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.