Organometallics

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Organometallics
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Electron Counting in the D block

• Link

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The 18 electron rule Just as organic chemists
have their octet rule for organic compounds, so
do organometallic chemists have the 18 electron
rule. And just as the octet rule is often
violated, so is the 18 electron rule. However,
both serve a useful purpose in predicting
reactivity. Each derives from a simple count of
the number of electrons that may be accommodated
by the available valence orbitals (one s and
three p for organic chemists organometallic
chemists get five bonus d-orbitals in which to
place their electrons).
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What are d-electrons, anyway? While we teach our
students in freshman chemistry that the periodic
table is filled in the order Ar4s23d10, this
turns out to be true only for isolated metal
atoms. When we put a metal ion into an electronic
field (surround it with ligands), the d-orbitals
drop in energy and fill first. Therefore, the
organometallic chemist considers the transition
metal valence electrons to all be d-electrons.
There are certain cases where the 4s23dx order
does occur, but we can neglect these in our first
approximation. Therefore, when we ask for the
d-electron count on a transition metal such as Ti
in the zero oxidation state, we call it d4, not
d2. For zero-valent metals, we see that the
electron count simply corresponds to the column
it occupies in the periodic table. Hence, Fe is
in the eighth column and is d8 (not d6) and Re3
is d4 (seventh column for Re, and then add 3
positive charges...or subtract three negative
ones). Now that we can assign a d-electron count
to a metal center, we are ready to determine the
electronic contribution of the surrounding
ligands and come up with our overall electron
count.
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Method 1 The ionic (charged) model The basic
premise of this method is that we remove all of
the ligands from the metal and, if necessary, add
the proper number of electrons to each ligand to
bring it to a closed valence shell state. For
example, if we remove ammonia from our metal
complex, NH3 has a completed octet and acts as a
neutral molecule. When it bonds to the metal
center it does so through its lone pair (in a
classic Lewis acid-base sense) and there is no
need to change the oxidation state of the metal
to balance charge. We call ammonia a neutral
two-electron donor. In contrast, if we remove a
methyl group from the metal and complete its
octet, then we formally have CH3-. If we bond
this methyl anion to the metal, the lone pair
forms our metal-carbon bond and the methyl group
acts as a two-electron donor ligand. Notice that
to keep charge neutrality we must oxidize the
metal by one electron (i.e. assign a positive
charge to the metal). This, in turn, reduces the
d-electron count of the metal center by one.
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Method 2 The covalent (neutral) model The major
premise of this method is that we remove all of
the ligands from the metal, but rather than take
them to a closed shell state, we do whatever is
necessary to make them neutral. Let's consider
ammonia once again. When we remove it from the
metal, it is a neutral molecule with one lone
pair of electrons. Therefore, as with the ionic
model, ammonia is a neutral two electron donor.
But we diverge from the ionic model when we
consider a ligand such as methyl. When we remove
it from the metal and make the methyl fragment
neutral, we have a neutral methyl radical. Both
the metal and the methyl radical must donate one
electron each to form our metal-ligand bond.
Therefore, the methyl group is a one electron
donor, not a two electron donor as it is under
the ionic formalism. Where did the other electron
"go"? It remains on the metal and is counted
there. In the covalent method, metals retain
their full complement of d electrons because we
never change the oxidation state from zero i.e.
Fe will always count for 8 electrons regardless
of the oxidation state and Ti will always count
for four. Notice that this method does not give
us any immediate information about the formal
oxidation state of the metal, so we must go back
and assign that in a separate step. For this
reason, many chemists (particularly those that
work with high oxidation state complexes) prefer
the ionic method.
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The two methods compared some examples The most
critical point we should remember is that like
oxidation state assignments, electron counting is
a formalism and does not necessarily reflect the
distribution of electrons in the molecule.
However, these formalisms are very useful to us,
and both will give us the same final answer.
Consider the following simple examples. Notice
how some ligands donate the same number of
electrons no matter which formalism we choose,
while the number of d-electrons and donation of
the other ligands can differ. All we have to do
is remember to be consistent and it will work out
for us.
Link
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• Commonly observed mechanistic steps in
  organometallic chemistry include
• Oxidative Addition
• Reductive Elimination
• Transmetallation
• Ligand Substitution
• Coordination
• Insertion
• b-Hydride Elimination

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Oxidative Addition (Note that the metal has
undergone a formal change in oxidation state of
2)
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An Example of an Oxidative Addition with a
Non-transition Metal is the Grignard Reaction
(Magnesium is an alkaline earth metal).
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The Grignard Reaction
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The Grignard Reagent is Highly Useful
Synthetically ! (as shown below, it reacts as a
carbanion equivalent)
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Please notice (from the table below) that
magnesium is an electropositive metal, thus the
C-Mg bond is highly polarized toward carbon. By
contrast, most transition metals (e.g. Pd) have
electronegativities closer to that of carbon,
thus the C-Transition metal bonds are less polar
(and the carbon less likely to mechanistically
behave as a carbanion).
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Oxidative addition of Pd
1) Oxidative Addition of Pd to C-X

• Notes
• The oxidative addition step increases the
  oxidation state of the metal by 2.
• Therefore, it occurs most readily with electron
  rich metals and when the metal is in a relatively
  low oxidation state (e.g. Pd(0)). It cannot
  occur when the metal is already at its highest
  oxidation level.
• The reaction proceeds most readily when the
  carbon is sp2 hybridized (i.e. an aryl halide, or
  a vinyl halide).

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Commonly used sources of Pd0 include
Tris(dibenzylideneacetone)dipalladium (0)
Pd2(dba)3
Tetrakis(triphenylphosphine)palladium (0)
Pd(PPh3)4
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Also, Pd(II) is very readily reduced to Pd(0)
Thus, it is often more convenient to use Pd(II)
complexes, which are more air-stable than
commercial Pd(0) complexes.
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2) Reductive Elimination

• Notes
• This reaction is the reverse of oxidative
  addition
• Therefore, the formal oxidation state of the
  metal decreases by 2.

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3) Transmetallation
In the above reactions, notice that the C-Sn bond
(top) and the C-Mg bond (bottom) have both been
exchanged for a C-Pd bond.
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Now we will put the three reactions together
(oxidative addition, transmetallation, and
reductive elimination) to produce some very
widely utilized processes which employ palladium
(as Pd(0)) as a catalyst.
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Negishi Coupling
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Mechanism of the Negishi Coupling
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The Stille Coupling
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The Suzuki Coupling
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Mechanism of the Suzuki Coupling
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Sonogashira Coupling
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To understand the Heck Reaction, two new
processes will be introduced The insertion
reaction The b-hydride elimination
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The insertion reaction
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The b-hydride Elimination
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The Heck Reaction
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Reductive Elimination (or deprotonation)
Oxidative Addition
b-Hydride Elimination
Complexation
Insertion
Rotation
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Notice that, although Pd(II) is most commonly
used in a Heck Reaction, the active species (for
the initial oxidative addition) is Pd(0), which
is believed to be produced by the action of Et3N
on the Pd(II).
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Buchwald-Hartwig Coupling
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Richard R. Schrock
Olefin Metathesis
Robert H. Grubbs
Yves Chauvin
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The Catalysts
Grubbs 1st Generation Catalyst
Grubbs 2nd Generation Catalyst
Schrock Catalyst
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Examples of Olefin Metathesis by Grubbs First
Generation Catalyst
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Simplified Mechanism
Link
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Examples of Olefin Metathesis by Grubbs Second
Generation Catalyst
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Examples of Olefin Metathesis by Schrock Catalyst
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Ring Opening Metathesis (and Ring Opening
Metathesis Polymerization, ROMP)
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The Wacker-Tsuji Oxidation
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Notice that, in the Wacker Oxidation, the active
catalyst is Pd(II), thus differentiating this
process from all the other Pd catalyzed processes
discussed in this presentation.
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Allyl esters and Allyl ethers to protect
carboxylic acids and alcohols, respectively
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Overview
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Hydrogenation
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Lindlars Catalyst, a poisoned form of
palladium, can selectively hydrogenate triple
bonds, generating double bonds selectively, with
the Z-geometry Common catalyst poisons include
quinoline and lead
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