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Title:Intramolecular Ligand Permutation at Tetracoordinate and Pentacoordinate Silicon (Pseudorotation, Hypervalent)
Author(s):Stevenson, William Hartin, Iii
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Organic
Abstract:The X-ray crystal structures of 3,3,3',3'-tetrakis(trifluoromethyl)-1,1'(3H,3'H)-spirobi{2,1-benzoxasilole}(silane 1) and tetramethylammonium bis {(alpha),(alpha)-bis(trifluoromethyl)benzene-methanolato(2-)-C('2),O} phenylsilicate (2) are described. Silane 1 is a distorted tetrahedron at silicon with a significant deviation towards the trans planar geometry; the 10-Si-5 phenylsiliconate, 2, is nearly trigonal-bipyramidal at silicon. Evidence that the bidentate ligands of 1 preferentially stabilize the trigonalbipyramidal geometry of 10-Si-5 compounds, 3, relative to 8-Si-4 and 12-Si-6 forms is presented. Rates of ligand permutation for a series of siliconates, 3, are consistent with a mechanism proceeding by nondissociative Berry-type pseudorotation. The energy barrier to inversion for phenylsiliconate 2 ((DELTA)G(,422K)('*) = 26.0 kcal/mol) is lower than that for the isostructural, isoelectronic phosphorane ((DELTA)G(,422K)('*) = 28.3 kcal/mol). The rate of monodentate Si-C bond cleavage of arylsiliconates 3 by bromine and triflic acid decreases markedly as the electron withdrawing ability of the monodentate ligand increases. Phenyl- and n-butylsilicontes 3 react with bromine at comparable rates to give selective cleavage of the monodentate Si-C bond.
The inversion of 1 at silicon observed in weakly nucleophilic media is proposed to occur by a mechanism involving pseudorotation of a 10-Si-5 intermediate, 3 (i.e. Nu + 1 (DBLARR) 3 (DBLARR) (')3 (DBLARR) (')1 + Nu, where (')1 and (')3 are enantiomers of 1 and 3). Supporting this mechanism are kinetic studies which indicate that the rate is first-order in nucleophile and has no apparent correlation with the solvent ionizing power of the nucleophile or of the nonnucleophilic solvent. An equilibrium between 1 and 3 is frozen out in the low temperature ('1)H, ('19)F, and ('29)Si NMR spectra of mixtures of 1 and nucleophiles p-dimethylaminobenzaldehyde (DMAB) or methanol. Low temperature rate studies of the inversion of intermediate 3 (Nu = DMAB) are consistent with inversion by intramolecular pseudorotation ((DELTA)G(,183K)('*) = 10.2 kcal/mol). The reaction is fast enough to explain the inversion of 1 observed at higher temperatures.
Measurement of the rate of silane 1 inversion in nonnucleophilic solvent decahydronaphthalene-d(,18) gives evidence for a unimolecular process via a planar 8-Si-4 transition state ((DELTA)G(,422K)('*) = 26.1 (+OR-) 0.3 kcal/mol). A bimolecular inversion process is also detected. This is proposed to proceed through pseudorotation of a 10-Si-5 intermediate formed by coordination of the weakly nucleophilic oxygen of 1 at the silicon of another molecule of 1.
Issue Date:1984
Description:157 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.
Other Identifier(s):(UMI)AAI8422162
Date Available in IDEALS:2014-12-15
Date Deposited:1984

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