Planarized triarylboranes: Stabilization by structural constraint and their plane-to-bowl conversion

Article abstract of DOI:10.1021/ja211944q

Triphenylborane and 9,10-diphenyl-9,10-dihydro-9,10-diboraanthracene, constrained to a planar arrangement with methylene tethers, were synthesized by intramolecular multi-fold Friedel-Crafts cyclization. These compounds were stable toward air, water, and amines, despite the absence of steric protection in the vertical direction with respect to the B atoms, and showed characteristic structural, electronic, and photophysical properties. In addition, upon treatment with a fluoride ion, these compounds underwent a plane-to-bowl conversion in a controlled manner.

Full text of DOI:10.1021/ja211944q

Communication
pubs.acs.org/JACS
Planarized Triarylboranes: Stabilization by Structural Constraint and
Their Plane-to-Bowl Conversion
Zhiguo Zhou,† Atsushi Wakamiya,*,‡ Tomokatsu Kushida,† and Shigehiro Yamaguchi*,†
†Department of Chemistry, Graduate School of Science, Nagoya University and JST-CREST, Furo, Chikusa, Nagoya 464-8602, Japan
‡Institute for Chemical Research, Kyoto University and JST-PRESTO, Uji, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: Triphenylborane and 9,10-diphenyl-9,10-
dihydro-9,10-diboraanthracene, constrained to a planar
arrangement with methylene tethers, were synthesized by
intramolecular multi-fold Friedel−Crafts cyclization. These
compounds were stable toward air, water, and amines,
despite the absence of steric protection in the vertical
direction with respect to the B atoms, and showed
characteristic structural, electronic, and photophysical
properties. In addition, upon treatment with a fluoride
ion, these compounds underwent a plane-to-bowl
conversion in a controlled manner.
Figure 1. Molecular design. (a) Schematic representation of planarized
triarylborane. (b) Planarized triphenylborane 1 and a diboron
homologue 2.
riarylborane represents an important electron-accepting
building unit for π-conjugated materials.1 A number of
the π conjugation through the vacant p orbital of the B atom.
T
Whereas a triphenylborane constrained with ethylene tethers
was reported by Okada and Oda,6 a completely planarized
derivative is unprecedented. As a fundamental character of the
planarized boranes, we are also interested in whether these
compounds still maintain reactivity toward a strong Lewis base.
If so, we can expect an additional possibility, such as a stimuli-
responsive structural change. The addition of an external Lewis
base would induce the coordination number change of the
boron from tri-coordination to tetra-coordination,7 resulting in
a plane-to-bowl conversion accompanied by a significant
electronic perturbation (Figure 1a).8 We now report the
efficient synthesis of these planarized triarylboranes and discuss
the impacts of the planar constraint on their structures,
electronic properties, and reactivity.
Our synthetic strategy to construct the planarized borane 1
was to conduct a two-fold intramolecular Friedel−Crafts
cyclization in a simultaneous fashion using a di(2-propenyl)-
substituted triarylborane 4 as the key precursor (Scheme 1a).
This route has already proven effective for the synthesis of
constrained triphenylphosphines and -arsines, as reported by
Hellwinkel and co-workers, in which they conducted the
proton-promoted cyclization.9 Compound 4 was prepared by
the reaction of a bromoborane 3 with 2,6-di(2-propenyl)-
phenyllithium in toluene in 80% yield. For the cyclization of 4,
the choice of Lewis acid was crucial. All of our initial attempts
using various proton acids as well as Lewis acids, such as AlCl3,
FeCl3, BF3·OEt2, and TiCl4, only gave complex mixtures.
Alternatively, we focused on metal triflates as the Lewis acid.
fascinating materials with the triarylborane substructure have
been developed for various applications, such as organic light-
emitting diodes, two-photon absorption materials, and anion
sensors.1 In the conventional molecular designs for these
materials, a key issue is to sterically protect the boron moiety
due to its inherent high reactivity. Most of the organoboron
materials so far reported employed at least one or two bulky
aryl groups on the B atom, such as mesityl,1,2 triisopropyl-
phenyl,3 and supermesityl groups.4 However, their steric
bulkiness sometimes has a detrimental effect on the solid-
state properties that rely on the intermolecular interaction, such
as charge carrier transporting properties.
We now introduce a new idea for the stabilization of the tri-
coordinated boron-containing π skeleton, which is “kinetic
stabilization not by steric bulkiness, but by structural
constraint”. On the basis of this concept we designed two
target compounds, a planarized triphenylborane 1 and its
diboron homologue 2 (Figure 1). Although these compounds
do not have any steric protection for the B atom in the vertical
direction, they should be stabilized, because the rigidly fixed
cyclic skeleton around the B atom would retard a
decomposition process through the reaction with Lewis basic
species, due to the destabilization of a tetra-coordinated
intermediate and/or prevention of a C−B bond cleavage
from the intermediate by the chelating effect. In addition, these
compounds can be regarded as a boron congener for planarized
trityl cations and triphenylamines, which are known to be a
useful core for dyes5a or hole-transporting materials,5b
respectively. In this regard, the planarized boranes should
serve as useful two-dimensional π scaffolds with electron-
accepting character. The planarized structure would enhance
Received: December 22, 2011
Published: February 28, 2012
© 2012 American Chemical Society
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Communication
Scheme 1. Intramolecular Multi-fold Friedel−Crafts
Cyclization
Whereas the use of Sn(OTf)2 and Bi(OTf)3 gave similar results,
Sc(OTf)3 could cleanly promote the cyclization. Thus,
treatment of 4 with 2 equiv of Sc(OTf)3 in 1,2-dichloroethane
at 95 °C gave a planar triarylborane 5 in 68% yield. The use of a
catalytic amount (0.5 equiv) of Sc(OTf)3 also gave the product,
but the yield decreased to 40% (see Supporting Information
(SI)). To synthesize the all-methylated D3h-symmetry product
1, the methylene bridge in 5 was further functionalized. Thus,
oxidation of 5 with CrO3 in refluxing acetic acid produced a
carbonyl-bridged planar borane 6 in 65% yield without
decomposition despite the harsh conditions. Treatment of 6
with Me2Zn in the presence of TiCl4 successfully gave the D3h
product 1 in 76% yield.
This synthetic strategy was also applicable to the synthesis of
a more extended π-conjugated compound 2 that has two B
atoms. Thus, treatment of a tetra(2-propenyl)-substituted 8,
which was prepared from 7,10 with 4 equiv of Sc(OTf)3
promoted the four-fold intramolecular Friedel−Crafts cycliza-
tion to produce the planarized dihydro-dibora-anthracene a
dihydro-diboraanthracene 2 in 25% yield (Scheme 1b).
Compounds 1 and 2 were unambiguously characterized by
NMR spectroscopy, mass spectrometry, and X-ray crystallog-
raphy (vide infra). As expected, both compounds are quite
stable against air and water and could be purified by silica gel
column chromatography in an open atmosphere. This high
stability is in stark contrast to that of Ph3B, which is easily
decomposed on silica gel. These compounds also have a high
thermal stability. The decomposition temperature with a 5%
weight loss (Td5) of 2 was 353 °C (see SI).
Figure 2. Crystal structures and photophysical properties of planarized
triphenylboranes. (a) ORTEP drawings of 1 (left) and 2 (right) (50%
probability for thermal ellipsoids). (b) UV−vis absorption (solid lines)
and fluorescence spectra (dashed lines) for 1 (blue), 2 (red), and
Mes3B (black) in THF.
(1.554−1.589 Å) in the literature.12 The short B−Cipso bond
distances (1.506(3)−1.528(3) Å) were also observed in the
intermediate product 6. These data suggest that this is a
common feature of the planarized triarylboranes with
methylene bridges. In the packing structure, no special
intermolecular interaction was observed in both 1 and 2,
presumably due to steric hindrance of the peripheral methyl
groups at the bridge moieties. However, in the case of 6 with a
carbonyl bridge, this compound forms a face-to-face dimeric π-
stacked structure with a distance of 3.46 Å (see SI).
̀
The photophysical properties of the planarized boranes 1 and
2 were investigated (Figure 2b). Their data are summarized in
Table 1, together with data for Mes3B,13 for comparison. In the
Table 1. Photophysical and Electrochemical Data
a
a
absorption
fluorescence
reduction potential
b
compd
λabs [nm]
log ε
λem [nm]
ΦF
E1/2[V]
1
289
4.40
4.11
3.97
407
0.10
−2.59 (−2.69)
310(sh)
320(sh)
Single crystals of the planar boranes 1 and 2 were obtained
by recrystallization from CH2Cl2/hexane solutions. X-ray
structural analysis confirmed that both compounds indeed
have nearly planar structures (Figure 2a). The central B atoms
have an ideal trigonal planar geometry with the sum of the
three Cipso−B−Cipso angles of 360.0° for both 1 and 2. The
dihedral angles between the phenyl groups on the B atom are
0.0−0.15° and 0.0−2.6° for 1 and 2, respectively. All the
methylene-bridge carbon atoms sit in the same plane with the
boron planes. Noteworthy is that the B−Cipso bond distances
(1.519(2)−1.520(2) Å) of 1 are much shorter than those of
normal triarylboranes, such as Ph3B (1.571−1.589 Å)11a and
trimesitylborane (Mes3B, 1.573−1.580 Å).11b,c The B−Cipso
bonds (1.520(2)−1.532(2) Å) in 2 are also shorter than those
of other 9,10-dihydro-9,10-diboraanthracene derivatives
2
377
4.35
384
400
0.10
0.08
−2.04 (−2.09)
−2.56 (−2.61)
c
Mes3B
332
4.17
374
−
−2.57 (−2.63)
a
b
In THF. In THF with Bu4N+PF6 (0.1 M) at a scan rate of 100
mVs−1. Potentials vs ferrocene/ferrocenium. Peak reduction potential
c
(Epc) in parentheses. Reported in ref 13.
UV−vis absorption spectra in THF, 1 showed the strongest
absorption band at λmax = 289 nm, with two shoulder bands at
310 and 320 nm. Notably, this longest λmax is slightly shorter
than the λmax of Mes3B, despite the highly coplanar structure of
1. On the other hand, the diboron homologue 2 showed its λmax
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Communication
at 377 nm, which is red-shifted by 28 nm compared to that of
9,10-dimesityl-9,10-diboraanthracene (349 nm).12c The red-
shifted absorption maximum for 2 compared to 1 demonstrates
the effective expansion of π conjugation through the two boron
moieties in 2. In contrast, in the fluorescence spectra, while the
monoboron compound 1 showed a bluish purple emission with
the maximum at 407 nm, the diboron homologue 2 showed a
fluorescence at 384 nm, which is 23 nm shorter than 1. The
Stokes shift for the diboron compound 2 was 484 cm−1, which
is reasonably small for its rigid skeleton. Instead, the significant
shift (Δλ = 6680 cm−1) of the emission band of 1 from the
absorption band is unusual for its rigid structure. Although the
origin of this emission spectrum is unclear at this stage, this
should be one of the features of this skeleton, since the
propeller-shaped Mes3B only shows an intense emission band
at 374 nm with a reasonable Stokes shift of 3380 cm−1.
To study the effect of the planar constraint on the electronic
structure, we conducted cyclic voltammetry (CV) measure-
ments in THF. As envisioned, the planarized borane 1 showed
reversible redox waves, indicative of sufficient stability even in
the reduced state under the measurement conditions, despite
the absence of steric protection over the boron moiety. Its
reduction potential (E1/2) was −2.59 V (Epc = −2.69 V) vs the
ferrocene/ferrocenium ion couple (Fc/Fc+), which is slightly
more negative than that of triphenylborane (Epc = −2.57 V)
and comparable to that of Mes3B (E1/2 = −2.57 V).13 This
comparison demonstrates that planarization of the triphenyl-
borane skeleton does not contribute significantly to decreasing
the LUMO level, despite its coplanar structure. In the CV of
the π-expanded diboron 2, two reversible reduction waves were
observed at E1/2 = −2.04 and −2.64 V. Interestingly, 6 with one
carbonyl bridge showed an irreversible reduction wave at the
more positively shifted potential of Epc = −1.82 V, indicative of
electronic tunability by structural modification at the bridge
moiety (see SI).
A question we had here was why the planarization of the
triphenylborane skeleton is not particularly effective to increase
the electron affinity. This is likely due to the effect of π
donation from the ipso-carbon atoms to the B atom via p−π
conjugation, which may also contribute to the short B−Cipso
bonds observed in the crystal structure as well as the high
chemical stability. Natural bond orbital analysis for 1 (B3LYP/
6-31G(d)) suggested that in the planar-constrained triarylbor-
anes, not only the p−π* interaction but also the p−π
interaction between B and ipso-carbon atoms effectively
occur to a greater extent than for conventional triarylboranes.
These electronic effects result in the moderate increment of
electron-accepting ability in 1. The π-donating effect from the
ipso-carbon atoms was also experimentally demonstrated by the
11B NMR spectrum. Thus, 1 features a broad signal at a higher
magnetic field (48.6 ppm) compared to those of the
nonplanarized triarylboranes (Ph3B, 67.4 ppm;14a Mes3B, 79.0
ppm14b).
react with a fluoride ion to form a fluoroborate complex 1·F−.
Thus, treatment of 1 with 1 equiv of [Me3SiF2]−·[S(NMe2)3]+
(TASF) in THF immediately formed a fluoroborate salt of 1·F−
+
with S(NMe2)3 as a white precipitate, which was isolated in
77% yield by filtration (Figure 3a). As expected for a tetra-
Figure 3. Plane-to-bowl conversion by fluoride ion coordination in 1
+
and 2. (a) Reaction of 1 with a fluoride ion to form 1·F−/S(NMe2)3 .
+
(b) Crystal structure of 1·F−/S(NMe2)3 . (c) Reactions of diborane 2
+
with 1 equiv or an excess of fluoride ion to produce 2·F−/S(NMe2)3
+
or 2·2F−/2S(NMe2)3 , respectively. (d) Crystal structures of 2·F−/
+
+
S(NMe2)3 and 2·2F−/2S(NMe2)3 . In the ORTEP drawings in (b)
and (d), the thermal ellipsoids are drawn at the 50% probability level,
and the countercations are omitted for clarity.
coordinated B atom, in the 11B NMR spectrum, 1·F− showed a
sharpened signal at the higher magnetic field of −4.2 ppm (h1/2
= 510 Hz) compared to that of 1 (48.6 ppm, h1/2 = 1000 Hz).
The 19F NMR spectrum features a broad signal at −176.4 ppm,
which is comparable to the chemical shifts observed for other
dimesitylfluoroborates.1f To determine the binding constant,
we conducted a UV titration experiment. The binding constant
(K) of 1 in THF toward a fluoride ion was determined to be 7.0
× 105 M−1 by a standard curve-fitting method with a 1:1
binding isotherm, which is comparable to that of Mes3B (3.3 ×
105 M−1).15,16 This result demonstrates that 1 still has a
significant Lewis acidity despite the planar constraint.
The X-ray crystal structural analysis revealed that the
fluoroborate complex 1·F− has a bowl-shaped structure (Figure
3b). Thus, the coordination number change of the boron center
from tri-coordination to tetra-coordination results in a plane-to-
bowl conversion. The depth of the resulting bowl structure,
defined by the distance between the B atom and the plane that
consists of three Cpara atoms, is 1.65 Å. The countercation
S(NMe2)3+ sits on the concave side of the bowl structure. This
complex has several notable structural features. First, the B
atom indeed takes a tetra-coordinate tetrahedral geometry with
the sum of the Cipso−B−Cipso angles of 326.7°. The Cipso−B
bond distances (1.603(3)−1.610(3) Å) in 1·F− are elongated
by ∼0.07−0.09 Å compared to those (1.519(2)−1.532(2) Å) in
1. The B−F bond length of 1.494(2) Å is slightly longer than
those of the known fluoroborates, such as (9-anthryl)3BF−/
K+[2.2.2]cryptand (1.466 Å)7 and Mes2PhBF−/nBu4N+ (1.481
Å).1f Second, the boracyclohexadiene ring takes a boat
conformation. This structural flexibility likely allows the
boron center to take the tetra-coordinated geometry. Third,
The shortened B−Cipso bond distances observed in the
crystal structures of 1 and 2 suggest that the rigidity of the
triarylborane skeletons is enhanced by the planar constraint
with the methylene tethers. This rigid structure would retard
reactions of the boron center with certain nucleophiles that
result in the formation of a tetrahedral tetra-coordinated
species. Indeed, 1 was inert even toward highly Lewis basic
1
amines, such as DBU or DABCO. The H NMR spectra of 1
did not show any change upon the addition of an excess of
these amines. However, we found that this compound can still
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Journal of the American Chemical Society
Communication
in this boat conformation, the F atom and one H atom of the
axial methyl group are in close proximity. Its distance is in the
range of 2.28(2)−2.32(2) Å, which is substantially shorter than
the sum of van der Waals radii (rvdw = 1.5 Å for F, 1.2 Å for H),
indicative of the formation of C−H···F−B H-bonds.17
AUTHOR INFORMATION
Corresponding Author
wakamiya@scl.kyoto-u.ac.jp; yamaguchi@chem.nagoya-u.ac.jp
■
Notes
The authors declare no competing financial interest.
Similarly, diborane 2 also reacted smoothly with the fluoride
ions to form the corresponding fluoroborates. Thus, treatment
with 1 equiv of TASF produced the corresponding mono-
fluoroborate 2·F−, which was isolated in 55% yield. Notably,
treatment of 2 with an excess of TASF (2.5 equiv) selectively
gave a cis isomer of difluoroborate 2·2F− in 78% yield (Figure
ACKNOWLEDGMENTS
■
This work was partly supported by JST. Z.Z. is grateful to the
Japan Society for the Promotion of Science (JSPS) for a
postdoctoral fellowship. The authors thank Prof. K. Itami
(Nagoya University), Prof. T. Sasamori, Dr. T. Ago, Dr. Y.
Nakajima, and Prof. Y. Murata (Kyoto University) for fruitful
discussion and generous support of this work.
1
3c). In the H NMR spectrum of the reaction mixture, the
formation of a trans isomer was not observed at all (see SI). To
elucidate the origin of this high selectivity, we conducted DFT
calculations at the B3LYP/6-31+G(d,p) level of theory.
According to the results, the cis isomer is more stable than
the trans isomer by 6.7 kcal/mol. This thermodynamic stability
of the cis isomer is presumably responsible for the cis selectivity.
The structures of these fluoroborates were also confirmed by
X-ray crystal structural analysis (Figure 3d). In the structure of
2·F−, while one B atom maintains the planar geometry
(∑Cipso−B−Cipso = 359.6°), the other B atom changes to the
tetrahedral geometry (∑Cipso−B−Cipso = 330.9°). In the
structure of cis-2·2F−, the combination of two tetrahedral
fluoroborate units (∑Cipso−B−Cipso = 329.4° and 329.1°)
results in the formation of a larger and deeper bowl structure
than in the monofluoroborate 1·F−. The depth of the bowl
structure in cis-2·2F− is 2.49 Å, which is much deeper than that
in 1·F− (1.65 Å). This large and deep bowl structure with a
dianionic electron-donating character suggests the potential use
of this system as a host for a specific guest molecule with an
electron-accepting character. In this regard, the reversibility of
the plane-to-bowl conversion should render this molecular
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, theoretical calculations, and X-ray
structural analysis (PDF, CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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