"Halophilic Activation" of Chlorosilanes: Allylation of Aldehydes Catalyzed by Platinum Dichloride or Silver Triflate
Alois Fürstner*, David Voigtländer
*Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim/Ruhr, Germany, Email: fuerstnermpi-muelheim.mpg.de
A. Fürstner, D. Voigtländer, Synthesis, 2000, 959-969.
PtCl2 efficiently catalyzes the addition of allyldimethylchlorosilane to aldehydes. The reaction is highly aldehyde selective and tolerates ketones, esters, nitriles, alkenes, aryl halides, ethers and nitro groups. If substituted allylchlorosilanes are used, the resulting homoallyl alcohols are obtained with good to excellent diastereoselectivity.
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With few exceptions, Sakurai reactions are carried out with stoichiometric amounts of Lewis acids as promoters activating the aldehyde. In contrast, the new method is based on the activation of allyl(crotyl) dimethylchlorosilane 1 by a "halophilic" catalyst, leading to an increased electrophilicity at silicon. The resulting species A/A’ then acts as a Lewis acid, coordinates onto the aldehyde and delivers its allyl(crotyl) group in a chairlike transition state B via a hetero-Cope type process. Among various "halophilic" promotors screened, PtCl2 turned out to be optimal in terms of efficiency, catalytic turn-over, and ease of handling. In contrast to conventional Sakurai reactions, this PtCl2-catalyzed procedure allows the preparation of syn- as well as anti-configurated homoallyl alcohols because it translates the (E/Z)-geometry of the substrate into the relative stereochemistry of the product.