Pinacol Rearrangement
The Pinacol rearrangement is the acid-catalyzed transformation of vicinal diols to afford ketones or aldehydes. Besides protic acids (H2SO4, HClO4, TsOH), Lewis acids (BF3·OEt2, TMSOTf) are also used.
General features:
1. Cyclic or acyclic vicinal glycols can undergo the rearrangement providing aldehydes and/or ketones. 2. When all the substituents are equal the rearrangement yields a single product. 3. The regioselectivity of the reaction is based on the migratory ability of the substituents attached. 4. Cyclic systems may rearrange via ring-expansion or ring-contraction, depending on the ring size. 5. Side reactions such as β-eliminations may occur, yielding dienes and allylic alcohols.
Pinacol Rearrangement
Reaction Mechanism
1. Protonation of one hydroxyl group. 2. Loss of water. 3. [1,2]–shift to give a more stable carbocation. 4. Loss of the proton gives the product.
Experimental Procedure
The heterosubstituted alcohol (1.7 mmol, 1.0 eq) and silver carbonate (6.0 eq) were placed in a flame-dried sealed tube, and dry toluene (16 mL) was added under argon. After the mixture was heated at 110 ºC for 10 h, it was allowed to cool down to room temperature and diluted with ethyl acetate. The reaction mixture was then filtered through a pad of celite, and the filtrate was concentrated and purified by silica-gel column chromatography to afford the desired product (86% yield).
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