Ruthenium alkylidene metathesis
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Date
01/07/2019Author
Forcina, Veronica
Metadata
Abstract
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.