Toluene → Triethylpentylphosphonium bromide

77 g (0.5 moles) of 1-bromopentane (a reagent available from Tokyo Chemical Industry Co., Ltd.) was added dropwise in 236 g (0.5 moles) of a 25% toluene solution of triethylphosphine (HISHICOLIN (registered trademark) P-2, a trade name of Nippon Chemical Industrial Co., Ltd.), and the mixture was allowed to react at 70 to 80° C. for 6 hours. After completion of the reaction, hexane was added for crystallization, thereby obtaining 113 g (percent yield: 84%) of a crystal of triethylpentylphosphonium bromide. 27 g (0.3 moles) of sodium dicyanamide (a reagent available from Wako Pure Chemical Industries, Ltd.) was added to 81 g (0.3 moles) of this triethylpentylphosphonium bromide, and the mixture was allowed to react in an aqueous system. Subsequently, the reaction mixture was stirred for aging at room temperature for 3 hours. After completion of stirring, a product was extracted with dichloromethane and passed through a silica gel column for purification. The solvent was distilled off, and the residue was dried in vacuo at 100° C. and at a degree of vacuum of 0.5 kPa for 5 hours. Confirmation of the thus obtained product was carried out by means of 1H-NMR, 13C-NMR and 31P-NMR. A yield of the product (pale yellow transparent liquid) was 77 g (percent yield: 69%), and it was confirmed by 31P-NMR that its purity was 98% or more.