Acylsilanes Thioacylsilanes

The reaction proved to be highly chemoselective, allowing thionation of aldehydes in the presence of other carbonyl groups. As a noticeable exception acylsilanes were converted to thioacylsilanes [123]. 

The acid-catalyzed reaction of acylsilanes 2 with hydrogen sulfide in ether at low temperatures allowed the synthesis of thermally unstable thioacylsilanes 1 (Scheme 1) in good yields (Table 1). 

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). 

Ricci and co-workers showed that it is possible to transform a wide range of acylsilanes 2 into the corresponding thioacylsilanes 1 by reaction of bis(trimethyl-silyl)sulfide in the presence of CoC12-6H20 under very mild conditions and in good yields (Table 2)19 (Scheme 2). 

The interesting and unusual spectroscopic properties of acylsilanes have received considerable attention.6a,b Unfortunately, a similar systematic study for thioacylsilanes has not been done because of their low thermal stability. These compounds are blue materials [ -max(CCl4) = 678 nm for lb, 692 nm for la,13 and 612 nm for lg16], and are generally liquids except thiobenzoyl-triphenylsilane la, 2,4,6-trimethyl-thiobenzoyltrimethylsilane Is, and 3,5-di-terf-butylthiobenzoyltriphenylsilane le which are solids. 

Disilylated 477-thiopyrans are formed by the thionation of l,5-bis(acylsilanes) presumably via the thioacylsilane (Scheme 22) <04TL87>. 

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