High-efficiency microphotooxidation using milliwatt LED sources

Article abstract of DOI:10.1016/j.tetlet.2010.11.071

Inexpensive milliwatt light emitting diode (LED) sources allow energy- and atom-efficient microphotochemical reactions. Thus, sources constructed from three 120 mW 5 mm diameter 627 nm LED's enable μmol-mmol scale methylene blue-sensitized singlet oxygen photooxidations of various arenes and cyclopentadienones using a 3-5 M excess of oxygen in 82-98% yields.

Full text of DOI:10.1016/j.tetlet.2010.11.071

Tetrahedron Letters 52 (2011) 352–355
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
High-efficiency microphotooxidation using milliwatt LED sources
⇑
John M. Carney, Reagan J. Hammer, Martin Hulce , Chad M. Lomas, Dayna Miyashiro
Department of Chemistry, Creighton University, 2500 California Plaza, Omaha, NE 68178-0323, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Inexpensive milliwatt light emitting diode (LED) sources allow energy- and atom-efficient microphoto-
chemical reactions. Thus, sources constructed from three 120 mW 5 mm diameter 627 nm LED’s enable
Received 27 September 2010
Revised 10 November 2010
Accepted 12 November 2010
Available online 18 November 2010
l
mol–mmol scale methylene blue-sensitized singlet oxygen photooxidations of various arenes and cyclo-
pentadienones using a 3–5 M excess of oxygen in 82–98% yields.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Photooxidation
Endoperoxide
Light-emitting diode
Arene
1. Introduction
was considered important. We report here that static atmospheres
of oxygen at modest pressures result in efficient production of end-
operoxides using commercial milliwatt light emitting diode (LED)
sources.
Arene endoperoxides, the first of which was described in 1926
by Dufraisse,1 usually are prepared by photosensitized [4+2] cyclo-
addition of arenes with singlet oxygen.2,3 Depending on the arene,
the corresponding endoperoxide may thermally revert; for this
reason arene endoperoxides have been demonstrated to act as sin-
glet oxygen storage batteries.2a,4 Interested in preparing a series of
17O labeled arene endoperoxides5 for spectroscopic studies of this
phenomenon, we sought a general, label-efficient, high-yielding,
small-scale photochemical method for endoperoxide synthesis.
Operationally, two factors stand out as potential roadblocks to
photooxidations of arenes to produce thermally labile, 17O labeled
arene endoperoxides. First, even on a small scale, expense of 17O la-
beled oxygen preempts the normal, ‘open system’ practice of bub-
bling oxygen through solutions of substrate in the presence of a
sensitizer.6 Even were this not the case, open systems passing rel-
atively large volumes of dry gas through volatile solvents can expe-
rience significant solvent loss: This is particularly undesirable
when reaction rates are being examined. For these reasons, use
of a closed system was considered essential. Secondly, some com-
mon light sources employed for photooxidation generate signifi-
cant heat from which thermally unstable photoadducts must be
protected. Use of such sources requires inconvenient infrared fil-
ters, ventilating devices, and dewars with illumination windows;
in any case, these sources are capable of warming any closed
system and, therefore, pose an additional safety risk. Thus, for
the intended application a low thermal output visible light source
2. Results and discussion
To assay optimum conditions for NMR-scale 17O labeled arene
endoperoxide synthesis, 30–60 lmol of 9,10-dimethylanthracene
(1a) was chosen as a representative substrate (Fig. 1). The extent
of 9,10-dimethylanthracene-9,10-endoperoxide7 (2a) formation
was easily determined by integration of its methyl signals (d
2.19) relative to that of the starting arene (d 3.15) in the crude reac-
tion product.8
In initial experiments to design a closed, recirculating system, a
gas buret was used to deliver a known volume of excess oxygen to
a small gas manifold, attached via capillary and return tubes to a
10 mL round bottomed flask containing the substrate dissolved in
5 mL of a 25
lM solution of methylene blue in dichloromethane.
hν, sens, 3 ºC
10 O2
+
CH2Cl2, 23 min
1a
O
O
2a
⇑
Corresponding author. Tel.: +1 402 280 2271; fax: +1 402 280 5737.
E-mail address: mhulce@creighton.edu (M. Hulce).
Figure 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.071

J. M. Carney et al. / Tetrahedron Letters 52 (2011) 352–355
353
The oxygen was recirculated in the manifold using a miniature DC
diaphragm pump to bubble it through the reaction solution using
the capillary tube.
A
A simplification of this recirculating closed atmosphere concept
was achieved using a straightforward static atmosphere of oxygen
over a previously degassed reaction solution. A 10 mL hypovial
B
C
-
+
equipped with stirring bar was charged with 30–60 lmol of
9,10-dimethylanthracene, then crimped shut using a silicone sep-
tum stopper and aluminum seal. An inverted 14/20 septum stop-
per was placed over the top of the sealed hypovial to insure that
it was gas-tight, and the entire vial was wrapped in aluminum foil
D
to exclude light. After charging with 5 mL of a 25 lM solution of
methylene blue in dichloromethane, the vial was subjected to
two À78 °C freeze-aspirator pump-N2 purge-thaw cycles; in a last
cycle, it was injected with 5–15 mL of oxygen, corresponding to a
ca. 10-fold molar excess, and resulting in a 1–2.8 atm pressure in-
side the vial. Rapid stirring of the vial was sufficient to saturate the
solution with oxygen, and no endoperoxide formation was ob-
served as long as the vial remained foil-wrapped.
Figure 2. (A) 3 LED source in 1.5 cm glass tube; (B) 12 VDC power supply; (C)
sealed hypovial with stirring bar and O2 atmosphere; (D) magnetic stirrer.
Table 2
Conversion of 1a–2a using various sensitizers versus LED peak emission wavelength
LED wavelength (nm)
Conversion %
(sens MBe)
Conversion %
(sens TPPf)
Conversion %
With a satisfactory closed system available, attention turned to
identifying the best sources of visible light for photooxidation. In
order to consider monochromatic as well as polychromatic
sources, a baseline assay of common commercially available or
easily prepared sensitizers used in dichloromethane was per-
formed (Table 1). Using a traditional 250 W tungsten–halogen light
source projected through a 10 cm, 3 °C recirculating water wall and
a source-to-reaction vessel distance of 30 cm, 23 min irradiations
indicated that methylene blue9 was the most effective sensitizer
choice; tetraphenylporphyrin (TPP)10 and the bis-triethylammo-
nium11 or bis(cetyltributylphosphonium)12 salts of rose bengal,
while performing about the same, were less effective. Phase trans-
fer conditions using aqueous methylene blue or rose bengal and
cetyltributylphosphonium bromide12 were least effective. Based
on this assay further investigations were confined to source opti-
mization for endoperoxide formation using methylene blue, rose
bengal bis(triethylammonium) salt, and tetraphenylporphyrin.
Struck by the gross energy inefficiency of incandescent light
sources, which not only generate more heat than light but, for
use in the work at hand, require additional active cooling to re-
move that heat from the reaction environment, more energy-effi-
cient light sources were considered. Aware of the recent
introduction of a high-power multiwatt light emitting diode
(LED) photoreactor13 and the use of LED arrays in photodynamic
therapy14 and photodynamic therapy sensitizer development,15
in NOx photocatalytic oxidation,16 in oxygen sensing devices,17
and in annular flow, microfluidic and batch photoreactor design,18
we hypothesized that inexpensive, energy-efficient, low working
temperature, narrow band (full width at half maximum (FWHM)
typically 630 nm) milliwatt LED’s would be superior light sources
for small-scale photooxidations of arenes. To test this hypothesis,
LED light sources to irradiate 10 mL hypovials were constructed
from three 5 mm narrow viewing angle LED’s of the same peak
emission wavelength inserted into 1.5 Â 11 cm piece of glass tub-
ing so that the LED’s lenses were flush with the end of the tubing.
Power was provided with a standard 12 VDC power supply (Fig. 2;
photographs are available in the Supplementary data). Depending
(sens Et3NH)2RBg)
405a
98
36
12
6
11
24
32
98
48
51
420b
24
26
32
77
13
8
59
16
33
10
54
37
75
5
1
1
0
35
470b
505c
525b
588d
605b
627b
660a
250 W W/X
a
b
c
d
e
f
3 Â 80 mW 5 mm LED lamps.
3 Â 120 mW 5 mm LED lamps.
3 Â 100 mW LED lamps.
3 Â 125 mW LED lamps.
MB, methylene blue.
TPP, tetraphenylporphyrin.
(Et3NH)2RB, bis(triethylammonium) salt of rose bengal.
g
on peak emission wavelength used, total power consumption of
these sources ranges from 240 to 375 mW; over 30 min irradiation
with the LED source touching the side of the hypovial, heating of
the vial contents was observed to be minimal.
Using methylene blue as the sensitizer, photooxidation of 1a
was investigated as a function of LED source peak emission wave-
length (Table 2). Two LED sources—those with peak emission
wavelengths at 627 (red) and 405 (violet) nm—provided P98%
conversion to endoperoxide. The increased energy efficiency of
the photooxidation was notable: Lower source energy input, cou-
pled with low source working temperature obviating the need
for IR filtering result in these LED sources providing more than
an 8000-fold increase in efficiency of watts used per unit
conversion.
The rapidity and ease of formation of 2a using the 627 nm
source for 23 min is explained by the nearly complete overlap of
the 627 nm (FWHM = 20 nm) LED emission spectrum and the
absorption spectrum of methylene blue (kmax 656 nm, FWHM =
37 nm) in dichloromethane: Essentially all of the incident light is
absorbed by the photosensitizer, allowing maximal singlet oxygen
production (Fig. 3a). At the 405 nm source wavelength, methylene
blue is transparent, but 1a is not: with a significant absorption
band at 400 nm (FWHM = 12 nm), overlap of the 405 nm
(FWHM = 14 nm) LED emission spectrum allows the arene itself
to act as a sensitizer.19 Thus, P98% conversion to endoperoxide
also was observed after irradiation for 23 min in the absence of
methylene blue. In either case, the technique provides rapid access
to 2a with conversions equal to or better than other known
methods.6–8,20
Table 1
Conversion of 1a–2a versus sensitizer
Sensitizer
27 M methylene blue
190 M methylene blue—phase transfer
Conversion %
l
l
51
9
33
51
31
18
l
M rose bengal bis(triethylammonium) salt
35
24
34
33
l
l
l
M rose bengal—phase transfer
M rose bengal bis(cetyltributylphosphonium) salt
M tetraphenylporphyrin

354
J. M. Carney et al. / Tetrahedron Letters 52 (2011) 352–355
Table 3
3a: 627 nm LED Emission / Methylene Blue Absorbance
Micro- and milliscale 627 nm LED photooxidations using 25
sensitizer
l
M methylene blue as
Yielda (%)
2.5
2
600
500
400
300
200
100
0
Substrate
Time (min)
90
Product
O
1.5
1
O
83
2b8,25
1b
Ph
Ph
0.5
0
23
23
96
O
98b
O
300 350 400 450 500 550 600 650 700 750
Ph
1c
Ph
-100
-0.5
2c5,6,23c,26
nm
3b: 525 nm LED Emission / RB(Et3NH)2 Absorbance
O
1440
O
96
2
70
60
50
40
30
20
10
0
45,27
1.5
1
3
5
23
98
O
O
210c
94c,d
618a,c;23
O O
O
0.5
0
Ph
Ph
Ph
Ph
Ph
Ph
23
82d
Ph
Ph
822,23c
300 350 400 450 500 550 600
650
700 750
7
-0.5
-10
nm
a
b
c
1H NMR yield unless otherwise noted.
405 nm LED source; no sensitizer used.
Reaction performed on 0.2 g (1.5 mmol) scale with 3.8 equiv O2.
Isolated yield.
3c: 525 nm LED Emission / TPP Absorbance
d
2
70
60
50
40
30
20
10
0
1.5
1
and dienes was examined (Table 3). 9,10-Diphenylanthracene
(1c), -terpinene (5) and tetracyclone (7) all rapidly form endoper-
a
oxides in good to excellent yields. As was the case for 1a, 1c itself
acts as a photosensitizer, so that no additional sensitizing dye is
necessary when a 405 nm LED source is used. Yield of 2c was the
same whether the reaction was run at 3 or 23 °C. Also, note that
the endoperoxide formed from 7 undergoes further elimination
of carbon monoxide, resulting in the observed (Z)-alkene, 8.22,23c
Finally, these microphotooxidation conditions are at least mod-
erately scalable. Straightforward scale-up of the photooxidation of
0.5
0
300 350 400 450 500 550 600
650
700 750
-0.5
-10
nm
a
-terpinene to millimole quantities allowed for production of a
Figure 3. Solid lines, LED emission spectra. Dashed lines, sensitizer absorption
spectra in CH2Cl2.
quarter-gram of ascaridole (6)18a,c,23 with no change in illumina-
tion by the LED source.
Slower-reacting arenes also produced endoperoxides in good to
excellent yields, anthracene (1b) requiring 90 min to react, while
1,4-dimethylnaphthalene required 24 h of irradiation at 3 °C for
high levels of conversion to endoperoxide 4.
Using rose bengal bis(triethylammonium) salt instead of meth-
ylene blue as sensitizer resulted in 0–75% conversions of 1a–2a
when the LED source peak emission wavelength was varied. Max-
imum conversion was achieved using the 525 nm (green,
FWHM = 35 nm) source, the emission spectrum of which overlaps
substantially with the visible absorption spectrum of the dye (kmax
523, 551 nm, FWHM ca. 60 nm; Fig. 3b). Similarly, using tetraphen-
ylporphyrin as sensitizer resulted in 8–77% conversions to endo-
peroxide 2a. Two conversion maxima were observed: 77%
conversion using the 525 nm LED source, and 59% using the
627 nm source. Both sources substantially overlap with one or
more of the sensitizer Q bands (k = 514, 549, 589, 648 nm;
Fig. 3c).21 From this set of sensitizer vs. source studies, then, in
all cases narrow band milliwatt LED sources demonstrate them-
selves to be superior to a 250 watt incandescent source for the
photooxidation of 9,10-dimethylanthracene to 9,10-dimethylan-
thracene-9,10-endoperoxide.
In other experiments, photooxidations of the Diels–Alder dimer
(9)24 of 2,5-dimethyl-3,4-diphenyl-2,4-cyclopentadienone and 2,5-
diethyl-3,4-diphenyl-2,4-cyclopentadienone (10)28 were briefly
investigated. Room temperature photooxidation of 9 using the pre-
viously determined optimum reaction conditions for formation of
2a was unsuccessful: only starting material was recovered. This
outcome is not unexpected: retro-Diels–Alder reaction of colorless
9 to produce the presumably photooxizable red 2,5-dimethyl-3,4-
diphenyl-2,4-cyclopentadienone occurs only at temperatures well
above room temperature (ca. P80 °C). On the other hand, under
the same reaction conditions cyclopentadieneone 10 underwent
rapid photooxidation. In this case, a complex mixture of products
formed. Gas chromatography–mass spectrometry indicated signifi-
cant formation of an adduct with oxygen (m/z = 292). There are,
however, at least two routes by which 10 may undergo additions
with singlet oxygen: Cycloaddition to provide the endoperoxide,
Having determined the optimal combination of LED source and
sensitizer for microphotooxidation, applicability to other arenes

J. M. Carney et al. / Tetrahedron Letters 52 (2011) 352–355
355
and ene or Schenck reaction to provide an allylic hydroperoxide.29
References and notes
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3. Conclusion
Commercial narrow viewing angle 5 mm milliwatt light emit-
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tion. Generating little waste heat, they are particularly useful in
the preparation of thermally labile arene endoperoxides. Demon-
strated to be scalable in the micro- to millimolar range using an ar-
ray of three LED’s, larger arrays may provide access to larger scale
reactions or rate enhancements of slow reactions.
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9,10-Dimethylanthracene (1a, 10.3 mg, 49.9
in an aluminum foil-wrapped, magnetic stirring bar-equipped
10 mL hypovial. The vial was charged with 5 mL of a 27 M solu-
lmol) was sealed
l
tion of methylene blue in dichloromethane. After two À78 °C
freeze-aspirator pump-1 atm N2 purge-23 °C thaw cycles, 12 mL
(ca. 0.5 mmol, 10 equiv) of O2 was injected into the vial using a
gas-tight syringe. The foil was removed, the contents of the reac-
tion vial stirred rapidly at 3 °C in a cold room as the vial was irra-
diated with three 125 mW, 627 nm LED’s. After 23 min, the vial
was vented by inserting a syringe needle through the septum.
The septum was removed and the contents of the vial filtered
through a Pasteur pipette containing a 2 cm plug of neutral chro-
matographic alumina into a tared, 25 mL round bottomed flask.
The alumina plug was washed five times with 1 mL aliquots of
dichloromethane and the washings combined with the filtrate.
Concentration by rotary evaporation afforded 11.8 mg (99%) of
9,10-dimethylanthracene 9,10-endoperoxide (2a) which contained
no 9,10-dimethylanthracene (1a, Rf = 0.63) when analyzed by TLC
(petroleum ether). 1H NMR (CDCl3) d 7.44 (m, 4H), 7.31 (m, 4H),
2.19 (s, 6H) ppm. 13C NMR (CDCl3) d 141.11, 127.64, 120.91,
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Acknowledgments
The authors gratefully acknowledge funding from the Creighton
University Department of Pathology and the Creighton University
Graduate School for a Summer Faculty Research Fellowship. James
T. Fletcher, Robert C. Allen, and Ahsan U. Khan are thanked for use-
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General experimental conditions and apparatus, detailed exper-
imental procedures and spectroscopic data for all compounds pre-
pared. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2010.11.071.