Reactivity of the Carbocations Stabilized by Divalent Sulfur and Application to Organic Synthesis

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A new simple method for isolation of sulfur-stabilized carbocations, 1,3-dithiolan-2-ylium and 1,3-dithian-2-ylium ions, as salts was presented, and the isolated salts were converted to ketene dithioacetals and ortho esters. Reactions of the salts with nucleophiles, such as thiols and sulfinates afforded adducts, which were used as precursors for photochemical generation of dithio carbocations.Reactions of the isolated salts in aqueous solution were kinetically examined and a detailed mechanism of the breakdown of the tetrahedral intermediate was presented. Hydrolysis of Mixed O,S-ortho esters also provided useful information on the mechanism of the breakdown of the tetrahedral intermediate.Kinetic investigations on the acid-catalyzed hydrolysis of ketene dithioacetals showed that the dithio carbocation intermediates easily deprotonate; i. e., the protonation is reversible. Effects of an internal hydroxyl group on the hydrolysis of ketene dithioacetal resulted in a discovery of a new mode of anchimeric assistance; an internal solvation mechanism.Flash photolysis was carried out with a xenone lamp on aqueous solutions of 2-(p-toluene-sulfonyl)-1, 3-dithiolane and 2-(p-chlorophenylthio)-1,3-dithiolane, and their 2-phenyl derivatives. UV spectra, rapidly scanned with an MCPD detector, showed that the transient intermediate is dithiolanylium ion. Decay curves of the transient cation were more rapid than those expected for the first-order decay and recombination of the cation and the counter anions formed was considered to contribute to this reaction. In the presence of thiol or sulfinate, the decay of the cation followed the first-order kinetics and the second-order rate constants for the reactions with the nucleophiles were obtained.Further determinations of rate constants for reactions of a variety of nucleophiles and carbocations are planned.