Regioselective Transformation of Long π‐Conjugated Backbones: From Oligofurans to Oligoarenes

Abstract

We demonstrate the transformation of oligofurans through sequential Diels-Alder cycloaddition reactions to provide oligoarenes in two chemical steps, regardless of the oligomer length. By this method, oligonaphthalenes containing up to six units were obtained in high yield through the formation of up to 12 new C-C bonds. The versatility of this method was demonstrated for various polyaromatic hydrocarbons. The regioselectivity of this process enabled the synthesis of a library of substituted triarylenes from a single terfuran precursor by modification of the dienophile strength and the order of addition. Overall, this study demonstrates that long oligofurans are interesting not only as organic electronic materials, but also as starting materials for the formation of various conjugated systems.

Multiple Diels–Alder Transformations in Linear π-Conjugated Systems

“…Recently, we demonstrated the transformation of long oligofurans to oligonaphthalenes using DA cycloaddition followed by deoxygenation (Scheme 1, d). 8 The current dis-cussion will focus on multiple Diels-Alder reactions as a method for obtaining novel π-conjugated backbones. For a general overview discussing the synthesis of polyaromatic systems, we refer the reader to an extensive, recent review by Stępień.…”

“…25 The aforementioned regioselectivity can be utilized to transform terfuran into a substituted triphenyl moiety, by sequentially increasing the dienophile strength (Scheme 5, a). 8 We achieved this by modifying the order of dienophile addition using N-methyl maleimide 10 and N-ethyl maleimide 11 as model dienophiles with moderate reactivity, and benzyne precursor 4 as a dienophile with strong reactivity. As we showed previously, the addition of one equivalent of 10 to DM-3F proceeds selectively through cycloaddition-aromatization to the terminal rings.…”

“…The versatility of this approach allows the formation of a library of oligoarenes starting from a single symmetric oligofuran precursor. 8 The addition of even a large excess of a moderate dienophile (maleimide) led solely to cycloaddition at the two terminal positions. Steric factors can account for the difficulty of a reaction of the third dienophile with the central ring which is more crowded after cycloaddition in the two adjacent terminal positions.…”

“…In contrast, the less electron-poor nature of the synthesized epoxynaphthalene (which does not aromatize spontaneously) does not lead to such deactivation. 8 Our previous studies demonstrated that the highly conjugated nature of the furan monomer means that its activation energy is lower than that of longer units. This can account for the ease of cycloaddition in the case of benzyne (as the resulting epoxynaphthalene disrupts the conjugation).…”

“…This can account for the ease of cycloaddition in the case of benzyne (as the resulting epoxynaphthalene disrupts the conjugation). 8 Consequently, the addition of benzyne to furan oligomers is expected to proceed in a sequential manner.…”