Stereoisomerization of Hexacoordinate Sulfur and Tellurium
Michalak, Ronald Stanley
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https://hdl.handle.net/2142/70201
Description
Title
Stereoisomerization of Hexacoordinate Sulfur and Tellurium
Author(s)
Michalak, Ronald Stanley
Issue Date
1982
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 reaction of bromine trifluoride with sulfurane 1 gives rise to the first reported isolation of geometrical isomers of 12-S-6 sulfur, the trans- and cis-persulfuranes 2 and 3. The cis isomer is 2.0 (+OR-) 0.1 kcal/mol more favored in free energy than the all trans isomer in methylene chloride solution at 25(DEGREES)C. Calorimetry showed the trans-to-cis isomerization to be exothermic by 1.6 (+OR-) 0.5 kcal/mol. The rapid, acid-catalyzed isomerization of trans isomer 2 to give cis isomer 3 occurs by a dissociative mechanism through a pentacoordinate cationic sulfur intermediate, a 10-S-5 persulfonium ion, which was isolated as its hexafluorophosphate salt. A lower limit for the activation barrier of the unobserved nondissociative isomerization of 2 to 3 is 50 kcal/mol in quinoline solution. Polarization and other effects on the relative bond lengths and energies of the isomers are discussed. The results of complete X-ray structure determinations for 2 and 3 are described together with a PMO argument rationalizing the observed order of bond lengths.
The reactions of bromine trifluoride with telluranes 9 and 10 give rise to the all-trans 12-Te-6 species, pertelluranes 3 and 13. The rate of isomerization from the all-trans geometry of 3 to the more stable isomer 4 with cis fluorines and oxygens and trans carbons was determined in different solvents and at different concentrations at 61(DEGREES)C. Failure to trap fluoride ion by added hexamethyldisilazane provides evidence against a dissociative mechanism involving Te-F bond heterolysis to give dissociated ions. The similarity of first-order rates of reaction {2.0 x 10('-5) to 2.5 x 10('-5) sec('-1) at 61(DEGREES)C} in solvents of differing ionizing power (Z-values in the range 54 to 64.2) provides evidence against any heterolytic dissociation mechanism for the isomerization of 3 to 4. The most probable mechanism for the permutational isomerization is concluded to be a nondissociative twist mechanism involving no cleavage of any of the six bonds to tellurium. The isomerization of 13 to a stereoisomer was not observed, either thermally or in the presence of Lewis acids. This could result from a higher activation barrier for the isomerization, or 13 could represent the most thermodynamically stable isomer in the set of possible stereoisomers. The results of complete X-ray crystallographic structure determinations of pertellurane 4 and tellurane 5 are described.
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