Transcript Insertion and Elimination

Insertion and elimination
Peter H.M. Budzelaar
Insertion reactions
If at a metal centre you have
a) a s-bound group (hydride, alkyl, aryl)
b) a ligand containing a p-system (olefin, alkyne, CO)
the s-bound group can migrate to the p-system.
R
M
R
M
CO
O
2
R
M
M
R
Insertion and elimination
Insertion in MeMn(CO)5
insertion
HOMO
TS
agostic
LUMO
CO
CO adduct
3
h2-acyl
Insertion and elimination
Insertion reactions
The s-bound group migrates to the p-system.
But if you only see the result, it looks like the p-system has inserted
into the M-X bond, hence the name insertion.
To emphasize that it is actually (mostly) the X group that moves, we
use the term migratory insertion.
The reverse of insertion is called elimination.
Insertion reduces the electron count, elimination increases it.
Neither insertion nor elimination causes
a change in oxidation state.
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Insertion and elimination
1,1 insertions
In a 1,1-insertion, metal and X group "move" to the same atom of
the inserting substrate.
The metal-bound substrate atom increases its valence.
CO, isonitriles (RNC) and SO2 often undergo 1,1-insertion.
Me
M
M
CO
5
O
Me
Me
M
SO2
M
S Me
O O
Insertion and elimination
Insertion of CO and isonitriles
• CO insertion is hardly exothermic.
• An additional ligand may be needed to trap the acyl
and so drive the reaction to completion.
• In the absence of added ligands often fast equilibrium.
• CO insertion in M-H, M-CF3, M-COR endothermic.
– no CO polymerization.
– but isonitriles do polymerize!
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Insertion and elimination
Double CO insertion ?
Deriving a mechanism from a reaction stoichiometry
is not always straightforward.
The following catalytic reaction was reported a few years ago:
2 R2NH + 2 CO + ArI
"Pd"
R2NCOCOAr + R2NH2+ I-
This looks like it might involve double CO insertion.
But the actual mechanism is more complicated.
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Insertion and elimination
No double CO insertion !
R2NCOCOAr
L2Pd(CO)n
ArX
- n CO
ox add
CO red elim
X
COAr
L2Pd
L2Pd
Ar
CONR2
HNR2 nucl attack
- H+
ins CO
+
COAr
L2Pd
CO
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COAr
subst
CO
- X-
L2Pd
X
Insertion and elimination
Promoting CO insertion
• "Bulky" ligands
O
CO
M
requires more space than M
R
R
• Lewis acids
Coordinate to O, stabilize product
AlCl3
AlCl3
O
O
C
vs
M
M
R
R
Drawback: usually stoichiometric
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Insertion and elimination
Sometimes it only looks like insertion
Nucleophilic attack at coordinated CO can lead to the same
products as standard insertion:
Ir
OMe
Ir
CO
Ir
OMe
CO
OMe
Ir
COOMe
Main difference: nucleophilic attack does not require an empty site.
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Insertion and elimination
1,2-insertion of olefins
Insertion of an olefin in a metal-alkyl bond produces a new alkyl.
Thus, the reaction leads to oligomers or polymers of the olefin.
Me
M
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M
M
Insertion and elimination
1,2-insertion of olefins
Insertion of an olefin in a metal-alkyl bond produces a new alkyl.
Thus, the reaction leads to oligomers or polymers of the olefin.
Best known polyolefins:
• polyethene (polythene)
• polypropene
In addition, there are many specialty polyolefins.
Polyolefins are among the largest-scale chemical products made.
They are chemically inert.
Their properties can be tuned by the choice
of catalyst and comonomer.
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Insertion and elimination
Why do olefins polymerize ?
Driving force: conversion of a p-bond into a s-bond
– One C=C bond: 150 kcal/mol
– Two C-C bonds: 285 = 170 kcal/mol
– Energy release: about 20 kcal per mole of monomer
(independent of mechanism!)
Many polymerization mechanisms
–
–
–
–
Radical (ethene, dienes, styrene, acrylates)
Cationic (styrene, isobutene)
Anionic (styrene, dienes, acrylates)
Transition-metal catalyzed (a-olefins, dienes, styrene)
Transition-metal catalysis provides the best opportunities
for tuning of reactivity and selectivity
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Insertion and elimination
Mechanism of olefin insertion
Standard Cossee mechanism
M
M
R
R
M
R
M
R
Green-Rooney variation (a-agostic assistance):
M
M
CH2P
M
CH2P
H
CH2P
M
H
P
Interaction with an a C-H bond could facilitate tilting of the migrating alkyl group
The "fixed" orientation suggested by this picture is probably incorrect
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Insertion and elimination
Insertion in M-H bonds
Insertion in M-H bonds is nearly always fast and reversible.
 Hydrides catalyze olefin isomerization
Regiochemistry corresponds to Markovnikov rule (with Md+-Hd-)
To shift the equilibrium to the insertion product:
• Electron-withdrawing groups at metal
alkyl more electron-donating than H
• Early transition metals
M-C stronger (relative to M-H)
• Alkynes instead of olefins
more energy gain per monomer, both for M-H and M-C insertion
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Insertion and elimination
Catalyzed olefin isomerization
Metals have a preference for primary alkyls.
But substituted olefins are more stable!
Cp2Zr
Cl
dominant
alkyl
Cp2ZrHCl
dominant
olefin
Cp2Zr
Cl
In isomerization catalysis, the dominant products and the dominant
catalytic species often do not correspond to each other.
For each separately, concentrations at equilibrium reflect
thermodynamic stabilities via the Boltzmann distribution.
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Insertion and elimination
Catalyzed olefin isomerization
xs
Most stable alkyl
Cp2ZrCl
Cp2ZrHCl
or
or
+
+
Most stable olefin
+ little
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Insertion and elimination
M-H vs M-C insertion
Insertion in M-C bonds is slower than in M-H.
Barrier usually 5-10 kcal/mol higher
Factor 105-1010 in rate !
Reason: shape of orbitals (s vs. sp3)
M
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M
Insertion and elimination
Repeated insertion
Multiple insertion leads to dimerization,
oligomerization or polymerization.
M H
M Et
kprop
M Bu
kCT
M H +
Key factor: kCT / kprop = 
  1: mainly dimerization
  0.1-1.0: oligomerization
(always mixtures)
 « 0.1: polymerization
  0: "living" polymerization
kprop
M Hx
kCT
M H +
kprop
M Oc
kprop
etc
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kCT
M H +
For non-living polymerization:

N n2 
( n  0)
n 1
  1
Wn  2
(n  2) 2

n 1
(2  1)  1
Insertion and elimination
Schulz-Flory statistics
0.45
Mole fraction
0.40
Weight fraction
0.35
0.30
 = 0.7
0.25
0.20
0.15
0.10
0.05
0.00
2
5
8
11 14 17 20 23 26 29 32 35 38 41 44 47
Mole fraction
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Weight fraction
 = 0.1
2
5
8
11 14 17 20 23 26 29 32 35 38 41 44 47
Mole fraction
0.03
Weight fraction
0.02
0.02
 = 0.02
0.01
0.01
For non-living polymerization:

N n2 
( n  0)
n 1
  1
Wn  2
0.00
2
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Key factor: kCT / kprop = 
  1: mainly dimerization
  0.1-1.0: oligomerization
(always mixtures)
 « 0.1: polymerization
  0: "living" polymerization
5
8
11 14 17 20 23 26 29 32 35 38 41 44 47
(n  2) 2

n 1
(2  1)  1
Insertion and elimination
Applications of oligomers and polymers
• Ethene and propene come directly from crude oil "crackers"
– Primary petrochemical products, basic chemical feedstocks
• Dimerization rarely desired
– Making butene costs $$$ !
• Oligomers: surfactants, comonomers
– High added value, but limited market
• Polymers: plastics, construction materials, foils and films
– Very large market, bulk products
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Insertion and elimination
Selective synthesis of trimers etc ?
• 1-Hexene and 1-octene are valuable co-monomers.
• Selective synthesis of 1-hexene from ethene is not possible
using the standard insertion/elimination mechanism.
• There are a few catalysts that selectively trimerize ethene
via a different mechanism ("metallacycle" mechanism).
– Redox-active metals (Ti, V, Cr, Ta) required
– Cr systems are used commercially
• There are also one or two catalysts that preferentially produce
1-octene. The mechanism has not been firmly established.
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Insertion and elimination
Trimerization via metallacycles
+
Key issues:
M=
• Geometrical constraints
prevent b-elimination
in metallacyclopentane.
Ti
(and others)
M II
subst
M II
coord
• Formation of 9-membered
rings unfavourable.
M II
• Ligand helps balance (n)
and (n+2) oxidation states.
red
elim
M? H
M IV
M IV
H
coord
b-elim
M IV
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ins
M IV
Insertion and elimination
CO/olefin copolymerization
M
O
• CO cheaper than ethene
• Copolymer more polar
than polyethene
– much higher melting point
CO
P
P
O
M
• Chemically less inert
M
O
CO
P
O
P
• No double CO insertion
O
M
uphill
• No double olefin insertion
O
CO
CO binds more strongly, inserts more quickly
• Slow b-elimination from alkyl
CO
M
O
5-membered ring hinders elimination
O
P
M = L2Pd, L2Ni
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Insertion and elimination
Hydroformylation
• Used to make long-chain alcohols and acids from 1-alkenes
– Often in situ reduction of aldehydes to alcohols
– Unwanted side reaction: hydrogenation of olefin to alkane
• Main issue: linear vs branched aldehyde formation
• It is possible to make linear aldehydes from internal olefins !
H
H
M
M
O
O
M
O
H
M
H
M
O
H2
M
CO
CO
M
H2
H
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Insertion and elimination
Insertion of longer conjugated systems
Attack on an h-polyene is always
at a terminal carbon.
 LUMO coefficients largest
 Usually a,w-insertion
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M
M
R
R
Insertion and elimination
Insertion of longer conjugated systems
A diene can be h2 bound.
 1,2-insertion
M
M
R
R
Metallocenes often do not have enough space for h4 coordination:
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Insertion and elimination
Diene rubbers
• Butadiene could form three different "ideal" polymers:
cis 1,4
trans 1,4
1,2
• In practice one obtains an imperfect polymer
containing all possible insertion modes.
• Product composition can be tuned by catalyst variation.
• Polymer either used as such or (often)
after cross-linking and hydrogenation.
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Insertion and elimination
Addition to enones
• RLi, Grignards: usually 1,2
– "charge-controlled"
R OH
• OrganoCu compounds often 1,4
– or even 1,6 etc
– "orbital-controlled"
– stereoregular addition possible
using chiral phosphine ligands
– frequently used in organic synthesis
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O
R
O
Insertion and elimination
Less common elimination reactions
a-elimination:
Cp2Zr
Cp2Zr
H
Probably via s-bond metathesis: Zr
H
H
tBu
Other ligand metallation reactions:
-
Zr
tBu
H
Zr
L2Pt
L2Pt
Via s-bond metathesis or oxidative addition/reductive elimination
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Insertion and elimination
Less common elimination reactions
b-elimination from alkoxides of late transition metals is easy:
M
O CH
3
M
+ CH2O
H
The hydride often decomposes to H+ and reduced metal:
alcohols easily reduce late transition metals.
Also, the aldehyde could be decarbonylated to yield metal carbonyls.
For early transition metals, the insertion is highly exothermic
and irreversible.
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Insertion and elimination