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https://hdl.handle.net/2142/19791
Description
Title
The addition of oxygen to coordinated thiophene
Author(s)
Skaugset, Anton Eric
Issue Date
1992
Doctoral Committee Chair(s)
Rauchfuss, Thomas B.
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, Inorganic
Language
eng
Abstract
The complex (Cp*Rh(TMT)) $\sp{2+}$ (Cp* = $\eta\sp5$-C$\sb5$Me$\sb5$, TMT = tetramethylthiophene) can be reduced with two electrons to give the novel compound Cp*Rh(TMT), which features an $\eta\sp4$-bound tetramethylthiophene ligand with the sulfur atom oriented away from the metal center. The thiophenic sulfur atom in this complex is extremely nucleophilic, as shown by the reaction of Cp*Rh(TMT) with molecular oxygen, which cleanly yields the $\eta\sp4$-tetramethylthiophene-S-oxide complex, Cp*Rh(TMT-1-O).
When (Cp*Rh(TMT)) $\sp{2+}$ is treated with three equivalents of hydroxide ion in aqueous solutions, a ring opened acetylpropenethiolate complex is isolated, Cp*Rh(Me$\sb3$APT) (APT = acetylpropenethiolate) featuring an $\eta\sp4$-bound ligand which has a pendant acetyl group. Similar reactivity was observed for ((cymene)Ru(TMT)) $\sp{2+}$, ((cymene)Ru(DMT)) $\sp{2+}$ (DMT = dimethylthiophene), and ((TMT)$\sb2$Ru) $\sp{2+}$. In contrast, upon treatment with aqueous base, the complexes (Cp*Ir(TMT)) $\sp{2+}$ and ((C$\sb5$Me$\sb4$H)Rh(TMT)) $\sp{2+}$ yield tetramethylthiophene-1-oxide complexes. Base hydrolysis of ((cymene)Os(TMT)) $\sp{2+}$ yields mixtures of the two products.
When the complex Cp*Rh(Me$\sb3$APT) was treated with a single equivalent of H$\sp+$, the 2-hydroxythiophenyl complex (Cp*Rh(TMT-2-OH)) $\sp+$ was formed. The proposed mechanism for this recyclization invokes nucleophilic character for the terminal thiolate in the acetylpropenethiolate ligand. This reactivity is supported by the reaction of MeOTf with Cp*Rh(Me$\sb3$APT), which yielded the S-Me adduct, (Cp*Rh(MeSC$\sb3$Me$\sb3$C(O)Me)) $\sp+$.
The 2-hydroxythiophenyl complex was shown to be intermediate in the base hydrolysis of (Cp*Rh(TMT)) $\sp{2+}$. The monoprotonation of (C$\sb5$Me$\sb4$H)Rh(TMT-1-O) also yielded the 2-hydroxythiophenyl complex. Upon addition of base, however, (C$\sb5$Me$\sb4$H)Rh(TMT-2-OH) did not reform the thiophene-1-oxide species, but rather yielded the acetylpropenethiolate complex, (C$\sb5$Me$\sb4$)Rh(Me$\sb3$APT). This reactivity indicates that the initial attack of hydroxide at bound thiophene occurs at the sulfur atom, with competition occurring between deprotonation of the hydroxide (resulting in the thiophene-1-oxide complex) or migration of the hydroxide group to the $\alpha$-carbon before deprotonation (resulting in the acetylpropenethiolate complex).
The complex Cp*Rh(Me$\sb3$APT) undergoes facile and clean thermolysis to give tetramethylfuran and (Cp*$\sb4$Rh$\sb4$S$\sb4$), a rhodium-sulfur cubane. The reaction is first order in concentration of Cp*Rh(Me$\sb3$APT), and presence of phosphine does not affect the rate of the reaction.
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