Studies on the reactivity of sulfonium salts

Abstract

The reactivity of $\alpha$ -and $\gamma$ -alkoxydimethylsulfonium salts derived from the reaction of saturated aliphatic acetals, $\alpha,\beta$,-unsaturated acetals, and 3,4-dihydro-2H-pyran with dimethyl sulfide in the presence of trimethylsilyl trifluoromethanesulfonate was investigated, and it was found that alkoxysulfonium salts underwent substitution reaction with various nucleophiles such as Grignard reagents, allylsilanes, silyl enol ethers, allyltins, and lithium thioalkoxides, etc. Aryl and alkyl magnesium bromides reacted efficiently with alkoxysulfonium salts at low temperature to affored the corresponding coupled products, whereas vinyl and acetylenyl magnesium bromides were less reactive. With allylsilanes and silyl enol ethers, the reaction of alkoxysulfonium salt was slower than the corresponding Lewis acid catalyzed reaction of acetals. Allyltins and tin enol ethers were reactive but highly unstable O-alkylated products were obtained in the latter case to give unsymmetrical mixed acetals. Lithium thioalkoxides reacted with alkoxysulfonium salts very fast at low temperature to afford the corresponding monothioacetals in good yields.