Applications of novel silica supporting palladium catalysts in coupling reactions
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.