Chapter 5 Oxidative addition and reductive elimination

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Chapter 5
Oxidative addition and reductive elimination
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Outline A. Introduction to oxidative addition
reactions B. Mechanism of oxidative addition
reactions C. Examples of addition of specific
molecules D. Reductive elimination E. Oxidative
coupling
References and suggested readings

• The Organometallic Chemistry of the Transition
  Metals, Robert H. Crabtree, 3rd Edition, 2001,
  Chapter 6.
• Organometallic Chemistry, G. O. Spessard, G. L.
  G. L. Miessler, Prentice-Hall New Jersey, 1997,
  Chapter 7.
• Advanced Inorganic Chemistry, F. A. Cotton, G.
  Wilkinson, John Wiley Sons, 5th ed. 1988,
  Chapter 27, 1186.
• Organotransition Metal Chemistry, Akio Yamamoto,
  1986, Chapter 6.2.

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• Introduction to oxidative addition reactions

Q1. What is an oxidative addition reaction?
Most common cases
Example
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There are a few other types reactions that can be
also regarded as oxidative addition reactions.
a. Electrophilic attack of metals. e.g.
No change in total valence electrons count !
Examples
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b. Addition of XY or X?Y to LnM.
Examples
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c. Binuclear oxidation reactions.
Note no change in valence electron count
Examples
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Q2. What types of complexes LnM can undergo
oxidative addition reactions?
Higher oxidation state is accessible
stable. Availability of vacant site for newly
formed bonds.
Exercise. Can the following complexes undergo
O.A. reactions? TiR4 WR6
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Q2. What types of complexes LnM can undergo
oxidative addition reactions?
Higher oxidation state is accessible
stable. Availability of vacant site for newly
formed bonds.
Exercise. Can the following complexes undergo
O.A. reactions? TiR4 WR6
No, Ti(IV) W(VI)
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Q3. In general, what are the factors affect the
following equilibrium?
Q3A. Rank the order of Keq for the reactions
below.
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Q3. In general, what are the factors affect the
following equilibrium?
Q3A. Rank the order of Keq for the reactions
below.
Order in Keq P(OPh)3 lt PPh3 lt PMe3 Electron-rich
metal center favor oxidative addition product.
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Q3B. Which of the following reactions has a
larger Keq?
In fact, for isostructural d8 complexes
This is the trend for d orbital energy and
ability for back-donation.
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Q3B. Which of the following reactions has a
larger Keq?
(a). The energy of d electron of Ir is relatively
higher.
In fact, for isostructural d8 complexes
This is the trend for d orbital energy and
ability for back-donation.
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Q3C. Which of the following reaction is least
likely?
Weak X-Y bond favors O.A. reaction.
Stable M-X, M-Y bonds favor O.A. reaction.
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Q3C. Which of the following reaction is least
likely?
(c). Pt-CH3 bond is weaker, H3C-CH3 bond is
strong.
Weak X-Y bond favors O.A. reaction.
Stable M-X, M-Y bonds favor O.A. reaction.
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Q4. What bonds can be oxidative added?
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B. Mechanisms of oxidative addition reactions
1). Concerted pathway
or
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Q1. What are the most important features? of
valence e- count of reactant LnM DS gt 0 or
lt 0? Any solvent effect on reaction
rate? Q2. What is the character of substrate
X-Y bonds? X-Y ---gt H-H, R3Si-H, R3C-H,
C-C H-X in nonpolar solvents.
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Q1. What are the most important features? of
valence e- count of reactant LnM 16 DS gt
0 or lt 0? lt 0 Any solvent effect on reaction
rate? no Q2. What is the character of
substrate X-Y bonds? X-Y ---gt non-polar or low
polar, e.g. H-H, R3Si-H, R3C-H, C-C H-X in
nonpolar solvents.
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Q3. Stereochemistry. If via concerted mechanism,
which one should be the product?
Example.

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Q3. Stereochemistry. If via concerted mechanism,
which one should be the product?
B. X, Y have to be cis to each other
Example.

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Q4 If via concerted mechanism, which one should
be the product?
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Q4 If via concerted mechanism, which one should
be the product?

• Sterochemistry of Si should be retained,
• H, Si should be cis to each other.

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2). Non-concerted mechanism
a). SN2 reactions
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18
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Q1. What are the most important features?
of valence e- count of reactant LnM DS gt
0 or lt 0? Any solvent effect on reaction
rate? e.g in benzene, acetone same reaction
rate? Charge separation in TS, reaction is faster
in polar solvent
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Q1. What are the most important features?
of valence e- count of reactant LnM 18 or
less DS gt 0 or lt 0? lt 0 Any solvent
effect on reaction rate? yes e.g in benzene,
acetone same reaction rate? Charge separation in
TS, reaction is faster in polar solvent
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Q2. What kinds of substrates may undergo O.A. via
SN2 mechanism? e.g. X2 Cl2, Br2, I2 R-X
Ar-X, CH3X, PhCH2X, CH2CH-CH2X Q3.
Stereochemistry at M. If via SN2 mechanism, which
product should one get?
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Q2. What kinds of substrates may undergo O.A. via
SN2 mechanism? Good electrophiles that can
produce stable anions. e.g. X2 Cl2, Br2,
I2 R-X Ar-X, CH3X, PhCH2X, CH2CH-CH2X Q3.
Stereochemistry at M. If via SN2 mechanism, which
product should one get?
Both are possible. Experimentally A was obtained.
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General cases
Q4. Stereochemistry at R. If via SN2 mechanism,
what would be the product?
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General cases
Q4. Stereochemistry at R. If via SN2 mechanism,
what would be the product?
B. The configuration at C should be inverted.
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Q5. Relative reaction rate. For the pair of
oxidative addition reactions below, select the
one with highest reaction rate. Assume that the
reactions are though SN2 mechanism.
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Q5. Relative reaction rate. For the pair of
oxidative addition reactions below, select the
one with highest reaction rate. Assume that the
reactions are though SN2 mechanism.
b. The complex is more e- rich and therefore more
nucleophilic.
R- CH3. CH3 has least steric hindrance.
Therefore CH3I is more reactive.
X OTs. OTs- is a better leaving group. In
general, the order of reactivity of R-X
are R-OTs gt R-I gt R-Br gt R-Cl
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b). Other non-concerted process (i) Radical
mechanisms
Some possibilities (ia) Non-chain radical
mechanism, e.g.
Pt(PPh3)3 --------------gt Pt(PPh3)2 PPh3
fast Pt(PPh3)2 R-X ---------gt .PtR(PPh3)2
X. slow .PtR(PPh3)2 X.---------gt
R-PtX(PPh3)2 fast
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b) Chain radical mechanism, e.g.
R-X -----------------gt R. R. IrCl(CO)(PMe3)2
------gt RIrCl(CO)(PMe3)2 RIrCl(CO)(PMe3)2 R-X
----gt RIrXCl(CO)(PMe3)2 R. R. R.
----------gt R2
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Q1. What kinds of substrates may undergo O.A. via
radical mechanism? Simple thought weak X-Y
bonds, radical is stable X2 Cl2, Br2,
I2 R-X Ar-X, CH3X, PhCH2X, CH2CH-CH2X
Binuclear oxidative addition (usually 1e
process). e.g.
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Major difference between free radical and SN2
mechanisms
SN2 Free radical stereochemistry
at carbon Relative reactivity RCl, RBr, RI,
ROTs reactivity of R-X MeX, 1o,2o,3o
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Major difference between free radical and SN2
mechanisms
SN2 Free radical stereochemistry
at inverted lost carbon Relative
reactivity R-OTs gt R-I R-I gt R-Br RCl, RBr,
RI, ROTs gtR-Br gt R-Cl gtR-ClgtR-OTs reactivity
of R-X MeXgt1ogt2ogt3o MeXlt1olt2olt3o MeX, 1o,2o,3o
(steric) (radical stability)
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• Exercise. Oxidative addition (O. A.) to compound
  A is found to take place with MeOTs, but not with
  i-PrI. Complex B reacts with i-PrI, but not with
  MeOTs.
• What is the mechanism in each case?
• Which of the complexes, A or B, will be more
  likely to react with MeI?

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• Exercise. Oxidative addition (O. A.) to compound
  A is found to take place with MeOTs, but not with
  i-PrI. Complex B reacts with i-PrI, but not with
  MeOTs.
• What is the mechanism in each case?
• (b) Which of the complexes, A or B, will be more
  likely to react with MeI?

MeOTs is good for SN2, but not good for radical
reactions i-PrI is not good for SN2, good for
radical reactions Therefore Compound A undergo
SN2 O.A. Compound B undergo radical
O.A. Since CH3I is good for SN2 O. A., A will
react with it.
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ii). Ionic mechanism Some molecules (e.g. HCl,
HBr, HI) dissociate into ions in polar solvents.
HCl H2O ----------gt H(aqua) Cl-
(aqua) Therefore oxidative addition of these
molecules in polar solvents must involve ionic
species.
There are two possible pathways, depending on
electron-richness of metal complexes
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If LnM is electron rich, the following sequence
is likely
Example
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If LnM is electron poor, the following sequence
is likely
Example
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C. Examples of addition of specific molecules
1) R-X bonds
Q1. Predict the product for the following
reactions.
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C. Examples of addition of specific molecules
1) R-X bonds
Q1. Predict the product for the following
reactions.
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2) C-H bonds
Intramolecularly, relatively easy
"cyclometallation"
Intermoleclar, more difficult. Need very
electron-rich LnM
Examples of substrates that undergo C-H oxidative
addition
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How to generate reactive 16e LnM? Many ways.
e.g.
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Examples of C-H bond oxidative addition reactions
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C-H bond activation is involved in many
reactions. e.g.
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C-H bond activation is involved in many
reactions. e.g.
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3) C-C bonds
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3) C-C bonds
favored
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4) Other bonds
X-X
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4) Other bonds
X-X
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O-H
Cy Cyclohexanyl
N-H
(Pyrrole)
(
)
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D. Reductive elimination
Important rxn in catalysis, last step
Examples
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Examples
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Examples
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Detailed mechanism? Could be very
complicated. The following seems true for
mononuclear complexes. 1). Transition state
2) Stereochemistry at R is retained.
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3). The two groups which reductively eliminate
must be cis to each other.

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More complicated cases. 1). Binuclear reductive
elimination.
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2). Apparent oxidative addition and reductive
elimination reactions.
Which pathway is more likely?
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2). Apparent oxidative addition and reductive
elimination reactions.
Which pathway is more likely?
(b). The O.S. of Zr in (a) is unlikely.
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Which pathway is more likely?
(b). The O.S. of Hf in (a) is unlikely.
(a)
(b)
Similar mechanism can explain the following rxn.
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Which pathway is more likely?
(a)
(b)
Similar mechanism can explain the following rxn.
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E. Oxidative coupling
Related reactions

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Examples
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These reactions are useful in synthesis and
catalysis, e.g.
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These reactions are useful in synthesis and
catalysis, e.g.
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Exercise. Suggest reagents or products.
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Exercise. Suggest reagents or products.