Selenium-carbon double bonds selenocarbonyl compounds

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

The relatively poor 2p-4p r-overlap of carbon-selenium double bonds makes selenocarbonyl groups less stable than the corresponding thiocarbonyl and carbonyl analogs. Until the mid-1970s, only those selenocarbonyl compounds where the bond order of the C=Se bond was lowered by coordination or resonance were known. However, the isolation of two deep blue, thermally stable selones (46) and (47) proved that such... 

The reactivity and reaction patterns of selenocarbonyl compounds vary considerably from the compounds in categories A-C in Scheme 1. The substituents adjacent to the selenocarbonyl group attenuate the polarity of the compounds. As a result, less polar selenoaldehydes and selenoketones behave as olefins, and the carbon selenium double bond of heteroatom-substituted selenocarbonyl compounds becomes more polar. In this section, a variety of transformations of selenocarbonyl compounds have been classified based on the reagents reacted with selenocarbonyl compounds. 

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