Selenation-oxidation

H. J. Reich, S. Wollowitz, Preparation of a,/3-Unsaturated Carbonyl Compounds and Nitriles by Selen-oxide Elimination, Org. React. 1993, 44, 1-296. 

Benzyl-phenyl- IV/ld, 547 (Selen-oxid-Red.) IX, 983 (alkylier, Se-Se-Spalt.)... 

Selenium forms weaker a-bonds than sulfur and many reactions which involve the cleavage of such bonds are faster than for analogous sulfur compounds and proceed under milder reaction conditions. The sj -elimination of selen-oxides was discovered in 1970 [9] and had a major impact on organoselenium chemistry. This reaction is about three orders of magnitude more rapid than the elimination of the corresponding less polar and less basic sulfoxides. Sigmatropic rearrangements proceed at markedly lower temperatures. These reactions are discussed in detail in Chap. 8 by Y. Nishibayashi and S. Uemura. 

Step. In fact, several asymmetric reactions via this process have recently been reported for the preparation of chiral allylic alcohols. In this section, typical results of asymmetric [2,3]sigmatropic rearrangement via chiral allylic selen-oxides to afford the corresponding chiral allylic alcohols are described. 

This chapter is devoted exclusively to the alkylation of the above mentioned organometallic reagents with alkyl halides, epoxides and oxetanes. It includes a large variety of organosulfur- and organoselen-ium-stabilized carbanions derived from saturated and unsaturated sulfides, selenides, sulfoxides, selen-oxides and sulfones as well as those carbanions bearing another heteroatomic moiety. The chapter excludes, however, those organometallics which can be viewed as a-thio and a-seleno enolates. 

Selective reduction of the C(2)=C(3) bond in naphthyridone 379 followed by treatment with BuLi in THF and methylation affords compound 380. Subsequent regeneration of the double bond occurs as a result of successive selenation, oxidation and yyn-elimination to form naphthyridone 381 (1991JHC541). Earlier, an analogous scheme has been used for the synthesis of ethyl 7-chloro-l-ethyl-6-fluoro-2-mrthyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate (1989JHC1675). 

The ratio of these ligand exchange and reduction products also depends largely upon the temperature and molar ratio of the RLi reagents. In the selen-oxides, the Se-0 bond energy is lower than that of S-O, and hence Se-O bond cleavage should take place concomitantly with C-Se bond cleavage. 

A more general route for this type of chiral bicyclic lactam is based on a metalation from a Meyers lactam (198) followed by alkylation, acylation, or sulfenylation to give the a-substituted bicyclic unsaturated lactams. These compounds were converted to a, -unsaturated derivatives (199) by a metalation-selenation-oxidative elimination sequence that proceeded in good overall yields (Scheme 35) <90T495l>. 

The anti-Markownikoff adducts (1) on oxidation (H2O2, pyridine) form selen-oxides that decompose to either vinyl or allyl bromides, depending on the structure of the R group ... 

This reaction has been modified by the use of different oxidants to generate the selen-oxides. In addition, an antibody-catalyzed selenoxide elimination has been developed. Moreover, this reaction has been performed under basic conditions. 

A new, high-yield version of the ester Claisen rearrangement involves selen-oxide elimination in generation of the required keten-acetal system (Scheme 31). Two other new applications of the Claisen rearrangement (to prepare 2,4-dienals " or -dienones from allyl alcohols) are outlined in Scheme 32. 

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