Design, Synthesis and Aromaticity of an Alternating Cyclo[4]Thiophene[4]Furan

Abstract

A new class of conjugated macrocycle, the cyclo[4]thiophene[4]furan hexyl ester (C4TE4FE), is reported. This cycle consists of alternating α-linked thiophene-3-ester and furan-3-ester repeat units, and was prepared in a single step using Suzuki-Miyaura cross-coupling of a 2-(thiophen-2-yl)furan monomer. The ester side groups help promote a syn conformation of the heterocycles, which enables formation of the macrocycle. Cyclic voltammetry studies revealed that C4TE4FE could undergo multiple oxidations, so treatment with SbCl₅ resulted in formation of the [C4TE4FE]²⁺ dication. Computational work, paired with ¹ H NMR spectroscopy of the dication, revealed that the cycle becomes globally aromatic upon 2e^- oxidation, as the annulene pathway along the outer ring becomes Hückel aromatic. The change in ring current for the cycle upon oxidation was clear from ¹ H NMR spectroscopy, as the protons of the thiophene and furan rings shifted downfield by nearly 6 ppm. This work highlights the potential of sequence control in furan-based macrocycles to tune electronic properties.

Keywords: conjugated macrocycles; cyclo[n]furans; cyclo[n]thiophenes; global aromaticity; homogeneous catalysis.