One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

Abstract

The isoelectronic replacement of C=C bonds with ⁻B=N⁺ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh₃)NTf₂]-catalyzed 6-endo-dig cyclization of BN–H and BO–H bonds across suitably positioned C≡C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B₂-2,6-E₂-anthracenes (E = N, O) as well as B,O₂-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)₂-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C–H borylation approach.

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Introduction

The targeted doping of polycyclic aromatic hydrocarbons (PAHs) with p-block atoms is a powerful tool to create new molecules for applications as diverse as drug development, materials science, and catalysis.¹ Especially the isoelectronic replacement of a non-polar C=C bond with a polar ⁻B=N⁺ bond can substantially influence the π-electron distribution and frontier-orbital energies of the resulting B,N-PAH and thus lead to chemical and physical properties that differ considerably from those of the corresponding carbonaceous congener (“BN/CC isosterism”).² By varying the positions,³ orientations, and number of ⁻B=N⁺ units within the molecular framework, a greatly expanded structural and chemical space becomes accessible.^4–15 In addition, breaking the symmetry of a molecular scaffold through the introduction of a ⁻B=N⁺ unit often allows late-stage functionalizations with higher regioselectivities than in the case of the parent PAH.¹⁶ Because of these appealing features, considerable research efforts are currently being made to develop novel B,N-PAHs. Although many synthesis approaches are still based on individual solutions of limited scope, the following more general strategies have already emerged. (i) B–N/B–C-bond formation cascades: the B–N bond is formed first, followed by an intramolecular electrophilic borylation reaction with a suitably positioned aryl, vinyl, or alkynyl group (Scheme 1a).^17–21 (ii) C–C-bond formation/oxidation cascades: a ring-closing metathesis (RCM) is conducted on an already established B–N moiety and a subsequent oxidation step generates the conjugated π system (Scheme 1b).^22,23 (iii) Photochemical or exciton-driven C=B-bond formation: starting from B–N adducts with N-heteroaryl-benzyl backbones, the elimination of HR′ (R′ = alkyl, aryl), which occurs either upon UV/vis irradiation or upon application of an electric current to a corresponding electroluminescent device, closes the conjugation pathway (Scheme 1c).^24,25

Scheme 1 Common strategies for the synthesis of B,N-doped PAHs. (RCM = ring-closing metathesis).

Compared to B,N-PAHs, analogous B,O-PAHs have been largely neglected, although a first derivative, 10-hydroxy-9,10-oxaboraphenanthrene, was published by Dewar et al. as early as in 1960.²⁶ Still today, the majority of known B,O-PAHs contains the structural motif of a 9,10-oxaboraphenanthrene,^27–42 likely due to its convenient accessibility via B–O-bond formation/electrophilic aromatic borylation sequences (Scheme 2a; compare the analogous B,N case shown in Scheme 1a).^26–33

Scheme 2 Common strategies for the synthesis of B,O-doped PAHs.

A second class of B,O-PAHs with a reasonably large number of members are 1,2-B,O-naphthalenes. Sheppard et al. and Gong et al. selected o-alkynylphenyl boronic acids as the starting materials and used the Au- or Pd-catalyzed intramolecular addition of a BO–H bond to the C≡C bond in the final cyclization step (Scheme 2b).^43–46 Yorimitsu et al. prepared an isomeric 2,1-B,O-naphthalene through Ni- or Mn-catalyzed O–C-bond activation of benzofurans and subsequent insertion of a [Bpin]⁻ fragment generated by heterolysis of bis(pinacolato)diboron, pinB–Bpin (“aromatic metamorphosis”; Scheme 2c).^47,48

Even the limited information available on the chemical properties of B,O-PAHs reveal some distinct patterns: (i) due to the higher electronegativity and weaker π-donor ability of O relative to the N atom, B,O-PAHs tend to be less aromatic and more Lewis acidic than corresponding B,N-PAHs.^{28,34,49–55} (ii) The 1,2-B,O-naphthalenes shown in Scheme 2b possess a relatively low stability due to the electron-rich C atom at the position β to the O atom (“boron enolates”).^43–45 The associated peculiar reactivities of certain B,O-PAHs make them valuable synthetic intermediates for aldol reactions, aminations, Suzuki–Miyaura C–C couplings, Chan–Lam C–O couplings, and the formation of lactones by deborylative CO insertion.^{29,35–37,43,44,47,56}

On the other hand, if the β position is embedded into an aromatic benzene ring (as in the cases of the isomeric 2,1-B,O-naphthalenes shown in Scheme 2c) and the B atom is equipped with an appropriate substituent,⁵⁷ the resulting B,O-PAHs can become sufficiently inert to serve as luminescent materials for optoelectronic applications.^{31–33,38,39,41} It is also important to note in this context that the steric demand of an O atom is even lower than that of a CH or NH fragment in analogous B- or B,N-PAHs. This promotes a coplanar conformation between the respective B,O-heterocycle and, e.g., B-bonded phenyl substituents and can thus contribute to the increase of conjugation lengths and the extension of the frontier orbitals from the oxaborin moiety to the phenyl ring.^{38,47,52,58–60}

If one proceeds further from B,E-PAHs to (B,E)_n-PAHs with E = N or O and n > 1, the available information again becomes dramatically less,^{9,11,13,30,32,39,54,55,61–78} which is unfortunate, because some evidence has already been gathered that the number n of embedded ⁻B=E⁺ units significantly influences the chemical and physical properties of corresponding compounds.^{30,54,68,72,79} Thus, new atom-economic strategies for the construction of (B,E)_n-PAHs, which ideally avoid the use of sensitive boron halides, sophisticated organometallic reagents, or forcing reaction conditions, are still in demand. The most time and cost-efficient approach would have to be sufficiently modular to implement both E = N and O via closely related reaction protocols. Some of these conditions are met by recently discovered ring-expansion reactions on five-membered ring precursors, which result in the insertion of one E atom into an endocyclic B–C bond. This way, boroles or 9-borafluorenes have been converted to 2,1-B,N/2,1-B,O-benzenes or 10,9-B,N/10,9-B,O-phenanthrenes, respectively.^{41,53,54,80–85}

Inspired by the well-established oxypalladation of alkynes and Sheppard's above-mentioned work,⁴³ our group herein discloses a straightforward protocol for the synthesis of (B,N)_n- and (B,O)_n-PAHs. The main structural motif present in all synthetic intermediates employed is that of an o-alkynyl-substituted aryl boronic or borinic acid (Scheme 3). These intermediates can either be cyclized directly with the help of a [Au^I] catalyst or first transformed into the corresponding aminoboranes and then cyclized using the same [Au^I] catalyst. Our approach offers the following advantages: (i) the chemistry of boronic and borinic acids is nowadays very well explored and most derivatives can easily be handled, purified, and stored.⁸⁶ (ii) The possibility to decide in the last step before cyclization whether a B,N- or B,O-PAH should be prepared guarantees maximum efficiency. (iii) The protocol is applicable to the synthesis of both single and multiple B,E-doped PAHs. Our approach differs from all other strategies in that the cyclization step relies on the formation of C–N/O bonds.

Scheme 3 Synthesis of compounds 4a,b–6a,b. Reagents and conditions: (i) 1–2 equiv. (Me₃Si)₂NMe, C₆D₆, 120 °C, 2–3 d, sealed NMR tube. (ii) 1–2 mol% [Au(PPh₃)NTf₂], CHCl₃, room temperature, 10 min. (iii) 5 mol% [Au(PPh₃)NTf₂], CDCl₃, 60 °C, 4–48 h. [a] Overall yield over two steps from commercially available chemicals. [b] Overall yield over three steps from commercially available chemicals. [c] Overall yield over four steps from commercially available chemicals. [d] Yield of the final cyclization step. [e] Yield of the final cyclization step without isolation of 1b–3b.

As a proof-of-principle, we have already produced (B,N)₂- and (B,O)₂-perylenes via our method.⁵⁵ In our present work, we extend the scope of the reaction to (double) B,N/O-doped naphthalenes and anthracenes. It will also be shown that OBO- and NBO-doped phenalenyls are becoming accessible, the latter having no precedent in the literature.

Results and discussion

Syntheses

The o-alkynyl-substituted aryl borinic and boronic acid precursors 1a–3a and 7a required for the preparation of the (B,O)_n- and (B,N)_n-PAHs 4a,b–6a,b and 8a–c (Schemes 3 and 4) were synthesized starting from readily accessible mixed bromo(iodo)arenes. Negishi-coupling protocols were employed for the selective introduction of the alkynyl substituents at the iodinated C atoms. The subsequent borylation reactions were performed by Br/Li exchange with tBuLi, quenching of the resulting aryl lithium intermediates with MesB(OMe)₂ or B(OMe)₃, and aqueous acidic workup (see the ESI† for full details). The electronegative F₃C group in 7a,b and 8a–c is a remnant of the synthesis of 2-bromo-1,3-diiodo-5-(trifluoromethyl)benzene from 1-bromo-4-(trifluoromethyl)benzene and served to direct the two I atoms to the ortho positions of the Br substituent during electrophilic aromatic iodination. F₃C also proved to be a useful ¹⁹F NMR-spectroscopic handle. The tBuC₆H₄ moieties were chosen with the aim to increase the solubilities of the obtained intermediates and products in organic solvents. Moreover, these substituents offer some steric shielding to the otherwise fully exposed reactive β-CH sites (see above). Finally, the intense tBu signals in the sparsely populated alkyl regions of the ¹H NMR spectra of 1a,b–8a–c effectively reveal the presence of possible side products and are thus a good measure of the selectivities of the desired conversions.

Scheme 4 Synthesis of compounds 8a–c. Reagents and conditions: (i) 1.5 equiv. (Me₃Si)₂NMe, C₆D₆, 120 °C, 2–3 d, sealed NMR tube. (ii) 1 mol% [Au(PPh₃)NTf₂], CHCl₃, room temperature, 4–6 h. (iii) 2 mol% [Au(PPh₃)NTf₂], CDCl₃, 60 °C, 4 h. (iv) 5 drops TfOH (approx. 1 equiv.), CH₂Cl₂, room temperature, 5 min. [a] Yield over three steps from commercially available chemicals. [b] Yield over four steps from commercially available chemicals. [c] Yield of the final cyclization step. [d] Yield of the final cyclization step without isolation of 7b.

The B,O precursors 1a–3a were cyclized using the catalyst [Au(PPh₃)NTf₂] in CHCl₃.^43,55 Regardless of the presence of the bulky Mes substituents, which serve to protect the B atoms during workup, the reactions took place already at room temperature, required no more than a few minutes for completion, and gave yields in the range 82–99% (Scheme 3; all cyclizations were quantitative according to NMR).

The mono- or ditopic borinic acids 1a–3a were transformed into the corresponding (methylamino)boranes by heating with (Me₃Si)₂NMe in C₆D₆ (1–2 equiv., 120 °C, 2–3 d, sealed NMR tube). After the quantitative conversion was confirmed by NMR spectroscopy, the solvent was changed to CDCl₃, [Au(PPh₃)NTf₂] was added and the mixture heated to 60 °C for 4–48 h while monitored by NMR spectroscopy. After workup, 4b–6b were isolated in yields of 64–72% over both steps (Scheme 3).

The boronic acid 7a turned out to be a special case: although a double cyclization reaction is possible and affords the B,O₂-PAH 8a in 86% yield, it requires a reaction time of several hours rather than minutes at room temperature (Scheme 4).

Even upon heating of 7a with 2.5 equiv. of (Me₃Si)₂NMe, the amination reaction stopped at the stage of the methylamino(hydroxy)borane 7b (Scheme 4); for the subsequent targeted synthesis of 7b, 1.5 equiv. of (Me₃Si)₂NMe were used (C₆D₆, 120 °C, 2–3 d). Treatment of 7b with 2–25 mol% of [Au(PPh₃)NTf₂] resulted in a 6-endo-dig addition of the BN–H bond across the C≡C bond (60 °C, 4–48 h; 8c), whereas no B,O-doped ring was formed; the optimized protocol for the synthesis of 8c uses 2 mol% [Au(PPh₃)NTf₂] and a reaction time of 4 h. The complete cyclization of 8c to the B,N,O-phenalenyl 8ab was finally achieved by addition of F₃CSO₃H (TfOH).^87,88

These results are remarkable in several respects: (i) 8ab is the first B,N,O-PAH available so far. (ii) When the reactions are independent, the ring closure to the (B,O)_n-PAHs with the [Au(PPh₃)NTf₂] catalyst is generally faster than the formation of the corresponding (B,N)_n-PAHs. However, for 7b, when there is now intramolecular competition between N–H and O–H, cyclization to the B,N heterocycle is preferred.⁸⁹ (iii) TfOH can also be a suitable cyclization catalyst and in this particular example it is even superior to [Au(PPh₃)NTf₂].

The closest relatives of our B,O-naphthalene 4a are Sheppard's boron enolates shown in Scheme 2b.⁴³ Some of these derivatives (e.g., R′′ = H,⁴⁵nBu⁴³) were characterized by NMR spectroscopy, IR spectroscopy, and high resolution mass spectrometry. The optoelectronic properties were not investigated. Yet, the presence of a strongly π-donating OH substituent at the B atom of Sheppard's enolates should lead to a lesser extent of double bonding in the endocyclic B–O bond than in the case of 4a, which carries an electronically more innocent Mes substituent.⁹⁰ A doubly benzannulated congener of the B,O₂-doped phenalenyl 8a was prepared by Hatakeyama via demethylative direct borylation of 2,2′′-dimethoxy-1,1′:3′,1′′-terphenyl (BBr₃/TMP, 180 °C, 18 h; TMP = 2,2,6,6-tetramethylpiperidine; see Scheme 2a for a related reaction).³¹ No precedence exists for the B,O- and (B,O)₂-anthracenes 5a and 6a.

Fig. 1 shows the sequence of relative thermodynamic stabilities of all conceivable parental B,N-naphthalene isomers according to quantum-chemical calculations (R¹–R⁸ = H);⁹¹ substitution has been achieved at the indicated positions. We note that the energetically favorable 2,1-B,N- and 1,2-B,N-naphthalenes differ from the other known isomers by the presence of unperturbed aromatic C₆ rings.³

Fig. 1 Relative thermodynamic stabilities of B,N isosters of naphthalene containing one B,N unit. Substitutions have been achieved at the indicated positions.

The most stable 2,1-B,N-naphthalenes are also by far the most thoroughly studied isomers.^{18,19,92–103} The dominant synthesis strategies rely on ring-closing reactions between o-vinyl-/o-alkynylanilines and R′BCl₂ or K[R′BF₃]/SiCl₄/NEt₃ (cf.Scheme 1a; R′ = halogen, alkyl, alkenyl, alkynyl, (hetero)-aryl).^{18,19,92,93,95,99,102} An “aromatic metamorphosis” approach (cf.Scheme 2c) is based on reductive ring opening of indoles with excess Li metal, trapping of the dilithiated intermediate with RBpin, and aqueous workup.¹⁰³

Apart from our current work on the second most stable 1,2-B,N-naphthalenes, the only other report comes from the Cui group.¹⁰⁴ They used benzylic imines as key intermediates (3 synthesis steps) and converted them into the corresponding enamidyl dibromoboranes, which were cyclized via an Et₃N-promoted intramolecular electrophilic aromatic borylation reaction at 80 °C (2 steps; yields: 30–51% with respect to the corresponding benzylic imine). The authors demonstrated a scope of their reaction sequence across the substituents R¹ = alkynyl/(hetero)aryl, R² = aryl, R³ = tBu/Ph, R⁵ = F and R⁶ = Me.

Apart from the B,N-anthracenes 5b and 6b, their orientational isomers 2,1-B,N-anthracene and 2,6-B₂-1,5-N₂-anthracene are known. The air-stable compounds are accessible through (double) borylative cyclization of 2-amino-3-vinylnaphthalene or 1,4-diamino-2,5-divinylbenzene with BCl₃. Subsequent Cl/H exchange using LiAlH₄ afforded the target molecules in about 80% yield over the last two steps.⁷⁹

To conclude, our novel synthesis approach for (B,N)_n- and (B,O)_n-doped PAHs (i) provides previously inaccessible positional isomers in yields competitive with any alternative cyclization reaction developed to-date, (ii) is modular, as it allows the synthesis of (B,E)_n derivatives with E = N and O starting from the same late precursors, and (iii) takes advantage of well-established borinic and boronic acid chemistry, avoiding the use of corrosive, air- and moisture-sensitive haloboranes.

NMR-spectroscopic and X-ray crystallographic characterization of 4a,b–6a,b and 8a–c

All spectra were run in CDCl₃.¹⁰⁵ In the ¹H NMR spectra of 4a,b–6a,b and 8a–c, the successful cyclization reactions are indicated by the presence of singlet resonances in the aromatic regions, which are due to the newly generated H^β protons (see Schemes 3 and 4 for the position labels). The ¹³C{¹H} NMR spectra of 4a,b–6a,b no longer contain signals characteristic of C(sp) atoms; instead, new resonances appear that belong to C^β (4a–6a: av. 106.2 ppm; 4b–6b: av. 113.4 ppm) and C^α (4a–6a: av. 151.8 ppm; 4b–6b: av. 145.7 ppm). Intense cross peaks between the NCH₃ proton signals and the C^α signals are observed in all ^H,CHMBC spectra of 4b–6b. The ¹¹B NMR shift values of 4a,b–6a,b are found in the expected region of 39–46 ppm.¹⁰⁶

The B,O₂-phenalenyl 8a shows NMR features similar to those of the B,O-naphthalene 4a (8a: δ(H^β) = 6.97, δ(C^β) = 104.2, δ(C^α) = 154.2), but its ¹¹B signal is significantly shifted to higher field as a result of better magnetic shielding by the two π-donating O atoms (8a: 29 ppm vs.4a: 46 ppm). For the singly ring-closed compound 8c, we observe simultaneously resonances attributable to CH^β and C^α units and those attributable to two alkynyl-C atoms. An ^H,CHMBC experiment proved the formation of a 1,2-B,N-naphthalene core with dangling OH substituent (δ(OH) = 7.14). The ¹H NMR spectrum of the B,N,O-phenylene 8ab, in contrast, lacks an OH signal and instead shows two well-resolved H^β resonances at 6.34 and 6.89 ppm. While two C^β resonances appear at 104.2 and 108.1 ppm, alkynyl-C signals are not detectable in the ¹³C{¹H} NMR spectrum of 8ab.

Further comparison of the NMR spectra of 4a–6a on the one hand and 4b–6b on the other reveals additional remarkable differences in the molecular and electronic structures as a function of B,O- vs. B,N-doping. While the ¹H and ¹³C chemical shift values of the Mes substituents are largely the same, especially the CH^γ values of the tBuC₆H₄ groups of 4a–6a (δ(H^γ) = av. 7.94 ppm; δ(C^γ) = av. 125.2 ppm) are significantly different from those of 4b–6b (δ(H^γ) = av. 7.39 ppm; δ(C^γ) = av. 129.1 ppm). The same is true for the B,O-half of 8ab (δ(H^γ) = 7.87 ppm; δ(C^γ) = 125.3 ppm) compared to its B,N-half (δ(H^γ) = 7.36 ppm; δ(C^γ) = 128.9 ppm). These differences can be explained by different conformations of the tBuC₆H₄ substituents with respect to the heterocyclic cores in 4a,b–6a,b: the small O atom allows an essentially coplanar arrangement in solution, whereas the larger NMe group enforces a twist between the two moieties (the bulky Mes rings are consistently orthogonally positioned). Different conformations between 4a–6a and 4b–6b are also observed in the solid state and are relevant for the interpretation of the optoelectronic properties of the B,O- vs. B,N-doped species (see below).

Looking at 4a,b/5a,b, we find that the C^ε atoms are deshielded by about 10 ppm with respect to the C^δ atoms, regardless of whether the compounds contain O or N atoms (δ(C^δ) = 126.4 (4a), 126.1 (4b), 124.0 (5a), 123.4 ppm (5b); δ(C^ε) = 136.5 (4a), 135.5 (4b), 138.9 (5a), 136.8 ppm (5b)). Given that the δ(C^δ) values are close to that of C₆H₆ (128.4 ppm), the +M effect of O and N does not seem to reach out to this position, whereas the π-charge density at C^ε is apparently reduced due to the −M effect of B. In line with this interpretation, the two equivalent CH atoms of the benzene cores of 6a,b, which should experience the ±M effects of both dopant elements, are also considerably deshielded (134.7 (6a), 133.2 ppm (6b)). We also note that plots of the highest occupied molecular orbitals (HOMOs) of 4a,b/5a,b consistently indicate a larger contribution of the p_z orbital located at C^δ than of the p_z orbital located at C^ε (see the ESI†).

The product molecules 4a, 6a, 8a and 4b, 5b have been characterized by X-ray crystallography (Fig. 2; the solid-state structures of numerous intermediates are compiled in the ESI†). As a general feature, the B–O distances (1.368(4)–1.388(2) Å) are significantly shorter than the B–N distances (1.417(5)–1.435(7) Å), and this is even true for the boronic acid ester 8a, in which two π-donating O atoms compete for the same p_z(B) acceptor orbital. Apart from different grades of ⁻B=E⁺ double-bond character (E = O, N), other influencing factors are the smaller covalent radius of the O atom compared to the N atom (which also carries a Me substituent here) and a lower degree of intramolecular steric repulsion in the B,O-doped compounds. In agreement with this latter factor and the NMR data discussed above, we find much smaller dihedral angles between the B,O heterocycles and the tBuC₆H₄ substituents in 4a, 6a, 8a (14.4(2)–33.6(1)°) than between the B,N heterocycles and the tBuC₆H₄ ring in 4b, 5b (82.1(2)–83.0(1)°).

Fig. 2 Crystallographically determined solid-state structures of 4a, 4b, 5b, 6a, and 8a; H atoms were omitted for clarity. Selected bond lengths [Å] and dihedral angles [°]: 4a: B–O = 1.388(2), C(α)–C(β) = 1.345(2); BO ring//mesityl = 83.1(1), BO ring//tBuC₆H₄ = 17.0(1). 4b: B–N = 1.417(5), C(α)–C(β) = 1.356(6); BN ring//mesityl = 78.0(1), BN ring//tBuC₆H₄ = 83.0(1). 5b: B–N = 1.435(7), C(α)–C(β) = 1.328(7); BN ring//mesityl = 85.3(2), BN ring//tBuC₆H₄ = 82.1(2). 6a: B–O = 1.374(3), C(α)–C(β) = 1.340(3); BO ring//mesityl = 75.7(1), BO ring//tBuC₆H₄ = 28.3(1). 8a: B–O = 1.368(4)/1.381(5), C(α)–C(β) = 1.344(5)/1.345(4); BO ring//tBuC₆H₄ = 14.4(2)/33.6(1).

A comparison of the C–C distances in 4a, 6a, 8a and 4b, 5b also provides insight into the degree of cyclic π-delocalization along the molecules' cores: all C^α–C^β distances fall in the range 1.328(7)–1.356(6) Å, which is indicative for largely isolated C^α=C^β double bonds (ideal value: 1.34 Å).¹⁰⁷ The adjacent C^β–Ar distances, in turn, are consistently longer (1.437(6)–1.447(7) Å) than C^α=C^β and also longer than all C–C distances in the annulated benzene rings. Taken together, these data point toward Clar's sextets in the carbonaceous six-membered rings and attached –B=E–C=C– heterobutadiene fragments. We finally note that the ⁻B=N⁺ bond lengths found in our 1,2-B,N-naphthalene 4b and 1,2-B,N-anthracene 5b do not differ from those in corresponding 2,1-B,N-naphthalenes/anthracenes and are thus not influenced by the orientation of the B,N pairs.^95,96,108

Optoelectronic properties of 4a,b–6a,b and 8a, 8ab

Of the newly synthesized B,O- and B,N-PAHs compiled in Schemes 3 and 4, only the B,E-anthracenes 5a,b and the (B,N)₂-anthracene 6b show (quasi)reversible redox waves in their cyclic voltammograms (vs. FcH/FcH⁺; THF, room temperature, supporting electrolyte: 0.1 M [nBu₄N][PF₆]; Table 1). 5a (E_1/2 = −2.58 V) is easier to reduce by 220 mV than 5b (E_1/2 = −2.80 V), likely due to the higher electronegativity of the O atom compared to the N atom and the extension of the conjugation pathway into the coplanar tBuC₆H₄ ring (cf. the conformations adopted by 4avs.4b in the solid state; Fig. 2). For a comparable 2,1-B,N-anthracene bearing a mesityl group on boron as the sole substituent, a peak cathodic potential of E_p,c = −2.59 V (DMF) was reported.¹⁰⁸ These data indicate that the electrochemical properties of the B,E-anthracenes are not only influenced by the choice of E = N or O but also by subtle effects resulting from the orientation of the introduced B,E pairs.¹² Reduction of the (B,N)₂-anthracene 6b occurs at E_1/2 = −3.00 V and is thus somewhat harder to achieve than the reduction of the B,N-anthracene 5b (E_1/2 = −2.80 V).

Table 1 Photophysical, electrochemical, and computational data of the compounds 5a,b–6a,b and 8a, 8ab. Optical measurements were performed in CHCl₃, and electrochemical measurements were performed in THF (room temperature, supporting electrolyte: [nBu₄N][PF₆] (0.1 M), scan rate: 200 mV s⁻¹)

	λ abs [nm] (ε [M^−1 cm^−1])	λ onset [nm]	λ em [nm]	Φ PL [%]	E ^optG [eV]	E′^DFTG^e [eV]	E 1/2 [V]	E ^CVLUMO [eV]	E′^DFTLUMO^g [eV]	E′^DFTHOMO^h [eV]
a Each onset wavelength (λonset) was determined by constructing a tangent on the point of inflection of the bathochromic slope of the most red-shifted absorption maximum. b Resolved vibrational fine structure. c Quantum yields were determined by using a calibrated integrating sphere. d Optical band gap E^optG = 1240/λonset. e For better comparability with the E^optG values, the computed energy gaps (E^DFTG) have been scaled according to the following linear equation: E′^DFTG = 0.65 × E^DFTG + 0.48. f E ^CVLUMO = −4.8 eV − E1/2 (FcH/FcH^+ = −4.8 eV vs. vacuum level). g For better comparability with the E^CVLUMO values, the computed LUMO energies (E^DFTLUMO) have been scaled according to the following linear equation: E′^DFTLUMO = 0.65 × E^DFTLUMO −1.20. h Scaled HOMO energies (E′^DFTHOMO) were calculated from the scaled LUMO energies (E′^DFTLUMO) and the scaled HOMO–LUMO energy gaps (E′^DFTHOMO = E′^DFTLUMO − E′^DFTG). i Quantum yields measured in c-hexane. sh = shoulder.
5a	290 (45 925)	419	426 (sh)	20	2.96	2.87	−2.58	−2.22	−2.23	−5.10
	297 (45 316)		445	31^i
	328 (sh)
	339 (34 508)
	351 (20 644)
	379 (sh)
	395 (sh)
5b	268 (44 824)	422	428 (sh)	32	2.94	2.93	−2.80	−2.00	−2.03	−4.96
	326 (9621)		448	51^i
	341 (12 571)		470 (sh)
	379 (sh)
	405 (sh)
6a	312 (sh)	447	452	41	2.77	2.73	—	—	−2.27	−5.00
	326 (sh)		475	49^i
	343 (sh)		507 (sh)
	360 (75 215)
	373 (56 253)
	404 (13 108)
	425 (8628)
6b	347 (27 027)	422	426	16	2.94	2.98	−3.00	−1.80	−1.80	−4.78
	388 (sh)		446	52^i
8a	270 (13 015)	399	397	43	3.11	3.02	—	—	−2.20	−5.22
	314 (25 351)		419	52^i
	329 (sh)		443
	366 (5963)		468
	386 (4312)		510 (sh)
8ab	307 (30 228)	412	398 (sh)	34	3.01	2.98	—	—	−2.03	−5.01
	315 (sh)		415	44^i
	329 (27 393)		434
	374 (10 481)		461 (sh)
	393 (sh)		491 (sh)

---

UV/vis spectra were recorded for 4a,b–6a,b and 8a, 8ab (CHCl₃); photoluminescence spectra of these compounds were measured both in CHCl₃ and c-hexane (Fig. 3 and Table 1). The naphthalene derivatives 4a,b show low photoluminescence quantum efficiencies of Φ_PL < 10% and will therefore not be discussed further. We first focus on the onsets of absorbance (λ_onset), since these are directly correlated with the HOMO–LUMO energy gaps (E^opt_G). The λ_onset values of 5a/5b (419/422 nm) are almost identical, which is also true for the emission wavelengths λ_em (426/428 nm). Thus, despite their different LUMO levels, E^opt_G is the same for the two B,E-anthracenes. In contrast, both the onset of absorbance and the emission wavelength are bathochromically shifted for 6a compared to 6b (λ_onset = 447 vs. 422 nm; λ_em = 452 vs. 426 nm). Here, the impact of two vs. zero coplanar tBuC₆H₄ substituents and an associated enlarged π-electron system in 6a may play a role.

Fig. 3 (a) Normalized UV/vis absorption (solid lines) and emission (dashed lines) spectra in CHCl₃ of compounds 5a,b–6a,b. (b) Normalized UV/vis absorption (solid lines) and emission (dashed lines) spectra in CHCl₃ of compounds 8a and 8ab.

If recorded in c-hexane, all emissions are detected at somewhat shorter wavelengths compared to measurements in CHCl₃ (Δ(λ_em) = 260–920 cm⁻¹), the vibrational fine structures are even better resolved, and quantum efficiencies of Φ_PL = 31% (5a), 51% (5b), 49% (6a), and 52% (6b) are obtained. The trends in the experimentally determined optical band gaps E^opt_G(5a) ≈ E^opt_G(5b) ≈ E^opt_G(6b) > E^opt_G(6a) are well reproduced by quantum-chemical calculations (cf. the scaled values E′^DFT_G in Table 1). The calculations also confirm a less cathodic reduction potential of 5a compared to 5b, reflected by the lower computed LUMO energy of 5a.

The gas-phase structures and frontier-orbital configurations of 6a/6b and their carbonaceous congener 6 are exemplarily shown in Fig. 4. Orthogonally positioned Mes rings are seen throughout, while the tBuC₆H₄ substituents in 6a are less twisted with respect to the heterocyclic core than in 6b, which agrees well with the X-ray crystallography results (Fig. 2). Compound 6 possesses a smaller energy gap E′^DFT_G = 2.63 eV than 6a (2.73 eV) and 6b (2.98 eV). As an important difference between our B,N-anthracenes 5b/6b and Liu's positional isomers 2,1-B,N-anthracene and 2,6-B₂-1,5-N₂-anthracene, the π systems of the C^α=C^β bonds contribute strongly to the HOMOs of the former compounds (Fig. 4) but only negligibly to those of the latter.⁷⁹

Fig. 4 DFT-calculated nodal structures and scaled energy levels of the frontier orbitals of the parental 1,5-dimesityl-3,7-di(tBuC₆H₄)anthracene (6), 6a, and 6b (HOMOs bottom, LUMOs top; isovalue of the isosurface plots: 0.05 a₀^−3/2; B3LYP/6-31G*).

The optical properties of the B,O₂- and B,N,O-phenalenyls are similar and therefore do not require further discussion (Table 1 and Fig. 3b). However, we note pleasingly high photoluminescence quantum efficiencies of Φ_PL = 52% (8a) and 44% (8ab) in c-hexane.

Late-stage derivatization of the (B,O)₂-perylene 9

With increasing knowledge about B,N-PAHs it becomes more and more obvious that functional groups do not necessarily have to be introduced before the cyclization step. Late stage derivatization, a widely used tool in PAH chemistry, is also possible and often remarkably regioselective (see corresponding reactions on 2,1-B,N-benzenes and 2,1-B,N-naphthalenes).¹⁶ Given the higher reactivity of B,O- compared to B,N-PAHs (see above), we were interested in exploring whether late-stage functionalizations of the former are also possible, while maintaining the structural integrity of the heterocyclic scaffold. For this purpose, we selected the parent (B,O)₂-perylene 9 (Scheme 5), which allows a direct comparison with its highly symmetric carbonaceous counterpart. As functional group to be introduced, the pinacolatoboryl (Bpin) substituent was chosen.

Scheme 5 Transformation of (B,O)₂-perylene 9 to the doubly Bpin-substituted (B,O)₂-perylene 10 by Ir-catalyzed borylation and their structures in the solid state. Reagents and conditions: (i) 3.1 equiv. B₂pin₂, 3 mol% [Ir(COD)(μ-OMe)]₂, 6 mol% dtbpy, THF, 80 °C, 42 h.

Similar to other (B,O)₂-perylenes,⁵⁵ compound 9 was synthesized by double cyclization of the diethynylated 9,10-dihydroxy-9,10-dihydro-9,10-diboraanthracene derivative using the [Au(PPh₃)NTf₂] catalyst (Scheme 5).¹⁰⁹ The successful preparation of 9 thus demonstrates the applicability of our method to substrates carrying terminal alkynes (see the ESI† for full details). Treatment of 9 with 3.1 equiv. of B₂pin₂ in the presence of the catalyst system 1/2 [Ir(COD)(μ-OMe)]₂/dtbpy^110–112 led to a quantitative (TLC) double C–H borylation at the two C atoms in α-position to the O atoms and gave the corresponding product 10 in 63% yield after workup (Scheme 5; COD = 1,5-cyclooctadiene, dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridine). No mono-, tri-, or tetraborylated derivatives were observed; an increase in the amount of added B₂pin₂ to 5 equiv. did not lead to higher borylation levels either. The regioselectivity of the functionalization reaction was confirmed by X-ray crystallography (Scheme 5) and NMR spectroscopy. 10 gives rise to two resonances in the ¹¹B NMR spectrum and four resonances in the aromatic region of the ¹H NMR spectrum. Three doublets of doublets (each integrating 2H) can be assigned to the C₆H₃ fragments and the remaining singlet (2H) to the protons at the positions β to the O atoms (^H,CHMBC experiment).

The regioselectivity of the borylation of 9 is based on two factors: (i) the key intermediate of the catalytic cycle, fac-[Ir(dtbpy)(Bpin)₃], is a rather bulky metal complex that avoids performing the rate-determining C–H-activation step at sterically encumbered positions ortho to ring junctions or other substituents.^113,114 Activation of the peri-C–H bonds of 9 (or perylene) is therefore unfavorable. (ii) Of the four remaining C–H bonds, the two adjacent to the electronegative O atom, which is also smaller than a neighboring C–H unit, react preferentially.¹¹⁵ In contrast to the selective diborylation of 9, the corresponding reaction between carbonaceous perylene and 2.2 equiv. of B₂pin₂ furnished a mixture of mono- (11), di- (three isomers, 12a–c), tri- (13), and tetraborylated (14) products (Scheme 5). While the perylenes carrying different numbers of Bpin substituents have been separated by HPLC, it has been found impossible to separate 12a–c. On the other hand, the 2,5,8,11-tetraborylated perylene 14, which does not (yet) have a (B,O)₂-analogue, was accessible in 83% yield by using 4.4 equiv. of B₂pin₂ and the Ir catalyst (Scheme 5).^{114,116–118}

Conclusions

We have shown that the mild Au(I)-mediated cyclization of aryl(amino)boranes or aryl boronic and borinic acids with ortho-positioned C≡C bonds is an ideal tool for the synthesis of singly and doubly B,N- or B,O-doped polycyclic aromatic hydrocarbons (PAHs). We see the following advantages of this new method over many of the existing protocols: (i) boronic and borinic acids, which are nowadays standard reagents in every synthesis-oriented laboratory, serve as starting materials. (ii) The use of sophisticated organometallic reagents or corrosive boron halides is avoided. (iii) Modularity is achieved by the fact that the aminoboranes required for the synthesis of the B,N-PAHs are accessible in one step from the boronic and borinic acids required for the fabrication of the B,O-PAHs. The cyclization protocols presented herein thus represent a significant addition to the currently available toolbox for doping PAHs with main-group elements. Moreover, by using an unsubstituted (B,O)₂-perylene as the substrate for Ir-catalyzed C–H borylation, we (i) present a rare example of late-stage functionalization of a B,O-PAH and (ii) show that the exchange of two C=C for ⁻B=O⁺ bonds leads to dramatically improved product selectivity compared to the case of the carbonaceous perylene.

Author contributions

T. K. performed the late-stage derivatization of compound 9; O. O. synthesized and characterized all other compounds. M. B. performed the X-ray crystal structure analyses. H.-W. L. and M. W. supervised the project. The manuscript was written by M. W. and O. O. and edited by all the co-authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge Aleksandar Lucic for his assistance with manuscript preparation.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2058862–2058872, 2068160, and 2068161. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc00543j

‡ Contributed equally.

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