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Preparation of Functionalized Dialkylzincs via a Boron−Zinc Exchange. Reactivity and Catalytic Asymmetric Addition to Aldehydes

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journal contribution
posted on 15.11.1996, 00:00 by Falk Langer, Lothar Schwink, Arokiasamy Devasagayaraj, Pierre-Yves Chavant, Paul Knochel
The hydroboration of olefins with Et2BH provides diethyl(alkyl)boranes 2 which readily undergo a boron−zinc exchange with Et2Zn providing a range of polyfunctional primary, secondary, and benzylic diorganozincs. The resulting diorganozincs 3 have been reacted with various electrophiles (allylic halides, acid chlorides, alkylidenemalonates, ethyl propiolate, nitroolefins) in the presence of CuCN·2LiCl with excellent yields. With secondary dialkylzincs prepared from diastereomerically pure diethyl(alkyl)boranes, the boron−zinc exchange occurs with loss of stereochemistry. The asymmetric addition of 3 to aldehydes in the presence of the chiral catalyst 55 furnishes optically active polyfunctional secondary alcohols (50 to over 96% ee).