Reduction phosphine selenides

Several of the procedures discussed in the sulfoxide section describe the successful extension of the method to the reduction of selenoxides, - - and there is little doubt that many of the other procedures cited earlier could be used likewise. Sakaki and Oae used triphenylphosphine selenide and similar se-lenides to reduce selenoxides to selenides in 79-93% yield (equation 19). Using a chiral phosphine selenide, these workers showed that the phosphine oxide formed had suffered predominant inversion, with a stereospecificity of over 80%. Detty has reported the application of the silane PhSeSiMes (12) to the reduction of selenoxides and telluroxides. The reactions are rapid and proceed essentially quantitatively, even in the presence of a hydroxy or carbonyl group. 

Selenolates prepared from diphenyl or dimethyl diselenide by reduction with NaBH4 smoothly transform various benzylic alcohols 24 into the corresponding selenides 25 in the presence of aluminum chloride (Scheme 30 a) [41]. AICI3 is considered to activate the alcohol substrate by coordinating to the oxygen. Similar transformations are possible by the reaction of alcohols with phenyl selenocyanate in the presence of tributyl phosphine [52]. When the selenolate is reacted with aromatic aldehydes or ketones 26 in the presence of AICI3, the corresponding benzylic selenides 27 are obtained in moderate yields (Scheme 30b) [41]. 

Several papers have appeared describing the cleavage of disulphides and diselenides with phosphines. A two-step mechanism applies to the reduction of a disulphide with Ph3P in aqueous media, and the intermediate Ph3PSAr ArS hydrolyses to ArSH + PhaPO. More complex disulphides containing acetamido-alkyl and o a-di(ethoxycarbonyl)alkyl substituents give oxazole and trisulphide side-products during reduction with PhaP. Conversion of dimethyl diselenide with (R2N)aP into dimethyl selenide, and photochemical deselenation of diethyl diselenide to diethyl selenide with PhaPMe, which involves an EtSe radical chain mechanism, have been described. 

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