Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Abstract Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All‐ortho‐fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φ fl) of 18–24 % in solution, green or yellow solid‐state emission (Φ fl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10−2. Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φ fl of up to 47 % in CH2Cl2 and 25 % in the solid state. As revealed by the DFT calculations, their P–M interconversion pathway is more complex than that of H1. Single‐crystal X‐ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.


Introduction
In recent years,c hirality emerged as ac entral concept in the field of p-conjugated compounds.R apid progress in synthetic methodology of polycyclic aromatic hydrocarbons (PAHs) and nanobelts [1] contributed to the development of al arge variety of curved, contorted, and bent congeners. [2] Interest in these compounds is motivated by their unique solid-state packing,d ynamic nature,a nd chiroptical properties of configurationally stable derivatives.T uning of their properties is achieved mainly by substitution of their periphery with functional groups or, more recently,incorporation of heptagons and octagons. [3] In general, the performance of all-carbon PA Hs can be improved by utilization of heteroatoms. Introduction of main group elements into PA Hs entails significant perturbation of their electronic structures.P olycyclic heteroaromatics with fine-tuned properties are explored as functional chromophores and charge transport materials.S uch structural variation can also be used to achieve compounds with attractive features for coordination chemistry and catalysis. [4] Primary representatives of chiral PA Hs are screw-shaped molecules called helicenes. [4d, 5] In materials science,they were studied in the context of their chirality-determined organization in the solid state and its implications for charge transport in transistor and photovoltaic devices. [6] They were also identified as promising compounds for chiral light emission, since they exhibit high differential emission of right-and left-handed circularly polarized (CP) light quantified by the dissymmetry factor. [7] Yet, for application as CP luminescence emitters,f or instance in CP-organic light emitting diodes (CP-OLEDs), high dissymmetry factors are not sufficient. Organic materials should also show intense emission, preferably at high concentration or in the solid state.H owever,i ntersystem crossing typically lowers F fl of helicenes, [8] hence limiting their potential use in chiral optoelectronics.
p-Conjugated boron compounds have received recognition for their outstanding optical properties and are intensively studied in OLED devices. [9,10] Embedding boron into pconjugated scaffolds can provide materials with high electron affinity,electron mobility,and photovoltaic performance. [10,11] Moreover,t he versatility of BÀNd ative bonds enables the construction of stimuli-responsive materials and dynamic systems. [12] Thus,m erging benefits of boron with chirality could give rise to materials with unique characteristics and improved properties versus all-carbon analogues.
Although aplethora of organoboron molecules have been synthesized to date, [9][10][11][12][13][14] the availability of helically chiral congeners with boron in the p-conjugated core is still limited. Three-coordinate boron-fused helicenes were synthesized by Hart reaction and boron-assisted demethylative cyclization as the key steps, [15] at andem bora-Friedel-Crafts-type reaction, [16] intramolecular Yamamoto coupling of triarylborane, [17] or intramolecular electrophilic borylation. [18] Likewise, four-coordinate boron helicenes are rare.Inaddition to chiral O-BODIPYs and O-aza-BODIPYs with boron on the inner helicene rim, [19] only few other organoboron helicenes have been reported. Boron-bridging of [4]-and [6]helicenes with one or two flanking pyridine units [20] elongated the framework by two or four fused rings,whereas configurational stability in azabora [5]helicenes was achieved by substitution of terminal positions with sterically demanding groups. [21] Our objective is the synthesis of long helicenes consisting of multiple boron atoms.The attractiveness of such extended structures lies beyond the structural curiosity.T hese entities should display large circular dichroism, efficient symmetrybreaking spin transport, [22] and allow studies of exciton transport pathways in discrete molecules.A ccess to such elongated structures is,h owever,l imited due to synthetic limitations.T here are only few reports on long well-defined helicenes [23] with the record number of 19 fused rings for oxahelicenes. [24] To construct organboron helices,wepropose amodular approach in which flexible oligoaryl precursors are prepared in aconvergent synthesis from small achiral building blocks.L ewis acidic boron is introduced as "glue" to these species to join two or more subunits into fully fused scaffolds by formation of dative bonds with nitrogen or other heteroatoms.N oteworthy,c hirality cannot be ensured by bulky substituents at sterically hindered positions of the substructures in this case,n or can the boron atoms be included only in the terminal parts of the helical backbone. Thec onstruction of fully ortho-fused helices is energetically costly due to introduced steric strain. Areasonable means to facilitate the closure of azaborole rings would be incorporation of meta-fused or ac ombination of both ortho-a nd meta-fused units.T hus,w eh erein present the synthesis and properties of two types of molecules differing in the fusion point:f ully p-conjugated all-ortho-fused azabora [7]helicenes H1 and azbora [9]helicenes H2,which embody both angularly and linearly fused rings,a sp rimary substructures of organoboron helices,such as EH in Figure 1. As we will demonstrate, ac ombination of an on-planar geometry of helicenes with boron has as ynergistic effect on the emission of these emitters both in solution and in the solid state.

Results and Discussion
Synthesis. Biaryl BA and triaryl TA were synthesized by cross-coupling of 1-chlorobenzo[h]isoquinoline (BIQ-Cl) with borylated phenanthrene (PHE-Bpin)a nd benzene (BEN-Bpin)d erivatives (Scheme 1). BEN-Bpin was prepared by Miyaura borylation from commercially available 1,3-dibromobenzene.The syntheses of the other two building blocks are more demanding. PHE-Bpin was synthesized in seven steps from 2-bromobenzaldehyde and 2-methoxyphe-nylboronic acid. Cross-coupling thereof afforded formylsubstituted biphenyl, which was converted into the corresponding alkyne via Corey-Fuchs reaction. Thes ubsequent Pt-catalyzed ring closure produced methoxy-phenanthrene. Thef ollowing cleavage of the methyl ether,s ynthesis of ap seudohalide,a nd Suzuki-Miyaura reaction furnished PHE-Bpin. BIQ-Cl could be obtained in five steps.T he synthesis started from coupling of 3-bromo-4-methylpyridine with 2-formylphenylboronic acid, followed by base-promoted cyclization to benzo[h]isoquinoline (BIQ). Oxidation of BIQ, rearrangement of N-oxide to the corresponding lactam, and, finally,chlorination thereof with POCl 3 afforded BIQ-Cl.F or reproducibility and ease of purification, it is advised to perform the last three reactions in as tepwise manner (method B, Supporting Information) rather than in one pot (method A). Thed etailed synthesis of the small building blocks is presented in the Supporting Information. To execute the introduction of boron atoms into these intermediates,we adapted the method reported by Murakami [25] with some modifications.A ccording to this protocol, BA and TA were reacted with BBr 3 in the presence of i-Pr 2 NEt to yield complexes H1-Br 2 and H2-Br 4 ,respectively.The synthesis was accomplished by the exchange of the bromide with alkyl or aryl ligands in overall yields of 7-8 %f or H1 (10 steps) and 30-32 %f or H2 (8 steps). Whereas substitution with Me and Et could be performed under mild conditions,introduction of Ph groups required elevated temperature.T riorganylaluminum reagents proved superior to diorganylzinc complexes for this transformation. Not only was the reaction of H1-Br 2 with Et 2 Zn lower-yielding than the analogous reaction with Et 3 Al, but it was also more sluggish and had to be performed at higher temperature (see the Supporting Information). All compounds except H2-Et 4 feature excellent stability against

Angewandte Chemie
Research Articles light, moisture and air. H2-Et 4 ,o nt he other hand, decomposed over time.F or this reason, its further characterization was not carried out.
Solid state structural analysis. Single crystals of racemic H1-Me 2 and H2-Me 4 suitable for X-ray analysis were obtained by slow evaporation of chloroform solutions,a nd those of H1-Ph 2 and H2-Ph 4 by diffusion of hexane into CH 2 Cl 2 solutions. H1-Me 2 and H2-Me 4 crystallized in the P2 1 /n space group,t he other two in the P 1s pace group.T he B À Nb ond lengths of 1.597(3)-1.612 (6) in the azaborole rings confirm strong Lewis pair interactions (Figures S63-S66 in the Supporting Information). In the solid state,compounds H1 and H2 adopt helical conformations.T he sums of the five dihedral angles for the inner helicene rim (f)ofH1-Me 2 and H1-Ph 2 are 94.28 8 and 88.48 8,r espectively,w hich are intermediate values between those of phospha-and sila- [7]helicenes (95-1008 8) [26] and other hetero [7]helicenes (79-888 8). [27] Thedistortion is largely determined by the geometry of the five-membered rings closely related to the type of ah eteroatom. Thea ngles between two formal C=Cd ouble bonds of azaborole rings are approximately 388 8,large enough to ensure asubstantial overlap of terminal rings and, in turn, excellent configurational stability of H1.
Thed ihedral angle between the mean planes of terminal rings in H1-Me 2 is 28.18 8 (Figure 2), smaller than in other hetero [7]helicenes.Aslightly larger q AG (33.38 8)was observed in H1-Ph 2 ,w hich is comparable to that in pristine carbo- [7]helicene (32.38 8). [28] Such small splay angles indicate en-hanced intramolecular p-p interactions in both molecules. Thea ngles defined by rings A-E (formally azabora- [5]helicene) are 27.48 8 and 36.78 8 (Figure 2). Thecorresponding angles in H2 molecules are generally larger.I nH2-Me 4 one BIQ wing is more strongly bent than the other unit (q AE and q EI of 38.1 and 45.68 8). H2-Ph 4 is even more distorted (q AE and q EI of 27.6 and 50.48 8), which results in alarger q between the terminal rings (q AI of 23.68 8 vs.9 .28 8 for H2-Me 4 ). Since the helical cores of the optimized geometries are almost symmetrical, these differences must originate from the crystal packing modes.
In the packing arrangements of all four helicenes,stacks of (P)-and (M)-enantiomers could be observed. Themolecules are arranged in as lipped fashion forming stacks with interplanar distances of 3.35-3.59 . H1-Me 2 and H2-Me 4 arrange in as andwich herringbone pattern through C À H···p interactions with adjacent dimers. H1-Ph 2 and H2-Ph 4 share ad ifferent packing arrangement. Thei somers are packed in an alternating fashion forming sheet structures with multiple CÀH···p interactions also involving Ph rings ( Figure S67 in the Supporting Information).
Absorption and emission properties. Thep hotophysical data are summarized in Table S1 (Supporting Information). H1 show moderate molar absorption coefficients (e)( 7.6-9.7 10 3 m À1 cm À1 ). Thelowest-energy absorption bands of H1 are centered at 426-432 nm with well-resolved vibronic progressions at 407-412 nm and correspond to the yellow color of CH 2 Cl 2 solutions (Figure 3a). Absorption maxima of H1 are bathochromically shifted vs.a ll-carbon analogues [29] and related hetero [7]helicenes. [26,27] l abs of the compounds bearing two boron atoms are blue-shifted to 404-406 nm. This pronounced shift is accompanied by an increase in intensity (e of up to 19.7 10 3 m À1 cm À1 ). In contrast to H1,t he fine structure is almost entirely lost. Thesignificant hypsochromic shift vs. H1 is likely due to asomewhat disrupted conjugation along the helical core.S ince H2 features high flexibility (see below), it is possible that various conformers coexist in solution differing in the effective p-conjugated pathway.T he HOMOs and LUMOs of H1 are delocalized over the entire helicene cores with somewhat larger coefficients on PHE and BIQ moieties,r espectively ( Figure S74 in the Suppporting Information). TheH OMOs of H2 involve the whole pconjugated systems with larger coefficients at the pyridine and central benzene rings,a nd small contributions from the Ph substituents for H2-Ph 4 .T he LUMO and LUMO + 1o f H2 are more or less uniformly delocalized over both BIQ and BEN moieties.A ccording to the time-dependent density functional theory (TD-DFT) calculations at the CAM-B3LYP [30] -D3BJ [31] /def2-TZVP [32] level (solvent CH 2 Cl 2 , PCM model) the lowest energy absorption bands of H1 mainly correspond to the HOMO!LUMO transitions (84 %), while those of H2 are superpositions of two transitions and are predominantly attributed to the HOMO! LUMO (76-80 %, oscillator strength f % 0.45), and HOMO! LUMO + 1( % 70 %, f % 0.72-0.10) transitions ( Figure S75 and Tables S3-S6 in the Supporting Information) so that very little charge transfer is to be expected. Thecompounds show blue (H1-Me 2 , H1-Et 2 ,a nd both H2)o rg reen (H1-Ph 2 ) fluorescence with maxima at 459-477 nm (Figure 3b), which translates to Stokes shifts of 1700-1800 cm À1 for H1-Me 2 and H1-Et 2 ,and 2200-2250 cm À1 for H1-Ph 2 ,and H2 compounds. l fl of H2 are,l ike the absorption bands,b lue-shifted versus emission maxima of H1.T he emission spectra are devoid of vibronic structures.F luorescence quantum yields (F fl )f all in the range of 18-24 %a nd are markedly higher compared to carbohelicenes consisting of only six-membered rings, [33] whereas F fl of structurally similar compounds strongly depend on the atom at the fusion point of ac entral fivemembered ring (from 0.1 to 23 %f or heteroatoms and up to 40 %f or carbon). [26,27,29,34] H2 are highly emissive with F fl of 43-47 %.
Absorption of spin-coated films is slightly red-shifted ( Figure S68 and Table S1 in the Supporting Information), probably due to somewhat increased intramolecular interactions in the solid state,w ith the absorption maxima of H1 located between 433 and 438 nm and of H2 at 402 nm. In general though, the line shapes resemble those of spectra in solution. Only small variations in the intensity ratios of the 0-0to0-1 vibronic transitions of the S 0 !S 1 transition from ca. À5% for H2-Me 4 to + 9% for H1-Ph 2 could be observed. Emission was measured for amorphous powder samples of H1 and H2.The fluorescence spectra are presented in Figure S69, while the images of the powders under visible and UV irradiation are shown in Figures 3a and S70 in the Supporting Information. H1-Me 2 , H1-Ph 2 ,a nd H2-Ph 4 show impressive F fl values of 17, 23, and 25 %, respectively. [35] To our knowledge,t hese F fl values are among the highest quantum yields reported for helicenes to date. [36] However,emission of H1-Et 2 and H2-Me 4 is substantially weaker (3 %a nd 8%, respectively). As opposed to H2,s howing blue fluorescence both in solution and in the powder with only small shifts of the emission spectra, the spectra of powder samples of H1 are red-shifted by approximately 2000 cm À1 for both alkyl derivatives and almost 3000 cm À1 for H1-Ph 2 as compared to their spectra in CH 2 Cl 2 .T hese pronounced spectral shifts result in achange of the emission color from blue to green and green to yellow,r espectively.
Essentially,t he F fl values of H1-Me 2 and H1-Ph 2 do not decrease upon going from solution to the solid state.I n contrast to other popular emitters,s uch as BODIPY [37] or perylene bisimide (PBI) dyes, [38] these organoboron helicenes do not undergo aggregation-caused quenching of fluorescence.Whereas for PBIs,extensive molecular engineering via introduction of voluminous substituents is necessary in order to retain high emission properties in the solid state, [39] we could achieve this for H1-Me 2 and H1-Ph 2 without any special treatment, since their inherent non-planar geometry effectively reduces intermolecular p-p interactions.I na ddition, the advantage of this molecular design manifests itself in the fact that the change in the emission color of H1 could be obtained by simply replacing Me with Ph substituents,hence without any modification of the p-conjugated core.
Electrochemistry. Thee lectrochemical behavior of H1 and H2 was investigated by cyclic voltammetry (CV) and pulse techniques in CH 2 Cl 2 in the presence of Bu 4 NPF 6 as as upporting electrolyte and calibrated versus ferrocenium/ ferrocene (Fc + /Fc). As shown in Figures 3c and S71 in the Supporting Information, all H1 compounds exhibit one reversible reduction wave at À2.11-À2.13 Vf or alkyl derivatives.The reduction potential of H1-Ph 2 is anodically shifted by ca. 0.1 V. Thee ffect of substituents on boron is more pronounced for oxidation. Exchange of alkyl with Ph substituents results in an anodic shift of ca. 0.2 V. For H1-Et 2 ,the second oxidation at + 1.57 Vcould be recorded. The differences in redox potentials are rather small. Thus,t he band gaps differ only slightly,w hich coincides with the shifts in the absorption spectra of these compounds.Avoltammogram of H2-Me 4 reveals two oxidation processes at + 0.86 and + 1.22 V, and one irreversible reduction at À2.24 V. On the contrary,o nly one oxidation (+ 1.08 V) and two reduction processes at À2.09 and À2.31 Vw ere observed for the Ph congener.
Chiroptical properties. Enantiomers of H1-Me 2 , H1-Et 2 , and H1-Ph 2 were resolved by HPLC on ac hiral stationary phase (for details see SI). As shown in Figure 4, their electronic circular dichroism (ECD) spectra recorded in CH 2 Cl 2 revealed perfect mirror-image relationships.T he absolute configuration of the enantiomers was assigned by comparison of the experimental ECD with the TD-DFTsimulated ECD spectra ( Figure S76 in the Supporting In- formation). Thus,the first and second fractions correspond to (P)-and (M)-enantiomers,r espectively.A se xpected, the ECD spectra of (P)-H1-Me 2 and (P)-H1-Et 2 have similar profiles,t hey differ,h owever,i ni ntensity.A ccordingly,t heir spectra exhibit positive Cotton effects (CEs) in the ranges of 272-405 nm (De =+158 m À1 cm À1 at 323 nm; De = + 19 m À1 cm À1 at 282 nm) and 273-407 (De =+115 m À1 cm À1 at 324 nm; De =+31 m À1 cm À1 at 284 nm), respectively.N egative CEs are observed in the ranges of 229-272 (De = À148 m À1 cm À1 at 248 nm) and 405 to ca. 450 nm (De = À7 m À1 cm À1 at 426 nm) for (P)-H1-Me 2 and 231-273 (De = À173 m À1 cm À1 at 247 nm) and 405 to ca. 450 nm (De = À6 m À1 cm À1 at 428 nm) for (P)-H1-Et 2 .T he ECD spectrum of (P)-H1-Ph 2 revealed ad ifferent profile to those of alkyl derivatives with negative ECD at 294 nm (De = À22 m À1 cm À1 ) and astrong negative CE at 254 nm (De = À119 m À1 cm À1 ). A positive CE appears in the range of 307-405 nm (De = + 100 m À1 cm À1 at 325 nm), and aweak negative CE between 405 and 459 nm (De = À11 m À1 cm À1 at 432 nm). Thei ntensities of the longest wavelength bands of all compounds are low and so are the corresponding anisotropy factors (g abs ) (0.7 10 À3 -1.4 10 À3 ). On the other hand, the strong ECD bands located at 323, 324, and 325 correspond to the highest j g abs j of 1.12 10 À2 ,9 .1 10 À3 ,a nd 7.6 10 À3 ,r espectively.I n particular, H1-Me 2 exhibits excellent chiroptical performance with j g abs j exceeding those of carbo [6]helicene, [40] an umber of multipoles, [3a, 41] and approaching j g abs j of helicene nanoribbons [42] and double [8]helicene. [43] P-M interconversion. As opposed to H2 (see below), helicenes H1 are configurationally stable.N or acemization was observed for as olution of (P)-H1-Me 2 over am onth at room temperature.A ccording to DFT calculations (B3LYP-D3BJ/def2-SVP, [28] solvent CH 2 Cl 2 ,P CM model), the P-M interconversion of H1-Me 2 proceeds via one transition state of C 1 symmetry ( Figure 5). Thei nversion barrier (DG ¼ 6 )i s 152.3 kJ mol À1 (36.4 kcal mol À1 )a nd is comparable to the configurationally stable hexahelicene (36.2 kcal mol À1 ). [44] For comparison, DG ¼ 6 for azabora [5]helicenes H3 of 57.8 kJ mol À1 (13.8 kcal mol À1 )( Figure S77 in the Supporting Information) is considerably lower and the formation of the configurationally stable helicene would require introduction of ab ulky substituent into as terically hindered position of the Nheterocycle or an all-carbon subunit. Thermal racemization of (P)-H1-Me 2 in 1,2-dichlorobenzene at 180 8 8Cw as monitored by HPLC following the decay of the enantiomeric excess.The Gibbs free energy of activation for racemization was determined to be 142.6 kJ mol À1 (34.1 kcal mol À1 ), which corresponds to ar acemization half-life of 70.2 min at 180 8 8Ca nd approximates to the calculated value.S uch ah igh barrier indicates that the devices incorporating these materials would not be adversely affected by racemization during the fabrication process,e ven at relatively high temperatures.I n contrast to H1,t he interconversion of H2 occurs via three transition states due to the presence of ahydrogen atom of the central benzene ring on the inner rim of H2.Inprinciple, H2 can be considered as two azabora [5]helicenes (H3)w ith one joint benzene ring, each undergoing P-M interconversion. In  the first step, (P,P)-H2-Me 4 converts to al ocal minimum (P,M)-H2-Me 4 (LM1)with both BIQ on the same side of the benzene ring. Thes table conformation is ca. 23.1 kJ mol À1 (5.5 kcal mol À1 )l ower in energy than LM1.T he second process occurs via at ransition state in which two BIQ moieties are in co-facial arrangement. This process is accompanied by the lowest energy penalty.F rom this state, the molecule relaxes to as econd local minimum LM2-an enantiomer of LM1.F inally,t he molecule reaches as table form (M,M)-H2-Me 4 via TS3 which has an enantiomeric relationship with TS1.T he activation barriers for TS1, TS2, and TS3 are 59.9, 12.0, and 36.8 kJ mol À1 (14.3, 2.9, and 8.8 kcal mol À1 ), respectively.T he first value is markedly smaller than ab arrier of 100 kJ mol À1 (23.9 kcal mol À1 )f or carbo [5]helicene, [45] which racemizes slowly at ambient temperature. [46] Thus,the interconversion of H2 occurs rapidly at room temperature,w hich prevents the resolution of the stereoisomers.Because both azabora [5]helicenes are apart of the same system, the interconversion of one of them affects the geometry of the second subunit and hence,t he whole molecule.Nevertheless,the presence of the second BIQ unit leads to an egligible increase in DG ¼ 6 (+ 2.1 kJ mol À1 )a s compared with H3.A ccordingly,t he limiting process in the interconversion of H2 is defined by the interconversion of the azabora [5]helicene subunit.

Conclusion
In summary,wehave synthesized azabora [7]helicenes and azabora [9]helicenes as primary substructures of extended helical structures.T hese compounds were prepared by aconceptually simple modular approach in which the helical structure was obtained by boron-bridging of conformationally flexible biaryl BA and triaryl TA.Configurational stability of angularly fused H1 was achieved without any additional blocking groups at the terminal positions.C onfigurationally flexible H2 is anew type of building block consisting of both angularly and linearly fused rings. H1 feature moderate (high among helicenes) fluorescence quantum yields and superior chiroptical properties with j g abs j of up to 1.12 10 À2 .E xcellent F fl of 43-47 %w ere recorded in CH 2 Cl 2 solution for H2.I ntense fluorescence (F fl of up to 25 %) was retained in the solid state for H1-Me 2 , H1-Ph 2 ,a nd H2-Ph 4 affording green, yellow,and blue emitters,respectively.T hus,introduction of boron into helical scaffolds provided helicenes with outstanding optical properties both in solution and in the solid state.T hese features along with high chemical and photostability make these fluorophores attractive for applications as pristine materials or (chiral) emissive dopants in polymer matrices in OLEDs,f luorescent solid-state sensors,a nd fluorescent probes for bioimaging.
Thef lexibility of our synthetic approach opens up the opportunity to prepare heterohelices with precisely modulated properties.I ncorporation of both types of units should facilitate modification of their helical pitch, the extent of the intersystem crossing and optical properties.O ur current efforts are focused on the application of this concept to the synthesis of extended systems.