Chem 634 Fall 2013
Classical Organometallic Chemistry and Introduction to Transition Metal Catalysis
Reading: Heg Ch 1–2
CS-B 7.1, 8.2–8.3, 11.3
Grossman Ch 6
Outline
•  Classical organometallic chemistry
•  Introduction to transition metal complexes
•  Overview of organometallic mechanisms
Classical Organometallic Chemistry
Grignard Reaction
Me
+ Mg(0)
X
Me
MgX
X: I > Br >> Cl ≠ F
Mechanism (surface reaction):
R X + Mg(0)
R X
+
Mg+
R +I
Similar chemistry with Li and Zn
Mg+
R Mg I
Wurtz Coupling
Wurtz coupling can compete with formation of Grignard reagent:
Mg(0)
R X
1/2 R R +MgX2
Via:
R + R
R R
Ratio of Wurtz to Grignard is substrate dependant
Metal-Halide Exchange
Lithium-Halogen Exchange:
pKa'=50
Br
BuLi
Li
+ BuBr
-78 °C
pKa'=43
Also aryl iodides, vinyl bromide and iodides
Note: Sometimes BuX is a problem
Li
Bu
+ BuBr
More on Lithium Halogen Exchange
pKa'=53
I
O
O
t-BuLi (2 equiv)
Li
O
+ t-BuI
O
-78 °C
n-BuLi will not work in this case. Why?
Why 2 equiv?
H
Me
Me
H
H
H
+
I
2nd equiv of t-BuLi prevents product from quenching on t-BuI
Magnesium-Halogen Exchange
I
(LiCl)
Me
MgI
i-PrMgCl
Me
•  typically low temp (R=EWG), ~-20°C
•  EWG not required, but helps
•  Grignards slow to reaction with EWG at this temperature
R=CO2Me, C(O)R, NO2, etc.
Note on LiCl:
Me
Mg
Me
Me
Cl
2
Mg
Cl
Me
aggregates in soln
Me
Cl
Mg
Me
Li
Cl
more reactive
Knochel, ACIE, 2003, 42, 4302
Tin-Lithium Exhange
or other stabilized, allyl, or vinyl tin
RO
SnR'3
R'= Me or Bu
BuLi
RO
Li
+ R'3SnBu
Directed Metallation
DG
DG
BuLi
DG
+BuH
H Li
Bu
DG=directing group
Li
Examples:
O
O
Me
O
BuLi
"MOM"
O
Me
Li
NHBoc
BuLi
Li
N
Li
O
O
Sniekus (Queens Univ, Canada), Chem. Rev., 1990, 90, 879
Cuprates
2 RLi
CuX
R2Cu(I)Li•LiX
X=I, Br, Cl
"Gilman Cuprate"
Upsides:
•  soft nucleophiles
•  less basic than RLi or RMgX
•  also works with RMgX
Downsides:
•  thermal sensitivity
•  typically only 50% of “R” groups transfer.
Solution Structure:
Me
Li
Cu
Me
Me
Cu
Me
dimeric
Li
Modern Organocopper Chem, Ed. Krausse, Wiley, 2002
Other Organocuprates
External ligands can affect cuprate reactivity:
R
Cu + R'Li
RLi + CuCN
2 RLi + CuCN
R
Cu R' Li
dummy ligand
RCu(CN)Li
R2CuLi + LiCN
"higher order" gilman organocuprate
Modern Organocopper Chem, Ed. Krausse, Wiley, 2002
Organometallic Complexes
2–
PPh3
Pd
PPh3
PPh
PPh3 3
Ph2 H2
P Cl N
Ru
P Cl N
Ph2 H2
M
st
0
1 row
nd
2 row
rd
3 row
III
3
d
Sc
Y
La
early
IV
4
d
Ti
Zr
Hf
V
5
d
V
Nb
Ta
Cl
Cl Pt Cl
Cl
Ph3P Rh Cl
PPh3
PPh3
Ph
O
otol2P
Ph
VI
6
d
Cr
Mo
W
CF3
N
B
Ir
CF3
Ph
VII
7
d
Mn
Tc
Re
dividing line
VIII
8
d
Fe
Ru
Os
IX
9
d
Co
Rh
Ir
X
10
d
Ni
Pd
Pt
late
4
XI
d s
Cu
Ag
Au
10 1
XII
10 2
d s
Zn
Cd
Hg
How Many Valence Electrons?
Consider CH4… Recall the Octet Rule
2p x 3
E
LCAO's of 4 H 1s orbitals
Bonding orbitals filled...
stable configuration.
2s
C
CH4
4xH
How Many Valence Electrons?
Transition metals have 5 d orbitals (plus s & 3 p’s)!
x M–L antibonding orbitals
(n+1)p x 3
(n+1)s
E
nd x 5
(9–x) nonbonding orbitals
Bonding and nonbonding orbitals filled...
stable configuration.
x M–L bonding orbitals
M
MLx
Lx
Determining Electron Count of TM Complex
Ionic Method: All ligands are dissociated with their electrons.
L-type ligands = neutral, 2 e- donor
X-type ligands = anionic 2 e- donor
n
X
X
L
M
n+y
L
L
M
X
X
mnemonic:
X
L
M
X
X
L
L
M+ L
X–
Electron counting is a formalism.
L
where y = number
of x-type ligands
Example of Ionic Method Electron Counting
H–
O
C
H
OC
OC
Fe
H
CO
O C
Fe2+
CO
C
O
4 X CO
2 X H–
Fe(2+) (d6)
4 X 2e2 X 2e-
C O
H–
8e4e6e-
________________________________________
18e-
Note: Both Ionic and Neutral Methods give the same electron count,
but differ in how they treat the metal center.
Charged Complexes
2– 2K+
Cl
Cl
Cl
Pd
Cl
4 X ClPd(2+) (d8)
4 X 2e-
8e8e-
________________________________________
16e-
Oxidation State
•  Formalism to account for “oxidation state” of metal center.
•  This is only a formalism, in reality electrons (and thus charge) are
shared over both the ligand and metal.
•  Nonetheless, oxidation state guides understanding of metal
reactivity.
•  Using ionic method of electron counting arrives directly at metal
oxidation state.
•  Example:
H
OC
OC
Fe
H
CO
CO
4 X CO
2 X H–
Fe(2+) (d6)
4 X 2e2 X 2e-
8e4e6e-
________________________________________
Oxidation State = Fe(II)
18e-
Examples of X-type Ligands
Ligand
Disconnection
Complex
ligand charge
ligand e-
hydride
M H
M+ +
H
-1
2
halide
M Cl
M+ +
Cl
-1
2
alkyl
M CR3
M+ +
CR3
-1
2
aryl (σ bound)
M
M+ +
-1
2
-1
2
M+ +
acetylide
M
silane
M SiR3
M+ +
SiR3
-1
2
amido
M NR2
M+ +
NR2
-1
2
alkoxide
M OR
M+ +
OR
-1
2
R
R
Examples of X-type Ligands (More Complex)
Ligand
Disconnection
Complex
O
acetate (η1)
acetate (η3)
allyl (η1)
allyl (η3)
cyclopentadienyl (Cp)
M O
M
M
M
ligand e-
O
Me
O
M
ligand charge
Me
M+ +
-1
2
-1
4
M+ +
-1
2
M+ +
-1
4
-1
6
M+ +
O
Me
O
O
Me
O
M+ +
•  𝜼 (eta) denotes “hapticity” = how many atoms of ligand are bond to metal from
single donor site.
Examples of 2e– Donor “L”-type Ligands
ligand charge
ligand e-
PPh3
0
2
M+
NEt3
0
2
M+
N CR
0
2
0
2
0
2
0
2
0
2
0
2
Ligand
Complex
Disconnection
phopshine
M PPh3
M+
amine
M NEt3
nitrile
M N CR
H
sigma bond (H–H)
sigma bond (C–H)
H
M
M+
H
H
H
H
M
M+
CR3
alkene
CR3
M+
M
carbonyl
M C
ether
M OR2
O
M+
C
M+
OR2
O
“L”-type Ligands Donating More than 2e–
Ligand
Disconnection
Complex
diene
M
ligand charge
ligand e-
M+
0
4
arene (η6)
M
M+
0
6
arene (η2)
M
M+
0
2
0
4
Ph
Ph
Ph
P
bisphosphine (κ2)
Ph
P
M+
M
P
Ph
Ph
Ph
Ph
R
R
R
R
N
diamine (κ2)
2 X 2e– L-type ligands
P
N
M+
M
P
R
R
4
2 X 2e– L-type ligands
N
R
0
R
•  𝜅 (kappa) denotes “denticity” = how many binding sites of a polydentate ligand
are bound .
X2 Ligands
Ligand
Complex
Disconnection
2+ CR2
ligand charge
ligand e-
-2
4
triplet carbenes
(alkylidines)
M CR2
M2+
oxo
M O
M2+
+ O
-2
4
imido (bent)
M N
M2+
2+ NR
-2
4
2-
R
NHC-Ligands
Ligand
Disconnection
Complex
R
N
N-heterocyclic carbenes
ligand charge
R
N
M
0
M+
N
R
N
R
Sigma Bond
Pi-Backbond
R
M
N
C
N
R
M
C
N
N
ligand e-
2
18 Electron “Rule”
Many, but not all, stable, isolatable TM complexes are 18e– complexes.
PPh3
Ph3P Pd PPh3
PPh3
Pd = group 10
10e- + 8e- = 18e-
Ph2 Cl
P
Ru
P
Ph2 Cl
H2
N
N
H2
Ru = group 8
+2 = d6
6+12 = 18
NOTE: 18e– “rule” is more of a suggestion. Many of the most interesting, i.e.
reactive, complexes do not follow this “rule”.
Basic Organometallic Mechanisms
•  To understand transition metal catalysis, we need to understand
something about the mechanisms by which the catalysts operate.
•  Many TM catalyzed reactions proceed via a series of elementary
steps.
Dissociative Ligand Substitution
L
L
L
L
M
L
L
L
-L
L
M
L
L
L'
L
common with:
•  coordinately saturated
•  18e- complexes
L
L
L
M
L
L'
L
Associative Ligand Substitution
L'
L
L
M
L'
L
L
L
L
M
L
L
L
L
equitorial ligand w/
strongest pi-interactions
common with:
•  16 e- complexes
•  requires open coordination site
L
L
L'
Y* M L
Y* M L'
L
L
trans effect
M
L
L'
Oxidative Addition
[Mn]
R
X
[Mn+2]
R
X
•  formal oxidation state of metal center increases by two
•  favored by electron-rich metal centers
Example:
Ph
Ph3P
0
Pd
Ph3P
I
Ph
Ph3P
II
Pd
Ph3P
I
Reductive Elimination
[Mn]
•
•
•
•
R
[Mn-2] + R R
R
formal oxidation state of metal center decreases by two
promoted by electron poor metal centers
large ligands accelerate process
microscopic reverse of oxidative addition
Example:
Ph
N
NiII
N
N
Ni0 +
N
O
Ph
N
N
O
Transmetallation
[Mn]
R
M' R'
X
[Mn]
R
R'
+ M' X
•  Exchange of ligands between different metal centers
•  No change in oxidation state at metals
•  Exact mechanisms are complex, some are still unknown.
Example:
Ph Bu3Sn
Ph3P
II
Pd
Ph3P
I
Ph
Ph
Ph3P
+ Bu3SnI
PdII
Ph3P
Ph
Migratory Insertion
[Mn]
R
[Mn]
X
R
X
•  Ligand inserts into adjacent metal-ligand bond
•  No net change of oxidation state at the metal
Examples:
PPh3
OC
RhI
PPh3
CO
OC
PPh3
PPh3
R
Ar
Ph3P
PdII
PPh3
O
RhI
+
R
+
Ph3P
PdII
PPh3
Ar
β-Elimination
R
[Mn]
[Mn]
R
•
•
•
•
Microscopic reverse of migratory insertion
No net change of oxidation state at the metal
Most often observed as β-hydride elimination from alkyl ligand
Requires open coordination site on the metal center
Example:
+
PPh3
Ph3P
note open
coordination site
Ar
PdII
H
PPh3
Ph3P
PdII
H
+
β-Elimination
Note: most of the time, β-hydride elimination is superfacial. This is
because the mechanism is inner-sphere, involving formation of a
metal-hydride bond.
H1
[Pd]
Ph
Ph
H3
note open
coordination site
H3
+
PPh3
Ph3P
H2
H2
Ar
PdII
H
PPh3
Ph3P
PdII
H
+
α-Elimination
R
R'
R
[Mn]
[Mn]
H
+ HR'
Via:
R
R'
[Mn]
H
or:
R
[Mn]
R'
‡
H
not common in organic synthesis
Oxidative Coupling/Cyclization
‡
[Mn]
[Mn]
[Mn+2]
σ−Bond Metathesis
[M] R'
+
R R''
R' R''
[M] R
Via:
[M]
R
R'
R''
‡
[M]
R
R'
R''
Common mechanism of C-H bond activation.
π−Bond Metathesis
X Y
[M] X + R Y
[M] R
via:
[M]
X
R
Y
[2+2]
[M]
R
X
Y
[retro 2+2]
Common in alkene metathesis.
[M]
R
X
Y