Ion-supported Triphenylphosphine

1-Methyl-1-[4-(diphenylphosphino)benzyl]pyrrolidinium bromide (1) is an air-stable ion-supported triphenylphosphine developed by Togo et al. In the halogenation of alcohols and the esterification of carboxylic acids with the Mitsunobu reaction using 1, co-product, ion-supported triphenylphosphine oxide 2 can be easily separated from the product by ether extraction due to its poor solubility in ether. 2 is recovered by filtration in over 90% yield. After the O-methylation of the recovered 2 with dimethyl sulfate and subsequent reduction with LiAlH₄, 1 can be regenerated and reused for the same reactions.

1 is also useful as a ligand of metal catalysts for the Mizoroki-Heck reaction and the Sonogashira reaction. The ionic liquid reaction media containing 1 and a palladium catalyst can be reused for the same reactions, maintaining the high yield and the high purity of the products.

Typical Procedure: 1-Methyl-1-[4-(diphenylphosphino)benzyl]pyrrolidinium bromide (1) (1320 mg, 3.0 mmol) is dried by a vacuum pump for 2 h at 70 °C. To the flask containing 1 is added a solution of 3-phenyl-1-propanol (272 mg, 2.0 mmol) and CBr₄ (729 mg, 2.2 mmol) in dichloromethane (6 mL). The obtained mixture is stirred for 2 h at 40 °C under an argon atmosphere. After the reaction, ether (10 mL) is added and the obtained mixture is stirred for 40 min at room temperature. Then, the mixture is filtered and washed with ether. Removal of the solvent from the filtrates gives 3-phenyl-1-bromopropane in a crude state. The purity is in the range of 70-75%, due to a co-product, CHBr₃. Pure 3-phenyl-1-bromopropane is obtained by short column chromatography on silica gel (CHCl₃). Co-product, ion-supported phosphine oxide (2) is recovered by the above filtration in 93% yield.