Unique reaction behavior of organozincs toward pi-allylpalladium complexes: Palladium catalyzed synthesis of unsymmetrical ketones and umpolung of pi-allylpalladium

Alkylorganozinc reagents, non-functionalized and functionalized with electrophilic functional groups, e.g., ester, ketone, nitrile, etc., display unique reactivity toward pi-allylpalladium complexes :They do not undergo usual coupling reaction, providing allylalkanes, instead, they form rather stable (reductive elimination resistant) alkylallyipalladium complexes, which either undergo a reductive carbonylative coupling reaction to provide alkyl allyl ketones under carbon monoxide (1 atm; unsymmetrical ketone synthesis via palladium catalyzed three-component coupling reaction of allylic benzoates, carbon monoxide, and alkylorganozincs) or react further with another mole of organozinc reagents to furnish an equilibrium mixture with dialkylpalladium and allylzinc species (umpolung of pi-allylpalladium via transmetailation with organozincs). Stereo-defined allylzincs, generated by this umpolung technique with inversion of configuration at the allylic center of allylic benozoates, react with aldehydes with high regio- and stereoselectivities, Mechanistic details of these unique reactions are discussed.