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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?

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

Mozingo reaction

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 $\pu{20 kcal/mol}$ 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-sensitive functional groups are present in the starting carbonyl compound, the Mozingo reaction is likely the pathway of choice to reduce the carbonyl.

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  • $\begingroup$ Thanks, but what I am searching for is really the mechanism of the reaction : does the formation of the thioketal exactly matches the mechanism of the protection of an alcohol by an acetal ? What is the mechanism of the hydrogenation ? $\endgroup$ Mar 19 '16 at 19:53
  • $\begingroup$ Edited the answer to include mechanism discussion. $\endgroup$
    – ron
    Mar 19 '16 at 20:16
  • $\begingroup$ What are the side products of the reaction? $\endgroup$
    – user44900
    Jan 5 '19 at 18:13
  • $\begingroup$ @user44900: The side products should be propane and hydrogen sulfide. In the absence iof hydrogen, it could give the coupled product, $\ce{R1R2C-CR1R2}$. $\endgroup$ Sep 28 at 19:51
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Although, the Mozingo reaction, which describes the hydrogenolysis of Sulfur compounds by Raney nickel catalyst, is discovered in 1943 (Ref.1), the removal of sulfur from an organic molecule using Raney nickel is known even before. For example, the traces of catalyst poisons (mostly sulfur containing compounds) may be removed by treating compounds containing them with pyrophoric Raney nickel catalyst at room temperature or slightly above (Ref.2). However, the fate of the sulfur containing compounds, when these are the poisons, has not been determined. it was reported that Raney nickel catalyst in neutral or alkaline solution removes sulfur from aliphatic thiols $(\ce{R-SH})$ and disulfides $(\ce{R-S-R'})$, forming first nickel mercaptides $(\ce{(R-S)_nNi})$, which then decompose to yield sulfur-free compounds.

The reduction mechanism of these reactions, Ron suggested elsewhere, which occurs on metal surfaces is relatively unknown. During their work, Mozingo et al. has found that an active Raney nickel catalyst alone, in the presence of a solvent at moderate temperature, removes either reduced or oxidized sulfur by cleavage from the remainder of the organic molecule. Based on their observations, they have suggested two courses for the reaction of a sulfide with Raney nickel. In the first of these, the nickel is considered to function as a metal, removing the sulfur in a Wurtz-type reaction according to equation $(1)$: $$\ce{R-S-R' + Ni(H) -> R-R' + R'-R'} \tag1$$

Alternately, in the presence of sufficient Raney nickel catalyst to contain an excess of hydrogen, the reaction may take the course represented by equation $(2)$: $$\ce{R-S-R' + Ni(H) -> R-H + R'-H} \tag2$$

Briefly, when all the sulfur containing compounds reacted with sufficient Raney nickel catalyst to contain a large excess of hydrogen, only the reaction $(2)$ has been observed. In every case, the rupture of the carbon-sulfur bond was accompanied by the formation of a new carbon-hydrogen bond. During these reactions, products from the combination of the organic radicals, the coupled products as shown in the equation $(1)$, did not occur. Thiol compounds, on the other hand, would in deed give the same product by either mechanism $(1)$ or $(2)$ since $\ce{R'}$ is a hydrogen atom in this case. All of these observation suggest that radical mechanism, which happening on the metal surface. For instance, exactly that was suggested in the recent reference, which is a review of Raney nickel reduction (Ref.3):

Desulfurization of thiophenes on Ni surfaces

Note: The $\ce{Ni}$-promoted desulfurization transformation of thioketals, now known as Mozingo reaction, was firstly described in 1944 by Wolfrom and Karabinos (Ref.4), which used Raney nickel to reduce a thioacetal function into a methylene group.


References:

  1. Ralph Mozingo, Donald E. Wolf, Stanton A. Harris, and Karl Folkers, "Hydrogenolysis of Sulfur Compounds by Raney Nickel Catalyst," J. Am. Chem. Soc. 1943, 65(6), 1013–1016 (ODI: https://doi.org/10.1021/ja01246a005).
  2. Homer Burton Adkins, In Reactions of hydrogen with organic compounds over copper-chromium oxide with nickel catalysts; University of Wisconsin Press: Madison, WI, 1937 (ASIN: B0006ANUWE).
  3. Jana Rentner, Marko Kljajic, Lisa Offner, and Rolf Breinbauer, "Recent advances and applications of reductive desulfurization in organic synthesis," Tetrahedron 2014, 70(47), 8983-9027 (ODI: https://doi.org/10.1016/j.tet.2014.06.104).
  4. M. L. Wolfrom and J. V. Karabinos, "Carbonyl Reduction by Thioacetal Hydrogenolysis," J. Am. Chem. Soc. 1944, 66(6), 909–911 (ODI: https://doi.org/10.1021/ja01234a021).
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