Study On The Asymmetric Hydrosilylation Of Aromatic Ketones

Chiral secondary aromatic alcohols are the key intermediates for the synthesis of pharmaceuticals, pesticides and materials. In the view of reaction efficiencies, atom economy and environmental benignancies, the asymmetric hydrosilylation of aromatic ketones is one of the most effective accesses to chiral secondary alcohols. This reduction method should be of value to industrial applications with respect to its mild reaction condition, no high-pressure condition and simple operating procedures. Therefore, this thesis studied the asymmetric hydrosilylation of aromatic ketones to prepare chiral secondary aromatic alcohols.Firstly, the chemical reduction was chosen for a series of aromatic ketones using sodium borohydride as reducing agent and the corresponding racemic products were obtained. After characterized by IR and 1H NMR, the racemic products were also analyzed by GC with the chiral capillary column. And the corresponding retention times of(R)-configuration and(S)-configuration products were confirmed, which provided a reference for the asymmetric hydrosilylation of aromatic ketones.Secondly, in this thesis, we aimed to develop an economical and practical catalytic system which was effective for asymmetric hydrosilylation in air atmosphere and at room temperature(25 °C). Therefore, the homogenous chiral catalytic system(R)-BINAP/ Cu(OAc)2·H2O/PMHS was studied and used in the asymmetric hydrosilylation of aromatic ketones. The effecting factors of reaction, such as reaction temperature, air, metal salts, chiral ligands and so on, were investigated. Conversion of substrates and enantiometric excess values of(R)-1-arylethanols were used as evaluation indexes. The suitable reaction conditions for the asymmetric hydrosilylation were as follows: 1 mmol of the aromatic ketone, 0.03 mmol of the catalyst precursor Cu(OAc)2·H2O, 0.01 mmol of the chiral ligand(R)-BINAP, 3 mmol of the reducing agent PMHS and 3 m L of the solvent toluene were included in the reaction system, and the reaction lasted for 4 h at 25 °C and in air atmosphere. The conversion of the substrates and the enantiomeric excesses of the product(R)-1-arylethanols were respectively up to 99% and 93%. Also, it was proved that the electronic effect and steric hindrance of the substituents on aromatic ring played an important role in the asymmetric hydrosilylation of aromatic ketones.Lastly, based on the study of the Cu(II) homogeneous catalysis, an economical and efficient heterogeneous catalytic system(R)-BINAP/Cu Fe2O4/t-Bu OK/t-Bu OH/PMHS was developed which showed a high catalytic activity in the asymmetric hydrosilylation of aromatic ketones at room temperature(25 °C) and in air atmosphere. In this thesis, Cu Fe2O4 nanoparticles were synthesized respectively by coprecipitation, sol-gel method and solvothermal method, and the catalytic activities of them were also investigated. It was indicated that compared to the others, the Cu Fe2O4 nanoparticles prepared by solvothermal method had spherical shape, small size, better dispersion, uniform distribution and excellent catalytic activities. Therefore, the Cu Fe2O4 nanoparticles prepared by solvothermal method were chosen for the study of heterogeneous catalytic systems. The effecting factors of reaction including metal precursors, additives, solvents, reaction time and substrates, were investigated. Conversion of substrates and enantiometric excess values of(R)-1-arylethanols were used as evaluation indexes. It was indicated that with the use of t-Bu OK and t-Bu OH, the catalytic activity of the system(R)-BINAP/Cu Fe2O4/PMHS was significantly improved and the reaction could almost be completed in 2 hours. The conversion of the substrates and the enantiomeric excesses of the product(R)-1-arylethanols were respectively up to 99% and 92%. Furthermore, it was demonstrated that based on the magnetic property of Cu Fe2O4 catalyst, it could be easily separated in an external magnetic field. And after reused for four times, the catalyst could also have a high catalytic activity for the asymmetric hydrosilylation.