Iodocyclization of (Z)-1-(butyltelluro)-1,4-diorganylbut-1-en-3-ynes. Synthesis and reactions of 3-iodotellurophenes

The iodocyclization of(Z)-tellurobutenynes 5a-g by reaction with I-2/petroleum ether was studied in detail. 3-Iodotellurophenes 7a-f were formed, and optimum conditions to obtain these compounds in high yields were established. The reaction involves attack of iodide at the initially formed intermediate of type 10 followed by ring closure that is favored by the strong aromatic character of the resulting products. Two possible and alternative pathways for the ring closure are proposed to explain our observations: (a) transformation of 10 to 11 that undergoes further cyclization to give 7a-f (pathway a) or (b) direct cyclization of 10 to give tellurophene diiodide 9 (pathway b). The products and side products obtained are in agreement with the proposed mechanisms. Formation of 7a from ditelluride 14 and iodine provides additional evidence of the intermediacy of 11 (in pathway a). Structures of 7a and 5g were elucidated by X-ray crystallography. In the case of compound 5e, where a terminal triple bond was present, the resulting intermediate of type 10 also underwent the attack of iodide directly at the terminal carbon to give compound 12 together with 7e. Ring-opening of 3-iodotellurophene 7a occurs by reaction with n-butyllithium to form acyclic ditelluride 14 or monotelluride 16 depending on. the alkyllithium amounts employed. Plausible mechanisms for these novel reactions are proposed and supported. Conversion of 7a to 3-(butyltelluro)-2,5-diphenyltellurophen (22) was carried out readily by an unusual ''aromatic nucleophilic substitution'' using the butyl tellurolate anion.