BN-Phenanthrenes: Synthesis, Reactivity, and Optical Properties

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

BN-Phenanthrenes: Synthesis, Reactivity, and Optical Properties Chen Zhang,† Lei Zhang,‡ Chao Sun,† Wenfang Sun,†,∥ and Xuguang Liu*,†,§ †

Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, People’s Republic of China ‡ School of Science, Tianjin Chengjian University, Tianjin 300384, People’s Republic of China § Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China ∥ Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States

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S Supporting Information *

ABSTRACT: Two series of BN-phenanthrenes, including the parental compounds, have been synthesized in an efficient manner from commercially available materials. Both parental BN-phenanthrenes are strongly fluorescent in solution. Their absorption and emission spectra are greatly dependent on the BN position and orientation. Moreover, the reactivities of the two series of BN-phenanthrenes toward electrophiles are completely different, and these species can be further functionalized by cross-coupling reactions.

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oping with heteroatoms is an effective way to tune the properties of polycyclic aromatic hydrocarbons (PAHs).1 In particular, BN-codoped PAHs, in which pairs of C atoms have been replaced by BN units, have attracted great interest, because of their potential to create new optoelectronic materials.2 In fact, several BN-PAHs have been reported to be highly effective and sometimes superior to their carbonaceous analogues in fields such as organic field-effect transistors (OFETs).3 While many BN-perturbed analogues of PAHs now exist,2,3 the fundamental understanding of the effect of BN/CC replacement on the structure and properties of the PAHs must be improved. Thus, the basic knowledge obtained from simple BN-PAHs is highly valuable. Phenanthrene, one of the simplest PAHs, is of great interest to organic and material chemists because of its high stability and electroluminescent properties.4a−c Moreover, phenanthrenes are also found as core structure in natural products.4d Only four BN-phenanthrene derivatives have been synthesized and isolated to date (Figure 1). Dewar first reported BNphenanthrene 1 in 19585a and 2 in 1962.5b Vaquero and Garcı ́a-Garcı ́a provided an improved synthetic route to 2 in 2017 and studied its properties and reactivity. 6 BNphenanthrene 3, with an internal BN unit, was synthesized by the Piers group in 2007.7 In 2013, Wang’s group reported the synthesis of BN-phenanthrene 4 by a photoelimination reaction.8 The presence of BN units in phenanthrenes has a substantial influence on their properties.5−8 For instance, most BN-phenanthrenes (1, 3, and 4) are strongly emissive (2 is not). As shown in Figure 1, the BN units in the reported BNphenanthrene derivatives are mainly located on the middle B ring of the phenanthrene. To the best of our knowledge, BN/ CC replacements in the terminal A/A′ ring have not been disclosed. Moreover, unsubstituted BN-PAHs (“parental”) are © XXXX American Chemical Society

Figure 1. Reported BN-phenanthrenes and this work.

highly desirable, because their electronic structures are not influenced by substituent effects.9 Herein, we describe the synthesis and properties of the two missing isomers of BNphenanthrene, 5 and 6, including the parental molecules.10 Undaunted by this challenge, the key step in the synthesis was identifying a suitable starting material that can be easily transformed to the target molecule by organic transformations. The syntheses of parental BN-phenanthrenes 5a and 6a are shown in Scheme 1. The key step involves the borylative cyclization of the corresponding amino vinyl naphthalene amines. The synthesis of BN-phenanthrene 5a commenced with the conversion of the hydroxyl group of commercially available 2-nitronaphthalen-1-ol to trifluoromethanesulfonate (OTf) by treatment with trifluoromethanesulfonic anhydride Received: February 11, 2019

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DOI: 10.1021/acs.orglett.9b00530 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Synthetic Route to Parental BN-Phenanthrenes 5a and 6a

Figure 2. NICS (1) values of phenanthrene and parental BNphenanthrenes 5a and 6a. Determined at the B3LYP/6-311+G(2d,p) level of theory.

the middle ring of 5a (−9.17) and 6a (−9.29) are more negative than those of the corresponding ring in carbonaceous phenanthrene (−8.00),13 which indicates the middle rings of BN-phenanthrene 5a and 6a have stronger aromaticity. We also investigated the reactivity of the parental BN-phenanthrenes 5a and 6a. The bromination of these isomers could not be achieved using either bromine (Br2) or N-bromosuccinimide (NBS) as the brominating reagent, presumably because of the effect of the B−H bond.14 We then focused on preparing BN-phenanthrenes bearing a phenyl group on the boron atom, which should improve their stability (Scheme 2), facilitating further manipulations.

(Tf2O). Suzuki cross coupling of intermediate 8 with 4,4,5,5tetramethyl-2-vinyl-1,3,2-dioxaborolane furnished 2-nitro-1vinylnaphthalene (9). The reduction of 9 using iron powder gave 1-vinylnaphthalen-2-amine (10). Treatment of 10 with boron trichloride followed by lithium aluminum hydride (LiAlH4) afforded parental BN-phenanthrene 5a. 5a is relatively stable in the air for several hours and can be purified by chromatography on silica gel. We found that 5a is not stable in air for long periods and should be stored under an inert atmosphere. The initial synthesis of another isomer of BN-phenanthrene, 6a, was staring from the 2-bromonaphthalen-1-amine. After a substantial effort, we found that the cross coupling of 2bromonaphthalen-1-amine with either vinylboronic acid pinacol ester, vinylmagnesium bromide or tributyl(vinyl)tin failed in our hand. Ultimately, the successful synthesis of 6a started with 1-nitro-2-naphthaldehyde (11), which can be easily prepared by a previously reported procedure (see Scheme S10 in the Supporting Information).11 The Wittig reaction between 11 and a triphenyl phosphonium ylide (generated in situ from Ph3PMeI) afforded 1-nitro-2-vinylnaphthalene (12), which can be reduced to 2-vinylnaphthalen1-amine (13) by iron powder. Similarly, the borylative cyclization/reduction sequences using BCl3/LiAlH4 furnished parental BN-phenanthrene 6a in 70% yield over two steps. We found that 6a is less stable than 5a and decomposed gradually during purification. Pure 6a could be obtained by preparative thin-layer chromatography and should be stored under an inert atmosphere immediately after purification. The 1H NMR chemical shifts of the N−H protons in parental BN-phenanthrenes 5a and 6a are in the aromatic region (5a, 8.48 ppm; 6a, 9.30 ppm), which suggests these species are aromatic. The chemical shifts of B−H in 5a (5.18 ppm) and 6a (5.23 ppm) are ∼5.20 ppm, which is the shift typical of BN aromatics containing B−H bonds.9 Unfortunately, we were not able to obtain X-ray crystal structures of 5a and 6a. To understand the effects of the location and orientation of the BN unit on the aromaticity of BNphenanthrene, we performed nucleus-independent chemical shift (NICS) calculations (Figure 2).12 The NICS (1) values of the BN rings in 5a and 6a are comparable (5a, −6.58; 6a, −6.52), indicating the BN-containing rings in the two isomers have similar aromaticities (Figure 2). The NICS (1) values of

Scheme 2. Synthetic Route to BN-Phenanthrenes 5b and 6b

Gratifyingly, the electrophilic borylation of 1-vinylnaphthalen-2-amine (10) or 2-vinylnaphthalen-1-amine (13) with dichlorophenylborane (PhBCl2) produced BN-phenanthrenes 5b (99%) and 6b (94%) in excellent yields. Both 5b and 6b are quite stable toward chromatography on silica gel. Compounds 5b and 6b were smoothly brominated using bromine (Br2) as the brominating reagent, which is remarkably different from the inertness of 5b and 6b toward NBS as a brominating reagent; using NBS as brominating reagent resulted in either poor reactivity (with 5b) or no reaction at all (with 6b). Interestingly, we found that the regioselectivities of the halogenation reactions of these two isomers are completely different. The bromination of BN-phenanthrene 5b occurs exclusively at the α-carbon adjacent to boron, even when using more than 2 equiv of Br2. On the other hand, under the same conditions, BN-phenanthrene 6b yielded a mixture of the mono- (6c) and dibrominated products (6c′), which can be separated by careful chromatography. When the amount of Br2 was increased to two equivalents, dibrominated product 6c′ became the only product. Notably, when using 3 equiv of Br2, 6c′ was not further brominated. The different behaviors of isomers 5b and 6b highlight the tremendous influence of the location of the BN unit on the reactivity of the BNphenanthrene. The difference between 5b and 6b is that 6b has a C−H in para position of the NH group on the central B


Letter

Organic Letters

shorter than the typical B−C single bond covalent radius (1.58 Å).16a,b Shorter N−C bonds were also observed in the inner ring (5c, 1.376 Å; 6c′, 1.393 Å). Interestingly, the C7−C8 bond connecting the two terminal rings is longer in 5c (1.361 Å) than in 6c′ (1.309 Å), which is also consistent with the calculated NICS (1) values, in which the middle ring of 6a showed greater aromaticity than that of 5a. Both BNphenanthrenes have a planar framework, and the dihedral angles between the C4BN ring and the other terminal benzene ring are quite small (5c, 5.6°; 6c′, 1.3°). The phenyl groups connected to the B atom are out of the main plane with dihedral angles of ∼32° for 5c and 54° for 6c′, which indicate their diminished conjugation with the planar BN-phenanthrene core (see Tables S3−S8 in the Supporting Information). The normalized UV-vis absorption and emission spectra of BN-phenanthrenes 5a, 5b, 6a, and 6b and carbonaceous phenanthrene in cyclohexane are shown in Figure 5, and the

ring, which can be brominated, but for 5b this position is fused to another benzene ring and therefore not amenable to bromination. To provide a qualitative explanation of the regioselectivity of the bromination reactions, we performed theoretical calculation on the relative stability of arenium ions (after attack of Br+) on BN-phenanthrenes 5b and 6b (see Figure 3, as well as

Figure 3. Calculated stability of arenium ions, corresponding to the attack of Br+ on different positions of BN-phenanthrenes 5b and 6b, determined at the B3LYP/6-311+G(2d,p) level of theory.

Figure S6 in the Supporting Information). In the case of 5b, the arenium ion intermediate generated by E+ attacking at the carbon (C-12) adjacent to boron was treated as the zeroenergy reference, which is also the typical position where bromination of 1,2-dihydro-1,2-azaborine occurred.15 The relative energies of the arenium ions in all of the remaining positions in 5b are >10 kcal/mol, which is consistent with the experimental results, in which electrophilic aromatic substitution occurs exclusively at the C-12 position of 5b. In the case of 6b, the second lowest-lying arenium ions was observed at C-7 (2.23 kcal/mol), which is consistent with the dibromination results observed experimentally. Thanks to the high stability of the B-phenyl BNphenanthrene derivative, we were able to obtain the crystal structures of 5c and 6c′, which allowed us to determine the solid-state structures of both scaffolds, although the quality of the crystal structure of 6c′ is poor. [The structures of 5c and 6c′ are depicted in Figure 4.] The results confirmed the regioselectivity of the bromination reactions. The BN bond lengths of 5c (1.426 Å) and 6c′ (1.408 Å) are close to the localized BN double bonds (1.403(2) Å),16a,b and they are much shorter than typical BN single bonds (1.56 Å). The B−C bonds in the inner rings (5c, 1.528 Å; 6c′, 1.509 Å) are also

Figure 5. Normalized absorption (left) and emission spectra (right) of phenanthrene, parental BN-phenanthrenes 5a and 6a (top), and phenyl substituted BN-phenanthrenes 5b and 6b (bottom) in cyclohexane at a concentration of 10−5 M.

maxima of their absorption and emission bands are presented in Table 1 (see Table S9 in the Supporting Information). The low energy bands of parental BN-phenanthrenes 5a and 6a are substantially red-shifted (5a, λabs = 346 nm; 6a, λabs = 356 nm) compared to the carbonaceous phenanthrene (294 nm). The overall absorption spectrum of 5a is blue-shifted, compared to 6a. The trend is consistent with the calculated results (see Table S2 in the Supporting Information). The introduction of a phenyl ring on the boron red-shifted the absorption (5b, λabs = 352 nm; 6b, λabs = 360 nm), because of conjugation between the B-Ph moiety and the main scaffold. Phenanthrene is a well-known fluorophore with a relatively weak emission at 348 nm (Φ = 0.09).7 We also measured the emission properties of the BN-phenanthrenes, and all four BNphenanthrenes showed strong emission in solution (Figure 5, Table 1). The emission maxima of 5a are comparable to those of carbonaceous phenanthrene, and the emission maxima of 6a were red-shifted. Again, B-Ph 5b (λem = 360 nm) and 6b (λem = 365 nm) displayed red-shifted emission bands, compared to their corresponding B−H analogues 5a (λem = 348 nm) and 6a (λem = 354 nm). This trend is consistent with that observed in their absorption spectra. The solution-state fluorescence quantum yields of BN-phenanthrenes (5a, 0.79; 5b, 0.92; 6a,

Figure 4. Solid-state structures of BN-phenanthrenes 5c and 6c′ with views parallel and perpendicular to the polycyclic planes. Thermal ellipsoids are set at the 50% probability level. H atoms have been omitted for the sake of clarity. C


Letter

Organic Letters Table 1. Photophysical Properties of 5a, 5b, 6a, and 6b compound Phen 5a 5b 6a 6b

λabsa (nm)

ε (M−1 cm−1)

294 345 356 352 360

λonset (nm)

11926 12016 14877 6811 9908

λemb (λex) (nm)

301 358 364 363 369

348 348 360 354 365

(294) (301) (300) (300) (283)

Φplc d

0.09 0.79 0.92 0.56 0.85

EGopt (eV)e 4.12 3.46 3.41 3.42 3.36

a

Refers to the lowest-energy peak maxima values. bRefers to the highest-energy peak maxima values. cRelative to 9,10-diphenylanthracene in cyclohexane (λex = 336 nm, Φpl = 0.93 ± 0.03).17 dAdopted from ref 7. eOptical band gap Eopt G = 1240/λonset

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0.56; 6b, 0.85) are higher than that of all-carbon phenanthrene (0.09). Notably, the quantum yield of BN-phenanthrene 5 is higher than that of BN-phenanthrene 6. Transition-metal-catalyzed cross-coupling reactions have been widely used to construct the C−C bonds of aromatic compounds.18 In particular, the cross coupling of halogenated BN-PAHs has been well-documented.6,9f,19 The reactivities of brominated BN-phenanthrenes 5c, 6c, and 6c′ were evaluated (see Scheme 3). To our delight, brominated BN-phenan-

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00530. Full details on the synthesis of all BN-phenanthrenes 5 and 6; NMR spectra, UV-vis and photoluminescence data, electrochemical data, X-ray crystallographic data, and theoretical calculations; experimental and computational details (PDF)

Scheme 3. Further Functionalization of 5c, 6c, and 6c′a

Accession Codes

CCDC 1896392 and 1896393 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lei Zhang: 0000-0003-0100-8357 Wenfang Sun: 0000-0003-3608-611X Xuguang Liu: 0000-0002-7859-3139

a

The reaction conditions were not optimized.

Notes

threnes 5c could participate in Suzuki-type (5e, 72%), Hecktype (5d, 78%) and Sonogashira-type reactions (5f, 39%) and afforded the corresponding products in good yields. Brominated BN-phenanthrenes 6c and 6c′ could also undergo Suzuki-type cross-coupling reactions to produce the corresponding products in decent yields (6d, 43%; 6e, 66%). The photophysical properties of functionalized products 5c−5f and 6c−6e can be found in the Supporting Information (Figures S8−S10 and Table S9). In summary, we have developed an efficient synthesis of two types of BN-phenanthrenes, including the parental species, by selecting the appropriate starting material. All the BNphenanthrenes were fully characterized, and two of them were unambiguously confirmed by single-crystal X-ray analysis. The aromaticities of the parental BN-phenanthrenes were quantified by experimental and computational studies. The BN position and orientation substantially impact the optical properties of the compound. Both types of BN-phenanthrenes are strongly emissive in solution. Notably, the position of the BN unit has a significant effect on the reactivity of the compound toward bromination, which was further explained by DFT calculations. The exciting photophysical properties and successful metal-catalyzed coupling reactions make these BN-phenanthrenes broadly applicable in future works.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Natural Science Foundation of Tianjin (No. 17JCZDJC37700), National Natural Science Foundation of China (No. 21502140), Tianjin University of Technology, and the Shanghai Institute of Organic Chemistry (No. K2015-10). We also thank the Training Project of Innovation Team of Colleges and Universities in Tianjin (No. TD13-5020) and the support from the University Student Innovation Program.

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BN-Phenanthrenes: Synthesis, Reactivity, and Optical Properties

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