Lecture 12 CATALYSIS 2. TRANSFORMATION OF ALKENES AND ALKYNES Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Lecture 12 CATALYSIS 2. TRANSFORMATION OF ALKENES AND ALKYNES

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

TRANSFORMATION OF ALKENES AND ALKYNES

I. METATHESIS OF ALKENES, ALKYNES AND CYCLOALKENESA. Alkene metathesisB. Alkyne metathesisC. ROMPD. Alkyne polymerization

II. ALKENE DIMERIZATION AND OLIGOMERIZATIONA. Ethylene dimerizationB. Oligomerization by successive insertions

III. ALKENE ISOMERIZATIONA. Double bond migration by -elimination B. Allylic C-H activationC. Cis-trans isomerization via metallocarbenes

IV. OLEFIN POLYMERIZATION

Metathesis – derived from the greek word meaning “to place differently” or “to transpose”

Metal-catalyzed exchange of alkylidene and alkylidyne units in alkenes and alkynes:

METATHESIS OF ALKENES AND ALKYNES

Depending on the nature of the applied alkene and the reaction conditions, metathesis reactions can give different results, allowing a structuring of the area:

ALKENE METATHESIS

ALKENE METATHESIS

ALKENES METATHESIS

Chauvin Mechanism (1970)

A pathway that involves a metallacyclobutane intermediate

A metallacyclobutane intermediate was isolated and characterized by Schrock (1989)

ALKENES METATHESIS

A metal alkylidene unit M=CH2 functions as the catalytically active center

Mo, W, Re, Rh have proven to be particularly useful central metal atoms

Early metathesis catalysts were derived from transition metal halides and carbanion donors (WCl6 / Et2Al / EtOH)

ALKENE METATHESIS

ALKENE METATHESIS - CATALYSTS


Both complexes have low coordination number (CN = 4) allows a facile access to the central metal atom.

Spectator ligands aids (imido , oxo ) in the formation of the metallacycle intermediate

CM has been used extensive the industry in the form of Higher Olefin Process (SHOP) – a combination process consisting of oligomerization, isomerization, and metathesis steps.

The metathesis step:

CROSS METATHESIS (CM)

If a shop process is followed by hydroformylation, fatty alcohols with 8-22 carbon atoms are formed!

In the laboratory, CM has a limited application. Products are obtained as mixtures of Z/E isomers

CROSS METATHESIS

In contrast to CM, Ring Closing Metathesis (RCM) has become a standard method in organic chemistry.

RING-CLOSING METATHESIS

Newer catalysts has inspired natural products synthesis:

RING CLOSING METATHESIS

Assymetric ring closure metathesis (ARCM) has also been developed using chiral catalysts:

RING CLOSING METATHESIS

Ring-opening metathesis is the reverse of RCM. A cross-metathesis with ethylene forms terminal dienesRing strain favors ring opening and are specially

common with nobornenes and cyclobutenes.

Synthetic utility is limited by the formation of different CM and self-methathesis products

RING OPENING METATHESIS

In some cases selectivity are achieved:

RING OPENING METATHESIS

Self metathesis; use of open chain alkene substrate is avoided.

The C=C double bond in the monomer is conserved in the polymer.

The catalytically active species is fixed to the end of the growing chain (“living polymer”)

As soon as a certain monomer is consumed, a different monomer can be used to make block coplymers.

Can be deactivated by reaction with a carbonyl group to yiled M=O (Wittig reaction).

RING OPENING METATHESIS POLYMERIZATION

ROMP mechanism:

RING OPENING METATHESIS POLYMERIZATION

ROMP in the industry:


The C=C double bond in this norbornene rubber allows for cross-linking

ROMP in the industry:


Metathesis involving C C triple bonds can proceed symetrically (Yne YneM) and in the mixed form (Ene YneM)

Metathesis using dissymetrical alkynes using MoO3 or WO3 as heterogeneous catalysts:

ALKYNE METATHESIS

Prototype Catalyst: [W(C-tBu)(O-tBu)3]


Mechanism:


The presence of a double and triple bonds in the reactants presents challenges: selective metathesis


Metathesis also applies to nitriles:


Mixed (EneYneM):

ALKENE – ALKYNE METATHESIS

Example:. [Ru(CO)3Cl2l2 catalyzes the skeletal rearrangement of 1,6- and 1,7-enynes to form vinylcycloalkenes (Murai, 1994) :

ALKENE – ALKYNE METATHESIS

SAMPLE PROBLEM:

Assume 2-pentene and 2-hexene undergo metathesis. AT equilibrium what are all the possible alkenes that would be present, neglecting stereochemistry about the double bond? Remember to consider self metathesis reactions.

SAMPLE PROBLEM:

What is the product of cyclooctene metathesis?

Two mechanisms:

- via metallacyclopentane intermediate

- insertion of 2 alkene molecules into M-H

ALKENE DIMERIZATION AND OLIGOMERIZATION

ALKENE DIMERIZATION VIA METALLACYCLOPENTANE

Involves coordination of two ethylene molecules, oxidative coupling, -elimination then reductive elimination.

Metal has a low number of valence electron and at least two non-bonding electrons, - necessary for oxidative coupling


Same as polymerization but limited to insertion of two monomeric olefins into the M-H bond.

This system is made catalytic by the -elimination step.

Key point: -elimination should be faster than 3rd

alkene insertion.

ALKENE DIMERIZATION BY SUCCESSIVE INSERTION

Isomerization can occur via migration of the double bond – terminal olefin to an internal olefin.

ALKENE ISOMERIZATION

ALKENE ISOMERIZATION by -ELIMINATION

16e hydride complexes isomerize terminal olefins via reversible insertion of the olefin into the M-H bond followed by -elimination.

Mixture of cis and trans is obtained with the more stable trans form being major.

ALKENE ISOMERIZATION by -ELIMINATION

Example:

ALKENE ISOMERIZATION by ALLYLIC C-H ACTIVATION

Catalyst do not contain a hydride ligand.

M should have at least two vacant coordination sites like the 14e species Fe(CO)3

CIS TRANS or Z/E ISOMERIZATION via METALLOCARBENES

Ziegler and Natta Polymerization catalyst:TiCl3/Et2AlCl – a heterogenous mixture

Proposed mechanism: (Cosse, 1975)

ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION

Watson, 1982, DuPont: soluble in initiator, LuCp*2CH3

























Documents

Lecture 12 CATALYSIS 2. TRANSFORMATION OF ALKENES AND ALKYNES Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display