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  • Title: Development of a highly alpha-regioselective metal-mediated allylation reaction in aqueous media: new mechanistic proposal for the origin of alpha-homoallylic alcohols.
    Author: Tan KT, Chng SS, Cheng HS, Loh TP.
    Journal: J Am Chem Soc; 2003 Mar 12; 125(10):2958-63. PubMed ID: 12617663.
    Abstract:
    This paper described a general method to obtain alpha-adduct homoallylic alcohols using indium, zinc, and tin in water. A new mechanism was proposed to account for the formation of these synthetically difficult-to-obtain molecules. Generally, this method can be performed with a wide range of aldehydes and allylic halides with just 6 equiv of water added, giving the alpha-adduct in high selectivities. To account for the origin of the alpha-homoallylic alcohol, the reaction mechanism was carefully studied using (1)H NMR, a crossover experiment, and the inversion stereochemical studies of 22 beta gamma-adduct homoallylic sterol to the 22 alpha alpha-adduct homoallylic sterol. From the results of mechanism studies, it is possible that two mechanism pathways coexisted in the metal-mediated alpha-regioselective allylation. The metal salts formed from the metal-mediated allylation can catalyze the gamma-adduct to undergo a bond cleavage to generate the parent aldehyde in situ followed by a concerted rearrangement, perhaps a retroene reaction followed by a 2-oxonia[3,3]-sigmatropic rearrangement to furnish the alpha-adduct. The alpha-adduct can also be synthesized via the formation of an oxonium ion intermediate between the gamma-adduct and the unreacted aldehyde. The proposed mechanisms were further supported by experimental findings from the addition of InBr(3) to gamma-adduct under similar conditions.
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