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=CH 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. Metathesis = double replacement reaction
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