Two BN isosteres of anthracene: Synthesis and characterization

Article abstract of DOI:10.1021/ja508813v

The synthesis of two parental BN anthracenes, 1 and 2, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spectroscopy studies revealed the following HOMO energy trend: anthracene, -7.4 eV; BN anthracene 1, -7.7 eV; bis-BN anthracene 2, -8.0 eV. The λ_max of the lower energy band in the UV-vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm. Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO-LUMO band gap remains unchanged across the anthracene series. The emission λ_max values for the three investigated anthracene compounds are at 403 nm. The pK_a values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN anthracenes 1 and 2 do not undergo heat- or light-induced cycloaddition reactions or Friedel-Crafts acylations. Electrophilic bromination of BN anthracene 1 with Br₂, however, occurs regioselectively at the 9-position. The reactivity behavior and regioselectivity of bromination of BN anthracenes are consistent with the electronic structure of these compounds; i.e., (1) the lower HOMO energy levels for BN anthracenes stabilize the molecules against cycloaddition and Friedel-Crafts reactions, and (2) the HOMO orbital coefficients are consistent with the observed bromination regioselectivity. Overall, this work demonstrates that BN/CC isosterism can be used as a molecular design strategy to stabilize the HOMO of acene-type structures while the optical band gap is maintained.

Full text of DOI:10.1021/ja508813v

Article
pubs.acs.org/JACS
Two BN Isosteres of Anthracene: Synthesis and Characterization
Jacob S. A. Ishibashi,†,‡,⊥ Jonathan L. Marshall,†,⊥ Audrey Maziere,§ Gabriel J. Lovinger,†,‡ Bo Li,‡
̀
Lev N. Zakharov,† Alain Dargelos,§ Alain Graciaa,§ Anna Chrostowska,*,§ and Shih-Yuan Liu*,†,‡
‡Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467-3860, United States
†Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States
§Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mater
́ ́
iaux, UMR CNRS 5254, Universite de Pau
et des Pays de l’Adour, Avenue de l’Universite, 64 000 Pau, France
S
* Supporting Information
ABSTRACT: The synthesis of two parental BN anthracenes,
1 and 2, was developed, and their electronic structure and
reactivity behavior were characterized in direct comparison
with all-carbon anthracene. Gas-phase UV-photoelecton spec-
troscopy studies revealed the following HOMO energy trend:
anthracene, −7.4 eV; BN anthracene 1, −7.7 eV; bis-BN
anthracene 2, −8.0 eV. The λmax of the lower energy band in
the UV−vis absorption spectrum is as follows: anthracene, 356
nm; BN anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm.
Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO−LUMO band gap remains unchanged
across the anthracene series. The emission λmax values for the three investigated anthracene compounds are at 403 nm. The pKa
values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN
anthracenes 1 and 2 do not undergo heat- or light-induced cycloaddition reactions or Friedel−Crafts acylations. Electrophilic
bromination of BN anthracene 1 with Br2, however, occurs regioselectively at the 9-position. The reactivity behavior and
regioselectivity of bromination of BN anthracenes are consistent with the electronic structure of these compounds; i.e., (1) the
lower HOMO energy levels for BN anthracenes stabilize the molecules against cycloaddition and Friedel−Crafts reactions, and
(2) the HOMO orbital coefficients are consistent with the observed bromination regioselectivity. Overall, this work demonstrates
that BN/CC isosterism can be used as a molecular design strategy to stabilize the HOMO of acene-type structures while the
optical band gap is maintained.
1
comprehensive study of 1,2-BN naphthalenes, including H,
13C, and 11B NMR characterization of two dozen derivatives,
complexation studies with (MeCN)3Cr(CO)3, and establishing
the equilibrium between the bis(azoniaboratanaphthyl) oxide
and its corresponding monomeric BOH structure.14 Ashe and
co-workers used established synthetic methods to investigate
the haptotropic migration of 1,2-BN naphthalenes,15 and most
recently, Molander’s group established Suzuki−Miyaura and
reductive cross-couplings of 3-bromo-1,2-BN naphthalenes.16
The original synthesis of 9,10-BN naphthalene by Dewar via
dehydrogenation of octahydro 9,10-BN naphthalene inter-
mediate was low-yielding (∼35%).6a Ashe and co-workers
improved the synthetic access to 9,10-BN naphthalene by
developing new synthetic methods based on the ring-closing
metathesis (RCM)/dehydrogenation sequence.17
1. INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) and their derivatives
have received significant attention in materials research because
of their modular synthetic access and their versatile, structure-
dependent properties that make them useful as semiconductors
in organic electronics applications.1 BN/CC isosterism, i.e., the
replacement of a CC unit with the isosteric B−N unit, has
emerged as a viable strategy to expand the chemical space of
PAHs, leading to compounds with similar geometric parame-
ters but quite distinct electronic structures.2,3 Dewar pioneered
the synthesis of BN isosteres of simple PAH in the 1950s and
1960s.4 Among the compounds that Dewar and his co-workers
prepared were 1,2-5 and 9,10-BN naphthalenes,6 9,10-BN
phenanthrenes,7 1,2-8,7-bis BN anthracenes,8 4a,10a-BN
phenanthrene,9 4,5-9,10-bis BN pyrenes,8 5,6- and 6,5-BN
benz[a]anthracenes,10 13,14-BN triphenylene,11 4,5-BN pyr-
ene,12 and 5,6-BN chrysene12 (Scheme 1). Since Dewar’s
pioneering work, new methods for the preparation of existing
and new BN isosteres of PAHs have been developed. With
regard to bicyclic BN naphthalenes, Paetzold in 1968 prepared
1,2-BN naphthalenes via the condensation of arylchloro-
(phenylamino)borane with phenylethyne.13 In 2004, Paetzold
adapted Dewar’s original synthetic strategy to provide a
With regard to the tricyclic BN isosteres of PAHs, in addition
to Dewar’s extensive work from 1958 to 1961, Williams
developed the oxidative photocyclization of anilinoboranes to
prepare 9,10-BN phenanthrenes.18 Additionally, Philp and co-
workers investigated the hydrogen-bonding behavior of the
Received: August 26, 2014
© XXXX American Chemical Society
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Scheme 1. Survey of Six-Membered-Ring-Containing BN
Isosteres of PAHs Bearing Isolated BN Units
Scheme 2. BN Heterocycles as Charge Transport Materials
than the corresponding carbonaceous dibenzo[g,p]chrysene.24
In 2013, Pei and co-workers synthesized BN-substituted
tetrathienonaphthalenes C and reported a hole mobility of up
to 0.15 cm2 V−1 s−1 for organic field-effect transistor (OFET)
devices based on those compounds.25 Most recently in 2014,
Pei and co-workers successfully synthesized BN coronene
derivative D and fabricated OFET devices which exhibit a hole
mobility of up to 0.23 cm2 V−1 s−1.26
Acenes, or linear benzofused polyaromatics, are a special class
of PAH which possesses favorable properties for use in organic
materials applications.27 For instance, single-crystal organic
field-effect transistors (SC-OFETs) based on rubrene
(5,6,11,12-tetraphenyltetracene) have achieved a benchmark
carrier mobility of 20−40 cm2 V−1 s−1.28 Similarly, pentacene
derivatives substituted at the 6- and 13-positions with alkynes
have demonstrated a field-effect mobility of up to 0.4 cm2 V−1
s−1 in organic thin-film transistors (OTFTs).29 Furthermore,
higher acenes, such as tetracene and pentacene derivatives, are
uniquely capable of serving as singlet fission materials.30
Despite the useful properties of acenes and the important
role they play in PAH chemistry, surprisingly few BN isosteres
of higher acenes have been reported to date. Parental BN acene
structures beyond the bicyclic naphthalene have remained
elusive. To the best of our knowledge, the only example of a
BN isostere of a higher acene is Dewar’s 1,2-8,7-bis BN
anthracene reported in 1960 (Scheme 1).8 In this paper, we
describe the synthesis of two parental BN-isosteres of
anthracene (1 and 2) and their characterization, which includes
ultraviolet photoelectron spectroscopy (UV-PES), optical
properties, and chemical reactivity.
NH-BOH-9,10-BN phenanthrene system.19 In 2007, Piers
prepared the “internal” BN phenanthrene isostere 4a,4b-BN
phenanthrene and found that its photophysical properties are
distinct from the carbonaceous phenanthrene and Dewar’s
“external” 9,10-BN phenanthrene.20 More recently, Wang
prepared 8a,9-BN phenanthrene via the photoelimination of
the corresponding cyclic pyridylborane precursor.21 In addition
to Dewar’s initial work in the 1960s on tetracyclic BN isosteres
of PAHs, Philp’s group investigated the hydrogen-bonding
behavior and dehydration reactivity of NH-BOH, 5,4-9,10-bis
BN pyrene.22 Furthermore, Piers and co-workers developed the
synthesis of 10a,10b-BN pyrene, in which the BN unit is
internalized.23
It was not until after 2010 when BN isosteres of PAHs have
been demonstrated to have favorable charge transport proper-
ties. In 2011, Nakamura’s group disclosed the synthesis of BN
dibenzochrysene A and BN hexabenzotetracene B (Scheme 2)
via an intramolecular electrophilic arene borylation and
demonstrated that the intrinsic hole mobility of A (0.07 cm2
V−1 s−1) measured by time-resolved microwave conductivity is
similar to that of rubrene and one order of magnitude better
2. EXPERIMENTAL AND COMPUTATIONAL METHODS
Coupled UV-Photoelectron Spectroscopy−Mass Spectrom-
etry Measurements. The UV-PES spectra were recorded on a
home-built (IPREM/ECP), three-part spectrometer equipped with a
main body device, He−I radiation source (21.21 and/or 48 eV), and a
127° cylindrical analyzer. The spectrometer works at constant analyzer
energy under 5 × 10−6 hPa working pressure and ≤10−7 hPa for
channeltron (X914L) pressure. The monitoring is done by a
microcomputer supplemented by a digital-analogue converter (AEI
spectrum). The spectra resulting from a single scan are built from 2048
points and are accurate within 0.05 eV. Spectra are calibrated with
lines of xenon (12.13 and 13.44 eV) and of argon (15.76 and 15.94
eV). The accuracy of the ionization energies is 0.03 eV for sharp
peaks and 0.05 eV for broad and overlapping signals. Mass spectra
were recorded on a modified quadrupole mass spectrometer
(PFEIFFER Prisma QMS200) with an electron impact at 50 eV
(mass range, 200 amu; detection limit, ≤10−14 hPa; working pressure,
2 × 10−7 hPa; operating temperature, 200 °C; electronic amplifier in
working conditions, 10−10 A; QUAD STAR422 software for recording
and treatment of MS data). The samples were slowly vaporized under
low pressure (10−6 hPa) inside a handmade three-valve injector (3/4 in.
B
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diameter; 10 cm length; working temperature, −190 °C ≤ T ≤ 300
°C), and the gaseous flow was then continuously and simultaneously
analyzed by both UV-photoelectron and mass spectrometers.
hot chlorobenzene. Starting with 10 g of 3, we were able to
synthesize up to 3.4 g of BN anthracene 1 in one batch without
involvement of column chromatography and with an overall
yield of 36%.44
Computational Methods. All calculations were performed using
the Gaussian 0931 program package with the 6-311G(d,p)32 basis set.
Extra diffuse functions [6-311++G(d,p)] are included in the basis set
to improve the description of the electron affinities (EAs). DFT has
been shown to predict various molecular properties of similar
compounds successfully.33 All geometry optimizations were carried
out with the CAM-B3LYP34 functionals and were followed by
frequency calculations in order to verify that the stationary points
obtained were true energy minima. Ionization energies (IEs) were
calculated with ΔSCF-DFT, which means that separate SCF
calculations were performed to optimize the orbitals of the ground
state and the appropriate ionic state (IE = Ecation − Eneutral). The
advantages of the most frequently employed ΔSCF-DFT method of
calculations of the first ionization energies have been demonstrated
previously.35 The TD-DFT36,37 approach provides a first-principal
method for the calculation of excitation energies within a density
functional context taking into account the low-lying ion calculated by
the ΔSCF method (the excitation energies of the radical cation
obtained from a TD-DFT treatment were added to the ionization
energy that was computed with the ΔSCF-DFT method). The vertical
ionization energies were also calculated at the ab initio level according
to the OVGF38 (in this case the effects of electron correlation and
reorganization are included beyond the Hartree−Fock approximation,
and the self-energy part was expanded up to third order) and SAC−
CI39 (the symmetry adapted cluster/configuration interaction method
of Nakatsuji and Hirao, which describes accurately and efficiently the
electronic structures of the excited, ionized, and electron-attached
states of molecules) methods. MOLEKEL40 and GaussView were used
as visualization tools for MOs.
The synthesis of bis-BN anthracene 2 commenced with the
oxidation of commercially available 1,4-dibromo-2,5-dimethyl-
45
benzene 7 with KMnO4 followed by Curtius rearrangement
to produce N-Boc-protected dibromide 8 (Scheme 4). Cross-
Scheme 4. Synthesis of Bis-BN Anthracene 2
coupling of 8 with potassium vinyltrifluoroborate furnished the
bis-vinyl adduct 9. Removal of the N-Boc protecting group was
accomplished with trifluoroacetic acid to reveal the key
cyclization precursor 10. Gratifyingly, double cyclization with
boron trichloride proceeded smoothly, and subsequent treat-
ment of the resulting crude mixture with LiAlH4 gave bis-BN
anthracene 2 in 78% yield over two steps. Alternatively, when
the initial cyclized intermediate was treated with MeLi instead
of LiAlH4, bis-BN anthracene 2a was produced.
UV-Photoelectron Spectroscopy. UV-PES experimen-
tally determines the gas-phase ionization energies of molecules
that can be correlated to the energies of occupied molecular
orbitals. For a reliable assignment of UV photoelectron
spectroscopic bands and for the interpretation of spectra, a
combined UV-PES/theoretical approach is necessary. Chros-
towska’s group has calibrated different computational methods
[e.g., the standard outer valence green function (OVGF),
density functional theory (DFT), self-consistent field/time-
dependent density functional theory (ΔSCF/TD-DFT), TD-
DFT, complete active space second-order perturbation theory
(CASPT2), and statistical average of different orbital model
potential exchange−correlation functional (SAOP XC)] against
the experimentally determined UV-PES IEs.36 The combined
UV-PES/computational modeling approach developed by
Chrostowska and co-workers is used to investigate the
electronic structure of BN anthracenes 1 and 2.
3. RESULTS AND DISCUSSION
Synthesis. The syntheses of targeted compounds 1 and 2
were adapted from Dewar’s original protocol for the
preparation of 1,2-BN naphthalenes.5 The key step involves
the borylative cyclization of an aminostyrene. Commercially
available starting material 3-amino-2-naphthoic acid 3 under-
went a Sandmeyer reaction to generate the iodinated
compound 4 in good yield (Scheme 3).41 Subsequent Curtius
Scheme 3. Synthesis of 1,2-BN Anthracene 1
Figure 1 illustrates the UV-PE spectra and the highest
occupied MOs (HOMOs) corresponding to the experimentally
determined IEs of each of the three anthracene molecules. The
UV-PES of anthracene has been reported.46 For the sake of
consistency and to allow for a direct comparison among the
compounds under investigation, we collected its UV-PES data
again (Figure 1a). Its first ionization occurs at 7.4 eV and
corresponds to ejection of an electron from the HOMO (b2g
symmetry). The second, third, and fourth ionizations occur at
8.6, 9.2, and 10.2 eV and correspond to HOMO−1 (b1g),
HOMO−2 (au), and HOMO−3 (b2g), respectively. The first
band of the PE spectrum of BN anthracene 1 (point group Cs)
can be found at 7.7 eV and corresponds to its HOMO. This
orbital has A″ symmetry and can be compared with the HOMO
rearrangement of 4 using diphenylphosphoryl azide (DPPA) as
the azide source resulted in the formation of 3-iodonaphthalen-
2-amine 5.42 Suzuki cross-coupling of intermediate 5 with
potassium vinyltrifluoroborate furnished 2-amino-3-vinylnaph-
thalene 6.43 Treatment of precursor 6 with boron trichloride
followed by lithium aluminum hydride (LiAlH4) produced 1,2-
BN anthracene 1. Alternatively, when MeLi is used as the
nucleophile instead of LiAlH4, the B-Me substituted 1,2-BN
anthracene 1a is generated. The parent 1,2-BN anthracene 1 is
air-stable and can be readily purified by recrystallization from
C
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Figure 1. UV-PE spectra of (a) anthracene, (b) BN anthracene 1, and (c) bis-BN anthracene 2. The vertical axis is defined as the negative value of
the experimentally determined ionization energy (−IE). The top four occupied molecular orbitals associated with the corresponding ionization
energies for each of the molecules are also depicted.
of anthracene. The second (8.3 eV), third (9.6 eV), and fourth
ionizations (10.2 eV) correspond to HOMO−1 (A″),
HOMO−2 (A″), and HOMO−3 (A″), respectively. For bis-
BN anthracene 2 (point group C2h), the first ionization band
was found at 8.0 eV (corresponding to the HOMO, bg),
followed by bands at 8.2 eV (bg), 9.7 eV (au), and 10.4 eV (au).
It is worth noting that the four highest occupied MOs for each
anthracene molecule are π-type orbitals.
According the frontier molecular orbital theory,47 the
electronic structure determination of the HOMO and LUMO
is most significant in terms of property and reactivity
predictions. Figure 1 shows that for the anthracene system, a
trend of decreasing HOMO energy levels with more BN units
is observed: anthracene (−7.4 eV) > 1,2-BN anthracene 1
(−7.7 eV) > bis-BN anthracene (−8.0 eV). This is in stark
contrast to simple monocyclic arenes, where BN/CC isosterism
results in significant destabilization of the HOMO.48,49 The
stabilization of the HOMO is an important objective, as
molecules with a high HOMO energy level typically lead to
reactive, air sensitive materials.50,51 Thus, our UV-PES analysis
suggests that BN/CC isosterism could potentially serve as a
design strategy to stabilize otherwise unstable PAHs com-
pounds (e.g., higher acenes).
Absorption and Emission Spectra. Figure 2 illustrates
the absorption spectra of anthracene, BN anthracene 1, and bis-
BN anthracene 2 in cyclohexane. The lowest-energy absorption
peaks for BN anthracenes and the carbonaceous anthracene are
all centered around 377 nm. This observation is again in
contrast to the behavior of simple monocyclic arenes, for which
it has been determined that BN/CC isosterism causes a
significant bathochromic shift of the lowest-energy absorption
band.48,52 Figure 2 also shows that in contrast to the
carbonaceous anthracene, a second electronic transition can
Figure 2. Absorption spectra of anthracene, BN anthracene 1, and bis-
BN anthracene 2 measured in cyclohexane at 5.5 × 10−6
M
concentration.
∼310 nm is predicted to have a relatively strong oscillator
strength and originates mostly from the transition of HOMO−
1 → LUMO with some minor involvement of the LUMO+1
(for BN anthracene 1) and LUMO+2 (for bis-BN anthracene
2).
Figure 3 shows the emission spectra of anthracene, BN
anthracene 1, and bis-BN anthracene 2 in cyclohexane. The
highest-energy emission peaks for the three anthracene
molecules are located around 387 nm with vibrational energy
levels being resolved. The difference between the lowest-energy
absorption peak (377 nm) and the highest-energy emission peak
(387 nm) is relatively small, suggesting a small structural
reorganization going from the ground state to the first excited
state.
The excitation spectra correlate with the absorption spectra
for BN anthracenes 1 and 2, and when the BN anthracenes are
excited at either the low- or the high-energy absorption band in
a nonpolar solvent, the resulting emission peaks are the same
without loss of vibrational fine structure (see the Supporting
Information for details). These observations are consistent with
Kasha’s rule; i.e., the observed emission bands originate from
the lowest singlet excited state.54
be observed for BN anthracenes (λmax(1) = 327 nm, λmax(2)
=
311 nm) in the 280−400 nm window. As is commonly seen
with PAHs in nonpolar solutions, the vibrational fine structure
for the electronic transitions illustrated in Figure 2 is well-
resolved. According to TD-DFT calculations (see the
Supporting Information for details), the lower-energy absorp-
tion band at ∼360 nm can be ascribed to the S0 → S1 transition
for all anthracene compounds.53 This excitation mainly involves
a HOMO → LUMO transition with relatively small oscillator
strength. The high-energy band observed for BN anthracenes at
The emission intensity at the highest-energy peak was linear
from 4.5 × 10−6 to 1.8 × 10−5 M. At higher concentrations, the
D
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(corresponding to the low-energy peak)58 remain similar for
all three anthracenes: anthracene, 3.26 eV; BN anthracene 1,
3.22 eV; and bis-BN anthracene, 3.23 eV. TD-DFT calculations
show that the S0 → S2 transition mostly originates from the
excitation of electrons in the HOMO−1 orbital, and UV-PES
data show a destabilizing trend of the HOMO−1 energy with
increasing BN units. Thus, the observation of an additional
absorption band within the 280−400 nm window for the BN
anthracenes 1 and 2 is consistent with their high-lying
HOMO−1 and low-lying LUMO orbitals relative to those of
anthracene (Figure 4).59
Survey of Reactivity. We have determined that both BN
anthracenes 1 and 2 exhibit good air stability, much like
Dewar’s 1,2-BN naphthalene.5a Samples of each 1 and 2 do not
show appreciable decomposition by 1H NMR analysis when left
open to air and light on the benchtop for 7 days. We attempted
to determine the pKa of 1 using a bracketing study. However,
when we treated 1 with potassium hexamethyldisilazide
(KHMDS) to deprotonate the N-H proton, we observed
addition to boron. To avoid complications associated with the
reactivity of the B−H group, we performed all further reactivity
studies on the B−Me derivatives 1a and 2a.
Figure 3. Emission spectra of anthracene, BN anthracene 1, and bis-
BN anthracene 2 measured in cyclohexane at 5.5 × 10−6
M
concentration (excitation wavelength: 360 nm).
fluorescence did not increase linearly with concentration (see
the Supporting Information for details), indicating concen-
tration-dependent quenching. There was no evidence for
excimer formation, as the emission profile remained unchanged
over the range of concentrations that we investigated.55,56
Compounds 1 and 2 have fluorescence quantum yields of 0.38
and 0.68, respectively; anthracene has a reported quantum yield
of 0.36.57
The table and the orbital diagram in Figure 4 summarize key
experimental data that describe the electronic structure of the
anthracene series. From UV-PES studies we learned that the
energy of the HOMO decreases in the order of anthracene >
BN anthracene > bis-BN anthracene. According to the UV−vis
absorption spectra, the estimated HOMO−LUMO gaps
KHMDS cleanly deprotonates 1a at nitrogen to furnish the
corresponding conjugate base of 1a. By quenching this anion
with a variety of potential proton sources, we determined that
the pKa of 1a is approximately 26; the anion partially
deprotonated pentamethylcyclopentadiene (Cp*H) (pKa = 26
−
in DMSO), as evidenced by peaks for both C5Me5 and
C5Me5H in the 1H NMR spectrum. Ashe and co-workers
similarly observed partial deprotonation of Cp*H when they
treated it with the anion of monocyclic B-phenyl-1,2-azaborine
and assigned its pKa at approximately 26.60 Using the same
method, the pKa of bis-BN anthracene 2a was determined to be
also ∼26.
The cycloaddition chemistry of anthracene is well-known.61
Both catalyzed and uncatalyzed Diels−Alder reactions occur
across the central ring (Scheme 5).62 We replicated these
Scheme 5. Survey of Cycloaddition and Friedel−Crafts-Type
Reactivity
reaction conditions to attempt a [4 + 2] cycloaddition. When
we treated 1a or 2a with a variety of dienophiles, we did not
observe cycloaddition reactions (see the Supporting Informa-
tion for details). The very electron deficient tetracyanoethylene
dienophile, for example, did not react with 1a or 2a when
heated up to 130 °C in benzene-d6. Under identical conditions,
anthracene fully converted to the cycloaddition product within
hours.63 Iron(III)- and aluminum(III)-catalyzed conditions64
also did not produce any cycloadducts. We also attempted to
use compound 1a as a dienophile instead of a diene, and no
reaction was observed.
Figure 4. Summary of key experimental data.
E
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It is known that UV light induces a [4 + 4] dimerization
across anthracene’s central ring, yielding the so-called
“butterfly” dimer (Scheme 5).61 We did not observe a [4 +
4] dimerization reaction when we irradiated 1a or 2a with UV
light. Similarly, neither compound 1a nor 2a undergoes a
Friedel−Crafts alkylation when treated with ethyl bromide in
the presence of a Lewis acid (e.g., iron trichloride or aluminum
trichloride). We were able to recover only starting materials
with no evidence of reaction, even with excess Lewis acid
present. Attempted Friedel−Crafts acylation65 reactions of 1a
or 2a using acetic anhydride and aluminum trichloride were
similarly ineffective. The generally observed lack of cyclo-
addition and Friedel−Crafts reactivity for BN anthracenes is
consistent with their relatively low-lying HOMO energy levels
as determined by UV-PES.
Scheme 6. Synthesis and Crystallographic Characterization
of 1b-TPA
Despite the relatively low propensity of BN anthracenes to
undergo cycloaddition and Friedel−Crafts reactions, they do
react with halogens.66 For instance, treatment of 1a with
elemental bromine at 0 °C generates the brominated BN
anthracene 1a-Br regioselectively in good yield (Figure 5).67
HOMO energy trend: anthracene, −7.4 eV; BN anthracene 1,
−7.7 eV; bis-BN anthracene 2, −8.0 eV. In stark contrast to
monocyclic arenes, BN/CC isosterism in the context of
anthracene leads to stabilization of the HOMO. The absorption
behaviors of the low-energy band of the anthracene series (i.e.,
anthracene, 1, and 2) are similar, which is also in contrast to the
monocyclic system, where a bathochromic shift was observed
for BN isosteres of benzene and toluene vs their carbonaceous
counterparts. The λmax of the lower energy band in the UV−vis
absorption spectrum is as follows: anthracene, 356 nm; BN
anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm. The
HOMO−LUMO band gap remains unchanged across the
anthracene series. Consistent with Kasha’s rule, the emission
for the anthracene series originates from the corresponding
singlet excited states. The emission λmax for the three
investigated anthracene compounds are all at 403 nm. We
determined the pKa of the N-H proton for BN anthracene 1
and bis-BN anthracene 2 to be approximately 26. In contrast to
anthracene, its BN isosteres 1 and 2 do not undergo thermal or
photoinduced cycloaddition reactions and Friedel−Crafts
acylations. BN anthracene 1a, however, undergoes regioselec-
tive electrophilic bromination with Br2 at the 9-position. The
reactivity behavior of BN anthracenes is consistent with the
electronic structure of these compounds; i.e., (1) the lower
HOMO energy levels for BN anthracenes stabilize the
molecules against cycloaddition and Friedel−Crafts reactions,
and (2) the HOMO orbital coefficients are consistent with the
observed bromination regioselectivity. Overall, this work
demonstrates that BN/CC isosterism can be used as a
molecular design strategy to stabilize the HOMO of acene-
type structures while the optical band gap is maintained.
Current efforts are directed at developing BN isosteres of
higher linear acenes.
Figure 5. Regioselective halogenation of BN anthracenes 1.
The electrophilic halogenations of 1a preferentially take place
at the 9-position, which is in stark contrast to what has been
observed for 1,2-BN naphthalenes,5b,16a 9,10-BN naphthale-
nes,6b and monocyclic 1,2-azaborines,68 where the carbon
adjacent to boron has been determined to be the most reactive
position. As can be seen from Figure 5, the HOMO orbital
coefficient at the 9-position for 1a is relatively large while the
corresponding coefficients at the C(3) position are diminish-
ingly small (Figure 5, bottom left). On the other hand, while
the electrostatic potential map for 1a indicates a significant
negative charge at the 9-position, substantial negative charge is
also located at the C(3) position (Figure 5, bottom right).
Thus, it appears that the reaction of BN anthracenes 1a with
Br2 is orbitally controlled, which is consistent with the “soft”
nature of the electrophilic Br2 reagent.47b,69
To further support our assignment of the regioselective
bromination reaction, we prepared BN anthracene 1b-TPA
from BN anthracene 1b by using the previously employed
bromination conditions followed by Suzuki cross-coupling with
(4-(diphenylamino)phenyl)boronic acid (Scheme 6). Crystals
of 1b-TPA suitable for single-crystal X-ray diffraction analysis
were grown from a pentane/Et2O solution of 1b-TPA, and the
structure illustrated in Scheme 6 unambiguously confirms our
regiochemical assignment of the bromination reaction.70
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, spectroscopic data, additional
computational details, complete ref 31, and crystallographic
information, including CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
4. CONCLUSION
AUTHOR INFORMATION
Corresponding Authors
shihyuan.liu@bc.edu;
In summary, we prepared the first parental BN anthracenes 1
and 2 and characterized their electronic structure and reactivity
behavior in direct comparison with the carbonaceous
anthracene. Gas-phase UV-PES studies revealed the following
■
anna.chrostowska@univ.pau.fr
F
dx.doi.org/10.1021/ja508813v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
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Author Contributions
⊥J.S.A.I. and J.L.M. contributed equally to this paper.
Notes
The authors declare no competing financial interest.
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III Organometallics 2006, 25, 513−518. (b) Rohr, A. D.; Kampf, J. W.;
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ACKNOWLEDGMENTS
■
This paper is dedicated to Prof. Lawrence T. Scott on the
occasion of his 70th birthday. Support has been provided by the
Defense Threat Reduction Agency (Grant HDTRA1-11-1-
0045). We thank Prof. Michael M. Haley for helpful
discussions. A.C. and A.M. are grateful to the Communaute
de Communes de Lacq (France) for financial support.
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