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Palladium-Catalyzed Asymmetric Silaboration of Allenes

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journal contribution
posted on 2006-10-25, 00:00 authored by Toshimichi Ohmura, Hiroki Taniguchi, Michinori Suginome
An enantioselective silaboration of allenes was achieved using an achiral silylborane in the presence of a palladium catalyst bearing a chiral monodentate phosphine ligand. (R)-2-Bis(3,5-dimethylphenyl)phosphino-1,1‘-binaphthyl gave the highest enantioselectivities in the addition of (diphenylmethylsilyl)pinacolborane to the internal CC bond of terminal allenes at 0 °C, giving the corresponding β-borylallylsilanes in high yields with high enantiomeric excesses. The enantioselectivity depended on the bulkiness of substituents of allenes:  the enantiomeric excesses were found to be 91−93% ee (R = tert- and sec-alkyl), 88−90% ee (R = aryl), and 80−82% ee (R = prim-alkyl and Me) at 0 °C. Perfect chirality transfer was observed in the intramolecular cyclization reactions of the functionalized allylsilanes, affording highly enantioenriched cyclic alkenylboranes, which underwent Suzuki−Miyaura coupling with aryl halides.

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