Continuous-Flow Preparation of γ-Butyrolactone Scaffolds from Renewable Fumaric and Itaconic Acids under Photosensitized Conditions

Article abstract of DOI:10.1021/acs.oprd.7b00314

The method and results described herein concern the photosensitized addition of various alcohols to renewable platform fumaric and itaconic acids under scalable continuous-flow conditions in glass micro- and mesofluidic reactors. Alcohols were used both as reagents and as solvents, thus contributing to a reduced environmental footprint. Process parameters such as the temperature, light intensity, and the nature as well as amount of the photosensitizer were assessed under microfluidic conditions and, next, transposed to a lab-scale mesofluidic reactor connected with an in-line NMR spectrometer for real-time reaction monitoring. Substituted γ-butyrolactones, including spiro derivatives with unique structural features, were obtained with quantitative conversion of the starting materials and in 47-76% isolated yields. The model photoaddition of isopropanol to fumaric acid was next successfully transposed in a pilot-scale continuous-flow photoreactor to further demonstrate scalability.

Full text of DOI:10.1021/acs.oprd.7b00314

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Full Paper
Continuous-flow preparation of #-butyrolactone scaffolds from
renewable fumaric and itaconic acids under photosensitized conditions
Romaric Gérardy, Marc Winter, Clemens R Horn, Alessandra
Vizza, Kristof Van Hecke, and Jean-Christophe M Monbaliu
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00314 • Publication Date (Web): 21 Nov 2017
Downloaded from http://pubs.acs.org on November 21, 2017
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Continuousꢀflow preparation of γꢀbutyrolactone
scaffolds from renewable fumaric and itaconic acids
under photosensitized conditions
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Romaric Gérardy,† Marc Winter,‡ Clemens R. Horn,‡ Alessandra Vizza,‡ Kristof Van Hecke§ and
JeanꢀChristophe M. Monbaliu*,†
† Center for Integrated Technology and Organic Synthesis, Department of Chemistry, University
of Liège, Bꢀ4000 Liège (Sart Tilman), Belgium
‡ Corning Reactor Technologies, Corning SAS, 7 bis Avenue de Valvins, CS 70156 Samois sur
Seine, 77215 Avon Cedex, France
§
XStruct, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281ꢀ
S3, Bꢀ9000 Ghent, Belgium
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KEYWORDS
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Platform molecules, Photochemistry, Continuousꢀflow chemistry, Metalꢀfree generation of
radicals, Paraconic acids, Inꢀline NMR monitoring
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ABSTRACT
The method and results described herein concern the photosensitized addition of various alcohols
to renewable platform fumaric and itaconic acids under scalable continuousꢀflow conditions in
glass microꢀ and mesofluidic reactors. Alcohols were used both as reagents and as solvents, thus
contributing to a reduced environmental footprint. Process parameters such as the temperature,
light intensity, as well as the nature and amount of the photosensitizer were assessed under
microfluidic conditions, and next transposed to a labꢀscale mesofluidic reactor connected with an
inꢀline NMR spectrometer for realꢀtime reaction monitoring. Substituted γꢀbutyrolactones,
including spiro derivatives with unique structural features, were obtained with quantitative
conversion of the starting materials and in 47ꢀ76% isolated yields. The model photoaddition of
isopropanol to fumaric acid was next successfully transposed in a pilotꢀscale continuousꢀflow
photoreactor to further demonstrate scalability.
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INTRODUCTION
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Carboxylic acids are abundant in Nature, and have been widely utilized as renewable and
sustainable building blocks.1ꢀ4 Among them, fumaric (1) and itaconic (2) acids are listed in the
top 12 valueꢀadded chemicals from biomass by the U.S. Department of Energy.5 Although 1 is
still manufactured from fossil resources, fumaric acid can also be synthesized by fungi through
fermentation of sugars.6 By contrast, 2 is already produced on the industrial scale using a mature
biotechnology based on fungi to transform carbohydrates.6 Fumaric and itaconic acids have thus
a bright forecast as sustainable chemical building blocks.5 The presence of an alkene moiety in
1,2 is an asset for designing sequences of reactions towards more complex molecular
architectures, such as reactions with carbon radicals.
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Photochemistry can be advantageously exploited for metalꢀfree radical generation.7 For
instance, terebic acid (3a) was synthesized by Schenck and colleagues using a photochemical
addition of isopropanolꢀderived radicals to maleic or fumaric acid, followed by in situ
lactonization in a conventional macroscopic batch reactor. In the report, irradiation of the
medium over 18 h was necessary to obtain 4.56 g of 3a, and benzophenone was utilized as a
photosensitizer (PS).8 The scope of the methodology was later broadened to other alcohols, and
the corresponding radicals were generated by thermal or photolytic treatment of peroxides,
giving various γꢀbutyrolactone scaffolds, i.e. paraconic acids 3a-c or the corresponding esters
(Scheme 1).9ꢀ11 Phenylglyoxylic acid was also recently reported as an effective PS for the
preparation of 3a-c benzyl esters in batch.12 Paraconic acids are synthetic or naturally occurring,
and many of them are endowed with promising biological activities.13ꢀ14 Compounds 3a-c are
nowadays commercially available, and are reported as building blocks for the preparation of
biologically relevant molecules.15ꢀ16
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Scheme 1. Addition of alcohol radicals to maleic or fumaric acid.
Most batch photochemical processes are limited to small scale as a consequence of
superficial light penetration. Long irradiation times are often required, hence triggering sideꢀ
reactions due to overexposure. The combination of photochemistry and continuousꢀflow microꢀ
and mesofluidic reactors is a blooming research area since it enables efficient and scalable
photochemical processes.17ꢀ18 The acetoneꢀphotosensitized addition of isopropanol (iPrOH) to
maleic acid was reported in continuous microfluidic setups, as well as in a batch reactor, but only
the influence of the residence time was studied, and the scalability was not addressed.19 The most
performant microflow reactor (800 µm internal diameter) sustained a low productivity for 3a
(1.5 g dꢀ1), but emphasized the superiority of continuousꢀflow reactors for such photochemical
processes.19 The scope was later extended to the photoaddition of iPrOH to furanones.20ꢀ23
We report herein on the development of a scalable continuousꢀflow photosensitized addition
of alcohols to renewable platform molecules such as fumaric and itaconic acids. Substituted γꢀ
butyrolactones, including paraconic acids with unique structural features, were prepared
accordingly in commercial glass microꢀ and mesofluidic reactors. Fast optimization of the
process parameters was ensured by inꢀline NMR spectroscopy. Screening of the reaction
parameters revealed a positive impact of an increase in temperature, as well as a moderate effect
of light intensity. Under optimized conditions, γꢀbutyrolactones were prepared on an
unprecedented scale (up to ~12 g dayꢀ1) with short residence time (10 min) and moderate to high
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isolated yields (47ꢀ76%) in a labꢀscale mesofluidic reactor. The model photoaddition of iPrOH
onto fumaric acid (1) was eventually translated in a pilotꢀscale glass mesofluidic reactor, which
sustained a productivity of 83 g dayꢀ1. This work is part of our ongoing research program
dedicated to the continuousꢀflow upgrading of biomassꢀderived platform molecules24 and to the
preparation of valuable scaffolds under photosensitized continuousꢀflow conditions.25
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RESULTS AND DISCUSSION
Assessing feasibility. This investigation started with a preliminary study using a commercial
glass microfluidic photoreactor (FutureChemistry® FlowStart EvoTM, 92 ꢁL internal volume)
equipped with a photochemical module comprising 2 LEDs (365 nm) mounted on a
thermoregulated support (Supporting Information). The photoaddition of iPrOH onto fumaric
acid (1) under irradiation at 365 nm was selected as a model reaction, and process parameters
including the residence (irradiation) time, the concentration in 1 ([1]) and in PS as well as the
temperature (Supporting Information for details) were assessed. Promising results were
collected, and the process was translated to a commercial glass mesofluidic photoreactor
(Corning® AdvancedꢀFlow Lab Photo ReactorTM, Figure 1).
Figure 1. a. Photograph of a labꢀscale Corning® AdvancedꢀFlow Lab Photo ReactorTM. b.
Details of the hub hosting the compact glass mesofluidic module integrated with a high capacity
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heatꢀexchanger and the thermoregulated LED panels (2 x 20 × 365 nm LEDs). c. Side view of
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the glass module upon irradiation at 365 nm. Corning proprietary.
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The increase in scale facilitated the integration of inꢀline NMR spectroscopy, which in turn
drastically accelerated the optimization of the photoaddition under continuousꢀflow conditions.
A thorough screening of process parameters such as the temperature, residence (irradiation) time,
light intensity, as well as the nature of the PS and the substrate/PS ratio was performed, and
dozens of experimental data points were generated within one day. The reactor configuration
included a thermoregulated glass fluidic module (2.6 mL internal volume) sandwiched between
two 365 nm LED panels (Figure 2, see also Experimental Section and Supporting Information).
A domeꢀtype backꢀpressure regulator (Zaiput Flow Technologies®) was inserted after the reactor
(pressure set at 2 bar), and the reactor effluent was redirected towards a benchꢀtop inꢀline NMR
spectrometer (43 MHz SpinsolveTM Carbon NMR spectrometer from Magritek®).
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O
O
O
O
O
O
3b
3a
3c
1 or 2
in alcohol
+ PS
2 bar
CO2H
CO2H
CO2H
O
NMR
analysis
O
O
O
4a
O
4c
O
4b
HO2C
HO2C
HO2C
O
CO2H
HO2C
CO2H
HO2C
R
R
2
PS
1
Figure 2. Simplified flow chart for the continuousꢀflow preparation of γꢀbutyrolactones 3,4a-c
through a photosensitized addition of alcohols to renewable platform fumaric (1) and itaconic
acids (2). PS stands for photosensitizer (R = H, benzophenone; R = OMe, 4,4’ꢀ
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dimethoxybenzophenone). Compounds 3(4)a, 3(4)b and 3(4)c were obtained in iPrOH,
CyOH/MeOH (9:1) and MeOH, respectively, from 1(2). Heat exchangers are omitted for clarity.
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A solution containing both 1 and the PS in iPrOH was subjected to irradiation at 365 nm in
the continuousꢀflow setup. Various residence (irradiation) times ranging from 1 to 20 min were
screened using 0.2 equiv. of either dimethoxybenzophenone (DMBP) or benzophenone (BP) as
PS (Figure 3). The residence time was adjusted by changing the set point of the HPLC pump.
Faster reactions and higher conversions were observed for the photoaddition of iPrOH on 1 (0.05
M) with DMBP as PS than with BP, in agreement with previous literature reports.23 Despite these
promising results, the lower solubility of DMBP affected negatively the productivity of the
continuousꢀflow process since it imposed low concentrations of 1. The higher solubility of BP in
iPrOH (up to 0.04 M) enabled more concentrated feed solutions (up to 0.2 M for 1). With a low
concentration feed solution (0.05 M in 1) and with 0.2 equiv. in BP, the photoaddition of iPrOH
on 1 plateaued at 67% for the longest residence time (20 min). Upon increasing the concentration
in 1 to the limit of solubility in iPrOH (0.2 M) and keeping the BP/1 ratio at 0.2, the conversion
increased with the residence time, and reached 88% after 20 min irradiation. BP can undergo
photoreduction and photopinacolization in the presence of a hydrogen donor such as iPrOH,
leading to its deactivation.26 It was suggested that in the case of diluted solutions, the larger
amount of iPrOH induced PS deactivation to a greater extent. A 0.2 M solution of 1 in iPrOH
containing benzophenone as a photosensitizer was thus selected for further optimization.
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0.2 M substrate, BP
0.05 M substrate, DMBP
0.05 M substrate, BP
0
5
10
Residence time (min)
15
20
Figure 3. Conversion of 1 as a function of the residence time (labꢀscale experiments performed
in a Corning® AdvancedꢀFlow Lab Photo ReactorTM). Conditions: fumaric acid (1) + 0.2 equiv.
of PS in iPrOH, temperature = 25 °C, LED power (365 nm) = 100%. BP = benzophenone,
DMBP = dimethoxybenzophenone.
The influence of the temperature, light intensity and substrate/PS ratio were studied at a
residence time of 10 min (Table 1). The conversion was improved from 35% at 25 °C to 69% at
55 °C, and plateaued at 68% at higher temperature (entries 1ꢀ4). A similar positive impact of
temperature for a photosensitized electronꢀtransfer deoxygenation reaction27 and for an analog
photosensitized addition of iPrOH to furanones were previously noticed.28 As the latter did not
involve a lactonization step, the radical addition is most likely the rate limiting step.
Temperatureꢀdependent rates were also reported for the addition of carbon radicals to alkenes
and alkynes.29ꢀ30 Decreasing the LED intensity to 60% had a minimal impact on the conversion,
but it dropped from 35% to 22% conversion when the intensity was further diminished to 40%
(entries 1, 5ꢀ7). The BP/1 ratio had a dramatic impact on the conversion: it increased from 12%
with a BP/1 ratio of 0.05 to 78% with a BP/1 ratio of 0.4 (entries 1, 8ꢀ11).
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Table 1. Conversion of 1 as function of the temperature, LED power and PS amount (labꢀscale
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experiments performed in a Corning® AdvancedꢀFlow Lab Photo ReactorTM).
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Entry
Temperature
LED
Power (%)
100
Benzophenone
Conv.
(%)
35
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(°C)
25
40
55
70
25
25
25
25
25
25
25
(equiv.)
0.2
1
2
100
0.2
54
3
100
0.2
69
4
100
0.2
68
5
80
0.2
36
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60
0.2
35
7
40
0.2
22
8
100
0.05
0.1
12
9
100
18
10
11
100
0.3
49
100
0.4
78
Conditions: 0.2 M fumaric acid + BP in iPrOH, res. time = 10 min.
Substrate scope. This set of results clearly emphasized the positive impact of increasing both
the temperature and the BP/1 ratio. The scope of this continuousꢀflow procedure was next
broadened to the addition of iPrOH, cyclohexanol (CyOH) and methanol (MeOH) to fumaric (1)
and itaconic (2) acids (Figure 2). To keep the process as efficient as possible, the conditions were
further optimized to reach maximal conversion while balancing temperature and reaction time.
Actually, the BP/1 ratio appeared as a powerful leverage to reach quantitative conversion under
mild process conditions (T = 40 °C, res. time = 10 min). The most representative results are
summarized in Table 2 (Supporting Information for full details of the optimization).
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The addition of iPrOH and CyOH to 1 required 0.4 equivalent of BP to reach quantitative
conversion. To avoid crystallization of CyOH and to improve the solubility of the substrate, a 9:1
v/v mixture of CyOH/MeOH was utilized as solvent in the CyOH feed. Terebic acid (3a) and its
spiro derivative 3b were obtained in 76% and 75% isolated yields, respectively (entries 1ꢀ2). The
addition of MeOH was more challenging:11,12 a larger excess of BP (3 equiv.) gave only 64%
conversion. At higher temperature (55 °C) and longer irradiation time (20 min), quantitative
reaction of 1 with MeOH was observed, and the simplest paraconic acid 3c was isolated in 47%
yield (entry 4). Alternatively, some of these compounds could be purified by recrystallization.
Quantitative conversion of compounds 1 and 2 was obtained for the photoaddition of MeOH,
iPrOH and CyOH (entries 1,2,4,6, and 10, Table 1), in which case only the desired γꢀ
butyrolactones were formed. Material loss occurred upon chromatography on silica gel.
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Table 2. Optimization of isopropanol, cyclohexanol and methanol photoaddition to fumaric and
itaconic acids (labꢀscale experiments performed in a Corning® AdvancedꢀFlow Lab Photo
ReactorTM).
Entry
Substrate
Solvent Benzophenone Product
(equiv.)
Conv.a
(%)
>99
>99
64
Yieldb Productivity
(%)
76
75
ꢀ
(g dayꢀ1)
1
2c
3
iPrOH
CyOH
MeOH
MeOH
iPrOH
iPrOH
CyOH
CyOH
0.4
0.4
3
9.7
11.6
ꢀ
1
1
1
1
2
2
2
2
3a
3b
3c
3c
4a
4a
4b
4b
4d
3
>99
64
47
ꢀ
2.6
ꢀ
5
0.4
0.7
0.4
0.5
6
>99
85
52
ꢀ
7.2
ꢀ
7c
8c
95
72
11.5
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9d
MeOH
MeOH
3
clogging
>99
ꢀ
ꢀ
ꢀ
3
4
5
6
2
2
4c
4c
10d,e
3.5
ꢀf
7
8
Conditions: 0.2 M substrate + BP in alcohol, res. time = 10 min, T = 40 °C, LED power = 100%.
9
a
1
b
c
Offꢀline H NMR. Isolated yield after purification by column chromatography. The solvent
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was a 9:1 v/v mix of CyOH/MeOH. d Res. time = 20 min, T = 55 °C. e 0.1 M substrate. f Traces
of compound 4c in the crude, no purification attempted.
Scheme 2. Photosensitized addition of alcohols to itaconic acid.
The unprecedented radical addition of alcohols to 2 potentially leads to two plausible
products, after either 5-exo-trig or 6-exo-trig lactonization (Scheme 2). Under these conditions,
only γꢀbutyrolactones 4a-b were detected in the crude NMR spectra, in accordance with Brown’s
1
postulate.31 Previous literature H NMR data for 4a did not match those reported in the present
study.32 The structural features of compounds 4a-b were unambiguously confirmed by 2D NMR
experiments, and by Xꢀray diffraction on a single crystal (Figure 4 and Supporting Information).
The photoaddition of alcohols to 2 required an increased amount of BP, as expected from its
lower electrophilicity.9 With a BP/2 ratio of 0.4 to mediate the photoaddition of iPrOH, only 64%
conversion was reached (entry 5). With a BP/2 ratio of 0.7, quantitative conversion was
observed, and the corresponding product 4a was isolated in 52% yield (entry 6). Similarly, the
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photoaddition of CyOH onto 2 required a BP/2 ratio of 0.5 to proceed with near quantitative
conversion, and the desired compound 4b was isolated in 72% yield (entries 7ꢀ8). CyOH
appeared thus slightly more reactive than iPrOH for the photoaddition onto 2. The addition of
MeOH was attempted with the same conditions as used in entry 4. However, clogging occurred
downstream the reactor (entry 9). The concentration of substrate 2 was reduced to 0.1 M, and
quantitative conversion was reached at larger BP/2 ratio of 3.5. However, a complex mixture was
obtained and only traces of compound 4c were observed in the crude. No further purification was
attempted (entry 10). Experiments were also conducted using cinnamic acid as substrate in an
iPrOH feedstock, but low conversion (<5%) was obtained in each cases, emphasizing its poor
electrophilicity as substrate.
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Figure 4. Single crystal XꢀRay diffraction structure of 4a.
Scaling-out. Finally, the model photoaddition of iPrOH onto 1 was implemented in a pilotꢀscale
mesofluidic photoreactor (41 mL internal volume, Corning® AdvancedꢀFlow G1 Photo
ReactorTM, see Figure 5) (Supporting Information). The reactor configuration included 5
thermoregulated glass fluidic modules (8.2 mL internal volume) fluidically connected in series.
The G1 photo reactor used for assessing the scalability was not fully optimized, and the LED
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density was decreased by 53% compared to the Lab Photo Reactor utilized in the preliminary
labꢀscale trials. Each fluidic module was irradiated on both sides by a thermoregulated LED
panel (365 nm, 30 LEDs per panel, 300 LEDs in total). With such design, the conditions are
comparable in terms of light intensity to an experiment run in an AdvancedꢀFlow Lab Photo
ReactorTM (see labꢀscale experiments above) at 47% of LED power.
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Figure 5. a. Pilotꢀscale Corning® AdvancedꢀFlow G1 Photo ReactorTM composed of 5
thermoregulated glass fluidic modules connected in series. b. Details of the glass fluidic modules
(left) (8.2 mL internal volume) that are surrounded by 2 thermoregulated LED panels (30
LEDs@365 nm per panel, right). Corning proprietary.
Similar processing conditions were used as for the optimized reaction in the 2.6 mL
mesofluidic reactor (Table 2), and 3a was obtained accordingly with a daily productivity of 83 g
(60% conversion, 10 min residence time), demonstrating the scalability of the photosensitized
addition of alcohols to fumaric and itaconic acids. Based on the findings at lab and pilot level, an
optimized production scale photo reactor (Advancedꢀflow G3 Photo ReactorTM) would have an
expected daily productivity of 547 g.
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CONCLUSION
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This work reports an efficient photosensitized addition of isopropanol, cyclohexanol and
methanol to renewable fumaric and itaconic acids under scalable continuousꢀflow conditions.
The implementation of inꢀline NMR spectroscopy enabled fast optimization of the reaction
parameters. The positive impact of both the temperature and the substrate/photosensitizer ratio
was demonstrated on the model photoaddition of isopropanol to fumaric acid. Upon
optimization, various γꢀbutyrolactones, including novel spiro derivatives, were prepared within
short residence times and in 47ꢀ76% isolated yields. This continuousꢀflow procedure is amenable
to the preparation of libraries of structurally unique building blocks, including spiro derivatives
and, most importantly, the method is scalable.
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EXPERIMENTAL SECTION
Continuous-flow setup.
The feed solution was conveyed to the photoreactor with a FLOM HPLC pump (0.01 – 100 mL
minꢀ1; wettedꢀparts: PTFE, PCTFE, FFKM and ruby) through a section of 1/8” PFA tubing
(Swagelok®). The feed solution was installed on a precision scale for accurate flow rate
monitoring. The reactions were conducted in a commercial continuousꢀflow reactor (Corning®
AdvancedꢀFlow™ Lab Photo Reactor). The photoreactor included a compact glass mesofluidic
module (0.4 mm channel height, 2.6 mL internal volume) sandwiched in a high capacity heatꢀ
exchanger (2 layers, 22 mL, 1 W mLꢀ1 Kꢀ1). LED panels were mounted on both sides of the
fluidic module (40 mm from the center of the reactive layer), and each LED panel was equipped
with 20 × 365 nm LEDs and a heat exchanger (T = 10 °C). The thermoregulation of both the
glass fluidic module and the LED panels was carried out with LAUDA® Proline RP 845
thermostats. The reactor effluents were conveyed through PFA capillaries (1/8” O.D.) towards an
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inline 43 MHz Spinsolve™ Carbon NMR spectrometer from Magritek® equipped with the flowꢀ
through module, and collected. A Zaiput Flow Technologies® domeꢀtype backꢀpressure regulator
(BPR) was inserted between the photoreactor and the inꢀline NMR spectrometer and connected
to a cylinder of compressed argon (set point: 2 bar).
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Typical experimental procedure (lab mesoscale). A solution of the substrate (1 or 2, 0.2 M)
and the appropriate amount of benzophenone in 250 mL of an alcohol (iPrOH, CyOH or MeOH)
was prepared under sonication, and degassed with argon for 15 min. For the cyclohexanol
solution, a 9:1 v/v mixture of CyOH/MeOH was used as solvent. The feed solution was loaded in
a brown glass feed tank and kept under an argon atmosphere. The solution was delivered to the
glass mesofluidic reactor under irradiation at 365 nm through a HPLC pump set at the
appropriate flow rate, and the reaction was monitored by inꢀline NMR spectroscopy (Supporting
Information). After equilibration and collection, the solvent was removed under reduced pressure
and the sample was analyzed by offꢀline high field NMR. Analytical samples were obtained after
column chromatography on silica gel (ethyl acetate/petroleum spirit 40ꢀ60/acetic acid 10:10:1).
ASSOCIATED CONTENT
Supporting Information.
The following files are available free of charge. Details of the equipment, microfluidic and
mesofluidic (pilot scale) procedures, additional experimental data, structural assignments (NMR
and Xꢀray diffraction) and characterization of compounds 3a-c and 4a,b. (PDF).
Accession Codes
CCDC 1554500 contains the supplementary crystallographic data for this paper and can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
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Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44ꢀ1223ꢀ
336033; or deposit@ccdc.cam.ac.uk).
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AUTHOR INFORMATION
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Corresponding Author
* jc.monbaliu@ulg.ac.be (JeanꢀChristophe M. Monbaliu)
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources
This work was supported by the University of Liège (Welcome Grant WGꢀ13/03, JCMM). KVH
thanks the Hercules Foundation (project AUGE/11/029 “3DꢀSPACE: 3D Structural Platform
Aiming for Chemical Excellence”) and the Research Foundation ꢀ Flanders (FWO) for funding.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors gratefully acknowledge Dr. Cédric Malherbe (University of Liège) for the FTIR
spectra and Prof. C. Damblon (University of Liège) for the NMR data.
ABBREVIATIONS
NMR, nuclear magnetic resonance; PS, photosensitizer; BP, benzophenone; DMBP,
dimethoxybenzophenone; LED, light emitting diode; CyOH, cyclohexanol; MeOH, methanol;
iPrOH, isopropanol; BPR, back pressure regulator
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