Allylation Involving Umpolung

Tamaru and co-workers have shown that Et3B displays two distinctive behaviors in Pd-catalyzed reaction of allylic alcohols [118a]  

activation of allyl alcohols for electrophilic allylation (a-allylation of ketones and aldehydes) and 

Activation of allyl alcohols is treated in Chapter 4.1. Concerning nucleophilic allylation, it has been shown that ciimamyl alcohol (324) reacts smoothly with benzaldehyde in the presence of Pd catalyst and Et3B as a stoichiometric reagent in THE at room temperature to afford homoallylic alcohol 326 [119]. In this reaction, nucleophilic allylboron 325 is generated by transmetallation of 7r-allylpalladium with Et3B and reacts with benzaldehyde. 

When aliphatic aldehydes are used under similar conditions, a-allylation and aldol condensation of aldehydes occur. 

Nucleophilic allylation of aldehydes with allyl alcohol proceeds smoothly in the presence of Et2Zn via the formation of nucleophilic allylethylzinc 327 to provide the homoallyl alcohol 328 [120], 

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In contrast to the above reactions of donor-acceptor cyclopropanes involving electrophrhc n-allyl intermediates, umpolung of cyclopropanes was observed in iridium-catalyzed coupling of VCP and alcohols (Scheme 2.59) [87]. In the reaction, nucleophilic 3t-allyliridium species were used to achieve diastereo- and enan-tioselective carbonyl allylation. 

The reversal in mode of reactivity or umpolung of ir-allylpalladium complexes can be accomplished by two strategies. The first involves conversion of the ir-allyl species to an allyl-M species, where M is a... 

Allylstannane 176 is formed by the reaction of allyl acetate with distannanes [97,98], In this reaction, umpolung of the electrophilic 7r-ally] palladium to the nucleophilic allylstannane occurs. Allylation of bromoindole 198 to give allylindole 199 involves the oxidative addition of 198 to Pd, transmetallation with the allylstannane 176, and final reductive elimination [99],... 

PhS stabilise carbanions, these allyl compounds 12,13, and 15 form anions which can be used in synthesis. This article describes further developments in our work with these two groups, involving both migration and transposition, leading to regiospecific reagents for synthons with normal reactivity or umpolung and their application to natural product synthesis. 

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