Composition of catalyst resting states of hydroformylation catalysts derived from bulky mono-phosphorus ligands, rhodium dicarbonyl acetylacetonate and syngas

Abstract

The paper describes the composition of the resting states of several catalysts for alkene hydroformylation derived from bulky monophosphorus ligands. The results presented assess how bulky ligands compete with CO for the rhodium, and hence the role of 'unmodified' catalysts in alkene hydroformylation in the presence of these ligands. High Pressure Infra-Red (HPIR) spectroscopy was carried out at the rhodium and syngas concentrations typically used during catalysis experiments. These HPIR studies revealed that two ligands previously studied in Rh-catalysed hydroformylation react with [Rh(acac)(CO)2] and H2/CO to give the unmodified rhodium cluster, [Rh6(CO)16], as the only detectable species. Both less bulky phosphoramidites, and 1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane, on the other hand, do not show the presence of [Rh6(CO)16], and hence catalysis proceeds by purely ligand modified species under normal conditions. In the case of the Rh/ phosphaadamantane catalysts, anecdotal evidence that this only forms a particularly useful catalyst above a certain pressure threshold can be understood in terms of how the catalyst composition varies with pressure. The ligands discussed have all been assessed in the hydroformylation of propene to separate their innate branched selectivity from their ability to isomerise higher alkenes to internal isomers. 2

Understanding a Hydroformylation Catalyst that Produces Branched Aldehydes from Alkyl Alkenes

“…Enantioselective hydroformylation is slowly coming of age as a potentially clean, low cost, and atom-efficient method to make chiral aldehydes. Studies on enantioselective hydroformylation generally focus on specific classes of substrate that have an inbuilt propensity to primarily form branched aldehydes. − In fact, forming branched aldehydes from alkyl alkenes, even using achiral catalysts, is very unusual . Alongside projects exploiting substrate-controlled regioselectivity, sporadic attempts were made in our laboratories to address the highly desired enantioselective hydroformylation of alkyl alkenes.…”

Iodide-Assisted Pd Catalysis as an Attractive Alternative to Rh Catalysis for the Industrially Relevant Isoselective Hydroformylation of Simple Aliphatic Alkenes

“…Below, we present the progress in this domain exploiting different approaches that promote isoselectivity, from the earliest examples up to the most recent studies. This includes the works from Reek’s 14–17 and Clarke’s 18 19 20 21 22 23 laboratories, that established the current state of the art for the isoselective Rh-catalyzed hydroformylation of propylene and other unbiased alkenes. For the isoselective hydroformylation of activated alkenes, such as vinyl arenes, vinyl ethers, or acrylates, that show significantly different reactivity patterns, we refer the readers to the corresponding literature.…”

“…22 Besides the Rh catalysts with phosphine-phosphite ligands, Clarke and co-workers reported a series of Rh catalysts with phosphine-phosphoramidite ligands that displayed a moderate level of isoselectivity in the hydroformylation of propene while conducting the reactions in perfluorinated solvents. 20…”

Isoselective Hydroformylation of Propylene by Iodide‐Assisted Palladium Catalysis

“…The commercial processes occur in the presence of either Rh or Co catalysts whose inherent selectivity favors the formation of n ‐butanal over isobutanal [2–4] . The increasing demand for isobutanal‐derived materials, such as plasticizers, coatings, solvents, cleaners, food additives, or extractants for pharmaceutical products, as well as the unmatched challenge of developing catalysts featuring the isoselectivity, have driven significant research [5–20] . However, limited progress has been achieved thus far and no methods have been commercialized.…”