Chapter 5 Oxidative addition and reductive elimination

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Chapter 9
Homogeneous Catalysis Transition Metal Catalyzed
Reactions
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Outline A. Introduction to catalysis B.
Hydrogenation of unsaturated compounds C.
Reactions involving carbon monoxide D. Oxidation
(Wacker process and related reactions) E.
Metathesis reactions F. Reactions of unsaturated
hydrocarbons involving C-C bond formation G.
Other reactions involving C-C bond formation
References and suggested readings

• The Organometallic Chemistry of the Transition
  Metals, Robert H. Crabtree, 3rd Edition, 2001,
  Chapters 9, 11

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A. Introduction to homogeneous catalysis
Catalysts containing d-block metals are of
immense importance to the chemical industry. In
1990 in the US, the value of chemicals (including
fuels) produced with at least one manufacturing
catalytic step was 890 billion dollars. The
search for new catalysts is one of the major
driving forces behind organometallic
research. Catalysts fall into two categories. A
homogeneous catalyst is in the same phase as the
reactants (normally in solution) a heterogeneous
catalyst is in a different phase (usually solid)
from the reactants. Heterogeneous catalysts are
difficult to characterize. Catalytic mechanisms
are considerably easier to study in solution
where such powerful methods as NMR can be used to
both assign structures and follow reaction
kinetics. In this part, we are going to look at
some homogeneously catalyzed organic reactions.
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What is a catalyst? A catalyst is a substance
which speed up the rate of a reaction without
itself being consumed.
Co2(CO)8 H2 ? HCo(CO)4
Co2(CO)8 is a catalytic precursor for
hydrogenation.
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Why catalytic activity?
The overall activation energy is lowered.
For a reversible reaction, a catalyst alters the
rate at which the?G state of equilibrium is
attained it does not alter the equilibrium
constant. An important property of a catalyst is
its selectivity, i.e. ability to lower only one
reaction pathway to the desired product.
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Catalytic cycle.
A catalytic cycle consists of a sequence of
stoichiometric reactions (normally reversible)
that form a closed loop the catalyst must be
regenerated so that it can participate in the
cycle of reactions more than once.
For efficient catalysis, the intermediates must
have short life times.
The catalytic turnover number (TON) is the number
of moles of product per mole of catalyst it
indicates the number of catalytic cycles, e.g.
one can say after 2 h, the TON is 2400. The
catalytic turnover frequency (TOF) is the number
of moles of product per mole of catalyst per unit
time, e.g. TOF 20 min-1.
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Common reactions
Basic ideas to rationalize metal catalyzed
reaction
18e rule. 18e ltgt 16e 16e ltgt 14e ( for
d8 metals)
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B. Hydrogenation of unsaturated compounds
Many unsaturated compounds can be hydrogenated.
e.g.
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Many transition metal complexes catalyze such
reactions. e.g.
Rh(diene)(PR3)2 RuHCl(PPh3)3,
RuH2(PPh3)4 H2PtCl6 SnCl2 ......
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A). Features 1) Hydrogen is added in a syn
manner.
2). Selectivity. Less hindered olefin (better
ligand) is hydrogenated first
Why?
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B). Major mechanisms
Dihydride mechanism
Let's use Wilkinson's catalyst RhCl(PPh3) as an
example.
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Modification of the mechanism
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Monohydride mechanism
Cat. RhH(PPh3)3.
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Implications of the catalytic cycles. 1). The
metal center should be able to form
2). The H2 is syn added to substrates.
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dihydrogen mechanism
The mechanism of alkene hydrogenation catalyzed
by RuHCl(PPh3)3
RuHCl(PPh3)3 serves usefully in the catalytic
hydrogenation of alkenes, showing very high
selectivity for terminal over internal double
bonds.
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C). Asymmetric hydrogenation. Different optical
isomers show different biological activity. Thus
there is need to develop method to selective
synthesize one of the optical isomers.
Chiral compounds can be synthesized from
asymmetric catalysis. Asymmetric hydrogenation
is one of such type of reactions.
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(-)-carvoneoil of spearmint
()-carvoneoil of caraway
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Basic principle
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if MLn is not chiral
if MLn is chiral
MLn is chiral, if L is chiral. Many chiral
ligands are phosphine ligands.
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Examples of chiral ligands
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Examples
Enantiomeric excess (ee) is defined as
follows ee 100 (R-S)/(RS), where R and S
relative quantities of R and S enantiomers. An
enantiomerically pure compound has 100 ee.
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The first commercial application of this idea was
the synthesis of L-Dopa (by Monsanto) which is
used in the treatment of Parkinsons disease.
This seminal work by William Knowles was
recognized with the Nobel prize in 2001.
The anti-inflammatory drug Naproxen (active in
the S-form) is prepared by asymmetric
hydrogenation. Enantiopurity is essential,
since the R-enantiomer is a liver toxin
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C. Reactions involving carbon monoxide. There are
many such reactions. Here we only discuss
Hydroformylation and related reactions.
Typical catalysts Co2(CO)8, HCo(CO)4,
CoH(CO)x(PR3)4-x RhH(CO)(PPh3)3
RhH(CO)x(PR3)4-x PtCl2(PR3)2 SnCl2
Historically, cobalt compounds were
preferentially employed as the catalysts, because
of their low cost. Today, most new plants use
rhodium complexes, despite the higher cost of
rhodium vs. cobalt Co2(CO)8 500g/290,
RhH(CO)(PPh3)3 5g/300 CoCl2 1000g/430, RhCl3
5g/500
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Typical Mechanism
How is the branched aldehyde is formed?
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Using Co2(CO)8, propene is converted with H2 and
CO into a mixture of butyraldehyde and
iso-butyraldehyde.
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Improving the selectivity of the catalyst.
The linear aldehyde is preferred for some
applications, such as for the synthesis of
biodegradable detergents, and then it is
desirable to suppress the isomerization. It is
found that addition of an alkylphosphine to the
reaction mixture gives much higher selectivity
for the linear product.
The replacement of CO by a bulky ligand disfavors
the formation of complexes of sterically crowded
2-alkanes.
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Related reactions.
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D. Oxidation (Wacker process and related
reactions)
Many terminal olefins can be oxidized similarly.
e.g.
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Mechanism? Let's use the following reaction to
illustrate
Q1. Can one uses PdCl2/CuCl? Q2. If one runs the
reaction in D2O, will D be in the product? Q3.
If one uses 18O2/16OH2, the product contains 18O
or 16O?
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E. Metathesis reactions 1. Olefin metathesis.
Olefin metathesis is the metal-catalyzed
disproportion of alkenes. This reaction has been
known for nearly 50 years, but only in the past
decade has it become widely used in both
industrial and academic labs for the synthesis of
polymers and complex organic compounds. The
2005 Nobel prize was award to Yves Chauvin,
Richard Schrock, and Robert Grubbs for their work
in developing olefin metathesis into a widely
used process.
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There are several classes of olefin metathesis
reactions.
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All of the above reactions are reversible, so
equilibrium mixtures are obtained. To produce
high yields of a given product a suitable driving
force must be present. Cross metathesis
Mixtures of products are produced unless a
volatile byproduct (ethylene) is produced that
can be removed from the reaction mixture. RCM
is favored for the production of unstrained rings
and is driven both entropically and by the
elimination of a volatile alkene. ROM is only
favored at very high olefin concentrations, or
more commonly with strained olefins.
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Many complexes catalyze such reactions. e.g.
WCl5/Bu4Sn , MoCl2(NO)2(PPh3)2/Al2Me3Cl3, M(CO)6
promotors
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Some typical precursors to carbene species
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Mechanism. Take the right reaction as an example
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Propose a mechanism for the right reaction.
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Asymmetric Ring Closing Metathesis
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2. Alkyne metathesis
Carbyne complexes catalyze such reactions,
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Mechanism
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Nitrile Metathesis
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• F. Reactions of unsaturated hydrocarbons
  involving C-C bond formation
• Oligomerization of olefins.
• Many later transition metal complexes catalyze
  dimerization and oligomerization of olefins.

(hfacac)2Ni/PCy3/MAO/EtAlCl2, CrCl33H2O/Ligand/MA
O
The actual catalysts are in fact M-H complexes.
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MAO is a poorly defined mixture of linear and
cyclic oligomers produced by the controlled
hydrolysis of trimethylaluminum. MAO is a magic
ingredient that provided much more active
catalysts than were obtained with typical
alkylaluminum Lewis acids.
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Catalytic cycle for dimerization of olefin.

Why M-alkyl, or M-Ar could also be used?
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How higher oligomers are obtained?
Oligomerization can be explained by multiple
insertion of CH2CH2 before b-H elimination.
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A variety of Nickel catalysts can be used to
oligomerize ethylene into mixtures of higher
alkenes. This chemistry is part of the Shell
Higher Olefin Process (SHOP), which converts
ethylene to alkenes, which can be sold or
converted to long chain aldehydes or alcohols by
hydroformylation. The Ni complex used in the SHOP
process is a P-O anionic chelate that can be
prepared in situ from the ligand and (COD)2Ni.
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2. Polymerization of olefins Many transition
metal (especially early) complexes catalyze
polymerization of olefins.
Ziegler-Natta catalysts TiClx/AlR3, The catalyst
thus formed is a solid of complex constitution.
The Ziegler-Natta catalysts, for which Ziegler
and Natta won the Nobel prize in 1963, account
for more than 15 million tones of polyethylene
and polypropylene annually.
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Homogeneous polymerization catalysts
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Late TM complexes. It was long thought that high
molecular weight polymer could not be produced.
Maurice Brookhart (UNC) has shown that this is
not the case as long as electrophilic late
transition metal complexes are used as the
catalysts. Review Chem. Rev., 2000, 100,
1199-1203
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The active species are M-R, M-H Most likely
mechanism
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3. Polymerization of cyclic olefins
Cyclic olefin can be polymerized by carbene
complexes. e.g.
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mechanism
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Mechanism?
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4. Oligomerization and polymerization of
alkynes a). Trimerization and tetramerization. Ma
ny metal complexes catalyze such reactions. e.g.
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General mechanism
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Pyridine can also be synthesized if R-C?N is
used. e.g.
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b). Polymerization. Acetylenes can be polymerized
by M-alkyl or MC complexes.
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Mechanism?
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G. Other reactions involving C-C bond
formation 1. cross-coupling.

• Cross-coupling of an organometal (R2M) with an
  organic electrophile (R1X) has emerged over the
  past 30 years as one of the most general and
  selective methods for C-C bond formation

Typical catalysts PdL4, PdCl2(L)2, NiL4,
NiCl2(L)2, L PR3
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Industrial production of styrene derivatives
(Hokka Chemical Industry, Japan)
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Mechanism. MX2L2 as catalyst. e.g. Ar-X
RMgBr -----------gt Ar-R MgBrX, cat NiCl2L2
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ML4 catalyzed reactions. e.g. Ar-X RMgBr
-----------gt Ar-R MgBrX, cat PdL4
Mechanism?
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2. C-C bond formation involving oxidative
addition combined with CO insertion
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Mechanism?
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3. C-C bond formation involving olefin insertion
and b-hydrogen elimination. (Heck's reaction).
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Mechanism?