title: Coordination and Reactivity in Organosilicon Compounds
Creators: name: Jiang, Jianxiong
Publisher: name: The Open University
Year: 1991

description: The coordination chemistry of silicon in silane-nucleophile interactions is examined by solution NMR spectroscopy ( <sup>29</sup>Si, <sup>1</sup>H and <sup>13</sup>C ) and conductivity measurements for acyclic, simple, multifunctional silanes R<sub>4-n</sub>SiX<sub>n</sub>( R = Me, Ph, H; X = F, Cl, Br, I, OSO<sub>2</sub>CF3; n = 2, 3, 4 ) and a choice of nucleophiles ( Nu = NMI, HMPA, DMAP, DMF, DMPU ). <br><br> Coordination at silicon involves delicate balance between the steric and electronic effects of all the groups involved. Tetra-, penta- and hexa-coordinated silicon species, neutral or ionic, intermediates or final adducts, are all observed in solution, depending on the number and nature of the leaving and the R groups, the nature and quantities of the nucleophiles. No evidence of heptacoordination is found. <br><br> The ability to expand coordination at silicon increases with the increasing of the number of electronegative functional groups. The sequence of coordinate capability is SiX<sub>4</sub>, R'SiX<sub>3</sub> (hexa-) &gt; R'<sub>2</sub>SiX<sub>2</sub> (penta-) &gt; R'<sub>3</sub>SiX (tetra-) when R' = Me, Ph. The nature of the leaving groups does not affect the maximum coordination number of the most stable adduct of a silane but determines the equilibrium constants K, which is in an Older of I &gt; OTf, Br » Cl &gt; F » OR" ( R" = Et, <i>i</i>Pr ). <br><br> The H-containing silanes present a distinct example of the steric effect of the R groups. A replacement of R' by H often increases the maximum coordination number of the most stable silane-nucleophile adducts by one, together with an enhanced reactivity of the silane. The coordinate ability of H-containing silanes is in the order: HSiX<sub>3</sub>, R'HSiX<sub>2</sub>, SiX<sub>4</sub> (hexa-) &gt; R'<sub>2</sub>HSiX, R'<sub>2</sub>SiX<sub>2</sub> (penta-) &gt; R'<sub>3</sub>SiX (tetra-). <br><br> The coordination at silicon is also determined by the steric hindrance resulting from the nucleophiles, which can be divided into two categories. Nucleophiles of group one with smaller steric hindrance ( Nu<sup>I</sup> = NMI, DMAP, DMF ) usually form silane-Nu<sup>I</sup> complexes with coordination numbers greater by one than those of nucleophiles of group two with larger steric hindrance ( Nu<sup>II</sup> = HMPA, DMPU ). The nucleophilicities of the nucleophiles only contribute to the equilibrium constant K. <br><br> The competitive processes between association and substitution in the interactions of silanes with nucleophiles result in the coordination at silicon changing with the quantities of the nucleophile presented. A number of reaction mechanisms are suggested. Some of the intermediate complexes are proved less reactive than their parent silanes or adducts to substitution. <br><br> Me<sub>2</sub>HSiOTf and MeHSiOTf<sub>2</sub> are found to be effective reducing agents for aldehydes and ketones. They are the first simple silanes reported to be promising reducing agents without the presence of a catalyst Factors affecting the reduction are discussed.