Introduction and Range of Leaving Groups

Pd-catalyzed reactions of various allylic compounds via the formation of tt-allylpalladium complexes offer many synthetically useful methods. The following allylic compounds are known to form 7r-allylpalladium complexes by oxidative addition. 

Reaction of jr-allylpalladium chloride with malonate and acetoacetate was reported in 1965, showing that r-allylpalladium complexes are electrophilic [1]. Most importantly, electrophilic r-allylpalladium complexes react with various kinds of pronucleophiles of carbon, oxygen, and nitrogen. Then Pd(0) is regenerated after the reactions. The generation of Pd(0) offers the possibility of a catalytic process. This is the characteristic feature of r-allylpalladium chemistry. 

Palladium Reagents and Catalysts—New Perspectives for the 21st Century J. Tsuji 2004 John Wiley Sons, Ltd ISBNs 0-470-85032-9 (HB) 0-470-85033-7 (PB) 

As shown above many allylic compounds can be used for catalytic reactions with different reactivity. Allylic esters such as carbonates, acetates, and phosphates are widely used. Allylic acetates and phosphates react in the presence of bases such as EtsN and AcONa. However, Giambastiani and Poli reported that allylation of -keto esters, but not malonates, with allylic acetates can be carried out under neutral conditions, although the reaction is slower [5]. 

Allylic carbonates are more reaetive than acetates. In addition, reaction of carbonates proceeds in the absence of bases [6]. Formation of jr-allylpalladium 9 from allyl methyl carbonates 8 proceeds by oxidative addition, followed by decarboxylation, and TT-allylpalladium methoxide 9 is generated at the same time, which abstracts a proton from a pronucleophile to form 10. In situ formation of methoxide is a key in the allylation under neutral conditions. Allylation under neutral conditions is useful for the reaction of base-sensitive compounds. For example, exclusive chemoselective reaction of the carbonate group in 4-acetoxy-2-butenyl methyl carbonate (11) occurred in the absence of a base to yield 12. Similar chemoselective reaction of the allyl carbonate group in the chiral cyclopentenyl methyl carbonate 13 with the jS-keto ester 14 without attacking the allylic acetate group to give 15 was observed even in the presence of NaH. As expected, retention of stereochemistry (see Chapter 4.2.1) was observed in this substitution [7]. 

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