Organometallic Chemistry an overview of structures and reactions Peter H.M. Budzelaar.
Insertion and elimination Peter H.M. Budzelaar. Insertion and elimination 2 Insertion reactions If...
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Transcript of Insertion and elimination Peter H.M. Budzelaar. Insertion and elimination 2 Insertion reactions If...
Insertion and elimination2
Insertion reactions
If at a metal centre you have
a) a -bound group (hydride, alkyl, aryl)
b) a ligand containing a -system (olefin, alkyne, CO)
the -bound group can migrate to the -system.
M
R
MR
MR
COM
O
R
Insertion and elimination4
Insertion reactions
The -bound group migrates to the -system.
But if you only see the result, it looks like the -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 causesa change in oxidation state.
Insertion and elimination5
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.
MMe
SO2
MS Me
O O
MMe
CO
MMe
O
Insertion and elimination6
Insertion of CO and isonitriles
• CO insertion is hardly exothermic.
• An additional ligand may be needed to trap the acyland 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!
Insertion and elimination7
Double CO insertion ?
Deriving a mechanism from a reaction stoichiometryis not always straightforward.
The following catalytic reaction was reported a few years ago:
This looks like it might involve double CO insertion.
But the actual mechanism is more complicated.
"Pd"R2NCOCOAr + R2NH2
+ I-2 R2NH + 2 CO + ArI
Insertion and elimination8
No double CO insertion !
subst
nucl attack
red elim
ins
ox addCO
- H+HNR2
- X-CO
L2PdCONR2
COAr
+
L2PdCO
COArL2Pd
X
COAr
CO
L2PdAr
X
- n COArXL2Pd(CO)n
R2NCOCOAr
Insertion and elimination9
Promoting CO insertion
• "Bulky" ligands
• Lewis acidsCoordinate to O, stabilize product
Drawback: usually stoichiometric
MCO
RM
O
Rrequires more space than
MC
R
O
AlCl3
M
O
R
AlCl3
vs
Insertion and elimination10
Sometimes it only looks like insertion
Nucleophilic attack at coordinated CO can lead to the same products as standard insertion:
Main difference: nucleophilic attack does not require an empty site.
Ir OMe Ir OMe
Ir CO OMeCO
Ir COOMe
Insertion and elimination11
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.
MMe
M M
Insertion and elimination12
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 choiceof catalyst and comonomer.
Insertion and elimination13
Why do olefins polymerize ?
Driving force: conversion of a -bond into a -bond– One C=C bond: 150 kcal/mol– Two C-C bonds: 285 = 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 (-olefins, dienes, styrene)
Transition-metal catalysis provides the best opportunitiesfor tuning of reactivity and selectivity
Insertion and elimination14
Mechanism of olefin insertion
Standard Cossee mechanism
Green-Rooney variation (-agostic assistance):
Interaction with an C-H bond could facilitate tilting of the migrating alkyl group
The "fixed" orientation suggested by this picture is probably incorrect
MR
MR
M
R
MR
M
HPH
MCH2P
M
CH2P
MCH2P
Insertion and elimination15
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 M+-H-)
To shift the equilibrium to the insertion product:• Electron-withdrawing groups at metal
alkyl more electron-donating than H
• Early transition metalsM-C stronger (relative to M-H)
• Alkynes instead of olefinsmore energy gain per monomer, both for M-H and M-C insertion
Insertion and elimination16
Catalyzed olefin isomerization
Metals have a preference for primary alkyls.
But substituted olefins are more stable!
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.
Cp2ZrHCl
Cp2ZrCl
Cp2ZrCl
dominantalkyl
dominantolefin
Insertion and elimination17
Catalyzed olefin isomerization
Cp2ZrHClxs
or
or
Cp2ZrCl
+ +
+ little
Most stable alkyl
Most stable olefin
Insertion and elimination18
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-H vs M-C insertion
MM
Insertion and elimination19
Repeated insertion
Multiple insertion leads to dimerization,oligomerization or polymerization.
M H
M Et
M Bu
M Hx
M H +
+M H
etc
kCT
kCT
kprop
kprop
kprop
M Oc +M HkCT
kprop
For non-living polymerization:
1
2
2
12
1)12(
)2(
)0(1
nn
nn
nW
nN
Key factor: kCT / kprop = 1: mainly dimerization 0.1-1.0: oligomerization
(always mixtures) « 0.1: polymerization 0: "living" polymerization
Insertion and elimination20
Schulz-Flory statistics
Key factor: kCT / kprop = 1: mainly dimerization 0.1-1.0: oligomerization
(always mixtures) « 0.1: polymerization 0: "living" polymerization
For non-living polymerization:
1
2
2
12
1)12(
)2(
)0(1
nn
nn
nW
nN
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47
Mole fraction
Weight fraction
0.00
0.01
0.01
0.02
0.02
0.03
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47
Mole fraction
Weight fraction
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47
Mole fraction
Weight fraction
= 0.7
= 0.02
= 0.1
Insertion and elimination21
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
Insertion and elimination22
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 ethenevia 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.
Insertion and elimination23
Trimerization via metallacycles
M = Ti
+
Key issues:
• Geometrical constraintsprevent -eliminationin metallacyclopentane.
• Formation of 9-memberedrings unfavourable.
• Ligand helps balance (n)and (n+2) oxidation states.
MIV
MIV
MII
MIV
HMIV
MIIMII
redelim
subst coord
coord
ins
-elim
M? H
(and others)
Insertion and elimination24
CO/olefin copolymerization
• CO cheaper than ethene• Copolymer more polar
than polyethene– much higher melting point
• Chemically less inert
• No double CO insertionuphill
• No double olefin insertionCO binds more strongly, inserts more quickly
• Slow -elimination from alkyl5-membered ring hinders elimination
MO P
M
O
P
O
M
O
P
CO
MCO
O P
O
M
O P
O
CO
CO
M = L2Pd, L2Ni
Insertion and elimination25
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 !
HM M
HM
COCO
M
M
O
MH2
O
M
H
O
HH
O
H2
Insertion and elimination26
Insertion of longer conjugated systems
Attack on an -polyene is alwaysat a terminal carbon.
LUMO coefficients largest
Usually ,-insertion
M
R
R
M
Insertion and elimination27
Insertion of longer conjugated systems
A diene can be 2 bound. 1,2-insertion
Metallocenes often do not have enough space for 4 coordination:
MR
MR
Insertion and elimination28
Diene rubbers
• Butadiene could form three different "ideal" polymers:
• In practice one obtains an imperfect polymercontaining all possible insertion modes.
• Product composition can be tuned by catalyst variation.• Polymer either used as such or (often)
after cross-linking and hydrogenation.
cis 1,4
trans 1,4
1,2
Insertion and elimination29
Addition to enones
• RLi, Grignards: usually 1,2– "charge-controlled"
• OrganoCu compounds often 1,4– or even 1,6 etc– "orbital-controlled"– stereoregular addition possible
using chiral phosphine ligands– frequently used in organic synthesis
O
R OH
O
R
Insertion and elimination30
Less common elimination reactions
-elimination:
Other ligand metallation reactions:
Cp2Zr
- Cp2Zr
Zr H
HHtBu
tBuH
Probably via -bond metathesis:
-
L2Pt L2Pt-
ZrZr
Via -bond metathesis or oxidative addition/reductive elimination
Insertion and elimination31
Less common elimination reactions
-elimination from alkoxides of late transition metals is easy:
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 exothermicand irreversible.
MO CH3 M
H+ CH2O