A Very Readily Prepared Ligand for Rhodium Catalyzed Propene Hydroformylation

In the search for an easy-to-make ligand that would enable Rh catalyzed hydroformylation of propene to favor the branched aldehydes, iso-butanal, bis-phosphonites and bis-phosphoramidites were designed. Catalysts derived from bis-phosphonites gave the desired low n:iso ratios, but were unstable to hydrolysis. A bis-phosphoramidite ligand (EasyDiPhos) that can be made in just one synthetic step has been discovered that enables Rh-catalyzed hydroformylations to proceed with unusually low n:iso ratios. They were also found to have significant stability to moisture, air, and to relatively high temperatures during hydroformylation conditions. The latter was studied using a combination of high pressure infra-red (HPIR) spectroscopy and NMR analysis of the catalyst resting state after 1 week under relevant conditions. Propene hydroformylation with n:iso ratios well below 1 was carried out and with turnover numbers around 1000 mol/mol in 1 hour reaction time. One of the ligands, whilst not being the most iso-selective example, had a partially flexible n:iso ratio depending on temperature and pressure. Two Pt(II) complexes were prepared and their X-ray crystal structures determined. The most iso-selective catalyst was examined at low temperature (prioritizing iso-selectivity over rates) in hydroformylation of some other terminal alkenes with significant branched selectivity.