Practical evaluation of compact fluorescent lamps for dye-sensitized photooxidation reactions

Article abstract of DOI:10.1055/s-0033-1340976

Energy-efficient compact fluorescent lamps (CFL) have been evaluated as a light source for the sensitized generation of singlet oxygen used in the oxidation of 1,3-butadienes and furfural derivatives using a range of dyes including rose bengal, methylene

Full text of DOI:10.1055/s-0033-1340976

1168▌
LETTER
letter
Practical Evaluation of Compact Fluorescent Lamps for Dye-Sensitized
Photooxidation Reactions
Sensitized Photooxidations Using Compact Fluorescent Lamps
Kieran P. Stockton,a Joseph P. May,a Dennis K. Taylor,b Ben W. Greatrex*a
a
School of Science and Technology, University of New England, Armidale, New South Wales, 2351, Australia
Fax +61(0)267735011; E-mail: ben.greatrex@une.edu.au
b
School of Agriculture, Food and Wine, University of Adelaide, South Australia, 5065, Australia
Received: 02.02.2014; Accepted: 18.02.2014
much of the light is wasted as heat which needs to be re-
moved by cooling of reaction mixtures. A typical multi-
Abstract: Energy-efficient compact fluorescent lamps (CFL) have
been evaluated as a light source for the sensitized generation of sin-
glet oxygen used in the oxidation of 1,3-butadienes and furfural de-
rivatives using a range of dyes including rose bengal, methylene
gram-scale photooxidation
using
rose bengal
bis(triethylammonium) salt in CH2Cl2 can require 500–
blue, and tetraphenylporphyrin. An ambient temperature water 1500 W of tungsten halogen lamps in close proximity to
jacket was sufficient to cool reaction mixtures due to the low heat
output of lamps and the reaction rates using 110 W of CFL were
slower but comparable to those obtained using a 500 W tungsten
time,9 and the majority of the energy applied is heat so that
halogen lamp. The conditions were applied to the photooxidation of
a series of furan aldehydes and 1,3-butadienes to give products in
the reaction to achieve reaction times in the order of 5–8
hours.8,4b,4d High light intensity can affect sensitizer life-
cooling requirements can limit the practical scalability of
photooxidations using these lamps.
good yield in 4–96 hours.
In the consumer lighting market there has been a move to
compact fluorescent lamps (CFL) from incandescent
bulbs due to their energy efficiency as a source of lighting.
CFL are inexpensive, readily available, and generate up to
fivefold more visible light than incandescent bulbs for the
equivalent energy input. Light is generated in a so-called
tricolor CFL by the passage of current through mercury
vapor with the light emitted consisting of visible and UV
light.7,10 The UV light then strikes a rare earth phosphor
coating on the inside of the bulb resulting in visible wave-
length emission. To match the sensitivity of the human
eye, red, green, and blue emitting materials are combined
to generate white light, which has a series of maxima cor-
responding to the different phosphor emissions. Light
emitted from a CFL measured using a diode array detector
as well as the UV spectra of dyes used in this study are
shown in Figure 1.
Key words: peroxides, photooxidation, photochemistry, pericyclic
reaction, oxygenations, sensitizer
Singlet-oxygen chemistry is of great interest due to its
utility in synthesis. Applications include the [4+2]
cycloaddition1 and the Schenck–Ene reaction2 generating
useful endoperoxides and allylic hydroperoxides, respec-
tively. The most common method for the production of
singlet oxygen involves irradiating an air- or oxygen-sat-
urated solution of substrate containing a photosensitizing
dye.1 The dye absorbs a photon of light and is excited to a
singlet state (Sn) and then intersystem crossing gives rise
to the triplet-state-excited dye. Transfer of energy from
the triplet-excited-state dye to ground-state oxygen
(3ΔO2) yields singlet-state oxygen (1ΔgO2) or the excited
state 1Σ+g which relaxes to 1ΔgO2.3 Oxygen saturation can
3
1
improve the conversion of ΔO2 to ΔgO2 depending on
the dye used and so reactions are usually performed with
oxygen saturation.
The excitation of dyes used for the generation of singlet
oxygen requires light with wavelength corresponding to
the absorption bands of the dye.3b For photooxidation re-
actions, reported light sources are most often tungsten hal-
ogen,4 xenon,5 or incandescent lamps.6 Tungsten halogen
and incandescent lamps have emission maxima at 970 nm
and 1050 nm, respectively.7 Both emit a range of light en-
ergies corresponding to black-body radiation of objects
between 2750–3000 K, with the majority of light emitted
in the near infrared region. The absorption bands for many
of the dyes used for photooxidations are in the visible re-
Figure 1 Normalized emission spectra for CFL and normalized ab-
gion of the spectrum between 500–700 nm and therefore sorption spectra for photosensitizing dyes (see also Table 1)
The application of energy-efficient light sources for
SYNLETT 2014, 25, 1168–1172
chemical reactions is gaining momentum. CFL have pre-
viously been used for the photolysis of α-diazo Weinreb
Advanced online publication: 25.03.2014
DOI: 10.1055/s-0033-1340976; Art ID: ST-2014-D0091-L
© Georg Thieme Verlag Stuttgart · New York
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LETTER
Sensitized Photooxidations Using Compact Fluorescent Lamps
1169
amides for the preparation of β-lactams via a Wolff rear-
rangement,11 and CFL lights have been used to initiate po-
lymerization reactions.12 Seeberger has reported the use of
LED lights for the production of singlet oxygen in a con-
tinuous-flow reactor system, however, only μmol/min
quantities could be produced using this light source due to
the low light intensity.13 To the best of our knowledge,
CFL have not been reported for the generation of singlet
oxygen.
O
O
1O2
Ph
Ph
Ph
Ph
1
3
2
H
1O2
MeO
O
O
O
MeOH
O
4
Scheme 1 Photooxygenation of dienes
Table 1 Absorption Maxima and Quantum Yields of the Sensitising
Dyes Used for Singlet Oxygen Generation
We found that CFL were an effective light source for the
sensitizer TPP with the endoperoxide 2 formed in 93%
conversion from butadiene following photolysis for 14
hours (Table 2, entry 1). Complete reaction was seen after
24 hours with no apparent decomposition of the product
Sensitising dye
Solvent
λ
max (nm)
Quantum
yield
rose bengal (RB)
MeOH
548
0.7614
0.6315
0.4014
0.6915
0.5215
1
observable by H NMR spectroscopy. The reaction rate
tetraphenylporphyrin (TPP) CH2Cl2 419, 514, 549
obtained using a 500 W tungsten halogen globe was
slightly higher than that of the 110 W CFL (Table 2, en-
tries 1 and 5). Rose bengal disodium salt is insoluble in
CH2Cl2 and so a mixture of MeOH–CH2Cl2 (2:1) was
used for the reaction of 1 with only 34% conversion into
eosin B (EB)
MeOH
EtOH
514
532
653
erythrosin (EN)
methylene blue (MB)
MeOH
Table 2 Photooxidation of 1,3-Diphenylbutadiene and 5-Methylfur-
fural
The absorption maxima and quantum yields for singlet-
oxygen generation of common dyes used for photooxida-
tion reactions are listed in Table 1. The major green emis-
sion band in CFL at 546 nm directly overlaps the Q-band
absorption of TPP at 549 nm. The same emission band
overlaps the absorptions of rose bengal at λmax 548 nm and
the shoulder of erythrosin at λmax 532 nm. The red emis-
sion band in CFL at 620 nm overlaps the broad methylene
blue absorption band at λmax 653 nm. Due to these over-
laps we hypothesized that CFL could be used with these
sensitizers for singlet-oxygen generation.
Quantum yields,15 the lifetime of singlet oxygen,9,16 the
dye spectra,17 and the rates of dye photobleaching
reactions17 all show solvent dependence. As such, we con-
sidered that the evaluation of reaction outcomes would be
the most appropriate method for examining the suitability
of CFL for singlet-oxygen generation. Commercially
available 55 W and 48 W CFL in reflective housings were
used for photooxidation reactions in a 50 mm diameter
jacketed Pyrex cylindrical flask cooled with water at
20 °C. The [4+2] addition of singlet oxygen to 1,4-diphe-
nyl-1,3-butadiene (1) giving rise to 3,6-diphenyl-3,6-di-
hydro-1,2-dioxine (2) was used as a model reaction and
the reactions followed by 1H NMR (Scheme 1). A second
substrate, 5-methylfurfural (3), suitable for use with alco-
hol-soluble dyes, was also used, and the reaction to give
5-methoxy-5-methyl-furan-2(5H)-one (4)18 followed by
GC–MS. Reactions were sampled regularly to determine
time to completion, and the results are summarized in Ta-
ble 2.
Entrya Diene Dye Solvent
Light
source (h)
Time Conv.
(yield, %)
1
2
1
1
1
1
1
1
1
3
3
3
3
3
3
3
TPP CH2Cl2
TPP CH2Cl2
CFLb
CFL
CFL
14
24
6
93
100 (72)
3
3
RB
RB
MeOH
4
MeOH/CH2Cl2 CFL
24
6
34
5
TPP CH2Cl2
CH2Cl2
MB CH2Cl2
TH
73
6c
7
7
CFLb
CFL
CFL
TH
26
24
5
86
1
8
RB
RB
RB
MeOH
MeOH
MeOH
100
100
100 (70)
65
9
4
10d
11
12d
13
14
CFL
3
TPP MeOH–CH2Cl2 CFL
23
5
EB
EN
MeOH
MeOH
CFL
CFL
CFL
89
24
5
100
100
MB MeOH
a Reactions were carried out using 2 g of substrate in 150 mL of sol-
vent in a tubular reactor at 20 °C and 100 mg of sensitizing dye (Table
1) bubbled with oxygen. Reactions of 1 were followed by 1H NMR
spectroscopy, and 2 was isolated by crystallization. Reactions of 3
were followed by GC–MS, and 4 was isolated by reduced pressure
distillation. CFL = 158 W CFL array, TH = 500 W tungsten halogen
lamp.
b Reaction performed with 4 g of substrate and 110 W lamps in 150
mL of solvent.
c An additional portion of sensitizer (50 mg) was added at the 20 h
mark (81% conversion).
d Reaction performed using a rectangular cuboid reactor 300
mm × 200 mm × 3 mm using 110 W CFL.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1168–1172

1170
K. P. Stockton et al.
LETTER
the endoperoxide product 2 after 24 hours (Table 2, entry MS showed that the rate of consumption of 3 was inde-
4). Performing the reaction in methanol, a common sol- pendent of the concentration of 3 until ca. 90% of 3 had
vent for rose bengal photooxidations gave only trace been consumed.
amounts of endoperoxide 2 due to the insolubility of the
starting butadiene (Table 2, entry 3).
Erythrosin has a lower quantum yield than rose bengal
and a red-shifted absorption maxima. Using erythrosin B
Methylene blue which is used in aqueous conditions for as the photosensitizer, rapid consumption of the starting
singlet-oxygen generation produced virtually no product furfural was observed with 50% consumption after 80
from the reaction of 1 in CH2Cl2 after irradiation for 24 minutes, but the reaction slowed due to photobleaching of
hours (Table 2, entry 7). This was surprising as CH2Cl2 the dye. The reaction was incomplete after eight hours al-
has been used for methylene blue sensitized photooxida- though 100% conversion to the product 4 was still seen af-
tions in high yield1d and the dye–CFL combination was ter 24 hours (Table 2, entry 13).
suitable for the oxidation of 3 in methanol (Table 2, entry
14).
The overnight reaction times for some of the substrates
prompted us to examine alternative reaction conditions.
Lamberts and Neckers have reported a number of organic- The use of a cylindrical reaction vessel 40 mm
soluble rose bengal derivatives that exhibit high quantum (I.D.) × 130 mm (length) gave a total reactor surface area
yields, and some of these derivatives such as the bis(tri- of 160 cm2 and due to the shape of the vessel, not all light
ethylammonium)salt of rose bengal have found wide- impacted the reaction. An alternative reactor suitable for
spread use.14 We decided to investigate one of these alcoholic solvents was constructed using 300 mm × 200
derivatives with favorable quantum yield in conjunction mm glass plates spaced 3 mm apart, cooled with a water
with CFL and so rose bengal salt 7 (λmax = 561 nm) was jacket, and sealed using cross-linked polydimethylsilox-
synthesized (Scheme 2). The alkylation of rose bengal di- ane giving a rectangular cuboid reactor with a 1200 cm2
sodium salt 5 with ethyl iodide and then treatment of the surface area or ca. 800 cm2 when partially filled. When
product 6 with triethylamine yielded material with spec- placed close to the lamps, this reactor configuration re-
troscopic properties matching those reported.11 Irradiation sulted in greatly reduced stray light and shortened the time
of diene 1 sensitized with 7 using 110 W of CFL gave for the photooxidation of 5-methylfurfural sensititized
86% conversion to product although an extended reaction with rose bengal from five hours to three hours using
time was required (Table 2, entry 6).
2 × 55W CFLs (Table 2, entry 10). Eosin also gave 89%
conversion of 3 into 4 in five hours using this configura-
tion.
Cl
Cl
Cl
Cl
A series of butadienes were prepared to demonstrate the
scope of the reaction. The reaction of allyl triphenylphos-
phonium bromide with the appropriate substituted benzal-
dehyde in the presence of KOt-Bu gave 9–11 as E/Z
mixtures19 while 8 and 12 were prepared from (E)-cinna-
maldehyde using Wittig chemistry.20
CO2R1
I
I
O
O
OR2
I
I
Photooxidation of 8–13 using CFL sensitized with rose
bengal or TPP afforded endoperoxides on gram scale (Ta-
ble 3). Extended reaction times were required for dienes
9 and 10, and some polymerization of the diene was evi-
dent as insoluble material was present at the end of the re-
action. In the case of cyclopentadiene 13, the reaction
progress was inferred by treating an aliquot of the reaction
mixture with (R,R)-N,N′-bis(3,5-di-tert-butylsalicyli-
dene)-1,2-cyclohexanediaminocobalt(II) and monitoring
the appearance of epoxide 20 as the endoperoxide violent-
ly decomposes at room temperature when neat.
R1 = Na, R2 = Na
rose bengal
5
6
7
EtI, Δ
Et3N
R
1 = Et, R2 = H
1 = Et, R2 = HNEt3
R
Scheme 2 Synthesis of sensitizer 7
The reaction of 5-methylfurfural (3) with singlet oxygen
in methanol yielded furanone 4 as the sole product as
judged by GC–MS and NMR spctroscopy (Table 2, en-
tries 8–14). The reaction proceeds through a [4+2] cyclo-
addition to the furan giving an ozonide which
decarbonylates presumably following hemiacetal forma-
tion on the side chain. Using 158 W of CFL as the light
source and rose bengal as the sensitizer in methanol, com-
plete transformation of the starting 5-methylfurfural (3) to
the furanone 4 was seen in five hours (Table 2, entry 8).
By comparison, irradiation with a 500 W tungsten halo-
gen lamp took four hours for completion (Table 2, entry
9). No reaction was seen when the solution of dye and fur-
fural was bubbled with nitrogen and stored in the dark
confirming that singlet oxygen was the reactive species.
Sampling the reaction every hour and analyzing by GC–
O’Shea and Foote have demonstrated that in the presence
of singlet oxygen, the double bonds in (E,E)-, (E,Z)- and
(Z,Z)-2,4-hexadiene rapidly isomerize.21 The recovered
diene from the photooxidations of 9–11 were depleted
in E-isomer indicating that isomerization is slower
than [4+2] addition for these aryl-substituted dienes. The
E-isomer would be expected to yield more 1,2-dioxine as
it is able to adopt the required s-cis conformation and this
may partly explain the lower yields of product that were
obtained from 9–11 relative to (E)-1-phenyl-1,3-butadi-
Synlett 2014, 25, 1168–1172
© Georg Thieme Verlag Stuttgart · New York

LETTER
Sensitized Photooxidations Using Compact Fluorescent Lamps
1171
ene (8). The mixture of (1E,3Z)- and (1E,3E)-1-phenyl- This study has demonstrated that inexpensive CFL are a
1,3-pentadiene (12) reacted completely and afforded the suitable source of light for the singlet-oxygen-mediated
cis-peroxide containing 3% of the trans-isomer after [4+2] reaction sensitized by methylene blue, erythrosin,
isomerization of the butadiene. The ratio indicating fast rose bengal, and TPP. Other energy-efficient light sources
isomerization of 12 agree with the findings of Motoshiya such as LED can be used to selectively generate a useable
et al. obtained using a tungsten halogen lamp.22 Taken to- wavelength, however, there is significant cost associated
gether, the results suggest that aryl-substituted butadienes with high wattage LED arrays. Long reaction times were
should be synthesized in such a way that the aryl-substi- required for some substrates, however, CFL are much
tuted alkene has the geometry required for the addition of cooler than tungsten halogen lamps and pose a much re-
singlet oxygen as isomerization is slow.
duced risk of fire. It is envisaged that CFL could be used
in conjunction with the dyes examined here to improve
Table 3 Sensitized Photooxidations of Butadienes Using 110 W Compact Fluorescent Lamps
Entrya
Starting material
Conditionsb Time
Solvent
Sensitizer Product
Yield (% conv.)c
72 (100), Lit: 304b
O
O
O
O
1
A
A
16
96
CH2Cl2
TPP
8
15
2
3
4
CH2Cl2
CH2Cl2
CH2Cl2
TPP
26 (57),d Lit: 98b
Cl
9 22:78 E/Z
O
Cl
16
O
O
O
O
A
A
72
16
TPP
TPP
20 (70)e
O
10 51:49 E/Z
17
O
O
28 (75)f
18
11 57:43 E/Z
O
O
5
6
A
A
24
4
CH2Cl2
CH2Cl2
TPP
TPP
61 (100)g
(100)h
12 21:79 E/Z
19 97:3 cis/trans
O
H
O
13
20
O
O
O
HO
O
H
7
8
B
B
6
4
MeOH
EtOH
RB
RB
78 (90)
21
14
O
EtO
O
O
O
57 (100)h
H
22
3
a Reactions were performed on 2 g of starting material in 150 mL of solvent bubbled with oxygen.
b Conditions A used a 40 mm (I.D.) × 130 mm (length) cylindrical reactor. Conditions B used a rectangular cuboid reactor 300 mm × 200
mm × 3 mm.
c Based on recovered diene.
d 7:93 E/Z ratio in the recovered diene.
e 9:91 E/Z ratio in the recovered diene.
f 12:88 E/Z ratio in recovered diene.
g 1.4 g scale.
h Measured using GC–MS.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1168–1172

1172
K. P. Stockton et al.
LETTER
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