Download Organometallic Chemistry

Tacticity
isotactic
• Isotactic and
syndiotactic polymers
are crystalline,
atactic is amorphous.
• NMR spectroscopy is a
syndiotactic
atactic
powerful tool for studying
polymer stereochemistry.
Triad Tacticity
• Ziegler/Natta catalysts
Dyad Tacticity
allow control of tacticity.
m
•Tacticity of polymer is
determined by % m or r
dyads. (Perfectly isotactic
polypropylene has 100%
m dyads)
m
m
isotactic(mm)
isotactic (meso, m)
m
r
r
atactic (mr)
syndiotactic (racemic, r)
r
r
syndiotactic (rr)
Site vs. chain-end control
Site control mechanism:
the catalytic center determines
the stereospecificity of the polymer.
Chain-end control mechanism: the polymer chain determines the
stereospecificity of the final polymer.
Chain-end control
13C
NMR spectroscopy
Atactic: 9 signals (10 expected, 2 coincidental) based on pentads
Late transition metal catalysts
M=C complexes
Textbook H: Chapter 13.1 – 13.2
Textbook A: Chapter 9
2005 Nobel prize in chemistry
"for the development of the metathesis method in organic synthesis"
Richard Schrock
Massachusetts Institute
of Technology (MIT)
Cambridge, MA, USA
Yves Chauvin
Institut Français du Pétrole
Rueil-Malmaison, France
Robert Grubbs
California Institute of Technology
(Caltech) Pasadena, CA, USA
http://nobelprize.org/chemistry/laureates/2005/index.html
Outline


Carbene complexes
 Bonding
 Structural and spectroscopic features
 Synthesis
 Reactivity
Olefin metathesis
 Mechanism
 Reaction overview


Grubbs catalysts
 Stereochemistry
 Catalyst decomposition
Alkyne metathesis
Reactions related to olefin metathesis



ROMP
RCM
M=C complexes: comparison
Property
Fischer-type
(carbene)
Schrock-type
(alkylidene)
Nature of carbene C
Typical R groups
Typical metal
Typical ligands also attached
to the metal
Electrophilic
p donor (e.g. OR)
Mo(0), Fe(O), Cr(0)
Good p acceptor
(e.g. CO)
Nucleophilic
Alkyl, H
Ti(IV), Ta(V), W(VI)
Good s or p donor
(Cl, Cp, Alkyl)
Fischer carbenes: synthesis

Nucleophilic attack at a carbonyl ligand: most common method

Activation of a neutral acyl complex

From activated olefins
Schrock carbenes: synthesis

a-abstraction: induced by steric bulk

Rearrangement of coordinated ligands

From activated olefins
M=C complexes: comparison
Property
Fischer-type
(carbene)
Schrock-type
(alkylidene)
Nature of carbene C
Typical R groups
Typical metal
Typical ligands also attached
to the metal
Electrophilic
p donor (e.g. OR)
Mo(0), Fe(O), Cr(0)
Good p acceptor
(e.g. CO)
Nucleophilic
Alkyl, H
Ti(IV), Ta(V), W(VI)
Good s or p donor
(Cl, Cp, Alkyl)
Carbene complexes: Fischer type
Alkylidene complexes: Schrock type
Structural and spectroscopic features
Fischer type
Schrock type
NMR: 5 – 15 ppm
13C NMR: 220 – 260 ppm
JCH typical: 150 – 160 Hz
JCH agostic: 90 – 120 Hz
1H
MCC angle: 160 – 170°
MC distance: longer than M=C, but shorter than M-C
CX distance: shorter than C-X
Fischer carbenes: reactivity

Heteroatom substitution: related to the aminolysis of esters to give amides

Cyclopropanation

The Dötz reaction
Mechanism of the Dötz reaction
Schrock carbenes: reactions with electrophiles
Other reactions
Olefin metathesis
Mechanism:
Such cycloaddition reactions between two alkenes to give cyclobutanes are
symmetry forbidden and occur only photochemically. However, the presence of dorbitals on the metal alkylidene fragment breaks this symmetry and the reaction is
quite facile.
Normally, the products are statistical, unless the reaction can be driven
in some way or the two alkenes have different reactivities.
Reference: Chem. & Eng. News 2002, Dec 23, 34-38
http://nobelprize.org/nobel_prizes/chemistry/laureates/2005/animation.html
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