Preparation of phenyl selenides

Compounds of selenium are highly toxic and in addition have a tendency, like their sulfur analogues, to have an unpleasant odour. Care is therefore necessary when handling selenium reagents, and it is advisable to avoid the more pungent chemicals where an alternative is available. In particular, selenophenol itself is extremely noxious and should be avoided if possible. As phenyl selenoglycosides are stable to both basic [41] and acidic [40] conditions it is often possible for the functionality to be introduced early in a given synthesis. 

More recently Sinay showed that the reaction of a C-1,2 orthoester with selenophenol in the presence of mercury(II) bromide as a Lewis acid catalyst gave the corresponding selenoglycoside [44]. This leaves an acetate at the 0-2 position of 

Other methods of selenoglycoside sythesis include the use of selenostannane in the presence of catalytic Bu2Sn(OTf)2 [48] and the opening by selenophenol of a 1,2-epoxide (Brigl anhydride) [49], however, these methods are less frequently employed. 

Another useful method for the preparation of phenyl selenoglycosides is the azidoselenation reaction, in which a glycal is reacted with sodium azide, diphenyldiselenide and (diacetoxyiodo)benzene to give a 2-azido-2-deoxy-seleno-glycoside. The reaction is believed to proceed through a radical mechanism, and 

Generation of a phenyl selenoglycoside from 2,3,4,6-tetra-O-acetyl-a-D-galacto-pyranosyl bromide via displacement of the anomeric halide with a selenophenol anion generated by reduction of diphenyldiselenide [44]. 

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Diphenyl diselenide, (CeH5)2Se2, is formed as a by-product in the preparation of phenyl selenide when magnesium phenyl bromide in ether solution reacts with selenium.5 A 2 per cent, yield is obtained when benzene, aluminium and selenium bromides react in dilute carbon disulphide solution. The product melts at 62° C.6 The corresponding oxide is an oil, B.pt. 230° C. at 65 mm.,7 and the sulphide, (C6H5)2Se2S, M.pt. 55° C., is obtained from sulphur chloride and phenyl selenide in carbon disulphide solution.8... 

The method used to prepare phenyl selenide can have a pronounced effect on the selectivity of the reagent. It was observed that Michael addition or acetate displacement could be selectively achieved by changing the method of preparation of phenyl selenide. In the illustrated case, the reagent prepared using sodium hydride gave the conjugate addition product (eq 46). 

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