Modellering en sintese van Grubbs-tipe komplekse met imienligande

Abstract

The development of the well-defined ruthenium carbene complex 1 is one of the most important contributions to the use of the metathesis reaction in organic synthesis. The compound exhibits a high degree of selectivity for the metathesis of 1-alkenes but tends to have a limited lifespan at elevated temperatures. This concern was partially addressed by the development of the second generation Grubbs catalyst 2. Unfortunately this compound tends towards low selectivity at elevated temperatures. Several research groups attempted to increase the stability and activity of these metathesis catalysts by synthesising complexes with hemilabile O,N-bidentate ligands from 1 and 2. Complex K1 is an example of this approach. The nitrogen dentate dissociates from the metal centre of K1 to expose a free coordination site when a suitable competing substrate is available, but prevents degradation of the complex by occupying the site otherwise. The balance between availability and stabilisation of the coordination site may show the way to the development of catalysts with high activity and stability. The initial aim of this study was the synthesis of new complexes with hemilabile O.N-bidentate ligands from 1 and to test these for metathesis with 1-octene. Suitable ligands for this synthesis were identified with the support of molecular modelling. The imine ligands L3, L4, L6. L7. L 14, L 15 en L 16 were synthesised by the condensation reaction of aldehydes or ketones with amines. The literature catalyst K1 was successfully prepared and tested for metathesis activity with 1-octene. The purpose of this was to establish a benchmark with which the experimental results of the new catalysts could be compared. Molecular modelling was used to identify ligands that could be suitable for catalyst synthesis. The phenyl substituted ligands L4, L7 and L 16 were selected, but several attempts at the synthesis of the corresponding catalysts K4, K7 and K16 were unsuccessful. Molecular modelling was used in an attempt to shed some light on the results of this study. Possible explanations are based on tlie observation of specific HOMO-LUMO interactions, N-O distances, bite angles, HOMO orientations and the orientations of specific groups within these compounds. Conclusions from these observations are drawn by inter alia comparison with a model ligand and catalyst.