Selenoxide Elimination - Scope and Limitations - Selenylating and Oxidizing Reagents

Selenylating and Oxidizing Reagents

α-Selenylation of carbonyl compounds can be accomplished with a variety of reagents, using either electrophilic or nucleophilic selenylating reagents. Usually, simple phenylseleno compounds are used in elimination reactions; although 2-nitrophenylselenides react more quickly, they are more expensive to prepare, and phenylselenides typically react in minutes. Electrophilic selenylating reagents can be used in conjunction with enols, enolates, or enol ethers. Phenylselenating reagents include:

• Diphenyl diselenide
• Benzeneselenyl chloride
• Benzeneselenyl bromide
• Benzeneselinyl chloride
• Sodium benzeneselenolate
• Trimethylsilyl phenyl selenide

The most common oxidizing agent employed is hydrogen peroxide (H₂O₂). It is sometimes used in excess, to overcome catalytic decomposition of H₂O₂ by selenium; however, undesired oxidation of starting material has been observed under these conditions. Oxidation of products (via the Baeyer-Villiger reaction, for instance) has also been observed.

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For substrates whose product olefins are sensitive to oxidation, meta-Chloroperoxybenzoic acid (mCPBA) can be employed as an oxidant. It oxidizes selenides below the temperature at which they decompose to alkenes; thus, all oxidant is consumed before elimination begins. Buffering with an amine base is necessary before warming to avoid acid-mediated side reactions.

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Ozone, which gives only dioxygen as a byproduct after oxidation, is used to oxidize selenides when special conditions are required for thermolysis or extreme care is necessary during workup. Quinones can be synthesized from the corresponding cyclic unsaturated carbonyl compounds using this method.

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