Formation of ruthenium hydrides via Fischer carbene intermediates

Abstract

Olefin metathesis is a key C-C bond formation reaction and is used in a variety of fields including natural product synthesis, fine chemical synthesis and active pharmaceutical ingredient synthesis. However, progress has been hampered in part due to unwanted side reactions such as olefin isomerisation. Research has shown the culprit for these side reactions is a ruthenium hydride species. Once identified it was realised that if the formation of such a species could be controlled, there was scope for tandem catalytic reactions in which metathesis could be followed by olefin isomerisation or hydrogenation.

A common synthetic route to ruthenium hydrides from metathesis catalysts, specifically Grubbs first and second generation catalysts, is through reaction with vinyl ethers. It has been shown that Grubbs catalysts react with alkyl vinyl ethers to form Fischer carbene complexes, which can be thermally decomposed into ruthenium hydrides capable of isomerising alkenes. A more convenient route was developed by Nishida utilising vinyloxy trimethylsilane in place of alkyl vinyl ethers, resulting in shorter reaction times and lower temperatures. Despite the importance of such reactions there are limited studies on the mechanism of formation of the ruthenium hydride from Fischer carbene complexes.

This research has focussed on elucidating the mechanism of Nishida’s reaction in the hope of shedding light on the factors that affect such a system and that could be extended to alkyl vinyl ether systems. This has been achieved using in situ 1H and 31P{1H} NMR spectroscopy, kinetic modelling, isotopic labelling and 2D NMR spectroscopy to aid in the characterisation of intermediates and products. The results of these studies have confirmed that the reaction proceeds via a Fischer carbene intermediate and in the case of CH2Cl2 or in toluene with excess water via the desilylated Fischer carbene too. Addition of a chloride source enhanced the rate of the formation of ruthenium hydride from the Fischer carbene indicating an intermolecular mechanism in which a chloride anion from another molecule attacks the silicon centre of the TMS group releasing TMSCl as a side product. A key finding of this study has been the second product formed during the reaction: RuCl2(CO)(PCy3)2. This dihalocarbonyl species is little reported in the literature and is an area of future research into its potential catalytic properties.