Bodipy fluorophore toolkit for probing chemical reactivity and for tagging reactive functional groups

Article abstract of DOI:10.1002/ejoc.201400052

The photophysical and synthetic studies of new tether-functionalized boron dipyrromethene (BODIPY) fluorophores as probes for chemical reactions are described. These compounds differ from typically reported probes in that they provide a way to tag and indicate chemical reactions without chemical transformation of the BODIPY core itself; instead the dyes are spectators. The introduction and modification of the tether has expanded the available chemistry and yet these new BODIPY derivatives have similar photophysical properties to their parent substrates. As a result of the chemistry enabled by these probes, a single step of a multistep metal-catalyzed reaction was revealed by a change in the fluorescence resonance energy transfer (FRET) signal. The fundamental knowledge of quantum yield, FRET efficiencies, and reagent compatibility is critical to enabling the broader application of fluorescent probes to study chemical reaction mechanisms by cutting-edge microscopy techniques. Copyright

Full text of DOI:10.1002/ejoc.201400052

FULL PAPER
DOI: 10.1002/ejoc.201400052
BODIPY Fluorophore Toolkit for Probing Chemical Reactivity and for
Tagging Reactive Functional Groups
Eva M. Hensle,[a] N. Melody Esfandiari,[a] Sung-Gon Lim,[a] and Suzanne A. Blum*[a]
Keywords: Fluorophores / Reagent compatibility / Fluorescence / Gold
The photophysical and synthetic studies of new tether-func-
tionalized boron dipyrromethene (BODIPY) fluorophores as
probes for chemical reactions are described. These com-
properties to their parent substrates. As a result of the chem-
istry enabled by these probes, a single step of a multistep
metal-catalyzed reaction was revealed by a change in the
pounds differ from typically reported probes in that they pro- fluorescence resonance energy transfer (FRET) signal. The
vide a way to tag and indicate chemical reactions without
chemical transformation of the BODIPY core itself; instead
the dyes are spectators. The introduction and modification of
the tether has expanded the available chemistry and yet
these new BODIPY derivatives have similar photophysical
fundamental knowledge of quantum yield, FRET efficienc-
ies, and reagent compatibility is critical to enabling the
broader application of fluorescent probes to study chemical
reaction mechanisms by cutting-edge microscopy tech-
niques.
probes are spectators.[14] The determination of the photo-
physical properties of these tethered fluorophore probes is
critical for the interpretation of dye behavior in single-mole-
cule or -particle fluorescence microscopy studies.[9,10,14] We
herein describe an expansion of the BODIPY toolkit
through synthesis and ensemble photophysical studies. In
one application that underscores the chemical information
available through the exquisite sensitivity of fluorescence
resonance energy transfer (FRET), the tethered spectator
BODIPY probe signals exactly one fundamental chemical
step out of many in a multistep gold-catalyzed reaction.
Introduction
An emerging application area for organic fluorophores is
as probes for chemical reactions on both the ensemble[1,2]
and, in the past 8 years, single-molecule and -particle levels
by microscopy.[3–12] Nascent applications include investi-
gations into the mechanisms of organic[13] and transition-
metal[1] reactions and homogeneous and heterogeneous ca-
talysis by fluorescence microscopy.[3,4,6,7,12] These studies
have been enabled by recent advances in boron dipyrro-
methene (BODIPY) dye synthesis.[14] These dyes are
uniquely suited to probing chemical reactions because un-
like other common high-quantum-yield fluorophore dyes,
they do not have accessible lone pairs that could bind to
transition metals nor protic functional groups that could
interfere with a chemical reagent’s native reactivity.[15–17]
Consistent with these recent advances, the ability to gain
insight into reactivity by fluorescence microscopy is a grow-
ing application area with less than 10 reported examples.[14]
Essential to the goal of enabling these studies through syn-
thesis is the development of strategies to attach these dyes
through tethers, namely unreactive chains such as alkyl
groups, to remote reactive groups, for example, alkynes, ole-
fins, alcohols, esters, epoxides, phosphines, and transition-
metal complexes. These tethered probes differ from typically
reported probes in that they enable tagging and fluores-
cence indication of chemical reactions without chemical
transformation of the BODIPY core itself; instead the
Results and Discussion
First, a BODIPY FRET pair,[2] 6, was synthesized
(Scheme 1). Molecule 6 exemplifies the strategy of tethering
BODIPY fluorophores through inert alkyl tethers to a core
reactive functional group. Specifically, the tether removes
both BODIPY dyes from the electronic and steric environ-
ment of the 1-ester-2-alkynylphenyl group. The key syn-
thetic step in the construction of 6 was a Sonogashira cou-
pling of green BODIPY alkyne 2 (with a tetramethylene
tether) and orange BODIPY aryl iodide 5 (with a penta-
methylene tether). This cross-coupling reaction showed the
compatibility of both tethered BODIPY building blocks
towards both copper and palladium catalysts as well as
amine bases. Compound 6 displayed a FRET efficiency of
0.94 at an excitation of 488 nm, producing strong emission
in the orange region.
[a] Department of Chemistry, University of California,
2046A Reines Hall, Irvine, CA 92697-2025, USA
E-mail: blums@uci.edu
We next tested the ability of FRET pair 6 to serve as a
color-changing probe for a chemical reaction (Scheme 2).
The ability of these dyes to inform on a single step in a
multistep gold-catalyzed cyclization to yield isocoumarins
http://www.chem.uci.edu/~blumlab/main.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201400052.
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Scheme 1. Synthesis of green BODIPY 2, orange BODIPY 5, and BODIPY FRET pair 6 by a key Sonogashira coupling reaction that
attaches the fluorophores to the reactive molecular core through spectator tethers.
was investigated. The addition of [PPh3AuCl]/AgOTF actions[19] to their untagged counterparts, a key finding that
(which generated the corresponding gold triflate in situ) to permitted the interpretation of fluorescence spectra (see be-
6 in wet dichloromethane successfully catalyzed isocouma- low). This result underscores the potential utility of these
rin cyclization and hydrolysis to form 9, as confirmed by 1H dyes as probes for individual reaction steps in complicated
1
NMR spectroscopy. After 15 min, the H NMR spectrum catalytic reactions, once the appropriate tagging chemistry
showed full consumption of 6 and generation of the proto- is developed.
demetalated lactone product 9. This hydrolysis reaction
Ensemble fluorescence measurements suggested the via-
demonstrated that adventitious water in the solvent was bility of the color-change-monitoring strategy to detect one
adequate to hydrolyze the oxocarbenium ion intermediate 7 specific step in the multistep catalytic reaction that forms 9.
and protodeaurate the gold–carbon bond[18] to generate the First, the emission spectrum of probe molecule 6 was ac-
demetalated lactone 9. Overall, the cyclization/hydrolysis of quired before the reaction (Scheme 2, left). Secondly, the
BODIPY-tagged alkynyl esters demonstrated that the emission spectrum of the product mixture containing 9 was
tagged reagents participated in analogous gold-catalyzed re- acquired after the reaction (Scheme 2, right), which re-
Scheme 2. Application of FRET probe 6 to the ensemble study of gold-catalyzed cyclization/hydrolysis reaction. The fluorescence emission
spectra of 6 before the gold-catalyzed cyclization/hydrolysis and after the formation of isocoumarin (9) show a fluorescence change that
indicates one chemical step (hydrolysis) in the multistep reaction.
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BODIPY Fluorophore Toolkit
Scheme 3. Synthesis of 11 bearing an electron-rich styrenyl moiety.
vealed a shift in the fluorescence that confirmed that
Next, the compatibility of olefin-containing BODIPY
alcohol 8 was liberated and that the acceptor fluorophore compounds with ruthenium carbenes was leveraged
was no longer attached to the substrate, thus permitting (Scheme 4). Specifically, the functionalization of a terminal
emission of the donor molecule [reaction (1) in Scheme 2]. olefin with butadiene monoepoxide was achieved by the
Thus, the change in color from green to orange indicated cross-metathesis reaction between BODIPY 12 and epoxide
that a specific single reaction step, alcohol release upon hy- 13 catalyzed by Grubbs’ second-generation catalyst, yield-
drolysis, had occurred.
ing BODIPY-tagged epoxide 14. The fluorescence exci-
tation and emission spectra of compound 14 were similarly
blueshifted relative to the parent tetramethyl-BODIPY, with
λex = 495 nm and λem = 505 nm.
As can be discerned from the fluorescence spectrum of
the product mixture of 8 and 9 at an excitation of 488 nm,
orange 8 displays reduced but nonzero fluorescence, which
indicates overlap between the excitation spectra of the do-
nor and acceptor. Excitation at 488 nm was chosen to
mimic the 488 nm argon ion laser line commonly available
for microscopy studies. The excitation spectrum overlap is
due to the small Stokes shift of BODIPY dyes and reveals
an inherent disadvantage of employing these fluorophores
as FRET pairs for chemical reaction probes.
BODIPY dyes bearing olefins recently have been em-
ployed as spectator probes to investigate ring-opening me-
tathesis polymerization at the single-particle and -molecule
level by fluorescence microscopy.[6,12] To broaden the appli-
cation of olefin-containing BODIPY dyes to studying other
types of polymerization processes and mechanisms, dyes
bearing pendent styrene moieties (e.g., see Scheme 3) would
be desirable given the significant representation of this sub-
stitution pattern in polymers.[20]
Scheme 4. Synthesis and photophysical properties of BODIPY
vinylepoxide 14.
At first, BODIPY dyes appeared compatible with the
mildly basic conditions of lithium acetate (Scheme 5). For
A potential complication in the synthesis of these com- example, this reagent smoothly transformed bromoalkyl
pounds would be the incompatibility of the highly electro- compound 15a to acetate compound 16a.[11] Subsequently,
philic reagents common to BODIPY synthesis with elec- the more strongly basic hydrolysis conditions of LiOH
tron-rich aromatic rings (Scheme 3). The synthesis of 11 transformed acetate compound 16a into alcohol 17a in high
proceeded satisfactorily (9% yield over four steps), however, yield and without apparent disruption of the BODIPY core
with the employment of thionyl chloride, POCl3, and BF3 or tether.
as stoichiometric reagents, and in the presence of reaction
Although the BODIPY core and tether appeared un-
byproduct amine-buffered HF. Compound 11 exhibited touched under these basic conditions, an attempt to synthe-
slightly blueshifted fluorescence excitation and emission size the shorter-chain alcohol 17b from chloropropyl-
spectra relative to the parent tetramethyl-BODIPY, with λex BODIPY 15b yielded only cyclopropyl-BODIPY 19. The
= 500 nm and λem = 511 nm (parent tetramethyl-BODIPY: generation of compound 19 is consistent with deproton-
λex = 505 nm and λem = 515 nm). The small blueshift estab- ation of the pseudo-benzylic α position (yielding intermedi-
lished that the tethered aromatic ring did not significantly ate 18), followed by SN2 displacement of the chloride. Thus,
alter the photophysical properties of the core. Thus, the the acidity of this α methylene group can be estimated on
electronic separation provided by the saturated ethylene the basis of its accessibility to deprotonation by acetate
tether provided sufficient separation of the reactive aro- (pKa of acetic acid in DMSO = 13).[21] The deprotonation
matic ring from the BODIPY core for the BODIPY to serve of the α methylene is also likely to be occurring reversibly
as a spectator in subsequent reactions.
for 15a under the conditions, however, ring closure is slower
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Scheme 5. Synthesis of bromoalkyl-BODIPY compounds and their conversion to the corresponding acetate and cyclopropyl under weakly
basic conditions.
for the 10-membered ring involved in the formation of 16a with weakly coordinating triflate anions are often produced
than for the three-membered ring.[22] The slower cyclization in situ from the corresponding chloride for employment as
reaction would then make this α-methylene deprotonation “cationic” gold catalysts.[19,23–26]
reversible and nonproductive.
Monitoring solutions of triflate 22 over the course of 6 h
Gold complexes, especially the formally cationic gold(I) by 1H and 31P NMR spectroscopy probed the chemical sta-
complexes, are an expanding class of catalysts in organic bility of the BODIPY core towards the electrophilic gold(I)
synthesis. Therefore the ability to synthesize and gain fun- center. Although mostly inert, the compound began to de-
damental knowledge of the photophysical properties of compose in solution after 2 h, as characterized by a small
fluorophore-tagged gold complexes would facilitate appli- decomposition peak in the 31P NMR spectrum that in-
cation to the study of catalysis by fluorescence microscopy. creased with time (Ͻ4% after 2 h and 8% after 6 h). Thus,
Ligand exchange between BODIPY-phosphine 20 and di- the BODIPY core was substantially inert to irreversible
methyl sulfide in the complex [Me2SAuCl] cleanly produced single-electron transfer to the electrophilic gold center[27]
BODIPY-tagged gold complex 21 (95%; Scheme 6). Coor- and gold Lewis acid promoted attack of nucleophiles on the
dination of 20 to gold resulted in minor shifts of the ab- core during the timescale of many gold-catalyzed reactions
sorbance and fluorescence maxima (Δλ = 2 and 6 nm, (including the reaction shown in Scheme 2, which was com-
respectively). The high quantum yield of the core was also plete in less than 15 min). Long-scale chemostability, how-
mostly retained upon coordination (Φ = 0.92 and 0.80 for ever, was lacking. Due to the instability of 22, this complex
20 and 21, respectively). The reduction in quantum yield was not isolated for further photophysical study.
suggests the possibility that partial quenching of the
For comparison, the excitation and emission spectra of
BODIPY core by gold(I) occurs despite the tetramethylene BODIPY-styrene 11, BODIPY-vinyl epoxide 14, and
tether and the full solubility of the complex in dichloro- BODIPY-gold(I) complex 21 are shown in Figure 1. The
methane; this photophysical effect contrasts the absence of modification of the meso position of the BODIPY core led
quenching in compound 20. To the best of our knowledge, only to a marginal change in the photophysical properties
this is the first BODIPY-tagged gold complex.
compared with the parent substrates and other similarly
Treatment of gold chloride 21 with AgOTf produced the tethered functional groups. López Arbeloa and co-workers
formally cationic gold(I) complex 22, as observed by 1H reported that an ester tethered through a single methylene
NMR spectroscopy (Ͼ95% conversion); gold complexes unit at the meso position changed the spectral properties
Scheme 6. Synthesis of 21 and 22, showing retention of the high quantum yield of the BODIPY core upon coordination to gold(I)
chloride and the stability towards salt metathesis conditions and a π Lewis acidic gold(I) cation.
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BODIPY Fluorophore Toolkit
and quantum yield significantly.[28] In contrast, the longer
methylene linker examined in this study left the photophysi-
cal properties largely intact.
Ͻ5 ppb was obtained from a Milli-Q Gradient A-10 water purifier
(Millipore, Billerica, MA) using a Q-Gard 2 purification pack and
a Quantum EX Ultrapure Organex cartridge. Fluorescence spectra
were recorded with an F-4500 fluorescence spectrofluorimeter
maintained at the facility operated by the Department of Chemis-
try at the University of California, Irvine. All solvents used for
single-molecule studies were of spectrophotometric grade.
BODIPY Donor 2: An oven-dried 100-mL round-bottomed flask
was charged with 6-heptynoic acid (1; 1.00 g, 7.92 mmol) and a
stirring bar, and then placed under nitrogen. Dry CH2Cl2 (20 mL)
and 2 drops of dry DMF were added to the flask. Thionyl chloride
(0.860 mL, 11.8 mmol) was added through a syringe causing a slow
evolution of gas. After the evolution of gas had subsided, the solu-
tion was stirred at room temperature for 2.5 h. It was then concen-
trated in vacuo to yield a yellow oil and then placed on a high-
vacuum line for 2 h to remove excess thionyl chloride. Without fur-
ther purification, the acid chloride was redissolved in dry CH2Cl2
(25 mL), and 2,4-dimethylpyrrole (2.00 mL/g, 19.8 mmol) and
phosphorus oxychloride (0.810 mL, 8.71 mmol) were added under
N2. The resulting solution was heated at reflux with stirring over-
night. The solution was then cooled to room temperature and con-
centrated in vacuo. To remove nonpolar impurities, the residue was
completely submerged in reagent-grade hexanes (90 mL) and stored
at –35 °C overnight. The hexanes then were decanted and the resi-
due was placed under high vacuum for 1 h and used in the next
step without further purification.
Figure 1. Fluorescence emission and excitation spectra of BODIPY
styrene 11 (λex = 500 nm) in CH2Cl2, BODIPY-vinyl epoxide 14
(λex = 495 nm) in CH3CN, and BODIPY-gold complex 21 (λex
=
500 nm) in CH2Cl2 (red: emission of 11, black: excitation of 11,
green: emission of 14, dark blue: excitation of 14, light blue: emis-
sion of 21, orange: excitation of 21).
Conclusions
The residue was dissolved in dry toluene (55 mL) under N2 and
treated with diisopropylethylamine (4.14 mL, 23.7 mmol). The
We have expanded the toolkit of spectator BODIPY
fluorophores tethered through unreactive methylene chains solution was stirred for 2 h at 80 °C. Boron trifluoride–dimethyl
etherate (2.55 mL, 27.7 mmol) was added and the mixture was
stirred at 80 °C for an additional 3.5 h. The red solution was cooled
to room temperature and transferred to a separatory funnel. Rea-
gent-grade toluene was swirled over the residue remaining in the
flask to extract residual product and added to the separatory fun-
nel. The solution was washed with brine, dried with sodium sulfate,
and concentrated in vacuo. The resulting residue was purified by
flash chromatography [CH2Cl2/hexanes (60:40), Rf = 0.3] to afford
2. The product was then recrystallized from reagent-grade toluene/
pentanes to yield 2 as dark-green/orange crystals (500 mg, 20%).
For recrystallization, the product was dissolved in reagent-grade
toluene (1 mL), layered with reagent-grade pentanes (4 mL), and
stored at –35 °C overnight. The product was spectroscopically iden-
tical to the compound reported in the literature:[29] λem = 498 nm.
(excitation at λ = 488 nm).
to reactive functional groups. The introduction of methyl-
ene chains prevented changes in the photophysical proper-
ties of the modified fluorophores. It was possible to reveal
a single step of a multistep metal-catalyzed reaction with
these probes. The reported findings provide fundamental
compatibility, reactivity, and photophysical information
that enable the broader application of fluorescent probes to
the study of chemical reactions by state-of-the-art fluores-
cence microscopy techniques.[3,4,6,9,11,12]
Experimental Section
General: Unless otherwise noted, all reagents and solvents were
used as received from their respective suppliers. Reactions were
monitored by TLC on precoated glass-backed plates (Merck F250)
and components were visualized by using UV light. Flash
chromatography was performed on Dynamic Absorbents 43–60 mi-
cron silica gel. tert-Butyl 6-hydroxyhexanoate was purchased from
Santa Cruz Biotechnology, 2-iodobenzioc acid and chloro(dimethyl
sulfide)gold were purchased from Sigma Aldrich, and [2-(diphenyl-
phosphanyl)ethyl]triethoxysilane was purchased from Gelest.
6-(tert-Butoxy)-6-oxohexyl 2-Iodobenzoate (3): In an oven-dried
100-mL round-bottomed flask, 2-iodobenzoic acid (1.2 g,
4.8 mmol), tert-butyl 6-hydroxyhexanoate (1.0 g, 5.3 mmol), and 4-
(dimethylamino)pyridine (0.060 g, 0.48 mmol) were dissolved in
dry dichloromethane (45 mL). The mixture was cooled in an ice
bath, dicyclohexylcarbodiimide (1.1 g, 5.5 mmol) was added, and
the mixture was stirred for 48 h with warming to room temperature
as the ice-bath melted. Dicyclohexenylurea (DCU) precipitated and
was removed by filtration. The filtered solution was concentrated
in vacuo to yield a yellow oil with some unremoved DCU as a
white powder. The residue was then dissolved in diethyl ether
(100 mL) and residual DCU was filtered off. Traces of DCU were
then removed on a short silica gel column (diethyl ether) to yield
1.98 g (99%) of 3 as a pale-yellow oil. The product was then used
1H and 13C NMR spectra were acquired with either a Bruker
DRX-500 or DRX-500 spectrometer equipped with a cryogenic
probe, and 31P NMR spectra were acquired with a Bruker DRX-
400 spectrometer. Chemical shifts (δ) are reported in parts per mil-
lion [ppm] and referenced to residual protiated solvent peaks as
reported in the literature. The spectral data are reported with the
following abbreviations: app, apparent; br., broad; m, multiplet; q,
quartet; s, singlet; t, triplet; td, triplet of doublets, sept, septet.
HRMS was performed at the facility operated by the Department
of Chemistry at the University of California, Irvine. Ultrapure
water with Ͼ18 MW resistance and a total organic content of
1
in the next step without further purification. H NMR (500 MHz,
CDCl3): δ = 7.96 (d, J = 7.9 Hz, 1 H), 7.78 (dd, J = 1.5, 7.7 Hz, 1
H), 7.38 (t, J = 7.4 Hz, 1 H), 7.13 (dt, J = 1.5, 7.7 Hz, 1 H), 4.32
(t, J = 6.6 Hz, 2 H), 2.23 (t, J = 7.4 Hz, 2 H), 1.81–1.75 (m, 2 H),
1.67–1.61 (m, 2 H), 1.50–1.45 (m, 2 H), 1.42 (s, 9 H) ppm.
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6-(2-iodobenzoyl)oxyhexanoic acid (4): In a glovebox, an oven-dried
25-mL round-bottomed flask was charged with 6-(tert-butoxy)-6-
oxohexyl 2-iodobenzoate (3) (0.50 g, 1.20 mmol) dissolved in dry
dichloromethane (15 mL). Trifluoroacetic acid (5.5 mL, 72 mmol)
was added and the resulting solution was stirred for 6 h. The reac-
tion mixture was removed from the glovebox, concentrated in
vacuo, and the residue was stripped with toluene (3ϫ 10 mL) to
afford a clear oil. The oil was placed under high vacuum overnight
and a white solid was formed. The product 4 (0.43 g, quantitative)
was then used in the next step without further purification. 1H
sion. The reaction could not be run in neat triethylamine due to
the insolubility of BODIPY donor 2 in triethylamine. The solution
was stirred at reflux under N2 overnight. The resulting mixture was
filtered through a glass frit and then added to a separatory funnel.
It was then washed with saturated NH4Cl (2ϫ 15 mL) and brine
(2ϫ 15 mL). The organic layer was dried with Na2SO4 and concen-
trated in vacuo. The crude product was purified by flash
chromatography with 10% ethyl acetate in hexanes (Rf = 0.3). The
resulting red oil was then further purified by recrystallization from
reagent-grade DCM/hexanes to afford 30 mg (19% containing less
NMR (500 MHz, CDCl3): δ = 11.5 (br. s, 1 H), 8.01 (d, J = 7.9 Hz, than 1 ppm hydrocarbon grease) of FRET pair 6. 1H NMR
1 H), 7.80 (dd, J = 1.5, 7.7 Hz, 1 H), 7.43 (t, J = 7.4 Hz, 1 H), 7.19
(dt, J = 1.5, 7.8 Hz, 1 H), 4.37 (t, J = 6.6 Hz, 2 H), 2.42 (t, J =
(500 MHz, CDCl3): δ = 7.85 (d, J = 7.8 Hz, 1 H), 7.49–7.47 (m, 1
H), 7.45–7.41 (m, 1 H), 7.36–7.32 (m, 1 H), 6.77 (s, 2 H), 6.04 (s,
7.4 Hz, 2 H), 1.87–1.81 (m, 2 H), 1.79–1.70 (m, 2 H), 1.59–1.48 (m, 2 H), 4.24 (t, J = 6.6 Hz, 2 H), 3.04–3.01 (m, 6 H), 2.75 (t, J =
2 H) ppm.
7.5 Hz, 2 H), 2.57–12.51 (m, 12 H), 2.42 (s, 6 H), 1.83–1.85 (m, 6
H), 1.78–1.73 (m, 6 H), 1.55–1.52 (m, 12 H) ppm. 13C NMR
(126 MHz, CDCl3): δ = 186.2, 156.5, 153.9, 146.0, 140.4, 134.3,
133.8, 131.9, 131.5, 131.4, 130.1, 128.6, 127.4, 124.2, 122.8, 121.6,
94.7, 64.7, 80.0, 64.8, 33.2, 31.6, 30.9, 30.1, 29.2, 28.4, 28.0, 26.4,
24.6, 23.2, 22.9, 22.4, 19.7, 16.4, 14.5 ppm. HRMS (ESI): calcd. for
[M + Na]+ 839.4282; found 839.4286. λem = 547 nm, FF = 0.94
(excitation at λ = 488 nm).
BODIPY Acceptor 5: Dry DMF (2 drops) and thionyl chloride
(0.120 mL, 1.65 mmol) were added through a syringe to a solution
of 4 (0.400 g, 1.10 mmol) in dry CH2Cl2 (15 mL) causing a slow
evolution of gas. After the evolution of gas had subsided, the solu-
tion was stirred at room temperature for 2 h. The solution was then
concentrated in vacuo to yield a pale-yellow oil and then put on a
high-vacuum line for 2 h to remove excess thionyl chloride. Without
further purification, the acid chloride was redissolved in dry
BODIPY-Styrene 11: Dry DMF (2 drops) and thionyl chloride
CH2Cl2 (6 mL) and tetrahydroindole (0.340 g, 2.8 mmol) in dry (0.019 mL, 0.26 mmol) were added through a syringe to a solution
CH2Cl2 (6 mL) was added under N2. Phosphorus oxychloride
(0.120 mL, 1.28 mmol) was added and the solution was heated to
40 °C and stirred for 2.5 h. The mixture was then concentrated in
vacuo to yield a red/green gum. To remove nonpolar impurities,
the residue was then covered with reagent-grade hexanes (90 mL)
and stored at –35 °C overnight. The mixture was decanted and the
residue placed on a high-vacuum line for 1 h to remove residual
hexanes. The remaining residue was dissolved in dry toluene
(10 mL) and dry triethylamine (0.500 mL, 3.58 mmol) was added.
The mixture was then heated to 80 °C and stirred for 10 min. Boron
trifluoride–dimethyl etherate (0.320 mL, 3.50 mmol) was added
and the solution stirred for an additional 1.5 h. The resulting dark-
red/pink solution was transferred to a separatory funnel and rea-
gent-grade toluene was swirled over the residue remaining in the
flask to extract residual product and added to the separatory fun-
nel. The solution in the separatory funnel was washed with brine
(3ϫ 50 mL) and the organic layer was dried with sodium sulfate,
filtered, and concentrated in vacuo to yield a dark-red residue. The
crude product was purified by flash chromatography [ethyl acetate/
hexanes (1:9), Rf = 0.4] and the product recrystallized from reagent-
grade DCM/hexanes to yield 5 as dark-green/orange crystals
(122 mg, 18%). For recrystallization, the product was dissolved in
reagent-grade DCM (1 mL), layered with reagent-grade hexanes
(4 mL), and stored at –35 °C overnight. The product was then used
of 10 (0.050 g, 0.21 mmol) in dry CH2Cl2 (5 mL) causing a slow
evolution of gas. After the evolution of gas had subsided, the solu-
tion was stirred at room temperature for 1.5 h. The solution was
concentrated in vacuo to yield a pale-yellow oil and then put on a
high-vacuum line to remove excess thionyl chloride. Without fur-
ther purification, the acid chloride was redissolved in dry CH2Cl2
(5 mL) and 2,4-dimethylpyrrole (0.057 mL, 0.53 mmol) was added
under N2 to give a yellow solution. Phosphorus oxychloride
(0.022 mL, 0.23 mmol) was added and the solution was heated to
50 °C and stirred for 12 h. After cooling to room temperature, the
mixture was concentrated in vacuo. To remove nonpolar impurities,
the residue was then covered with reagent-grade hexanes. The mix-
ture was decanted and the thick red oil was placed on a high-vac-
uum line, redissolved in dry toluene (5 mL) and heated to 80 °C.
Dry triethylamine (0.095 mL, 0.682 mmol) was added and the mix-
ture was stirred at 80 °C for 1 h. Boron trifluoride–dimethyl
etherate (0.059 mL, 0.639 mmol) was added and the solution
stirred for an additional 2 h. The resulting red solution was cooled
to room temperature and washed with brine. The organic layer was
dried with sodium sulfate, filtered, and concentrated in vacuo to
yield a dark-red/black oil. The crude product was purified by flash
chromatography [ethyl acetate/hexanes (1:10)]. The product was
recrystallized from reagent-grade DCM/hexanes to yield 11 as
dark-orange crystals (8 mg, 9%). For recrystallization, the product
was dissolved in reagent-grade DCM, layered with reagent-grade
hexanes, and stored at –35 °C overnight. 1H NMR (500 MHz,
CD2Cl2): δ = 7.36 (d, J = 2.1 Hz, 1 H), 7.12 (dd, J = 2.25, 8.4 Hz,
1
in the next step without further purification. H NMR (500 MHz,
CDCl3): δ = 8.05 (d, J = 7.9 Hz, 1 H), 7.78 (dd, J = 1.5, 7.3 Hz, 1
H), 7.46 (t, J = 7.4 Hz, 1 H), 7.72 (dt, J = 1.5, 7.7 Hz, 4 H), 6.78
(s, 2 H), 4.40–4.37 (m, 2 H), 3.0 (t, J = 6.1 Hz, 4 H), 2.82 (t, J = 1 H), 7.03 (dd, J = 11.2, 17.8 Hz, 1 H), 6.86 (d, J = 8.4 Hz, 1 H),
7.7 Hz, 2 H), 2.60 (t, J = 6.2 Hz, 4 H), 1.84–1.73 (m, 9 H), 1.59– 6.08 (s, 2 H), 5.71 (dd, J = 1.4, 17.8 Hz, 1 H), 5.23 (dd, J = 1.4,
1.54 (m, 2 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 186.8, 156.5, 11.2 Hz, 1 H), 4.53 (sept, J = 6.1 Hz, 1 H), 3.32–3.29 (m, 2 H),
143.1, 141.2, 1345.6, 133.8, 132.6, 130.8, 128.6, 127.9, 122.9, 93.9,
65.4, 33.0, 30.1, 28.4, 26.5, 24.6, 23.2, 22.9, 22.4 ppm. HRMS
2.93–2.89 (m, 2 H), 2.50 (s, 12 H), 1.34 (d, J = 6.1 Hz, 2 H) ppm.
Observable 13C NMR (126 MHz, CD2Cl2): δ = 154.3, 132.5, 132.3,
128.5, 128.2, 126.3, 122.0, 114.9, 114.3, 71.4, 37.0, 30.1, 22.3, 16.9,
14.6 ppm. HRMS (ESI): calcd. for [M + Na]+ 495.2400; found
495.2385. λex = 500 nm, λem = 511 nm.
(ESI): calcd. for [M + Na]+ 639.1473; found 639.1458. λem
540 nm (excitation at λ = 488 nm).
=
FRET Pair 6: A 50-mL round-bottomed flask was charged with of
[Pd(PPh3)4] (0.009 g, 0.008 mmol), CuI (0.003 g, 0.02 mmol), and
dry triethylamine (8 mL). A solution of BODIPY acceptor 5
(0.120 g, 0.194 mmol) and BODIPY donor 2 (0.076 g, 0.23 mmol)
dissolved in dry DCM (4 mL) was added to the resulting suspen-
BODIPY-Vinyl Epoxide 14: Butadiene oxide (13; 0.230 g,
3.28 mmol) and Grubbs II catalyst (0.054 g, 0.066 mmol) were
added to a solution of 12 (0.200 g, 0.661 mmol) in C6H6 (25 mL).
The resulting solution was stirred at room temperature for 20 h.
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BODIPY Fluorophore Toolkit
The reaction was then quenched with vinyl ethyl ether (1.5 mL).
The crude product was purified by flash chromatography [CH2Cl2/
hexanes (60:40), Rf = 0.4] to yield the title product 14 (15%). H
tion mixture was stirred at room temperature for 1 h. The solvent
was removed under reduced pressure and the orange solid obtained
was dried under high vacuum to yield 0.125 g (95%) of 21. 1H
1
NMR (500 MHz, CDCl3): δ = 6.10–6.04 (m, 3 H), 5.31 (dd, J = NMR (500 MHz, CD2Cl2): δ = 7.70–7.64 (m, 4 H), 7.57–7.49 (m,
7.7, 15.5 Hz, 1 H), 3.38–3.35 (m, 1 H), 3.09–3.06 (m, 2 H), 3.00–
2.98 (m, 1 H), 2.69–2.67 (m, 1 H), 2.52 (s, 6 H), 2.45–2.39 (m, 8
6 H), 7.35–7.33 (m, 3 H), 6.08–6.06 (m, 2 H), 2.97–2.94 (m, 2 H),
2.52–2.50 (m, 2 H), 2.48–2.44 (m, 6 H), 2.35–2.34 (m, 6 H), 1.79–
H) ppm. Observable 13C NMR (500 MHz, CDCl3): δ = 134.1, 1.78 (m, 4 H) ppm. 13C NMR (500 MHz, CD2Cl2): δ = 154.3,
129.1, 121.8, 52.1, 48.9, 34.0, 27.2, 16.5, 14.5 ppm. HRMS (ESI): 145.8, 141.0, 133.6, 133.5, 132.5, 132.4, 131.6, 129.8, 129.7, 129.2,
calcd. for [M + Na]+ 367.1773; found 367.1768. λex = 495 nm, λem
= 505 nm.
122.0, 33.1, 32.9, 28.3, 28.0, 26.7, 26.6, 16.8, 14.7 ppm. 31P NMR
(400 MHz, CD2Cl2): δ = 29.5 ppm. HRMS (ESI): calcd. for [M +
Na]+ 743.1621; found 743.1592. λex = 500 nm, λem = 510 nm, Φ =
0.80.
Chloropropyl-BODIPY 15b: Dry DMF (50 μL) and thionyl
chloride (0.66 mL, 9.1 mmol) were added to a solution of 4-bromo-
butyric acid (1.23 g, 7.40 mmol) in dry CH2Cl2 (40 mL) and the
solution was stirred at room temperature for 1 h. It was then con-
centrated in vacuo to yield a pale-yellow oil, which was redissolved
in toluene (few drops) and then subsequently put on a high-vacuum
line to remove the solvent and excess thionyl chloride. Without fur-
ther purification, the acid chloride was redissolved in dry CH2Cl2
(50 mL) and 2,4-dimethylpyrrole (1.94 mL, 18.4 mmol) was added
under N2 to give a yellow solution. Phosphorus oxychloride
(0.78 mL, 8.3 mmol) was added and the solution heated to 50 °C
and stirred for 8 h. After cooling to room temperature, the mixture
was concentrated in vacuo. To remove nonpolar impurities, the resi-
due was then covered with reagent-grade hexanes. The mixture was
decanted and the thick red oil was placed on a high-vacuum line,
redissolved in dry toluene (50 mL), and heated to 80 °C. Dry trieth-
ylamine (1.6 mL, 11 mmol) was added and the mixture was stirred
at 80 °C for 1 h. Boron trifluoride–dimethyl etherate (1.0 mL,
11 mmol) was added and the solution was stirred for an additional
1 h. The resulting red solution was cooled to room temperature
and washed with brine. The organic layer was dried with sodium
sulfate, filtered, and concentrated in vacuo to yield a dark-red/
black oil. The crude product was purified by flash chromato-
BODIPY–Au–OTf Complex 22: AgOTf (0.013 g, 0.050 mmol,
1.6 equiv.) was added to a solution of 21 (0.022 g, 0.031 mmol) in
CD2Cl2 (0.6 mL) and the reaction mixture was stirred at room tem-
perature. After 1 h the precipitated Ag salts were filtered through
a Whatman® filter and the formation and stability of the AgOTf
complex 22 were monitored in situ by 1H and 31P NMR spec-
1
troscopy. H NMR (400 MHz, CD2Cl2): δ = 7.67–7.54 (m, 10 H),
6.09 (s, 2 H), 2.95 (s, 2 H), 2.76–2.28 (m, 14 H), 1.80 (s, 2 H) ppm.
31P NMR (400 MHz, CD2Cl2): δ = 25.5 ppm. HRMS (ESI): calcd.
for [M – SO3CF3]+ 685.2035; found 685.2013.
FRET Experiment – Gold-Catalyzed Isocoumarin Rearrangement:
A 20-mL scintillation vial was charged with [PPh3AuOTf] (2.2 mg,
0.0036 mmol), a solution of 6 (3.0 mg, 0.0036 mmol) dissolved in
CD2Cl2 (2 mL) was added, and the mixture stirred at room tem-
perature. After 3 h, MeOD (0.7 mL) and H2O (0.1 mL) were added
and the mixture was stirred at room temperature for an additional
15 min. The reaction progress was monitored by 1H NMR spec-
troscopy, which confirmed the formation of protodemetalated lact-
one product 9 after 15 min. The NMR spectra from this experiment
are included in the Supporting Information. The fluorescence of 6
and the product 9 was evaluated following the procedure described
below.
1
graphy [ethyl acetate/hexanes (1:4)] to yield 15b (0.55 g, 23%). H
NMR (500 MHz, CDCl3): δ = 6.07 (s, 2 H), 3.70 (t, J = 6.1 Hz, 2
H), 3.13–3.10 (m, 2 H), 2.52 (s, 6 H), 2.43 (s, 6 H), 2.10–2.04 (m,
2 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 154.4, 144.5, 140.4,
131.5, 121.9, 44.8, 34.0, 25.9, 16.62, 14.51 ppm. HRMS (ESI):
calcd. for [M + Na]+ 347.1277; found 347.1279. C16H20BClF2N2
(324.14): C 59.20, H 6.21, N 8.63; found C 59.12, H 6.36, N 8.58.
λex = 495 nm, λem = 502 nm, Φ = 0.84.
Fluorescence Spectrophotometer: Fluorescence spectra were re-
corded with a Hitachi F-4500 FL spectrophotometer. Solutions of
BODIPY donor 2, BODIPY acceptor 5, and BODIPY FRET pair
6 were prepared in spectrophotometric-grade DCM at concentra-
tions of 2ϫ10–6 m (intramolecular FRET is not significant at this
concentration).[2] The solutions were excited at 488 nm and the
emission spectra recorded. The samples were scanned from 300 to
700 nm at a scan speed of 240 nm/min.
Compounds 16a and 17a were synthesized as previously re-
ported.[11]
Supporting Information (see footnote on the first page of this arti-
1
cle): H, 13C, and 31P NMR spectra.
Cyclopropyl-BODIPY 19: Chloropropyl-BODIPY 15b (0.20 g,
0.54 mmol) and potassium acetate (0.21 g, 2.2 mmol) were dis-
solved in dry DMF (10 mL) and the solution was stirred at 50 °C
for 3 d. After cooling to room temperature, the reaction mixture
was diluted in EtOAc (40 mL) and washed with H2O (30 mL). The
organic phase was washed with 80% brine (3ϫ 50 mL) and also
satd. brine. The organic phase was dried with Na2SO4 and concen-
trated under reduced pressure. 1H NMR (500 MHz, CDCl3): δ =
6.07 (s, 2 H), 2.52 (s, 6 H), 2.48 (s, 2 H), 1.94–1.91 (m, 1 H), 1.29–
1.25 (m, 2 H), 0.84–0.81 (m, 2 H) ppm. 13C NMR (126 MHz,
CDCl3): δ = 154.0, 147.0, 141.4, 134.8, 121.2, 29.8, 16.6, 14.6, 13.1,
12.1 ppm.
Acknowledgments
The authors thank the U. S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-08ER15994) for funding. Mr. Trevor P.
Cornell and Dr. Neeladri Das are thanked for the synthesis of 21
and 19, respectively.
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E. M. Hensle, N. M. Esfandiari, S.-G. Lim, S. A. Blum
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Received: January 13, 2014
Published Online: April 28, 2014
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Eur. J. Org. Chem. 2014, 3347–3354