Download Steric protection of alkylidene is not needed:

M=CH2 carbenes:
(also M=CH2 species can be isolated)
The bimolecular reaction of M=CH2 to ethylene species is one of the
major pathways for decomposition of metathesis catalysts!
First Metal-Carbenes where not active in
metathesis:
Coincidental discovery of active W-Carbene:
Active in
Metathesis
(only with
minor
amounts of
AlCl3)
Design of active W-Carbene catalyst:
- carbene should be neopentylidene (quite stable)
- complex should be well-accessible for alkenes (low coordination of
4 => free coordination site)
- X should NOT be O (dimer formation) but NR with large R group
- other substituents should be electron-withdrawing
(important for alkene coordination)
Synthesis:
Problems with W-Carbene catalyst:
- Reactions sometimes slow
- Intermediate can be isolated
 Mo-Carbene catalysts
(Remember: bonds to first-row transition metals are weaker = more dynamic)
Selective Cross Metathesis:
Aim = selectively obtain product R1CHCHR2
Alkyne metathesis:
Nitrile-Alkyne cross metathesis:
Ring opening metathesis:
(ROM)
Ring closure metathesis:
(RCM)
(Grubbs, JACS 1992)
Mechanism ring closure metathesis:
-
Reaction is driven by entropy: 1 molecule => 2 molecules
Even 7-membered rings are possible
By-product is easily removable gas
No polymer by-products: intramolecular reactions are faster than
intermolecular reactions
- Also heavily substituted olefins can be converted
Nowadays metathesis catalysis is extremely important in
organic synthesis, especially for making ring
compounds!
ADMET:
(acyclic diene metathesis)
polymers
Ring opening metathesis polymerization:
(ROMP)
Mechanism:
ROMP with chiral enantioselective catalysts:
isotactic
Chiral enantioselective catalysts:
Grubbs catalysts:
- coincidence; it was planned to polymerize the cyclopropene
- Cl2(PPh3)2Ru=CH-CH=CPh2 is not very active in metathesis but
Cl2(PCy3)2Ru=CH-CH=CPh2 is!
- BIG ADVANTAGE: not air-sensitive (reactions can be run in air)
- Disadvantage: large scale synthesis not possible...
1st generation Grubbs catalyst:
-
One-pot synthesis
From Cl2Ru(PCy3)2 + PhCHN2 + P(Cy)3
Multi-kg scale!
Fundamental difference with Schrock carbene: electron-rich metal
that likes soft ligands like ethylene but not Lewis-bases like H2O
- => water and functional group tolerant
2nd generation Grubbs catalyst:
- substitution of PCy3 for Arduengo Carbene (highly electrondonating)
 Ru even more electron rich => highly active catalyst (very
functional group tolerant!)
Self-healing polymers
Grubbs, Nature 2001, 409, 794.
Cross-linked dicyclopentadiene polymer:
First proposed mechanism for metathesis:
(Calderon)
Bis-olefine mechanism (even number of carbon atoms):
Chauvin mechanism for metathesis:
M-Carbene mechanism (odd number of carbon atoms):
.Exchanging dance partners
Grubbs experiment to prove mechanism:
FOUND: complete scrambling of deuterium