The addition of oxygen to coordinated thiophene

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.U of I OnlyETDs are only available to UIUC Users without author permissio