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Olefin Metathesis

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.

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Olefin Metathesis

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  1. Olefin Metathesis Mechanism – involves a 4-centered intermediate (a metallacyclobutane): Olefin Metathesis Catalysts - it doesn’t matter what the carbene (M=CR2) is on the catalyst that you start with, as the first step is loss of the CR2 group on the way to the real catalytic cycle. Commercially available catalysts (Aldrich etc.):

  2. 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.). • 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).

  3. Applications: • Synthesis of housefly pheromone by CM: Me(CH2)7CH=CH2 + H2C=CH(CH2)12Me  Me(CH2)7CH=CH(CH2)12Me desired product + H2C=CH2 + other products. • Neohexene process: Step 1 (acid cat dimerization): Me2C=CH2 + H+ tBuHC=CMe2. Step 2 (metathesis): tBuHC=CMe2 + H2C=CH2 tBuHC=CH2 desired product + Me2C=CH2 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

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