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Ch 14 Organometallic Catalysis

Ch 14 Organometallic Catalysis. Catalysis Basics Catalyst speeds up a reaction by changing E a , but is not used up itself Commercial Importance: conversion of cheap feedstocks (coal, oil, water) into more useful molecules Stoichiometric Reactions are not as economical

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Ch 14 Organometallic Catalysis

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  1. Ch 14 Organometallic Catalysis • Catalysis Basics • Catalyst speeds up a reaction by changing Ea, but is not used up itself • Commercial Importance: conversion of cheap feedstocks (coal, oil, water) into more useful molecules • Stoichiometric Reactions are not as economical • Some reactions are just too slow to be useful without catalysts • Forms of Catalysts • Heterogeneous = solid material (Pt/C, Raney Ni) with reactive surfaces • Homogeneous = soluble molecular species • Easier to study • Easier to modify • Representative Catalytic Processes • Catalytic Deuteration: (Cp)2TaD is the catalytic species

  2. Reductive Elimination Oxidative Addition Reductive Elimination Oxidative Addition Reductive Elimination

  3. Hydroformylation = Oxo Process • Industry: CH3CH=CH2 CH3CH2CH2CHO • HCo(CO)3 is the catalytic species

  4. HRh(CO)2(PPh3)2 is faster, more selective catalyst for Hydroformylation Rh(I), d8, 18 e- species Dissociation of CO Rh(I), d8, 16 e- species Addition of alkene ligand Rh(I), d8, 18 e- species 1,2-Insertion Rh(I), d8, 16 e- species Addition of CO ligand Rh(I), d8, 18 e- species Alkyl Migration Rh(I), d8, 16 e- species Oxidative Addition Rh(III), d6, 18 e- species Reductive Elimination of Product, Reforming Catalyst

  5. Monsanto Acetic Acid Process

  6. Monsanto Acetic Acid Process: Alternative View

  7. Wacker (Smidt) Process Substitution Substitution 1,2-Insertion

  8. Hydrogenation by Wilkinson’s Catalyst Rh(I), d8, 16 e- species Rh(III), d6, 18 e- species Rh(III), d6, 16 e- species Rh(III), d6, 18 e- species Oxidative Addition Rh(III), d6, 16 e- species Rh(I), d8, 14 e- species

  9. Steric bulk slows down this reaction • Selective for least hindered alkene

  10. Olefin Metathesis (Nobel Prize 2005: Grubbs, Schrock, Chauvin) • Metathesis = double replacement reaction • Several Mechanisms Have Been Proposed

  11. Evidence for/against Alkyl Exchange • Reacting H3C—HC=CH—CH3 and D3C—DC=CD—CD3 • =CH—CD3 and =CD—CH3 products would be expected if this mechanism was correct: it’s not • None of these “mixed” products were observed • “Diolefin” or “Pairwise” Mechanism • Both alkenes (olefins) coordinate the metal ion • A cyclobutane-like intermediate forms • The cyclobutane decomposes to the new alkenes

  12. Evidence for/against Pairwise Mechanism • The kinetic (early) product mixture of the following reaction should contain primarily product A • Two alkenes have to bind prior to reaction • The thermodynamic (late) product mixture should contain a statistical mixture of A, B, and C • Result: the statistical mixture was found at all time periods (early or late), which is not consistent with this Pairwise Mechanism • Carbene/non-pairwise mechanism (Chauvin Mechanism) • A metal carbene forms first • Only one alkene is needed to initiate reaction • The key intermediate is a metallocyclobutane • This mechanism expects an equilibrium mixture of products at all times • Results above are consistent with this mechanism

  13. Additional early/late experiments confirm: • Carbene complex shown to undergo metathesis • Schrock Metathesis Catalyst • Most reactive (fastest) and effective • Highly sensitive to H2O and O2, which reduces their usefulness for many reactions • Can buy commercially • If M = Mo and R = isopropyl = Schrock’s Catalyst

  14. Grubb’s Metathesis Catalyst • Less active (slower) than Schrock • Less sensitive to water and oxygen • Less expensive because Ru << Mo in expense Grubbs I: less stable, slower, still the best for some reaction Grubbs II: more stable, less sensitive to water and air, generally faster Grubb-Hoveyda: can be modified to even work in water (green chemistry) • Applications of Methathesis Catalysts a) Ring Closing Metathesis (RCM) Grubbs I Grubbs II Grubbs-Hoveyda

  15. b) Ring Opening Metathesis Polymerization (ROMP)

  16. Alkyne Methathesis is also possible • Heterogeneous Catalysts 1) Very important commercially

  17. Mechanisms are very difficult to decipher • Zeigler-Natta Polymerizations • Aluminum Alkyls + Ti catalysts polymerized olefins • Two major proposed mechanisms appear to be valid in different cases

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