Catalysis Research Of Tetraarylphosphonium Salts Supported Chiral Imidazolidin-4-ones

Recently, asymmetric catalysis is getting to be the most prevalent way in preparation of chiral compounds, as the reaction can conducted in mild conditions, relatively low dosage as well as the resulted yields and stereoselectivities are excellent. Among the metal-free chiral organic catalysts, chiral imidazolinone plays a dominant role in asymmetric catalysis and can be well implemented in asymmetric Diels-Alder cycloaddition, asymmetric1,3-dipolar cycloaddition, asymmetric Michael addition, asymmetric transfer-hydrogenation reaction etc. However, the difficulty of recover and recycle of the catalysts restricted its promising application.For the sake of recovering and recycling of the chiral imidazolinone catalysts, it has been established in the literature that immobilization of imidazolinones on the soluble or insoluble supports, facilitating the separation of the title products and the catalyst via the physical characteristic of the support. Tetraarylphosphonium salts are one of the soluble supports that have the common nature as soluble supports, promoting the reaction reacted homogeneously and rapidly in liquid state, enabling the products measured on-line. Furthermore, tetraarylphosphonium salts not only have their own feature, but also displaying remarkable chemical and thermal stabilities, demonstrating relatively high loadings for its low molecular weight, showing good solubility in moderate and high polar solvents while bad solubility in small polar solvents, which makes the recovering and recycling of the catalyst in reality.In this paper, we chose tetraarylphosphonium salts as the supports and synthesized tetraarylphosphonium supported chiral imidazolinones, applying in asymmetric1,3-dipolar cycloaddition and asymmetric transfer-hydrogenation reaction. Details are as follows:Firstly, small molecular chiral imidazolidin-4-one can be synthesized in two steps from L-Tyrosine methyl ester hydrochloride, the chiral raw material, then immobilized on the soluble carriers, tetraarylphosphonium salts. By changing the substituent group of the imidazole ring and the anion of tetraarylphosphonium salts, we have designed and synthesized five kinds of tetraarylphosphonium salts supported chiral imidazolidin-4-ones, the products have also been tested by IR,1H NMR and13C NMR analysis and the data are in accordance with the structure of target compounds.Secondly, The above5catalysts, tetraarylphosphonium salts supported chiral imidazolidin-4-ones, had been used in enantioselective1,3-dipolar cycloadditions of nitrones with unsaturated aldehydes. To evaluate the capacity of the tetraarylphosphonium supported chiral imidazolidinones, the synthesis of the isoxazolidine aldehyde via N-benzyl-C-phenyl nitrone with E-crotonaldehyde was chosen as a model reaction. By discussing the reaction solvent, auxiliary acid, reaction temperature and time,5catalysts and their catalyst loadings, we found2-8a was the best catalyst for this model reaction, resulted in89%yield,94/6endo/exo,94%endo ee. After that, we used2-8a to search the scope of substrates in optimal reaction conditions, of which the applicability was verified. Furthermore, the catalyst can be simplely seperated by recrystallization at the end of the reation, and the catalytic effects was not significant lower in the successtive4cycle using.Thirdly, the tetraarylphosphonium salt supported chiral imidazolidin-4-one2-8a had been applied in asymmetric transfer hydrogenation of unsaturated aldehydes. To evaluate the capacity of the tetraarylphosphonium supported chiral imidazolidinones, the transfer hydrogenation of β-methyl cinnamic aldehyde was chosen as a model reaction. We had discussed the effect of temperature in0℃and25℃, found the catalytic results in0℃was better,99%conversation and98%endo ee.Overall, our tetraarylphosphonium salts supported chiral imidazolidin-4-ones can catalyze the enantioselective1,3-dipolar cycloadditions of nitrones with unsaturated aldehydes and asymmetric transfer hydrogenation of unsaturated aldehydes effectively. In this way, we can easily see that these catalysts not only retained the excellent catalytic effect of small molecular chiral catalyst, but also achieved the aim of recovering and recycling of the chiral catalysts.