Regioselective Hydroformylation of an α,β‐Unsaturated Ester: Spectroscopic Studies on Catalytic Species and the Influence of Ligands on Regioselectivity

Abstract Hydroformylation of α,β‐unsaturated esters is an atom‐economical route towards aldehydes as precursors for pharmaceuticals. In this work, regioselective hydroformylation of an α,β‐unsaturated ester to the β‐aldehyde was initially re‐evaluated using a commercial Rh catalyst sample named “[HRh(PPh 3 ) 4 ]” due to reported high selectivity for the β‐aldehyde with this type of catalyst. However, while such high selectivity was achieved, subsequent investigation of this sample by high‐pressure NMR and in situ IR spectroscopy under process conditions revealed a behavior like a phosphine‐free system, indicating that the sample is decomposed and may be present as rhodium carbonyl clusters. This hypothesis has been supported by hydroformylation experiments with and without phosphine ligands. While a strong decrease in regioselectivity to the β‐aldehyde was observed in the presence of triphenylphosphine as added ligand, phosphine‐free rhodium complexes led to high regioselectivities. In detail, the hydroformylation showed high regioselectivity (91 %) and overall selectivity to the β‐aldehyde (70–77 %) with [Rh 6 (CO) 16 ] as well as [Rh(acac)(CO) 2 ] as ligand‐free rhodium complexes. With further unmodified, phosphine‐free rhodium compounds as catalysts, again high regioselectivities (90–93 %) towards the β‐aldehyde were obtained. Thus, our results indicate that non‐modified triphenylphosphine‐free Rh catalysts are efficient catalysts for regioselective hydroformylation of β‐aryl‐substituted α,β‐unsaturated carboxylates to the β‐aldehyde.