Abstract
De novo design of boron heterocycles represents a cornerstone of synthetic chemistry, driven by their significant potential in biology, catalysis, and materials science, alongside fundamental investigations into structure and bonding from organic and inorganic perspectives. This thesis explores the design, synthesis, reactivity, and applications of boron-containing macrocycles, polycyclic aromatic hydrocarbons (PAHs), and organic cages.Chapter 1 reviews the historical development of organoboron chemistry, encompassing small-molecule activation, catalysis, and the evolution of boracyclic materials for diverse applications, highlighting key breakthroughs. Section 1.1 surveys organoboron reagents, emphasizing nucleophilic boron species and boron radicals and their roles in small-molecule activation and catalytic transformations. Key nucleophilic and boron-centered radical species are classified, and representative compounds and seminal contributions are discussed. Section 1.2 introduces boron-doped polycyclic aromatic hydrocarbons (PAHs), macrocycles, and cages, summarizing pioneering studies, major developments, and emerging applications. Section 1.3 outlines the general experimental and characterization methodologies employed throughout Chapters 2–4. Section 1.4 presents the research objectives,scope, and contributions of this thesis.
Chapter 2 describes the design, synthesis, and characterization of a new diborencine macrocycle 2.5 capable of trapping a single electron. A silicon- and tin-bridged diphenyl precursor (2.4) was prepared, and subsequent transformations afforded the target diborencine macrocycle (2.5). One- and two-electron reductions produced the corresponding radical and dianion species (2.6 and 2.7), which were isolated and fully characterized, including definitive structural assignment by single-crystal X-ray diffraction (SC-XRD). Electron paramagnetic resonance (EPR) spectroscopy, supported by computational analysis, was used to elucidate the nature of the unusual B–B one-electron σ-bond in compound 2.6. The reactivity of radical 2.6 was investigated with chalcogen sources (O2, PhSSPh, PhSeSePh), 1,4-benzoquinone, and selected additional substrates. Density functional theory (DFT) calculations provided mechanistic insight into the reaction of 2.6 with PhSSPh. Overall, this chapter highlights how the one-electron σ-bond enables single-electron transfer (SET) reactivity.
Chapter 3 explores the reaction of precursor 2.4, introduced in the previous chapter, with boron tribromide (BBr3) to afford a distinctive tetraboron compound (3.1). This compound was fully characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction (SC-XRD). The chapter examines the photophysical properties and Lewis acidity of 3.1, evaluating its potential as a building block for polyboron-doped polycyclic aromatic hydrocarbons (PAHs). Reactivity studies with a range of ligands L1-L5, including N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs), are also detailed, alongside preliminary investigations into the reduction of the corresponding adducts. The second part of the chapter describes synthetic efforts toward a triborasumanene derivative, in which three-coordinate boron atoms are incorporated into the bay regions of the sumanene framework. The key precursor tristannasumanene 3.6 was synthesized and characterized. Multiple boron reagents were attempted and a boronic ester byproduct was isolated. The targeted structure is anticipated to exhibit an unprecedented bowl depth for this class of compounds.
Chapter 4 investigates 9-borafluorene precursors as platforms for building complex architectures such as macrocycles and cages. Direct halogenation of 9-borafluorene is precluded by the deactivating boryl group, and although dihalogenated derivatives 4.1 and 4.2 are accessible via a modified tin-based route, Suzuki–Miyaura coupling consistently fails due to the electrophilic boron center. Anhydrous alternatives such as Negishi and Kumada couplings prove also impractical for macrocyclic or three-dimensional targets. To circumvent these limitations, a "build-then-borylate" strategy was developed using dibenzosilole as a robust fluorene surrogate: a key scaffold 4.5 bearing both halide and boronate handles was prepared on gram scale and elaborated through sequential Suzuki couplings, and a subsequent [2+2] cyclization delivered a strained dibenzosilole-based macrocycle 4.6. Si/B exchange, however, unexpectedly triggered tert-butyl C–H activation on the Mes* substituent to afford a borole-fused product 4.7, while attempted Scholl-type π-extension led exclusively to decomposition. Together, these outcomes expose an intrinsic incompatibility of the borafluorene/silole motif with boron-containing cage construction, motivating the redirection of the strategy toward more robust scaffolds in future work.
Overall, this thesis summarizes efforts toward isolating boron-containing macrocycles, PAHs, and cages using strategies involving organic synthesis, reduction, and neutral ligand coordination chemistry. These molecules represent boron-incorporated architectures synthetically inaccessible by other means, yielding new materials with unique reactivity profiles and versatile functional properties.
| Date of Award | 11 May 2026 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Zhenpin LU (Supervisor) & Zhengtao XU (Supervisor) |
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