Gas-Phase Nucleophilic Reactivity of Alkoxysilanes

The gas-phase reactivity of the Me4-nSi(OEt)(n) (n = 1-3) alkoxysilanes was investigated by Fourier transform ion cyclotron resonance (FT-ICR) technique in order to characterize the fundamental mechanisms associated with nucleophilic attack on these important precursors of new materials. Typical nucleophiles such as F-, MeO- and EtO- react readily and preferentially by attack at the silicon center with formation of a pentacoordinated siliconate that undergoes elimination processes initiated by either a nascent methide ion, Me, or an ethoxide ion, EtO, that can abstract a proton to yield either a carbanion or a siloxide-type anion. Carbanions of the type X(Me)(3-n)Si(OEt)(n)CH2- (X = F, MeO, EtO and n = 1-3) and siloxide ion of the type X(Me)(4-n)Si(OEt)(n-1)O- (X = F, MeO, EtO and n = 2-3) can be easily dissociated by infrared multiphoton excitation with a CO2 laser to give rise to a large variety of simple siloxide and silicate-type anions that are reminiscent of intermediate species in the early stages of sol-gel processes. For the higher-ethoxy containing substrates, a competing reaction is observed that is analogous to an E2 elimination reaction in which the nucleophile abstracts a beta-proton from the ethoxy group leading to elimination of ethylene. The kinetics, relative product distribution and thermochemistry of these reactions are also reported for some specific cases.