Migratory Insertion & Elimination Rxns

A migratory insertion reaction is when a cisoidal anionic and neutral ligand on a metal complex couple together to generate a new coordinated anionic ligand. This new anionic ligand is composed of the original neutral and anionic ligands now bonded to one another.There is NO change in the oxidation state or d electron-count of the metal center.

MnOC

OC CO

CH3

C

C

MnOC

OC CO

C

O CH3

MnOC

OC CO

C

O CH3

L

+L

migratory insertion ligand addition

Mn(+1)18e-

Mn(+1)16e-

Mn(+1)18e-

elimination

O

O O O

acyl

General Features of Migratory Insertions:

1) No change in formal oxidation state (exception: alkylidenes)

2) The two groups that react must be cisoidal to one another

3) A vacant coordination site is generated by the migratory insertion. Therefore, a vacant site is required for the back elimination reaction (e.g., b-hydride elimination). A trapping ligand is often needed to coordinate to the empty site formed from a migratory insertion in order to stop the back elimination reaction.

4) Migratory insertions are usually favored on more electron-deficient metal centers.

The following are common anionic and neutral ligands that can do migratory insertion reactions with one another:

Anionic: H-, R- (alkyl), Ar- (aryl), acyl-, O2- (oxo)

Neutral: CO, alkenes, alkynes, carbenes

CO and alkyl migratory insertions (as shown on previous slide) are extremely important and are often generically referred to as carbonylation reactions.

Hydride and CO migratory insertions to produce formyl groups are not common due to the thermodynamic instability of the formyl-metal interaction.

Some Electronic effects

C FeC

C

R

CO

OO

OC Fe

L

C

CO

OO

+ L

R O

THF

ZZ

Z+ = Li+ > Na+ > (Ph3)2N+

L = PMe3 > PPhMe2 > PPh2Me > CO

R = n-alkyl > PhCH2

best Lewis acid - can coordinate to electron-richCO ligands and drain off some e- density

strongest coordinating ligand - best trapping ligand

most electron-rich alkyl group makes the best nucleophile formigrating to the electron-deficient CO

Migration vs. Insertion

MnOC

OC CO

CH3

C

C

O

O

Mn(+1)18e-

MnOC

OC CO

CO

O CH3

Mn(+1)16e-

MnOC

OC CO

CH3

C

C

O

O

Mn(+1)18e-

MnOC

OC CO

CO

O

CH3

Mn(+1)16e-

a MIGRATION rxn involves theanionic ligand doing a nucleophillic-like attack on theneutral ligand. This involves the anionic ligand moving to the site where the neutral ligand is coordinated. An empty coordination site is left behind.

an INSERTION rxn involves the neutral ligand moving over to where the anionic ligand is coordinated and "inserting" into the anionic ligand-metal bond to generate the new anionic ligand. An empty coordination site is left behind from where the neutral ligand originally was located.

Migration

Insertion

While most systems studied have been shown to do migrations, both are possible. The following is a system where both are very similar in energy and the solvent used favors one or the other.

Fe

CEtPh3P

O

*

FePh3P

*

OEt

*CO FeCPh3P

*

OEtO*

inversion

FePh3P

*O

Et

*COFe

CPh3P

*O

EtO*

retention

Et migrates

CO inserts

HMPA

CH3NO2

Alkene Migratory Insertions

ZrCH3

ZrCH3

Zr

CH3

Zr

CH3

ZrCH3

M H

HH

M H

HHH

HH

H‡

M

HH

H

H H

migratory insertion

-hydride elimination

Alkene Migratory Insertion – b-Hydride Elimination

Nb H R

*

*

Nb

*

*

RH

CONb

*

*RH

CO

NMR ir radiat ion of the Nb-hydr ide resonance af f ects the NMR resonance f or the alkyl hydr ide,demonstrating that they are connected by the migratory inser tion mechanism

Problem: Why don’t either of the complexes shown below do alkene-hydride migratory insertions at room temperature?

IrPh3P

Cl PPh3

CO

HEt3P

Pt

PEt3H

Problem: Sketch out and label the two mechanistic steps (in the correct order) that are occurring for the following reaction.

Ru

CH

O

+ PPh3 Ru

CPh3P

O

RhMe3P

RCN PMe3

H

CH3

RhMe3P

RCN PMe3

H

CH3

RhMe3P

RCN PMe3

CH3

Ph3P

Pd

PPh3Cl

OO

Me

Ph3P

Pd

Cl

O

O

Me

Ph3P

Pd

Cl OO

Ph3P

Pd

Cl

O

O+PPh3

PPh3

-PPh3

Agostic C-H to Metal Interactions – “Frozen Migratory Insertion”

Co

R3P

H

Co

R3P

+ H+

* *

Co

R3P

*H

H

H

One of the C-H bonds of the methyl group is within bonding distance to the Co center. This is called an Agostic C-H bond interaction.

Because the C-H bond is sharing some of its s-bond electron density with the metal, the C-H bond is weakened. This produces some relatively clear-cut spectroscopic characteristics:

1) nC-H infrared stretching frequency is lowered to the mid-2500 cm-1 region from a normal

value of 2900-3000 cm-1

2) the JC-H coupling constant in the 13C NMR is lowered to around 70-90 Hz from a normal

value of 150 Hz.

3) the 1H chemical shift of the agostic proton is in the -10 to -15 ppm region, much like a metal-hydride resonance.

Carbene-Alkylidene Migratory Insertions

M CH2

X

+ L M CH2

L X

X = H , R , OR , halide

Normally a migratory insertion refers to a neutral ligand reacting with an anionic ligand to produce a new anionic ligand. But if we electron-count the carbene as a dianionic ligand (alkylidene), we are reacting a monoanionic ligand (X) with a dianionic ligand (alkylidene) to make a new monoanionic ligand. This changes the oxidation state of the metal center and is now formally a reductive coupling reaction.

In the case of X = H-, the reverse reaction is called an a-hydride abstraction or elimination.

Re

CH3Ph3P

H3C

*+ Ph3C+

HRe

Ph3P

H3C

*

CH2

Re

Ph3P

*

Re

Ph3PH

*

Ph3C+ = good hydride abstracting reagent

WCH3

PhPh3C

HW

CH2

PhNCMe

WCH2

NCMe

Ph

Eliminations

M

H H

HM

H

-hydride elimination

M

O

M

R

CO

carbonyl eliminationor decarbonylation

R

M M

H

-hydride elimination

R

HH

R

The key points are:1) No change in formal oxidation state (exception: alkylidenes)

2) You must have an empty orbital that is cisoidal to the group that you are doing an elimination reaction on. Alternatively, a cisoidal labile ligand that can easily dissociate to open up an empty orbital.

M CH3

HH

M CH3

HHH

HH

H‡

M

CH3H

H

HH

migratory insertion

M

CH3H

H

HH

M

CH3

H

HH

H

M

CH3H

HH

H

M CH3

HHHH

‡

M

HH

HH

CH3

-hydride elimination

methyl elimination

rotation of C-C bond to move CH3

group away from metal to avoid steric

effects

VeryDifficult!

But the reverse methyl elimination rxn is very difficult:

One unusual example of what is believed to be a methyl elimination reaction is involved in the following transformation (Bergman, JACS, 2002, 124, 4192-4193):

RhN

H3C

Me3P

*

CCH3

+ HSiPh3 RhC

H3C

Me3P

*

NSiPh3

+ CH4

RhN

H3C

Me3P

*- NCCH3

+ HSiPh3

Rh

H3C

Me3P

*

H

SiPh3Rh

Ph3Si

Me3P

*- CH4

Rh(+5)C

CH3

+ NCCH3

NC

CH3

RhMe3P

*

CN

Ph3Si

CH3

RhMe3P

*

CN SiPh3

H3C

RhC

H3C

Me3P

*

NSiPh3

SiPh3Rh

CN

H3C

Rh

C

N

CH3

SiPh3

ligand dissociation

methylelimination

Note that the isocyanide ligand ismore strongly donating and a better

-backbonding ligand than the startingacetonitrile. This keeps this from being

a catalytic reaction.

One reason that the methyl elimination reaction occurs here is that the b-hydride elimination reaction generates a high energy ketene-imine:

RhMe3P

*

CN SiPh3

H3C

RhMe3P

*

C

N SiPh3

H2CH

Problem: Identify each step in the following mechanism. Some steps may have several things occurring.

Co

H3C

H3C

*

Co

H3C

*

Co

H3CH

*

Co

Me3P

* CH2=CH2

PMe3

+ CH4 +

1 2

3

Problem: Sketch out a detailed mechanism and label each step for the following overall reaction.

RhCH3H3C

Ph3P RhPPh3

OC+ 2CO

H3C

O

CH3

+