Influence of solvents on the direction of the reactions of vinylacetylene with nucleophilic reagents

Abstract

We have found that the nature of the solvent substantially changes the direction of the reaction of vinylaee~ylene (VA) with thiols under nucleophilic conditions. While in ethanol, the main reaction is stereospecific addition to the triple bond of VA with the formation of cis-l-alkyl(aryl)thiobuta-l,3-dienes (I), and the reaction at its double bond, giving 4-alkyl(aryl)thiobuta-l,2-dienes (II) and 4-alkyl(aryl)-thiobut-l-ynes (111) has a subsidiary nature [I], in an aprotic solvent (AS) (dioxane, THF), the allene compounds (]7:) are obtained a lmost exclusively. These resu l t s confirm the hypothesis put forward previously [1] that i1:~ ethanol the react ion takes place through the formation of a complex with the part icipat ion of the C=C bond of VA and a proton f rom the solvent. Under these conditions the electrophil ici ty of the tr iple bond is irLcreased while its tendency to conjugate with the C= C bond is s imultaneously decreased . In an AS, the react ion apparently takes place by an attack of the nucleophile f rom the direct ion of the C=C bond in accordance with the polar izat ion of the VA molecule [2]