Tautomerism, enethiol: thioketone

For simple aliphatic thioketone-enethiol tautomerism it has been assumed that the tautomers are similar in energy, and therefore one can anticipate only small differences, if any, in basicities of the sulphur and carbon sites191,192. 

Aliphatic thioketones result tautomerically pure from the reaction of H2S with ketals in the presence of ZnCl2. Deprotonation with LDA gives the enethiolate which can be silylated to form the silyl vinyl sulphide. These are fairly stable to traces of water but react with MeOH to give the enethiol (equation 3)7. 

For simpler, less hindered aliphatic thioketones, the method of Mayer and Berthold [132] of thionation of acetals by H2S was shown [133] to lead to tautomerically pure thioketones when the transformation was carried out under sufficiently mild conditions. Thus the reaction at 0°C in ethanol or THF in the presence of 0.05-0.01 eq of zinc chloride yielded reasonable yields of thioketones (2) devoid of the isomeric enethiols (2 ). 

Enethiols 61 are tautomerically stable isomers of thioketones 60 and, in contrast to enols, enethiols have been isolated and characterized. This method provides a route for the selective synthesis of enethiols by deprotonation of 60 and quenching with trimethylchlorosilane, since under these conditions no isomerization takes place. 

This study was complemented by a selective synthesis of the tautomeric enethiols [77]. Aliphatic thioketones were deprotonated by LDA, silylated, and the resulting silyl vinyl sulfides were smoothly converted to enethiols by simple addition of methanol. These are stable compounds which do not equilibrate with thioketones, this behaviour probably related to the extremely mild conditions of the (easy) cleavage of the silicon-sulfur bond. 

Energy units 2 Enethiols, formation 252 tautomerism with thioketone 395 Entropy 151... 

A number of unsaturated thioketones, bearing only aliphatic substituents, could be isolated [61, 62]. When the products possess protons on the a carbons, they were accompanied by the tautomeric enethiols [62]. When this form was favored, some authors carried out the [3,3] shift in acetic anhydride as a solvent and isolated the corresponding thiolester [63, 64]. Recently, the rearrangement of tung-sten(pentacarbonyl) complexes of sulfides was reported to lead to the corresponding thiocarbonyl derivatives [65]. 

Cyclic a 3-unsaturated thioketones appeared to exist exclusively in the thione form. A peak at 8 = 1.88 p.p.m. in the n.m.r. spectrum of diiomor-pholine-3-thione was attributed to the presence of an enethiol tautomer (516). On the basis of a study of the tautomerism of simple thio-amides by means of the basicity method, Kjellin and Sandstrom found the pKt values for thiOacetamide, N-methylthioacetamide, thiobenzamide, and N-methyl-thiobenzamide to be -8.6, -9.6, -8.3, and -8.9, respectively... 

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