Reaction pathway and stereoselectivity of olefin metathesis at high temperature

The reactions of ethylene and propylene catalyzed by molybdenum model catalysts are examined at high temperature (800-880 K). Hydrocarbon products up to C-6 are present in measurable amounts for both reactions, and products up to C-8 are observed. The product distributions obtained are well described by a Schulz-Flory distribution, where a plot of ln(r(n)/n) versus n yields a straight line, indicating that products are formed by a chain-polymerization mechanism. Analysis of the slopes and intercepts of the Schulz-Flory distributions allows the kinetic parameters for high-temperature metathesis to be estimated. For ethylene, degenerate metathesis is determined to proceed with a reaction order of 1.3 +/- 0.2 and an activation energy of 55 +/- 7 kcal/mol. The metathesis of propylene is determined to have a reaction order of 1.02 +/- 0.05 and an activation energy of 53 +/- 3 kcal/mol. Propylene metathesis exhibits stereoselectivity for cis-2-butene formation that is two to three times greater than that predicted thermodynamically. This is suggested to be the result of the geometry necessary for methylcarbene recombination in the context of the proposed mechanism. (C) 1998 Academic Press.