1. Tacticity Isotactic and syndiotactic polymers are crystalline,
atactic is amorphous.
NMR spectroscopy is a powerful tool for studying polymer stereochemistry.
Ziegler/Natta catalysts allow control of tacticity.
Tacticity of polymer is determined by % m or r dyads. (Perfectly isotactic polypropylene has 100% m dyads)

2. 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.

3. Chain-end control

4. 13C NMR spectroscopy
Atactic: 9 signals (10 expected, 2 coincidental) based on pentads

5. C2 symmetric catalysts: isotactic polymers
C2 symmetric catalysts give mainly isotactic polymer.
Control is exerted by the ancillary ligands (site control), which direct the metal-bound polymer, which in turn directs the monomer (chain-end control).

6. Cs symmetric catalysts: syndiotactic polymers
Cs symmetric catalysts give mainly syndiotactic polymer.

7. Late transition metal catalysts

8. M=C complexes
Textbook H: Chapter 13.1 – 13.2
Textbook A: Chapter 9

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

10. Outline Carbene complexes Olefin metathesis Alkyne metathesis
Bonding
Structural and spectroscopic features
Synthesis
Reactivity
Olefin metathesis
Mechanism
Reaction overview
ROMP
RCM
Grubbs catalysts
Stereochemistry
Catalyst decomposition
Alkyne metathesis
Reactions related to olefin metathesis

11. 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)

12. Fischer carbenes: synthesis
Nucleophilic attack at a carbonyl ligand: most common method
Activation of a neutral acyl complex
From activated olefins

13. Schrock carbenes: synthesis
a-abstraction: induced by steric bulk
Rearrangement of coordinated ligands
From activated olefins

14. 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)

15. Carbene complexes: Fischer type

16. Alkylidene complexes: Schrock type

17. Structural and spectroscopic features
Fischer type
Schrock type
1H NMR: 5 – 15 ppm
13C NMR: 220 – 260 ppm
JCH typical: 150 – 160 Hz
JCH agostic: 90 – 120 Hz
MCC angle: 160 – 170°
MC distance: longer than M=C, but shorter than M-C
CX distance: shorter than C-X

18. Fischer carbenes: reactivity
Heteroatom substitution: related to the aminolysis of esters to give amides
Cyclopropanation
The Dötz reaction

19. Mechanism of the Dötz reaction

20. Schrock carbenes: reactions with electrophiles

21. Other reactions

22. Reference: Chem. & Eng. News 2002, Dec 23, 34-38
Olefin metathesis
Mechanism:
Such cycloaddition reactions between two alkenes to give cyclobutanes are symmetry forbidden and occur only photochemically. However, the presence of d-orbitals 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

23. Timeline