Olefin Metathesis Mechanism – involves a 4-centered intermediate (a metallacyclobutane): Olefin Metathesis Catalysts - it doesn’t matter what the carbene (M=CR 2 ) is on the catalyst that you start with, as the first step is loss of the CR 2 group on the way to the real catalytic cycle. Commercially available catalysts (Aldrich etc.):

Control over product distribution:  Drive reaction to a single product by constantly removing a volatile alkene (possible for any olefin metathesis involving two alkenes elimination of ethene or propene or butene etc.). (ADMET)  Drive reaction in a particular direction by using a large excess of one alkene (particularly easy if that alkene is a gas – e.g. ethene).  Drive ring opening reactions using ring strain. Especially useful for ROMP (ring opening metathesis polymerization).

Applications:  Synthesis of housefly pheromone by CM: Me(CH 2 ) 7 CH=CH 2 + H 2 C=CH(CH 2 ) 12 Me  Me(CH 2 ) 7 CH=CH(CH 2 ) 12 Me desired product + H 2 C=CH 2 + other products.  Neohexene process: Step 1 (acid cat dimerization): Me 2 C=CH 2 + H +  t BuHC=CMe 2. Step 2 (metathesis): t BuHC=CMe 2 + H 2 C=CH 2  t BuHC=CH 2 desired product + Me 2 C=CH 2 starting material.  Shell Higher Olefin Process (SHOP).  ROMP to make new polymers – can be done in a living fashion.  RCM in total synthesis - K.C. Nicolaou- Epothilone A - Alois Fürstner- Amphidinolides, Herbamurin, Pyrenophorin