Development of Highly Slective Reactions Using Allylic Barium Reagents

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The organomatallic compounds of heavier alkaline earth metals have found little application in organic synthesis, since they do not offer any advantages over Grignard reagents. We have been interested in using barium or strontium reagents with the anticipation that such species should exhibit markedly different stereochemical stability from that the ordinary magnesium reagent. We have already reported that the allylic barium reagents can be easily prepared from in situ generated reactive barium and allylic chlorides, and react with a variety of carbonyl compounds at-78ﾟC to produce the homoallylic alchols with remarkably high alpha-selectivity and retention of the configuration of the starting chloraide.In this research, we further examined the reactions of allylic barium reagents with other electrophiles and thus highly alpha-selective allylation reactions with allylic halides, epoxides, and imines were found. Noteworthy was the fact that aldimines were transformed into homoallylic amines by treatment with allylic barium reagents in which both the alpha-and gamma-adducts were selectively obtained by simply changing the reaction temperature. Bis(2,2,2-trifluoroethyl)phoshate was found to be a superior leaving group for alpha, alpha'-selective cross-coupling reaction of allylic alcohol derivatives with allylic barium reagents. Selective Barbier-type cross-coupling reation was also achieved using this leaving group. For example, when a 1 : 1 mixture of (E) -2-octenyl 1-bis (2,2,2-trifluoroethyl) phosphate and (E) -2-decenyl chloride was teated with 1 equiv of reactive barium, the cross-coupling products were obtained with 86% selectivity and an alpha, alpha'/alpha, gamma' ratio of 94 : 6 . For practical use, largescale carboxylation of an allylic barium reagent was also investigated and over 70% yield of the product was constantly obtained in the 7 mmol-scale reaction.