Modelling of the cross-metathesis of cyclopentene and 1,7-octadiene

Abstract

Experiments were carried out to study the cross-metathesis of cyclopentene and 1,7-octadiene initiated by WCl 6/SnMe 4 homogeneous catalyst. Concentration changes of the products and those of the starting olefins were monitored by withdrawing samples from the reaction mixtures and analyzing them by GC-MS. A chemical mechanism was constructed from carbene reactions such as polymerization, termination, initiation, chaintransfer, etc. to describe the formation of the products, and a kinetic model was developed to determine their quantitative relations. Using a computer program, the reactions could be simulated over a wide range of the conversion. Relative rate constants were determined for the elementary reactions of the mechanism. It was found that initiation by methylene carbene is approx. eight times as fast as initiation by alkylidene carbenes and the propagation of ring-opening polymerization of cyclopentene. The rate constant of chain-transfer to terminal olefin by formation of an alkylidene carbene is somewhat greater than the rate constant of the propagation. However, the rate constant of chain-transfer involving the reverse orientation of the terminal olefin and resulting in the formation of a methylene carbene is 1.5–2 orders of magnitude smaller than the former ones. The results also confirm the earlier conclusions that the complex alkylidenes, W = CHR, are much preferred to the complex methylene, W = CH 2, as chain-carriers in degenerate metathesis of terminal alkenes.