Studies of a dirhodium tetraphosphine catalyst for hydroformylation and aldehyde-water shift catalysis

Abstract

Research into the dirhodium tetraphosphine catalyst precursor [rac-Rh2(nbd)2(et,ph-P4)](BF4)2 shows it is capable of forming a highly active and regioselective hydroformylation catalyst in situ when using an acetone or acetone/water solvent. Hydroformylation experiments (using 1-hexene), FT-IR studies, and acid-base studies were performed to better understand the various complexes of the dirhodium catalyst cycle. These studies lead to the newly proposed catalyst mechanism when performed in an acetone/water solution, using the monocationic [rac-Rh2(H)(µ-CO)2(CO) (et,ph-P4)]+ as the proposed active catalyst complex for hydroformylation. For the conversion of 1-hexene to heptanal, it is capable of performing an initial rate of 30 turnovers per min, 33:1 linear-to-branched aldehyde regioselectivity, and less than 0.5% isomerization/hydrogenation. A remarkable new catalytic reaction, termed Aldehyde-Water Shift Catalysis, can occur under the proper conditions only when water is added to the acetone solvent. Under mild hydrogen deficient conditions the reaction of aldehyde and water can produce carboxylic acid and H2. The net stoichiometry of hydroformylation combined with this unusual aldehyde-water shift catalysis is that of hydrocarboxylation, an extremely difficult reaction to perform selectively or under mild conditions. DFT calculations and experimental studies indicate [Rh2(µ-CO)2(CO)2(et,ph-P4)]2+ as the likely active catalyst for this aldehyde-water shift catalysis. We believe bimetallic cooperativity plays an important role in this catalysis as it does in hydroformylation. Various designs of autoclave reactors were built in order to maintain and monitor the new reaction conditions, including pressure, temperature, and gas flow rate. The best tandem catalysis results yielded a rate of 50 turnovers per hour with a regioselectivity of 60:1 linear-to-branched carboxylic acid, operating at 50 psig (3.4 atm) and 90°C.