4 - Tellurium in Organic Synthesis

Abstract

This chapter focuses on the application of tellurium (Te) compounds as reducing reagents of organic substrates. Inorganic species—such as hydrogen telluride, sodium hydrogen telluride, sodium telluride—and organic derivatives—such as aryltellurols (and tellurolates) and diorganyl tellurides—are widely employed in several reduction processes—such as reduction of carbonyl compounds, nitro derivatives, epoxides, aryl alkenes, and arylalkynes—as well as in the reductive cleavage of several types of carbon–heteroatom bonds. In several cases, the use of Te reagents is clearly advantageous compared to the usual methods because of the mildness of the experimental conditions and the great selectivity. The first reported radical reaction promoted by Te reagent was probably the conversion of allylic halides into coupled 1,5-dienes by treatment with telluride anions. The reaction, which gives the best results when employing the reagent prepared in situ from elemental Te and lithium triethylborohydride, proceeds through the intermediacy of the thermally unstable bis-allylic telluride, followed by the extrusion of Te and coupling of the formed allylic radicals. Additional advantages are the possibility of generating the reducing agents in situ as well in catalytic amounts in the presence of an inexpensive coreductant and the recovery of the Te material.