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# Controllable 1,4-Palladium Aryl to Aryl Migration in Fused Systems Application to the Synthesis of Azaborole Multihelicenes

Felix Full, Albert Artigas, Kevin Wiegand, Daniel Volland, Klaudia

Szkodzińska, Yoann Coquerel, Agnieszka Nowak-Król

► To cite this version:

Felix Full, Albert Artigas, Kevin Wiegand, Daniel Volland, Klaudia Szkodzińska, et al. Controlled 1,4-Palladium Aryl to Aryl Migration in Fused Systems Application to the Synthesis of Azaborole Multihelicenes. Journal of the American Chemical Society, 2024, 146 (42), pp.29245-29254. 10.1021/jacs.4c12562 . hal-04831000

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Submitted on 11 Dec 2024

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# Controllable 1,4-Palladium Aryl to Aryl Migration in Fused Systems – Application to the Synthesis of Azaborole Multihelicenes

Felix Full,† Albert Artigas,∥ Kevin Wiegand,† Daniel Volland,† Klaudia Szkodzińska,† Yoann Coquerel,‡ Agnieszka Nowak-Król†,∗

**Keywords:** Acetate, Aromaticity, Azaboroles, Chiroptical Properties, Fluorescence, Helicenes, Miyaura Borylation, Multihelicenes 1,4-Palladium Migration

**ABSTRACT:** Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura borylation tandem reaction in fused systems. The Pd shift occurred in the bay region of the dibenzo[g,p]chrysene building blocks, giving rise to a thermodynamically controlled mixture of 1,8- and 1,9-borylated compounds that allowed the preparation of regioisomeric azaborole multihelicenes from the same starting material. The outcome of this synthesis can be controlled by the choice of reaction conditions, allowing the migration process to be turned off in the absence of an acetate additive and the target multiheterohelicenes to be prepared in a regioselective manner. The target compounds show bright green fluorescence in dichloromethane with emission quantum yields ($Φ$) of up to 0.29, $lg_{lum}$ values up to $2.7 \times 10^{-3}$, and green or green-yellow emission in the solid state, reaching $Φ$ of 0.22. Single crystal X-ray diffraction analyses gave insight into their molecular structures and the packing arrangement. Evaluation of aromaticity in these multihelicenes revealed a non-aromatic character of the 2H-1,2-azaborole constituent rings.

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## INTRODUCTION

Molecules embedding more than one (hetero)helicene¹ unit are referred to as multiple (hetero)helicenes or multi(hetero)helicenes.²⁻⁶ To date, a plethora of these fascinating molecules have been reported, including double, triple, quadruple, quintuple and even sextuple analogues.⁷⁻¹⁵ Interest in these compounds evolved naturally from research on planar and contorted polycyclic aromatic

<footnote>†Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Universität Würzburg, Am Hubland, Würzburg, Germany</footnote>

<footnote>∥Facultat de Ciències, Campus Montilivi, Carrer de Maria Aurèlia Capmany i Farnès, 69, Girona, Catalunya, Spain</footnote>

<footnote>‡Aix Marseille Univ, CNRS, Centrale Med, ISM2, Marseille, France</footnote>
hydrocarbons (PAHs).16-18 The incorporation of helical moieties into π-conjugated structures adds a new dimension to their functionality, enabling the realization of new-generation devices, such as circularly polarized organic-light emitting diodes, polarization-selective photodetectors, and spintronic devices.19-22 To improve their performance, a large variety of chiral materials have been developed and studied, contributing to a better understanding and control of their chiroptical properties, the orientation of the molecules relative to other device components, and the packing arrangement.23-26 Dibenzo[g,p]chrysene (DBzC) with its inherently twisted structure constitutes a unique representative of contorted PAHs,27 and an appealing candidate as a building block for the synthesis of various single and multiple helicene frameworks.28-31 Over the past decade, several beautiful DBzC multihelicenes have been reported,32-34 with only a few examples of multiheterohelicenes.33, 35 Doping of PAHs with heteroatoms is an effective method for fine-tuning their optoelectronic properties.36-37 For instance, the introduction of boron or a B-N motif leads to increased electron affinity, and improved photophysical properties.38-55 However, the incorporation of heteroatoms into π-extended systems often requires functionalization of the all-carbon cores at positions that are difficult to access by conventional methods, which hinders the synthesis of these structures.

Notably, 1,n-Pd migrations coupled with subsequent reactions are powerful tools to generate synthetically challenging targets. To date, diverse Pd migration processes have been developed,56-64 but surprisingly, a relatively little attention was given to the aryl to aryl migration. Since the seminal works of Larock65 and Gallagher,66 this type of migration has been observed in a narrow range of molecular scaffolds, i.e. conformationally labile biaryl substrates, which facilitate the incorporation of a voluminous metal atom.67-69 The only example of employing a fused system in this process was the synthesis of a Pd complex of dibenz[a,c]anthracene with a stoichiometric amount of Pd(PEt3)4, where the Pd 1,4-shift at 160 °C was rather sluggish (10 days) and the desired complex was accompanied by several other products.70 Except for this work, the 1,4-palladium migration has not been reported for rigid systems.

Herein, we describe a controllable through-space 1,4-Pd aryl to aryl migration on the **DBzC** platform, which allowed the synthesis of configurationally stable regioisomeric quadruple helicenes containing two azaborole units via a single synthetic route. This is the first example of a 1,4-Pd migration occurring on condensed PAH systems, where the generated Pd species is trapped by another reaction. We also show that this process can be turned off, providing access to the target molecules in a regioselective manner. The properties of these multihelicenes were studied by absorption and emission spectroscopy in solution and in the solid state, cyclic voltammetry and pulse techniques. We also provide insights into their chiroptical properties, configurational stability, molecular structures and packing arrangement. Finally, the aromatic character of the target compounds has been evaluated based on magnetic and electronic criteria, addressing this intriguing aspect of azaborole chemistry.

# RESULTS AND DISCUSSION

## Synthesis and Optimization Studies

The synthesis of multihelicenes **MH1** and **MH2** requires stitching two benzoiso[h]quinoline (BIQ)71 moieties onto the **DBzC** core at 1,9- and 1,8-positions, respectively. The key **DBzC** building blocks, functionalized in the bay areas, are challenging to be obtained by traditional methods. To date, only a few bay-dibrominated examples have been reported.72-73 They are however incompatible in cross-coupling reactions with sterically encumbered α-pyridine derivatives, which are usually ineffective in Suzuki coupling,74 while their metallated derivatives are inaccessible. Therefore, the reaction of choice was the Suzuki coupling between the chlorinated *N*-heterocycle and the borylated **DBzC** partners. Their precursors were 1,9- and 1,8-substituted nonaflates **DBzC-1,9-ONf** and **DBzC-1,8-ONf** (Schemes S1 and S2, Supporting Information), as the ONf group proved to be the most effective leaving group in the synthesis of other azaborole helicenes.71, 75-78 Conversion of these compounds to the Bpin analogs by Pd-catalyzed Miyaura borylation with B2pin2 was performed under standard conditions using
potassium acetate and Pd(dppf)Cl₂ in dry and degassed N,N-dimethylformamide under argon atmosphere. To our surprise, regardless of the precursor used, each reaction produced a mixture of **DBzC-1,8-Bpin** and **DBzC-1,9-Bpin** in combined yields of 0-54 % (Scheme 1a). ¹H NMR revealed two sets of signals with essentially equal intensity. Thus, the regioisomers were formed in a 50:50 ratio (Figure 1a), indicating a thermodynamic control in their formation. Two distinct well-separated doublets of doublets at 8.75 and 8.58 ppm were assigned to the aromatic signals of **DBzC-1,9-Bpin**, while those at 8.68 and 8.64 ppm to the second isomer.

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Controllable 1,4-Palladium Migration/Miyaura Borylation for Model Compound **1** and **DBzC** Derivatives.</figcaption></figure>

In addition, the spectrum shows two singlets at ca. 1.50 ppm corresponding to the non-equivalent Bpin methyl groups. We reasoned that this unexpected outcome can be attributed to a thermodynamically controlled 1,4-Pd migration in the bay region of the **DBzC** derivatives. However, it should be noted that the **DBzC-1,9-ONf** and **DBzC-1,8-ONf** can in principle undergo two-fold 1,4-Pd migration, leading to the compounds with substitution patterns equivalent to those of the starting materials due to the molecular symmetry. That means that only one-fold Pd-migration gives rise to the regioisomeric mixture. This process, most likely promoted by the acetate base, involves the oxidative addition of the organic nonaflate to Pd(0) and the remote C-H activation of the other bay position, leading to the formation of the regioisomeric products. Our synthesis of Bpin-substituted **DBzC** is the first practical demonstration of using Pd 1,4-migration in fused systems to prepare complex PAHs, where the generated Pd species is trapped with the diboron reagent in the Miyaura borylation. This tandem reaction has not been studied so far not only for fused systems, but also for flexible biaryls.

Due to the lack of sufficient separation of the resulting Bpin regioisomers by conventional purification methods, the first approach to **MH1** and **MH2** was the conversion of the regioisomeric mixture into
**DBzC-1,8-BIQ and DBzC-1,9-BIQ (Scheme S3).** The Suzuki-Miyaura cross-coupling of **DBzC-1,8-Bpin** and **DBzC-1,9-Bpin** with **BIQ-Cl** afforded the target compounds in a 59% yield, also as a 1:1 regioisomeric mixture. Their further purification was extremely difficult due to the presence of several atropisomers, i.e. the corresponding ($R_a, R_a$) and ($S_a, S_a$)-enantiomers and the meso-compounds with similar or nearly identical retention times. Therefore, the mixture of teraryls was directly submitted to the electrophilic borylation with $BBr_3$ in the presence of Hünig's base to form the helical scaffolds, followed by a ligand exchange on boron with $AlMe_3$. Surprisingly, the reaction control via TLC indicated the formation of only two products. The compounds isolated by standard column chromatography and subsequent preparative TLC in a total yield of 38% were identified as the racemic mixture of multihelicones **MH1** and **MH2**.⁷⁹ Thus, the 1,4-Pd migration process can be utilized to access the desired helicones from the same starting material, reducing the synthetic effort.

The multihelicones **MH1** and **MH2** embed two azabora[7]helicones with high resistance to helical inversion at room temperature and two fluxional carbo[4]helicene moieties, resulting in up to six possible conformations in each case (see Computations section in the Supporting Information, the nomenclature used for the *M* and *P* stereodescriptors starts with the left azabora[7]helicene and continues counter-clockwise).

<figure><img src="image_4.png" /><figcaption>**Scheme 2.** Synthesis of Target Multihelicones **MH1** and **MH2** from Regioisomerically Pure Precursors.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** a) ¹H NMR spectra of the regioisomeric mixture of **DBzC-1,9-Bpin** and **DBzC-1,8-Bpin** and the isolated regioisomers in CD₂Cl₂ at 298 K. b) Molecular structure of **DBzC-1,8-Bpin** determined by X-ray diffraction analysis at 100 K (CCDC 2353062). Thermal ellipsoids are shown with 50% probability. Hydrogen atoms were omitted for clarity.</figcaption></figure>

As indicated by DFT calculations (ωB97XD⁸⁰/def2-TZVP⁸¹⁻⁸²//ωB97XD/def2-SVP⁸¹⁻⁸²), the most stable diastereomer of **MH1** (PMPM)-C1-MH1, possesses azabora[7]- and carbo[4]helicene moieties of opposite helicities. This result is consistent with the single crystal X-ray analysis (vide infra) of **MH1** (Figure 2), showing the thermodynamically most stable conformation. Likewise, the (PMPM)-C1-MH2 is the thermodynamically preferred diastereomer. However, it is only slightly lower in energy (by 1.7 kJ mol⁻¹ in the gas phase) than (PPPM)-C4-MH2, observed in the solid state (Figure 2). The absence of **MH1** and **MH2** with mixed *P* and *M* helicities of the azaborahelicene moieties in a single molecule can be rationalized by their higher relative energies. Due to the presence of as many as four helical moieties, the enantiomerization mechanisms of both **MH1** and **MH2** are complex, with the preferred pathways computationally characterized by ΔGênant‡ of ca. 149 kJ·mol⁻¹ for the rate-determining steps (gas phase at 298 K, see Figures S94 and S95). Experimentally, the configurational stability of **MH1** and **MH2** was studied by thermal racemization at 170 °C in *ortho*-dichlorobenzene or at 190 °C in 1,2,4-trichlorobenzene, following the decay of the enantiomeric excess either by electronic circular dichroism (ECD) or by analytical chiral HPLC to provide ΔGênant‡ of 141 and 143 kJ·mol⁻¹, respectively (see Figures S87 and S88).

The intrinsic limitation of the non-regioselective synthesis, i.e. the overall low yield of each regioisomer, as the key ONf intermediate is evenly distributed between the two synthetic pathways, and the challenging purification, prompted us to develop a regioselective synthesis of **MH1** and **MH2**. The key to achieving this goal is suppressing the 1,4-Pd migration process during the Miyaura borylation of **DBzC-1,8-ONf** and **DBzC-1,9-ONf**. To identify the suitable reaction conditions, we studied this transformation on the readily accessible unsymmetrical biphenyl prototype **1** bearing a nonaflate functionality at the *ortho* position (Scheme 1b and Table S1). When **1** was allowed to react with B₂pin₂ under the conditions we applied for the synthesis of **DBzC-1,8-Bpin** and its 1,9 regioisomer, both the direct borylation product **2a** and the migration/borylation product **2b** were formed in a 63:37 ratio
(Scheme 1b and Table S1, entry 1). This suggests that, under comparable reaction conditions, the 1,4-Pd migration is faster in the rigid palladium complexes derived from **DBzC-1,8-ONf** and its 1,9 regioisomer than in the flexible palladium complex derived from **1**. Already the change of the catalyst from Pd(dppf)Cl₂ to Pd(PPh₃)₄ resulted in a significant shift toward the retention product **2a** (87:13, entry 2). Afterwards, the impact of the solvent and the base was investigated. Replacing potassium acetate with KF, CsF and K₃PO₄ (entries 6 and 7) yielded **2a** as the exclusive product. These results support the hypothesis of the critical role of acetate in promoting the 1,4-Pd shift. To gain further insight into this process, we have carried out DFT calculations for the ONf-substituted biphenyl under the conditions described in entries 4 and 5 (Table S1). The most plausible mechanism appears to involve a reversible acetate-mediated deprotonation of the remote bay position, the formation of the five-membered palladacycle, and the subsequent protonation, which results in net 1,4-Pd migration (see Scheme S5, Figure S96 and the discussion in the Supporting Information).

Ultimately, the optimized catalytic system consisting of B₂pin₂, CsF and Pd(PPh₃)₄ in 1,4-dioxane at 110°C provided exclusively **2a** in 80% yield, with no detectable amount of **2b** (Scheme 1b and Table S1, entry 7). Under these conditions, the 1,4-Pd migration process is totally suppressed for prototype **1**. Noteworthy, we did not manage to significantly shift the outcome of the model reaction to the migration product (Table S1, entries 9–12).

The optimized conditions were then transferred to the borylation of **DBzC-1,8-ONf** and **DBzC-1,9-ONf** with B₂pin₂. To our delight, **DBzC-1,8-Bpin** and **DBzC-1,9-Bpin** were isolated in 96% and 80% yields, respectively, with no migration products observed (Scheme 1a). The molecular structure of **DBzC-1,8-Bpin** was unambiguously confirmed by solid state structural analysis (Figure 1b).

With pure **DBzC-1,8-Bpin** and **DBzC-1,9-Bpin** in hand, we proceeded with the synthesis of multihelicenes, following the same synthetic route, as described above for the regioisomeric mixture (Scheme 2). Accordingly, two-fold Suzuki-Miyaura cross-coupling of the Bpin-substituted compounds with **BIQ-CI** selectively furnished teraryls **DBzC-1,8-BIQ** and its 1,9 regioisomer in 31 and 30% yields, respectively. In both cases, the ¹H NMR spectra of the isolated products revealed two sets of signals with a 1:1 ratio, which we have assigned to a mixture of the racemates and the *meso*-compounds. This assumption was confirmed by analytical HPLC on a chiral stationary phase. The chromatograms of both products showed clearly visible three peaks (Figure S82). Their deconvolution and integration provided the ratios of 2:1:1 and 1:2:1 for **DBzC-1,9-Bpin** and **DBzC-1,8-Bpin**, respectively, which are attributed to the (*S*<sub>a</sub>*S*<sub>a</sub>)-, (*R*<sub>a</sub>*R*<sub>a</sub>)-, and (*R*<sub>a</sub>*S*<sub>a</sub>)-diastereomers (Figure S85). The conversion of these diastereomeric mixtures to the target multihelicenes was efficient, selectively yielding **MH1** and **MH2** in 57% and 69% yields, respectively, over two steps.
<figure><img src="image_6.png" /></figure>

**Figure 2.** Molecular structures of **MH1** (CCDC 2353061) and **MH2** (CCDC 2353060) determined by X-ray analysis at 100 K. Thermal ellipsoids are shown with 50% probability. Hydrogen atoms were omitted for clarity.

**Optical Properties**

To elucidate the effect of the presence of two azabora[7]helicene moieties and their mutual arrangement on the photophysical properties of the multihelicenes, their absorption and emission spectra were measured in solution and in the solid state. The results are summarized in Table 1. Both regioisomers show intense yellow color in dichloromethane (Figure 3b) and yellow-orange color in the solid state. These observations are in good agreement with the recorded absorption spectra (Figure 3a). The lowest-energy absorption bands in dichloromethane solutions are located at 460 nm and 461 nm for **MH1** and **MH2**, respectively. Moreover, a slightly hypsochromically shifted shoulders (vibronic progressions) are observed for both compounds at 445 nm (**MH1**) and 443 nm (**MH2**). The corresponding molar extinction coefficients ($ε$) are moderate, in the range of $4.6–8.6 × 10^3 M^{-1} cm^{-1}$. According to DFT calculations (ωB97XD/def2-SVP, solvent CH<sub>2</sub>Cl<sub>2</sub>), these absorption bands are superpositions of several transitions (Figures S98 and S101, Tables S9-S11, Supporting Information). In contrast to **MH1**, its regioisomer **MH2** exists as a mixture of two relevant conformers **C1-MH2** and **C4-MH2**, differing in the configuration of the labile carbo[4]helicene moiety, with the relative population of 79 and 21% in dichloromethane at 298 K. The first, most intensive transitions in the spectra of these two species and of **MH1** (oscillator strengths ($f$) of 0.46-0.51) are predominantly attributed to the HOMO → LUMO transitions (59-64%). In general, the LUMOs are uniformly delocalized over the whole molecules, while the HOMOs have larger coefficients on the **DBzC** central core (Figure S97, Supporting Information).

The bright green fluorescence of dichloromethane solutions of **MH1** and **MH2** (Figure 3b) coincides with the emission maxima at 527 nm and 523 nm for **MH1** and **MH2**, resulting in the large apparent Stokes shifts of $2760 cm^{-1}$ and $2570 cm^{-1}$, respectively. The emission quantum yields of 0.26 (**MH1**) and 0.29 (**MH2**) are relatively high for helicenes built from *ortho*-fused rings and the fluorescence lifetimes ($τ_{fl}$) reach the averaged values of 7.1 ns (**MH1**) and 6.7 ns (**MH2**).

In the solid state, the regioisomers differ in their absorption behavior. **MH1** shows a blue-shift of its first absorption maximum, while the band of **MH2** is bathochromically shifted, which indicates different packing arrangements of these regioisomers. This assumption is further reinforced by the shifts of the emission bands, following the same trend as observed for the solid-state absorption
spectra (Figure 3b). Accordingly, the maxima are centered at 523 nm (MH1) and 553 nm (MH2) and correspond to green or green-yellow emission color. The bathochromic shift in the case of MH2 is pronounced (1040 cm⁻¹) compared to the small hypsochromic shift for MH1 (150 cm⁻¹). The Φ values in the solid state for both MH1 and MH2 are lower than the corresponding values measured in solution (0.16 and 0.22, respectively).

Compared with the recently reported single azabora[7]helicene **EH3**⁷⁵ built on a **DBzC** core, the additional helical units in **MH1** and **MH2** result in a bathochromic shift of both absorption (1040-1090 cm⁻¹) and emission maxima (490-630 cm⁻¹) in solution, while the Φ<sub>fl</sub> values in dichloromethane are comparable. Thus, the introduction of an additional azaborole helicene allows the modulation of the optical response toward higher wavelengths, while maintaining the emission efficiency in solution. On the other hand, the emission of **MH2** in the solid state is significantly higher than that of **EH3** (Φ<sub>PL</sub> = 0.13).⁷⁵

**Single Crystal X-Ray Diffraction Analysis**

To get insight into the molecular structures and the packing arrangements of the target multihelicenes and their intermediates, we grew crystals of **DBzC-1,8-Bpin** (for the packing arrangement, see the Supporting Information), rac-**MH1** and rac-**MH2** by diffusion of *n*-hexane into their dichloromethane solutions. The compounds crystallized in the space groups *P*1, *P*2₁/c, and *P*2₁/n, respectively. The analysis of **MH2** (Figure 2) showed B-N bond lengths of 1.618(1) and 1.622(2) Å, indicating strong Lewis pair interactions and stable azaborole rings. It also revealed the (*PPPM*)-conformer (and enantiomeric (*MMMP*)-conformer), referred to as **C4-MH2**, which was identified by DFT calculations to be slightly higher in energy than the thermodynamically preferred (*PMPM*)-conformer **C1-MH2** in the gas phase (*vide supra*). Along the long molecular edge, the neighboring homochiral molecules are interchangeably rotated by 90° (Figure S80a,b, Supporting Information), resulting in pronounced face-to-edge C–H···π interactions between the homochiral molecules (Figure S80d). In contrast, the interactions between the heterochiral molecules with a significant offset are less distinctive (Figure S80c). These include only C–H···π interactions, and no π–π interactions. Apparently, the highly twisted structure of the multihelicene in combination with the inherent distortion of the **DBzC** core effectively reduces the π–π interactions, which is also reflected in the high solubility of **MH2**.

Numerous attempts to grow single crystals of **MH1** suitable for an X-ray diffraction analysis provided crystals showing complete molecular disorder over the inversion center, and despite our efforts, it was not possible to obtain direct evidence on the packing arrangement of **MH1**. However, the analysis unambiguously revealed the (*PMPM*)-conformer **C1-MH1** (Figure 2).
<figure><img src="image_7.png" /><figcaption>**Figure 3.** a) Top: Absorption spectra of **MH1** and **MH2** in CH₂Cl₂ solutions (**MH1**: *c* = 5.7 × 10⁻⁵ M, **MH2**: *c* = 5.4 × 10⁻⁵ M). Middle: Normalized absorption and emission spectra in CH₂Cl₂ solutions (**MH1**: *c* = 2.0 × 10⁻⁵ M, **MH2**: *c* = 1.2 × 10⁻⁵ M, *λ*<sub>ex</sub> = 440 nm). Bottom: Normalized absorption (thin films obtained from CH₂Cl₂ solutions on quartz glass) and emission (amorphous powder) spectra in the solid state. b) Photographs of CH₂Cl₂ solutions of **MH1** and **MH2** under visible light and UV light c) ECD (**MH1**: *c* = 3.8 × 10⁻⁵ M, **MH2**: 2.6 × 10⁻⁵ M) and CPL (**MH1**: *c* = 9.0 × 10⁻⁶ M; **MH2**: 9.2 × 10⁻⁵ M) spectra in CH₂Cl₂.</figcaption></figure>

<table><caption>**Table 1.** Optical Properties of **MH1** and **MH2**.</caption><thead><tr><th rowspan="2">Cpd.</th><th colspan="2">λ<sub>abs</sub><sup>[a]</sup> / ε<sub>max</sub><sup>[b]</sup></th><th rowspan="2">λ<sub>em</sub><sup>[c]</sup>/Stokes<br>nm</th><th rowspan="2">shift / cm<sup>-1</sup></th><th rowspan="2">Φ<sub>fl</sub><sup>[d]</sup></th><th colspan="3">τ<sup>[e]</sup>/ns</th><th rowspan="2">λ<sub>abs</sub><sup>[g]</sup>/λ<sub>em</sub><sup>[h]</sup></th><th rowspan="2">Φ<sub>PL</sub><sup>[i]</sup></th></tr><tr><th>nm</th><th>/ 10<sup>3</sup> M<sup>-1</sup><br>cm<sup>-1</sup></th><th>nm</th><th>(% amp)</th><th>τ<sup>[f]</sup>/ns</th><th>nm</th><th>/ nm</th></tr></thead><tbody><tr><td rowspan="2"><b>MH1</b></td><td>460</td><td>5.1</td><td rowspan="2">527</td><td rowspan="2">2760</td><td rowspan="2">0.26</td><td rowspan="2">2.8 (0.046)</td><td rowspan="2">7.3 (95.4)</td><td rowspan="2">7.1</td><td rowspan="2">454</td><td rowspan="2">523</td><td rowspan="2">0.16</td></tr><tr><td>445</td><td>4.6</td></tr><tr><td rowspan="2"><b>MH2</b></td><td>461</td><td>8.6</td><td rowspan="2">523</td><td rowspan="2">2570</td><td rowspan="2">0.29</td><td rowspan="2">2.3 (0.049)</td><td rowspan="2">6.9 (95.1)</td><td rowspan="2">6.7</td><td rowspan="2">464</td><td rowspan="2">553</td><td rowspan="2">0.22</td></tr><tr><td>443</td><td>7.9</td></tr></tbody></table>

[a] Absorption maximum in CH₂Cl₂. [b] Molar absorption coefficient. [c] Fluorescence maximum in CH₂Cl₂. [d] Absolute fluorescence quantum yield in CH₂Cl₂. [e] Fluorescence lifetime (relative % amplitude). [f] Amplitude-weighted average fluorescence lifetime. [g] Absorption maximum in the solid state. [h] Emission maximum in the solid state. [i] Photoluminescence quantum yield in the solid state.

<table><caption>**Table 2.** Chiroptical Properties of **MH1** and **MH2**.</caption><thead><tr><th>Cpd.</th><th>lg<sub>abs</sub>l<sup>[a]</sup> / 10<sup>-3</sup></th><th>lg<sub>lum</sub>l<sup>[b]</sup> / 10<sup>-3</sup></th><th>B<sub>CPL</sub><sup>[c]</sup> M<sup>-1</sup> cm<sup>-1</sup></th></tr></thead><tbody><tr><td><b>MH1</b></td><td>4.9 (328 nm), 1.7 (460 nm)</td><td>2.7 (512 nm)</td><td>1.79</td></tr><tr><td><b>MH2</b></td><td>4.6 (335 nm), 1.9 (460 nm)</td><td>2.2 (515 nm)</td><td>2.75</td></tr></tbody></table>

[a] Absorption dissymmetry factor at a given wavelength in parentheses. [b] Luminescence dissymmetry factor at a given wavelength in parentheses. [c] CPL brightness calculated as $B_{CPL} = ε \times Φ \times lg_{lum}l/2$.
## Chiroptical Properties

To investigate the chiroptical properties of the multihelicenes, the racemic mixtures of **MH1** and **MH2** were resolved via semi-preparative HPLC on a chiral stationary phase (for details, see the Supporting Information). By comparing the experimental electronic circular dichroism (ECD) spectra with the simulated spectra (TD-DFT calculations, Figures S98, S100, S101, and S103), the first isolated fraction was assigned to the (**PP**)-enantiomer, while the second fraction to the (**MM**)-enantiomer (note: only the descriptors for the two configurationally stable azabor[7]helicene fragments are used). Their ECD and circularly polarized luminescence (CPL) spectra were recorded in dichloromethane (Figure 3c and Table 2). The ECD of (**PP**)-**MH1** shows a negative Cotton effect at 463 nm ($\Delta\varepsilon = -16 \text{ M}^{-1}\text{cm}^{-1}$) and a positive Cotton effect at 328 nm ($\Delta\varepsilon = 71 \text{ M}^{-1}\text{cm}^{-1}$). (**PP**)-**MH2** reveals a similar line shape with a negative Cotton effect at 458 nm ($\Delta\varepsilon = -14 \text{ M}^{-1}\text{cm}^{-1}$) and a positive Cotton effect at 334 nm ($\Delta\varepsilon = 94 \text{ M}^{-1}\text{cm}^{-1}$). The enantiomers manifest a mirror image relationship, and the dissymmetry factors $lg_{\text{abs}}$ reach the highest values of $4.9 \times 10^{-3}$ at 328 nm for **MH1** and $4.6 \times 10^{-3}$ at 335 nm for **MH2**. The $lg_{\text{abs}}$ corresponding to the lowest-energy bands are smaller, i.e. $1.7 \times 10^{-3}$ and $1.9 \times 10^{-3}$, respectively. The compounds also show CPL activity. Like the CD spectra, the CPL spectra of their (**MM**)- and (**PP**)-enantiomers unfold mirror image relationship, with the maxima for (**MM**)-**MH1** and (**MM**)-**MH2** positioned at 515 nm and 513 nm, respectively. These positive bands correspond to the $lg_{\text{lum}}$ factors of $2.7 \times 10^{-3}$ for **MH1** and $2.2 \times 10^{-3}$ for **MH2**.

CPL brightness ($B_{\text{CPL}}$) of **MH1** and **MH2**, defined as $B_{\text{CPL}} = \varepsilon \times \Phi \times \sqrt{lg_{\text{lum}}}/2$, combines the absorption coefficient, the emission quantum yield in solution, and the dissymmetry factor efficiencies to give a more accurate evaluation of the overall efficiency of a chiral emitter.⁷⁵ The values for **MH1** and **MH2** of $1.79 \text{ M}^{-1}\text{cm}^{-1}$ and $2.75 \text{ M}^{-1}\text{cm}^{-1}$, respectively, are within the typical range for most of the azaborole helicenes.

## Electrochemistry

The electrochemical properties of **MH1** and **MH2** were investigated by cyclic voltammetry (CV) and square wave voltammetry (SW) in dichloromethane in the presence of *n*-Bu₄NPF₆ as a supporting electrolyte and calibrated against ferrocenium/ferrocene (Fc⁺/Fc). The redox potentials are summarized in Table S8 and the voltammograms are depicted in Figure S93 (Supporting Information). Both compounds undergo one irreversible reduction and three oxidation processes. The oxidation potentials of **MH1** are located at +0.56, +0.72 and +1.03 V (SW) and are comparable to those of **MH2** (ca. +0.62, +0.74 and ca. 1.03, SW). The reduction of **MH1** and **MH2** occurs at -2.24 V (SW) and -2.22 V (SW), respectively. This differs from the behavior of single azabor[7]helicene **EH3**, for which two oxidation and two reduction events can be observed. The addition of a second azaborole [7]helicene fragment to the **DBzC** core results in a slight cathodic shift of both $E^{\text{ox1}}$ and $E^{\text{red1}}$ by 0.10-0.16, leading to the elevation of the HOMO and LUMO energy levels for both compounds compared to **EH3**.⁷⁵

## Aromaticity

An intriguing question is the aromaticity of azaboroles and PAHs that incorporate this heterocycle. While the aromatic character of simple five-membered boracycles containing several heteroatoms (B, N, O, S) has been studied,⁸³⁻⁸⁴ this aspect remained unresolved for 2H-1,2-azaborole, a constituent heterole of our helicenes. To address this question, we examined aromaticity in multihelicenes **MH1** and **MH2** using 3D isocontours maps of isotropic magnetic shielding (IMS)⁸⁵ and the electron density of delocalized bonds (EDDB).⁸⁶ These analyses revealed that both **MH1** and **MH2** are aromatic molecules: the 3D IMS isocontours maps (Figure 4a) show only positive values of IMS (visualized by the blue color and attributed to aromaticity) over the six-membered rings, and the fraction of delocalized electrons is ca. 76.1–77.2% according to the EDDB<sub>H</sub>(r) analyses (Figure 4b). The non-equivalent magnetic and electronic environment of both sides of each ring, inherent to the chirality of **MH1** and **MH2**, is clearly visualized in these analyses. Expectedly, the values of IMS over the azaborole rings are of small
magnitude (|IMS| < 5.5 ppm, visualized by the neutral color) indicating a non-aromatic character of these rings, as opposed to the structurally related antiaromatic boroles.⁸⁷⁻⁸⁸ The non-aromaticity of azaboroles was confirmed by the EDDB isovalue plots showing no electron density over the B–N and B–C bonds. Thus, the Clar structures⁸⁹⁻⁹⁰ of **MH1** and **MH2** contain eight π-sextets and three isolated double bonds (Figure 4c).

<figure><img src="image_8.png" /><figcaption>**Figure 4.** a) 3D isocontours maps of isotropic magnetic shielding (IMS) of **C1-MH1**, **C1-MH2** and **C4-MH2**. b) Plots of the electron density of delocalized bonds [EDDB<sub>H</sub>(r)] of **C1-MH1**, **C1-MH2** and **C4-MH2**; isovalue = 0.025. c) The Clar structures of **MH1** and **MH2**.</figcaption></figure>

## CONCLUSIONS

We have developed the synthesis of two original regioisomeric quadruple helicenes **MH1** and **MH2** embedding two azaborole [7]helicene units, the first configurationally stable azaborole multihelicenes. The key step of the synthetic sequence was the borylation of 1,9- and 1,8-nonaflate-substituted **DBzC** derivatives. Their reactions with B<sub>2</sub>pin<sub>2</sub> under conditions that are well-known to facilitate the C-H activation induced the 1,4-Pd aryl to aryl migration, leading to the formation of equal amounts of both regioisomers and in turn, allowed the synthesis of the target helicenes from the same starting material. To our knowledge, this is the first example of a tandem reaction on fused systems involving a 1,4-Pd shift.

Importantly, the outcome of this reaction can be controlled by the choice of the base and the catalytic system. Our experiments demonstrated the central role of the acetate, presumed to act as a proton shuttle, in promoting the through-space activation of the C-H bond at a remote bay position of **DBzC**. The reactions employing Pd(PPh<sub>3</sub>)<sub>4</sub> as catalyst in absence of acetate bases enabled the controlled C-B bond formation at the positions originally occupied by the nonaflate groups, giving access to the regioisomerically pure azaborole multihelicenes **MH1** and **MH2** in excellent yields.

These compounds exhibit green fluorescence in dichloromethane with appreciable quantum yields of up to 0.29 and 0.22, respectively, and lg<sub>lum</sub>l of up to 2.7 × 10<sup>-3</sup>. In the solid state, **MH1** and **MH2** show green or green-yellow emission with Φ<sub>PL</sub> of up to 0.22 for the latter compound. While the spectra of **MH1** are hypsochromically shifted versus the spectra in solution, those of **MH2** show a pronounced bathochromic shift. Compared to a single azaborole helicene derived from **DBzC**,<sup>75</sup> the incorporation of a second helicene fragment proved particularly beneficial for the chiroptical responses of **MH1** and the emission intensity in the solid state of **MH2**, while shifting the spectra bathochromically. The presence of a second azaborole helicene fragment also altered the electrochemical response compared to monohelicene analog, albeit **MH1** and **MH2** have comparable electrochemical profiles. Thus, increasing the number of azaborole helicene fragments in combination with an extended π-system can serve as a viable strategy to gain access to materials whose properties may be further modulated by the mutual arrangement of helical fragments. In addition, using magnetic and electronic criteria, the
aromaticity patterns of **MH1** and **MH2** can be visualized and compared, which revealed the non-aromatic character of 2H-1,2-azaborole constituent rings.

Overall, these results enrich the structural diversity of dibenzochrysene PAHs while improving some of their properties. We also envision that the conditions of 1,4 palladium migration can be applied to other fused systems, allowing the synthesis of PAHs with substitution patterns that may be difficult to access otherwise. Efforts in this direction are underway in our laboratories.

## ASSOCIATED CONTENT

**Supporting Information.** Synthetic procedures, characterization details, HPLC resolution, cyclic voltammetry and square wave voltammetry data, crystal structural data, racemization studies, photophysical and chiroptical studies, computational details. The 3D IMS isocontours maps are interactive and are available for visualization (and personalization) in the Supporting Information as .vtk files. This material is available free of charge via the Internet at http://pubs.acs.org.

## AUTHOR INFORMATION

### Corresponding Author

* Agnieszka Nowak-Król – Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, 97074 Würzburg, Germany; orcid.org/0000-0003-0454-2491;
Email: agnieszka.nowak-krol@uni-wuerzburg.de.

### Notes

The authors declare no competing financial interest

### Accession Codes

CCDC 2353062 (DBzC-1,8-Bpin), 2353061 (MH1), and 2353060 (MH2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge *via* www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

## ACKNOWLEDGMENT

A.N.-K. thanks the German Research Foundation (DFG) for an Emmy-Noether Fellowship (NO 1459/1-1). A.N.-K. and F.F. thank the Hector Fellow Academy (HFA) for financial support. A.A. acknowledges the European Union's Framework Programme for Research and Innovation Horizon Europe under the Marie Skłodowska-Curie Grant Agreement No. 101106492 (project title: Full-Fission). Y.C. thanks the French Agence Nationale de la Recherche—ANR for financial support (ANR-19-CE07-0041). The authors thank Dr. Krzysztof Radacki for providing a solution for the crystal structure of **MH1**, Prof. Todd B. Marder for use of his fluorimeter, and Dr. Yannick Carissan for the ims3d.py program.

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