The palladium complex 1, bearing the Trost 'Standard' Ligandi (TSL) 2 is an efficient and highly selective catalyst used to facilitate asymmetric allylic alkylation. Earlier research conducted by Lloyd Jones and co-workers suggested that the selectivity of the catalyst 1 monomeric species may arise via ligand-accelerated catalysis pathway. TSL 2 has a degree of flexibility that is crucial for the selectivity, but also responsible for the rapid equilibrium of the Pd-TSL monomers 1 with the low selectivity and diminished activity oligomers (I)n. The oligomers (I)n are known to be the dominant catalyst 1 species at higher concentrations responsible for effects observed and are memory in allylic alkylation catalytic cycle. The aim of this work was to further develop the understanding of the aggregated catalyst (I)n. n 1 {(1 )4}n Small-angle neutron scattering (SANS) revealed that Pd-TSL oligomers (I-BArF)n readily aggregate to form cylinders in common organic solvents. According to molecular mechanics modelling at MM3 theory level the architecture is the most consistent with linear stacks of cyclic disc-shaped tetramers {(I-BArF)4}n. In low polarity solvents the rods can grow up to ~200 A long and may contain between 10 to 14 tetramer discs (1)4, sandwiched with bulky BArF anion layers. In polar organic media the solvation is more effective and shorter (30 A) stacks were detected by SANS. Molecular dynamics simulations also supported these observations. Finally, the kinetic SANS experiments showed that the architecture remains effectively unchanged during the catalytic cycle. 1< ~ 200A >1 16-20 A 31p NMR measurements, however, indicated that the highest concentration of monomer 1 with only trace amounts of oligomer {(I)4}n is accessible in very dilute dichloromethane solution of < 4 mM. In contrast, high dielectric constant solvents favour almost complete oligomerisation. Further experiments focused on the structure of {(I-BArF)4}n racemic mixtures. SANS measurements required the complementary perdeuterated complex [Dd-I-BArF. An enantiopure, deuterium labelled [Dd-I-BArF pseudo-enantiomer was prepared by a new divergent synthetic route of 8 steps in the longest linear branch and employing simple commercially available starting materials. SANS data analysis indicated that [Dd-I-BArF formed shorter cylinders than I-BArF. However, the measurements were not sensitive enough to resolve the exact composition of pseudo-racemic mixtures [DoID47I-I-BArF.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:680110 |
| Date | January 2014 |
| Creators | Racys, Daugirdas |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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