Ruthenium alkylidene metathesis
The olefin metathesis of oleochemicals is a promising method to produce valuable compounds from renewable feedstocks. However, several issues hinder the industrial application of this methodology such as low catalyst turn over number (TON), selectivity and substrate conversion. Using the ethenolysis of methyl oleate (MO) as a model reaction, the optimisation of this valuable process was investigated. A range of commercially available metathesis pre-catalysts were examined and evaluated based on several key criteria: conversion of methyl oleate, selectivity towards ethenolysis over self metathesis and TON. After identifying the best performing catalyst (Hoveyda-Grubbs 1st generation pre-catalyst), the remaining reaction conditions (ethylene pressure, catalyst loading, reaction time and temperature) were investigated using design of experiments. The optimised conditions obtained gave excellent conversion of methyl oleate. It is proposed that the remaining issues of selectivity and TON may be improved by applying a membrane separation technique. The initiation mechanism of Grubbs 3rd generation pre-catalyst (GIII) was also investigated to unravel the difference in selectivity compared to Grubbs 2nd generation pre-catalyst (GII) (12 % and 17 % respectively), as both pre-catalysts should give the same catalytically-active alkylidene species. The model reaction of GIII with ethyl vinyl ether (EVE) was followed by stopped-flowed UV spectroscopy (SF-UV). Due to the extreme sensitivity of the GIII pre-catalyst, substantial optimisation of the instrument set-up was required in order to achieve reproducible kinetic data and anaerobic conditions. The model reaction was studied by SF-UV at different concentrations of EVE and the effect of 3-bromopyridine addition was also investigated. The kinetic data suggests that the initiation of GIII proceeds via a combination of associative and dissociative pathways. Accordingly, two different alkylidene active species are able to react with an olefin (MO) when GIII is used as pre-catalyst compared with one alkylidene species when GII is employed as pre-catalyst. This is proposed to be crucial for the difference in selectivity between GII and GIII pre-catalysts in the ethenolysis of MO.