Selenium—oxygen bonds reactions with

Fig. 12.10. Mechanism of the a-oxygenation of ketones in reactions with selenium dioxide an electrophilic substitution reaction (—> —> C) is followed by a /(-elimination at the C-0 single bond.

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

Selenium dioxide is able to a-oxygenate ketones via their enol tautomers. As is demonstrated in Figure 12.10 by the reaction of selenium dioxide with cyclohexanone, the actual electrophilic substitution product C is unstable. The latter contains selenium in the oxidation state +2 that takes the opportunity to transform into selenium in the oxidation state 0, i.e., elemental selenium, by way of the fragmentation reaction indicated. Thereby, the a-C O single bond of the primary product C is transformed into the a-C=0 double bond of the final product B (which, however, is largely present as the tautomeric enol A). 

The cnc reaction is now considered to include not only all carbon enophiles, as in equation 1, but also those with one heteroatom (equation 2) and with two heteroatoms (equation 3). Those enophiles with one heteroatom invariably react in the fashion depicted, i.e., the formation of a C —C bond, as of course is the case with all carbon enophiles. Eue reactions that form C-C bonds (equations 1 and 2) are covered in Section D. 1.6.2. Heteroatom-carbon bond formation using singlet oxygen is treated in Section D.4.9. and ene reactions of selenium dioxide are covered in Section D.4.10. Ene reactions that establish a bond between carbon and a nitrogen are covered here. 

The first step in both cases is an ene reaction with the Se=0 bond (pp. 1270-1). The electrophilic selenium attacks the less substituted end (largest HOMO coefficient) of the alkene and a proton is removed from the methyl group trans to the main chain. Then a [2,3]-sigmatropic rearrangement puts the double bond back where it was (trans selectively) and functionalizes the old methyl group with an oxygen atom. 

One-Atom Insertions. Siliranes also undergo one-atom insertion reactions with elemental sulfur, oxygen, or selenium. Similarly, the insertion of aryl or alkyl isocyanates provides iminosi-lacyclobutanes in high yield as single stereoisomers (eq 7). The reaction proceeds with the retention of silacyclopropane configuration and the insertion occurs at the more substituted C-Si bond when unsymmetrical siliranes are employed, similar to the regios-electivity observed for aldehydes and formamides. 

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