Nucleophilic Attack of R-lithium at Tetrahedral Silicon in Alkoxysilanes. An Alternate Mechanism

The currently accepted mechanism for nucleophilic attack at silicon in tetraalkoxysilanes, e.g. Si(OEt)4 is suggested to involve formation of penta- and then hexacoordinated intermediates as supported by the apparent exclusive formation of R3SiOR′ and R4Si from nucleophilic attack by RLi and RMgX. Our recent discovery of a direct route from biogenic silica to tetraalkoxyspirosiloxanes prompted us to revisit this reaction as a potential route to diverse silicon-containing species with single Si–C bonds as early studies demonstrate that spirosiloxanes form quite stable pentacoordinated alkoxysilane compounds. As anticipated, Si(2-methyl-2,4-pentanediolato)2 (SP) reacts with RLi (R = Ph, anthracene, phenylacetylene, etc.) at −78 °C to form pentacoordinated Si, e.g. LiPhSP equilibrates with the starting reagents even at 3:1 ratios of PhLi:SP with no evidence for formation of hexacoordinated species by mass spectral, NMR and quenching studies. Thus, quenching with MeI or Me3SiCl allows isolation of monosubstituted products from RLi:SP; RSi(OR′)3 including some ring-opened oligomers. Comparative studies of reactions of PhLi with Si(OEt)4 allows isolation of mono- and disubstituted products again even at 1:1 ratios of PhLi:Si(OEt)4. However, on standing at −78 °C for long periods of time or on warming to 0 °C, the primary product for both reactions is Ph4Si even with 0.5 equivalents of PhLi. At reaction temperatures ≥0 °C the primary product is again Ph4Si. These results suggest that hexacoordinated intermediates are not part of the substitution mechanism and may suggest that the higher-substituted compounds arise from disproportionation processes. We also briefly describe the conversion of anthracenylSP and 9,9-dimethylfluoreneSP to silsesquioxanes.