Letter Cite This: Org. Lett. 2019, 21, 109−113
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Dibora[10]annulenes: Construction, Properties, and Their RingOpening Reactions Weidong Zhang,† Demei Yu,† Zhijun Wang,† Bingjie Zhang,† Letian Xu,† Guoping Li,† Ni Yan,‡ Eric Rivard,§ and Gang He*,†
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†
School of Science jointly with Frontier Institute of Science and Technology, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Key Laboratory of Sustainable Energy Materials Chemistry, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710049, China ‡ Polymer Materials & Engineering Department, School of Materials Science & Engineering, Engineering Research Center of Transportation Materials, Ministry of Education, Chang’an University, Xi’an, Shaanxi 710064, China § Department of Chemistry, University of Alberta, Edmonton, Alberta T6G2G2, Canada S Supporting Information *
ABSTRACT: The selective construction of various dibora[10]annulenes through mild boron−tin exchange reactions is reported. Dibora[10]annulenes exhibit optical and electrochemical properties of value for future sensing applications. Controlled addition of the Lewis base pyridine to dibora[10]annulenes instigates a selective ring-opening reaction. This work explores a new area of boron chemistry that represents the first step in the potential formation of dibora[10]annulene-derived polymers.
B
and optoelectronic behavior of possible value for applications in analyte sensing. Our detailed exploration of BTE chemistry10 started with the preparation of the phenyl- and thiophene-fused stannoles 1, 2, 3a, and 3b (Scheme 1). Structures of 3a and 3b were confirmed by X-ray crystallography (Figures S3 and S4),11 which revealed the presence of planar “stannole” SnC4 heterocyclic units. The previously known borole 5 was synthesized through the BTE reaction between stannafluorene 1 and PhBCl2 in toluene, according to the method reported in the literature.5b Crude 5 crystallized from hexane/THF as the yellow adduct 5·THF in 87% isolated yield (Scheme 1 and Figure S5); formation of a THF adduct highlights the high Lewis acidity of the boron center in 5.12 An analogous BTE reaction between 2 and PhBCl2 in toluene afforded the unsymmetric thiophene-fused borole 6 (Scheme 1); as with 5, the empty pz-orbital of the boron atom in 6 can interact with THF to yield the 1:1 adduct 6·THF (Figure S6). As shown in Figure S6, 6·THF crystallizes as a mixture of diastereomers, as also evidenced by NMR (Figure S49). The successful formation of the antiaromatic borole 6 shows that incorporation of one ring-fused thiophene unit within a stannole does not hinder the BTE reaction. However, BTE between bithienylstannoles 3a,b and PhBCl2 in toluene led to the unexpected formation of the ring-expanded dibora[10]annulenes 7a and 7b in 80 and 85% yield, respectively (Scheme
oron-containing materials are continuing to attract considerable attention for the development of next generation organic electronics.1 Of particular interest are antiaromatic boroles that possess high Lewis acidity and environmentally sensitive optical properties due to the presence of a formally vacant p-orbital at boron.2 Preparation of these heterocycles usually transpires under mild conditions via boron−tin exchange (BTE) between a preformed stannole ring and an arylboron dihalide, ArBX2.3 BTE reactions have also been used to prepare highly antiaromatic perfluoropentaphenylboroles4 and borafluorenes,5 and application of these boron heterocycles as Lewis acidic activators has been reported.6 Heteroarene-fused boroles with even higher degrees of antiaromaticity (e.g., dithienoborole, 9a, NICS(1)zz = 40.3; Scheme 1) were reported by Yamaguchi and co-workers.7 Such heteroarene-fused boroles could not be prepared by commonly used BTE reactions, leading us to consider if antiaromaticity may correlate with the ease of borole formation via BTE.8 Furthermore, the preparation of new macrocyclic boron heterocycles bearing multiple Lewis acidic sites for the cooperative binding of analytes would be valuable to the scientific community.9 In this paper, we show that formation of new ring-expanded bora[10]annulenes can be achieved through a BTE reaction involving stannafluorene- or thiophene-fused stannole precursors. Selectivity of the BTE reaction appears to be affected by the degree of antiaromaticity in predesigned boroles. Previously unknown bora[10]annulenes also display versatile coordination © 2018 American Chemical Society
Received: November 5, 2018 Published: December 18, 2018 109
DOI: 10.1021/acs.orglett.8b03538 Org. Lett. 2019, 21, 109−113
Letter
Organic Letters
explained by the presence of the electron-donating and sterically hindered −SiMe3 groups in 7b. We developed an alternative route to the dibora[10]annulenes 7a,b (Scheme 2).16 Specifically, reaction between
Scheme 1. BTE Reaction and the Selective Synthesis of Boroles and Dibora[10]annulenes
Scheme 2. Synthesis of 7a and 7b from 4a and 4b
1), instead of the predesigned boroles, 9a and 9b. Yellow crystals of 7a and 7b were obtained from hexanes/THF mixtures. According to X-ray crystallography (Figures 1, S7 and S8), the
the bis(stannyl)dithiophenes 4a,b and PhBCl2 gave dibora[10]annulenes 7a,b in 76 and 70% isolated yield, respectively. Attempts to prepare selenium-containing dibora[10]annulenes via the interaction of either diselenophene-fused stannoles or bis(stannyl)diselenophenes17 with PhBCl2 were unsuccessful, and in each case, complex product mixtures were obtained. The synthesis of dibora[10]annulenes between 3b and MesBCl2 was also unsuccessful due to the low reactivity of MesBCl2.18 The mechanism for the formation of the 7a from BTE was analyzed by high-level DFT calculations (Figures S12 and S13). To help clarify the relationship between the antiaromaticity of the predesigned boroles (9a,b) and the outcome of the BTE reactions, we also conducted NICS calculations on various boroles and their ring-expanded dibora[10]annulene counterparts. As shown in Figure 2, the thiophene-fused boroles (9a,b)
Figure 1. ORTEP of 7b with thermal ellipsoids presented at a 50% probability level.
Figure 2. Plots of the computed NICS(1)zz values and Gibbs free energy associated with the formation of boroles. Specific BTE investigated is shown in the SI (Figures S15 and S16).
boron centers in these species adopt distorted trigonalpyramidal geometries with C−B distances in the range of those present in electron-deficient triarylboranes.13 Each structure consists of a nonplanar 10-membered ring core with transannular B···B distances of 2.979(51) Å for 7a and 2.995(2) Å for 7b. Notably, THF from the crystallization solvent does not coordinate to the boron centers in 7a or 7b, indicating a lower degree of Lewis acidity in comparison with that of ring-fused boroles 6 and 9a shown above.14 The Lewis acidities of 7a and 7b were evaluated using the Gutmann−Beckett method.15 Due to the formation of the corresponding 7a−Et3PO adduct, a 31P NMR signal appeared at δ = 81.2 ppm, which was shifted from the resonance of 41.0 ppm for free Et3PO, indicating weak Lewis acidity in 7a (acceptor mumber (AN) = 89.1; for comparison, the AN of BF3 is 89).15 However, no sign of adduct formation between Et3PO and 7b could be found by 31P NMR spectroscopy, indicating a lower degree of Lewis acidity for 7b compared with that of 7a (Figure S67). This observation can be
showed more positive NICS(1)zz values (higher antiaromaticity) compared with that of other boroles, consistent with Yamaguchi’s results.7 Interestingly, the Gibbs free energies (ΔG) associated with the computed BTE reactions (starting from the respective stannoles and PhBCl2) showed an overall trend similar to that of the NICS(1)zz values. Accordingly, ΔG for formation of dibora[10]annulenes, such as 7a,b, became favorable (Figure S17). Therefore, we propose that the outcome of BTE (dibora[10]annulene vs borole) may correlate with the antiaromaticity of predesigned boroles.19 Based on our computations, the allowed mutual rotation of thiophene− thiophene units plays an important role in promoting the formation of dibora[10]annulenes. The proposed mechanism for the synthesis of 7a from 3a and 4a is listed in Figure S15 of the SI. The unexpected formation of new electron-deficient dibora[10]annulenes prompted us to further explore their reactivity toward Lewis bases (Scheme 3).6d,20 Accordingly, 110
DOI: 10.1021/acs.orglett.8b03538 Org. Lett. 2019, 21, 109−113
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428 nm, τ = 3.1 ns) showed a slight blue shift in comparison with 7b in THF (Figure 3).
Scheme 3. Synthesis of 7b·2Py and 8 (Top) and Aromatic Region of the 1H NMR Spectra of 7b, 7b·2Py, and 8 (Bottom)
Figure 3. Emission spectra of 7b, 7b·2Py, 8, and 7b·2MePy in THF (7b, λex = 346 nm; 7b·2Py, λex = 340 nm, 8, λex = 350 nm; 7b·2MePy, λex = 340 nm).
addition of more than 2 equiv of pyridine (entry 1 in Table S1) or 4-methylpyridine to 7b in hexanes gave boron-bound pyridine adducts 7b·2Py and 7b·2MePy (Figures S9 and S10).21 Notably, the transannular B···B distances became significantly elongated upon binding of pyridine: 4.608(58) Å for 7b·2Py vs 2.995(2) Å for 7b. This may be attributed to the geometry change at boron (4-coordination in 7b·2Py and 3coordination in 7b) and the concomitant increase of steric hindrance about boron upon coordination of pyridine or 4methylpyridine. Interestingly, when 7b/pyridine reactant ratios were decreased to 1:1 and 1:0.5 (entries 2 and 3 in Table S1), the ring-opened complex 8 (Scheme 3) was unexpectedly formed, along with pyridine adduct 7b·2Py. When the 7b/pyridine ratio was decreased to 1:0.25 (entries 4 and 5 in Table S1), 8 was obtained as a pure product, and its ring-opened structure with both borole and borane environments was confirmed by X-ray crystallography (Scheme 3 and Figure S11). We hypothesize that 8 is generated through an intramolecular nucleophilic reaction followed by pyridine coordination-triggered phenyl transfer (route 2, Figure S2). Another possible route involves pyridine-promoted phenyl group transfer from a tetracoordinate boron to an adjacent tricoordinate boron center22 (route 3, Figure S2). To collect higher quantities of 8, a flow injection method was used to control the amount of pyridine added (the mole ratio of residual of 7b and pyridine added each time was fixed to 1:0.25). After slow addition of pyridine over 14 h (0.34 mmol pyridine was added totally; 7b was 0.2 mmol, and final 7b/pyridine ratio was 1:1.7), a maximum isolated yield of 77% was obtained (Figure S1).11 Figures S27 and S29 show solution UV−vis spectra of 7a with a λmax = 302 nm in toluene, whereas in more polar solvents such as THF and DMF, the maximum absorption bands were slightly red-shifted to 310 nm and λmax = 315 nm, respectively. Compared to that of 7a, absorption peaks of 7b showed a λmax = 320 nm in toluene, THF, and DMF. Dibora[10]annulenes 7a and 7b afforded sky-blue fluorescence (e.g., λem = 416 nm for 7a; λem = 426 nm for 7b), with slightly blue-shifted emission in more polar solvents, such as DMF (Figures S28 and S30). This phenomenon is attributed to the coordination of the donor solvent to the boron atom, which prevents the establishment of B(p)−ring (π*) conjugation in the LUMO.23 Similarly, emission of 7b·2Py (λem = 406 nm, τ = 3.2 ns) and 8 (λem =
Considering the generally strong binding of fluoride ions to 3coordinate boron centers,24 titration of 7b with F− (in the form of nBu4NF in THF) was conducted and monitored by UV−vis and photoluminescence (PL) spectroscopy (Figures S31 and S32). After binding with F−, the emission maxima at 417 nm was quenched and a new emission profile appeared at 394 nm due to the formation of the fluoride-bound compound 7b·2F‑. An estimated fluoride ion binding constant of 1 × 109 M−1 was extracted from these fluorescence titration experiments (Figure S33).25 The bis(fluoride) adduct (7b·2F−) can be converted back into starting 7b by treatment with a stronger fluoride scavenger, such as BF3·OEt2 (Figure S34). These results suggest that the binding of fluoride to 7b does not lead to side reactions or decomposition and highlights the potential of dibora[10]annulenes to act as fluorescence-based fluoride sensors. The electron-accepting properties of 7a and 7b were studied by cyclic voltammetry in CH2Cl2, and in each case, only one reversible peak was observed (7a, E1/2 = −1.72 V; 7b, E1/2 = −1.85 V; Figure S35).26 To investigate the influence of different solvents, CV data was also collected in THF (Figure S36). Interestingly, one quasi-reversible and another reversible redox wave were observed, which could be interpreted as a stepwise reduction of individual borane moieties. The more negative potential of 7a (E1/2 = −2.69 V) relative to 7b (E1/2 = −2.02 V) was attributed to the higher Lewis acidity of 7a. Our computational results were consistent with the experimental data (Table S3 and Figure S37). In conclusion, a series of 10-membered dibora[10]annulene heterocycles were synthesized via two convergent BTE routes. Based on experimental data and accompanying DFT calculations, the outcome of the BTE reaction between stannoles and PhBCl2 appears to correlate with the degree of antiaromaticity in the predesigned boroles; in addition, rotational conformational freedom within bithiophene skeletons likely promotes dibora[10]annulene formation. Moreover, when 7b was combined with substoichiometric quantities of pyridine, the unexpected ring opening of this dibora[10]annulene transpired. The title dibora[10]annulenes also have useful fluoride binding (Lewis acidic) and tunable optoelectronic properties that are valuable for sensing applications.27 Overall, this study provides key insights into the nature of the widely used BTE reaction and 111
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introduces a new type of multifunctional boron-containing material to the community.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03538. Detailed experimental procedures, NMR spectra, and Xray crystallographic data for new compounds (PDF) Accession Codes
CCDC 1542346−1542347, 1542349−1542350, 1542370, 1542383, 1542386, 1542482, and 1542484 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]. uk, 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
Eric Rivard: 0000-0002-0360-0090 Gang He: 0000-0002-5319-7084 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (21603016, 21704081, 21875180), Natural Science Foundation of Shaanxi province (2017JQ2023, 2018JM2026), Key Laboratory Construction Program of Xi’an Municipal Bureau of Science and Technology (201805056ZD7CG40), and the Cyrus Chung Ying Tang Foundation. E.R. thanks the Alexander von Humboldt Foundation for a fellowship. We also thank Prof. Yu Fang of Shaanxi Normal University for helpful discussions. We are especially grateful to Prof. Yanzhen Zheng of Xi’an Jiaotong University for his help with X-ray crystallographic analysis. We thank Dr. Gang Chang of Xi’an Jiaotong University and Yu Wang at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with acquiring PL spectra.
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REFERENCES
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