Synthesis, Properties, and Reactivity of Bis-BN Phenanthrenes: Stepwise Bromination of the Main Scaffold

Article abstract of DOI:10.1021/acs.orglett.0c00021

Two bis-BN phenanthrenes have been synthesized. Their photophysical properties are different from those of the reported mono-BN phenanthrenes. Moreover, the stepwise bromination of bis-BN phenanthrene 8 gave a series of brominated bis-BN phenanthrenes, wi

Full text of DOI:10.1021/acs.orglett.0c00021

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Synthesis, Properties, and Reactivity of Bis-BN Phenanthrenes:
Stepwise Bromination of the Main Scaffold
Lingjian Zi, Jinyun Zhang, Chenglong Li, Yi Qu, Bin Zhen, Xuguang Liu,* and Lei Zhang*
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ABSTRACT: Two bis-BN phenanthrenes have been synthesized. Their photo-
physical properties are different from those of the reported mono-BN phenanthrenes.
Moreover, the stepwise bromination of bis-BN phenanthrene 8 gave a series of
brominated bis-BN phenanthrenes, with all of the mono-, di-, tri-, and
tetrabrominated bis-BN phenanthrenes being successfully isolated and characterized.
In addition, preliminary results showed that mono- and dibrominated species can be
further functionalized by cross-coupling reactions.
mbedding a boron atom into polycyclic aromatic
hydrocarbons (PAHs) is an effective way to alter their
E
structural and electronic properties.1 In particular, BN/CC
isosterism resulted in boron/nitrogen-containing PAHs (BN-
PAHs), in which the boron and nitrogen atoms have
complementary electron-accepting and -donating properties,
and they have attracted great attention due to their potential to
create new optoelectronic materials.2 The mono-BN PAHs are
the most common examples of BN-PAHs,3 whereas bis-BN
PAHs have often existed in larger sized PAHs.4 In terms of bis-
BN PAHs in small-size PAHs (three aromatic rings or fewer),
only one single example of bis-BN anthracene has been
reported.4a In BN-PAHs with more than one BN unit, the
fundamental question is the effect of the number of BN units
on their chemical and photophysical properties.
Figure 1. Reported BN-phenanthrenes and this work.
Phenanthrene, which is the isomer of linear anthracene, is of
great interest to organic and material chemists.5 In total, six
mono-BN phenanthrene derivatives have been reported to date
(Figure 1). The first BN-phenanthrene was reported by the
Dewar group in 1958.6a The BN-phenanthrene isomer 2 was
also reported by Dewar group,6b and Vaquero and coworkers
improved the synthesis of 2 using ring-closing metathesis as
the key step.7a−c BN-phenanthrene 3 with an internal BN unit
was synthesized by the Piers group using platinum-catalyzed
annulation relations.8 Wang’s group reported the BN-
phenanthrene 4 by a photoelimination reaction.9 Very recently,
Vaquero7d,e and our group10 independently reported the BN-
phenanthrenes 5 and 6 with a BN unit on the terminal ring of
phenanthrene. Despite the recent progress in the study of the
synthesis and properties of mono-BN phenanthrenes, BN-
phenanthrenes beyond the mono-BN isosteres have remained
elusive (Figure 1). In this Letter, we describe the synthesis and
properties of two bis-BN phenanthrenes 7 and 8 (Figure 1,
bottom right corner). We found that the properties and
reactivities of bis-BN phenanthrenes are intriguingly different
from those of mono-BN phenanthrenes.
The synthesis of bis-BN phenanthrenes 7 and 8 is shown in
Scheme 1. 3,6-Dibromobenzene-1,2-diamine11 was used as the
starting material. Two-fold Stille cross coupling of 9 with
tributyl(vinyl)tin yielded the 3,6-divinylbenzene-1,2-diamine
Received: January 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00021
© XXXX American Chemical Society
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Scheme 1. Synthetic Route to bis-BN Phenanthrenes 7 and
8
Figure 3. Solid-state structures of bis-BN-phenanthrenes 7 and 8 with
views parallel and perpendicular to the polycyclic planes. Thermal
ellipsoids are set at the 50% probability level. H atoms have been
omitted for clarity.
Table 1. Representing Bond Lengths and Dihedral Angles of
bis-BN Phenanthrenes 7 and 8
10 in moderate yield. Intramolecular cyclization of 9 with two
equivalents of dichlorophenylborane (PhBCl2) produced bis-
BN phenanthrene 7 in low yield (<5%), along with
unidentified byproducts. An attempt to isolate and purify the
byproducts failed. To avoid the formation of a byproduct, we
decided to connect the two nitrogen atoms with an ethylene
bridge (Scheme 1, bottom). Condensation of 10 with glyoxal
followed by hydrogenation of the resulting 5,8-dibromoqui-
noxaline 11 furnished 5,8-dibromo-1,2,3,4-tetrahydroquinoxa-
line 12. Stille cross coupling of 12 with tributyl(vinyl)tin gave
5,8-divinyl-1,2,3,4-tetrahydroquinoxaline 13. Borylative cycli-
zation of 13 with PhBCl2 produced bis-BN phenanthrene 8
with an ethylene bridge sitting in between two nitrogen atoms,
which can also be viewed as bis-BN dihydropyrene, in good
yield (85%). We found that both 7 and 8 are stable in air and
could be purified by chromatography on silica gel. Attempts
toward the dehydrogenation of 8 to get the fully aromatic bis-
BN pyrene failed (Scheme S17).
B1−N1
B2−N2
N−C
dihedral
a
B−C endo B−C exo
endo
angle
7
8
1.421(4)
1.418(4)
1.437(8)
1.449(8)
1.526(3)
1.518(4)
1.540(9)
1.545(9)
1.555(4)
1.576(4)
1.583(9)
1.582(9)
1.379(3)
1.385(3)
1.422(7)
1.401(7)
4.5°
6.8°
48.2°
44.0°
a
Dihedral angle between the B-phenyl and the main framework of bis-
BN phenanthrene plane.
(1.437(8), 1.449(8) Å) are much shorter than the typical B−N
single bond (1.56 Å), indicating a distinct double-bond
character (1.403(2) Å).13 Specifically, the B−N bond lengths
of 7 (1.421(4), 1.418(4) Å) are shorter than the B−N bond
lengths in 8 (1.437(8), 1.449(8) Å), which is in line with the
more negative calculated NICS(1) values of 7′ (−6.22)
compared with those of 8 (−5.73). The inner-ring B−C bonds
of 7 (1.526(3), 1.518(4) Å) and 8 (1.540(9), 1.545(9) Å) are
shorter than the typical B−C single-bond covalent radius (1.58
Å), which indicates its double-bond character as well.13
Interestingly, the dihedral angles between the phenyl group
attached to boron and the main scaffold of 7 (4.5 and 6.8°) are
quite small. In contrast, the phenyl groups on boron are
twisted and deviated from the main backbones by 48.2 and
44.0° in 8, presumably due to the bulky steric effect of the
relatively larger CH2 group attached to the nitrogen atom in 8
(Figure 3 and Table 1).
To understand the effects of the location and number of BN
units on the aromaticity of bis-BN phenanthrenes, we
performed nucleus-independent chemical shifts (NICS)
calculations (Figure 2);12 parental mono-BN phenanthrene 6
We then turn our interest to investigating the bromination of
bis-BN phenanthrene 8 (Scheme 2). As expected, we found
that the bromination products of 8 are dependent on the ratio
between 8 and Br2 used. The bromination of 8 took place
exclusively at the α-carbon adjacent to boron when using fewer
than two equivalents of Br2. Specifically, the mono-brominated
Figure 2. NICS(1) values of phenanthrene, mono-BN phenanthrene
6, and bis-BN phenanthrenes 7′ and 8 determined at the B3LYP/6-
311+G(2d,p) level of theory. [a]The NICS(1) values of phenanthrene
and 6 are adopted from ref 10.
Scheme 2. Stepwise Bromination of bis-BN Phenanthrene 8
and carbonaceous phenanthrene were included for direct
comparison.10 The NICS(1) values of the BN rings in 7′ (B−
H, −6.22) and 8 (−5.73) are less negative that those in 6
(−6.52), indicating that the BN rings in bis-BN phenanthrenes
have smaller aromaticity. The NICS(1) values of the middle
ring of 7′ (−10.17) and 8 (−9.70) are more negative than
those of the corresponding ring in carbonaceous phenanthrene
(−8.00) and 6 (−9.29), which indicates that the middle rings
of bis-BN phenanthrenes 7′ and 8 have stronger aromaticity.
The structures of bis-BN phenanthrenes 7 and 8 were
further confirmed by X-ray single-crystal analysis (Figure 3).
The typical bond lengths and dihedral angles are shown in
Table 1. B−N bond lengths in 7 (1.421(4), 1.418(4) Å) and 8
B
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product (1Br-8) could be isolated as the major product when
using 1.0 equiv of Br2 (53%). Further increasing Br2 to 2.1
equiv, the dibrominated product (2Br-8) became the major
product (85%). When using more than two equivalents of Br2,
the site-selective tribromination (3Br-8) and tetra-bromination
(4Br-8) of the 8 were observed. After the reaction conditions
were optimized, we found that using 4.2 equiv of Br2 gave the
tribrominated product (3Br-8) as the major product (48%),
along with a trace amount of the mono- and dibrominated
products, which can be carefully isolated by chromatography.
The tetrabrominated product (4Br-8) (10%) could be isolated
when using 6.0 equiv of Br2 along with some unidentified
byproducts. We did not isolate any further brominated
products in the reaction system.
The solid-state structures of mono- (1Br-8), di-(2Br-8), and
tetra-brominated (4Br-8) bis-BN phenanthrene derivatives
were successfully confirmed by X-ray single-crystal analysis
(Tables S12−S20), which confirmed the regioselectivity of the
bromination reactions. An attempt to grow single crystals of
tribrominated bis-BN phenanthrene 3Br-8 failed. Notably, the
reactivity of bis-BN phenanthrene 8 toward bromination is
different from the reactivity of mono-BN phenanthrenes 27a
and 5,7d,10 in which bromination exclusively occurs at the α-
carbon adjacent to boron. With regards to mono-BN
phenanthrene 6, mono-7e,10 and dibrominated10 products
could be isolated, and no further bromination was observed.
The different behaviors of bis-BN phenanthrene 8 toward
bromination highlight the influence of the number and the
location of the BN unit on the reactivity of BN phenanthrenes.
Resonance structures can be used to rationalize the
activation of C-3, C-5, C-6, and C-8 positions of 8 (Figure
4). Computational studies of brominated intermediates are
orbital (NBO) charge analyses15 on 8 demonstrated that the
electron density value increases in the order of C4/C7
(−0.114) < C5/C6 (−0.178) < C3/C8 (−0.507), which
should correlate more directly with the observed regioselec-
tivity. The calculated activation energy values and transition
states of each step for the formation of mono- (15.0 kcal/mol),
di- (17.0 kcal/mol), tri- (24.5 kcal/mol), and tetra-brominated
(25.5 kcal/mol) bis-BN phenanthrene derivatives and details
of the calculation can be found in the Supporting Information
(Figures S8−S11), which is consistent with experimental
observation that bromination occurs in a stepwise manner.
We also studied the photophysical properties of bis-BN
phenanthrenes 7 and 8, and we found that the ethylene bridge
in 8 has a negligible influence on its absorption and emission
energy (Figure S1). Both 7 and 8 showed blue-shifted
absorption spectra and red-shifted emission spectra compared
with mono-BN phenanthrene 6 (B-Ph) (Figure S1 and Table
S1).10 Furthermore, solvent polarity had little effect on the
absorption and emission energies of 7 and 8 (Figure S3). The
absolute quantum yields of 8 (Φpl = 0.19) are more than three
times those of 7 (Φpl = 0.06), and both of the values are
smaller than that of 6 (B-Ph, Φpl = 0.85). The differences in
their photophysical properties of 7/8 and mono-BN
phenanthrene 6 again highlight the tremendous effect of the
number of BN units on the properties of the BN-
phenanthrenes. The UV−vis absorption and emission spectra
of 1Br-8, 2Br-8, 3Br-8, and 4Br-8 are also recorded (Figure S2
and Table S1).
Aryl halides are wildly used in the transition-catalyzed cross-
coupling reactions.16 The cross-coupling reaction of halo-
genated BN-PAHs is an effective way to further tune their
photophysical properties.2o,7,10,14b,17 To our delight, Suzuki
and Heck cross coupling18 of 1Br-8/2Br-8 with phenylboronic
acid or styrene produced the corresponding coupling products
in low yield (Scheme 3).19 Attempts to utilize the
tribrominated (3Br-8) and tetra-brominated (4Br-8) bis-BN
phenanthrenes as cross-coupling partners resulted in complex
reaction mixtures.
The direct comparison of the photophysical properties of
1Ph-8, 2Ph-8, 1Sty-8, and 2Sty-8 is shown in Figure 5 and
Table 2. (See also Table S1.). The absorption bands of 1Ph-8,
2Ph-8, 1Sty-8, and 2Sty-8 are bathochromically shifted in the
a
Scheme 3. Further Functionalization of 1Br-8 and 2Br-8
Figure 4. Resonance structures of bis-BN phenanthrene 8 (left) and
the calculated stability of arenium ions, corresponding to the attack of
Br+ on different positions of 8, NBO values, and activation energies
determined at the B3LYP/6-311+G(2d,p) level of theory (right).
equally informative in rationalizing the site selectivity observed.
We performed theoretical calculations on the relative energies
of the arenium ions formed after the addition of bromine to 8
(Figure 4 and Figures S7−S11).The arenium ion intermediate
generated by Br+ attacking at the carbon (C-3/C-8) adjacent
to boron was treated as the zero energy reference, which is also
the most electron-rich position at which the bromination of
1,2-azaborine14a or BN-naphthalene14b took place. The second
lowest-lying arenium ion was observed at C-5/C-6 (3.50 kcal/
mol), in line with the site-selective tri- or tetra-bromination
results observed experimentally. The C-4/C-7 positions are not
located theoretically. The activation free-energy values of the
Br2 initial attack of each position were also calculated (C3/C8
= 15.0 kcal/mol; C5/C6 = 20.7 kcal/mol). Natural bond
a
Reaction conditions were not optimized.
C
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Letter
Accession Codes
CCDC 1974591−1974596 and 1975062 contain the supple-
mentary 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.
AUTHOR INFORMATION
Corresponding Authors
■
Figure 5. Normalized absorption (left) and emission spectra (right)
of 8, 1Ph-8, 2Ph-8, 1Sty-8, and 2Sty-8 in dichloromethane at a
concentration of 10−5 M.
Xuguang Liu − Tianjin Key Laboratory of Organic Solar Cells
and Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, 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, Shanghai 200032, People’s Republic of China;
orcid.org/0000-0002-7859-3139; Email: xuguangliu@
tjut.edu.cn
Table 2. Photophysical Properties of 8, 1Ph-8, 2Ph-8, 1Sty-
8, and 2Sty-8
a
a
b
c
comp
λabs (nm)
ε (M−1 cm−1
)
λem (λex) (nm)
Φpl
EGopt
8
307
333
348
369
400
25076
24674
45922
30746
62469
397 (322)
406 (333)
411 (348)
423 (369)
460 (400)
0.19
0.21
0.60
0.74
0.88
3.25
3.19
3.10
2.89
2.66
1Ph-8
2Ph-8
1Sty-8
2Sty-8
Lei Zhang − School of Science, Tianjin Chengjian University,
Tianjin 300384, People’s Republic of China; orcid.org/
0000-0003-0100-8357; Email: zhanglei-chem@tcu.edu.cn
a
b
Refer to the highest-intensity peak maxima values. Absolute
c
quantum yield in dichloromethane. Optical band gap EGopt
=
1240/λonset.
Authors
Lingjian Zi − Tianjin Key Laboratory of Organic Solar Cells
and Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
Jinyun Zhang − Tianjin Key Laboratory of Organic Solar Cells
and Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
Chenglong Li − Tianjin Key Laboratory of Organic Solar Cells
and Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
Yi Qu − Tianjin Key Laboratory of Organic Solar Cells and
Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
order of 1Ph-8 < 2Ph-8 < 1Sty-8 < 2Sty-8 (Figure 5, left),
demonstrating the extension of the π-conjugation by the
introduction of benzene or styrene moieties. The trend of the
emission fluorescence maxima of 1Ph-8, 2Ph-8, 1Sty-8, and
2Sty-8 is consistent with the trend of the UV−vis absorption
spectra (Figure 5, right).
In addition, four derivatives 1Ph-8 (0.21), 2Ph-8 (0.60),
1Sty-8 (0.74), and 2Sty-8 (0.88) displayed higher quantum
yields than the “parental” bis-BN phenanthrene 8 (0.19). The
band gaps estimated from the absorption onsets were found to
be 3.19, 3.10, 2.89, and 2.66 eV for 1Ph-8, 2Ph-8, 1Sty-8, and
2Sty-8, respectively (Table 2). These numbers are qualitatively
in agreement with the calculated values (Tables S4 and S5).
In summary, we have successfully synthesized two bis-BN
phenanthrenes. The aromaticities of the bis-BN phenanthrenes
were quantified by experimental and computational studies.
Notably, the number of the BN unit has a significant effect on
the reactivity of the BN phenanthrene toward bromination,
which was further explained by density functional theory
calculations. The stepwise bromination of bis-BN phenan-
threne 8 gave a series of brominated compounds. Preliminary
results showed that the mono- and dibrominated bis-BN
phenanthrenes can be further functionalized by cross-coupling
reactions, which can further tailor the photophysical properties
of these bis-BN phenanthrenes. Further studies using these bis-
BN phenanthrenes as building blocks in organic material are
currently underway in our laboratory.
Bin Zhen − Tianjin Key Laboratory of Organic Solar Cells and
Photochemical Conversion, Tianjin Key Laboratory of Drug
Targeting and Bioimaging, School of Chemistry and Chemical
Engineering, Tianjin University of Technology, Tianjin 300384,
People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00021
Notes
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00021.
■
The authors declare no competing financial interest.
sı
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
Natural Science Foundation of Tianjin (17JCZDJC37700,
18JCQNJC06900), the National Natural Science Foundation
of China (21502140), Tianjin University of Technology, and
the Shanghai Institute of Organic Chemistry (K2015-10). We
Full details of the synthesis of bis-BN phenanthrenes 7
and 8, NMR spectra, UV−vis and photoluminescence
data, X-ray crystallographic data, and theoretical
calculations (PDF)
D
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M. A.; Sucunza, D.; Frutos, L. M.; Salgado, A.; García-García, P.;
also thank the Training Project of Innovation Team of
Colleges and Universities in Tianjin (TD13-5020).
́
Vaquero, J. J. Org. Lett. 2018, 20, 4902−4906. (c) Abengozar, A.;
Sucunza, D.; García-García, P.; Vaquero, J. J. Beilstein J. Org. Chem.
́
2019, 15, 1257−1261. (d) Abengozar, A.; García-García, P.; Sucunza,
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(18) The Stille cross coupling of 1Br-8/2Br-8 with tributyl(4-
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solvent) resulted in a low yield as well.
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https://dx.doi.org/10.1021/acs.orglett.0c00021
Org. Lett. XXXX, XXX, XXX−XXX