Dibora[10]annulenes: Construction, Properties, and Their Ring-Opening Reactions

Article abstract of DOI:10.1021/acs.orglett.8b03538

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.

Full text of DOI:10.1021/acs.orglett.8b03538

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Dibora[10]annulenes: Construction, Properties, and Their Ring-
Opening Reactions
Weidong Zhang,† Demei Yu,† Zhijun Wang,† Bingjie Zhang,† Letian Xu,† Guoping Li,† Ni Yan,‡
Eric Rivard,§ and Gang He*,†
†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.
and optoelectronic behavior of possible value for applications in
analyte sensing.
oron-containing materials are continuing to attract
Bconsiderable 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
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
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 precur-
sors. 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
Received: November 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03538
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
explained by the presence of the electron-donating and sterically
hindered −SiMe3 groups in 7b.
Scheme 1. BTE Reaction and the Selective Synthesis of
Boroles and Dibora[10]annulenes
We developed an alternative route to the dibora[10]-
annulenes 7a,b (Scheme 2).16 Specifically, reaction between
Scheme 2. Synthesis of 7a and 7b from 4a and 4b
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 counter-
parts. As shown in Figure 2, the thiophene-fused boroles (9a,b)
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
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).
Figure 1. ORTEP of 7b with thermal ellipsoids presented at a 50%
probability level.
boron centers in these species adopt distorted trigonal-
pyramidal 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
showed more positive NICS(1)zz values (higher antiaromatic-
ity) 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.
to the formation of the corresponding 7a−Et3PO adduct, a 31
P
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
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,
B
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Letter
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 3-
coordination in 7b) and the concomitant increase of steric
hindrance about boron upon coordination of pyridine or 4-
methylpyridine.
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
Considering the generally strong binding of fluoride ions to 3-
coordinate 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 calcu-
lations, 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
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
=
C
DOI: 10.1021/acs.orglett.8b03538
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Letter
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introduces a new type of multifunctional boron-containing
material to the community.
ASSOCIATED CONTENT
* Supporting Information
■
S
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 X-
ray 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 data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
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(b) Williams, V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins,
S.; Marder, T. B. J. Am. Chem. Soc. 1999, 121, 3244.
(10) Zhang, W.; Li, G.; Xu, L.; Zhuo, Y.; Wan, W.; Yan, N.; He, G.
Chem. Sci. 2018, 9, 4444.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ganghe@mail.xjtu.edu.cn.
ORCID
(11) See the Supporting Information for details.
(12) For a study of the Lewis acidity of related fluorinated
counterparts to 5, see: Chase, P. A.; Romero, P. E.; Piers, W. E.;
Parvez, M.; Patrick, B. O. Can. J. Chem. 2005, 83, 2098.
Eric Rivard: 0000-0002-0360-0090
Gang He: 0000-0002-5319-7084
Notes
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(13) Cheng, F.; Bonder, E. M.; Jakle, F. J. Am. Chem. Soc. 2013, 135,
17286.
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J. Am. Chem. Soc. 2016, 138, 6224. (b) Kong, L.; Lu, W.; Li, Y.; Ganguly,
R.; Kinjo, R. J. Am. Chem. Soc. 2016, 138, 8623. (c) Hubner, A.; Kaese,
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bner, A.; Bolte, M.; Lerner, H.-W.; Wagner, M.
The authors declare no competing financial interest.
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T.; Diefenbach, M.; Endeward, B.; Bolte, M.; Lerner, H.-W.;
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R. Chem. Commun. 2006, 2875.
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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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