Synthesis and modelling of imine derivatives as ligands for Grubbs type pre–catalysts

Abstract

The Grubbs type pre–catalysts are widely used for olefin metathesis. The pre–catalysts exhibit high activity and selectivity, but low stability and short lifetimes Salicylideneanilines (SA) are Schiff bases that can be derived from the reaction of amine derivatives with salicylaldehyde and 2–hydroxy–1–naphthaldehyde. These ligands were synthesized to improve the activity, stability and lifetime of the Grubbs first and second generation pre–catalysts. In an attempt to improve on these properties both theoretical and experimental investigations were conducted. Molecular modelling was done using Material Studio, Dmol3 (PW91/GGA/DNP) to evaluate potential imine derivative as ligands for Grubbs type pre–catalysts, for the metathesis reaction with 1–octene. Forty five ligands were chosen to investigate properties for suitable ligands. The usefulness of the HOMO energy as preliminary criteria for screening suitable ligands was investigated. The HOMO energy of possible ligands was calculated against the well defined Verpoort ligand. In this study the ligand with lower energy than that of the Verpoort's ligand was considered. Analysis of the electron density, electrostatic potential, Fukui functions, HOMOLUMO orbitals and population analysis were used to address the research problem. The second criteria included being found in literature, synthesis procedure as well as a high yield. The two most promising ligands were chosen to be synthesized. These ligands were used in the synthesis of new Grubbs 1 and 2 type pre–catalysts. The third criteria were the dissociation energy (Ru–N) for the new Grubbs type pre–catalysts for activation properties that may influence the hemilability. Furthermore molecular modelling helped to gain insight into the mechanism of the 1–octene metathesis reaction by using the Potential Energy Surface (PES) scan. Both ligands were successfully synthesized according to literature methods. The ligands were characterized by MS, IR and NMR techniques. The synthesis of substituted Grubbs 1 type pre–catalysts was unsuccessful. The substituted Grubbs 2 type pre–catalysts were obtained and characterized by MALDI–TOF and NMR techniques. The two substituted Grubbs 2 type pre–catalysts were tested for metathesis activity with 1–octene. Only one substituted pre–catalyst was active for metathesis. This catalyst was less active and selective than the Grubbs first and second generation pre–catalysts but showed an increased lifetime.