Intramolecular Ligand Permutation at Tetracoordinate and Pentacoordinate Silicon (Pseudorotation, Hypervalent)

Stevenson, William Hartin, Iii

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Description

Title

Intramolecular Ligand Permutation at Tetracoordinate and Pentacoordinate Silicon (Pseudorotation, Hypervalent)

Author(s)

Stevenson, William Hartin, Iii

Issue Date

1984

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(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.

Graduation Semester

1984

Type of Resource

text

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