Selective hydrogenation of amides using bimetallic Ru/Re and Rh/Re catalysts

Heterogeneous Ru/Re and Rh/Re catalysts, formed in situ from Ru3(CO)12/Re2(CO)10 and Rh6(CO)16/Re2(CO)10 respectively, are effective for the liquid phase hydrogenation of cyclohexanecarboxamide (CyCONH2) to CyCH2NH2 in up to 95% selectivity without the requirement for ammonia to inhibit secondary and tertiary amine formation. Good amide conversions are noted within the reaction condition regimes 50–100 bar H2 and ⩾150 (Rh) – 160 °C (Ru). Variations in Ru:Re and Rh:Re composition result in only minor changes in product selectivity with no evidence of catalyst deactivation at higher levels of Re. In situ HP-FTIR spectroscopy has shown that catalyst genesis occurs via decomposition of the metal carbonyl precursors. Ex situ characterization, using XRD, XPS and EDX-STEM, has provided evidence for the active components of these catalysts containing bimetallic Ru/Re and Rh/Re nanoclusters, the surfaces of which become significantly oxidized after use in amide reduction. Potential mechanistic pathways for amide hydrogenation are discussed, including initial dehydration to nitrile, a pathway potentially specifically accessible to primary amides, and evidence for often postulated imine intermediates.

Graphical abstractRecyclable Ru/Re and Rh/Re bimetallic catalysts are effective for the highly selective (90%) reduction of primary amides such as CyCONH2 to the corresponding primary amine CyCH2NH2.Figure optionsDownload full-size imageDownload high-quality image (81 K)Download as PowerPoint slideResearch highlights► Selective (⩾ 90%) hydrogenation of RCONH2 to RCH2NH2 over Ru/Re, Rh/Re catalysts. ► Conventional requirement for NH3 addition to inhibit (RCH2)2NH formation unnecessary. ► Reaction conditions: 50–100 bar H2, ⩾150 (Rh), ⩾160 °C (Ru). ► Active (and recyclable) catalysts comprise bimetallic Ru/Re, Rh/Re nanoclusters. ► Potential amide reduction pathways include initial dehydration of CyCONH2 to CyCN.