Carbon-carbon bond cleavage in fluorescent pyronin analogues induced by yellow light

Article abstract of DOI:10.1021/ol302244f

A novel class of pyronin analogues, which undergoes a photochemically induced cleavage of the C-C bond in the presence of water in both solution and on a silica gel surface upon direct irradiation with visible light, is reported. The reaction course can be monitored by characteristic fluorescence of both the starting compound and the final product. This system could find useful applications in the field of photoremovable protecting groups or caged fluorophores.

Full text of DOI:10.1021/ol302244f

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
CarbonꢀCarbon Bond Cleavage in
Fluorescent Pyronin Analogues Induced
by Yellow Light
†,‡,§
ꢀ
Peter Stacko,
†,‡,§
†
,†,‡
ꢀ
Peter Sebej,
ꢁ
Aneesh Tazhe Veetil, and Petr Klan*
Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8,
625 00, Brno, Czech Republic, and Research Centre for Toxic Compounds in the
Environment, Faculty of Science, Masaryk University, Kamenice 126/3, 625 00 Brno,
Czech Republic
klan@sci.muni.cz
Received August 13, 2012
ABSTRACT
A novel class of pyronin analogues, which undergoes a photochemically induced cleavage of the CꢀC bond in the presence of water in both
solution and on a silica gel surface upon direct irradiation with visible light, is reported. The reaction course can be monitored by characteristic
fluorescence of both the starting compound and the final product. This system could find useful applications in the field of photoremovable
protecting groups or caged fluorophores.
Carbonꢀcarbon bond cleavage is among the most im-
portant chemical transformations in both the synthetic
laboratory and nature. Such reactions are usually con-
ducted in the presence of a catalyst,1 but they can also be
promoted by UV light.2 In general, photosensitizers,
photocatalysts, or photoinitiators are required for visible
light-promoted reactions.3 Photochemical processes often
involve electron or energy transfer in the primary step. For
example, Kutateladze and co-workers have developed a
strategy of aldehyde and ketone protection using the 1,3-
dithiane moiety, which is efficiently removed by oxidative
electron transfer in the presence of a sensitizer, such as
benzophenone, upon irradiation with UV light.4 Visible-
light initiated CꢀC bond scission reactions are rare.5
Rhodamines, pyronins, and xanthenones, well-known
dyes with strong absorption and fluorescence in the visible
region, have found wide application in molecular biology
and medicine.6 Wirz and co-workers have recently in-
vestigated the photochemistry of fluorescein analogues,
6-hydroxy-3-oxo-3H-xanthen-9-yl)methyl derivatives, which
release diethyl phosphate or carboxylate via CꢀO bond
scission upon irradiation at over 500 nm.7 Here, we report
on analogous fluorescent pyronin-based derivatives that
undergo CꢀC bond cleavage upon irradiation with visible
light in the absence of any photosensitizer.
† Department of Chemistry.
‡ Research Centre for Toxic Compounds in the Environment.
§ These authors contributed equally.
(1) Ruhland, K. Eur. J. Org. Chem. 2012, 2683–2706.
(2) Klan, P.; Wirz, J. Photochemistry of organic compounds: From
concepts to practice; John Wiley & Sons Ltd.: Chichester, 2009;
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A. N.; Kottani, R.; Gustafson, T. P.; Kutateladze, A. G. J. Am. Chem. Soc.
2005, 127, 12458–12459. (c) McHale, W. A.; Kutateladze, A. G. J. Org.
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ꢀ
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(7) Sebej, P.; Wintner, J.; Muller, P.; Slanina, T.; Al Anshori, J.;
Lovely Angel, P. A.; Klan, P.; Wirz, J. J. Org. Chem., DOI: 10.1021/
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jo301455n.
r
10.1021/ol302244f
XXXX American Chemical Society

The 1,3-dithian-2-yl derivatives of a 6-amino-3H-
xanthen-3-iminium moiety (pyronin analogues) 1a and
1b were synthesized from xanthen-9-one (2), prepared by
cyclizative condensation of 2,20,4,40-tetrahydroxybenzo-
phenone and its subsequent derivatization8 (Supporting
Information), and 1,3-dithiane (3a) or 5-phenyl-1,3-dithiane
(3b), respectively, according to Scheme 1. The synthetic inter-
mediate 3b was prepared by reduction of diethyl 2-phenyl
malonate (4) to give the diol 5 and successive steps in 55%
overall yield (Scheme 2). The model compound 1c was
synthesized according to a known procedure.9 The xanthe-
none 2 was treated with a 5-fold excess of 1,3-dithiane-2-yl
anion generated in situ from equimolar amounts of 3 and
n-BuLi (an excess of n-BuLi causes its addition to 1) at 0 °C.
The subsequent elimination of water upon addition of an acid
produced the target dark-purple products in good yields
(55ꢀ64%). In the dark, 1a and b are stable in the solid state
for months but show signs of decomposition in a methanolic
solution after several weeks.
101 and Rhodamine B6a or their analogues.10 The fluo-
rescence quantum yields were determined to be Φfl
=
(14.7 ( 0.1) and (16.1 ( 0.1)% for 1a and 1b, respectively,
in methanol. The absorbance of both 1a and mixed 1c/1,3-
dithiane solutions was found to be proportional to the
concentration (Beer’s law) over 3 orders of magnitude
(Figures S22 and S23, Supporting Information), which
ruled out conceivable intermolecular charge transfer inter-
actions of 1 in the ground state.
Scheme 1
Figure 1. Absorption and uncorrected emission spectra of 1a in
methanol (c = 10ꢀ5 M).
Irradiation of 1a and b (c ≈ 2 ꢁ 10ꢀ5 M) in methanol at
either 546 or 590 nm led to their decomposition. Figure 2
shows a decrease of the absorption band intensity of the
parent, red-fluorescing compound 1a (λmax = 584 nm, red
line; right cuvette) and the simultaneous appearance of a
new band at λmax = 499 nm (green line) during the course
of irradiation. This intermediate was then thermally (in the
dark; although a coincident photochemical pathway is not
ruled out) converted to a stable product (λmax = 382 nm,
blue line). The addition of water slowed down the photo-
reaction but accelerated the second process. The resulting
samples (>95% conversion) exhibited a strong blue fluo-
rescence (Figure 2, left cuvette), subsequently attributed to
the xanthenone 2. This compound was identified as an
exclusive chromophoric product by both NMR (Figure
S21) and LC-HRMS (the spectroscopic data were com-
pared to those of the authentic compound).
Scheme 2
The HRMS analysis of the irradiated degassed solutions
þ
of 1b identified a side product with the formula C10H11S2
,
whereas secondary, evidently oxidation products with
formulas C10H10OS2þ and C9H10OS2þ were found in the
nondegassedsamples. None of the spectra corresponded to
that of 3b. The observed species can be daughter ions of
(photo)products formed during the course of irradiation,
such as adducts with methanol or water. Figures S17ꢀ19
offer the proposedstructures, whichare analogous tothose
Compounds 1a and b exhibit strong absorption
;
water: λmax (abs) = 588 nm) and emission (methanol: λmax
(em) = 608 nm) bands (Figure 1), which are slightly
bathochromically shifted compared to those of Rhodamine
(methanol: λmax = 584 nm; εmax ≈ 6 ꢁ 104 Mꢀ1 cmꢀ1
(8) Wu, L.; Burgess, K. Org. Lett. 2008, 10, 1779–1782.
(9) Wu, L. X.; Burgess, K. J. Org. Chem. 2008, 73, 8711–8718.
(10) Ahn, Y.-H.; Lee, J.-S.; Chang, Y.-T. J. Am. Chem. Soc. 2007,
129, 4510–4511.
B
Org. Lett., Vol. XX, No. XX, XXXX

obtained by anodic oxidation of 1,3-dithiane derivatives.11
A concentration-dependent quantum yield12 could be an
indication of a photoinitiated bimolecular process. In this
work, the quantum yield of disappearance of 1b in metha-
nol, determined by ferrioxalate actinometry, was found to
be approximately the same ((0.029 ( 0.019)%) in the
concentration range of 5 ꢁ 10ꢀ7 to 2 ꢁ 10ꢀ4 M. Never-
theless, we still cannot fully disregard the contribution of a
self-photoinitiated bimolecular mechanism based on this
experiment. Addition of an excess of 1,3-dithiane did not
affect the quantum yield.
1
In addition, multiple H NMR signals attributed to the
phenyl ring in an exhaustively irradiated 1b solution
(Figure S21) demonstrate that the mixture of side products
derived from the phenyl-1,3-dithiane moiety is complex
and cannot be easily analyzed.
In order to identify a transient species observed by
absorption spectroscopy (Figure 2, green line), the metha-
nolic solution of 1b was irradiated using a 500 W halogen
lamp until more than 90% of the starting material was
consumed. The solvent was immediately removed under
reduced pressure at 20 °C to prevent photoproduct decom-
position. 1H, 13C, and 2D NMR analyses in conjunction
with the HRMS data (Figure S18) suggested that the
thermally unstable intermediate is the methoxy derivative
6 (Scheme 3). In addition, irradiation of 1b in CD3OD
provided the corresponding CD3-labeled compound 6
(Φ = 0.03%), and a subsequent addition of an excess of
D218O or benzylthiol resulted in thermal (dark) substitu-
tion of the methoxy group and formation of the adducts 2
or 7, respectively. Irradiation of 1b in CH3OH/D218O (4:1,
v/v) gave 2 with the isotopically labeled carbonyl group
(Figure S20). The carbonyl oxygen thus unambiguously
originatesfrom water present in a solvent. Thefactthatdry
acetonitrile had a detrimental effect on the photolysis rate
confirms that water is indispensable for the formation of 2.
Although irradiation of 1 in pure water also provided 2 as
an exclusive final product, the reaction was slower than
that carried out in solvent containing only small amounts
of water (Table S1).
Scheme 3. Formation of the Products from 1b
Electron transfer could be an initial step in the photo-
transformation of the excited pyronin chromophore. The
dithiane oxidation is known to take place in the presence of
a triplet sensitizer, such as benzophenone, resulting in the
mesolytic CꢀC bond cleavage.4c The triplet state of rho-
damine dyes can be photochemically reduced by electron
donors, such as thiols13 or amines.14 Its formation by
intersystem crossing is known to be inefficient (<1%),15
which could be related to our observation of a low
disappearance quantum yield of 1b and also to the fact
that the photolysis efficiency was unaffected by deoxy-
genation in the concentration range 10ꢀ6 to 2 ꢁ 10ꢀ4
M
(e.g., Table S1).
We also estimated that the driving force for the one-
electron reduction potential of pyronin Y (ꢀ0.81 V vs
SCE16), its singlet energy ES = 51.4 kcal molꢀ1 17 and the
,
oxidation potential of 1,3-dithiane (þ1.05 V vs SCE;18 Eq
S1, Supporting Information) is exergonic. However, due
to the relatively short singlet-state lifetime of rhodamine
(12) Megerle, U.; Wenninger, M.; Kutta, R. J.; Lechner, R.; Konig,
B.; Dick, B.; Riedle, E. Phys. Chem. Chem. Phys. 2011, 13, 8869–8880.
(13) van de Linde, S.; Krstic, I.; Prisner, T.; Doose, S.; Heilemann,
M.; Sauer, M. Photochem. Photobiol. Sci. 2011, 10, 499–506.
(14) Chibisov, A. K.; Slavnova, T. D. J. Photochem. 1978, 8, 285–297.
(15) Lessing, H. E.; Richardt, D.; Vonjena, A. J. Mol. Struct. 1982,
84, 281–292.
Figure 2. Absorption spectra measured following irradiation at
590 nm of a 1a solution in methanol (cinit ≈ 2 ꢁ 10ꢀ5 M; one
spectrum taken every 30 min). Inset: Blue fluorescence of the
irradiated mixture (left cuvette) compared to red fluorescence of
1a (right cuvette; excited by 366 nm light).
(16) Gould, I. R.; Shukla, D.; Giesen, D.; Farid, S. Helv. Chim. Acta
2001, 84, 2796–2812.
(17) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006.
(18) Evans, D. H.; Gruhn, N. E.; Jin, J.; Li, B.; Lorance, E.;
Okumura, N.; Macias-Ruvalcaba, N. A.; Zakai, U. I.; Zhang, S. Z.;
Block, E.; Glass, R. S. J. Org. Chem. 2010, 75, 1997–2009.
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Org. Lett., Vol. XX, No. XX, XXXX
C

dyes (units of ns19), only intramolecular electron transfer
from the sulfur atom to the excited pyronin moiety would
be feasible at such low concentrations (fluorescence of
Rhodamine 6G in methanol is quenched only at very high
concentrations (>10ꢀ2 M20)).
Photoinitiated formation of the products 2 and 6 from
1b indicates that nucleophilic attack at the pyronin
9-position is one of the key processes which either precedes
or follows the mesolytic CꢀC bond cleavage (subsequent
nucleophilic substitution at this position, such as in 6f2 or
6f7, is possibly a dark process). It has been shown that a
considerable amount of Rhodamine B exists in its lactone
form even in polar alcohols.21 Therefore, more than one
pyronin form can exist in nucleophilic solvents, and their
participation on the reaction should not be neglected. The
reaction mechanism of this reaction is currently under
investigation in our laboratory.
Figure 3. 1a adsorbed on a chromatographic silica gel plate
visualized by 366 nm light. The hν symbol was created by a green
laser light.
Due to its polar nature, 1a was adsorbed on silica gel
from its methanolic solutions. We tested whether it can be
photochemically converted into 2 after the solvent is
removed by evaporation. Thus 1a was deposited on a
nonfluorescent chromatographic silica gel plate, and green
light from a portable laser was used to create a macro-
scopic pattern (Figure 3). The blue fluorescence of 2 was
conveniently detected instead of the original red fluores-
cence of 1a at the site of irradiation. 2 was then selectively
eluted from the surface by acetonitrile resulting in a blue
fluorescent solution. Formation of 2 suggests that the
residual amount of water on a silica gel surface was
sufficient to enable the reaction.
In conclusion, we report on a novel class of pyronin
analogues 1a, b, which undergo photochemically induced
cleavage of the CꢀC bond, probably via intramolecular
electron transfer, in the presence of water in both solution
and on a silica gel surface upon direct irradiation with visible
light. The final chromophoric photoproduct is a stable
compound absorbing below 430 nm. The course of the
reaction can be monitored by characteristic fluorescence of
both the starting compound and the final product. As one of
the rare examples of a visible-light triggered system, it could
find useful applications in the field of photoremovable
protecting groups22 or caged fluorophores.
Acknowledgment. Support for this work was provided
by the Grant Agency of the Czech Republic (203/09/0748)
and the European Union (CETOCOEN, CZ.1.05/2.1.00/
01.0001; administered by the Ministry of Education,
Youth and Sports of the Czech Republic). The authors
ꢁꢀ
express their thanks to Tomas Hamsı
Lukas Maier, Andrea Valigurova, and Blanka Vrbkova
ꢀ
ꢁ
k, Jan Krausko,
´
ꢁꢀ
ꢁ
ꢁ
from Masaryk University and Filip Mravec and Tereza
ꢁ
Halasova from Brno University of Technology for their
technical and analytical help.
Supporting Information Available. Experimental de-
tails, calculations of the Gibbs energy of electron transfer,
epifluorescent microscopy images and NMR, UVꢀvis,
and HRMS characterization data for compounds are
provided. This material is available free of charge via
the Internet at http://pubs.acs.org.
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1999, 126, 51–58.
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A 1986, 42, 747–754.
(22) Pelliccioli, A. P.; Wirz, J. Photochem. Photobiol. Sci. 2002, 1,
441–458.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX