3- and 5-Functionalized BODIPYs via the Liebeskind-Srogl reaction

Article abstract of DOI:10.1039/b818390b

Chemoselective cross-coupling reactions were demonstrated for C-S bonds in the BODIPY dyes 1 and 4, and similar reactions were applied to make the two-dye cassette system 11.

Full text of DOI:10.1039/b818390b

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www.rsc.org/obc | Organic & Biomolecular Chemistry
3- and 5-Functionalized BODIPYs via the Liebeskind-Srogl reaction†
Junyan Han,a Oswaldo Gonzalez,b Angelica Aguilar-Aguilar,b Eduardo Pen˜a-Cabrerab and Kevin Burgess*a
Received 17th October 2008, Accepted 21st October 2008
First published as an Advance Article on the web 30th October 2008
DOI: 10.1039/b818390b
Table 1 Photophysical properties of compounds 1–3 in EtOAc
Chemoselective cross-coupling reactions were demonstrated
for C–S bonds in the BODIPY dyes 1 and 4, and sim-
ilar reactions were applied to make the two-dye cassette
system 11.
labs max
lfl max
(nm)
fwhm
(nm) a
b
compd (nm)
emax (L mol-1 cm-1)
Uf
1
577
571
555
559
581
46700
50600
52400
44000
51900
595
597
588
596
621
47
59
49
48
41
0.40 0.04
0.58 0.01
0.14 0.01
0.36 0.03
0.38 0.01
2b
3a
3b
3c
Organometallic cross coupling reactions are useful for extending
the conjugation of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, or
R
BODIPYꢀ (here abbreviated BODIPY) dyes to give probes that
fluoresce at longer wavelengths. Usually this is achieved via
inherently basic processes (Suzuki1 and Sonogashira2) that tend
to cause partial decomposition of BODIPY dyes. Furthermore,
chemoselective reactions involving different halides in the same
molecule are sometimes desirable, but hard to achieve via these
transformations. For instance, the C–Cl sites of systems like A3 are
reactive for Sonogashira and Suzuki reactions,4 but competitive
reactions of the aryl bromide site would be expected. This
paper describes how Liebeskind-Srogl reactions5–7 can be used to
achieve chemoselective couplings to BODIPY dyes under neutral
conditions; further, this transformation was used to give a water-
souble two dye cassette system.
a Full width at half maximum height of fluorescence (fwhm). b Rhodamine
101 (U = 1.0 in ethanol) as standard.11
It has been demonstrated that Liebeskind-Srogl couplings can
be used to substitute thioalkyl groups at the C8 position of the
BODIPY core.8 Chemoselective Liebeskind-Srogl couplings at
thioalkyl groups over aryl bromides have also been demonstrated.9
Consequently we felt that selective substitution at the 3,5-positions
as in compound 1 would be possible. In actuality, phenyltin (a)
and electron-rich aryl-tin (c) reagents cleanly gave the disubsituted
products 3, some monosubstituted intermediate was isolated for
the electron-deficient aryl group tested (b), but in no case was the
aryl-bromide affected (Scheme 1).
Fig. 1 shows normalized absorbance (a) and fluorescence
(b) spectra for compounds 1–3. The newly installed aryl groups
shift both maxima to the red, as previously described for 3,5-
diaryl-BODIPY dyes.10 More extensive spectroscopic data for
these products is shown in Table 1. Both para-electron withdrawing
(CO2Me) and donating (OMe) groups shifted the maximum
fluorescence wavelength to red (8 nm and 33 nm respectively,
compare 3a, 3b and 3c).
Scheme 1 Synthesis of 3- and 5-functionalized BODIPY via Liebe-
skind-Srogl reaction. CuMeSal = copper(I) 3-methylsalicylate.
meso-(4-Bromoaryl)BODIPY substituents like those in com-
pounds 2 and 3 can be further elaborated via Sonogashira
reactions. However, recent unpublished data from our labora-
tories have shown that there are advantages to using copper-
mediated alkyne-azide coupling reactions12,13 to join two BODIPY
fragments together to form two-dye cassette systems. Azido-
functionalized dyes are required to achieve this. Thus the sequence
outlined in Scheme 2a was performed to generate the azidodicar-
boxylic acid system 9. Coupling of the dithioether 4, which is syn-
thesized from B,3 to the appropriate 4 eq stannane gave 17% of the
corresponding monosubstituted material 5, and 66% the desired
disubstituted product 6 (see supporting information). However,
using 4 eq 4-methoxycarbonylphenylboronic acid afforded 6 in
quantitative yield. We think the boronic acid is much more stable
than the tin compound under the reaction conditions. A mild ester
hydrolysis14 of this material, and reduction of the azide gave the
amine 8 which was then converted to the azide 9.
aDepartment of Chemistry, Texas A & M University, P. O. Box 30012, College
Station, TX 77842, USA. E-mail: burgess@tamu.edu; Fax: +1 (979) 845
8839; Tel: +1 (979) 845 4345
bDepartamento de Quimica, Universidad de Guanajuato, Col. Noria Alta
S/N. Guanajuato, Gto. 36050, Mexico. E-mail: eduardop@quijote.ugto.mx
† Electronic supplementary information (ESI) available: Details of synthe-
sis of compounds 1–12. See DOI: 10.1039/b818390b
34 | Org. Biomol. Chem., 2009, 7, 34–36
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Fig. 1 (a) Normalized absorbance spectra of compounds 1–3 (1 ¥ 10-6 M)
in EtOAc. (b) Normalized fluorescence spectra of compounds 1–3
(1 ¥ 10-7 M) in EtOAc with excitation at 540 nm.
The sulfonated BODIPY alkyne 10 was formed from the known
starting material C15 via sulfonation conditions recently reported
by us (Scheme 2b).3 Finally, the appropriately functionalized
BODIPYs, azide 9 and alkyne 10, were joined via the click reaction
shown in Scheme 2c. The final product 11 is highly water soluble.
It was isolated via preparative reverse phase HPLC.
Cassette 11 is designed to function as a “through bond energy
transfer” (TBET) system. These feature two dye components
that are prevented from becoming completely planar because of
steric issues. Thus the UV-visible absorption spectrum of 11 and
BSA-11 has two maxima corresponding to the donor fragment
and the acceptor part (Fig. 2a).
Motivation for making TBET cassettes is derived from the fact
that they can be excited at a much shorter wavelength (donor
absorbance) than their fluorescence emission (from acceptor part).
This leads to enhanced spectral resolution of emission peaks if
several dyes are used together. However, three major obstacles
have emerged from our research efforts: (i) making the cassettes
in water soluble form; (ii) obtaining good energy transfer from
the donor to the acceptor in an aqueous medium (cassettes that
work well in less polar solvents can be poor in water); and,
(iii) maintaining good energy transfer when the cassette is
conjugated to a protein. Cassette 11 is quite water soluble, so
that parameter is satisfied. To quantitate the function of the
cassettes we measure their overall quantum yields for the acceptor
fluorescence emission when excited at the donor. We also use a
parameter called the energy transfer efficiency (ETE%) which is
defined as ETE% = (quantum yield of the acceptor fragment in
Scheme 2 Synthesis of energy transfer cassette 11.
the cassette excited at the donor)/(quantum yield of the acceptor
fragment in the cassette excited at the acceptor) ¥ 100.
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In summary, the work described here indicates that Liebeskind-
Srogl couplings provide another dimension for chemoselectivity
in construction of BODIPY dye derivatives.
Acknowledgements
We thank Mr LiangXing Wu for useful comments, Ms Lingling
Li and Mr Cliferson Thivierge for providing material A & B, and
10 respectively, the National Institutes of Health (GM72041) and
The Robert A. Welch Foundation for financial support, and the
TAMU/LBMS-Applications Laboratory directed by Dr Shane
Tichy for assistance with mass spectrometry. E. P-C wishes to
thank CONCYTEG-FOMIX 07 for support.
Notes and references
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Fig. 2 (a) Normalized absorption of 11 and BSA-11 in pH = 7.4 PBS
buffer. (b) Normalized fluorescence spectra of 11 and BSA-11 in pH =
7.4 PBS buffer.
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The overall quantum yield of 11, and its ETE% for 11 in pH 7.4
phosphate buffer were determined as 0.23 and 85%. Qualitatively,
the undesirable “leakage” of fluorescence from the donor is seen
at about 520 nm in Fig. 2b.
Cassette 11 was activated by forming an ester of N-
hydroxysuccinimide (see supporting information) and conjugated
to bovine serum albumin (BSA). This caused the absorbance
maxima to be red-shifted by about 8 mm, but had little impact
on the fluorescence maxima (Fig. 2). The overall quantum yield of
the BSA-11 conjugate, and its ETE% for pH 7.4 phosphate buffer
were determined as 0.10 and 75%.
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36 | Org. Biomol. Chem., 2009, 7, 34–36
This journal is
The Royal Society of Chemistry 2009
©