Lewis acid assisted decomplexation of F-BODIPYs to dipyrrins

Article abstract of DOI:10.1002/ejoc.201301662

A simple synthetic route was developed for the decomplexation of F-BODIPYs (fluorine-substituted boron-dipyrromethenes) to afford dipyrrins in high yields. This was achieved by treating the F-BODIPYs with different Lewis acids such as ZrCl₄, TiCl₄, AlCl₃, Sc(OTf)₃, or SnCl₄ in CH₃CN/CH₃OH under refluxing conditions. This synthetic strategy was efficient for different types of F-BODIPYs such as meso-aryl-substituted BODIPYs, 3-pyrrolyl BODIPYs, functionalized 3-pyrrolyl BODIPYs, π-extended pyrrolyl BODIPYs, sterically crowded BODIPYs, and the BF₂ complex of 25-oxasmaragdyrin. Copyright

Full text of DOI:10.1002/ejoc.201301662

FULL PAPER
DOI: 10.1002/ejoc.201301662
Lewis Acid Assisted Decomplexation of F-BODIPYs to Dipyrrins
Vellanki Lakshmi,[a] Tamal Chatterjee,[a] and Mangalampalli Ravikanth*[a]
Keywords: Synthetic methods / Nitrogen heterocycles / Lewis acids / Boron / Fluorine
A simple synthetic route was developed for the decomplex-
was efficient for different types of F-BODIPYs such as meso-
ation of F-BODIPYs (fluorine-substituted boron–dipyrro- aryl-substituted BODIPYs, 3-pyrrolyl BODIPYs, function-
methenes) to afford dipyrrins in high yields. This was
achieved by treating the F-BODIPYs with different Lewis ac-
ids such as ZrCl4, TiCl4, AlCl3, Sc(OTf)3, or SnCl4 in CH3CN/
CH3OH under refluxing conditions. This synthetic strategy
alized 3-pyrrolyl BODIPYs, π-extended pyrrolyl BODIPYs,
sterically crowded BODIPYs, and the BF2 complex of 25-oxa-
smaragdyrin.
rins.[9] Herein, we report a simple alternate synthetic route
to remove the BF2 moiety from F-BODIPYs to form dipyr-
rins in high yields. In our method, the BODIPY is treated
with different Lewis acids such as ZrCl4, TiCl4, AlCl3,
Sc(OTf)3, or SnCl4 in dry CH3CN/CH3OH, and the reac-
tion mixture is heated at reflux for 30–120 min under an
inert atmosphere followed by simple workup conditions and
recrystallization to afford the dipyrrins in high yields. As
demonstrated herein, the method is efficient for simple
meso-aryl-substituted BODIPYs,[6] sterically crowded
BODIPYs,[10] pyrrolyl BODIPYs,[11] functionalized pyrrolyl
BODIPYs,[12] π-extended BODIPYs,[13] and the BF2 com-
Introduction
The BF2 complexes of dipyrrins, which are popularly
known as boron–dipyrromethenes or BODIPYs, have
gained recognition as being one of the most versatile fluoro-
phores and are used extensively as labelling reagents,[1] fluo-
rescent switches,[2] chemosensors,[3] and laser dyes[4] because
of their remarkable spectral and photophysical properties.[5]
BODIPYs are generally prepared in two steps by starting
from dipyrromethanes.[6] In the first step, the dipyrrometh-
ane is treated with an oxidizing agent to give the dipyrrin,
and in the second step without isolation, the dipyrrin is
treated with BF3·OEt2 to afford the BODIPY. The dipyrrins
are versatile coordinating ligands and form several interest-
ing coordinating complexes with various geometries.[7d]
They are also useful precursors in the syntheses of dyes,
porphyrins, porphyrin-based biomimetic systems, diverse
porphyrin building blocks, and as intermediates in por-
phyrin transformations.[7] Although dipyrrins can be easily
prepared, these compounds are difficult to purify and pre-
fer to be stored in the form of either BF2 or metal com-
plexes. Thus, BF2 or a metal can be used as a masking moi-
ety for the dipyrrins, as these complexes are easy to prepare
and sufficiently robust for any modification to the dipyrrin
core. Later, the complexes can be unmasked to obtain the
desired modified dipyrrins for further chemistry. Lindsey
and co-workers developed strategies to protect the dipyrrin
as a zinc[8a,8b] or tin[8c] complex prior to functionalization
to facilitate purification, but these metal complexes are not
very stable under acidic conditions. In 2010, Thompson and
co-workers developed a new microwave-promoted deprotec-
tion strategy for the conversion of F-BODIPYs into dipyr-
plex of the macrocycle oxasmaragdyrin.[14]
.
Results and Discussion
The model reactions were carried out by taking 3-pyr-
rolyl meso-phenyl BODIPY[11] 1a as a reference compound.
Initially, we treated compound 1a with an excess amount
(ca. 10 equiv.) of different Lewis acids and metal salts such
as SnCl4, AlCl3, FeCl3, CuCl2, NiCl4, ZnCl2, CoCl3, and
ZrCl4 in dry CH3CN under reflux conditions and an inert
atmosphere (see Scheme 1). TLC analysis indicated that
either no reaction had occurred, or there was complete de-
composition of the starting material.
In case of ZrCl4, we used methanol to solubilize the
ZrCl4 and were pleasantly surprised to find that the TLC
analysis after 30 min showed the complete disappearance of
the fluorescent spot, which corresponded to 1a, along with
the appearance of a new, less polar, nonfluorescent spot,
which corresponded to 1b. NMR and HRMS analyses later
confirmed the structure of 1b. We re-examined the reaction
with other Lewis acids and salts by using dry methanol as
the cosolvent. Our studies showed that AlCl3, TiCl4, and
Sc(OTf)3 were equally efficient as ZrCl4 in the reaction, but
SnCl4 required a longer reaction time and gave partial con-
version. In the case of the other salts, the reactions did not
[a] Indian Institute of Technology (IIT),
Powai, Mumbai 400076, India
E-mail: ravikanth@chem.iitb.ac.in
http://www.chem.iitb.ac.in/~ravikanth/homepage/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301662.
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V. Lakshmi, T. Chatterjee, M. Ravikanth
FULL PAPER
Figure 1. The crystal structure of compound 2b.
followed by workup and recrystallization afforded the cor-
responding functionalized pyrrolyl dipyrrins 3b–5b in 96–
99% yield (see Scheme 3). The functionalized pyrrolyl di-
1
pyrrins 3b–5b were characterized by HRMS as well as H
and 13C NMR spectroscopic analysis (see Supporting Infor-
mation).
Scheme 1. Optimized studies for the conversion of 1a into 1b.
yield the dipyrrin, and the starting material was recovered.
Thus, our preliminary investigations indicated that meth-
anol was crucial for the decomplexation of the F-BODIPYs
in the presence of Lewis acids to give the dipyrrins. All
quantitative studies were carried out using ZrCl4.
To optimize the stoichiometric amounts of ZrCl4, 1a was
treated with 1 to 10 equiv. of ZrCl4 in dry CH3CN and
CH3OH under an inert atmosphere and reflux conditions
for 1 h . The studies showed that with up to 4 equiv. of
ZrCl4 the reaction was slow and required a longer reaction
time. However, employing 5 equiv. of ZrCl4 resulted in the
effective decomplexation of 1a[11a] to give 1b in the best
yield of 96% (see Scheme 2). Pyrrolyl BODIPY 2a[11b] was
converted into pyrrolyl dipyrrin 2b under similar reaction
conditions to afford a 94% yield of 2b, the structure of
which was confirmed by X-ray crystal structure analysis
(see Figure 1).
Scheme 3. Synthesis of functionalized 3-pyrrolyl dipyrrins 3b–5b.
We recently reported a synthesis that employed Wittig
reaction conditions to give π-extended pyrrolyl BODI-
PYs,[13] which have interesting photophysical properties. To
demonstrate a further use of our method, we carried out
the decomplexation of π-extended pyrrolyl BODIPYs 6a
and 7a by treating them with 5 equiv. of ZrCl4 under similar
reaction conditions to afford the corresponding π-extended
pyrrolyl dipyrrins 6b and 7b in 92 and 95% yield, respec-
tively (see Scheme 4). These sorts of π-extended pyrrolyl di-
pyrrins are not easily prepared through other conventional
synthetic routes.
Scheme 2. Synthesis of 3-pyrrolyl dipyrrins 1b and 2b.
The Lewis acid assisted BF2 decomplexation reaction
was used further to convert functionalized 3-pyrrolyl
BODIPYs[12] into functionalized pyrrolyl dipyrrins (see
Scheme 3), which are useful precursors for various applica-
Scheme 4. Synthesis of extended π-conjugated dipyrrins 6b and 7b.
We also tested our strategy with simple meso-aryl-substi-
tions.[15] The treatment of 3a–5a with 5 equiv. of ZrCl4 in tuted BODIPYs.[6] The reaction of meso-aryl-substituted
dry CH3CN and CH3OH at reflux for approximately 2 h BODIPYs 8a[16a] and 9a[16b] with 5 equiv. of ZrCl4 in dry
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Lewis Acid Assisted Decomplexation of F-BODIPYs to Dipyrrins
CH3CN/CH3OH under reflux conditions afforded the cor-
responding dipyrrins 8b[17] and 9b[17] both in approximately
90% yield (see Scheme 5, a). The characterization data of
dipyrrins 8b and 9b are in agreement with reported data.[17]
The reaction of 3,5-dimethyl meso-4-tolyl BODIPY[18] 10a
with ZrCl4 under the same reaction conditions yielded the
corresponding 1,9-dimethyl dipyrrin 10b in 90% yield (see
Scheme 5, b). We then extended our strategy to sterically
crowded hexaaryl BODIPY systems such as hexaphenyl
BODIPY 11a. Interestingly, the reaction of 4,4-difluoro-8-
(4-anisyl)-1,2,3,5,6,7-hexaphenyl-4-bora-3a,4a-diaza-s-
indacene[10] (11a) and 5 equiv. of ZrCl4 was facile and af-
forded stable hexaaryl dipyrrin 11b in 94% yield (see
Scheme 6, a). Thus, the method proceeded efficiently with
sterically crowded BODIPY systems, and the corresponding
dipyrrins, which require a multistep synthesis through any
other method, were easily obtained in good yields. Further-
more, the strategy was also applied to hexaalkyl
BODIPY[10] 12a under same reaction conditions to afford
the corresponding dipyrrin 12b in 91% yield (see Scheme 6,
b).
Scheme 6. Synthesis of (a) sterically crowded hexaphenyl dipyrrin
11b and (b) hexamethyl dipyrrin 12b.
Scheme 7. Synthesis of free base oxasmaragdyrin 13b from BF2
complex of oxasmaragdyrin 13a.
decomposition of the complex. Thus, the Lewis acid as-
sisted decomplexation of the BF2 complexes is only appli-
cable to meso-aryl-substituted F-BODIPY complexes.
Scheme 5. Synthesis of simple meso-aryl-substituted dipyrrins
(a) 8b and 9b and (b) 10b.
Finally, we extended our strategy to macrocycles such
as the BF2 complex of 25-oxasmaragdyrin 13a, which we
reported on recently.[14] The treatment of BF2–oxa-
smaragdyrin with ZrCl4 in dry CH3CN/CH3OH at reflux
for 5 h followed by workup and recrystallization afforded
the free base oxasmaragdyrin 13b in 95% yield (see
Scheme 7). The structure of oxasmaragdyrin 13b[19] was
confirmed by HRMS as well as NMR and absorption spec-
troscopy.
Scheme 8. Synthesis of (a) meso-free dipyrrin 14b and (b) meso-
methyl dipyrrins 15b and 16b.
Although the ZrCl4-assisted decomplexation of several
types of BF2 complexes proceeded efficiently, it failed for
the decomplexation of the meso-free F-BODIPYs. The
Conclusions
In summary, we developed an alternate Lewis acid as-
treatment of meso-free BODIPY 14a with 5 equiv. of either sisted synthetic strategy for the decomplexation of
ZrCl4 or AlCl3 in dry CH3CN/CH3OH under an inert at- F-BODIPYs to afford different types of dipyrrins in high
mosphere and varied temperatures resulted in decomposi- yields. Our strategy efficiently proceeded for simple meso-
tion (see Scheme 8, a). We also studies meso-methyl aryl-substituted BODIPYs as well as a BF2 complex of a
BODIPY 15a as well as meso-methyl 3-pyrrolyl BODIPY[20] macrocycle (i.e., 1a–13a). Some of the dipyrrins that were
16a (see Scheme 8, b). In both of these cases, we observed obtained by our strategy were not easily prepared through
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V. Lakshmi, T. Chatterjee, M. Ravikanth
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H), 6.88 (d, J = 4.77 Hz, 1 H), 7.04 (dd, J = 2.51, 1.25 Hz, 1 H),
7.19 (t, J = 1.88 Hz, 1 H), 7.37 (d, J = 8.44 Hz, 2 H), 7.59 (d, J =
8.44 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 110.1, 111.2,
113.4, 120.4, 122.8, 123.0, 123.4, 126.8, 128.2, 132.7, 132.9, 135.5,
136.7, 137.0 ppm. HRMS: calcd. for C19H15N3Br [M + H]+
364.0444; found 364.0448.
other synthetic routes. Although we are uncertain of the
mechanism for the decomplexation of the BODIPYs, we
assume that Lewis acids activate the B–F bonds by forming
a transition state as reported by Mély, Bonnet, and co-
workers,[21] In this transition state, the B–N bond weakens
and is subject to a nucleophilic attack by CH3OH to lead
to the required dipyrrin. Currently, we are exploring the
metal coordination chemistry of some selected dipyrrins
that we reported herein.
1
3b: Brown, shiny crystals (43 mg, 99% yield); m.p. 185–189 °C. H
NMR (400 MHz, CDCl3): δ = 6.30–6.37 (m, 1 H), 6.43 (d, J =
3.26 Hz, 1 H), 6.79 (d, J = 4.02 Hz, 1 H), 6.87 (q, J = 4.52 Hz, 2
H), 7.06 (d, J = 4.02 Hz, 1 H), 7.41–7.55 (m, 6 H), 9.60 (s, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 112.1, 113.2, 122.1,
123.0, 123.5, 127.8, 129.1, 130.0, 131.0, 133.4, 134.2, 136.0, 136.7,
137.6, 141.6, 180.0 ppm. HRMS: calcd. for C20H16N3O [M + H]+
314.1288; found 314.1274.
Experimental Section
General Methods: All NMR spectroscopic data (δ values, ppm)
were recorded with 400 and 500 MHz spectrometers. Tetramethyl-
silane (TMS) was used as an external reference for the H NMR
4b: Brown solid (42 mg, 96% yield); m.p. 220–223 °C. 1H NMR
(400 MHz, CDCl3): δ = 6.30 (dd, J = 3.76, 2.51 Hz, 1 H), 6.41 (dd,
J = 3.89, 1.13 Hz, 1 H), 6.70 (d, J = 4.02 Hz, 1 H), 6.85 (d, J =
4.52 Hz, 1 H), 6.92 (d, J = 4.27 Hz, 2 H), 7.30 (br. s. 1 H), 7.42–
7.55 (m, 5 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 102.9, 111.8,
112.0, 115.2, 121.4, 121.7, 123.5, 127.8, 129.2, 129.4, 131.1, 133.2,
137.4, 137.6, 141.5, 148.8, 157.7 ppm. HRMS: calcd. for C20H15N4
[M + H]+ 311.1291; found 311.1288.
1
(residual proton, δ = 7.26 ppm) and 13C NMR (δ = 77.2 ppm) spec-
tra in CDCl3. The multiplicities of the signals are reported as s
(singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). All
NMR measurements were carried out at room temperature in de-
uterochloroform (CDCl3). The HRMS spectra were recorded with
a Bruker maxis impact 282001.00081 using the electron spray ion-
ization method and a TOF analyzer.
5b: Brown solid (43 mg, 98% yield); m.p. 213-216 °C. 1H NMR
(400 MHz, CDCl3): δ = 6.33 (dd, J = 3.89, 2.38 Hz, 1 H), 6.46 (dd,
J = 4.02, 1.25 Hz, 1 H), 6.73 (d, J = 4.27 Hz, 1 H), 6.88 (d, J =
4.48 Hz, 1 H), 6.92 (d, J = 4.52 Hz, 1 H), 7.20 (d, J = 4.27 Hz, 1 H),
7.33–7.36 (m, 1 H), 7.44–7.54 (m, 5 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 112.6, 112.7, 113.0, 121.7, 124.5, 127.8, 129.3, 130.4,
131.0, 132.8, 133.3, 137.3, 137.4, 142.9, 156.8 ppm. HRMS: calcd.
for C19H15N4O2 [M + H]+ 331.1190; found 331.1176.
X-ray Crystal Structure Analysis: Single-crystal X-ray structure
analysis was performed on a Rigaku Saturn724 diffractometer that
was equipped with a low-temperature attachment. Data were col-
lected at 100 K using graphite-monochromated Mo-Kα radiation
(λα = 0.71073 Å). The data were collected by using the ω-scan tech-
nique and were scaled and reduced by using CrystalClear-SM Ex-
pert 2.1 b24 software. The structures were solved by direct methods
and refined by least-squares against F2 utilizing the software pack-
ages SHELXL-97,[22] SIR-92,[23] and WINGX.[24] All non-hydro-
gen atoms were refined anisotropically.
6b: Brown, crystalline solid (41 mg, 92% yield); m.p. 223 °C. 1H
NMR (400 MHz, CDCl3): δ = 6.06–6.07 (m, 1 H), 6.14–6.15 (m, 1
H), 6.34 (d, J = 3.76 Hz, 1 H), 6.59 (d, J = 8.03 Hz, 1 H), 6.69 (d,
J = 16.31 Hz, 1 H), 6.87 (d, J = 3.76 Hz, 1 H), 7.03 (d, J = 4.52 Hz,
1 H), 7.13 (d, J = 4.27 Hz, 1 H), 7.18–7.24 (m, 1 H), 7.26–7.46 (m,
10 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 113.4, 112.7, 118.5,
126.5, 127.5, 127.6, 128.6, 128.7, 131.1, 132.2, 137.5, 138.0,
138.4 ppm. HRMS: calcd. for C27H22N3 [M + H]+ 388.1808; found
388.1804.
CCDC-949053 (for 2b) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
General Procedure for the Synthesis of Dipyrrins 1b–13b: A solution
of ZrCl4 (5 equiv.) in methanol (2 mL) was added to the BODIPY
compound (i.e., 1a–13a, 50 mg, 1 equiv.) in acetonitrile (10 mL),
and the resulting mixture was stirred and heated at reflux for ap-
proximately 2 h. When TLC analysis showed the disappearance of
starting material (i.e., 1a–13a) and the appearance of a new nonflu-
orescent spot, which corresponded to the dipyyrin (i.e., 1b–13b),
the solvent was removed with a rotary evaporator under vacuum.
The resultant compound was dissolved in CHCl3, and the solution
was washed thoroughly with water. The combined organic layers
were dried with Na2SO4 and then filtered, and the solvents were
evaporated. The compound was recrystallized (CHCl3/pentane) to
afford a red-greenish crystalline solid in good yields.
1
7b: Shiny, brown crystals (43 mg, 95% yield); m.p. 230–234 °C. H
NMR (400 MHz, CDCl3): δ = 3.83 (s, 3 H), 6.06–6.11 (m, 1 H),
6.15–6.16 (m, 1 H), 6.30–6.40 (m, 2 H), 6.55–6.67 (m, 2 H), 6.82–
6.88 (m, 2 H), 7.01 (d, J = 4.58 Hz, 1 H), 7.10 (d, J = 4.58 Hz, 1
H), 7.29 (s, 1 H), 7.32–7.47 (m, 5 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 55.53, 111.2, 112.0, 114.2, 116.3, 116.6, 121.5, 123.5,
126.6, 127.4, 127.6, 127.7, 128.5, 129.0, 130.3, 131.2, 132.6, 137.2,
137.4, 137.7, 138.5, 159.3 ppm. HRMS: calcd. for C28H24N3O [M
+ H]+ 418.1914; found 418.1902.
8b: Yellow, sticky solid (37 mg, 88% yield); m.p. 117–110 °C. 1H
NMR (400 MHz, CDCl3): δ = 3.90 (s, 3 H), 6.46 (dd, J = 4.27,
1.51 Hz, 2 H), 6.71 (d, J = 4.02 Hz, 2 H), 6.99 (d, J = 8.68 Hz, 2 H),
7.45 (d, J = 8.53 Hz, 2 H), 7.72 (s, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 113.3, 114.1, 117.4, 129.5, 130.5, 133.1, 138.8, 139.3,
143.2 ppm. HRMS: calcd. for C16H15N2O [M + H]+ 251.1179;
found 251.1171.
1b: Green crystals (42 mg, 96% yield); m.p. 95–97 °C. 1H NMR
(400 MHz, CDCl3): δ = 6.24–6.28 (m, 2 H), 6.34–6.86 (m, 1 H),
6.78–6.79 (m, 1 H), 6.86 (m, 1 H), 7.02–7.03 (m, 1 H), 7.18 (s, 1 H),
7.43–7.46 (m, 3 H), 7.48–4.50 (m, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 111.4, 111.9, 114.7, 121.7, 123.0, 124.2, 126.5, 126.8,
127.8, 128.2, 128.7, 129.0, 131.2, 132.6, 137.2, 137.8, 138.1,
157.9 ppm. HRMS: calcd. for C19H16N3 [M + H]+ 286.1339; found
286.1332.
9b: Yellow, sticky solid (38 mg, 91% yield); m.p. 128–131 °C. 1H
NMR (400 MHz, CDCl3): δ = 2.45 (s, 3 H), 6.39 (dd, J = 4.12,
1.37 Hz, 2 H), 6.61–6.66 (m, 2 H), 7.22–7.27 (m, 2 H), 7.39 (d, J
= 7.93 Hz, 2 H), 7.64 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
1
2b: Green crystals (41 mg, 94% yield); m.p. 213-218 °C. H NMR
(400 MHz, CDCl3): δ = 6.24–6.27 (m, 2 H), 6.36 (dd, J = 3.51,
2.76 Hz, 1 H), 6.79 (dd, J = 3.51, 1.25 Hz, 1 H), 6.81–6.84 (m, 1 δ = 21.5, 117.6, 128.5, 129.1, 131.1, 134.6, 139.2, 141.0, 142.6,
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Lewis Acid Assisted Decomplexation of F-BODIPYs to Dipyrrins
143.6 ppm. HRMS: calcd. for C16H15N2 [M + H]+ 235.1230; found
235.1229.
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1
10b: Yellow, sticky solid (38 mg, 90% yield); m.p. 128–131 °C. H
NMR (400 MHz, CDCl3): δ = 2.43 (s, 3 H), 2.45 (s, 6 H), 6.15 (d,
J = 4.05 Hz, 2 H), 6.48 (d, J = 4.00 Hz, 2 H), 7.21 (d, J = 7.85 Hz,
2 H), 7.35 (d, J = 8.04 Hz, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 14.3, 16.5, 114.3, 117.4, 125.7, 128.3, 129.2, 130.9,
134.7, 139.5, 153.8 ppm. HRMS: calcd. for C18H19N2 [M + H]+
263.1543; found 263.1538.
11b: Red solid (44 mg, 94% yield); m.p. 278–282 °C. 1H NMR
(400 MHz, CDCl3): δ = 3.49 (s, 3 H), 5.87 (d, J = 8.53 Hz, 2 H),
6.55 (d, J = 7.03 Hz, 4 H), 6.65–6.75 (m, 8 H), 6.97 (dd, J = 6.27,
2.26 Hz, 4 H), 7.09–7.15 (m, 6 H), 7.27–7.32 (m, 6 H), 7.51 (d, J
= 6.27 Hz, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 55.1,
111.9, 124.8, 126.3, 126.7, 127.8, 128.3, 130.4, 130.8, 132.0, 133.6,
135.2, 135.8, 138.3, 143.2 ppm. HRMS: calcd. for C52H39N2O [M
+ H]+ 707.3057; found 707.3057.
[7]
[8]
1
12b: Orange solid (44 mg, 91% yield); m.p. 278–282 °C. H NMR
(500 MHz, CDCl3): δ = 1.41 (s, 6 H), 1.85 (s, 6 H), 2.31 (s, 6 H),
3.87 (s, 3 H), 6.94 (d, J = 7.80 Hz, 2 H), 7.19 (d, J = 8.55 Hz, 2
H) ppm. 13C NMR (125.7 MHz, CDCl3): δ = 22.9, 32.1, 34.0, 53.6,
114.1, 114.3, 116.1, 123.7, 124.2, 127.3, 129.4, 139.5, 150.9 ppm.
HRMS: calcd. for C22H26N2O [M + H]+ 335.2118; found 335.2119.
13b: Green solid (44 mg, 95% yield); m.p. Ͼ300 °C. 1H NMR
(400 MHz, CDCl3): δ = 2.73 (s, 9 H), 7.61 (d, J = 7.78 Hz, 4 H),
7.66 (d, J = 7.78 Hz, 2 H), 8.08 (d, J = 7.78 Hz, 4 H), 8.29 (d, J =
7.78 Hz, 2 H), 8.42 (d, J = 4.27 Hz, 2 H), 8.74 (s, 2 H), 8.96 (d, J
= 4.27 Hz, 2 H), 9.36 (d, J = 4.27 Hz, 2 H), 9.45 (d, J = 4.02 Hz, [9]
2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 21.2, 105.4, 107.2,
107.9, 108.3, 108.5, 115.3, 117.2, 117.3, 118.5, 115.3, 117.2, 117.3,
118.5, 119.4, 124.3, 125.2, 128.1, 128.4, 128.6, 129.3, 129.4, 133.8,
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Supporting Information (see footnote on the first page of this arti-
cle): Spectral data and crystallographic data.
Acknowledgments
M. R. and V. L. acknowledge the financial support from the Coun-
cil of Scientific and Industrial Research (CSIR), Government of
India.
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Received: November 6, 2013
Published Online: January 21, 2014
2110
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2105–2110