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This reaction is taken from: Pintér, Á.; Sud, A.; Sureshkumar, D.; Klussmann, M. Autoxidative Carbon-Carbon Bond Formation from Carbon-Hydrogen Bonds. Angew. Chem., Int. Ed. 2010, 49 (29), 5004–5007. DOI: 10.1002/anie.201000711.

Normally carbonyl groups are reduced to alcohols, why this reaction can go directly to methylene?

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    $\begingroup$ This is more like an amide than a ketone. $\endgroup$ Jan 30, 2022 at 15:19
  • $\begingroup$ Thanks for the edit, that's a meaningful idea you mentioned. THX a lot. $\endgroup$ Jan 30, 2022 at 15:54
  • $\begingroup$ If you got your answer, feel free to self answer it. Self answer are always appreciated and it also help future readers. $\endgroup$ Jan 31, 2022 at 4:01

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There is a hint in Wikipedia that the substrate, rather than the borane, may be the main factor. In this case the substrate is aromatic:

Aromatic carbonyls are more readily reduced to their respective alkanes than aliphatic compounds.[1] For example, ketones are reduced to their respective alkyl benzenes by catalytic hydrogenation[2,3] or by Birch reduction[4] under mild conditions.

While this passage does not mention specifically reduction by borane, the same effect would likely apply. If we suppose that the carbonyl compound is first reduced to an alcohol, removal of the hydroxyl group would require formation of a radical or ion, which in an aromatic compound such as the OP's substrate would be stabilized by conjugation with the aromatic ring(s). Such a species would then be reduced by accepting a hydrogen in place of the hydroxyl group.

With borane acting as a Lewis acid on the OP's substrate, an aromatic carbocation (isoelectronic with anthracene) would form and could then accept a hydride ion to give the methylene product.

Cited References

1. Nishimura, Shigeo (2001). Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis (1st ed.). New York: Wiley-Interscience. p. 583. ISBN 9780471396987.

2. Zaccheria, Federica; Ravasio, Nicoletta; Ercoli, Mauro; Allegrini, Pietro (2005). "Heterogeneous Cu-catalysts for the reductive deoxygenation of aromatic ketones without additives". Tetrahedron Letters. 46 (45): 7743–7745. https://doi.org/10.1016/j.tetlet.2005.09.041.

3. Walker, Gordon (1956). "Reduction of Enols. New Synthesis of Certain Methoxybenzsuberenes via Hydrogenation of Dehydroacetic Acids". Journal of the American Chemical Society. 78 (13): 3201–3205. https://doi.org/10.1021/ja01594a062.

4. Hall, Stan; Lipsky, Sharon; McEnroe, Frank; Bartels, Anne (1971). "Lithium-ammonia reduction of aromatic ketones to aromatic hydrocarbons". The Journal of Organic Chemistry. 36 (18): 2588–2591. https://doi.org/10.1021/jo00817a004.

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