General structure of a thioacetal.
General structure of a dithioacetal.
Thioacetals are the sulfur analogue of acetals. They are prepared in a similar way to acetals: by reacting a thiol with an aldehyde:
- RSH + R'CHO → R'CH(OH)SR
Dithioacetals are prepared similarly to thioacetals, which are intermediates:
- RSH + R'CH(OH)SR → R'CH(SR)2 + H2O
Such reactions typically employ either a Lewis or Brønsted acid catalyst.
Dithioacetals generated from aldehydes and 1,2-ethanedithiol are useful in organic synthesis:
- C2H4(SH)2 + R'CHO → R'CHS2C2H4 + H2O
Whereas the carbonyl carbon of an aldehyde is electrophilic, the deprotonated derivatives of dithioacetals feature nucleophilic carbon centers:
- R'CHS2C2H4 + R2NLi → R'CLiS2C2H4 + R2NH
Again, this reactivity is most commonly exploited in the 1,3-dithiolanes. The inversion of polarity between R'(H)Cδ+Oδ- and R'CLi(SR)2 is referred to as umpolung.