Markovnikov).

Oligomerisation, water–gas shift, Haber–Bosch and Fischer–Tropsch

In the Shell higher olefin process (SHOP) a nickel(II) hydride with a P,O chelate inserts ethylene repeatedly into Ni–H and Ni–alkyl bonds. β-Hydride elimination at any stage releases a linear 1-alkene and returns Ni–H. Because each alkyl faces nearly the same choice between growth and elimination at every length, the products form a broad (Schulz–Flory) distribution of chain lengths. The water–gas shift, CO + H₂O → CO₂ + H₂, raises the H₂ content of syngas. In a homogeneous model, hydroxide attacks a CO ligand of Fe(CO)₅, the metallacarboxylate loses CO₂ to leave [HFe(CO)₄]⁻, and protonation and H₂ loss close the cycle. The largest processes use solid catalysts. Haber–Bosch ammonia synthesis, N₂ + 3 H₂ ⇌ 2 NH₃ (ΔH ≈ −92 kJ/mol), runs over promoted iron at about 400–500 °C and 150–300 bar; dissociative chemisorption of N₂ is the slow step. Fischer–Tropsch synthesis turns syngas into mainly linear alkanes and 1-alkenes over cobalt or iron by stepwise growth of surface-bound chains. Iron also catalyses the water–gas shift and so suits H₂-poor, coal-derived syngas.

What to carry forward

Every industrial cycle is a sequence of the same few steps: ligand loss and binding, oxidative addition, migratory insertion, β-hydride elimination, nucleophilic attack on a coordinated ligand and reductive elimination. Track the metal's oxidation state and electron count around the cycle, find the slow step, and ask what the ligands, promoters and co-catalysts do: bulky phosphines set linear selectivity, chiral ligands set the enantiomer, iodide shuttles the methyl group, CuCl₂ and O₂ reoxidise palladium, and Lewis acids speed reductive elimination.

Problemas de prática (40)

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Desenhe o mecanismo 10

Hydroformylation and carbonylation

  • Palladium-catalysed methoxycarbonylation of ethyleneDraw the palladium hydride cycle that turns ethylene, CO and methanol into methyl propanoate: insertion, CO binding, migratory insertion, methanolysis and reprotonation of Pd(0).
  • Cativa process: promoted methyl migration to the acetyliridium anionDraw the promoter-assisted iodide loss, CO binding, methyl migration and iodide binding that turn [CH₃Ir(CO)₂I₃]⁻ into the acetyl anion [CH₃C(O)Ir(CO)₂I₃]⁻ in the Cativa process, with iridium staying Ir(III), and predict how acetyl iodide is then released to return iridium to Ir(I).
  • Cobalt-catalysed hydroformylation of but-2-ene to pentanalDraw how a cobalt hydride moves the double bond of but-2-ene to the chain end by β-hydride elimination and re-insertion, then completes hydroformylation to the linear aldehyde pentanal.
  • Water–gas shift catalysed by Fe(CO)₅ and hydroxideDraw the homogeneous water–gas shift cycle: hydroxide attack on a coordinated CO, decarboxylation to an iron hydride, protonation at iron and H–H reductive elimination.

Wacker oxidation and H–X additions

  • Platinum-catalysed hydrosilylation of hex-1-eneDraw the Chalk–Harrod cycle: Si–H oxidative addition to Pt(0), alkene insertion into Pt–H and Si–C reductive elimination to the terminal alkylsilane.
  • Nickel-catalysed hydrocyanation of butadiene to pent-3-enenitrileDraw HCN oxidative addition to Ni(0), butadiene insertion into Ni–H to form an allyl nickel cyanide, the allyl shift, and C–CN reductive elimination to the linear pentenenitrile.
  • Palladium-catalysed synthesis of vinyl acetate from ethyleneDraw the acetoxypalladation of ethylene, the β-hydride elimination that releases vinyl acetate, and the deprotonation that leaves Pd(0).
  • Wacker oxidation of ethylene to acetaldehydeDraw the Wacker oxidation of ethylene: attack of water on the palladium-bound alkene, β-hydride elimination, re-insertion, and loss of a proton and Pd(0) to give acetaldehyde.

Hydrogenation catalysts

  • Rhodium-catalysed asymmetric hydrogenation of an enamideDraw H₂ oxidative addition, alkene insertion and C–H reductive elimination for a chelating enamide at cationic rhodium, the route used in the Knowles L-DOPA process.

Bulk processes and process metrics

  • Nickel-catalysed ethylene oligomerisation (SHOP): growth and chain releaseDraw two ethylene insertions into a nickel hydride and the β-hydride elimination that releases a linear 1-alkene and regenerates Ni–H.

Preveja o produto 6

Hydrogenation catalysts

  • Which bond does a Noyori catalyst reduce in an enone?Predict the chemoselectivity of Ru diphosphine–diamine hydrogenation toward an α,β-unsaturated ketone.
  • Which C=C does Wilkinson's catalyst reduce in carvone?Predict which alkene of a diene is hydrogenated by Wilkinson's catalyst from the substitution of each C=C bond.
  • D₂ addition to dimethyl maleatePredict the diastereomer formed when Wilkinson's catalyst adds D₂ to a cis alkene, using syn addition.

Bulk processes and process metrics

  • What does the oxo-aldol-hydrogenation sequence make?Predict the product of linear hydroformylation followed by aldol condensation and hydrogenation, as in the industrial plasticiser-alcohol route.

Wacker oxidation and H–X additions

  • Product of platinum-catalysed hydrosilylationPredict the regiochemistry of platinum-catalysed hydrosilylation of a terminal alkene.
  • Product of a Tsuji–Wacker oxidationPredict the regiochemistry of the Wacker oxidation of a terminal alkene.

Classificar 1

  • What kind of step is hydroxypalladation?Classify the outer-sphere attack of water on palladium-bound ethylene among the elementary steps of organometallic chemistry.

Comparar 4

Hydroformylation and carbonylation

  • What changes when iridium replaces rhodium?Compare the rate-determining steps of the rhodium and iridium acetic acid processes and explain the role of the iridium promoters.
  • Cobalt or rhodium for propene hydroformylation?Compare unmodified cobalt and rhodium–phosphine hydroformylation catalysts by conditions, selectivity and cost.

Bulk processes and process metrics

  • Matching Fischer–Tropsch catalysts to syngasCompare cobalt and iron Fischer–Tropsch catalysts and match each to a syngas source by its water–gas shift activity.

Hydrogenation catalysts

  • Rank alkenes for Wilkinson hydrogenationRank alkenes by their rate of hydrogenation with Wilkinson's catalyst from their degree of substitution.

Calcular 8

Bulk processes and process metrics

  • Atom economy of acetaldehyde from ethanolCalculate the atom economy of making acetaldehyde by ethanol oxidation and compare it with the Wacker route.
  • Syngas consumed by iron Fischer–TropschBalance a Fischer–Tropsch reaction in which oxygen leaves as CO₂ and calculate the H₂ : CO consumption ratio.
  • Methane needed per mole of ammoniaCombine steam reforming, the water–gas shift and ammonia synthesis stoichiometry to find the methane needed per mole of ammonia.
  • Turnover frequency of a carbonylation catalystCalculate a turnover frequency in s⁻¹ from moles of product, moles of metal and reaction time.

Hydrogenation catalysts

  • Enantiomeric excess of an asymmetric hydrogenationCalculate the enantiomeric excess from the percentages of the two enantiomers in a hydrogenation product.

Hydroformylation and carbonylation

  • Count the active cobalt hydroformylation catalystCalculate the valence electron count of the coordinatively unsaturated cobalt hydride that binds the alkene in cobalt hydroformylation.
  • Linear-to-branched ratio in propene hydroformylationCalculate the linear-to-branched (n : iso) aldehyde ratio from a hydroformylation product composition.

Wacker oxidation and H–X additions

  • Copper bookkeeping in the Wacker processCalculate how much copper(II) is reduced per turnover of the Wacker oxidation from the electron count of the palladium step.

Encontre o erro 1

  • Why is ethylene not reduced?Diagnose why an unhindered alkene stalls Wilkinson hydrogenation by binding too strongly to rhodium.

Interprete as evidências 2

Hydroformylation and carbonylation

  • Find the slow step of the Monsanto cycleIdentify the rate-determining step of rhodium-catalysed methanol carbonylation from its rate law and resting state.

Wacker oxidation and H–X additions

  • What does the D₂O experiment show?Interpret a deuterium-labelling experiment to rule out free vinyl alcohol as a Wacker intermediate.

Trabalhe de trás para frente 1

  • Which alkene gives 3-methylbutanal?Work back from an aldehyde to the alkene that gives it as the main hydroformylation product.

Planeje uma síntese 2

Wacker oxidation and H–X additions

  • From butadiene to the nylon diamineSequence the steps of the nickel-catalysed route from butadiene to hexane-1,6-diamine.

Hydroformylation and carbonylation

  • Order the steps of the Monsanto acetic acid cycleSequence the organic and rhodium steps that turn methanol and CO into acetic acid in the Monsanto process.

Explique por quê 5

Bulk processes and process metrics

  • The slow step of ammonia synthesisIdentify the rate-determining step of Haber–Bosch ammonia synthesis on iron and relate it to bond strength.

Hydrogenation catalysts

  • How can the minor complex give the major product?Explain, with the Curtin–Hammett principle, why the minor catalyst–substrate diastereomer controls enantioselectivity in rhodium enamide hydrogenation.
  • How does a Noyori catalyst hydrogenate a ketone?Describe the outer-sphere, bifunctional hydrogen transfer of Ru diphosphine–diamine ketone hydrogenation.

Wacker oxidation and H–X additions

  • Why add a Lewis acid to the second hydrocyanation?Explain how a Lewis acid bound to the cyanide ligand accelerates C–CN reductive elimination in nickel-catalysed hydrocyanation.
  • Why add copper(II) chloride to the Wacker process?Explain the role of the CuCl₂/O₂ couple in making the Wacker oxidation catalytic in palladium.

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