Recyclability of water-soluble ruthenium–phosphine complex catalysts in multiphase selective hydrogenation of cinnamaldehyde using toluene and pressurized carbon dioxide

Abstract

The recyclability of water-soluble ruthenium -phosphine complex catalysts was investigated in water -toluene and in water -pressurized carbon dioxide systems for selective hydrogenation of trans-cinnamaldehyde (CAL). For the first hydrogenation run, the selectivity for cinnamyl alcohol (COL) is high for both toluene and dense CO 2 , because of interfacial catalysis in which the reaction mainly occurs at the interface between the aqueous phase and the other toluene or dense CO 2 phase. The total CAL conversion and the COL selectivity decrease on the second run, more significantly with dense CO 2 than toluene. On the subsequent runs, however, less significant changes were observed. During the first run, the active metal complexes should change to much less active ones such as Ru(H) 2 L n (TPPTS) m (L=COL) by accumulation of the main product of COL. This structural change may occur more easily in multiphase hydrogenation with dense CO 2 than that with toluene, probably because the solubility in the dense CO 2 gas phase is even smaller than that in toluene. For homogeneous reaction of COL in aqueous phase, Ru(H) 2 L n (TPPTS) m catalyzes the isomerization to HCAL compared with the hydrogenation to hydrocinnamyl alcohol. With those complexes, however, the selectivity for COL is still comparable to that for HCAL for multiphase hydrogenation reactions because the hydrogenation of an ampholytic substrate of CAL occurs mainly at interface between water and toluene or dense CO 2 gas phase. Interactions of CO 2 molecules with CAL would also increase the reactivity of carbonyl group of the substrate.

Hydrogenation of Nitrostyrene with a Pt/TiO2 Catalyst in CO2-Dissolved Expanded Polar and Nonpolar Organic Liquids: Their Macroscopic and Microscopic Features

“…As referred to in section , Eckert et al pointed out the importance of interactions with CO 2 cosolvent molecules in selective hydrogenation of nitriles to primary amines, interacting with the desired products (primary amines) and suppressing their further hydrogenation to secondary amines . The present authors also revealed the significance of interactions of CO 2 with carbonyl − and nitro − groups in selective hydrogenation of α,β-unsaturated aldehydes and aromatic nitro compounds. One can say that, in addition to such solvent properties as polarity, interactions of solvent molecules with reacting species would be an interesting factor for manipulating the product distribution.…”

“…The reactions in CXLs are complicated multiphase reactions and the pressurization with CO 2 should have different physical and/or chemical impacts, positive or negative, on the reactions. The authors investigated the hydrogenation reactions of carbonyl − and nitro − compounds in CXLs with homogeneous and heterogeneous catalysts. In addition to the roles of promoting the dissolution of H 2 and diluting the reacting species in the CXLs, the pressurization with CO 2 has another important role of modifying the reactivity of substrates and intermediates.…”

A biphase H2O/CO2system as a versatile reaction medium for organic synthesis

“…Fujita et al 97 also studied selective hydrogenation of transcinnamaldehyde in biphase H 2 O/CO 2 system in the presence of hydrogen (CO 2 12-20 MPa, H 2 2 MPa, 70 C, 2 h) using watersoluble ruthenium/phosphine complex catalysts. The selectivity for cinnamyl alcohol as well as total conversion decreased aer the rst run (i.e., aer initial passivation of the complex with cinnamyl alcohol product) but on the subsequent runs remained more stable.…”

Hydrophilic Ligands and Their Application in Aqueous-Phase Metal-Catalyzed Reactions

“…Hydrogenation of cinnamaldehyde under comparable conditions in toluene using Ru/PPh 3 complexes gave low selectivity with the fully reduced alcohol being the major product in most cases . Hydrogenation of cinnamaldehyde in toluene/water using RuCl 3 / m -TPPTS or Ru(H) 2 ( m -TPPTS) 4 gave high selectivity for cinnamyl alcohol, although low conversion . Adding supercritical CO 2 improved the conversion, but slightly lowered the selectivity.…”