Insertion and elimination

Peter H.M. Budzelaar

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 elimination3

Insertion in MeMn(CO)5

HOMO LUMOinsertion

TSagostic

2-acylCO adduct

CO

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