Carborane–arene fused boracyclic analogues of polycyclic aromatic hydrocarbons accessed by intramolecular borylation

Arenes are 2D aromatics while dicarbadodecaborane clusters are branded as 3D aromatic molecules. In this work we prepare molecules that feature fused 2D/3D aromatic systems that represent boron-doped analogues of polycyclic aromatic hydrocarbons. The electron withdrawing nature of the ortho-carborane substituent enables swift arene borylation on boron bromide or hydride precursors to furnish five- and six-membered boracycles in conjugation with the arene. The mechanism was modeled by DFT computations implying a concerted transition state and analyzing the photophysical properties revealed high quantum yields in the six-membered systems.


Introduction
Polycyclic aromatic hydrocarbons (PAHs) have garnered attention in electronic materials as the extended conjugation enables electron transport. 1Traditional PAHs are composed of 2D aromatic p-systems and imbedding a tricoordinate boron center in the network can alter the energetics of the frontier molecular orbitals to inuence the photophysical, electronic, and magnetic properties. 2 Icosahedral dicarbadodecaborane clusters (C 2 B 10 H 12 ) are considered 3D aromatics and are emerging as attractive motifs to incorporate into extended networks due to their high stability and electron withdrawing effects when C-bound. 3 The incorporation of carboranes into extended p-systems is rare and presents an opportunity to expand chemical space beyond purely 2D PAHs.
In unifying 2D aromatics with 3D aromatics, there are examples that feature fully carbon based arenes in conjugation, 3i,j,4 however, PAH systems that include a boracycle are limited to two examples.Siebert, Nie, and coworkers prepared a variant of anthracene with boron atoms linking a benzene and ortho-carborane (A) by reaction of dilithio-ortho-carborane with 1,2-C 6 H 4 ( i Pr 2 NBCl) 2 . 5Marder, Braunschweig, and coworkers reported the intramolecular isomerization of 1,2-bis-bora-uorenyl-ortho-carborane to the hybrid borole/arene carboranefused PAH B (Fig. 1). 6Both A and B feature the B 2 C 4 ring that share the carborane and arene system, but the B 2 C 4 motif is the only boracycle reported to link 2D and 3D aromatics.
The intramolecular C-H borylation of arenes has been leveraged to access conjugated boracycles, exemplied in the elegant work by Yamaguchi, 7 Hatakeyama, 8 Ingleson, 9 Feng, 10 and others. 11A highly electrophilic boron center is crucial for these transformations and oen requires harsh reaction conditions or external activators.11e, 12 An additional challenge is selectivity in discriminating the C-H bonds of the arene.In the pursuit of carborane-fused polyaromatic frameworks that feature BC 4 and BC 5 rings, we surmised that boranes featuring an arene-substituted carborane could undergo intramolecular arene borylations under mild conditions via C-H activation to access carborane-fused PAHs.

Results and discussion
Organoboron halides and hydrides are reagents effective in intramolecular arene borylation.Given that a borane bearing a halide and carborane can be accessed in a single step from an ortho-carborane precursor, we targeted BrB Ph oCb 2 .Lithiation of 1-phenyl-o-carborane ( Ph oCb) with n-BuLi and subsequent reaction with half an equivalent of BBr 3 provided BrB Ph oCb 2 and the identity was conrmed by single crystal X-ray diffraction studies (Fig. 2a).Upon heating BrB Ph oCb 2 at 110 °C in toluene for 36 hours, intramolecular arene borylation of the pendant phenyl group on one of the Ph oCb substituents occurred to furnish the ve-membered boracycle 1, as identied by single crystal X-ray diffraction (Fig. 3).This compound represents a hybrid 2D/3D aromatic analogue of 9-borauorene.The 2D analogue, 9-borauorene, was discovered in 1963 (C) 2n,13 with variants bearing an ortho-carborane on the boron (D) 14 disclosed last year and variants of the 3D analogue featuring orthocarboranes have been reported in the past 5 years (E, Fig. 2c). 15t has been reported that the reactive HBr byproduct in dehydrobrominative borylations can degrade the boracycle to result in mixtures and lowers the yield.7a,8b Additionally, high temperatures and long reaction times are typically required.Compound 1 was isolated in 60% yield from the dehydrobromination pathway that required heating to 110 °C for 36 hours.Hydride reagents are also competent for intramolecular arene borylation and oen occur under milder conditions and with a more benign byproduct, H 2 . 16In an attempt to prepare the requisite hydride HB Ph oCb 2 for borylation to access 1, BrB Ph oCb 2 was reduced by triethylsilane that resulted in spontaneous dehydrogenative arene borylation to give 1 in a higher isolated yield of 81%.Conducting the reaction in C 6 D 6 and analyzing by in situ 1 H NMR spectroscopy reveals the generation of H 2 (4.47 ppm) and BrSiEt 3 (0.68 ppm, qd; and 0.90 ppm, t), implying BrB Ph oCb 2 was reduced to HB Ph oCb 2 that undergoes a tandem intramolecular electrophilic arene borylation (Fig. S28 †).11e,17 It is known that extending conjugation can alter the properties of PAHs.Hybrid borauorene analogue 1 features a carborane, BC 4 ring, and benzene linked.We were curious if a variant of 1 with extended conjugation could be accessed by  the borylation route and if the reaction would be selective if two distinct C-H bonds are available for borylation.For this, 1-(2naphthyl)-o-carborane ( 2Np oCb) was installed on boron by the analogous lithiation route to generate BrB 2Np oCb 2 in 70% yield with the structure conrmed by single crystal X-ray diffraction (Fig. 3).Treatment with triethylsilane triggered the borylation to furnish the naphthyl-fused PAH variant of 1 in 80% yield (Fig. 2b) with the identity determined by an X-ray diffraction study.In 2 there are two ortho-hydrogen atoms on the 2-naphthyl group that could undergo C-H borylation to give two different isomers, but surprisingly only the less encumbered position was borylated.For 1, the 11 B{ 1 H} NMR resonance for the non-cluster boron atom is in the tricoordinate region and is similar to the bromide precursor (BrB Ph oCb 2 : 64.1 ppm, 1: 62.7 ppm).The corresponding tricoordinate boron signal for BrB 2Np oCb 2 is at 64.5 ppm, but we were not able to observe the signal for 2 attributed to the low solubility and peak broadening.
The selective borylation to generate borauorene PAH analogue 2 prompted us to investigate whether 5-or 6membered rings would be generated if from an arene with two distinct C-H bonds poised to generate either ring size.Additionally, the second spectator carborane ligand on boron may not be essential for borylation to occur, thus we prepared precursors with only one carborane.Boranes featuring 1naphthyl-and 9-phenanthryl-ortho-carborane ( 1Np oCb and Phe- n oCb, respectively) were synthesized by deprotonation of the carborane reagent followed by reaction with PhBBr 2 .Dehydrobrominative borylation of the putative bromo-borane occurred spontaneously at 23 °C for both species.Single crystal X-ray diffraction studies revealed that the reactions furnished 6membered boracycles 3 and 4 with no evidence of the vemembered product indicating that formation of the BC 5 ring system is preferred over the BC 4 system (Scheme 1, Fig. 4).This is also in line with the mild reaction conditions of the dehydrobromination to the 6-membered ring products 3 and 4 in comparison to the ve-membered products 1 and 2 requiring heating (>110 °C).Diagnostic tricoordinate 11 B{ 1 H} NMR signals were detected at 64.0 ppm (3) and 63.7 ppm (4).
Density functional theory calculations (DFT) were carried out using a B3LYP-D hybrid functional with 6-311+G(d,p) basis set to shed light on the reaction mechanisms and observed selectivity.In the formation of 1, the dehydrogenation pathway from HB Ph oCb 2 is both kinetically and thermodynamically favored over dehydrobromination of BrB Ph oCb 2 (HB Ph oCb 2 DG ‡ = 63.5 kJ mol −1 , DG°= −99.6 kJ mol −1 c.f. BrB Ph oCb 2 DG ‡ = 96.0kJ mol −1 , DG°= −50.9 kJ mol −1 , Fig. 5).Both pathways proceed via a concerted transition state by a s-bond metathesis between the arene C-H and the B-Br or B-H moiety.The results are in line with the experimental results with dehydrogenation of HB Ph oCb 2 occurring spontaneously while the dehydrobromination of BrB Ph oCb 2 required elevated temperature.
The selectivity of forming ve-membered boracycle 3 0 over the possible six-membered product 3 was also investigated as there are two possible C-H bonds available for borylation of the proposed Scheme 1 Synthesis of six-membered boracycles 3 and 4 by dehydrobromination.
Fig. 4 Solid-state structures of 3 (left) and 4 (right).Hydrogen atoms are omitted for clarity and thermal ellipsoids are drawn at the 50% probability level.Br(Ph) 1Np oCb-borane intermediate (Fig. 5b).Within this H a is the hydrogen that if activated would generate the six-membered boracycle and H b is the hydrogen that would generate the vemembered boracycle.From the calculations, the H a activation barrier is lower than that of H b (DG ‡ H a = 48.2kJ mol −1 cf.DG ‡ H b = 57.9kJ mol −1 ) with both proceeding through a concerted s-bond metathesis pathway.In addition to the kinetic preference for 3, the 5-membered product is also thermodynamically favored (3: DG°= −70.8 kJ mol −1 c.f. 3 0 : DG°= −56.1 kJ mol −1 ) with the DFT studies in support of the observed experimental results.
The electrochemical properties of 1-4 were investigated by cyclic voltammetry.For 1 and 2, a two electron reduction close to −2.0 V followed by two one electron oxidation events were observed that are similar to the reported redox activity of 1,2substituted ortho-carborane species (Fig. S39 †).3c,e,18 For 3, an irreversible reduction at −1.39 V and two irreversible oxidations at 1.26 V and 1.47 V were observed.In the cyclic voltammogram of 4, the same pattern was observed with a reduction at −0.27 V and oxidations at 1.39 V and 1.63 V (Fig. S39 †).
Carborane substituents on boron serve as powerful electron withdrawing groups when C-bound that enhance Lewis acidity at boron.The Lewis acidities of 1-4 were evaluated by the Gutmann-Beckett method revealing that 1 and 2 are more acidic than B(C 6 F 5 ) 3 , but less acidic than tris(ortho-carboranyl) borane (BoCb 3 , Table 1).3k,19 The compounds with only one carborane bound to boron, 3 and 4, are less Lewis acidic than B(C 6 F 5 ) 3 .This is in line with the additive effect of additional carboranes bound to the boron that has been demonstrated by computations.3l The frontier molecular orbitals of 1-4 as well as the fully 2D analogues that replace the o-carborane with a benzene ring were computed by DFT methods.The LUMOs of 1-4 have the greatest contribution on the tricoordinate central boron atom and have lower energies than that of their corresponding 2D analogues (Fig. 6).The HOMOs of 1-4 are delocalized on the arene p conjugated system (see ESI †).In comparing six membered boracyclic systems 3 and 4 to the reported 1-boraphenalenes by Ingleson, 9c the LUMOs of 3, 4, and 1-boraphenalene have similar contributions within the boraphenalene moiety.The LUMO energies of 3 and 4 (−2.79 eV and −2.67 eV) are in range of the reported 1-boraphenalenes (−3.07 to −2.55 eV).
The photophysical properties of 3 and 4 are listed in Table 2.No appreciable emission was observed for 1 or 2. Compound 3 exhibits an absorption maximum at 345 nm and emission maximum at 397 nm with a Stokes shi of 3796 cm −1 and good quantum yield of 0.74.The absorption maximum and emission maximum are blue shied in comparison to 1-hydroxy-1boraphenalene (l (abs)max = 374 nm, l (em) = 489 nm, F = 0.27) by Würthner 11d and the reported carbene stabilized boraphenalenes (l (abs)max = ∼360-450 nm, l (em) = 540-563 nm, F = 0.11-0.43)by Gilliard, 20 while the quantum yield is higher.The absorption maximum of 4 (355 nm) and emission maximum (452 nm) are red shied compared to those of 3, but blue shied compared to the aforementioned boraphenalenes, with a larger Stokes shi (6041 cm −1 ) and a quantum yield of 0.31, indicating 4 is a blue emitter.
To obtain more insight into the electronic properties of 1-4, TD-DFT calculations were carried out to determine the absorption spectra and the energies of the rst excited singlet at the CAM-B3LYP/6-31+G(d,p) level of theory using toluene as a solvent. 6The relevant frontier molecular orbitals of 1-4 and their energies are shown in Fig. S47, S49, S51, and S53, respectively (see ESI †).The calculated values for the lower energy S 0 /S 1 absorptions for 1-4 are in close agreement with the experimental values as the largest difference is 25 nm for 3 and the smallest is 2 nm for 1 (Table 2).The S 0 /S 1 transition in 1 and 2 display low oscillator strengths (1: f = 0.002, 2: f = 0.036), categorizing them as weak transitions.The S 0 /S 1 transition for compounds 3 and 4 display high oscillator strengths (3: f = 0.401; 4: f = 0.474), classifying them as strong transitions.

Conclusions
In conclusion, we reveal that selective intramolecular C-H borylation reactions are a facile route to generate rare 2D/3D fused   Multinuclear NMR spectra ( 1 H, 13 C{ 1 H}, 31 P{ 1 H}, 11 B{ 1 H}) were recorded on a Bruker Avance III HD 400 MHz or 600 MHz instrument.High-resolution mass spectra (HRMS) were obtained in the Baylor University Mass Spectrometry Center on a Thermo Scientic LTQ Orbitrap Discovery spectrometer using ESI.Melting points were measured with a Thomas Hoover Unimelt capillary melting point apparatus and are uncorrected.UVvis and uorescence data were collected on Varian UV-vis spectrometer and a Fluoromax-4 spectrometer, respectively.Single-crystal X-ray diffraction data were collected on a Bruker Apex III-CCD detector using Mo Ka radiation (l = 0.71073 Å).Crystals were selected under paratone oil, mounted on MiTe-Gen micromounts, and immediately placed in a cold stream of N 2 .Structures were solved and rened using SHELXTL, and gures produced using OLEX2.

Fig. 1
Fig. 1 Prototypical 2D aromatic and 3D aromatic systems and known fused 2D/3D variants A and B. The unlabelled vertices on the cluster represent carbon.

Fig. 3
Fig.3Solid-state structures of BrB Ph oCb 2 , BrB 2Np oCb 2 , 1, and 2 (from left to right).Hydrogen atoms are omitted for clarity and thermal ellipsoids are drawn at the 50% probability level.

Fig. 5
Fig. 5 Energy profile calculated for the conversion of (a) HB Ph oCb 2 and BrB Ph oCb 2 to 1, and (b) Br(Ph)B Np oCb to 3 and 3 0 (in kJ mol −1 ).

Table 2
Summary of photophysical properties of 1-4 of PAHs.Through this route we accessed unsaturated veand six-membered boracycles that link an arene and an orthocarborane.The ve-membered species represent analogues of 9-borauorene and six-membered systems analogues of borabenzoanthracene and borabenzotetraphene.Both dehydrobromination and dehydrogenation pathways were effective in the borauorene variants with the dehydrogenation pathway occurring spontaneously.The reaction barrier is lower for the 6membered boracycles with dehydrobromination occurring spontaneously.Computations support the experimental reaction conditions with pathway proceeding via a concerted s-bond metathesis transition state.The intramolecular borylation of arylcarboranyl boranes is demonstrated to be an effective method to incorporate carboranes into extended p-systems and presents opportunities to expand chemical space beyond purely 2D PAHs.All manipulations were performed under an inert atmosphere in a nitrogen lled MBraun Unilab glovebox or using standard Schlenk techniques.Chloroform-d and benzene-d 6 for NMR spectroscopy were purchased from Cambridge Isotope Laboratories, Inc., dried by stirring for over CaH 2 , distilled, and stored over 4 Å molecular sieves.All other solvents were purchased from commercial sources as anhydrous grade, dried further using a JC Meyer Solvent System with dual columns packed with solvent-appropriate drying agents, and stored over 3 or 4 Å molecular sieves.Triethylsilane, nBuLi, and BBr 3 were purchased from commercial sources and used without further purication.The o-carborane reagents and PhBBr 2 were synthesized using the literature procedures.21 a l Exp.abs (nm) b l Cal.abs (nm) c 3 a Experimental absorption maximum.bCalculated absorption maximum.c 3 is the molar extinction coefficient in M −1 cm −1 .dF n is the uorescence quantum yield.e F 5 and F 6 are calculated in toluene using quinine sulfate (quantum yield 0.546 in 0.5 M H 2 SO 4 ) as a reference.No signicant emission was observed for 1 or 2.analogues ppm; 11 B{ 1 H} NMR (128 MHz, CDCl 3 ): d = 64.1 (broad s), 4.8 (s), −2.8 (s), −7.2 (s), −9.6 (s) ppm; HRMS(−ESI): calcd 531.3703 for C 16 H 30 B 21 Br [M + H] − found 531.3743.