Selective Transfer Hydrogenation of Furfural to Isopropyl Levulinate: An In Situ One-Pot Cascade Approach

Within the current article, we were interested in describing the complete conversion of furfural (FF) as a green starting material to small oxygenates using isopropanol (IPA) as a solvent and as a H-donor. We applied a series of Ru-based complexes, and found that the most active Ru-complex in terms of isopropyl levulinate (IPL) formation at 130°C after 21 h was Ru-MACHO-BH (Ru-MACHO-BH = carbonylhydrido[2-(diphenylphosphino-κP)-N-[2-(diphenylphosphino-κP)ethyl]ethanamine-κN][tetrahydridoborato(1-)-κH]ruthenium(II)). We used two catalytic functions, one was the metal function to cover the hydrogenation of the aldehyde by Ru, and a Brønsted acidic function to secure the specific sub-steps of the reaction network (e.g.: hydrolysis of furfuryl alcohol as an intermediate, hydrolysis of 2-(isopropoxymethyl)furan (FE) to IPL). We observed that, the harmonization of these two functions leads to a highly selective reaction toward IPL (complete conversion, 67% yield). Furthermore, the reaction cascade can be directed until the final product, γ-valerolactone (GVL) (15% yield). The Brønsted acid-Ru-MACHO-BH catalyst system provides a unique reaction media where the usual humin/high molecular weight side products formation can be suppressed, while the nature of catalysis stays intact (homogeneous during the entire reaction coordinate).