Gold-catalyzed allene cycloisomerization for pyrrole synthesis: Towards highly fluorinated BODIPY dyes

Article abstract of DOI:10.1039/c4ob02671c

A novel synthetic strategy toward highly fluorinated BODIPY dyes with exceptional photostabilities relying on sustainable gold catalysis has been developed. A key to the tailored pyrrole precursors is the gold catalysis performed in ionic liquids as the reaction medium, allowing a facile recycling of the catalysts. The dyes prepared are well-matching with the spectral windows of popular rhodamine dyes and possess high brightness while showing a distinctly higher photostability than the rhodamines especially in aprotic solvents.

Full text of DOI:10.1039/c4ob02671c

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DOI: 10.1039/C4OB02671C
ARTICLE
Gold-catalyzed Allene Cycloisomerization for Pyrrole
Synthesis: Towards Highly Fluorinated BODIPY Dyes
Cite this: DOI: 10.1039/x0xx00000x
Linda Lempke,a Tobias Fischer,b Jérémy Bell,b Werner Kraus,b Knut
Rurack*b and Norbert Krause*a
Received 00th January 2012,
Accepted 00th January 2012
A novel synthetic strategy toward highly fluorinated BODIPY dyes with exceptional photostabilities
relying on sustainable gold catalysis has been developed. Key to the tailored pyrrole precursors is the
gold catalysis performed in ionic liquids as reaction medium, allowing for a facile recycling of the
catalysts. The dyes prepared are well-matching with the spectral windows of popular rhodamine dyes
and possess a high brightness while showing a distinctly higher photostability than the rhodamines
especially in aprotic solvents.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Results and discussion
Sustainable metal catalysis has attracted general attention during the
last years. Meanwhile also for gold catalysis powerful sustainable
methods are known.1 The use of ionic liquids (ILs) or micelles as
reaction medium enables the stabilization of catalysts, preventing
their undesired reduction to metallic gold. Previously, the unique
stability and recyclability of these catalyst solutions was
demonstrated.2
The cycloisomerization was carried out in ionic liquids (Table 1-3).
Different gold catalysts and ionic liquids were examined for this
purpose (Scheme 2).
Novel sustainable synthetic routes are often also a key to success
in advancing compounds of a certain class of specialty chemicals,
for instance with unprecedented substitution patterns or improved
reaction yields. In this regard, boron–dipyrromethene (BODIPY)
dyes as one of the fastest growing classes of dyes3 are very attractive
targets. The popularity of BODIPY dyes is rooted in their
outstanding absorption and fluorescence properties and high photo-
and chemical stabilities, having resulted in a wide variety of
applications. Besides bioimaging,4 fluorescent indicators,5
photodynamic therapy,6 and organic photovoltaics,7 highly
fluorinated BODIPYs have been found recently to constitute a very
promising class of multifunctional labeling agents.8 One of the
bottlenecks for novel BODIPYs is the availability of tailor-made
pyrroles which constitute their core building blocks.
Scheme 1 Synthesis of BODIPY dyes.
The expertise of our groups in sustainable metal catalysis2 and
the development of high-performance dyes9 prompted us to embark
on a collaborative approach toward new fluorinated BODIPY dyes 4
by gold-catalyzed cycloisomerization of α-aminoallenes 1 to 2,5-
dihydropyrroles 2 in ionic liquids (Scheme 1). Although fluorinated
BODIPYs possess many promising features, only a very limited
number of fluorinated BODIPYs is known in literature.8,10
Furthermore, the synthesis of pyrroles by cyclization reactions
allows extensive structural variation and represents a valuable
alternative compared to common strategies.11
The cyclization conditions were optimized with allene 1a and
cationic gold catalyst A (Table 1). Best results were achieved with
[BMIM][PF6] and [BMIM][HSO4].
With 1 mol% of catalyst A in [BMIM][PF6] complete conversion
was observed after 24 h, while 2 mol% led to a shorter reaction time
of 2 h (entries 1 & 2). The acidic ILs [BMIM][HSO4] and
[EMIM][HSO4] were used at 60°C to attain a lower viscosity. Short
reaction times of 15 min and 1h were obtained (entries 4 & 5), which
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Table 2 Influence of the gold catalyst on the cycloisomerization of
-aminoallene 1a in [BMIM][PF6]13
are comparable to [BMIM][PF6] at the same temperature (entry 3).
The acidic ILs do not mediate the cycloisomerization in the absence
of the gold catalyst (entries 6 & 7).[12] In [BMIM][BF4] and
[EMIM][BF4] no reaction was observed after 24 h at RT (entries 8 &
9). This may be due to a limited solubility of the gold catalyst in
these more hydrophilic ionic liquids.
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DOI: 10.1039/C4OB02671C
Entry
[Au]
Additive
t
Conv. (%)a
1
2
A
-
2 h
100
30
AuBr3
-
48 h
1 h
3
Ph3PAuNTf2
-
100
100
100
0
4b
5
Ph3PAuNTf2
-
5 min
1 h
Scheme 2 Gold catalysts and ionic liquids used in this work.
Ph3PAuCl
AgSbF6
-
6
Ph3PAuCl
24 h
7
B
B
B
B
-
AgSbF6
-
30 min
24 h
100
0
Table 1 Influence of ionic liquids on the gold-catalyzed cycloiso-
merization of -aminoallene 1a
8
9b
10c
11
AgSbF6
AgSbF6
AgSbF6
5 min
10 min
24 h
100
100
20
a Conversion was determined by 1H-NMR spectroscopy.
b Toluene (ca. 20 Vol-%) was added. c Reaction temperature of 50 °C.
Entry
Ionic liquid
A (mol%)
T (°C)
t (h)
Conv.a (%)
Previous investigations with AuBr3 in [BMIM][PF6] showed an
extremely low catalyst leaching using hexane as extraction solvent.2a
The pyrrolines 2 are not soluble in nonpolar solvents like hexane.
Unfortunately, polar solvents like Et2O cause a dramatic catalyst
leaching. Therefore, the recycling was tested for 1a with toluene as
extraction solvent and Ph3PAuNTf2 as gold catalyst. Over four runs
excellent reactivity was observed (Table 3, entries 1-4). The reaction
was stopped after 5 min in all runs, leading to quantitative
conversion and high yield. A loss of reactivity was observed in the
fifth run (entry 5), causing a lower yield of 80% (12% of allene 1a
were also recovered). Allene 1b was cyclized under similar
conditions with 90% yield in 5 min (entriy 6). A second run was
carried out with a similar result (entry 7).
1
2
3
4
5
6
7
8
9
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][HSO4]
[EMIM][HSO4]
[BMIM][HSO4]
[EMIM][HSO4]
[BMIM][BF4]
[EMIM][BF4]
1
2
2
2
2
-
25
25
60
60
60
60
60
25
25
24
2
100
100
100
100
100
0
0.25
0.25
1
24
24
24
24
-
0
2
2
0
0
a Conversion was determined by 1H-NMR spectroscopy.
Allene 1a and ionic liquid [BMIM][PF6] were used for screening
of the gold catalyst (Table 2). Except for AuBr3 (entry 2), all
catalysts examined gave short reaction times between 30 min and 2 h
(entries 1, 3, 5 & 7). Both precatalyst B and Ph3PAuCl were
transformed into the corresponding cationic gold species with
AgSbF6. Without addition of a silver salt no reaction was observed
(entries 6 & 8), whereas the silver salt alone induces only a very
slow cyclization (entry 11). Beside the possibility of warming up
(entry 10) and the addition of a small amount of toluene to give a
solvent film led to decreased reaction times of 5 min (entries 4 vs. 3,
9 vs 7). This acceleration is probably caused by the two-phasic
character of the reaction mixture. While the catalyst solvent system
is a viscous solution, the allene is a solid and affords an emulsion
during the reaction. The toluene seems to act as phase-transfer agent.
These conditions were chosen for recycling experiments (Table 3).
Table 3 Recycling of the catalyst/solvent system
Entry
Allene
Run
Yielda (%)
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1b
1b
1
2
3
4
5
1
2
91
91
98
95
80b
90
92
a Isolated yield. b 12% of 1a was recovered.
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The cyclization products are converted into the pyrroles using to C1 and C7, the pentafluorophenyl moiety is strongly twisted out
KtOBu in THF (Scheme 1).14 According to common strategies, these of the BODIPY mean plane, with dihedral angles of 88.35(05)° for
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were directly used in the BODIPY synthesis.10a In contrast to the 4a and 76.73(10)° for 4b, which is as expDeOctIe:d10m.10o3r9e/Cp4rOonBo0u2n67c1eCd
literature method, however, oxidation times were extended and a than for 1,7-H- and 3,5-diaryl-substituted dyes.10e,14-III An overlay of
second complexation step employed, finally providing the dyes in the two molecular configurations shows the differences in molecular
good yields of 68 and 70%.
conformation, i.e., the arrangement of the substituents attached at C3
and C5 (Figure S1). Whereas in 4b the two phenyl rings are almost
parallel with a dihedral angle of 28.09(22)°, the angle amounts to
86.24(07)° in 4a. Because the arrangement of the molecules is head-
to-tail, the short contacts from the pentafluorophenyl ring to the
phenyl rings at C3,5 lead to a bowl-like conformation of 4a (Figure
2). Such a bowl-type arrangement of head-to-tail aligned 3,5,8-
phenyl-substituted BODIPYs has also been reported by Wakamiya
very recently.14-III
Figure 1 Ortep representation of 4a (left) and 4b (right) with atomic
labeling shown with 30% probability displacement ellipsoids.
The molecular structures of 4a,b were determined by single-
crystal X-ray diffraction analysis and the corresponding molecular
configurations are shown in Figure 1. Selected crystallographic data
and structure refinement parameters are listed in Table S1. The
observed geometric parameters of all crystal structures are generally
comparable with those of other BODIPYs.14-I,14-II 4a crystallizes in a
monoclinic, 4b in a triclinic space group with similar crystal
Figure 3 Absorption and fluorescence spectra of 4a (black) and 4b
packing. There are no significant interactions like hydrogen bonds or (red) in ethanol; the corresponding spectra of rhodamine 101 (blue)
are included for comparison.
π-π interactions in the crystal lattice of both dyes. Only a weak π-π
interaction occurs in 4a with a plane-to-plane distance of 4.34 Å
between the same pyrrole rings (N1, C1, C2 ,C3, C4) of two
neighbouring pyrromethene fragments of the BODIPY core.
The spectroscopic properties of 4a and 4b are comparable with
very little dependence on the solvent. The spectral envelopes shown
in Figure
3 resemble classical BODIPY features. The high
fluorescence quantum yields (Φf >0.85), long fluorescence lifetimes
(τf ~5.5 ns) and high absorption coefficients (ελmax ~50000 M–1
cm–1), independent of solvent polarity and proticity, distinguish the
dyes as bright emitter15s (comprehensive spectroscopic data of 4a,b
are collected in the ESI). Compared to their parent dye, 8-
pentafluorophenyl-1,7,3,5-tetramethyl-BODIPY, that carries methyl
groups instead of the trifluoromethylphenyl residues in the 3,5-
positions,8 the absorption maxima are shifted by ca. 40 nm and the
absorption bands are significantly broadened. The Stokes shifts are
increased from ca. 300 to 1000 cm–1. In view of the color rules for
BODIPY π system substitution,16 these changes can be attributed to
the extension of the fluorophores’ π-system with the additional
trifluorophenyl moieties and their enhanced rotational freedom. The
comparatively small spectral differences between 4a and 4b despite
the introduction of two additional CF3 groups in the former is
tentatively ascribed to the enhanced steric hindrance in case of 3,5-
phenyl substitution at the BODIPY core.17 The latter results in
considerable (average) torsion angles around the C1–C1′ bond <θC2–
C1–C1′–C2′> = 39° for 4a and for 38° 4b (for atom numbering pattern,
see Scheme 1, bottom panel) and hence reduced electronic coupling
Figure 2 Short contacts in 4a lead to a bowl-like conformation
The BODIPY skeleton formed by three conjugated heterocyclic
rings is almost planar, with an rms deviation of 0.1260 and 0.1016
for 4a and 4b. Due to steric repulsion of the methyl groups attached
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between BODIPY core and phenyl rings, diminishing the electron lipophilic compartments, for instance in cell imaging. Current work
withdrawing effect of these substituents. These experimental in our laboratories is directed toward both the synthesis of other
findings are well reflected by quantum chemical calculations functional BODIPYs and the application of tDheOIt:it1l0e.1d0y3Ve9ise/.Cw4AOrtiBcl0e2O6n7l1inCe
employing the density functional theory (DFT) method, revealing
differences in the S1←S0 transition in the gas phase of only ca. 5 nm Acknowledgements
Financial support by SusChemSys (which is co-financed by the
Regional Development Fund (Investing in Your Future) of the
European Union and the state of North Rhine-Westphalia), the
European Metrology Research Programme (EMRP) (project
IND15 SurfChem) and the Adolf-Martens-Fonds is gratefully
acknowledged. The EMRP is jointly funded by the EMRP
participating countries within EURAMET and the European
Union.
(Table S2, ESI). The latter are also the only oscillator-strong
transitions in the 350–500 nm region for both dyes, involving
HOMO and LUMO which are both centered almost exclusively on
the 3,5-diphenyl-dipyrrin fragments for 4a,b (Figure S2, ESI).
Furthermore, with the corresponding triplet transitions being shifted
for ca. 1.25 eV, the theoretical studies fully support the favorable
spectroscopic properties found experimentally.
Dyes 4a and 4b absorb and emit in the green visible range,
matching well the spectral region of prominent rhodamine dyes such
as rhodamine 101, 6G, B or TRITC18 and fitting perfectly to the
output of green laser sources (e.g., the prominent 532 or 543 nm
lasers, Figure 3). Compared to rhodamines the zwitterionic yet net
uncharged BODIPYs are commonly well soluble in organic solvents
across the entire polarity range,3,19 broadening the areas of
applications and offering potent alternatives to the search for
hydrophobic rhodamine derivatives.20 The most important feature of
the newly synthesized fluorinated BODIPYs 4a,b with regard to
application is their excellent photostability, in particular when used
in aprotic solvents. Figure 4 exemplarily combines photobleaching
curves of 4a,b and rhodamine 101 upon irradiation at 532 nm.
Notes and references
a L. Lempke, Prof. Dr. N. Krause
Organic Chemistry
Dortmund University of Technology
Otto-Hahn-Str. 6, D-44227 Dortmund, Germany.
Fax: (+)49 231 755 3884
E-mail: norbert.krause@tu-dortmund.de
b T. Fischer, Dr. J. Bell, Dr. W. Kraus, Dr. K. Rurack
Dept. 1 Analytical Chemistry; Reference Materials
BAM Federal Institute for Materials Research and Testing
Richard-Willstätter-Str. 11, D-12489 Berlin, Germany.
Fax: (+) 49 30 8104 1157
E-mail: knut.rurack@bam.de
Electronic Supplementary Information (ESI) available: Full experimental
details, including X-Ray structure analysis, Optical spectroscopy,
Computational Studies, and NMR spectra. See DOI: 10.1039/b000000x/
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Figure 4 Photobleaching curves for irradiation at 532 nm (1W Laser
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In the work presented here, two highly fluorinated and photo-stable
BODIPY dyes with excellent spectroscopic properties have been
prepared via gold catalysis in ionic liquids. While the use of
sustainable gold catalysis is often strictly limited to the applied
system, the possibility of recycling the catalyst solution was
extended to more challenging allenes. Here [BMIM][PF6] was found
to be an excellent solvent system for gold catalysis and recycling.
The fluorinated dyes possess an enormous potential for further
applications such as functional group labeling or the staining of
7 A. Bessette, G. S. Hanan, Chem. Soc. Rev., 2014, 43, 3342.
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