Stereoselective reaction of selenium dihalides with hex-3-yne

Abstract

In recent time, the synthetic potential of new electrophilic reagents, selenium dichloride and selenium dibromide, has attracted much interest [1–6]. These reagents are more selective than selenium tetrahalides used previously [7]. For example, reactions of selenium dichloride and dibromide with acetylene follow stereoselective anti-addition pattern with formation of (E,E)-bis(2-halovinyl) selenides in nearly quantitative yield [2], whereas the addition of selenium tetrahalides to acetylene gives both (E,E)-bis(2-halovinyl) selenides and their halogenation products at one double bond [7]. The addition of selenium dihalides to acetylenic compounds is generally regioand stereoselective; however, the nature of substituents at the triple bond could considerably affect steric configuration of the product [1–5]. For instance, unexpected anti–syn-addition product, (Z,E)-bis(2-chloro-3-trimethylsilylprop1-en-1-yl) selenide was obtained in the reaction of selenium dichloride with 3-trimethylsilylprop-1-yne [3]. It was presumed that the first elementary step of this reaction occurs as anti-addition, but in the second step syn-addition of the primary adduct to the second 3-trimethylsilylprop-1-yne molecule appears to be energetically more favorable than the anti-addition due to strong steric effect of two trimethylsilyl groups [3]. Reactions of selenium dihalides with propargyl alcohols lead to the corresponding syn-addition products, (Z,Z)-bis(1-halo-3-hydroxyprop-1-en-2-yl) selenides [4]. Their steric structure is determined by the hydroxy group which acts as internal nucleophile [4].