Palladium-catalyzed regioselective silaboration of 1,2-dienes

Palladium complexes catalyzed regioselective addition of the Si-B bond of (dimethylphenylsilyl)pinacolborane to allenes in good yields. In the silaboration of terminal allenes having electron-donating substituents such as alkyl and methoxy groups, the Si-B bond added to the internal C=C bond with regioselective B-C and Si-C bond formation at the central and substituted carbon atoms of the allene, respectively. In contrast, the silylborane preferably added to the terminal C=C bond with exclusive Si-C bond formation at the terminal carbon atom in the silaboration of allenes bearing electron-withdrawing groups such as perfluoroalkyl group. 1,3-Disubstituted allenes also underwent the silaboration in good yields with varying regio- as well as stereoselectivity. The silyl and boryl groups of the 2-borylallylsilanes thus prepared were selectively utilized for further synthetic elaboration. Palladium-catalyzed coupling of the 2-borylallylsilanes with aryl iodides afforded the corresponding 2-arylallylsilanes in fair-to-good yields. 2-Boryl-pi-allylpalladium complexes were successfully prepared by the reaction of the 2-borylallylsilanes with PdCl2(CH3CN)(2). (C) 2000 Elsevier Science S.A. All rights reserved.