Supercritical and Compressed Carbon Dioxide as Reaction Medium and Mass Separating Agent for Hydrogenation Reactions using Organometallic Catalysts

Abstract

1366 Scheme 39.2 Examples of typical rhodium-based catalyst systems and modified derivatives of triphenylphosphine as used to control their solubility with scCO 2 in homogeneous or multiphase systems (BARF = tetrakis[3,5-bis (trifluoromethyl)phenyl]borate). troscopy [28 b]. In combination with the cationic precursor [Rh(cod) 2 ][BF 4 ], high conversion and excellent ee-values were obtained for substrates 4 and 15. Visual inspection of the reaction mixture indicated that fully homogeneous conditions could be achieved with 15, whereas small amounts of substrate 4 remained al-39.2 The Molecular and Reaction Engineering Basis of Organometallic-Catalyzed Hydrogenations 1367 Scheme 39.3 Product control by affecting the relative rates of competing reaction pathways in scCO 2 . Path A is preferred in scCO 2 , whereas path B prevails in conventional solvents. Scheme 39.4 Enantioselective hydrogenation in scCO 2 using a CO 2 -philic derivative of BINAPHOS. 39 Supercritical and Compressed Carbon Dioxide as Reaction Medium 1372 Scheme 39.8 Possible reactions of CO 2 in the presence of H 2 under transition metal-catalyzed hydrogenation conditions. Scheme 39.9 Deuterium labeling as a mechanistic probe for hydrogenation pathways in scCO 2 .