Reactions of Elemental Sulfur and Selenium with Dimethyl Acetylenedicarboxylate, Methyl Propiolate and Benzyne

Abstract

The reaction of dimethyl acetylenedicarboxylate (DMAD) with elemental selenium in a molar ratio of 2: 1 in refluxing o-dichlorobenzene for 6 h afforded tetramethyl 2, 3, 4, 5-selenophenetetracarboxylate in 54% yield. Although the use of DMAD in excess afforded an improved yield (65%) of the selenophene, the reaction was accompanied by the formation of an appreciable amount of trimethyl 5-methoxy-2, 3, 4-furantricarboxylate as by-product, which resulted from the self-condensation of DMAD. Heating of methyl propiolate with elemental sulfur in a molar ratio of 5: 1 in benzene at 205∼215°C for 14 h in a stainless steel autoclave afforded dimethyl 2, 4- and 2, 5-thiophenedicarboxylates in 41 and 19% yields, respectively, while heating with elemental selenium under the same conditions gave dimethyl 2, 4- and 2, 5-selenophenedicarboxylates in 45 and 30% yields, respectively. The proposed mechanism involves the initial formation of 1, 2-dithiete(1, 2-diselenete)intermediates from acetylenes and elemental sulfur (selenium). These intermediates, after ring-opening, undergo cycloaddition with acetylenes to give 1, 4-dithiins(1, 4-diselenins) which eliminate sulfur (selenium)to give the observed final products.
Benzyne, generated by t hermal decomposition of diphenyliodonio-2-carboxylate in boiling o-dichlorobenzene reacted with elemental sulfur to give thianthrene (5%) and benzopentathiepin (6%), and reacted with elemental selenium to give selenanthrene (26%). The formation of thianthrene and selenanthrene provides, though indirect, an evidence for the intermediacy of (1, 4-diselenins) as intermediates in the foregoing reactions.