Reduction of Sulfonic Acids and Related Organosulfur Compounds with Triphenylphosphine–Iodine System

Abstract

Arenesulfonic acids, their sodium salts, and alkyl arenesulfonates can be reduced readily to the corresponding arenethiols quantitatively by treatment with a mixture of triphenylphosphine and a catalytic amount of iodine, while alkanesulfonic acids, sulfinic acids, disulfides, thiosulfonic testers, and sulfonates are also readily reduced to the corresponding thiols similarly. Upon treatment with a mixture of triphenylphosphine and excess iodine, however, these aliphatic sulfur compounds are converted eventually to the corresponding alkyl iodides. The relative reactivities of these sulfonyl derivatives in the reaction with the triphenylphosphine–iodine system are the following. Aromatic series: ArSO2Cl, ArSO2SAr′>ArSO2H>ArSO3R>ArSO3−HNBu3+ (or PyH+)>ArSO3H>ArSO2SO2Ar>>ArSO2CH2C(CH3)3, ArSO3Ar′. Aliphatic series: RSO2Cl, RSO2SR′, RSO2−HNBu3+>RSO3−HNBu3+>RSSR, RSO2H>RSO3H>RSH>RSO3R′. In these reactions, the arenesulfonic acids bearing electron-donating substituents can be reduced more readily than those having electron-withdrawing substituents.