Ch 14 Organometallic Catalysis I.Catalysis Basics A.Catalyst speeds up a reaction by changing E a,...

Ch 14 Organometallic Catalysis

I. Catalysis BasicsA. Catalyst speeds up a reaction by changing Ea, but is not

used up itself

1) Commercial Importance: conversion of cheap feedstocks (coal, oil, water) into more useful molecules

2) Stoichiometric Reactions are not as economical

3) Some reactions are just too slow to be useful without catalysts

B. Forms of Catalysts

1) Heterogeneous = solid material (Pt/C, Raney Ni) with reactive surfaces

2) Homogeneous = soluble molecular species

a) Easier to study

b) Easier to modify

II. Representative Catalytic ProcessesA. Catalytic Deuteration: (Cp)2TaD is the catalytic species

Reductive Elimination

Oxidative Addition



Oxidative Addition

B. Hydroformylation = Oxo Process

1) Industry: CH3CH=CH2 CH3CH2CH2CHO

2) HCo(CO)3 is the catalytic species

HRh(CO)2(PPh3)2 is faster, more selective catalyst for Hydroformylation

Dissociation of CO

Rh(I), d8, 18 e- species






Rh(III), d6, 18 e- species

Addition of alkene ligand

1,2-Insertion

Addition of CO ligand

Alkyl Migration

Oxidative Addition

Reductive Elimination of Product, Reforming Catalyst

C. Monsanto Acetic Acid Process

Monsanto Acetic Acid Process: Alternative View

D. Wacker (Smidt) Process

Substitution

Substitution

1,2-Insertion

E. Hydrogenation by Wilkinson’s Catalyst







1) Steric bulk slows down this reaction

2) Selective for least hindered alkene

E. Olefin Metathesis (Nobel Prize 2005: Grubbs, Schrock, Chauvin)

1) Metathesis = double replacement reaction

2) Several Mechanisms Have Been Proposed

CH2

CH2H2C CHR

H2C CHRCHR

CHR+ +

3) Evidence for/against Alkyl Exchange

a) Reacting H3C—HC=CH—CH3 and

D3C—DC=CD—CD3

b) =CH—CD3 and =CD—CH3 products would be expected if this mechanism was correct: it’s not

c) None of these “mixed” products were observed

4) “Diolefin” or “Pairwise” Mechanism

a) Both alkenes (olefins) coordinate the metal ion

b) A cyclobutane-like intermediate forms

c) The cyclobutane decomposes to the new alkenes

5) Evidence for/against Pairwise Mechanism

a) The kinetic (early) product mixture of the following reaction should contain primarily product A

b) Two alkenes have to bind prior to reaction

c) The thermodynamic (late) product mixture should contain a statistical mixture of A, B, and C

c) Result: the statistical mixture was found at all time periods (early or late), which is not consistent with this Pairwise Mechanism

6) Carbene/non-pairwise mechanism (Chauvin Mechanism)

a) A metal carbene forms first

b) Only one alkene is needed to initiate reaction

c) The key intermediate is a metallocyclobutane

d) This mechanism expects an equilibrium mixture of products at all times

e) Results above are consistent with this mechanism

f) Additional early/late experiments confirm:

g) Carbene complex shown to undergo metathesis

7) Schrock Metathesis Catalyst

a) Most reactive (fastest) and effective

b) Highly sensitive to H2O and O2, which reduces their usefulness for many reactions

c) Can buy commercially

d) If M = Mo and R = isopropyl = Schrock’s Catalyst

8) Grubb’s Metathesis Catalyst

a) Less active (slower) than Schrock

b) Less sensitive to water and oxygen

c) 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)

9) Applications of Methathesis Catalysts

a) Ring Closing Metathesis (RCM)

Grubbs I Grubbs II Grubbs-Hoveyda

b) Ring Opening Metathesis Polymerization (ROMP)

5) Alkyne Methathesis is also possible

G. Heterogeneous Catalysts

1) Very important commercially

2) Mechanisms are very difficult to decipher

3) Zeigler-Natta Polymerizations

a) Aluminum Alkyls + Ti catalysts polymerized olefins

b) Two major proposed mechanisms appear to be valid in different cases

Ch 14 Organometallic Catalysis I.Catalysis Basics A.Catalyst speeds up a reaction by changing E a,...

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