Applications of novel silica supporting palladium catalysts in coupling reactions

Whiteman, Leon (2010) Applications of novel silica supporting palladium catalysts in coupling reactions. (PhD thesis), Kingston University,


This work describes the preparation of a range of silica supported palladium catalysts and their testing in cross coupling reactions. A silica supported ‘bis’-phosphine palladium catalyst has been prepared and characterised using solid state [sup]31P NMR, TGA, elemental analysis and SEM. The evaluation of the catalyst in copper-free Sonogashira reactions is explored in some depth. Good to excellent conversion rates were observed throughout a wide range of substrates and the catalyst proved to be recyclable over 5 cycles. Also presented is the preliminary screening of the ‘bis’-phosphine palladium catalyst in Heck and Suzuki reactions and the first solvent and copper-free 5-‘endo’-‘dig’ cyclisation reaction of unprotected (2-phenylethynyl)aniline derivatives to yield 2- substituted indole derivatives. The preparation of range of novel ‘bis’-NHC (‘N’-heterocyclic carbene) palladium dichloride complexes is also reported. Due to the fact that the ‘bis’-NHC complexes are prepared prior to immobilisation, the precatalytic species could be well characterised by high-resolution mass, spectrometry, [sup]1H and [sup]13C NMR. The silica supported ‘bis’-NHC complexes were generally capable of achieving good conversions in Suzuki reactions of aryl iodides and bromides with 0.2 mol% catalyst loading. The catalysts also showed modest levels of activity toward deactivated aryl chloride substrates. The steric properties of the ‘N’-substituents on the ‘bis’-NHC complexes proved to have a significant effect on catalytic activity. As observed in homogeneous catalysis, bulky NHC ligands such as ‘N’-mesityl and ‘N’-(2,6-diisopropyl)phenyl exhibit increased catalytic activity compared to the less-bulky ‘N’-benzyl ligand. This is significant because the use of sterically bulky NHC ligands in hybrid catalysis has received little attention in the scientific literature. Finally, the preparation of a novel, silica supported iminoalkyl-NHC palladium complex has been explored. Although preparation of the iminoalkyl NHC complex proved problematic, the prototype catalyst displayed modest activity in allylic alkylation reactions (up to 24% conversion), which is apparently the first example of a heterogeneous NHC catalyst used for this process.

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