Modelling and synthesis of Grubbs-type complexes with hemilabile ligands

Abstract

Metathesis is a valuable method for the production of new alkenes and in the last 50 years a lot of catalytic systems for alkene metathesis were developed. In an earlier study, a Grubbs 2-type of catalyst was developed, which contained a N^0 hemilabile ligand with two aromatic R groups. This catalyst, which is called the Puk-Grubbs 2 pre-catalyst, showed an increase in stability, activity, selectivity and a longer lifetime in a 1-octene self-metathesis reaction in comparison to Grubbs 2. In order to improve these properties even further, variation in R groups of these Iigands were investigated by theoretical and experimental means. By using the molecular modelling program Material Studio, 202 possible hemilabile Iigands were investigated. During the first screening the HOMO, LUMO and electron density with electrophilic Fukui function of the Iigands were compared to the diphenyl alcohol of the Puk-Grubbs 2 catalyst and 42 Iigands showed similar electronic and steric properties. In the second screening, the HOMO, LUMO and electron density with nucleophilic Fukui function of these Iigands, when coordinated to a simplified Grubbs 2 pre-catalyst, were again compared to the simplified Puk-Grubbs 2. From these results, 5 Iigands were chosen to investigate experimentally from which 3 Iigands showed similar electronic properties to the Puk-Grubbs 2 catalyst. The other two Iigands were chosen to see what influence their bulky R groups have on the coordination of the ligand to Grubbs 2 as well as on the activity during a metathesis reaction. All five Iigands could be synthesized according to methods from literature. One was obtained in a very low yield (16%). They were characterized by MS, IR and NMR. Due to the bulkiness and steric hindrance, two Iigands could not be coordinated to Grubbs 2 catalyst, but the other three complexes were synthesized in a relatively good yield. Because the three synthesized complexes were sensitive to air, these were only characterized by MS and NMR. These results correlated well with the modelling results. The three synthesized complexes were found to be active for 1-octene metathesis reactions. Only one synthesized complex showed a similar selectivity to that of the Puk-Grubbs 2 catalyst. However, a much longer lifetime and higher turnover number was observed for all three complexes. An increase in temperature and higher catalyst load showed an increase in activity. It was also found that with a higher activity, the selectivity of the synthesized complexes decreased.