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Hydrogenation and dehydrogenation reactions play a key role in synthetic organic chemistry. The discoveries that selenium and sulfur substituents could be easily introduced α to a variety of acidifying functional groups, and that these derivatives could be smoothly converted to olefins by selenoxide or sulfoxide syn elimination under mild conditions greatly broadened the range of α,β‐unsaturated carbonyl compounds that could be prepared from their saturated analogs. This chapter covers the preparation from enols or enolates of α‐RSe substituted carbonyl compounds and nitriles and their conversion to olefins by thermolysis of the derived selenoxides. Other procedures for preparing these compounds are discussed in a limited way, and they are included in the tables, but the two‐step dehydrogenation is by far the most frequently used. The rapid acceptance of the selenoxide elimination as a synthetic procedure was the result of two factors: the ease with which many organoselenium compounds could be prepared using readily available, powerful electrophilic and nucleophilic selenium reagents, and the mild conditions (−50° to 40°) under which selenoxides fragment to olefins. Recognized selenoxide eliminations to form olefins were reported in 1967 and 1970, well after the related sulfoxide eliminations had been studied extensively. The application of the sulfoxide and selenoxide eliminations were reported almost simultaneously by several groups in 1973. These reactions have been the subject of several reviews. The principal procedures and reagents for the preparation of the requisite selenium compounds were also identified at that point. Preparations discussed in this chapter are Method A: Reaction of enolates, enols, enol ethers, and enamines with benzeneselenenyl chloride or bromide, or diphenyl diselenide. Method B: Reaction of enols with seleninylating agents [C 6 H 5 Se(O)Cl, (C 6 H 5 SeO) 2 O]. Method C: Nucleophilic substitution of α‐halo carbonyl compounds with metal selenolates. Method D: Alkylation of α‐phenylseleno carbonyl compounds and acylation of α‐lithio selenides or selenoxides.

The thermodynamic relationships between the I1 and IV oxidation states (eq 1, Y = S, Se) play a dominant role in determining

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