Thiophenes, oxidation reduction

A variety of methods have been described for the isolation of thiophenes from fossil fuels (13.23-25). An oxidation-reduction procedure very similiar to the present method has been described (25). the difference being in the reagent used for the oxidation step. In the method of Willey et al. (25). a 30% I Ctyacetic acid/benzene mixture under reflux for 16 hours was used. This is satisfactory for some thiophenes, but in other cases further oxidation to quinones and phenols takes place and thus recoveries were often poor since these substances could not be reduced back to thiophenes (26.27). 

The catalysis cycle comprises the following steps oxidative addition of H2 to the metal coordination of the benzothiophene in the 77 -mode hydride migration and, finally, elimination of dihydrobenzo[3]thiophene by reductive coupling of the hydride and dihydrobenzothienyl ligands (Scheme 73). Based on this, various ruthenium and rhodium complexes have been developed, which exibit good catalytic activity. 

The bis-ruthenium species 45 (Chart 5.12) has been prepared and characterized via an X-ray crystal structure [54]. No splitting is observed in the reduction waves, suggesting that the interaction between metal centers is weak, but the thiophene oxidation is shifted positive due to the electron-withdrawing nature of the metal groups. 

Keywords Birch reduction Diels-Alder reaction Dienes Dienophiles Dimerization Ionic hydrogenation Oxidation Reduction Reductive desulfurization Thiophene 1,1-dioxides Thiophene 1-oxides Transition metal complexes... 

This chapter discusses the common and/or recent methods to prepare thiophene oxides and their Diels-Alder reactions, as well as the reduction chemistry of thiophenes. 

The common and/or recent methods to prepare thiophene oxides and their Diels-Alder reactions, along with the reduction chemistry of thiophenes, are described in this chapter. Besides the methods which have been known for a long time, additional development of methods that involve thiophene oxidation and cycloaddititMi reactions as well as thiophene reductirai reactions is also likely to continue to attract interest and research efforts. In addition, there is a huge potential for both applica-ti(Mis and further methodological developments in the field of Diels-Alder cycloaddition reactions for preparing key compounds of importance. 

---