Mozingo reduction mechanism

Question

I have been searching for the mechanism of the Mozingo reduction on Google but I haven't found anything relevant.

Is the mechanism known? If so, what is it; if not, what hypothetical mechanisms can we consider?

Answer 1

Here is the reaction scheme for the Mozingo reaction (or Mozingo reduction) taken from this Wikipedia link.

The reduction works for both aldehydes and ketones and involves two steps. First, the carbonyl compound is converted into a thioketal (or thioacetal). The mechanism for this step is analogous to the mechanism for ketal or acetal formation (see here) except sulfur replaces oxygen as the nucleophile attacking the carbonyl.

In a second step, the thioketal is subsequently reduced to the corresponding methylene compound. The reduction can be a standard Raney nickel hydrogenolysis or hydrazine can be used at higher temperatures. The mechanism of reactions on metal surfaces or at higher temperatures are usually not well understood. Note however that a typical $\ce{C-S}$ bond is around 20 kcal/m weaker than a $\ce{C-C}$ bond. It is also known that sulfur is readily bound at metal surfaces (catalyst poisoning by sulfur being an example). The complexation of sulfur and the metal likely weakens the $\ce{C-S}$ bond further. Whether a $\ce{C-S.}$ radical is formed that reacts with a hydrogen radical complexed at the metal surface, or whether the hydride anion is generated at the metal surface and displaces the sulfur at the weakened $\ce{C-S}$ bond is not known.

The Mozingo reaction is complementary to the Clemmensen (run under acidic conditions) and Wolff-Kishner (run under basic conditions) reductions in that it can be performed under neutral conditions. So if acid- or base-sentive functional groups are present in the starting carbonyl compound, the Mozingo reaction is likely the pathway of choice to reduce the carbonyl.