Synthesis of Stable Dianionic Cyclic Silenolates and Germenolates

In this contribution a convenient synthetic method to obtain the previously unknown dianionic cyclic silenolates and germenolates is described. These dianions 2a,b and 4a,b are easily accessible via a one-pot synthetic protocol in high yields. Their structural properties were analyzed by a combination of NMR, single-crystal X-ray crystallography, and DFT quantum mechanical calculations. Moreover, the reactivity of 2a,b and 4a,b with selected examples of electrophiles was investigated. 2a and 4a were reacted with ClSiiPr3 to give new examples of polysilanes and polygermanes with exocyclic double bonds. The reaction of 2b with ClSiMe2SiMe2Cl led to the formation of the acyl bicyclo[2.2.2]octasilane 6. Moreover, the reaction of 2a,b and 4a,b with MeI, as an example of a carbon-centered electrophile, led to selective alkylation reactions at the negatively charged silicon and germanium atoms. The corresponding methylated structures 9a,b and 10a,b were formed in nearly quantitative yields. The competitive reactivity of the silyl and silenolate anion toward 1 equiv of ClSiMe3 showed that the outcome of the reaction was strongly influenced by the substituent at the carbonyl moiety. 2a reacted with 1 equiv of ClSiMe3 to give the corresponding cyclic silenolate S1a, which demonstrated that the silyl anion is more nucleophilic than the silenolate with attached aromatic groups. 2b, on the other hand, reacted with 1 equiv of ClSiMe3 to give the bicyclic compound 11 via an intramolecular sila-Peterson alkenation reaction. These findings clearly showed that the alkyl-substituted silenolate is more nucleophilic than the silyl anion. This paper demonstrates that 2a,b and 4a,b have the potential to be used as unique building blocks for complex polysilane and polygermane frameworks.