High-efficiency microphotooxidation using milliwatt LED sources

Tetrahedron Letters 52 (2011) 352–355

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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

Article history: Received 27 September 2010 Revised 10 November 2010 Accepted 12 November 2010 Available online 18 November 2010

a b s t r a c t 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 lmol–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. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Photooxidation Endoperoxide Light-emitting diode Arene

1. Introduction Arene endoperoxides, the first of which was described in 1926 by Dufraisse,1 usually are prepared by photosensitized [4+2] cycloaddition 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 singlet oxygen storage batteries.2a,4 Interested in preparing a series of 17 O 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 labeled oxygen preempts the normal, ‘open system’ practice of bubbling oxygen through solutions of substrate in the presence of a sensitizer.6 Even were this not the case, open systems passing relatively large volumes of dry gas through volatile solvents can experience 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 common light sources employed for photooxidation generate significant heat from which thermally unstable photoadducts must be protected. Use of such sources requires inconvenient infrared filters, 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

was considered important. We report here that static atmospheres of oxygen at modest pressures result in efficient production of endoperoxides using commercial milliwatt light emitting diode (LED) sources. 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 reaction 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 O 2 +

CH2Cl2, 23 min 1a OO

2a

⇑ Corresponding author. Tel.: +1 402 280 2271; fax: +1 402 280 5737. E-mail address: [email protected] (M. Hulce). 0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2010.11.071

Figure 1.

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J. M. Carney et al. / Tetrahedron Letters 52 (2011) 352–355

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 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 equipped with stirring bar was charged with 30–60 lmol of 9,10-dimethylanthracene, then crimped shut using a silicone septum stopper and aluminum seal. An inverted 14/20 septum stopper 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 to exclude light. After charging with 5 mL of a 25 lM solution of methylene blue in dichloromethane, the vial was subjected to two 78 °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 inside the vial. Rapid stirring of the vial was sufficient to saturate the solution with oxygen, and no endoperoxide formation was observed as long as the vial remained foil-wrapped. 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 performed (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-triethylammonium11 or bis(cetyltributylphosphonium)12 salts of rose bengal, while performing about the same, were less effective. Phase transfer conditions using aqueous methylene blue or rose bengal and cetyltributylphosphonium bromide12 were least effective. Based on this assay further investigations were confined to source optimization 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 remove that heat from the reaction environment, more energy-efficient 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  11 cm piece of glass tubing 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 Table 1 Conversion of 1a–2a versus sensitizer Sensitizer

Conversion %

27 lM methylene blue 190 lM methylene blue—phase transfer 33 lM rose bengal bis(triethylammonium) salt 51 lM rose bengal—phase transfer 31 lM rose bengal bis(cetyltributylphosphonium) salt 18 lM tetraphenylporphyrin

51 9 35 24 34 33

A B

C

+ D

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

a b c d e f g

LED wavelength (nm)

Conversion % (sens MBe)

Conversion % (sens TPPf)

Conversion % (sens Et3NH)2RBg)

405a 420b 470b 505c 525b 588d 605b 627b 660a 250 W W/X

98 36 12 6 11 24 32 98 48 51

24 26 32 77 13 8 59 16 33

10 54 37 75 5 1 1 0 35

3  80 mW 5 mm LED lamps. 3  120 mW 5 mm LED lamps. 3  100 mW LED lamps. 3  125 mW LED lamps. MB, methylene blue. TPP, tetraphenylporphyrin. (Et3NH)2RB, bis(triethylammonium) salt of rose bengal.

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 wavelength (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, coupled 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

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3a: 627 nm LED Emission / Methylene Blue Absorbance

600

2.5

500

2

1

350

400

450

500

550

600

650

700

nm 3b: 525 nm LED Emission / RB(Et3NH)2 Absorbance

60

40

2c 5,6,23c,26

5 O

400

450

500

550

600

650

700

nm

Ph

750 -0.5

3c: 525 nm LED Emission / TPP Absorbance

c d

2

60

1.5

50 40

1 A

30

0.5

20 10

0

450

500

550

6 18a,c;23 O O

Ph

Ph

Ph 8

82d

22,23c

a 1

70

400

Ph

98 94c,d

7 b

350

Ph

Ph 23

0

350

O O

23 210c

Ph

96

4 5,27

3

A

20

OO

1440

0.5

10

microW

Ph

1c

1

30

96 98b

OO

Ph

2 1.5

50

microW

23 23

750 -0.5

83

2b8,25 Ph

0

70

0 300 -10

OO

1b Ph

0.5

100

Yielda (%)

Product

90

200

0 300 -10

Time (min)

1.5

300

0 300 -100

Substrate

A

microW

400

Table 3 Micro- and milliscale 627 nm LED photooxidations using 25 lM methylene blue as sensitizer

600

650

700

750 -0.5

nm

Figure 3. Solid lines, LED emission spectra. Dashed lines, sensitizer absorption spectra in CH2Cl2.

Using rose bengal bis(triethylammonium) salt instead of methylene blue as sensitizer resulted in 0–75% conversions of 1a–2a when the LED source peak emission wavelength was varied. Maximum 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 tetraphenylporphyrin as sensitizer resulted in 8–77% conversions to endoperoxide 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 themselves to be superior to a 250 watt incandescent source for the photooxidation of 9,10-dimethylanthracene to 9,10-dimethylanthracene-9,10-endoperoxide. Having determined the optimal combination of LED source and sensitizer for microphotooxidation, applicability to other arenes

H 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.

and dienes was examined (Table 3). 9,10-Diphenylanthracene (1c), a-terpinene (5) and tetracyclone (7) all rapidly form endoperoxides 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 moderately scalable. Straightforward scale-up of the photooxidation of a-terpinene to millimole quantities allowed for production of a quarter-gram of ascaridole (6)18a,c,23 with no change in illumination 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. In other experiments, photooxidations of the Diels–Alder dimer (9)24 of 2,5-dimethyl-3,4-diphenyl-2,4-cyclopentadienone and 2,5diethyl-3,4-diphenyl-2,4-cyclopentadienone (10)28 were briefly investigated. Room temperature photooxidation of 9 using the previously 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-3,4diphenyl-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 significant 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,

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and ene or Schenck reaction to provide an allylic hydroperoxide.29 Work is in progress to determine which route(s) lead to products, and if reaction conditions afford discrimination between them. O Ph

Ph Ph Ph

O

O

Et

Et Ph

9

Ph

10

3. Conclusion Commercial narrow viewing angle 5 mm milliwatt light emitting diodes are effective, energy-efficient sources for photooxidation. Generating little waste heat, they are particularly useful in the preparation of thermally labile arene endoperoxides. Demonstrated to be scalable in the micro- to millimolar range using an array of three LED’s, larger arrays may provide access to larger scale reactions or rate enhancements of slow reactions. 4. Representative experimental procedure 4.1. 9,10-Dimethylanthracene 9,10-endoperoxide (2a) using methylene blue as sensitizer 9,10-Dimethylanthracene (1a, 10.3 mg, 49.9 lmol) was sealed in an aluminum foil-wrapped, magnetic stirring bar-equipped 10 mL hypovial. The vial was charged with 5 mL of a 27 lM solution of methylene blue in dichloromethane. After two 78 °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 reaction vial stirred rapidly at 3 °C in a cold room as the vial was irradiated 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 chromatographic 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, 79.84, 13.95 ppm. IR 3041 (w), 2983 (w), 2935 (w), 1463 (m), 1378 (m), 756 (s) cm 1. MS (MALDI) m/z 239.12 (M+H+). 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 useful insights. Supplementary data General experimental conditions and apparatus, detailed experimental procedures and spectroscopic data for all compounds prepared. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2010.11.071.

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