1. Insertion and elimination
Peter H.M. Budzelaar

2. Insertion reactions If at a metal centre you have
a s-bound group (hydride, alkyl, aryl)
a ligand containing a p-system (olefin, alkyne, CO)
the s-bound group can migrate to the p-system.
Insertion and elimination

3. Insertion in MeMn(CO)5 insertion TS agostic HOMO LUMO CO CO adduct
h2-acyl CO
Insertion and elimination

4. 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.
Insertion and elimination

5. 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.
Insertion and elimination

6. 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!
Insertion and elimination

7. Double CO insertion ?
Deriving a mechanism from a reaction stoichiometry is 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.
Insertion and elimination

8. No double CO insertion !
Insertion and elimination

9. Promoting CO insertion
"Bulky" ligands
Lewis acids
Coordinate to O, stabilize product
Drawback: usually stoichiometric
Insertion and elimination

10. 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.
Insertion and elimination

11. 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.
Insertion and elimination

12. 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.
Insertion and elimination

13. 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
Insertion and elimination

14. Mechanism of olefin insertion
Standard Cossee mechanism
Green-Rooney variation (a-agostic assistance):
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
Insertion and elimination

15. Insertion in M-H bonds
Insertion in M-H bonds is nearly always fast and reversible.
Þ 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
Insertion and elimination

16. 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.
dominant alkyl
dominant olefin
Insertion and elimination

17. Catalyzed olefin isomerization
Insertion and elimination

18. 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 in rate !
Reason: shape of orbitals (s vs. sp3)
Insertion and elimination

19. Repeated insertion
Multiple insertion leads to dimerization, oligomerization or polymerization.
Key factor: kCT / kprop = k
k » 1: mainly dimerization
k » : oligomerization (always mixtures)
k « 0.1: polymerization
k » 0: "living" polymerization
For non-living polymerization:
Insertion and elimination

20. Schulz-Flory statistics
k = 0.7
Key factor: kCT / kprop = k
k » 1: mainly dimerization
k » : oligomerization (always mixtures)
k « 0.1: polymerization
k » 0: "living" polymerization
k = 0.1
For non-living polymerization:
k = 0.02
Insertion and elimination

21. 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 elimination

22. 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.
Insertion and elimination

23. Trimerization via metallacycles
Key issues:
Geometrical constraints prevent b-elimination in metallacyclopentane.
Formation of 9-membered rings unfavourable.
Ligand helps balance (n) and (n+2) oxidation states.
(and others)
Insertion and elimination

24. CO/olefin copolymerization
CO cheaper than ethene
Copolymer more polar than polyethene
much higher melting point
Chemically less inert
No double CO insertion
uphill
No double olefin insertion
CO binds more strongly, inserts more quickly
Slow b-elimination from alkyl
5-membered ring hinders elimination
M = L2Pd, L2Ni
Insertion and elimination

25. 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 !
Insertion and elimination

26. Insertion of longer conjugated systems
Attack on an h-polyene is always at a terminal carbon.
Þ LUMO coefficients largest
Þ Usually a,w-insertion
Insertion and elimination

27. Insertion of longer conjugated systems
A diene can be h2 bound.
Þ 1,2-insertion
Metallocenes often do not have enough space for h4 coordination:
Insertion and elimination

28. Diene rubbers Butadiene could form three different "ideal" polymers:
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.
Insertion and elimination

29. 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
Insertion and elimination

30. Less common elimination reactions
a-elimination:
Other ligand metallation reactions:
Probably via s-bond metathesis:
Via s-bond metathesis or oxidative addition/reductive elimination
Insertion and elimination

31. Less common elimination reactions
b-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 exothermic and irreversible.
Insertion and elimination