Studies on environmentally benign asymmetric catalytic reactions

Abstract

The air-stable iridium-complex catalyzed asymmetric hydrogenation of quinoline substrates in organic solvent and Polyethylene Glycol Dimethyl Ether (DMPEG)/hexane biphasic system was investigated in this project. Catalysts with chiral C2-symmetric ligands such as PPhos, Xyl-PPhos, Cl-MeOBiphep, BisbenzodioxanPhos, and DifluorPhos were found to be effective for this type of reaction. Ee's of up to 95% can be achieved. In addition, the reactivity and stereoselectivity of most tested catalysts can be retained in DMPEG (Mn = 500) for at least three reaction cycles. Furthermore, some catalyst systems can give similar results without using glovebox and degassed solvent. In our previous work, the results of Ru/P-Phos catalyzed asymmetric hydrogenation of a-ketoesters were not very fruitful. The effect of different additives has been investigated for improving the catalytic results of this reaction. High enantioselectivities (up to 94%) and conversions (over 99%) were obtained when Lewis acid, CeCl3.7H2O was added as the additive. We also extended the application of this catalyst system in room temperature ionic liquids. The catalyst can be recovered and reused for a few times. We have also successfully demonstrated that the Ru/BisbenzodioxanPhos complex can hydrogenate a series of a-phthalimide ketones to the corresponding alcohols with enantioselectivities up to 98%. P-Phos family was proven to be effective for many catalytic reactions, and one of its ortho-substituted examples (o-Ph-P-Phos) had performed poorly in some selected experiments. Here, we further explore its application in the asymmetric hydrogenation of dehydroamino acids. The chiral products produced can be offered enantioselectivities up to 90%. Polymethylhydrosiloxane (PMHS Mn = 1700-3200) is a byproduct in silicone industry and is a cheap and environmental friendly reducing agent for various type of reactions. Our previous work showed that the air-stable CuF2/XylPPhos catalyst can reduce acetophenone and benzophenone with phenylsilane with high enantioselectivites at high S/L ratio. Here, we tried to develop a greener approach for this hydrosilylation reaction using PMHS as the hydride source and sole solvent for this reaction. The enantioselectivities of the chiral products can attain up to 90%.