Iron‐Promoted Tandem Cyclization of 1,3‐Diynyl Chalcogen Derivatives with Diorganyl Dichalcogenides for the Synthesis of Benzo[b]furan‐Fused Selenophenes

Abstract

AbstractA double intramolecular 5‐endo‐dig cyclization of butyl[2‐(phenylbuta‐1,3‐diynyl)phenyl]chalcogens has been employed in a selective preparation of benzo[b]chalcogenopheno[2,3‐d]furans. Several reaction parameters were studied to determine the best reaction conditions and we observed that the reaction of butyl[2‐(phenylbuta‐1,3‐diynyl)phenyl]chalcogens (0.25 mmol) with iron(III) chloride hexahydrate (2.0 equiv.) and diorganyl diselenides (1.75 equiv.) at reflux of dichloromethane was the most appropriate to give the products in 35–89% yields. These standard reaction conditions were compatible with many functional groups in the substrates, such as methyl, chlorine, fluorine, methoxy and heteroaryl. This protocol was also efficient for diorganyl diselenides but it was ineffective with diorganyl disulfides and ditellurides. In a competition among selenium, sulfur and oxygen nucleophiles we observed that the nucleophilicity and steric effects of the competing functional groups were determinant factors for the selectivity of the cyclization. The benzo[b]chalcogenopheno[2,3‐d]furans had absorptions in the UV region (300–350 nm range) with molar absorptivity coefficient values ascribed to spin and symmetry allowed π–π* electronic transitions. An emission located in the purple region (380–440 nm range), with a Stokes shift of between 65–100 nm, is probably associated to the charge transfer character of the excited state.magnified image