posted on 2003-03-08, 00:00authored byOlov A. Wallner, Kálmán J. Szabó
Palladium-catalyzed electrophilic allylic substitution of functionalized allyl chlorides and allyl
acetates can be achieved in the presence of hexamethylditin under mild and neutral reaction
conditions. This efficient one-pot procedure involves palladium-catalyzed formation of transient
allylstannanes followed by generation of a bis-allylpalladium intermediate, which subsequently
reacts with electrophiles. Using this catalytic transformation, various aldehydes and imines can
be allylated providing highly functionalized homoallyl alcohols and amines. Furthermore, tandem
bis-allylation reactions could be performed by employing tosyl isocyanate and benzylidenemalonitrile
as substrates. A particularly interesting mechanistic feature of this reaction is that palladium
catalyzes up to three different transformations in each catalytic cycle. Various allylic functionalities,
including COOEt, CONH2, COCH3, CN, Ph, and CH3, are tolerated in the catalytic reactions due
to the application of neutral and mild reaction conditions. The substitution reaction occurs with
very high regioselectivity at the branched allylic terminus. Moreover, in several reactions, a high
stereoselectivity was observed indicating that this new catalytic process has a high potential for
stereoselective synthesis. The regioselectivity of the reaction can be explained on the basis of DFT
calculations. These studies indicate that the allylic substituent prefers the γ-position of the η1-allyl moiety of the reaction intermediate.