Molecular modelling study of alkene metathesis with phosphine ligated Grubbs-type precatalysts

Abstract

In this study, an attempt was made to identify the electronic and steric properties of the precatalyst ligands that determine the characteristics of phosphine ligated Grubbs-type precatalysts for alkene metathesis by means of molecular modelling. It was found from studying the literature that the possibilities for synthesising a wide range of phosphine ligands are almost unlimited. Additionally, it was found that there is no easy method to determine the electronic and steric properties of the precatalyst ligands in existence. Molecular modelling might provide a method to study potential ligands and precatalysts before tedious synthesis methods are attempted. It was found that the theoretically calculated structures of the commercially available precatalysts compared well with the experimental data reported in literature. It is also shown that the energy profiles for alkene metathesis of simplified model systems do not compare well with non-simplified systems. Correlations between these simplified model systems and experimental work have to be regarded as serendipitous at best. When the energy profiles of the various new and commercially available precatalysts are compared, similarities in the energy trends for 1-octene metathesis are observed. These similarities raise questions about the significance of the differences in the energy barriers. In an effort to better understand this, two low activity precatalysts were also investigated in an attempt to identify the area or trend of poor catalysis. Instead of providing the desired different result, trends very similar to that of the highly active precatalysts were observed. This led to the observation that, without a sufficiently large dataset, great care should be taken before conclusions are drawn from theoretical work. Since the electronic investigation did not provide the desired result of finding a fast and effective method of determining which ligand merits further investigation, some steric aspects were studied. Once again, the precatalysts proved to be remarkably similar and no definitive answer for the observed differences in the various precatalysts could be determined. A preliminary experimental study into the feasibility of the synthesis of the new potential ligands was done. The multi-step synthesis route resulted in low yields in some cases, with the need for large volumes of solvents to purify the products. The toxicity of phenylphosphine also has to be taken into account when considering these types of ligands. A new precatalyst obtained by using a new ligand should show a remarkable improvement over the current commercially available precatalysts to justify the additional cost to synthesise a new ligand. It would seem that for future projects more consideration should be given to the deactivation mechanism of the Grubbs-type precatalysts, since this seems to be the logical starting point to look for the answers to the experimentally observed differences. A deeper understanding of the mechanism of alkene metathesis can only be obtained if all aspects are investigated in as much detail as possible. While the results did not provide the initially expected outcome, some valuable insights were gained that challenge the current way of thinking about the alkene metathesis mechanism. It is also clear that to oversimplify a very complex reaction and using limited data will lead to false assumptions being made.