Thioacetal

File:Thioacetal.png

General structure of a monothioacetal

File:Dithioacetal.png

General structure of a dithioacetal

In organosulfur chemistry, thioacetals are the sulfur (thio-) analogues of acetals (Template:Chem2). There are two classes: the less-common monothioacetals, with the formula Template:Chem2, and the dithioacetals, with the formula Template:Chem2 (symmetric dithioacetals) or Template:Chem2 (asymmetric dithioacetals).

The symmetric dithioacetals are relatively common. They are prepared by condensation of thiols (Template:Chem2) or dithiols (two Template:Chem2 groups) with aldehydes (Template:Chem2). These reactions proceed via the intermediacy of hemithioacetals (Template:Chem2):

Thiol addition to give hemithioacetal:

Template:Chem2

Thiol addition with loss of water to give dithioacetal:

Template:Chem2

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.

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:

Template:Chem2

The inversion of polarity between Template:Chem2 and Template:Chem2 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