Phenylselenenyl trifluoroacetate

Phenylselenenyl trifluoroacetate, prepared in situ by the addition of phenylselenenyl chloride or bromide to silver trifluoroacetate in benzene, undergoes virtually instantaneous addition to alkenes giving /3-trifluoroacetoxy selenides 39 in quantitative yields51. When ( )- or (Z)-2-butene or cyclohexene is used as the alkene then anti or trans addition only is observed, however, 1,3-cyclohexadiene gives predominantly trans-1,2 addition together with a small amount of a 1,4-adduct (Table 6)51. 

The most useful method is reaction of ketone (and ester) hthium enolates, usually prepared by deprotonation of ketones with LDA, with either Ce H5 SeBr or Ce Hs SeCl. Enol acetates can be converted into a-phenylseleno ketones by reaction with phenylselenenyl trifluoroacetate, prepared in situ by treatment of CeHsSeCl or CeHsSeBr with silver trifluoroacetate or by conversion to the lithium enolate and reaction with CeHsSeBr. It is sometimes possible to obtain isomeric a-phenylseleno ketones by use of these two methods (equations 1 and II). 

The second example27 belongs to the category of addition via onium-type intermediates . It exemplifies how the choice of the electrophile may affect the stereochemical course of the cyclization. When the hydroxy cnones rac-4a, b were treated with mercury(II) trifluoroacetate, both (E)- and (Z)-enone afforded basically the same yield and distribution of the diastereomer-ic cyclization products rac-5a, b. However, when phenylselenenyl chloride was used as electrophile, the (Z/Zj-isomers rac-4a, b produced quite contrasting ratios of the products rac-5 a, b. This result was interpreted in terms of EjZ equilibration in a single-bonded cationic adduct of the mercuric ions vs. nucleophilic attack on a rigid epi-selenonium ion27. 

Phenyl neopentyl sulfide, 272 Phenyl oxime-O-sulfonates, 456 2-Phenyloxirane, 219 Phenylpropargyl aldehydes, 6 Phenylselenenyl bromide, 459-460 Phenylselenenyl chloride, 459-460 Phenylselenenyi trifluoroacetate, 459 Phenylselenlnyl chloride, 459-460 ot-Phenylseleno ketones, 459 Phenylselenol, 235, 362 Phenylsulfinylcarbene, 457... 

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