New catalysts for branched selective hydroformylation of alkenes

Abstract

Both products, n-butyraldehyde and iso-butyraldehyde from propene hydroformylation are key building blocks for the synthesis of many chemical intermediates, and although high linear selectivity has been achieved, any form of branched selectivity remains very difficult to attain. This project aims to deliver a catalyst that can selectively produce branched iso-butyraldehyde as the major product from propene hydroformylation. One approach discussed is to study terphenyl phosphines as ligands. The synthesis of substituted terphenyls through Suzuki-Miyaura coupling reactions between aryl boronic acids and 2,6-dichloroanisole was studied. Novel phosphine-phosphanamine ligands with bulky terphenyl substituents were synthesised and tested in propene hydroformylation, and also asymmetric hydroformylation of other alkenes. The synthesis of several ferrocene-based phosphine-phosphoramidite ligands is also discussed. These ligands were then tested in rhodium-catalysed propene hydroformylation and their reactivities and selectivities are reported. These ligands/Rh catalysts showed a moderate reactivity for propene hydroformylation and up to 56% branched selectivity, which is close to the best selectivity known under industrially relevant conditions. The introduction of bulky substituents on the phosphoramidite part of the ligand did not deliver any huge increases in regioselectivity, but a large improvement in catalyst thermal stability was observed in experiments conducted using in situ high pressure infrared spectroscopy. The reaction conditions for rhodium-catalysed propene hydroformylation using the BOBPHOS ligand were investigated, with unprecedented branched selectivity of up to 82% achieved. A variety of aspects was examined, including the solvent, reaction temperature, reaction pressure with varying partial pressure of CO and H₂, and rhodium to ligand ratio. BOBPHOS derivatives which are more synthetically accessible and economically attractive were synthesised and tested in rhodium-catalysed propene hydroformylation. Comparable results with their parent ligand/Rh catalyst were obtained and improved thermal stabilities were observed in selected catalysts. Different directions for potential future works are suggested, which hopefully, along with the findings in this thesis, can be a major contribution to the development of an efficient, branched selective catalytic system for industrial propene hydroformylation.