A-Silyl enethiols

Under strictly anhydrous conditions which minimize competing protiodesilylation, (Z)-a-silyl vinyl sulfides with an co-carbonyl moiety attached to the S atom undergo a fluoride-promoted intramolecular cyclization which yields 2-alkenyltetrahydropyran-3-ols. The substrates are derived from (Z)-a-silyl enethiols by alkylation with co-halocarbo-nyl compounds (Scheme 157) <2000EJ02391>. 

Long reaction times lead to the disappearance of the blue color of thione 1, related to the addition of a second molecule of hydrogen sulfide to give colorless gem-dithiols 3. Aromatic derivatives (R = Ar) are less prone to further addition of hydrogen sulfide, whereas with aliphatic derivatives (R = alkyl), gem-dithiols are generally the final reaction products and can be isolated and fully characterized.18 gem-Dithiols 3 can be converted into thioacylsilanes 1 by neutralization of the thionation solution with solid sodium hydrogen carbonate. With this procedure, enolizable acylsilanes 2 (R = R CH2) are stereoselectively transformed into Z-a-silyl enethiols 4 (vide infra). 

As indicated in Scheme 1, enolizable acylsilanes were transformed by thionation, into Z-a-silyl enethiols 4 (Scheme 30). The reaction gave excellent yields independently of the substitution at silicon and the nature of the R1 group, which can be either an alkyl or an aryl group and can contain a range of functional groups including chloro, bromo, and COOH (Table 6). The Z-stereochemistry was assigned to the enethiols 4 by NOE experiments.46... 

A large variety of Z-a-silyl vinyl sulfides 55 were prepared starting from Z-a-silyl enethiols 4a-d with two different procedures46,47 (Scheme 36). According to path a (Scheme 36), products 55 were stereoselectively obtained by reacting 4 with halides in acetone in the presence of dry K2C03 at room temperature. The... 

In 1988, Mukaiyama et al. reported the Sn(OTf)2-50d-catalyzed asymmetric Michael reaction of a trimethylsilyl enethiolate, CH2=C(SMe)SSiMej (up to 70% ee) [243]. It was proposed that the catalytic reaction proceeded via an Sn(II) enethiolate. They also demonstrated that a BINOL-derived oxotitaniurn catalyzes the Michael addition of ketene silyl thioacetals to a-enone with high enantioselectivity (up to 90% ee) [244]. After this pioneering work other research groups developed new reaction systems for enantioselective Mukaiyama-Michael reactions. 

In an extension of this method, >-haloacylsilanes were transformed by H2S/HCI into the corresponding silyl thiones, which underwent enethiolization on base treatment and subsequent intramolecular cyclization to afford a range of cyclic sulfides251. 

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. 

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