Transcript lecture 12 catalysis_transformation of alkenes_alkynes

Lecture 12
CATALYSIS 2.
TRANSFORMATION OF ALKENES AND ALKYNES
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I.
METATHESIS OF ALKENES, ALKYNES AND CYCLOALKENES
A. Alkene metathesis
B. Alkyne metathesis
C. ROMP
D. Alkyne polymerization
II.
ALKENE DIMERIZATION AND OLIGOMERIZATION
A. Ethylene dimerization
B. Oligomerization by successive insertions
III. ALKENE ISOMERIZATION
A. Double bond migration by -elimination
B. Allylic C-H activation
C. Cis-trans isomerization via metallocarbenes
IV. OLEFIN POLYMERIZATION
TRANSFORMATION OF ALKENES AND ALKYNES

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
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
ALKENES METATHESIS

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
TRANSFORMATION OF ALKENES AND ALKYNES


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
dienes
 Ring 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
TRANSFORMATION OF ALKENES AND ALKYNES
ROMP in the industry:
TRANSFORMATION OF ALKENES AND ALKYNES
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]
TRANSFORMATION OF ALKENES AND ALKYNES

Mechanism:
TRANSFORMATION OF ALKENES AND ALKYNES

The presence of a double and triple bonds in the reactants
presents challenges: selective metathesis
TRANSFORMATION OF ALKENES AND ALKYNES

Metathesis also applies to nitriles:
TRANSFORMATION OF ALKENES AND ALKYNES

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
ALKENE DIMERIZATION AND OLIGOMERIZATION

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
 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 -ELIMINATION
Catalyst do not contain a hydride
ligand.
M should have at least two vacant
coordination sites like the 14e
species Fe(CO)3
ALKENE ISOMERIZATION by ALLYLIC C-H ACTIVATION
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
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ZIEGLER – NATTA TYPE ALKENE POLYMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
ALKENE DIMERIZATION AND OLIGOMERIZATION
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES
TRANSFORMATION OF ALKENES AND ALKYNES