Studies Towards Aromatic Belts, Macrocycles, and Azaacenes

Abstract

1.1 Chapter Overview First proposed by Heilbronner in 1954, [n]cyclacene is a much sought-after, unnatural, synthetic target consisting of fused benzene rings forming a lateral benzenoid macrocycle. Comparable to acenes, [n]cyclacene is projected to exhibit a small HOMO-LUMO energy gap. Moreover, computational studies suggest an unstable diradical character and a unique form of aromaticity in the macrocycle. [n]Cyclacene is recognized as the shortest zigzag carbon nanotube (CNT) therefore, cyclacenes might act as a potential scaffold for the synthesis of diameter and chirality-specific CNTs. Additionally [n]cyclacenes are calculated to exhibit unique magnetic/optoelectronic properties. Though recent syntheses have succeeded in fully conjugating a system after macrocyclization, the modified pseudo-cyclacenes prepared to date do not exhibit the predicted characteristics. Inspired by past synthetic attempts, the Douglas group’s overarching objective was the total synthesis of a [n]cyclacene via stereoselective Diels-Alder cycloadditions and cheletropic decarbonylation to achieve full conjugation of the laterally fused benzenoid macrocycle. Discussed in this work are unique attempts at macrocyclization of polycyclic bridged quinoidal half-cycles via Diels-Alder cycloadditions and reductions to form the conjugated macrocycle. 2.1 Chapter Synopsis Building off this initial [n]cyclacene work, I recognized that the all-carbon connectivity limited the macrocycle synthesis and worried the predicted biradical character of these systems as a secondary restrictive factor. So, I postulated: Would a nitrogen-doped analogue of [n]cyclacene circumvent our previous difficulties via more favorable C-N bond formations? Encouraged by literature and some exciting C-N reactivity for macromolecular materials, I hypothesized that a nitrogen-doped analogue of [n]cyclacene could possibly overcome our previous difficulties with macrocyclization and tune the predicted electronics of the systems. Great interest has been shown in synthesizing N-doped acenic micro/macromolecules and studying the optoelectronic properties. This interest has recently extended to carbon nanoring and carbon nanobelt materials as nitrogen doped CNTs display desirable materials characteristics. Theory and experimental evidence have shown nitrogen-doped acenes exhibit far greater stability in air and light than their all-carbon analogues, as well as sequestering the biradical character of extended acenes. Additionally, an [n]azacyclacene would represent the shortest zigzag N-doped carbon nanotube, as a singular acene strip, and thus a possible n-type organic semi-conductive material. I proposed a chiral-assisted synthesis (CAS) of [24]azacyclacene via Buchwald-Hartwig aminations of enantiopure 2,6-diamino-3,5-dibromoanthracenic curved macromolecules in a stereo-selectively controlled manner. By pre-functionalizing building-blocks, I predicted that a controlled cross-coupling would utilize the inherent curvature of these macromolecules leading to macrocyclic precursors. Which would be followed by a global bis-decarbonylative conjugation event to yield the final [24]azacyclacene target. However, controlling the regioselectivity during installation of precursor nitro groups proved challenging. While I was able to synthesize the building blocks, the resulting regioisomers were not successfully separated, undercutting the primary goal of the route. I shifted my focus to an analogous route, leveraging Schiff-base coupling strategies, which have proven to be vastly effective in forming covalent organic frameworks. Theoretically, these would follow dynamic covalent chemical principles, which would allow the thermodynamically favorable imine formation to control macrocyclization, instead of relying on enantiocontrol. I began by attempting to synthesize a 2,3,6,7-tetraaminoanthracenic cycloadduct, followed by coupling to a 1,2,4,5-tetrahydroxybenzene. Successfully, the tetrahydroxy benzene was synthesized in three steps, but synthesis of the tetraamino building blocks proved difficult due to inability to nitrate all positions and failure of subsequent aminations steps. Reversing the functionality of the building-blocks to a 1,2,4,5-tetraammoniumbenzene salt and a 2,3,6,7-tetrahydroxyanthracenic cycloadduct yielded viable materials to perform the macrocyclizations. Oxidations and coupling experiments were carried out, however no productive macrocycles were ever formed. Instead, these labors produced interesting tetracene precursors from model-study couplings of ortho-phenylene diamine and the tetrahydroxy adduct material that is further discussed in Chapter 3. 3.1 Chapter Synopsis Linear acenes, a category of polycyclic aromatic hydrocarbons, are of great interest for organic materials and electronics due to their extended π-conjugation and optoelectronic properties. Nitrogen-embedded acene congeners, azaacenes, are a resurgent class of acene materials. Replacing [n]benzenoid rings with [n]pyrazine rings in an acene backbone modulates acene properties, while opening additional synthetic routes to the targets. Azaacenes have increased electron affinity, making them robust n-type semiconductors in thin-film transistors and emitters for organic diodes, and valuable synthetic targets. Already several motifs mentioned earlier dominate the literature, but continued innovation calls for novel acene structures and expanding existing synthetic techniques. In azaacene materials, solubility and photodegradation become problems as conjugation extends in acene backbones. To overcome these challenges, bulky edge groups or nonlinear fused benzenoid rings are added, forming “pseudo” acenes. Based on my studies towards pyrazine containing macrocycles, I was able to investigate and optimize a novel oxidative cyclocondensation reaction. Herein I report this new approach towards the synthesis of extended linear [n]azaacenes of varying acene lengths (n = 5, 7, 9) and edge-group functionalization via a one-pot oxidation/condensation reaction. In this method I attempted to construct an azaacene precursor that has a carbonate-containing two carbon bridge over the central ring. This bridge gives the intermediates solubility without relying on benzannulation or edge-protecting groups. Additionally, this bridged motif should enable a light-mediated decarbonylative aromatization at a late stage in the synthesis. Oxidative Schiff-base coupling between a scope of diamines and the bridged biscatechol 2.71 form the desired azaacene core. The choice of diamine sets the acene length, edge-groups, and possibly the nitrogen count, providing several opportunities to be explored.