Molecular Ribbons via Diels-Alder Cycloadditions : Synthesis of Models for Solubilized Polyacenes and Polyacene Polyquinones

Syntheses of linearly fused ribbons of carbocyclic six-membered rings are accomplished by Diels-Alder cycloadditions of a diene (2,3-diheptylidene-1,2,3,4-tetrahydronaphthalene) and a bis-diene (2,3,6,7-tetraheptylidene1,2,3,4,5,6,7,8-octahydroanthracene) to a bis-dienophile (1,4,5,8-anthradiquinone) and a dienophile (1,4-anthraquinone). The Diels-Alder adducts were dehydrogenated to several more highly unsaturated molecular ribbons.

The electronic structure of polyacene (1), a hypothetical polymer consisting of linearly fused unsaturated six-membered rings, has been of much theoretical interest due to its extended conjugation.For example, Kivelson and Chapman have proposed that this class of materials has the potential to exhibit high temperature superconductivity and ferromagnetism. 1However, polyacene should be highly insoluble, complicating its purification and handling.The synthesis of a soluble polyacene derivative would be a highly desirable alternative, since such a polymer might be castable as a thin film.
A synthesis of a solubilized polyacene (2) is proposed to be accomplished via reductive deoxygenation of polyacene polyquinone 3, which could be obtained by dehydrogenation of polymer 4. Polymer 4 could, in turn, be prepared by double Diels-Alder cycloaddition of bis-diene 5 to diquinone 6. 2 As a model study for this proposed synthesis, a series of molecular ribbons have been synthesized to qualitatively ascertain the solubilizing ability of hexyl groups in systems related to polyacene. 3uble Diels-Alder cycloaddition of diene 8 4 to diquinone 6 4 (in the presence of the anti-oxidant, BHT, to inhibit radical polymerization of the diene) gave adduct 9 in 73% yield, surprisingly as a single stereoisomer.This adduct was assigned a structure consistent with the endo rule for Diels-Alder cycloadditions and with the stereochemistry unambiguously determined for a related adduct. 5Furthermore, we believe that it must have resulted from addition of the two diene molecules to opposite sides of the diquinone.Presumably, cycloaddition of diene 8 to diquinone 6 initially gives intermediate monoadduct 10, although this was not observed.The electron-deficient aromatic moiety in 10 may interact favorably through space with the electron-rich one, thus imparting a preferred U-shaped conformation to the core ribbon structure of the molecule.Addition of diene 8 to monoadduct 10 would then occur preferentially, if not exclusively, to the less hindered face of 10, resulting in an overwhelmingly predominant formation of the diastereomer shown for 9.
Aromatization of diquinone 15 with DDQ in benzene at room temperature gave anthracene-containing diquinone 16, while treatment of 15 with DDQ in benzene at reflux gave a 68:32 mixture of naphthacene 17 and anthracene 16, from which a 12% yield of 17 could be obtained.An attempt to produce undecacene diquinone 12 from diquinone 15 by dehydrogenation using DDQ under forcing conditions was not successful.
A molecular ribbon containing nineteen linearly fused carbocyclic six-membered rings was synthesized by first preparing diene 18, the presumed intermediate in the formation of double adduct 14, in 39% yield via 1:1 Diels-Alder cycloaddition of bis-diene 13 to 1,4-anthraquinone.
Diene 18 then underwent double cycloaddition to diquinone 6 to give a 90% yield of the desired molecular ribbon, adduct 19, as a mixture of diastereomers.This mixture could be smoothly oxidized to tetraquinone 20, which was also characterized as a diastereomeric mixture.

Conclusions
The Diels-Alder cycloadditions used to synthesize these ribbons of carbocyclic six-membered rings are high yielding, and partial dehydrogenation of the adducts without aromatization is quite facile.However, aromatization of diquinone 11 to nonacene diquinone 7 is sluggish and low-yielding, and undecacene diquinone 12 could not be prepared from diquinone 15 under any of the conditions tried.On the basis of these model studies it appears that the number and size of the alkyl groups currently being employed are not adequate for solubilizing polyacene-polyquinone polymer 3.