{{short description|Organosulfur compounds of the form –CH(O–)S– or –CH(S–)2}} thumb|100px|right|General structure of a monothioacetal thumb|100px|right|General structure of a dithioacetal
In organosulfur chemistry, '''thioacetals''' are the sulfur (''thio-'') analogues of acetals ({{chem2|R\sCH(\sOR)2}}). There are two classes: the less-common '''monothioacetals''', with the formula {{chem2|R\sCH(\sOR')\sSR"}}, and the '''dithioacetals''', with the formula {{chem2|R\sCH(\sSR')2}} (symmetric dithioacetals) or {{chem2|R\sCH(\sSR')\sSR"}} (asymmetric dithioacetals).<ref>{{GoldBookRef |title=thioacetals |file=T06348 }}</ref>
The symmetric dithioacetals are relatively common. They are prepared by condensation of thiols ({{chem2|\sSH}}) or dithiols (two {{chem2|\sSH}} groups) with aldehydes ({{chem2|\sCH\dO}}). These reactions proceed via the intermediacy of hemithioacetals ({{chem2|R\sCH(\sOH)\sSR'}}): :Thiol addition to give hemi'''thioacetal''': ::{{chem2 | RSH + R'CH(O) -> R'CH(SR)OH }} :Thiol addition with loss of water to give dithioacetal: ::{{chem2 | RSH + R'CH(OH)SR -> R'CH(SR)2 + H2O }} Such reactions typically employ either a Lewis acid or Brønsted acid as catalyst.
Dithioacetals generated from aldehydes and either 1,2-ethanedithiol or 1,3-propanedithiol are especially common among this class of molecules for use in organic synthesis.<ref>{{cite journal | doi = 10.15227/orgsyn.056.0008 | author = P. Stütz and P. A. Stadler | title = 3-Alkylated and 3-Acylated Indoles from a Common Precursor: 3-Benzylindole and 3-Benzoylindole | journal = Organic Syntheses | date = 1977 | volume = 56 | page = 8 }}</ref>
:400px|Synthesis of a dithioacetal from acetaldehyde and 1,3-propanedithiol
The carbonyl carbon of an aldehyde is electrophilic and therefore susceptible to attack by nucleophiles, whereas the analogous central carbon of a dithioacetal is not electrophilic. As a result, dithioacetals can serve as protective groups for aldehydes. <!-- TODO: how to hydrolyze -->
Far from being unreactive, and in a reaction unlike that of aldehydes, that carbon can be deprotonated to render it nucleophilic:
:{{chem2 | R'CHS2C2H4 + R2NLi -> R'CLiS2C2H4 + R2NH }}
The inversion of polarity between {{chem2|R'(H)C^{δ+}\dO^{δ−} }} and {{chem2|R'CLi(SR)2}} is referred to as umpolung. The reaction is commonly performed using the 1,3-dithiane. The lithiated intermediate can be used for various nucleophilic bond-forming reactions, and then the dithioketal hydrolyzed back to its carbonyl form. This overall process, the Corey–Seebach reaction, gives the synthetic equivalent of an acyl anion.
==See also== * Mozingo reduction * Thioketal
==References== {{Reflist}}
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Category:Functional groups Category:Organosulfur compounds