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 pr oducts up to C-6 are present in measurable amounts for both reactions, and products up to C-8 are observed. The product distributions obtain ed 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 a re formed by a chain-polymerization mechanism. Analysis of the slopes and intercepts of the Schulz-Flory distributions allows the kinetic pa rameters 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 met athesis of propylene is determined to have a reaction order of 1.02 +/ - 0.05 and an activation energy of 53 +/- 3 kcal/mol. Propylene metath esis 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 methylcarben e recombination in the context of the proposed mechanism. (C) 1998 Aca demic Press.