Development of phosphorus-mediated reactions in organic synthesis / y Xia Xuanshu, B. Sc., Sun Yat-sen U

Abstract

Polymer-supported catalysts and reagents have been widely used in organic chemistry because they could facilitate the purification procedures and usually be recycled. Much research has been directed to polymer-supported catalysts and reagents, mainly focusing on these aspects, such as new polymer support, new application in organic chemistry, different modifications and so on. Many polymer-supported phosphine reagents have been developed for Wittig reaction. However, most of them suffer from swelling issue or low loading. A new polyethlyeneimine-supported triphenylphosphine has been synthesized and used as a highly loaded bifunctional homogeneous reagent in a range of one-pot Wittig reactions. All the substrates afforded desired products in high yields after only simple purification procedures. Furthermore, it also served efficiently in reaction cascades involving a one-pot Wittig reaction followed by conjugate reduction of alkene products. In these transformations the phosphine oxide generated in Wittig reaction served as the catalyst for activating trichlorosilane in the subsequent reduction reaction. Triphenylphosphine oxide is always considered as a byproduct of Wittig and Mitsunobu reactions which complicates the purification procedures. One option to utilize it is its application in halogenation reaction with oxalyl halide. Heterogeneous polymer-supported triphenylphosphine oxides based on the rasta resin architecture have been synthesized, and applied as reagent precursors in a wide range of halogenation reactions. The rasta resin-triphenylphosphine oxides reacted with either oxalyl chloride or oxalyl bromide to form the corresponding halophosphonium salts, and these in turn reacted with alcohols, aldehydes, aziridines and epoxides to form halogenated products in high yields after simple purification. The polymer-supported triphenylphosphine oxides formed as a byproduct during these reactions could be recovered and reused numerous times with no appreciable decrease in reactivity. Another option is to use triphenylphosphine oxide as catalyst in organic synthesis. A highly regioselective 1,4-reduction of conjugated polyunsaturated ketones catalyzed by triphenylphosphine oxide is described. In the presence of triphenylphosphine oxide, conjugated di-, tri-, and tetraenones were selectively α,β-reduced using trichlorosilane without over reduction or isomerization, and all the substrates rendered desired products in high yields. Furthermore, 1,4-reduction products were successfully obtained in sequential one-pot Wittig/conjugate reduction reaction, triphenylphosphine oxide generated in Wittig reaction served as the catalyst for reduction reaction. In addition, natural moth pheromones and their analogues were synthesized in high yields using this method. Finally, the synthesis of γ-sanshool and hydroxy-γ-sanshool is depicted. The synthetic route started from simple and commercially available building blocks using an alkyne for E,E-2,4-diene group of the key synthetic intermediate 2E,4E,8Z,10E,12E-tetradecapentaenoic acid, which in turn was converted into both γ-sanshool and hydroxy-γ-sanshool by reaction with the appropriate amines.