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.