Structural and Reactivity Studies of a Homologous Series of Crown Ether-Supported Alkali-Metal Silyl Complexes

Alkali-metal silyl reagents have proved useful in widespread applications in organic chemistry as powerful nucleophiles for the generation of C–Si bonds. Typically, these compounds are generated in situ, and the understanding of the influence of the alkali-metal on their structures and their reactivities remains limited. Herein, we report a systematic study on the synthesis, structural and spectroscopic characterization of a homologous series of alkali-metal silyl complexes (AMSiMe2Ph, AM = Li–Cs) stabilized by crown ether ligands (12-crown-4, 15-crown-5 or 18-crown-6). X-ray crystallographic studies established monomeric contacted ion pair structures for these complexes. Multinuclear NMR spectroscopy (1H, 13C, 133Cs, 7Li, 29Si), along with 1H DOSY NMR spectroscopic investigations, was employed to evaluate the aggregation and solvation state of the alkali-metal silyls in THF. The ability of these compounds to act as silicon nucleophiles toward organic substrates has been tested using 4-bromoanisole, para-fluorobiphenyl and pyridine as model substrates, uncovering a noticeable alkali-metal effect for the efficiency in the generation of new C–Si bonds.