Multifunctional Catalysis for Sequential Reactions

1,2-Diols and 1,2-ketoalcohols are important functional groups that can require multiple steps to synthesise, which increases yield losses, waste and often uses stoichiometric reagents and toxic/expensive catalysts. The ability to transform simple substrates into more complex products in one-pot with two or more catalysed steps is desirable, especially if a single catalyst can carry out both reactions.
A multifunctional catalyst system was designed to transform primary alcohols into 1,2-diols through three sequential catalysed transformations; transfer dehydrogenation, benzoin condensation, and transfer hydrogenation. Two families of novel ligands and iridium(III) complexes bearing either thiazole or triazole rings were synthesised towards creating a multifunctional catalyst and characterised using mass spectrometry, NMR spectroscopy and x-ray crystallography. During the synthesis of the ligands, the alkylation of thiazole compounds and the DIBAL-H reduction of a thiazole ester were developed.
The organo-organometallic complexes were tested for the transfer dehydrogenation of benzyl alcohol and the reaction was investigated to show that the equilibrium between the forward and reverse reaction, and not catalyst deactivation, was the cause of limited conversion. The hydrogen acceptor was varied and acetone was found to be the most effective when used as a dilute solvent with strictly inert reaction conditions, reaching up to 78% conversion. Four of the novel complexes gave comparable conversions to the control catalyst; however, the benzoin condensation reaction was not successful.
The transfer hydrogenation of benzaldehyde was tested and found give quantitative conversions with the majority of novel complexes. A substrate scope identified that the more electron rich substrates were more easily oxidised. With a secondary alcohol, the transfer dehydrogenations occurred at a faster rate compared to the primary alcohol, reaching quantitative conversion.