Synthesis of 2,3-dihydrobenzo[b][1,4]oxatellurane derivatives from propargyl ethers

The reaction of substituted phenyl propargyl ethers with tellurium tetrachloride resulted in the formation of (E)-3‐(chloromethylene)-2,3-dihydrobenzo[b][1,4]oxatelluranes, which were isolated as crystalline 4,4-dibromo derivatives. The products represent the first reported bicyclic [1,4]benzoxatelluranes. Their formation indicates preferential anti addition of tellurium tetrachloride across the CC triple bonds of the chosen propargyl ethers with anti-Markovnikov regioselectivity, contrasting earlier reports of preferential syn addition of tellurium tetrachloride to alkynes. This likely results from the intermediacy of tellurenium cations stabilized by intramolecular Te-O coordination. The resulting adducts underwent electrophilic cyclization under the conditions of their formation. X-ray analyses obtained for two product molecules showed substantial deviations from planarity for the heterocyclic part of these molecules. No Te-O secondary bonding interactions were observed, but Te-Br secondary bonding was present. (E)-3‐(Chloromethylene)‐6,7-dimethyl-2,3-dihydrobenzo[b][1,4]oxatellurane, prepared by reduction of its 4,4-dibromo derivative, presented as a thick oxidation prone oil. ¹²⁵Te NMR chemical shifts are compared to those of structurally similar compounds.