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Abstract
Boron-doped polycyclic aromatic hydrocarbons (PAHs) have emerged as a prominent class of compounds due to the unique properties that can be achieved through the incorporation of boron, often paired with another heteroatom, a combination that makes them attractive for a range of applications. The benefit of doping with these heteroatoms is also evident in 1,2-azaboroles, a subclass of B-containing compounds, consisting of five-membered unsaturated heterocycles with dative boron-nitrogen bonds. The donation of electron density from nitrogen to boron renders the molecule electronically saturated and endows it with the stability that is a prerequisite for its application in organic electronics, photovoltaics or bioimaging. The development of these compounds, first described in the 1960s, has been particularly intensive over the past two decades, driven by their photoresponsive and luminescent properties. This review aims to provide a comprehensive overview of the synthetic methodologies employed in the construction of 1,2-azaboroles. In addition to classical approaches, such as nitrogen-directed electrophilic C-H borylation or lithiation-transmetalation of pre-functionalized substrates, we discuss less commonly used methods and protocols that are limited to specific starting materials, thus demonstrating a large available repertoire of synthetic tools to access these compounds.
Highlights
2.1 Lithiation-Transmetalation 2.2 Electrophilic C-H Borylation 2.3 Transition Metal-Catalyzed C–H Borylation 2.4 Cycloaddition 2.5 Photoisomerization 2.6 Hydroboration 2.7 Coordination-Cyclization 2.8 Nucleophilic Aromatic Substitution 2.9 Silicon-Boron Exchange 3 Conclusion and OutlookKey words azaborole, borylation, cycloaddition, electrophilic borylation, polycyclic aromatic hydrocarbons (PAHs), transmetalation
Boron-doped polycyclic aromatic hydrocarbons (PAHs) have emerged as a prominent class of compounds due to the unique properties that can be achieved through the incorporation of boron, often paired with another heteroatom, a combination that makes them attractive for a range of applications
The donation of electron density from nitrogen to boron renders the molecule electronically saturated and endows it with the stability that is a prerequisite for its application in organic electronics, photovoltaics or bioimaging
Summary
Boron-doped polycyclic aromatic hydrocarbons (PAHs) have emerged as a prominent class of aromatic compounds, with a growing interest in recent years and a broad spectrum of applications. The 1,2 azaborole ring has been incorporated into a multitude of structures, including ladder compounds,[13] helicenes,[14] boron-centered spiro compounds,[15] photoswitches and stimuli-responsive materials.[16] These compounds due to their attractive luminescent (Figure 1b) and charge-transport properties have been applied e.g. in solar cells,[17] transistor devices,[11,18] organic light-emitting diodes,[19] bioimaging,[20] and as erasable inks (Figure 1c and 1d).[12]. Sometimes no bond is shown between nitrogen and boron, even though bond lengths determined by single crystal X-ray diffraction analysis[21] or chemical shifts in 11B NMR spectroscopy[22] indicate interactions between both atoms This lack of consistency indicates that a correct description of the bonding situation in 1,2-azaboroles is not trivial. We consistently present the structures with a dative bond and no formal charges notation
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