Synthesis of sterically crowded polyarylated boron-dipyrromethenes

Article abstract of DOI:10.1021/jo2010022

A rapid synthetic route for polyarylated boron-dipyrromethenes using hexabromo boron-dipyrromethene as the key synthon is described. The X-ray structure revealed that the polyarylated BODIPY adopts a propeller-like conformation. These compounds exhibit re

Full text of DOI:10.1021/jo2010022

NOTE
pubs.acs.org/joc
Synthesis of Sterically Crowded Polyarylated Boron-Dipyrromethenes
Vellanki Lakshmi and Mangalampalli Ravikanth*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
S Supporting Information
b
ABSTRACT:
A rapid synthetic route for polyarylated boron-dipyrromethenes using hexabromo boron-dipyrromethene as the key synthon is
described. The X-ray structure revealed that the polyarylated BODIPY adopts a propeller-like conformation. These compounds
exhibit red-shifted absorption and fluorescence bands with decent quantum yields and reversible oxidation and reduction waves
when compared to unsubstituted boron-dipyrromethenes.
oron-dipyrromethene dyes (BODIPY) are highly fluorescent
dyes and have diverse applications as biolabels, artificial light
groups to the pyrroles of the BODIPY core.5 These kinds of
systems are also found to be brightly fluorescent in the solid state.
These properties are very important for their potential use in the
field of optoelectronics, such as high-performance electrolumi-
nescent devices, light-emitting field-effect transistors, and lasers.6
Although the literature on BODIPYs with alkyl substituents at
the pyrrole carbons is exhaustive, the BODIPYs with aryl
substituents at the pyrrole carbons are limited due to a lack of
proper precursors and synthetic routes. It is shown in the
literature that the aryl-substituted BODIPYs can be synthesized
by using aryl-substituted pyrroles as key precursors, which are
synthetically not easily accessible.7 Dehaen and co-workers8
developed a simple synthetic strategy to introduce a range of
substituents at 3,5-positions by nucleophilic substitution using
3,5-dichloro-BODIPY as the precursor. However, this metho-
dology has not been explored for the synthesis of other poly-
arylated BODIPYs containing more than two aryl groups at the
pyrroles of the boron-dipyrromethene core. Recently, Wakamiya
et al.9 reported the synthesis of polyphenylated BODIPY I using
triphenyl-substituted pyrrole as the key precursor, as shown in
Scheme 1. However, this method requires several steps involving
protection/deprotection reactions and extensive chromato-
graphic purifications. Furthermore, to synthesize various poly-
arylated BODIPYs by using this method, one needs to prepare
the corresponding triaryl pyrroles whose synthesis is difficult. In
this note, we report the facile synthesis of polyarylated BODIPYs
1À4 using a hexabromo derivative of BODIPY 5 as a key
B
harvesters, sensitizers for solar cells, fluorescent sensors, molec-
ular photonic wires, and laser dyes.1 This popularity of BODIPY
dyes is presumably because of their advantageous spectroscopic
properties such as high molar absorption coefficients, narrow
band shapes with tunable wavelengths, high fluorescence quan-
tum yields, and considerably high photostability. Furthermore,
the BODIPY dyes can be easily functionalized, and the function-
alized dyes can be used to introduce substituents at all positions
of the BODIPY core, including the two fluorines of the boron
atom.2 Thus, the electronic properties of BODIPY dye can be
fine-tuned by suitable substitution on the boron-dipyrromethene
core. The extensive literature available on BODIPY dyes suggests
that the introduction of substituents at the meso position does not
alter the electronic properties of the dye significantly because the
meso group is perpendicularly oriented and electronically
decoupled.3 On the other hand, the introduction of substituents
at the 3,5-positions and 1,7-positions significantly alters the
electronic properties of the dye.4 For example, the introduction
of two phenyl groups at the 3,5-positions shifts the absorption
band to approximately 50 nm toward red, and introduction of
two styryl groups at the same positions shifts the absorption band
toward 100 nm red.4a Thus, various synthetic strategies were
developed to obtain BODIPYs exhibiting absorption and emis-
sion bands in the visible or near-infrared (NIR) region of the
spectrum. Few of the approaches reported in the literature to
obtain BODIPYs with absorption/emission in the visible and
NIR region with appreciable quantum yields are by introducing
bulky aryl substituents on the BODIPY core or by fusing aryl
Received: May 19, 2011
Published: September 12, 2011
r
2011 American Chemical Society
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|

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Scheme 1. Reported Synthesis of Polyphenylated BODIPY I
Scheme 2. Synthesis of Polyarylated BODIPY Compounds (1À4)
synthon. The unknown hexabromo derivative of BODIPY 5 can
be synthesized in a 500 mg batch easily in a two-step one-pot
reaction under mild reaction conditions. The advantages of our
method over the reported method are the use of readily available
precursors, mild reaction conditions, and simple column chro-
matographic purifications to afford polyarylated BODIPYs in
decent yields. Furthermore, the method is applicable to synthe-
size a set of substituted polyarylated BODIPYs using the single
precursor, hexabromo BODIPY 5 (Scheme 2).
To synthesize the hexabromo BODIPY 5, first we synthesized
the required meso-anisyl dipyrromethane10 6 by condensing
1 equiv of p-anisaldehyde with 25 equiv of pyrrole under mild
acid-catalyzed reaction conditions followed by flash column
chromatographic purification. The hexabromo meso-anisyl dipyr-
romethane 7 was synthesized by treating 6 with 10 equiv of
N-bromosuccinimide in THF overnight at room temperature.
The progress of the reaction was monitored by TLC analysis,
which clearly indicated the formation of 7 as a sole product.
Without isolation, compound 7 was subjected to a two-step one-
pot reaction by oxidizing it in the first step with DDQ and reacted
in a second step with BF3 OEt2 at room temperature. The TLC
3
and absorption spectral analysis of the crude reaction mixture
indicated the formation of hexabromo derivative of BODIPY 5.
The crude product was subjected to silica gel column chromato-
graphic purification and afforded pure 5 as nonfluorescent green
powder in 24% yield. Compound 5 was confirmed by (M À F)+
ion peak in both ES-MS and HR-MS mass spectra and by the
absence of signals corresponding to pyrrole protons of the
BODIPY core because of their substitution with bromo groups
in the 1H NMR spectrum. Furthermore, compound 5 showed a
typical quartet in 19F NMR at À145.7 ppm with no shift
compared to unsubstituted meso-anisyl BODIPY3 8 (Table 1).
However, in 11B NMR, compound 5 showed a triplet at 0.12
ppm, which experienced a ∼0.2 ppm upfield shift compared to 8
(0.35 ppm), indicating the electron-deficient nature of com-
pound 5. The absorption spectrum of 5 showed one strong
S0fS1 absorption band at ∼550 nm. Compound 5 showed
one reversible reduction at À0.34 V (ΔEp = 65 mv) and one
irreversible reduction at À1.40 V, which appeared less negative
by ∼450 mV (Table 1) compared to 8. This supports the
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NOTE
electron-deficient nature of 5 due to the presence of six bromo
groups. Compound 5 was also characterized by X-ray diffraction
analysis. The single crystal of 5 was obtained on slow evaporation
of compound 5 in a CH2Cl2/petroleum ether mixture over a
period of 3 days. The structure of compound 5 (Figure 2) shows
that the two pyrrole rings and the central six-membered ring
containing the boron are in one plane like any other BODIPY3
such as compound 8. The interesting feature of compound 5 is
the orientation of the meso-aryl group, which is nearly perpendi-
cular with respect to the plane defining various dipyrrin atoms
(88°) unlike meso-tolyl boron-dipyrromethene3 in which the
dihedral angle is 53°. This indicates that the bromine atoms
provide steric hindrance resulting in perpendicular orientation of
the meso-aryl group to the dipyrrin plane in compound 5 (CCDC
No. 823749). The presence of six bromine atoms also causes
slight variations in various bond lengths and bond angles in
compound 5 compared to compound 8.
The polyarylated BODIPYs 1À4 were synthesized by cou-
pling of compound 5 with corresponding arylboronic acids such
as phenylboronic acid, 4-fluorophenylboronic acid, 4-methyl-
phenylboronic acid, and 4-methoxyphenylboronic acid in a
THF/toluene/H2O (1:1:1) mixture in the presence of a catalytic
amount of Pd(PPh3)4/Na2CO3 overnight at 80 °C. TLC analysis
initially showed more numbers of spots corresponding to various
arylated BODIPYs, but as the reaction progressed, the spots
corresponding to the other arylated BODIPYs disappeared and
one major highly fluorescent spot corresponding to the required
polyarylated BODIPYs was observed. The progress of the
reaction can also be judged from clear color change of the
reaction mixture from pink to a brightly fluorescent red solution
and also by following absorption spectroscopy. The crude
compounds were purified by silica gel column chromatography
and afforded pure fluorescent solids 1À4 in 50À55% yields. The
longer reaction times are needed for maximum conversion of
hexabromo derivatives to polyarylated BODIPYs. The polyary-
lated BODIPYs 1À4 are stable and freely soluble in common
organic solvents. The compounds 1À4 were characterized by
mass, NMR, absorption, electrochemical, and fluorescence tech-
niques. The HR-MS mass spectra showed the corresponding
expected molecular ion peak confirming the identity of com-
1
pounds 1À4. In H NMR, the signals corresponding to the
expected aryl groups are present in the 6.00À8.00 ppm region.
Compounds 1À4 showed a characteristic quartet at ∼À131.0
ppm in 19F NMR (Figure 1a) and triplet at ∼1.1 ppm in 11B
NMR (Figure 1b), which are significantly downfield shifted
compared to 5 and 8 (Table 1) due to the presence of aryl
groups at the periphery of the BODIPY core.
We solved the X-ray crystal structure of polyarylated BODIPY
1 (CCDC No. 823750) to show the arrangement of the
peripheral aryl groups on the central BODIPY core, and the
structure is presented in Figure 2. The single crystal of compound
1 was obtained from CHCl3 solution on slow evaporation at
ambient temperature over a period of 1 week. The compound 1
crystallizes as triclinic with a P1 space group. Unlike compounds
5 and 8, which possess planar indacene planes, compound 1
displayed distorted, “propeller-like” conformation. In compound
1, the indacene plane deviates from planarity with an average
dihedral angle of 14.5° between the central six-membered ring
and the two pyrrole rings of the dipyrrin unit. The torsion angle
between the two pyrrole rings is 19.4°. The highly distorted
structure of compound 1 can be ascribed to the steric hindrance
caused by the six phenyl rings on the BODIPY core, leading to
the propeller-like conformation. The dihedral angles between the
aryl substituents and the plane defining various dipyrrin atoms in
compound 1 are in the range of 50À61°, and the meso-aryl group
is twisted at 57° relative to the BODIPY core.
Table 1. Comparison of Important Spectral and Electroche-
mical Data of Compounds 1, 5, and 8
8
5
1
19F (δ in ppm)
11B (δ in ppm)
λabs (nm)
À145.03
0.35
À145.73
0.12
À131.25
1.20
The electron-rich behavior of compounds 1À4 is evident in
their redox properties. The comparison of cyclic voltammograms
of compound 1 with those of compounds 5 and 8 is presented in
Figure 3b. Unlike compounds 5 and 8, which do not show any
oxidation, compounds 1À4 showed one reversible oxidation
(ΔEp = 60À70 mV) at 1.30 V. Interestingly, the first reversible
reduction (ΔEp = 60À70 mV) of compounds 1À4 falls in the
500
552
574
λemi (nm)
516
566
609
Stokes shift (cmÀ1
)
580
480
<10À4
1020
0.36
quantum yield (Φ)
0.03
reduction potential (V)
À0.80
À0.34
À0.86
Figure 1. (a) Comparison of 19F NMR spectra of compounds (i) 8, (ii) 5, and (iii) 1. (b) Comparison of 11B NMR spectra of compounds (i) 8, (ii) 5,
and (iii) 1 recorded in CDCl3 (δ in ppm).
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Figure 2. ORTEP diagrams of compounds 5 and 1.
strongly fluorescent and showed a relatively broad red-shifted
fluorescence band with decent quantum yields of 0.2À0.5.
Furthermore, compounds 1À4 exhibited relatively large Stoke’s
shift and showed similar photophysical behavior like any other
BODIPY dye with a change of solvent polarity. The fluorescent
decay profiles of compounds 1À4 in different solvents were
collected and fitted to a single exponential. The fluorescence
lifetime of compounds 1À4 in different solvents varies between
1.5 and 4 ns. These compounds also appear to be strongly
fluorescent in the solid state, which will be investigated in due
course of time.
In conclusion, we developed short and rapid route for the
synthesis of polyarylated BODIPYs. This method avoids the use
of aryl-substituted pyrroles whose synthesis is difficult. The
hexabromo BODIPY which is the key synthon used in this
method can be prepared in large quantities. Unlike the method in
the literature which requires the synthesis of different aryl-
substituted pyrroles, here we can use hexabromo BODIPY to
prepare a set of aryl-substituted BODIPYs by coupling hexabro-
mo BODIPY with appropriately substituted arylboronic acids
under Pd(0) coupling conditions. The X-ray analysis showed
that the structure is distorted, and electrochemical studies
indicated that these compounds exhibit reversible oxidation
and reduction waves. The photophysical studies revealed that
both absorption and emission bands experienced large red shifts
with decent quantum yields and an increase in lifetimes com-
pared to unsubstituted BODIPYs; also these compounds are
brightly fluorescent in the solid state (Supporting Information),
which needs further investigation.
Figure 3. (a) Oxidation wave of compound 1 and (b) comparison of
reduction waves of cyclic voltammograms of compounds (i) 8, (ii) 5,
and (iii) 1 recorded in CH2Cl2 containing 0.1 M TBAP as supporting
electrolyte recorded at 50 mV sÀ1 scan speed.
same range as that of compound 8 (Table 1), although we
expected that these electron-rich compounds 1À4 are more
difficult to reduce compared to 8. We attribute this to the
nonplanarity of the BODIPY core in compounds 1À4 induced
by steric crowding of peripheral phenyl groups that reduce the
effective electronic conjugation in compounds 1À4. This is also
reflected in their oxidation potential, which was observed at
higher potential in spite of the presence of several electron-rich
aryl groups at the periphery of the BODIPY core.
The absorption properties of compounds 1À4 along with
those of compounds 5 and 8 were studied in five different
solvents (Supporting Information). It is observed that introduc-
tion of six bromines at the pyrrole carbons of the BODIPY core in
compound 5 results in ∼50 nm bathochromic shift of S0fS1
transition compared to that of unsubstituted BODIPY 8. The
polyarylated BODIPYs 1À4 showed broad S0fS1 transition
which experienced a bathochromic shift of 70À100 nm (Table 1)
compared to 8. The fluorescence properties of compounds 1À4
are in line with the absorption properties (Supporting In-
formation). Compound 5 is weakly fluorescent due to the
presence of heavy bromine atoms, and the fluorescence band is
red-shifted compared to 8. The polyarylated BODIPYs 1À4 are
’ EXPERIMENTAL SECTION
General: All NMR spectra (δ values, ppm) were recorded with 300
or 400 MHz spectrometers. Tetramethylsilane (TMS) was used as an
external reference for recording 1H (of residual proton; δ = 7.26 ppm)
and 13C (δ = 77.0 ppm) spectra in CDCl3. Cyclic voltammetric (CV)
and differential pulse voltammetric (DPV) studies were carried out with
an electrochemical system utilizing a three-electrode configuration
consisting of a glassy carbon (working) electrode, platinum wire
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NOTE
3
(auxiliary) electrode, and a saturated calomel (reference) electrode. The
experiments were performed in dry CH2Cl2 with 0.1 M TBAP as the
supporting electrolyte. Half-wave potentials were measured with DPV
and also calculated manually by taking the average of the cathodic and
anodic peak potentials. All potentials were calibrated versus saturated
calomel electrode by the addition of ferrocene as an internal standard,
taking E1/2 (Fc/Fc+) = 0.42 V vs SCE.11 The quantum yields were
CDCl3, δ in ppm) 3.58 (s, 3 H), 6.02 (d, 2H, J(H,H) = 8.64 Hz),
6.47À6.50 (m, 8H), 6.55À6.58 (m, 4 H), 6.65À6.67 (m, 6H), 6.94 (t, 4H,
3J(H,H) = 8.76 Hz), 7.34À7.37 (m, 4H); 19F NMR (376.49 MHz, CDCl3,
δ in ppm) À131.30 (q, 3J(B,F) = 64.0 Hz), 111.33 (s), 114.98 (s), 115.74
(s); 11B NMR (128.38 MHz, CDCl3, δ in ppm) 1.05 (t, 3J(B,F) = 30.8 Hz);
13C NMR (100 MHz, CDCl3, δ in ppm) 55.40, 112.54, 114.4, 114.6, 114.9,
115.0, 115.1, 115.2, 123.4, 127.4, 128.7, 130.4, 131.8, 131.9, 132.3, 132.4,
132.8, 132.9, 133.4, 133.9, 143.2, 155.1, 159.8, 160.4, 160.65, 161.9, 162.9,
170.6; HRMS calcd for (C52H31BF8N2O) 843.2418 (M À F)+, found
843.2396 (M À F)+.
calculated using Rhodamine 6G as reference (Φ = 0.88 in ethanol, λexc
=
488 nm).8e All Φ values are corrected for changes in refractive index.
4,4-Difluoro-8-(4-methoxyphenyl)-1,2,3,5,6,7-hexabromo-
4-bora-3a,4a-diaza-s-indacene (5). To a solution of 8-(4-methox-
yphenyl)dipyrromethane 6 (0.5 g, 1.98 mmol) in dry THF was added an
exccess of N-bromosuccinimide (3.53 g, 19.8 mmol) under N2 atmo-
sphere, and the reaction mixture was allowed to stir at room temperature
overnight. TLC analysis indicated that the disappearance of spots
corresponds to 6 and and appearance of a new spot corresponds to
compound 7. The solvent was removed on a rotary evaporator under
vacuum, and the crude compound was passed through flash silica gel
column chromatography using dichloromethane. The resultant com-
pound was dissolved in freshly distilled dichloromethane and oxidized
with DDQ (190 mg, 0.833 mmol) for 30 min at room temperature. The
reaction mixture was then treated with a small amount of Et3N (3.9 mL,
4,4-Difluoro-8-(4-methoxyphenyl)-1,2,3,5,6,7-hexa(4-methyl-
phenyl)-4-bora-3a,4a-diaza-s-indacene (3). Compound 3 was
1
obtained as a red solid in 51% yield (51 mg): H NMR (400 MHz,
CDCl3, δ in ppm) 2.11 (s, 6H), 2.19 (s, 6H), 2.34 (s, 6H), 3.56 (s,
3H), 5.92 (d, 2H, 3J(H,H) = 8.4 Hz), 6.44 (d, 2H, 3J(H,H) = 8.4 Hz),
3
3
6.54 (d, 8H, J(H,H) = 8.54 Hz), 6.68 (d, 2H, J(H,H) = 8.76 Hz),
3
3
6.78 (d, 6H, J(H,H) = 8.9 Hz), 7.06 (d, 4H, J(H,H) = 8.2 Hz),
7.33 (d, 4H, J(H,H) = 8.9 Hz); 19F NMR (376.5 MHz, CDCl3, δ
3
in ppm) À131.21 (q, J(B,F)= 64.0 Hz); 11B NMR (128.38 MHz,
3
CDCl3, δ in ppm) 1.18 (t, 3J(B,F) = 32.0 Hz); 13C NMR (100 MHz,
CDCl3, δ in ppm) 21.12, 21.30, 21.67, 55.15, 112.07, 124.17, 127.82,
128.27, 128.38, 129.24, 130.02, 130.56, 130.72, 130.81, 132.07,
132.54, 133.44, 134.26, 134.89, 135.58, 138.45, 144.02, 155.97, 160.00;
HRMS calcd for C58H49BF2N2O 819.3922 (M À F)+, found 819.3945
(M À F)+.
27.9 mmol) followed by BF3 OEt2 (4.4 mL, 35.26 mmol), and the
3
mixture was stirred for an additional 30 min at room temperature. The
solvent was removed in a rotary evaporator, and the resultant crude
compound was purified by silica gel column chromatography with
petroleum ether/ethyl acetate (98:2) and afforded pure hexabromo
4,4-Difluoro-8-(4-methoxyphenyl)1,2,3,5,6,7-hexa(4-meth-
oxyphenyl)-4-bora-3a,4a-diaza-s-indacene(4). Compound 4was
1
1
BODIPY 5 as a green solid in 24% (0.35 g): H NMR (400 MHz,
obtained as a red solid in 49% yield (60 mg): H NMR (400 MHz,
CDCl3, δ in ppm) 3.9 (s, 3H), 7.07 (d, 2H, 3J(H,H) = 9.0 Hz), 7.14 (d,
CDCl3, δ in ppm) 3.53 (s, 3H), 3.61 (s, 6H), 3.66 (s, 6H), 3.77 (s, 6H),
5.94 (d, 2H, 3J(H,H) = 8.6 Hz), 6.26 (d, 4H, 3J(H,H) = 8.7 Hz), 6.44
(d, 4H, 3J(H,H) = 8.9 Hz), 6.49 (d, 6H, 3J(H,H) = 8.76 Hz), 6.54 (d,
2H, 3J(H,H) = 8.92 Hz), 6.66 (d, 2H, 3J(H,H) = 8.7 Hz), 6.72 (d, 4H,
3J(H,H) = 8.9 Hz), 7.35 (d, 4H, 3J(H,H) = 8.9 Hz); 19F NMR (376.5
3
2H, J(H,H) = 8.7 Hz); 19F NMR (376.5 MHz, CDCl3, δ in ppm)
3
À145.76 (q, J(B,F) = 57.8 Hz); 11B NMR (96.3 MHz, CDCl3, δ in
ppm) 0.88 (t, 3J(B,F) = 28.2 Hz); 13C NMR (100 MHz, CDCl3, δ in
ppm) 55.59, 115.26, 117.66, 121.56, 124.05, 130.27, 131.91, 135.00,
143.38, 161.79; HRMS calcd for C16H7BBr6F2N2O 746.5764 (M À F)+,
found 746.5736 (M À F)+.
MHz, CDCl3, δ in ppm) À131.25 (q, J(B,F) = 64.0 Hz); 11B NMR
3
(128.38 MHz, CDCl3, δ in ppm) 1.20 (t, 3J(B,F) = 32.0 Hz); 13C NMR
(100 MHz, CDCl3, δ in ppm) 55.12, 55.22, 112.27, 112.94, 113.14,
113.20, 124.54, 125.99, 127.58, 131.56, 132.05, 132.51, 133.61, 155.56,
157.46, 157.87, 159.89; HRMS calcd for C58H49BF2N2O7 915.3617
(M À F)+, found 915.3607 (M À F)+.
General Method for the Synthesis of 4,4-Difluoro-8-(4-
methoxyphenyl)-1,2,3,5,6,7-hexaaryl-4-bora-3a,4a-diaza-s-
indacenes (1À4). Samples of 5 (100 mg, 0.13 mmol), appropriate aryl-
boronic acid (387 mg, ∼2.6 mmol), and Na2CO3 (256 mg, 2.44 mmol)
were taken in a 1:1:1 mixture of water/THF/toluene (15 mL) in a 100 mL
round-bottomed flask fitted with a reflux condenser and stirred under N2
for 5 min. A catalytic amount of Pd(PPh3)4 (68 mg, ∼5À10 mol %) was
added and the reaction mixture was refluxed at 80 °C for 6À10 h. After
completion of the reaction as judged by TLC analysis, the reaction mixture
was diluted with water (5 mL) and extracted with diethyl ether. The
combined organic layers were washed with water and brine and dried
over MgSO4. The solvent was evaporated, and the crude product was
purified on a silica gel column using petroleum ether/ethyl acetate
(95:5) to afford polyarylated BODIPYs 1À4 in 50À55% yield.
4,4-Difluoro-8-(4-methoxyphenyl)-1,2,3,5,6,7-hexaphenyl-
4-bora-3a,4a-diaza-s-indacene (1). Compound 1 was obtained as a
red solid in 51% yield (53 mg): 1H NMR (400 MHz, CDCl3, δ in ppm)
Characterization Data for Compounds 9, 10, and 11. 9:
1H NMR (400 MHz, CDCl3, δ in ppm) 3.63 (s, 3H), 6.15À6.17
(m, 4H), 6.45À6.47 (m, 2H), 6.55À6.56 (m, 2H), 6.79À6.81 (m, 2H),
6.90À6.94 (m, 4H), 7.04À7.07 (m, 2H), 7.16À7.17 (m, 2H),
7.43À7.44 (m, 2H), 7.63 (d, 2H, 3J(H,H) = 2.84 Hz); 19F NMR
(376.49 MHz, CDCl3, δ in ppm) À133.17 (q, 3J(B,F) = 64.0 Hz); 11
B
3
NMR (128.38 MHz, CDCl3, δ in ppm) 1.14 (t, J(B,F) = 30.8 Hz);
ES-MS calcd for C40H25BF2N2OS6 m/z 790.0352, observed 771.0901
(M À F)+.
10: 1H NMR (400 MHz, CDCl3, δ in ppm) 3.18 (s, 3H), 5.89 (d, 2H,
3J(H,H) = 6.9 Hz), 6.59 (d, 2H, 3J(H,H) = 8.4 Hz), 6.66À6.70 (m, 4H),
3
3
6.83 (d, 2H, J(H,H) = 8.3 Hz), 6.91 (d, 2H, J(H,H) = 8.5 Hz),
7.07À7.12 (m, 8H), 7.14À7.20 (m, 8H), 7.24À7.30 (m, 8H),
7.32À7.39 (m, 8H), 7.44À7.53 (m, 8H), 7.57À7.63 (m, 4H), 7.75 (d,
3
3.51 (s, 3H), 5.89 (d, 2H, J(H,H) = 8.76 Hz), 6.54À6.56 (m, 8H),
2H, J(H,H) = 8.4 Hz); 19F NMR (376.49 MHz, CDCl3, δ in ppm)
3
6.70À6.79 (m, 8H), 6.89À6.93 (m, 6H), 7.28À7.39 (m, 6H), 7.42 (d,
3
4H, J(H,H) = 5 Hz); 19F NMR (376.49 MHz, CDCl3, δ in ppm)
À131.07 (q, J(B,F) = 64.0 Hz); 11B NMR (128.38 MHz, CDCl3, δ
3
À131.30 (q, 3J(B,F) = 64.0 Hz); 11B NMR (128.38 MHz, CDCl3, δ in
ppm) 1.20(t, 3J(B,F)= 30.8 Hz);13CNMR(100 MHz, CDCl3, δ in ppm)
55.22, 112.4, 123.9, 125.5, 125.7, 126.3, 127.2, 127.5, 127.6, 128.4, 128.8,
129.2, 130.4, 130.9, 131.9, 132.4, 133.3, 133.6, 134.7, 134.9, 144.2, 156,
160; HRMS calcd for C52H37BF2N2O 735.2983 (M À F)+, found
735.3001 (M À F)+.
in ppm) 1.19 (t, 3J(B,F) = 30.8 Hz); ES-MS calcd for C88H61BF2N2O
m/z 1210.4, observed 1211.5 (M + 1)+.
11: 1H NMR (400 MHz, CDCl3, δ in ppm) 2.98 (s, 3H), 6.72 (m,
4H), 6.79À6.83 (m, 2H), 7.13À7.19 (m, 12H), 7.24À7.29 (m, 10H),
3
7.32À7.40 (m, 6H), 7.46À7.54 (m, 6H), 7.59 (d, 2H, J(H,H) = 8.6
Hz), 7.66À7.69 (m, 4H); 19F NMR (376.49 MHz, CDCl3, δ in ppm)
À130.52 (q, 3J(B,F) = 64.0 Hz); 11B NMR (128.38 MHz, CDCl3, δ in
ppm) 1.43 (t, 3J(B,F) = 30.8 Hz); ES-MS calcd for C76H49BF2N2O m/z
1054.3, observed 1035.9 (M À F)+.
4,4-Difluoro-8-(4-methoxyphenyl)-1,2,3,5,6,7-hexa(4-fluoro-
phenyl)-4-bora-3a,4a-diaza-s-indacene (2). Compound 2 was ob-
tained as a red fluorescent solid in 53% yield (61 mg): 1H NMR (400 MHz,
8470
dx.doi.org/10.1021/jo2010022 |J. Org. Chem. 2011, 76, 8466–8471

The Journal of Organic Chemistry
NOTE
’ ASSOCIATED CONTENT
(8) (a) Rohand, T.; Baruah, M.; Qin, W.; Boens, N.; Dehaen, W.
Chem. Commun. 2006, 266–268. (b) Baruah, M.; Qin, W.; Vallꢀee,
R. A. L.; Beljonne, D.; Rohand, T.; Dehaen, W.; Boens, N. Org. Lett.
2005, 7, 4377–4380. (c) Baruah, M.; Qin, W.; Basariꢀc, N.; Borggraeve,
W. M. D.; Boens, N. J. Org. Chem. 2005, 70, 4152–4157. (d) Qin, W.;
Leen, V.; Dehaen, W.; Cui, J.; Xu, C.; Tang, X.; Liu, W.; Rohand, T.;
Beljonne, D.; Averbeke, B. V.; Clifford, J. N.; Driesen, K.; Binnemans, K.;
Auweraer, M. V. D.; Boens, N. J. Phys. Chem. C 2009, 113, 11731–11740.
(e) Qin, W.; Leen, V.; Rohand, T.; Dehaen, W.; Dedecker, P.; Van der
Auweraer, M.; Robeyns, K.; Van Meervelt, L.; Beljonne, D.; Van
Averbeke, B.; Clifford, J. N.; Driesen, K.; Binnemans, K.; Boens, N. J.
Phys. Chem. A 2009, 113, 439–447.
S
Supporting Information. Characterization data for all
b
new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ravikanth@chem.iitb.ac.in.
’ ACKNOWLEDGMENT
(9) Wakamiya, A.; Sugita, N.; Yamaguchi, S. Chem. Lett. 2008,
37, 1094–1095.
(10) Lee, C.-H.; Lindsey, J. S. Tetrahedron 1994, 50, 11427–11440.
(11) Masui, M.; Sayo, H.; Tsuda, Y. J. Chem. Soc. B 1968, 973–976.
We thank Department of Atomic Energy, Government of
India for funding the project, and V.L. thanks CSIR for a
fellowship. We thank the DST-funded National Single Crystal
X-ray Diffraction facility for diffraction data, and S.M. Mobin and
Dr. M. Rajeswara Rao for their help in crystal structure analysis.
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