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Thioacetal

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General structure of a monothioacetal.
General structure of a dithioacetal.

Thioacetals are the sulfur analogues of acetals. There are two classes: monothioacetals and dithioacetals. Monothioacetals are less common, have the functional group RC(OR')(SR")H. Dithioacetals have the formula RC(SR')2H (symmetric dithioacetals) and RC(SR')(SR")H (asymmetric dithioacetals).[1]

The symmetric dithioacetals are relatively common. They are prepared by condensation of thiols or dithiols with aldehydes. These reactions proceed via the intermediacy of hemithioacetals:

  1. Thiol addition to give hemithioacetal: RSH + R'CH(O) → R'CH(SR)(OH)
  2. Thiol addition with loss of water to give dithioacetal: 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.[2]

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.

Far from being unreactive, and in a reaction unlike that of aldehydes, that carbon can be deprotonated to render it nucleophilic:

R'CHS2C2H4 + R2NLi → R'CLiS2C2H4 + R2NH

The inversion of polarity between R'(H)Cδ+=Oδ− and 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

References

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "thioacetals". doi:10.1351/goldbook.T06348
  2. ^ P. Stütz And P. A. Stadler "3-alkylated And 3-acylated Indoles From A Common Precursor: 3-benzylindole And 3-benzoylindole" Org. Synth. 1977, 56, 8.doi:10.15227/orgsyn.056.0008
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