Electrophilic Cyclization Involving Carbon-Selenium/Carbon-Halide Bond Formation: Synthesis of 3-Substituted Selenophenes.

The butylselanyl propargyl alcohols reacted with iodine to afford 3-iodoselenophenes. The change of nucleophile position from propargyl to homopropargyl was crucial for the aromatization and formation of selenophene rings. The experiments revealed that bromine and N-bromosuccinimide were not able to cyclize the butylselanyl propargyl alcohols; however, when the bromine source was copper(II) bromide the corresponding 3-bromoselenophenes were obtained in good yields. In addition, the reaction of butylselanyl propargyl alcohols with diorganyl diselenides catalyzed by copper(I) iodide gave the 3-(organoselanyl)selenophenes. The reaction took place with aromatic rings substituted by either electron-donating or -withdrawing groups in the alkynes and propargyl positions. The steric effects of substituents were dominant in determining the yields, whereas electronic effects had only a minor influence. Furthermore, by monitoring the reaction by 1H NMR, we were able to identify the key intermediate, which supported the elaboration of a proposed reaction mechanism. The 3-iodoselenophenes prepared allowed the synthesis of multifunctional selenophenes via application in metal-catalyzed coupling reactions, such as Sonogashira, Ullmann and Suzuki type reactions.