Coordination and Reactivity in Organosilicon Compounds

Jiang, Jianxiong (1991). Coordination and Reactivity in Organosilicon Compounds. PhD thesis The Open University.

Abstract

The coordination chemistry of silicon in silane-nucleophile interactions is examined by solution NMR spectroscopy ( 29Si, 1H and 13C ) and conductivity measurements for acyclic, simple, multifunctional silanes R4-nSiXn( R = Me, Ph, H; X = F, Cl, Br, I, OSO2CF3; n = 2, 3, 4 ) and a choice of nucleophiles ( Nu = NMI, HMPA, DMAP, DMF, DMPU ).

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.

The ability to expand coordination at silicon increases with the increasing of the number of electronegative functional groups. The sequence of coordinate capability is SiX4, R'SiX3 (hexa-) > R'2SiX2 (penta-) > R'3SiX (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 > OTf, Br » Cl > F » OR" ( R" = Et, iPr ).

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: HSiX3, R'HSiX2, SiX4 (hexa-) > R'2HSiX, R'2SiX2 (penta-) > R'3SiX (tetra-).

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 ( NuI = NMI, DMAP, DMF ) usually form silane-NuI complexes with coordination numbers greater by one than those of nucleophiles of group two with larger steric hindrance ( NuII = HMPA, DMPU ). The nucleophilicities of the nucleophiles only contribute to the equilibrium constant K.

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.

Me2HSiOTf and MeHSiOTf2 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.

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