Palladium-Catalyzed gamma -Selective Cross-Coupling of Potassium Allyltrifluoroborates with Aryl and 1-Alkenyl Bromides

摘要

Transition metal-catalyzed cross-coupling reactions have proved to be one of the most powerful methods for selective C-C bond formation. Among the possible combinations of electrophiles and nucleophiles, coupling reactions of allylic metals with aryl, alkenyl and allyl electrophiles or their reversed combination provide an important class of compounds due to the frequent occurrence of these fragments in natural products. Although there have been few attempts at employing allylboron compounds for this purpose, perfect control of the coupling position by phosphine ligands was first achieved by Hiyama and Hatanaka by using allyltrifluorosilanes. They found that palladium-catalyzed coupling selectively occurs at the gamma-carbon when using PPh_3 or bisphosphines possessing a large bite angle such as dppb, and the reaction proceeds at the alpha-carbon with bisphosphines possessing a relatively small angle such as dppp. Such an effect of phosphine ligands was also demonstrated by Tsuji in regioselective Stille coupling of aryl halides with Me_3SiCH_2CH=CHCH_2SnBu_3 at the alpha- or gamma-carbon.