Ben Ward Flashcards
1
Q
Preparation of active catalyst
A
- Oxidative M-C cleavage
- Protonation
- Abstraction
2
Q
Explain how tacticity of a polymer can be determined experimentally
A
- Using 13C NMR
- NMR signal of the monomer unit X is determined by the neighbouring two monomer units
- The chemical shift for each combination have been determined for many polyolefins
3
Q
Types of chiral polymers
A
- Atactic (Same chirality for entire polymer)
- Isotactic (Random chirality for entire polymer)
- Syndiotactic (alternating stereocentres)
- Hemi-isotactic (alternating same stereocentre and random stereocentre)
4
Q
Two types of stereocontrol in polymerisation
A
- Chain end control: growing polymer chain determines the stereochemistry. Mis-insertion is propagated and error continues till next mis-insertion
- Enantiomorphic site control: chiral catalyst determines stereochemistry. Mis-insertions are ignored
5
Q
Hydroamination - how rate of cyclisation is affected by size of R group in chain
A
- Larger R groups enforce a larger C-C-C angle and therefore shrink neighbouring angles
- Angle compression give lower energy of TS and therefore higher rate
6
Q
why catalysts based upon main group elements tend to give lower enantioselectivity?
A
Equilibrium Position:
-Active selective catalyst may be in equilibrium with highly active, non selective starting material. Even in small quantities, still much more reactive lowering e.e.
Ligand redistribution:
- As supporting ligands redistribute, multiple metal containing species present
- different compounds give different product distribution, lowering e.e.
7
Q
How solvent effects activity of catalyst
A
Non polar solvents:
- Do not dissolve cationic complexes
- catalyst free to react with alkene
- Toulene
Non innocent (anything protic): -Chloroform. catalyst so acidic it pulls a chloride ion away from CHCl3
Semi-innocent:
- coordinating solvents that stop catalyst working, some activity still observed
- THF, py
Suitable:
-Chlorobenzene
