1-Heteroaromatic-substituted tetraphenylboroles: π-π Interactions between aromatic and antiaromatic rings through a B-C bond

Article abstract of DOI:10.1021/ja309935t

A series of 2,3,4,5-tetraphenylboroles substituted with different aromatic heterocycles (thiophene, furan, pyrrole, and dithiophene) in the 1-position were synthesized and characterized by means of NMR, elemental analysis, and X-ray crystallography. In contrast to known 2,3,4,5-tetraphenylboroles, X-ray diffraction revealed a nearly coplanar arrangement of the aromatic heterocycles and the antiaromatic borole scaffold as a result of π-conjugation, which could be substantiated by DFT calculations. Furthermore, the 2,2′-dithiophene-bridged bisborole (14) exhibits a large bathochromic shift in the absorption spectrum, demonstrating the exceptional Lewis acidity of the nonannulated borolyl moiety.

Full text of DOI:10.1021/ja309935t

Article
pubs.acs.org/JACS
1‑Heteroaromatic-Substituted Tetraphenylboroles: π−π Interactions
Between Aromatic and Antiaromatic Rings Through a B−C Bond
Holger Braunschweig,* Alexander Damme, J. Oscar C. Jimenez-Halla, Christian Horl, Ivo Krummenacher,
̈
Thomas Kupfer, Lisa Mailander, and Krzysztof Radacki
̈
Institut fur Anorganische Chemie, Julius-Maximilians Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: A series of 2,3,4,5-tetraphenylboroles substituted with different aromatic heterocycles (thiophene, furan, pyrrole,
and dithiophene) in the 1-position were synthesized and characterized by means of NMR, elemental analysis, and X-ray
crystallography. In contrast to known 2,3,4,5-tetraphenylboroles, X-ray diffraction revealed a nearly coplanar arrangement of the
aromatic heterocycles and the antiaromatic borole scaffold as a result of π-conjugation, which could be substantiated by DFT
calculations. Furthermore, the 2,2′-dithiophene-bridged bisborole (14) exhibits a large bathochromic shift in the absorption
spectrum, demonstrating the exceptional Lewis acidity of the nonannulated borolyl moiety.
contrast, the substituent at the boron atom has been varied over
a much wider range, including phenyl derivatives (1a−d, 1g),
halogens (1f), ferrocenyl (1h), cymantrenyl (1i), or platinum
groups (1j) (Chart 1).
The metallocene-substituted boroles (1h, 1i) display a
boron−metal through-space interaction, which induces the π-
INTRODUCTION
■
Conjugated organic systems have been established as an
important class of materials because of their interesting
applications in a range of technological areas.1 In particular,
dimesitylboryl-substituted heterocyclic compounds reported by
Marder, Lequan, and Shirota demonstrated the versatile
properties of boron-modified π-conjugated systems, such as
strong photoluminescence, electroluminescence, and nonlinear
optical qualities.2 With its empty pz orbital, the three-
coordinated boron atom is able to interact with organic π-
systems leading to planar conjugated frameworks.3,4 For
a
Chart 1. Several Nonannulated Boroles
instance, Jakle and co-workers reported on diborylated 2,2′-
̈
dithiophenes with different aryl substituents on boron featuring
a planarized geometry between the pz orbital of boron and the
organic spacer, enabling these compounds to display interesting
electron transfer phenomena.5 Investigations by Weber and co-
workers on the synthesis and characterization of 1,3,2-
diazaborolyl-functionalized thiophenes and dithiophenes re-
vealed desirable electrochemical and optical properties for
functional materials.6
Boroles are unsaturated, extremely electron-deficient, penta-
cyclic molecules with four π-electrons. Due to their photo-
physical characteristics and isoelectronic relationship to the
cyclopentadienyl cation, boroles are of fundamental interest.
Up to date, several differently substituted, nonannulated borole
derivatives have been isolated and characterized by X-ray
crystallography.7 The substitution pattern of the C4R4 back-
bone is mainly limited to phenyl (1a−i),8 but examples with
perfluorophenyl (2)9 or thienyl groups (3) are known.10 In
a
Mes = 2,4,6-trimethylphenyl; Fc = ferrocenyl; Cym = cymantrenyl;
Cy = cyclohexyl.
Received: October 8, 2012
Published: November 21, 2012
© 2012 American Chemical Society
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systems of the cyclopentadienyl and borole ring to lie in the
same plane. In contrast, the metal-free boroles share as a
common feature a propeller-like arrangement of the aryl
substituents around the BC4 scaffold. The phenyl substituent
on boron in pentaphenylborole (1a) features a torsion angle of
−32.9(2)°, thus not allowing an effective π-stabilization of the
boron atom. Some degree of π-conjugation was suggested in
tetrathienylborole (3), in which the thienyl groups in 2 and 5
position showed smaller dihedral angles (10.7° and 38.2°).
However, direct conjugation between a nonannulated borolyl
boron atom and an organic π-system has not been observed so
far.
Given our interest in borole chemistry, we attempted to
combine the highly Lewis acidic borolyl group (BC4Ph4) with
heteroaromatic π-conjugated systems. Herein, we disclose the
syntheses of the first 1-heteroaromatic-substituted 2,3,4,5-
tetraphenylboroles (heterocycles: 7 = thiophene, 8 = 5-
trimethylsilylthiophene, 9 = 5-methylfuran, 11 = N-methyl-
pyrrole, 14 = 2,2′-dithiophene) and provide a rationale for the
interactions between the two different types of five-membered
rings.
reveals no disorder in the thiophene unit due to the SiMe3
substituent. In comparison to 1-aryl-2,3,4,5-tetraphenylboroles
(1a−d), the most striking feature of the thiophene-function-
alized boroles 7 and 8 is the nearly coplanar alignment of the
two five-membered rings. Although 7 shows some disorder, the
interplanar torsion angles are found in a range between 6.4(2)°
and 13.0(2)°. In 8 we found a similar coplanar arrangement of
the borolyl moiety and the 5-trimethylsilylthiophene ring with
even smaller interplanar torsion angles (5.7(2)°, 10.1(2)°).
This indicates that some degree of π-conjugation and electronic
interaction is operative in these systems. With the exception of
tetrathienyl-substituted borole (3), which features a π-
interaction between two thienyl moieties and the borole
scaffold,10 all known nonannulated boroles display a propeller-
like arrangement throughout all substituents.7 The B1−C5
bond length of 8 (av. 1.527 Å) lies in the same range compared
to other 1-aryl-2,3,4,5-tetraphenylboroles (1c = 1.537(7) Å, 1d
= 1.513(8) Å) but is shorter than the B1−C5 bond in 1g
(1.560(2) Å) or 3 (1.552(6) Å) with a sterically demanding
mesityl group.8d,m,10 Compound 9 was crystallized from a
saturated benzene solution as dark purple needles. Similar to
the solid state structures of 7 and 8, the borolyl substituent and
the aromatic furan ring in 9 are coplanar in the solid state,
suggesting π-bonding interactions. The interplanar torsion
angle of 17.4(2)° is slightly larger compared to the thiophene
derivatives 7 and 8. The B1−C5 bond length of 9 (1.515(4) Å)
is comparable to those of 8 (Table 1).
RESULTS AND DISCUSSION
■
Two different routes to prepare 1-heteroaromatic-substituted
boroles can be envisaged. The possible salt-elimination reaction
of 1-chloroborole (1f) with the respective lithiated heterocycles
was not followed, as the preparation and handling of 1f is rather
cumbersome. Instead, we used the established tin−boron
exchange reaction to synthesize the target compounds. The
boron dihalide precursors 4, 5, and 6 have been prepared via a
silicon−boron exchange reaction by treating the corresponding
trimethylsilyl-substituted compounds with neat BCl3.11 Hence,
compounds 7, 8, and 9 were successfully obtained by reaction
with 1,1-dimethyl-2,3,4,5-tetraphenylstannole and the respec-
tive boron dihalides (Scheme 1).
Scheme 2. Synthesis of 11 via Salt Elimination and a
Proposed Boryl Migration Mechanism
Scheme 1. Synthesis of Various 1-Heteroaromatic-
Substituted 2,3,4,5-Tetraphenylboroles (4, 7 (E = S; R = H);
5, 8 (E = S; R = SiMe3); 6, 9 (E = O; R = Me))
After workup and recrystallization from toluene, 7, 8, and 9
were isolated as air- and moisture-sensitive deep purple solids
in moderate to good yields (7: 70%, 8: 77%, and 9: 39%). The
identity of 7, 8, and 9 was confirmed by means of NMR
spectroscopy and elemental analysis. The 11B NMR resonances
of 7 (58.2 ppm), 8 (58.1 ppm), and 9 (52.3 ppm) are,
compared to the known pentaphenylborole (1a) (65.4 ppm),
shifted to lower frequency, due to the higher π-donor strength
of the heterocycles.12
Crystallization of 7 and 8 from CH2Cl2/hexane solutions
yielded dark purple crystals that were suitable for X-ray analysis.
In compound 7 (monoclinic, P21/c), two independet molecules
were found in the unit cell, and the thiophene rings were
disordered over two positions (occupancies: 85:15; 79:21). The
unit cell of 8 (monoclinic, P21/c) contains two molecules and
To extend this series of 2-borolyl heterocycles, we attempted
the synthesis of 1-pyrrolyl-2,3,4,5-tetraphenylborole. However,
the required dichloro-N-methylpyrrol-2-ylborane could not be
isolated according to the literature procedure.11 Instead, we
found the selective formation of dichloro-N-methylpyrrol-3-
ylborane (10) in good yields. Comparable migrations are
known for 2-(arylsulfinyl)pyrroles and 2-(acyl)pyrroles, which
are reported to be acid-induced transpositions.13 Unlike the
thiophene- and furan-substituted boroles, no reaction was
observed by treatment of dihaloborane (10) with 1 equiv of
1,1-dimethyl-2,3,4,5-tetraphenylstannole even at elevated tem-
peratures (50 °C). Accordingly, the synthesis could only be
accomplished via a salt-elimination reaction of 10 with 1,4-
dilithio-2,3,4,5-tetraphenylbuta-1,3-diene (Scheme 2). As ex-
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Table 1. 11B NMR Chemical Shifts δ [ppm], Bond Lengths [Å], and Interplanar Torsion Angles [°] of 7, 8, 9, and 11
b
b
7
8
9
11
a
11B [ppm]
58.2
58.1
52.3
57.5
B1−C1
B1−C4
B1−C5
1.576(3)/1.574(3)
1.576(3)/1.577(3)
1.577(3)/1.589(3)
1.587(3)/1.589(3)
1.528(3)/1.525(3)
1.353(3)/1.353(3)
1.532(3)/1.528(3)
1.347(3)/1.352(3)
1.587(4)
1.574(4)
1.515(4)
1.358(4)
1.521(4)
1.352(4)
17.4(2)
1.579(4)
1.580(4)
1.509(6)
1.357(4)
1.516(5)
1.357(4)
6.7(4)
c
__
C1C2
C2−C3
C3C4
1.352(3)/1.352(3)
1.535(3)/1.529(3)
1.361(3)/1.360(3)
c
borole/heterocycle [°]
__
10.1(2)/5.7(2)
a
b
c
Referenced against external [Et2O·BF3]. Two independent molecules in the unit cell. Not discussed, due to disorder.
Figure 1. Molecular structures and side views of (a) 7, (b) 8, (c) 9, and (d) 11. Hydrogen atoms are omitted for clarity, and thermal ellipsoids are set
at 50% probability.
pected, the 11B NMR resonance of 11, detected at 57.2 ppm, is
comparable to those observed for the thiophene derivatives 7
and 8.
Single crystals of 11 could be obtained from a saturated
toluene solution as dark red crystals. Despite the fact that the
borolyl group resides in the 3-position, the solid structure
features a similar coplanar arrangement with torsion angles of
6.77(4)°. The B1−C5 distance of 11 (1.509(6) Å) is slightly
shortened compared to 8 and 9, which arises from the better π-
donor ability of the nitrogen atom.12 It is noteworthy that 7, 8,
9, and 11 exhibit an intermolecular interaction between the
boron atom of the borole and a phenyl group of a neighboring
molecule as previously reported for related 1-aryl-2,3,4,5-
tetraphenylboroles.8b,d Moreover, the enhanced π-conjugation
in 7, 8, 9, and 11 (Figure 1) does not affect the degree of bond
alternation in the borolyl moiety (Table 1).
To finally isolate N-methylpyrrole with a borolyl group in the
2-position, we used 1-chloro-2,3,4,5-tetraphenylborole (1f) as a
borole source since it is known that lithiation of N-
methylpyrrole with one equivalent butyllithium in the presence
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of tetramethylethylenediamine (TMEDA) occurs selectively in
the 2-position.14 Immediately after addition of the lithiated N-
methylpyrrole to 1f in benzene, two doublets of doublets at
6.04 (3J = 4.00 Hz, 3J = 2.24 Hz) and 6.78 ppm (3J = 4.00 Hz, 4J
= 1.44 Hz) and one multiplet at 6.43 ppm were found in the 1H
NMR spectrum, for the three protons of the pyrrole ring. A
comparison with literature data revealed that only pyrroles
substituted in the 2-position show coupling constants between
3.8 and 4.0 Hz, suggesting the formation of 12.15 Surprisingly,
we could observe a similar rearrangement reaction as found for
the dihaloborane 10. The migration of the borolyl moiety from
the 2- to 3-position is slower compared to the migration of 10
1
and was followed by means of H NMR spectroscopy. The
ratio between 12 and 11 was determined to be 1:10 after one
day in benzene at room temperature (Figure 2).
To prove the initial formation of 12, we added the electron-
rich base 4-dimethylaminopyridine (DMAP) to block the
empty pz-orbital of the boron atom and consequently inhibit
the migration process (Scheme 3). After addition, the color of
the reaction mixture turned from deep red to pale yellow, and
the 11B NMR resonance shifted to lower frequency (0.1 ppm).
Single crystals of 12′ were obtained from a saturated toluene
solution. The solid state structure clearly confirms the initial
formation of 12 since the boron atom (B1) is bonded to the
carbon atom in the 2-position (C5) of the N-methylpyrrole
(Figure 3). The borolyl moiety is planar, and the bond lengths
within the C4Ph4 backbone correspond to those of an isolated
diene system, such as cis,cis-1,2,3,4-tetraphenylbuta-1,3-diene.16
The environment of the boron center shows a distorted
1
Figure 2. Migration of the borolyl moiety as followed by H NMR
spectroscopy for one characteristic doublet of doublets (a: reaction
start; b: after 3 h; c: after 12 h; d: after 24 h).
Scheme 3. Synthesis of 12 by Direct Functionalization of the
Chloroborole and Inhibition of the Migration Process by
DMAP
Scheme 4. Synthesis of 14 via Tin−Boron Exchange and
Formation of the DMAP Adduct 15
Figure 3. Molecular structure of 12′ in the solid state with hydrogen
atoms and solvent molecules omitted for clarity. Thermal ellipsoids are
set at 50% probability. Selected bond distances [Å] and angles [°]:
B1−C1 1.647(2), B1−C4 1.635(2), C1−C2 1.349(2), C2−C3
1.502(2), C3−C4 1.360(2), B1−N2 1.610(2), B1−C5 1.610(2),
C5−C6 1.387(2), C6−C7 1.409(2), C7−C8 1.364(2), C8−N1
1.370(2), N1−C5 1.385(2), N1−C9 1.449(2); C1−B1−C4 99.3(1),
C1−B1−C5 116.2(1), C1−B1−N2 108.3(1).
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tetrahedral geometry with a shorter B1−N2 distance (1.610(2)
Å) as compared to the known Lewis acid−base adducts of
boroles due to the electron-rich nature of DMAP.8l,n
To extend the concept of borolyl-substituted π-conjugated
systems, we performed the synthesis of bisborole 14 with
dithiophene as a spacer between two borolyl moieties. 5,5′-
Bis(dichloroboryl)-2,2′-dithiophene (13) was prepared by
reaction of 5,5′-bis(trimethylsilyl)-2,2′-dithiophene with an
excess of BCl3 via the silicon−boron exchange route. In a
similar manner as for the boroles 7, 8, and 9, compound 14 was
obtained via a tin−boron exchange reaction and isolated in 75%
yield as an orange-brown solid. The 11B NMR resonance was
observed as a broad signal (56.5 ppm) in a region of slightly
higher frequency in comparison to 1-(thiophen-2-yl)-2,3,4,5-
tetraphenylborole (7). Structural characterization was achieved
as its DMAP adduct (Scheme 4). To this end, 2 equiv of
DMAP was added to a solution of 14 in CH2Cl2. The color of
the solution turned from orange to yellow, and the 11B NMR
resonance was found at 0.3 ppm, characteristic of four-
coordinate boron. Single crystals of 15 were obtained from a
saturated CH2Cl2/hexane solution. X-ray diffraction confirmed
a distorted tetrahedral geometry at the boron center and the
overall connectivity of the bisborole (Figure 4).
Figure 5. Cyclic voltammogram of 7 at room temperature in CH2Cl2.
Scan rate 200 mVs−1, Pt/[nBu4N][PF6]/Ag, quasi-reversible reduction
wave upon one-electron reduction (dashed lines).
2,3,4,5-tetraphenylborole (1g) (E0 = −1.69 V) and anodi-
1/2
cally shifted compared to 1-ferrocenyl-2,3,4,5-tetraphenylborole
(1h) (E01/2 = −1.96 V)8h,l,10 The second irreversible reduction
process is centered at −2.32 V and corresponds to the
formation of the dianion.
Bisborole 14 is able to accept up to four electrons. Therefore,
one would expect four reduction events for a fully conjugated
system. The cyclic voltammogram of 14, however, shows only
two broad irreversible reduction waves in THF (Epa = −2.35 V,
Epa = −2.97 V) which could not be further resolved using
square wave voltammetry. Due to the highly Lewis acidic boron
atoms, THF readily forms a Lewis acid−base adduct with 14,
resulting in a significant cathodic shift of the redox potentials
compared to the measurements in noncoordinating solvents
(for details, see Supporting Information).
UV−VIS SPECTROSCOPY
The π-electron donation from the heterocycles to the boron
atom is also evident in the electronic absorption spectra (Figure
6). The absorption maxima of the strongly colored boroles 7
■
Figure 4. Molecular structure of 15 in the solid state with hydrogen
atoms and solvent molecules omitted for clarity. Thermal ellipsoids are
set at 50% probability. Selected bond distances [Å]: B1−C1 1.625(3),
B1−C4 1.615(4), C1−C2 1.358(3), C2−C3 1.504(3), C3−C4
1.354(3), B1−N2 1.588(3), B1−C5 1.631(4), C5−C6 1.370(4),
C6−C7 1.424(4), C7−C8 1.363(4), C8−S1 1.732(3), C5−S1
1.742(2).
ELECTROCHEMICAL STUDIES
■
Cyclic voltammetry was used to investigate the reduction
behavior of the different boroles 7, 8, 9, and 14. Similar to
pentaphenylborole (1a), 7, 8, and 9 show a quasi-reversible
one-electron reduction and a second irreversible one-electron
reduction event in CH2Cl2 with [nBu4N]PF6 as the supporting
electrolyte. As shown in our previous work, the two reduction
events can be assigned to the formation of a paramagnetic 5π-
electron radical anion and an aromatic 6π-electron dianion.8l As
a representive example of the newly synthesized boroles, Figure
5 shows the cyclic voltammogramm of 7 with the quasi-
reversible reduction event at E0 = −1.63 V, indicating the
1/2
formation of a paramagnetic monoanion. The peak potential is
in the same region as that found for 1-mesityl-2,3,4,5-tetrakis(2-
Figure 6. UV−visible spectra of compounds 7 (black), 8 (red), 9
(blue), and 11 (green) in CH2Cl2 at concentrations of 1 mmol L−1.
methylthienyl)borole (3) (E0
= −1.57 V) or 1-mesityl-
1/2
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(λmax = 527 nm; ε = 384 L·mol−1·cm−1), 8 (λmax = 522 nm; ε =
292 L·mol−1·cm−1), and 9 (λmax = 530 nm; ε = 462
L·mol−1·cm−1) are blue-shifted compared to 1a (λmax = 567
nm; ε = 361 L·mol−1·cm−1). The lowest energy transition band
is in the same region as the cationic 1-metalloborole (1j) (λmax
= 524 nm; ε = 813 L·mol−1·cm−1), previously prepared in our
14 is distinctly red-shifted compared to 18, which suggests a
more pronounced Lewis acidity of the borolyl moiety in
comparison to the B(C F ) group. Jakle et al. performed DFT
̈
6
5 2
calculations and showed that the excitations for 17 and 18 arise
from the HOMO → LUMO transitions. DFT calculations for
14 display that the HOMO is centered at the borole, and
therefore the HOMO → LUMO energy separation gives rise to
a very weak absorption at 509 nm. The much more pronounced
calculated absorption at 507 nm is a consequence of the
HOMO-2 → LUMO transition in 14. This correlates well with
group.8i The considerable hypsochromic shift for 11 (λmax
=
470 nm; ε = 507 L·mol−1·cm−1) supports again the higher π-
donor strength of the pyrrolyl substituent.12 Calculated
excitation energies for 7, 8, 9, and 11 correlate well with the
experimental findings and display the same hypsochromic shift
for 11 (for details, see Supporting Information).
the findings of Jakle since the molecular orbital of the HOMO-
̈
2 in 14 resembles the HOMO of 17 and 18 (for details, see
The bisborolyl-substituted bithiophene 14 exhibits an
absorption band at 456 nm (ε = 53862 L·mol−1·cm−1) with a
red-shifted shoulder (Figure 7). Interestingly, these data
Supporting Information).
THEORY
■
DFT calculations were performed to gain further insight into a
possible π-conjugation between the borolyl moiety and the
heterocycles.17 The calculated orbital plots for compounds 7, 8,
9, and 11 display a certain amount of π-conjugation in the
HOMO-2 along the B−C5 bond (Figure 8). For the larger
system 14, we found π-conjugation in the HOMO-4 (for
details, see Supporting Information).
Figure 7. UV−visible spectra of 14 in CH2Cl2 at concentrations of 0.1
mmol·L−1.
correlate well with the diborylated bithiophenes 16, 17, and
18 reported by Jakle et al., who observed that the absorption
̈
bands of 16 (λmax = 387 nm; ε = 23 890 L·mol−1·cm−1), 17
(λmax = 396 nm; ε = 48 290 L·mol−1·cm−1), and 18 (λmax = 413
nm; ε = 31 400 L·mol−1·cm−1) experience a bathochromic shift
with increased Lewis acidity of the boryl substituents (Chart
2).5b
Chart 2. Various Diborylated Bithiophenes Reported by
Jakle et al
̈
Figure 8. Molecular orbitals (isovalue = 0.03 au) related to π-
conjugation found in the studied borole compounds calculated at the
PBE0/6-31+G(d,p) level.
To gain a better understanding of the π-conjugation, we
carried out energy decomposition analysis (EDA) for model
systems of 7′, 8′, 9′, and 11′ compared to a model of 1a′. For
the model compounds we ignored the phenyl substituents at
the borolyl moiety, and only the conjugated core has been
taken into account. The relative energy values for the π-
conjugation percentage ΔEπ in the model systems 7′ (20.7%),
8′ (20.4%), 9′ (22.7%), and 11′ (20.5%) are significantly larger
compared to 1a′ (18.2%) (see Supporting Information). In
other words, the energy difference regarding the π-conjugation
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between these compounds relative to 9′, which shows the
highest degree of π-interaction, is 4.8 (7′), 5.1 (8′), 2.9 (11′),
and 10.8 (1a′) kcal/mol, respectively. These results confirm the
experimental findings and show that π-conjugation in 1-
heteroaromatic-substituted boroles is much more pronounced
compared to aryl-substituted tetraphenylboroles.
In addition, nucleus-independent chemical shift (NICS)
calculations were conducted at the PBE0/6-31+g(d,p) level to
evaluate the antiaromatic character of the borole ring and the
aromatic nature of the heterocycles. The obtained NICS(0)
values are listed in Table 2 and compared to literature data for
the unsubstituted aromatic compounds.
CONCLUSION
■
In summary, the synthesis and structural characterization of the
first 1-heteroaromatic-substituted-2,3,4,5-tetraphenylboroles
and a dithiophene-spaced bis-2,3,4,5-tetraphenylborole was
carried out. An interesting migration process could be
elucidated for the N-methylpyrrole derivative. Absorption
data for bisborole 14 correlate well with known dithiophene-
bridged bisboryl compounds and underlines the high Lewis
acidity of the 2,3,4,5-tetraphenylborolyl group. All investiga-
tions (i.e., X-ray, UV−vis, and cyclic voltammetry) support that
a certain degree of π-conjugation is operative between the
aromatic heterocycles and the antiaromatic borole. These
findings were corroborated by DFT calculations and demon-
strate that the coplanarity in these systems can be explained by
an electronic interaction of the heterocycles with the empty pz
orbital of the boron atom. Studies to extend the concept of
borolyl-based π-conjugated systems are currently underway.
a
Table 2. NICS(0) Values at the PBE0/6-31+g(d,p) Level
EXPERIMENTAL DETAILS
■
General Information. All manipulations were performed under an
inert atmosphere of dry argon using standard Schlenk techniques or in
a glovebox (MBraun). Hexane, THF, and CH2Cl2 were dried by
distillation over Na/K alloy (hexane, THF) or phosphorus pentoxide
(CH2Cl2) under argon and stored over activated molecular sieves (4
Å). CD2Cl2 was degassed by several freeze−pump−thaw cycles and
stored over molecular sieves. (E,E)-(1,2,3,4-Tetramethyl-1,3-butadien-
1,4-ylidene)-dilithium, 1,1-dimethyl-2,3,4,5-tetraphenyl-stannole,8a 2,5-
bistrimethylsilyl-thiophene,14 dichlorothiophene-2-ylborane, and di-
chloro-5-methylfurane-2-ylborane11 were prepared according to
known methods. The NMR spectra were recorded on a Bruker AV
500 (1H: 500.1 MHz, 13C: 125.8 MHz, 11B: 160.5 MHz, 29Si: 99.4
1
MHz) FT-NMR spectrometer. H and 13C{1H} NMR spectra were
referenced to external TMS via the residual protons of the solvent
(1H) or the solvent itself (13C). 11B{1H} NMR spectra were referenced
to [BF3·OEt2]. Microanalyses for C, H, and N were performed on a
Leco CHNS-932 Elemental Analyzer. UV−vis spectra were measured
on a JASCO V-660 UV−vis spectrometer.
Synthesis of 1-(Thien-2-yl)-2,3,4,5-tetraphenylborole (7). A
solution of Me2SnC4Ph4 (3.06 g, 6.06 mmol) in 30 mL of
dichloromethane was treated dropwise with 0.60 mL of dichlor-
othiophene-2-ylborane (1.00 g, 6.07 mmol) at −40 °C. The reaction
mixture was stirred for 3 h and subsequently allowed to warm to rt,
resulting in a color change from yellow to red. All volatiles were
removed in vacuo, and the side product Me2SnCl2 was removed
quantitatively by sublimation at rt and 10−6 mbar within 5 h. The red
residue was recrystallized from toluene/hexane to yield 7 (1.90 g, 4.22
mmol, 70%) as a purple red solid. 1H NMR (500.1 MHz, CD2Cl2, 297
K): δ = 6.85 (m, 4H, Ph), 7.00−7.10 (m, 6H, Ph), 7.11 (m, 1H, H-
thienyl), 7.17−7.27 (m, 10H, Ph), 7.44 (m, 1H, H-thienyl), 7.90 (m,
1H, H-thienyl). 13C NMR (125.8 MHz, CD2Cl2, 297 K): δ = 126.13,
127.37, 127.43, 128.01 (CH), 129.12 (C-thienyl), 129.72, 130.19
(CH), 141.37, 145.43 (C-thienyl), 136.91, 140.91, 162.25 (Cq). 11B
NMR (160.5 MHz, CD2Cl2, 297 K): δ = 58.2 (br). Elemental analysis
(%) calcd. for C32H23BS: C, 85.33; H, 5.15; S, 7.12. Found: C, 84.40;
H, 5.48; S, 7.11.
a
The phenyl groups around the borole moiety in the chemical
drawings are omitted for clarity.
Interestingly, the comparison with literature NICS(0) values
clearly displays a π-interaction of the heterocycles with the
boron atom, which leads to a mutual compensation of
aromaticity and antiaromaticity. While the aromaticity of the
heterocycles is significantly reduced compared to literature
data, the value for the benzene ring in pentaphenylborole (1a)
remains unperturbed (C6H6: −7.2 ppm).18,19 A similar
behavior can be expected for boryl-substituted derivatives. In
the case of borolyl substituents, however, the π-interaction of
the aromatic heterocycle with the vacant pz orbital on boron
leads to a decrease in antiaromaticity of the borole ring. We
believe that this effect is more pronounced in boroles 7, 8, 9,
and 11 than in pentaphenylborole (1a) (+12.9 ppm) as a result
of the higher π-donating abilities of the heteroaromatic
groups.10,18
Synthesis of Dichloro-5-trimethylsilylthien-2-ylborane (5).
To a sample of neat BCl3 (5.00 mL, 6.63 g, 56.6 mmol) was added 2,5-
trimethylsilylthiophene (2.30 g, 10.1 mmol) at −50 °C. The mixture
was allowed to warm to room temperature and stirred for 1 h. All
volatiles were removed in vacuo, and the residue was distilled (1.4
mbar, 65 °C) to give 5 as a colorless liquid (2.08 g, 8.78 mmol, 87%).
1H NMR (500.1 MHz, C6D6, 297 K): δ = 0.13 (s, 9H, SiMe3), 7.00 (d,
3
3JH−H = 3.54 Hz, 1H, H-thienyl), 7.83 (d, JH−H = 3.54 Hz, 1H, H-
The π-electrons of the dithiophene spacer in bisborole 14 are
shared between the two borolyl moieties, which leads to an
increased antiaromatic character (NICS(0): 12.1 ppm), in a
region typical for pentaphenylborole 1a.
thienyl). 13C NMR (125.8 MHz, C6D6, 297 K): δ = −0.59 (SiMe3),
136.18, 143.91 (CH-thienyl), 158.64 (Cq). 29Si NMR (99.4 MHz,
CD2Cl2, 297 K): δ = −5.62. 11B NMR (160.5 MHz, C6D6, 297 K): δ =
47.8.
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Journal of the American Chemical Society
Article
Synthesis of 1-(5-Trimethylsilylthien-2-yl)-2,3,4,5-tetraphe-
nylborole (8). A solution of Me2SnC4Ph4 (554 mg, 1.10 mmol) in 15
mL of dichloromethane was treated dropwise with a solution of
dichloro-5-trimethyl-silylthiophene-2-ylborane (260 mg, 1.10 mmol)
in 5 mL of dichloromethane at −40 °C. The reaction mixture was
stirred for 3 h and subsequently allowed to warm to rt, resulting in a
color change from yellow to red. All volatiles were removed in vacuo,
and the side product Me2SnCl2 was removed quantitatively by
sublimation at rt and 10−6 mbar within 5 h. The red residue was
recrystallized from a mixture of toluene/hexane to yield 8 as a purple
solid (440 mg, 0.84 mmol, 77%). 1H NMR (500.1 MHz, CD2Cl2, 297
K): δ = 0.24 (s, 9H, SiMe3), 6.83−6.86 (m, 4H, Ph), 6.99−7.08 (m,
6H, Ph), 7.14−7.16 (m, 4H, Ph), 7.18−7.26 (m, 7H, Ph & H-thienyl),
Hz, 4J = 2.4 Hz, 1H, H-pyrrolyl), 6.37−6.39 (m, 2H, NC5H4-4-NMe2),
6.69−6.71 (m, 4H, Ph), 6.76−6.77 (m, 1H, H-pyrrolyl), 6.87−7.04
(m, 16H, Ph), 8.14 − 8.17 (m, 2H, NC5H4-4-NMe2). 13C NMR (125.8
MHz, CD2Cl2): δ = 35.23 (N-CH3), 39.62 (N(CH3)2), 106.25
(NC5H4-4-NMe2), 106.98, 113.98, 123.35 (CH-pyrrolyl), 124.56,
125.58, 127.47, 127.63, 129.23, 130.47 (CH), 145.40 (NC5H4-4-
NMe2), 141.13 (Cq), 143.76 (Cq, NC5H4-4-NMe2), 150.66, 155.85
(Cq). 11B NMR (160.5 MHz, CD2Cl2): δ = 0.08. Elemental analysis
(%) calcd for C40H36BN3: C, 84.35; H, 6.37; N, 7.38. Found: C, 83.66;
H, 6.41; N, 6.79.
Synthesis of 5,5′-Bis(dichloroboryl)-2,2′-bithiophene (13).
To a sample of neat BCl3 (4.80 g, 3.62 mL, 40.9 mmol) was added a
solution of 5,5′-bis(trimethylsilyl)-2,2′-bithiophene (1.92 g, 6.18
mmol) at −20 °C. The mixture was allowed to warm to room
temperature and was stirred for 2 h. All volatiles were removed in
vacuo, and the residue was recrystallized from toluene at −30 °C to
yield 4 as a yellow solid (1.81 g, 5.52 mmol, 89%).
3
7.42 (d, JH−H = 3.55 Hz, 1H, H-thienyl). 13C NMR (125.8 MHz,
CD2Cl2, 297 K): δ = −0.32 (SiMe3), 126.04, 127.27, 127.38, 127.89,
129.72, 130.18 (s, CH), 135.49, 145.62 (CH-thienyl), 136.99, 140.85
(Cq), 140.96 (Cq-thienyl), 146.59 (Cq), 159.23 (Cq-thienyl), 162.03
(Cq). 29Si NMR (99.4 MHz, CD2Cl2, 297 K): δ = −5.61. 11B NMR
(160.5 MHz, CD2Cl2, 297 K): δ = 58.6 (br). Elemental analysis (%)
calcd for C35H31BSSi: C, 80.44; H, 5.98; S, 6.14. Found: C, 80.06; H,
5.94; S, 5.71.
1H NMR (500.1 MHz, C63D6, 297 K): δ = 6.78 (m, 3JH−H = 3.90 Hz,
2H, H-dithienyl), 7.43 (d, JH−H = 3.90 Hz, 2H, H-dithienyl). 13C
NMR (125.8 MHz, C6D6, 297 K): δ = 128.2 (CH-dithienyl), 144.1
(CH-dithienyl), 149.8 (Cq-dithienyl). 11B NMR (160.5 MHz, C6D6,
297 K): δ = 47.7. Elemental analysis (%) calcd for C8H4B2Cl4S2: C,
29.32; H, 1.23; S, 19.57. Found: C, 29.81; H, 1.24; S, 19.39.
Synthesis of 5,5′-Bis-(2,3,4,5-tetraphenylborole)-2,2′-bithio-
phene (14). A solution of Me2SnC4Ph4 (514 mg, 1.02 mmol) in 5 mL
of dichloromethane was treated dropwise with a solution of 5,5′-
bis(dichloroboryl)-2,2′-bithiophene (200 mg, 0.61 mmol) in 5 mL of
dichloromethane at −50 °C. The reaction mixture was stirred for 3 h
and subsequently allowed to warm to rt, resulting in a color change
from yellow to orange. All volatiles were removed in vacuo, and the
side product Me2SnCl2 was removed quantitatively by sublimation at
rt and 10−6 mbar within 5 h. The red residue was recrystallized from
toluene/hexane to yield 5 as a orange brown solid (415 mg, 4.61
Synthesis of 1-(5-Methylfuran-2-yl)-2,3,4,5-tetraphenylbor-
ole (9). A solution of Me2SnC4Ph4 (1.00 g, 1.98 mmol) in 15 mL of
dichloromethane was treated dropwise with a solution of dichloro-5-
methylfurane-2-ylborane (322 mg, 1.98 mmol) in 5 mL of dichloro-
methane at −60 °C. The reaction mixture was stirred for 16 h and
subsequently allowed to warm to rt, resulting in a color change from
yellow to red. All volatiles were removed in vacuo, and the side
product Me2SnCl2 was removed quantitatively by sublimation at rt and
10−6 mbar within 5 h. The red residue was recrystallized from toluene
1
at −30 °C to yield 9 as a purple solid (348 mg, 0.78 mmol, 39%). H
NMR (500.1 MHz, CD2Cl2, 297 K): δ = 2.66 (s, 3H, CH3), 6.11 (dd,
3J = 3.5 Hz, 4J = 0.8 Hz, 1H, H-furanyl), 6.80−6.82 (m, 4H, Ph), 6.91
1
(dd, 3J = 3.5 Hz, 5J = 0.6 Hz, 1H, H-furanyl), 7.06−7.08 (m, 6H, Ph),
7.15−7.13 (m, 6H, Ph), 7.18−7.21 (m, 4H, Ph). 13C NMR (125.8
MHz, CD2Cl2, 297 K): δ = 14.4 (CH3), 110.2 (CH-furanyl), 125.7,
127.2, 127.4, 127.6, 129.9, 130.1 (CH), 138.3 (CH-furanyl), 137.2
(Cq), 140.6 (b, Cq), 140.9, 161.9, 164.7 (Cq). 11B NMR (160.5 MHz,
CD2Cl2, 297 K): δ = 52.3 (br). Elemental analysis (%) calcd for
C33H25BO: C, 88.40; H, 5.62. Found: C, 88.31; H, 5.60.
mmol, 75%). H NMR (500.1 MHz, CD2Cl2, 297 K): δ = 6.82−6.84
(m, 8H, Ph), 6.99−7.06 (m, 14H, Ph), 7.12−7.14 (m, 8H, Ph & H-
dithienyl), 7.21−7.26 (m, 14H, Ph & H-dithienyl). 13C NMR (125.8
MHz, CD2Cl2, 297 K): δ = 126.23, 127.41, 127.48, 128.01 (CH),
128.03 (CH-dithienyl), 129.68, 130.05 (CH), 146.26 (CH-dithienyl),
136.80, 140.60, 150.93, 162.14 (Cq). 11B NMR (160.5 MHz, CD2Cl2,
297 K): δ = 56.5 (br). Elemental analysis (%) calcd C64H44B2S2: C,
85.52; H, 4.93; S, 7.14. Found: C, 84.59; H, 4.84; S, 6.76.
Synthesis of 1-(N-Methylpyrrol-3-yl)-2,3,4,5-tetraphenylbor-
ole (11). A solution of dichloro-N-methylpyrrole-3-ylborane (0.20 g,
1.23 mmol) in 3 mL of benzene was treated dropwise with a
suspension of (E,E)-(1,2,3,4-tetramethyl-1,3-butadien-1,4-ylidene)-di-
lithium (0.50 g, 1.23 mmol) in 10 mL of benzene at 0 °C. The reaction
mixture was allowed to warm to rt, changing its color from light brown
to dark red, and stirred for 12 h. Lithium chloride was filtered off, and
all volatiles were removed in vacuo. The product was recrystallized
from toluene, yielding 11 as red crystals (246 mg, 0.55 mmol, 45%).
1H NMR (500.1 MHz, CD2Cl2): δ = 3.50 (s, 3H, CH3), 6.03 (dd, 3J =
Synthesis of 5,5′-Bis-(2,3,4,5-tetraphenylborole)-2,2′-bithio-
phene-bis(dimethylaminopyridine) (15). To a solution of 5 (20
mg, 22.3 μmol) in 3 mL of dichloromethane was added DMAP (5.44
mg, 44.6 μmol), causing an immediate color change from dark orange
to yellow. All volatiles were removed in vacuo, and the residue was
recrystallized in CH2Cl2/hexane to give 12 (22.4 mg, 19.6 μmol, 88%)
as yellow crystals. 1H NMR (500.1 MHz, CD2Cl2, 297 K): δ = 3.10 (s,
3
12H, NMe2), 6.55−6.58 (m, 4H, NC5H4-4-NMe2), 6.66 (d, JH−H
=
3.45 Hz, 2H, H-dithienyl), 6.73−6.76 (m, 8H, Ph), 6.87−7.04 (m,
34H, Ph & H-dithienyl), 8.24−8.27 (m, 4H, NC5H4-4-NMe2). 13C
NMR (125.8 MHz, CD2Cl2, 297 K): δ = 39.77 (NMe2), 106.99
(NC5H4-4-NMe2), 123.31 (CH-dithienyl), 124.43, 125.52, 127.38,
127.43, 129.36, 130.68 (CH), 131.92 (CH-dithienyl), 145.12 (NC5H4-
4-NMe2), 139.18, 140.85, 143.51, 150.04, 155.95 (Cq). 11B NMR
(160.5 MHz, CD2Cl2, 297 K): δ = 0.29. Elemental analysis (%) calcd
for C78H64B2N4S2·2(CH2Cl2): C, 73.18; H, 5.22; N, 4.27; S, 4.88.
Found: C, 73.33; H, 5.29; N, 4.23; S, 4.81.
2.6 Hz, 4J = 1.6 Hz, 1H, H-pyrrolyl), 6.49 (dd, 3J = 2.6 Hz, 4J = 1.9 Hz,
1H, H-pyrrolyl), 6.83−6.85 (m, 4H, Ph), 6.71 (m, 1H, H-pyrrolyl),
6.99−7.04 (m, 6H, Ph), 7.13−7.17 (m, 6H, Ph), 7.20−7.24 (m, 4H,
Ph). 13C NMR (125.8 MHz, CD2Cl2): δ = 36.54 (CH3), 118.55 (CH-
pyrrolyl), 123.87 (CH-pyrrolyl), 125.51, 126.83, 127.31, 127.72,
129.80, 129.97 (CH), 137.54 (Cq), 139.10 (Cq-pyrrolyl), 141.57,
142.22, 160.11 (Cq). 11B NMR (160.5 MHz, CD2Cl2): δ = 57.5 (br).
Elemental analysis (%) calcd for C33H26BN: C, 88.59; H, 5.86; N, 3.13.
Found: C, 88.39; H, 5.91; N, 3.30.
ASSOCIATED CONTENT
* Supporting Information
Synthesis of 1-(N-Methylpyrrol-2-yl)-2,3,4,5-tetraphenyl-
borole-dimethylaminopyridine (12′). 2-Lithio-N-methylpyrrole
(10.8 mg, 124 μmol) and 1-chloro-2,3,4,5-tetraphenylborole (50.0
mg, 124 μmol) were dissolved in 2 mL of benzene at rt. After 5 min,
DMAP (15.2 mg, 124 μmol) was dissolved in 2 mL of benzene and
added slowly, resulting in a color change from red to pale yellow.
Lithium chloride was filtered off, and all volatiles were removed in
vacuo. The residue was recrystallized from toluene, and 12′ was
isolated as pale yellow solid (42 mg, 73.9 μmol, 60%). 1H NMR (500.1
MHz, CD2Cl2): δ = 3.02 (s, 6H, N(CH3)2), 3.83 (s, 3H, N−CH3),
■
S
Crystallographic data of complexes 7, 8, 9, 11, 12′, and 15 in cif
format, cyclic voltammetry measurements, and details about
DFT calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
h.braunschweig@uni-wuerzburg.de
■
3
4
3
5.75 (dd, J = 3.4 Hz, J = 1.7 Hz, 1H, H-pyrrolyl), 6.04 (dd, J = 3.4
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Journal of the American Chemical Society
Article
(12) (a) Charton, M. Prog. Phys. Org. Chem. 1981, 13, 119.
(b) Katritzky, A. R. Handbook of Heterocyclic Chemistry; Pergamon
Press: Oxford, 1983; p 11.
Notes
The authors declare no competing financial interest.
(13) (a) Carmona, O.; Greenhouse, R.; Landeros, R.; Muchowski, J.
M. J. Org. Chem. 1980, 45, 5336. (b) Carson, J. R.; Davis, N. M. J. Org.
Chem. 1981, 46, 839.
(14) Brandsma, L.; Verkruijsse, H. Preparative Polar Organometallic
Chemistry; Springer Verlag: Berlin, Heidelberg, 1987; Vol. 1, p 115.
(15) (a) Barton, T. J.; Hussmann, G. P. J. Org. Chem. 1985, 50, 5881.
(b) Denat, F.; Gaspard-Iloughmane, H.; Dubac, J. J. Organomet. Chem.
1992, 423, 173. (c) Dvornikova, E.; Kamieska-Trela, K. Synlett 2002,
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(16) Bats, J. W.; Urschel, B. Acta Crystallogr., Sect. E: Struct. Rep.
Online 2006, E62, 0748.
(17) Our calculations were done using mainly Gaussian03 at the
PBE0/6-31+G(d,p) level. See Supporting Information for a detailed
discussion of our methodology used, scan-NICS maps, and other
analyses.
ACKNOWLEDGMENTS
■
We thank the German Science Foundation (DFG) for financial
support.
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