Combining engineering and chemistry for the selective continuous production of four different oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents

Article abstract of DOI:10.1016/j.tet.2017.11.048

A range of products is reported from the photo-oxidation of cyclopentadiene from photochemically generated singlet oxygen (¹O₂) using carbon dioxide (CO₂) as a solvent and 5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin (TPFPP) as a CO₂-soluble photosensitizer. The endo-peroxide intermediate, generated from the reaction with singlet oxygen, is transformed into one of several different products in good yield depending on the conditions applied and by adding different reactors and reagents downstream of the photo-reactor, allowing the reaction products to be switched in one streamlined process. The addition of a thermal reactor facilitated the rearrangement of the endoperoxide to form Z-4,5-epoxy-2-pentanal. Quenching with thiourea yielded the syn-diol, (1R,3S)-cyclopent-4-ene-1,3-diol. Treatment with acid or base afforded furfuryl alcohol and 4-hydroxy-2-cyclopentenone respectively. High productivities for all products were obtained when compared to traditional batch reactions.

Full text of DOI:10.1016/j.tet.2017.11.048

Tetrahedron xxx (2017) 1e6
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Combining engineering and chemistry for the selective continuous
production of four different oxygenated compounds by photo-
oxidation of cyclopentadiene using liquid and supercritical CO2 as
solvents
,
*
Lingqiao Wu a, Zahra Abada a, Darren S. Lee a, Martyn Poliakoff a
,
, b, **
Michael W. George a
a School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
b Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, 315100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A range of products is reported from the photo-oxidation of cyclopentadiene from photochemically
generated singlet oxygen (1O2) using carbon dioxide (CO2) as a solvent and 5,10,15,20-tetrakis-(penta-
fluorophenyl)porphyrin (TPFPP) as a CO2-soluble photosensitizer. The endo-peroxide intermediate,
generated from the reaction with singlet oxygen, is transformed into one of several different products in
good yield depending on the conditions applied and by adding different reactors and reagents down-
stream of the photo-reactor, allowing the reaction products to be switched in one streamlined process.
The addition of a thermal reactor facilitated the rearrangement of the endoperoxide to form Z-4,5-epoxy-
2-pentanal. Quenching with thiourea yielded the syn-diol, (1R,3S)-cyclopent-4-ene-1,3-diol. Treatment
with acid or base afforded furfuryl alcohol and 4-hydroxy-2-cyclopentenone respectively. High pro-
ductivities for all products were obtained when compared to traditional batch reactions.
© 2017 Elsevier Ltd. All rights reserved.
Received 5 October 2017
Accepted 16 November 2017
Available online xxx
Keywords:
Photo-oxidation
Photochemistry
Cyclopentadiene
Singlet oxygen
Supercritical CO2
Liquid CO2
Continuous flow
Flow chemistry
1. Introduction
species that can easily be converted into
compounds.1,5e11
a variety of
Molecular oxygen is a widely available, inexpensive and envi-
ronmentally acceptable reagent and is often considered as a highly
attractive oxidant. The photochemical generation of singlet oxygen
(1O2), a relatively reactive species, from molecular oxygen, has
gained interest in contemporary organic synthesis over the past
decade, 1O2 provides a green and sustainable route to access higher
oxidation state chemicals with high atom efficiency.1e4 From a
green chemistry aspect, the generation of 1O2 using low energy
visible light via photosensitisation with an appropriate catalytic
photosensitizer (e.g. anthracene, xanthones, or porphyrins) is both
simple and efficient. 1O2 reacts with 1,3-dienes through a [4 þ 2]
cycloaddition process to form endoperoxides, which are reactive
Larger scale processes involving 1O2 often require the use of
non-flammable halogenated solvents.12 Supercritical CO2 (scCO2)
or liquid CO2 (liqCO2) are non-flammable alternatives to more
traditional organic solvents, and have many attractive properties,
especially for oxidation reactions as CO2 is fully-oxidized, non-
flammable and non-toxic.13,14 Furthermore, O2 is completely
miscible with the CO2 solvent and photo generated 1O2 has been
shown to have a longer lifetimes in scCO2.15e18 However, the use of
CO2 as a solvent for reactions requires specialized engineering
compared to more conventional batch and flow synthetic ap-
proaches and requires high pressure. Nevertheless CO2 is an
attractive solvent for continuous flow reactions, especially as liquid
and supercritical CO2 have a low viscosities and higher diffusivities
than more conventional solvents and products may easily be
recovered at the end of the experiments by simply reducing the
pressure.19 Continuous flow reactors are highly suited for scaling up
processes as they can be operated almost indefinitely and have
added safety benefits as they minimize the volume of reactants
* Corresponding author.
** Corresponding author. School of Chemistry, University of Nottingham, Univer-
sity Park, Nottingham, NG7 2RD, UK.
E-mail addresses: martyn.poliakoff@nottingham.ac.uk (M. Poliakoff), mike.
george@nottingham.ac.uk (M.W. George).
https://doi.org/10.1016/j.tet.2017.11.048
0040-4020/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
doi.org/10.1016/j.tet.2017.11.048

2
L. Wu et al. / Tetrahedron xxx (2017) 1e6
inside the reactor at any given instant.
of the endoperoxide can be switched to produce four different
compounds 3e6.
We have reported the development of a number of flow tech-
niques for reactions in CO2.20e24 Reactions include acid-catalyzed
alkylation25 and etherification,26 as well as hydrogenation,27
a
2. Results and discussion
process, which was successfully scaled up to 1000 tons/yr.28 We
also pioneered the use of self-optimizing flow reactors in CO2.29e33
We have also demonstrated that appropriate reactor design can
permit switching between the selective production of five different
products from a single feedstock, namely furfural.34 Most relevant
to this paper, we have reported the use of scCO2, as an alternative
solvent, enabling both thermal oxidation35 and photo-
oxidation36e38 to be carried out safely in non-flammable and non-
2.1. High pressure continuous flow apparatus
Initially, we carried out very small-scale batch reactions in a
previously reported high pressure spectroscopic cell45 which
permitted FTIR monitoring and kinetic measurements for the
various reaction. These results indicated that it should be possible
to synthesize 3e6 following the photo-oxidation of 1 in scCO2 using
a scaled-up continuous flow setup, since the key endoperoxide
intermediate 2 was found to have a lifetime of the order of several
minutes, in scCO2 under a range of conditions at temperatures as
high as 40 ꢁC. A schematic diagram of the reactor system for
continuous photoreactions is shown in Fig. 1.
A key engineering challenge for safe operation is to ensure
efficient mixing because the organic substrate is flammable, even if
the CO2 solvent is not itself flammable. It can be seen from Fig. 1
that we premix the O2 and CO2 and pass them through a static
mixer (a stainless steel tube, filled with sand) before they contact
the organic stream containing 1. The combined streams are then
passed through a second static mixer before entering the reactor.
This strategy has previously been successfully used for catalytic
thermal oxidation.35 After the photo-reactor, different arrange-
ments of additional reactors and mixers as well as a quenching
reagent feed can be employed depending on the desired product
(different configurations are shown in Fig. 1 with dashed lines).
toxic solvent. Early work involved the photo-oxidation of a-terpi-
nene with 1O2 in scCO2 which was transferred from a small volume
batch reactor to a larger flow reactor, with excellent space-time
yields. More recently, we reported the photochemical synthesis of
artermisinin from dihydroartemisic acid in a reactor which com-
bined liquid CO2 with a dual-function solid acid/photosensitizer.39
In this paper we show that multiple products can be produced
by switching reactors and reagents applied to the photo-oxidation
of cyclopentadiene (1) in liqCO2 and scCO2 to selectively make one
of four products in a continuous flow system (Scheme 1).
The photochemical oxidation of 1 is a relatively well-known
reaction, proceeding via a [4 þ 2] cycloaddition reaction with 1O2
generating endoperoxide 2.40 This reaction has been widely re-
ported with generation of the endoperoxide 2 after irradiation at
low temperatures i.e. ꢀ20 to ꢀ130 ꢁC.41 However, the isolation of 2
has been reported under conditions where the irradiation and
work-up was carried out at ca. minus 100 ꢁC due to instability of the
endoperoxide 2, which decomposes in to 3 at ca. > minus 30 ꢁC).42
A one pot synthesis of 4 from the photo-oxidation of 1 was
demonstrated using dichloromethane or carbon tetrachloride as
solvents in a biphasic mixture, however the yield was low.43 The
use of a falling-film microreactor for the photo-oxidation of low
concentrations of 1 has been reported; however reduction of the
endoperoxide 2 with thiourea only yielded 20% of 4.44 Here we
report the use of a continuous flow reactor for the photo-oxidation
of cyclopentadiene in CO2 and show how the downstream reactions
2.2. Synthesis and reactions of the cyclopentadiene endoperoxide
(2) in CO2
Initially the photo-oxidation of 1 was carried out in the
continuous flow photo-reactor where complete consumption of the
starting material was obtained over a range of conditions (Table 1).
Under both liquid CO2 and supercritical conditions (Table 1, entry
4e6), the complete conversion of 1 was observed. The products
were a mixture of 2 and 3, but were often obtained in different
ratios due to the unstable nature of 2 above ꢀ30 ꢁC, but the reaction
appeared quantitative, with few side products detected.
In order to ensure that all of the endoperoxide 2 was converted
to the Z-aldehyde 3, a simple thermal reactor was added down-
stream after the photo-reactor. This reactor consisted of 4 ꢂ tubular
reactors in series which were filled with glass beads and heated to
40 ꢁC to promote the thermal isomerization of 2 to 3. The optimal
conditions from the photo-oxidation (Table 1, entry 5) were run
with this additional reactor, which gave quantitative conversion to
the aldehyde 3.
Cleavage of the OeO bond of an endo-peroxide to yield a syn-
diol, which is a attractive motif present in a range of prostaglan-
dins48 is often facilitated using thiourea as a quencher.43,49 In order
to synthesize 4 following the photo-oxidation of 1 in the contin-
uous CO2 photo-reactor, an additional reagent stream was added
downstream of the photo-reactor (See Fig. 1, with organic pump 2
in configuration A, shown in blue). Initially, supercritical conditions
were used (Table 2, entry 1) but the elevated temperature required
for the supercritical conditions resulted in inconsistent yields
throughout the experiments as the endo-peroxide 2 can decom-
pose to 3 at the elevated temperatures before it could react with
thiourea. Lowering the temperature to 0 ꢁC to increase the lifetime
of the endoperoxide 2 (Table 2, entries 2e3) meant operating in
liqCO2, but, under these conditions, a slight increase in the yield of
the diol 4 was observed. Diluting 1 by the addition of a co-solvent
proved to be successful in increasing the yield of 4, in both scCO2
Scheme 1. Compounds of interest from the different treatments after the photo-
oxidation of Cyclopentadiene (1); 2, 3-dioxabicyclo[2.2.1]hept-5-ene (2), 4,5-epoxy-
cis-2-pentenal (3), (1R,3S)-cyclopent-4-ene-1,3-diol (4), 4-hydroxy-2-cyclopentenone
(5), and furfuryl alcohol (6).
Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
doi.org/10.1016/j.tet.2017.11.048

L. Wu et al. / Tetrahedron xxx (2017) 1e6
3
Fig. 1. Simplified schematic of the high pressure continuous flow setup. CO2 is delivered from a chilled CO2 pump and O2 is dosed at a known rate via a Rheodyne dosage unit. Static
Mixer 1 consists of a stainless steel tubular reactor filled with sand and provides mixing for the CO2 and O2. Static Mixer 2, which is also a steel tube filled with sand provides mixing
for the organic feed and the gases. Organic Pump 1 is used to pump the mixture of 1 and photosensitizer. The mixture is then passed in to a high pressure sapphire tube reactor,
which is irradiated by LEDs in an upward flow. The sapphire tube is filled with glass balls (6 mm diameter) to provide mixing. After the Photoreactor is a thermal reactor which
consists of 4 ꢂ tubular reactors (1/400 O.D. 0.04900 wall thickness, 150 mm length) filled with glass beads (710e1180
mm) in series which could be heated with an aluminium heating
jacketed.46,47 A second Organic Pump is added either before (config. A, shown in blue) or after (config. B, shown in red) the thermal reactor depending on the reaction and in some
cases a third Static Mixer is added in (Static Mixer 3, shown in red). (A full diagram of each set-up is shown in the ESI).
Table 1
Optimization of the photo-oxidation of cyclopentadiene in the high pressure continuous flow reactor.
Entry
Solvent
TPFPP (mol-%)
CO2 Flow Rate (mL min-1)
CPD (1) Flow Rate (mL min-1)
O2 Ratio (molar equiv.)
Temp. (ꢁC)
Conv. (%)a
33
-
10
>99
>99
>99
1
2
3
4
scCO2
scCO2
liqCO2
liqCO2
scCO2
MeOH/scCO2
0.002
0.002
0.002
0.008
0.008
0.008
1
1
0.5
0.5
0.25
0.25
0.05
0.3
0.1
0.05
0.05
1
2:1
2:1
0.1:1
2:1
1:1
2:1
40
40
0
0
40
40
b
5
6c
All Experiments were carried out at 120 bar unless stated otherwise.
a
Conversions were calculated by 1H NMR using biphenyl as an internal standard.
At this flow rate, blockages in the system occurred.
1 was pumped as a solution in MeOH (0.1 M).
b
c
Table 2
Optimization of the synthesis of 4 from 1 in the continuous flow reactor.
Entry
Solvent
Co-solvent
TPFPP: CPD (mol%)
CO2 (min$mlꢀ1
)
Flow Rate 1 (min$mlꢀ1
)
Flow Rate thioureaa (min$mlꢀ1
)
T/(ꢁC)
Yieldb 4 (%)
1
2
3
4
5
6
scCO2
liqCO2
liqCO2
scCO2
liqCO2
liqCO2
e
0.002
0.002
0.002
0.008
0.008
0.008
1
1
0.5
1
0.5
0.5
0.2
0.2
0.05
1(0.1 M)
1(0.1 M)
0.1(0.1 M)
1
2
2
1
0.5
0.8
40
0
0
40
0
0
22
33
e
e
30
MeOH
MeOH
THF
60c
94c
80c
a
Thiourea is dissolved in MeOH (1 M).
Yield was calculated by 1H NMR, biphenyl as internal standard.
Full conversion. All experiments were conducted at 120 bar, unless otherwise stated. Entry 1: O2:1 ¼ 1:1; Entry 2 to 6: O2:1 ¼ 2:1.
b
c
Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
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4
L. Wu et al. / Tetrahedron xxx (2017) 1e6
Initially, photosensitizers supported on solid acid catalysts were
investigated as this approach had proved successful before.39 In the
reaction with 1, the photosensitizers quickly bleached when using
TPP supported on Amberlyst-15, because when the photo-reactor
was irradiated, the outer layer of the catalyst bleached rapidly
and yielded only a trace of the product. A longer path reactor would
be required to increase the residence time, or photo-reactor that
provides more mixing.51,52 A tubular reactor filled with a hetero-
geneous acid catalyst was added downstream of the thermal
reactor; this set-up enabled complete conversion of 1 to 3 before
reacting with the acid catalyst (see Fig. 1, organic pump 2 and static
mixer 3 in configuration B, shown in red). Amberlyst-15 gave a poor
yield for 6, whilst switching to the more acidic niobium phosphate
(NbOPO4) and oxide (NbO5) gave minor improvements in yield (see
Table 3). Exchange of the solid acid catalyst reactor with a tubular
Scheme 2. Treatment of endo-peroxides with base and the resultant product 5.
Table 3
Production of 6 from CPD in the continuous flow reactor.
Scheme 3. Reaction of 2 and 3 under acidic conditions and proposed intermediate for
formation of 6.
and liqCO2 (Table 2, entries 4e6). Using THF as a solvent proved
slightly detrimental compared to MeOH as the lower solubility of
thiourea occasionally led to blockages.
Treatment of endo-peroxides with a base leads to the formation
of a a,b-unsaturated carbonyl moiety containing an attached hy-
a
Entry
Solvent
OP 1 FR (mL$minꢀ1
)
Acid Catalystb
Yield of 6c
1
2
scCO2
scCO2
scCO2
scCO2
0.05
0.05
0.05
0.05
Amberlyst-15
NbOPO4
NbO5
24%
34%
30%
85%
3
4d
TFA
droxyl group (Scheme 2). This type of reaction has been demon-
strated enantioselectively using cobalt complexes,50 and more
recently with chiral cinchona alkaloid derived organocatalysts.8,10
Initial treatment of 2 with a strong base such as NaOH yielded
mainly a polymeric material, which is unsurprising given that the
product is a Michael acceptor with a nucleophilic hydroxyl group.
To prevent decomposition of 5 by the basic catalyst, we employed a
heterogeneous catalyst Amberlite (IR 120 Naþ form), which con-
tains a mildly basic sulfonate group. Also having the catalyst
immobilized prevents further reaction of 5. Whilst this catalyst is
often used as an ion exchange resin, it showed moderate activity for
this transformation. However when just liqCO2 was used as the
solvent, the system quickly blocked due to the build-up of poly-
meric material on the catalyst bed, which was insoluble in liqCO2.
Repeating the experiments using a polar co-solvent (methanol)
cleared the build-up of the polymeric material, but only a modest
yield of only 18% of 5 was obtained after the reactor. Switching to a
homogenous base, triethylamine (NEt3), gave more positive results.
Triethylamine was pumped in downstream of the photo-reactor
(see Fig. 1, organic pump 2 in configuration A, shown in blue) and
was passed through a reactor at ambient temperature (Fig. 1, static
a
FR: Flow Rate.
b
c
Solid catalysts were loaded with 1 g in R3 (Fig. 1) before starting reaction.
Yields were calculated from 1H NMR with mesitylene as internal standard.
Liquid TFA (50 mM) in PhMe was pumped by organic pump 2 at a flow rate of
d
0.5 mL minꢀ1. Other conditions: system pressure: 120 Bar, reaction temperature:
40 ꢁC, TPFPP (0.008 mol % of CPD), the ratio of O2 to CPD was 1:1, scCO2 flow rate was
0.25 mL minꢀ1, unless otherwise stated.
reactor filled with glass beads for mixing, allowed switching to a
homogeneous acid and the addition of a trifluoroacetic acid (TFA)
feed gave an excellent yield of the furfuryl alcohol 6, 85%. In some
cases prolonged exposure to TFA led to further reaction of 6 to form
the trifluoroacetate ester of 6 (see Table 3, entries 3-4).
3. Conclusion
We have demonstrated that four compounds, (1R,3S)-cyclo-
pent-4-ene-1,3-diol 4, 4-hydroxy-2-cyclopentenone 5, 5-epoxy-cis-
2-pentenal 3 and furfuryl alcohol 6, can be synthesised from a
single feedstock, cyclopentadiene 1 in a high pressure continuous
flow reactor. The kinetics of the photo-oxidation of 1 and the
thermal isomerization of endoperioxide 2 were probed by FT-IR
and NMR, respectively. Both high yields and productivities are
achieved by merely tuning the quenching reagents and reaction
temperatures (see Fig. 2).
mixer 3) that was filled with glass beads (710e1180 mm). A con-
centration of 0.8 M NEt3 in MeOH was found to be optimal giving
the highest yield of 5, 70%, when the base was delivered at
0.5 mL minꢀ1. Increasing the flow rate, and hence the concentration
of base, led to a reduction in yield of 5, possibly as the result of a
combination of decreasing the residence time in reactor and a
higher rate of polymerization of the product. (See ESI).
4. Experimental
We investigated the reaction using an acid catalyst instead of a
basic catalyst. In this case we found that little reaction took place;
however the Z-aldehyde 3, produced from the thermal decompo-
sition of 2, reacted instead. Furfuryl alcohol 6 was detected as the
major product in the initial batch experiments, presumably this
was formed via an acid-catalyzed opening of the epoxide by the
aldehyde, which undergoes subsequent rearrangement to yield 6
(Scheme 3).
CAUTION! The experiments described in this report involve the
use of relatively high pressures and require equipment with the
appropriate pressure rating and with due regard to the potentially
explosive reaction between O2 and organic compounds. A simpli-
fied schematic of the continuous flow reactor is shown in Fig. 1 and
the full diagram is provided in the ESI. TPFPP, thiourea, Amberlyst-
15, triethylamine (NEt3), trifluoroacetic acid (TFA) were purchased
Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
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L. Wu et al. / Tetrahedron xxx (2017) 1e6
5
Fig. 2. A summary of the conditions, yields and productivities for the four reaction sequences.
from Sigma Aldrich and used as received. Dicyclopentadiene was
purchased from Sigma Aldrich and distilled using standard distil-
lation setup. Pure cyclopentadiene was stored at ꢀ20 ꢁC and used
for no more than a week before re-distilling, it was stored in an ice
bath during operation of the continuous flow reactor. O2 and CO2
were obtained from BOC gases. NMR spectra were acquired on a
Bruker DPX300, AV400 and AVIII400 spectrometers. The chemical
shift reported are relative to CDCl3 (7.26 ppm). Biphenyl or mesi-
tylene were used as internal standard for quantification. IR spectra
were recorded on an Avatar 320 FT-IR spectrometer. The high
pressure continuous set-up (Fig.1) employed in described reactions
consists of a Jasco™ PU-1580-CO2 pump, Jasco™ PU-980 HPLC
pumps, a high pressure sapphire tube reactor (Saint- Gobain
Crystals, 10 mm o.d., 1 mm wall thickness, 120 mm long) and a
Jasco™ BP-1580-81 back pressure regulator (BPR). Irradiated was
provided by LED lights (3 banks of 5 ꢂ Citizen Electronics Co. Ltd
1000 Lumen chips). The reactors consisted of stainless tubes (1/400
tube, 1.83 mL volume) and were packed with sand, glass beads or
the appropriate catalyst. The pipelines were 1/1600 stainless steel
tubing.
0.9 Hz, 2H), 2.74 (dt, J ¼ 14.3, 7.1 Hz, 1H), 1.57 (dt, J ¼ 14.3, 3.7 Hz,
1H); 13C NMR (101 MHz, CDCl3)
agreement with the literature.43
d 136. 7, 75.2, 44.0. The data is
4.4. 4-Hydroxy-2-cyclopentenone (5)
The mixture after BPR was neutralized immediately by 1 M HCl
then was extracted with diethyl ether, dried over MgSO4 and
evaporated. Compound 4 could be further purified by column
chromatography on silica, using gradient eluent of cyclohexane/
EtOAc (6:4 to 4:6). 1H NMR (400 MHz, CDCl3)
d
7.57 (dd, J ¼ 5.6,
2.3 Hz, 1H), 6.26e6.17 (dd, J ¼ 1.3, 5.6 Hz 1H), 5.06 (m, 1H), 2.78 (dd,
J ¼ 18.5, 6.1, 1H), 2.28 (dd, J ¼ 18.5, 2.3 Hz, 1H). 13C NMR (100 MHz,
CDCl3)
d 206.3, 162.8, 135.4, 70.5, 44.3. The data is agreement with
commercially obtained samples.
4.5. Furfuryl alcohol (6)
The mixture after BPR was neutralized immediately into a basic
quench and was then extracted with diethyl ether, dried over
MgSO4 and evaporated. 1H NMR (400 MHz, CDCl3)
d 7.40 (dd,
4.1. 2,3-dioxabicyclo[2.2.1]hept-5-ene (2)
J ¼ 1.9, 0.9 Hz, 1H), 6.34 (dd, J ¼ 3.2, 1.9 Hz, 1H), 6.29 (dd, J ¼ 3.2,
0.7 Hz, 1H), 4.61 (d, J ¼ 5.0 Hz, 2H). The data is agreement with
commercially obtained samples.
1H NMR (300 MHz, CDCl3)
d
¼ 6.47 (t, J ¼ 2.1 Hz, 2H), 5.27 (ddd,
J ¼ 3.6, 2.0 Hz, 2H), 2.10 (t, J ¼ 1.7 Hz, 2H).
Acknowledgments
4.2. 5 -epoxy-Z-2-pentenal (3)
We thank EPSRC grant (EP/P013341/1) supporting this work and
the University of Nottingham Vice-Chancellor's Scholarship award
for funding the studentship for L.W. We also thank M. Dellar,
M.Guyler, D. Lichfield, R. Meehan and R. Wilson for technical sup-
port at the University of Nottingham. L.W. would also like to thank
Dr. Jie Ke for helpful discussions.
1H NMR (300 MHz, CDCl3)
d
¼ 10.17 (d, J ¼ 6.6 Hz,1H), 6.24e6.05
(m, 1H), 4.23e4.11 (m, 1H), 3.16 (dd, J ¼ 5.4, 4.2 Hz, 1H), 2.77 (dd,
J ¼ 5.4, 2.5 Hz, 1H). The data is agreement with the literature.53
4.3. (1R, 3S)-cyclopent-4-ene-1,3-diol (4)
Could be further purified by column chromatography using
Appendix A. Supplementary data
neturalized Al2O3 and CH2Cl2/MeOH (8:2) as the eluent.1H NMR
(300 MHz, CDCl3)
d
¼ 6.03 (d, J ¼ 0.9 Hz, 2H), 4.69 (ddd, J ¼ 7.1, 3.7,
Supplementary data related to this article can be found at
Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
doi.org/10.1016/j.tet.2017.11.048

6
L. Wu et al. / Tetrahedron xxx (2017) 1e6
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Please cite this article in press as: Wu L, et al., Combining engineering and chemistry for the selective continuous production of four different
oxygenated compounds by photo-oxidation of cyclopentadiene using liquid and supercritical CO2 as solvents, Tetrahedron (2017), https://
doi.org/10.1016/j.tet.2017.11.048