Reversible Reaction of Triphenylphosphine With Isothiazol‐3(2H)‐one‐1,1‐diones

ABSTRACT

The reaction of triphenylphosphine with isothiazol‐3(2H)‐one‐1,1‐diones is investigated. The reaction proceeds via conjugated addition to the C═C bond, occurring regioselectively at the C5 position. Heating the reagents in acetic acid at 140°C affords phosphonium salts in the betaine form. Under milder conditions with triflic acid (in dichloromethane at 0°C), the corresponding phosphonium salts are obtained, isolated, and fully characterized. These salts undergo base‐induced decomposition back to the starting materials, indicating the reversibility of the reaction. The obtained phosphonium salts are evaluated in the Mitsunobu reaction, where they exhibit reactivity similar to that of BEHT triflate, although with lower efficiency.