Studies on tellurium-containing heterocycles. Part 18. Preparation and structure of 2-benzotelluropyrylium salts and 2-benzoselenopyrylium salts

Abstract

The regioselective and stereospecific intramolecular ring closure reactions of o-ethynylbenzyl tellurols 5A and o-ethynylbenzyl selenols 5B, which were readily generated by the reaction of the o-ethynylbenzyl bromides 4 with sodium hydrogen telluride (NaHTe) or sodium hydrogen selenide (NaHSe), produced the isotellurochromenes 6A and isoselenochromenes 6B together with (Z)-1-methylidene-2-telluraindans 7A and (Z)-1-methylidene-2-selenaindans 7B, respectively. The obtained isochromenes 6A and 6B were transformed into the corresponding 2-benzotelluropyrylium tetrafluoroborates 9A and 2-benzoselenopyrylium tetrafluoroborates 9B by treatment with triphenylcarbenium tetrafluoroborate (Ph₃C⁺BF₄⁻) in excellent yields, respectively. An X-ray structural analysis of the tert-butyl derivatives 9Ac and 9Bc is also described.