Efficient Modular Synthesis of Substituted Borazaronaphthalene

Article abstract of DOI:10.1021/acs.organomet.6b00811

A highly efficient modular synthetic method for BN-fused polycyclic aromatic hydrocarbons (PAHs) composed of a borazaronaphthalene core along with multiple functionalized sites is reported. The halogenated 2,1-borazaronaphthalene cores are constructed thr

Full text of DOI:10.1021/acs.organomet.6b00811

Communication
pubs.acs.org/Organometallics
Efficient Modular Synthesis of Substituted Borazaronaphthalene
Fang-Dong Zhuang, Ji-Min Han, Sheng Tang, Jing-Hui Yang, Qi-Ran Chen, Jie-Yu Wang,* and Jian Pei*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry
of Education, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and
Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China
*
S Supporting Information
ABSTRACT: A highly efficient modular synthetic method for BN-fused polycyclic aromatic hydrocarbons (PAHs) composed of
a borazaronaphthalene core along with multiple functionalized sites is reported. The halogenated 2,1-borazaronaphthalene cores
are constructed through a ring-closing reaction between o-ethynylaniline derivatives and boron halides. The controllable halogen
substituents make further derivatization of 2,1-borazaronaphthalene cores feasible by using metal-catalyzed cross-coupling
reactions. On the basis of the carefully designed precursors, various functional aromatic rings can be fused to the azaborine core
via a one-pot nucleophilic tandem reaction, affording previously inaccessible PAHs in moderate yields.
olycyclic aromatic hydrocarbons (PAHs) have attracted
considerable interest, given their wide applications in
borazaronaphthalene with all three sites, B atom, C3, and C4,
to be modified remains difficult. Herein, we report for the first
time the synthesis of halogenated 2,1-borazaronaphthalene and
their derivatives through a nucleophilic reaction between o-
ethynylaniline derivatives and boron halides, which not only
provides a feasible way to modify B, C3, and C4 positions
effectively but also affords previously inaccessible larger BN-
PAHs through a modular synthetic strategy.
P
organic semiconducting materials and electronic devices, such
1a
as organic light-emitting diodes (OLEDs), organic field-effect
1b,c
1d,e
transistors (OFETs),
and organic solar cells (OSCs).
In
comparison with conventional inorganic materials, they have
several advantages, such as light weight, low cost, flexible
tailored properties, convenient large-area fabrication, and the
2
great possibility of postfunctionalization. For the past few
To demonstrate our design, we first chose 2-
(phenylethynyl)aniline and dichloro(phenyl)borane as the
model systems for simplicity. As shown in Scheme 1, the
target product, 4-chloro-2,3-diphenyl-2,1-BN-naphthalene (2a),
can be smoothly prepared in good yield by a one-pot reaction.
After examination of several conditions, the solvent, temper-
ature, and reaction time were carefully optimized. It is proved
that the best yield of 82% could be achieved in 1,2,4-
trichlorobenzene (1,2,4-TCB) at 220 °C for 12 h. The ring-
closing and halogenation reactions can be simply completed
simultaneously. Triethylamine was added as a weak base to
neutralize the HCl generated in the ring-closing reaction. To
our delight, the reaction can be easily applied to a wide range of
substrates. As shown in Scheme 1, cyclizations with electron-
neutral (2c), electron-poor (2d), and electron-rich (2e) 2-
(arylethynyl)anilines and even a terminal alkyne (2b)
proceeded in good yields, and most of them gave more than
80% yields except for 2e, which is probably due to the
instability of the precursor. All of the products were confirmed
years, the replacement of a CC unit with its isoelectronic and
isostructural BN unit in PAHs has become a promising strategy
to tune the molecular geometries, electronic properties, and
3
molecular packing patterns in solid phases, which could
decrease the gap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
LUMO), lower the reorganization energy in the single crystals,
(
and afford additional intermolecular dipole−dipole interac-
4
tions. Therefore, the synthesis of novel functionalized BN-
PAHs can provide new opportunities for high-performance
organic optoelectronic devices.
In particular, 2,1-borazaronaphthalene, which represents one
of the smallest BN-PAHs, is a good platform to construct larger
functional BN-PAHs. Pioneering efforts by several research
groups have demonstrated successful synthesis and post-
functionalization of 2,1-borazaronaphthalene. Dewar and his
co-workers synthesized 2,1-borazaronaphthalene via an electro-
philic substitution, where further functionalization at the B
atom could be achieved through nucleophilic substitution and
1
13
11
by HRMS and H, C and B NMR spectra. In addition, the
5
Stille coupling. On the basis of 2,1-borazaronaphthalene,
molecular structure of 2a was further confirmed by a single-
Molander brominated C3 near the B atom to achieve further
functionalization through Suzuki−Miyaura cross-coupling
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
6
reactions with potassium organotrifluoroborates. Paetzold
and co-workers also synthesized C4-substituted 2,1-borazar-
7
onaphthalene via a three-component reaction. Despite all of
Received: October 26, 2016
the pioneering work above, an efficient construction of 2,1-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00811
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Communication
Scheme 1. (a) Synthesis of 4-Chloro-2,1-BN-naphthalenes 2
and (b) Single-Crystal Structure of Compound 2a
Moreover, the chlorine substituent in compound 2 can be
easily replaced by other halogens simultaneously within the
ring-closing reaction, simply by adding the halogen ions as
nucleophiles in the reaction (Scheme 2b). Three equivalents of
tetrabutylammonium bromide (TBAB) and tetrabutylammo-
nium iodide (TBAI) was added in situ to generate the
corresponding Br- and I-substituted products in good yields,
respectively. 2a was also observed in these reactions, which
could be separated by means of gel permeation chromatog-
raphy (GPC). The controllable diverse halogen substitution at
BN-PAHs provides possibilities to tune the molecular packing
in the solid state toward high-performance organic electronic
materials, since halogen substituents could afford additional
interactions, such as halogen−halogen interactions and
10
halogen−π interactions. In addition, the intermolecular
displacements and distances could also be substantially varied
11
by tuning the substituent halogen substituents. For instance,
substitution with large halogens such as bromine and iodine
could increase the torsional angle of molecules and reduce their
crystallinity, resulting in the performance improvement of
12
OSCs. However, the synthesis of halogenated structures is
inconvenient. It is usually required to bring in the halogen
substituents in the initial step of the whole synthesis, which
would increase the synthetic complexity of target molecules
containing different halogens. In this context, our strategy
provides an efficient way to construct BN-PAHs with different
halogens, and the halogens can be achieved in the final step
together with a ring-closing reaction.
According to the experimental results and the literature
13
report, a proposed mechanism is illustrated in Scheme 2a.
First, BCl Ph combined with an amino group by eliminating
2
HCl to afford the B−N-bonded intermediate II. Then the
alkyne was activated by coordinating with the empty orbital of
B atom. Subsequently, the nucleophile attacked the electron-
deficient alkyne, followed by the formation of 2,1-BN-
naphthalene backbone IV. According to this mechanism,
when we use different halide nucleophiles, the substituent at
the C4 site can be changed correspondingly.
The different halogen substituents at the C4 site of 2,1-BN-
naphthalene could not only tune the electronic properties and
change the molecular arrangement but also allow postfunction-
alization at the C4 site. We then turned toward examining the
metal-catalyzed cross-coupling reactions. Taking compound 3a
for example, its reactivity toward a Suzuki cross-coupling
reaction was investigated with different arylboronic acids. As
crystal structure, which was obtained in chloroform solution by
slow evaporation. The X-ray crystal data associated with the
8
solid-state superstructures are summarized in the Supporting
Information and illustrated in Scheme 1b. The X-ray diffraction
data of 2a indicate that the B−N bond length (1.416(4) Å) is
shorter than the expected value of a B−N single bond (1.58
9a
Å), yet a little longer than the localized BN double-bond
length (1.403(2) Å). In comparison with the pristine BN-
9
naphthalene (1.461(1) Å), compound 2a has a much shorter
B−N bond, implying a stronger localization in this structure.
The C-substituted phenyl ring is approximately perpendicular
to the central BN ring with a dihedral angle of 68.1(4)°, and
between the B-substituted phenyl ring and BN ring, the
dihedral angle is smaller (27.2(5)°).
Scheme 2. (a) Proposed Mechanism of the Ring-Closing Reaction of o-Ethynylaniline Derivatives and (b) Halogen-Exchanging
Reactions
B
DOI: 10.1021/acs.organomet.6b00811
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Organometallics
Communication
shown in Table 1, the coupling reactions between compound
Scheme 3. Synthesis of Larger Conjugated Systems through
the One-Pot Nucleophilic Tandem Reaction
3
a and electron-neutral and electron-rich arylboronic acids gave
Table 1. Suzuki Cross-Coupling Reactions of Compound 3a
entry
substrate Ar
product
yield (%)
1
2
3
phenyl
thienyl
4a
4b
4c
90
91
75
p-methoxycarbonylphenyl
excellent yields of up to 91% (4a,b). The electron-poor
compound 4c can also be obtained in a satisfying yield of 75%.
Such preliminary results suggest a good chemical compatibility
of the borazaronaphthalene cores. In comparison with the
reported methods, our conditions of cross-coupling reactions
on 2,1-borazaronaphthalene do not need expensive catalysts
and strict operations. We believe that other cross-coupling
reactions such as Stille, Negishi, and Sonagashira reactions
could also occur in addition to Suzuki reactions. Thus, our
work for the first time realized tunable and effective
postfunctionalization at C4 in BN-naphthalene.
Second, a Friedel−Crafts-like electrophilic substitution reaction
occurred to close the second ring, after elimination of the third
halide anion. For compounds 6a,b, these two steps of the
reaction can be completed in one pot because of the electron-
The absorption and fluorescence spectra of these compounds
were recorded in CHCl solution (Figures S1 and S2 in the
3
14
Supporting Information). All of the compounds exhibited
rich properties of the bithiophene fragment. However, for
similar absorption maxima and onset, except for 2b,e (Figure
compound 8, adding AlCl is necessary in the second step for
3
1). The slight difference in absorption may be related to the
the electrophilic substitution, because the biphenyl group is less
15
electron rich than the bithiophenyl group.
The absorption and emission spectra of compounds 6 and 8
were investigated, as shown in Figure S3 in the Supporting
Information. Compounds 6 show a broad absorption band peak
at 428 nm which might be ascribed to intramolecular charge
16
transfer. In the emission spectra, compounds 6 show a
substantial red shift of the emission maxima in comparison to
compound 8 (from λ 436 to 522 nm). The results indicate that
the photophysical properties of these BN-fused PAHs could be
easily adjusted by changing the cyclization precursor, providing
a convenient way to synthesize BN-fused PAHs with different
properties.
From these examples, we believe that this method could be
used in more fused rings, which would enable the synthesis of
more complicated and functional BN-fused PAHs. Further
research on the application of these BN-fused PAHs in
electronic devices is ongoing.
−
5
Figure 1. UV−vis absorption (1 × 10 M) of compounds 2a,b,e in
In summary, we have developed an efficient method to
synthesize 2,1-borazaronaphthalene-based PAHs. Starting from
o-ethynylaniline derivatives, we obtained the 2,1-BN-naphtha-
lene cores in one step and different halogen anions could be
attached to the C4 site. The halogen at the C4 position could
be easily functionalized by a Suzuki cross-coupling reaction.
Larger BN-fused PAHs were also constructed through one-pot
nucleophilic tandem reactions. Taking advantage of the
chemical flexibility and extensibility, more BN-PAH-based
materials with diversiform structures can be synthesized
through such a modular one-pot tandem synthetic protocol.
Application of this chemistry to functional optical and
electronic materials is currently being explored in our
laboratory.
chloroform.
bulky aryl group, which restricts the rotation of the substituent
groups at the C3, C4, and B atom, and influences the π−π
interaction between the substituent aryl group and BN ring.
Interestingly, larger BN-fused conjugated backones could be
constructed through a tandem synthetic strategy derived from
the above synthetic method, which closed two six-membered
rings in one step (Scheme 3). Boron trichloride and boron
tribromide were used to introduce the chlorine and bromine
atoms in the final products, respectively. The formation of
compounds 6a,b and 8 involved two steps. First, the
nucleophile attacked the alkyne, closing the first ring, which
is identical with the mechanism described in Scheme 2a.
C
DOI: 10.1021/acs.organomet.6b00811
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Communication
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The Supporting Information is available free of charge on the
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met.6b00811.
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Experimental details, photophysical properties, crystallo-
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Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Authors
E-mail for J.-Y.W.: jieyuwang@pku.edu.cn.
E-mail for J.P.: jianpei@pku.edu.cn.
ORCID
Jian Pei: 0000-0002-2222-5361
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.organomet.6b00811
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