Umpolung of pi-Allylpalladium with Diethylzinc and Synthetic Application

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pi-Allylpalladium is usually recognized to serve as synthetically important electrophilic allylating agent toward many nucleophiles (organometals). This research mechanistically clarified that pi-allylpalladium complexes, catalytically generated in situ by the reaction of allylic esters and Pd(O), undergo the alkyl ligand exchange with diethylzinc and provide a mixture of allylzinc and diethylpalladium species. The thus formed allylzinc demonstrated unique regio-and stereoselectivity for the reaction with carbonyl compounds. For example, 1,3-disubstituted allylzinc reacted with aldehyde to provide (Z)-anti-homoallyl alcohols with high stereoselectivity. Furthermore, the present methodology turned out to be very useful and powerful to generate stereochemically defined allylzinc species, the species being very difficult to generate by other methods. The similar umpolung reaction proceeds between pi-allylpalladium and triethylborane (unpublished results).In the related research, it was demonstrated that pi-allylpalladium serves as an active species to promote beta-decarbopalladation reaction. This reaction also proceeds catalytically with respect to palladium and furnishes omega-dienyl aldehydes and ketones in good yields. Chemical & Engineering News picked up this report from the following points of view ; the uniqueness of the reaction type and the synthetic importance of the products as synthetic intermediates.