Neoteric solvents for asymmetric hydrogenation:supercritical fluids,ionic liquids,and expanded ionic liquids

摘要

Neoteric (new) solvents such as supercritical CO_2(scCO_2),ionic liquids (ILs),ILs with cosolvents,and CO_2-expanded ionic liquids (EILs) offer flexible physical properties,which allow chemists and engineers to select the optimal solvent system for a specific reaction process.Homogeneously-catalyzed asymmetric hydrogenation of alpha,beta-unsaturated carboxylic acids was chosen for its economic interest and its multiple H_2-concentration dependent behaviours.For exmaple,with ruthenium BINAP-type catalysts,type I substrates require high H_2 cocnentration in solution,while type II require low H_2 cocnentrations.ScCO_2,ILs and EILs are highly attractive because of their contrasting properties and their potential flexibiltiy in improving or reducing hydrogen transfer rates and thus concentrations.Several ILs were tested and compared with EILs,IL-cosolvent mixtures,scCO_2 and normal methanol as media for these reactions to establish the msot effective system for each substrate type.Atropic acid (type I) was hydrogenated up to 92% ee which is not better than in methanol.However,tiglic acid (type II) was hydrogenated up to 93% ee in the optimzied IL system,which is significantly better than was observed in MeOH.CO_2-expansion of ionic liquids affected the selectivity for both substrates,improving the selectivity for atropic acid and lowering it for tiglic acid.The solubility of the catalyst in scCO_2 was measured and the antisolvent effect of H_2 in scCO_2 was demosntrated and discussed.