Transition metal Catalyzed Reactions

Download Report

Transcript Transition metal Catalyzed Reactions

Electron Counting in the D block
• Link
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).
What are d-electrons, anyway?
While we teach our students in freshman chemistry that the periodic table is filled in the
order [Ar]4s23d10, 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 delectron 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.
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.
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.
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.
Commonly observed mechanistic steps in
organometallic chemistry include:
•Oxidative Addition
•Reductive Elimination
•Ligand Substitution
•b-Hydride Elimination
Oxidative Addition
(Note that the metal has undergone a formal change in oxidation
state of +2)
An Example of an Oxidative Addition with a Non-transition Metal is the Grignard
Reaction (Magnesium is an alkaline earth metal).
The Grignard Reaction
The Grignard Reagent is Highly Useful Synthetically !
(as shown below, it reacts as a carbanion equivalent)
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
Oxidative addition of Pd
1) Oxidative Addition of Pd to C-X
a) The oxidative addition step increases the oxidation state of the metal by 2.
b) 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.
c) The reaction proceeds most readily when the carbon is sp2 hybridized (i.e. an aryl
halide, or a vinyl halide).
Commonly used sources of Pd0 include:
Tetrakis(triphenylphosphine)palladium (0)
Tris(dibenzylideneacetone)dipalladium (0)
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.
2) Reductive Elimination
a) This reaction is the reverse of oxidative addition
b) Therefore, the formal oxidation state of the metal decreases by 2.
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.
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.
Negishi Coupling
Mechanism of the Negishi Coupling
The Stille Coupling
The Suzuki Coupling
Mechanism of the Suzuki Coupling
Sonogashira Coupling
To understand the Heck Reaction, two
new processes will be introduced:
The insertion reaction
The b-hydride elimination
The insertion reaction
The b-hydride Elimination
The Heck Reaction
Reductive Elimination
(or deprotonation)
Oxidative Addition
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).
Buchwald-Hartwig Coupling
Richard R. Schrock
Robert H. Grubbs
Yves Chauvin
Olefin Metathesis
The Catalysts
Grubbs 1st Generation Catalyst
Grubbs 2nd Generation Catalyst
Schrock Catalyst
Examples of Olefin Metathesis by Grubbs First Generation Catalyst
Simplified Mechanism
Examples of Olefin Metathesis by Grubbs Second Generation Catalyst
Examples of Olefin Metathesis by Schrock Catalyst
Ring Opening Metathesis
(and Ring Opening Metathesis Polymerization, ROMP)
The Wacker-Tsuji Oxidation
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.
Allyl esters and Allyl ethers to protect carboxylic
acids and alcohols, respectively
The catalyst most commonly used in hydrogenation is a finely divided form of Pd
metal, deposited on a charcoal support. Unlike the catalysts used in the previous
processes, this catalyst is not soluble, thus is called ‘heterogeneous’.
Lindlar’s 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