Pinacol coupling reaction

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A pinacol coupling reaction is an organic reaction in which a carbon-carbon covalent bond is formed between the carbonyl groups of an aldehyde or a ketone in presence of an electron donor in a free radical process ^[1]. The reaction product is a vicinal diol. The reaction is named after the product of this reaction with acetone, pinacol (also 2,3-dimethyl-2,3-butanediol or tetramethylethylene glycol). The reaction is usually a homocoupling but intramolecular cross-coupling reactions are also possible. Pinacol was discovered by Wilhelm Rudolph Fittig in 1859.

Contents 1 Reaction mechanism 2 Scope 3 External links 4 References

[edit] Reaction mechanism

The first step in the reaction mechanism is one-electron reduction of the carbonyl group by a reducing agent such as magnesium to a ketyl radical anion species. Two ketyl groups react in a coupling reaction yielding a vicinal diol with both hydroxyl groups deprotonated. Addition of water or another proton donor gives the diol. With magnesium as an electron donor, the initial reaction product is a 5-membered cyclic compound with the two oxygen atoms coordinated to the oxidized Mg²⁺ ion. This complex is also broken up by addition of water with formation of magnesium hydroxide. The pinacol coupling can be followed up by a pinacol rearrangement. The alternative McMurry reaction with titanium trichloride gives deoxygenation of the vicinal diol to an alkene.

[edit] Scope

The Pinacol coupling of benzaldehyde can be carried out in water with vanadium trichloride and aluminium as co-reductant in order to maintain the catalytic cycle ^[2]. This heterogeneous reaction in water at room temperature yields 72% after 3 days with 56:44 dl:meso composition. In another system with benzaldehyde, Montmorillonite K-10 and zinc chloride in aqueous THF under ultrasound the reaction time is reduced to 3 hours (composition 55:45) ^[3]. On the other hand certain tartaric acid derivatives can be obtained with high diastereoselectivity in a system of samarium(II) iodide and HMPA ^[4].

[edit] External links

• Conversion of benzophenone to benzopinacol in sunlight, Organic Syntheses, Coll. Vol. 2, p.71 (1943); Vol. 14, p.8 (1934).
• [Conversion of acetone to pinacol hydrate with magnesium and mercury(II) chloride, Organic Syntheses, Coll. Vol. 1, p.459 (1941); Vol. 5, p.87 (1925).
• Pinacol MSDS

[edit] References

1. ^  R. Fittig. Justus Liebigs Ann. Chem. 110, 23–45 (1859).
2. ^  Vanadium-Catalyzed Pinacol Coupling Reaction in Water Xiaoliang Xu and Toshikazu Hirao J. Org. Chem.; 2005; 70(21) pp 8594 - 8596 DOI:10.1021/jo051213f
3. ^  Pinacolization of aromatic aldehydes using Zn/montmorillonite K10-ZnCl2 in aqueous THF under ultrasound Zang Hongjun, Li Jitai, Bian Yanjiang, Li Tongshuang Chemical Journal on Internet Jan. 1, 2003 Vol.5 No.1 Online Article
4. ^  Asymmetric synthesis by stereocontrol Yong Hae Kim, Sam Min Kim, and So Won Youn Pure Appl. Chem., Vol. 73, No. 2, pp. 283–286, 2001. Online Article