Ch 14 Organometallic Catalysis
I.
II.
Catalysis Basics
A.
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
Representative Catalytic Processes
A.
Catalytic Deuteration: (Cp)2TaD is the catalytic species
Reductive Elimination
Oxidative Addition
Reductive Elimination
Oxidative Addition
Reductive Elimination
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
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
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
Rh(I), d8, 16 e- species
Rh(III), d6, 18 e- species
Rh(III), d6, 16 e- species
Rh(III), d6, 18 e- species
Rh(III), d6, 16 e- species
Rh(I), d8, 14 e- species
1)
2)
Steric bulk slows down this reaction
Selective for least hindered alkene
E.
Olefin Metathesis (Nobel Prize 2005: Grubbs, Schrock, Chauvin)
1)
Metathesis = double replacement reaction
H2C
CHR
+
H2C
2)
CHR
CH2
CH2
+
CHR
CHR
Several Mechanisms Have Been Proposed
3)
Evidence for/against Alkyl Exchange
a)
b)
c)
4)
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
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)
6)
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) A metal carbene forms first
b) Only one alkene is needed to initiate reaction
c) The key intermediate is a metallocyclobutane
d)
e)
This mechanism expects an equilibrium mixture of
products at all times
Results above are consistent with this mechanism
7)
f)
Additional early/late experiments confirm:
g)
Carbene complex shown to undergo metathesis
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)
Grubbs I
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 II
Grubbs-Hoveyda
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)
b) Ring Opening Metathesis Polymerization (ROMP)
5)
G.
Alkyne Methathesis is also possible
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