Ionic and Radical Cross-Couplings for Carbon-Silicon Bond Formation

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The formation of C-Si bonds at saturated carbon atoms by bimolecular nucleophilic substitution (SN2) with silicon nucleophiles seems like a long-solved synthetic problem. Effective methods were, however, only disclosed last year. It was found that, depending on the leaving group, the displacement occurs either by an ionic mechanism or by a radical process. With triflate as leaving group, the catalytically generated copper-based silicon nucleophile reacts under SN2 whereas the same nucleophile engages in a stepwise radical mechanism with alkyl halides as electrophiles. Performing these C(sp3)-Si coupling reactions in asymmetric fashion is the next goal. With chiral alkyl triflates derived from readily available enantioenriched alcohols, the nucleophilic displacement would be enantiospecific. Conversely, the cross-coupling of alkyl halides would start from racemic material and is, as such, an enantioselective transformation. Achieving high enantioselectivity in a radical reaction is a formidable challenge. However, preliminary experiments have already shown that both ionic enantiospecific and radical enantioselective couplings with silicon nucleophiles are possible. This project aims at the systematic investigation of these new asymmetric C(sp3)-Si bond formations. Moreover, the ability of the copper-based silicon nucleophile to act as a one-electron reductant could also open a door to an unprecedented C(sp2)-Si cross-coupling reaction of diazonium ions.