g-C3N4/metal halide perovskite composites as photocatalysts for singlet oxygen generation processes for the preparation of various oxidized synthons

Article abstract of DOI:10.1039/d0cy02352c

g-C₃N₄/metal halide perovskite composites were prepared and used for the first time as photocatalysts forin situ¹O₂generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The standardized methodology was made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated and the reactions afforded desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed, especially in the case of the alkene oxidations, and poor chemoselectivity was somewhere observed in this work which is the first application of MHP-based composites forin situ¹O₂generation. The experimental protocol can be used as a platform to further expand the knowledge and applicability of MHPs to organic reactions, since perovskites offer a rich variety of tuning strategies which may be explored to improve reaction yields and selectivities.

Full text of DOI:10.1039/d0cy02352c

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g-C3N4/metal halide perovskite composites as
photocatalysts for singlet oxygen generation
processes for the preparation of various oxidized
synthons†
Cite this: Catal. Sci. Technol., 2021,
11, 2292
Marco Corti,a Rossella Chiara,a Lidia Romani,a Barbara Mannucci,b
a
a
*
*
Lorenzo Malavasi and Paolo Quadrelli
g-C3N4/metal halide perovskite composites were prepared and used for the first time as photocatalysts for
1
in situ O2 generation to perform hetero Diels–Alder, ene and oxidation reactions with suitable dienes and
alkenes. The standardized methodology was made applicable to a variety of olefinic substrates. The scope
of the method is finely illustrated and the reactions afforded desymmetrized hydroxy-ketone derivatives,
unsaturated ketones and epoxides. Some limitations were also observed, especially in the case of the
alkene oxidations, and poor chemoselectivity was somewhere observed in this work which is the first
Received 7th December 2020,
Accepted 15th January 2021
1
application of MHP-based composites for in situ O2 generation. The experimental protocol can be used
as a platform to further expand the knowledge and applicability of MHPs to organic reactions, since
perovskites offer a rich variety of tuning strategies which may be explored to improve reaction yields and
selectivities.
DOI: 10.1039/d0cy02352c
rsc.li/catalysis
due essentially to the instability of the organic sensitizers or
not completely applicable to the experimental conditions
Introduction
Singlet oxygen (1O2)1–4 is a highly reactive and short-lived
oxygen species and can undergo a variety of reactions; among
them, the hetero Diels–Alder (HDA) [4 + 2] cycloadditions, the
[2 + 2] cycloadditions, the ene reactions and epoxidations are
representative of the most interesting ways to afford valuable
synthons to be used in remarkable approaches toward organic
molecules.5–8 Photocatalysis uses light and pure oxygen to
required by specific syntheses. For these reasons, the modern
chemical literature on 1O2 reports the photochemical
properties of new catalytic systems that proved to be nicely
applicable to the generation of such transient oxygen species.
For example, the use of a Pd-conjoined metallosquare with
encapsulated fullerenes produces an efficient and
photochemically stable 1O2 photosensitizer. The metallocage
with the encapsulated fullerenes can oxidize a series of cyclic
and acyclic alkenes at room temperature via visible light.
These reactions take advantage of the excellent spin-
converting properties of fullerenes, which makes them
excellent agents for 1O2 production.11
The need for oxygen-containing heterocycles or simply
oxidized organic synthons is a continuous growing task in
organic synthesis.12 A range of products were obtained from
the photo-oxidation of cyclopentadiene from photochemically
generated 1O2 using carbon dioxide (CO2) as a solvent and
5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP) as a
CO2-soluble photosensitizer. The endoperoxide intermediate
was 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. Quite remarkably, quenching
with thiourea yielded the syn-diol, (1R,3S)-cyclopent-4-ene-1,3-
insert oxygenated functionalities on
a carbon skeleton,
avoiding the use of, in some cases, extremely toxic heavy
metal catalysts; for these and other reasons, photocatalysis is
an intrinsically environment friendly methodology.
1O2 is photochemically produced by the use of organic
molecules (sensitizers)9 or, alternatively, through classical
hydrogen peroxide decomposition promoted by hypochlorite
or hypobromite and ozonide decomposition. Moreover, 1O2
can also be obtained by thermal decomposition of unstable
molecules, such as arene endoperoxides, or by photolysis of
oxone.10 These methodologies are not devoid of problems
a University of Pavia, Department of Chemistry, Viale Taramelli 12, 27100 – Pavia,
Italy. E-mail: lorenzo.malavasi@unipv.it, paolo.quadrelli@unipv.it
b University of Pavia, Centro Grandi Strumenti (CGS), Via Bassi 21, 27100 – Pavia,
Italy
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy02352c
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diol, a valuable synthon for a variety of natural products.
Treatment with an acid or a base afforded furfuryl alcohol
through a variety of modified g-C3N4 catalysts in the presence of
metal halide perovskites (MHPs), thus forming a heterojunction
composite. In particular, we have recently reported the favorable
coupling of a lead-free 2D MHP, namely PEA2SnBr4, with g-C3N4
for hydrogen photogeneration, demonstrating the synergic
effect of coupling the two semiconductors in such a catalytic
system.18–20 Considering the superior optical properties of
MHPs, particularly the reduced charge carrier recombination
and the strong absorbance, it is an extremely appealing strategy
to create novel catalytic systems where MHPs improve the
efficiency of traditional materials such as graphitic carbon
nitride through the formation of a heterojunction composite.
For these reasons, in the present work, we exploited the possible
application of various catalytic systems, most of them already
tested by our group for hydrogen photogeneration, composed of
2D or 3D MHPs, coupled to g-C3N4, to investigate the scope of
the method in HDA, ene and oxidation reactions with
representative dienes and alkenes. We increased the number of
substrates used to perform the reactions and found interesting
as well valuable limitations occurring with some of them. The
obtained value-added compounds are commonly the synthons
for a remarkably high range of synthetic transformations, widely
applied in modern organic synthesis and here derived from a
and
4-hydroxy-2-cyclopentenone,
respectively.
High
productivities for all products were obtained when compared
to traditional batch reactions.13,14
Moreover, continuous flow photooxidation of several
conjugated dienes and subsequent rearrangement were
engineered, involving endoperoxidation, Kornblum–DeLaMare
rearrangement, and additional rearrangements to obtain useful
hydroxyenones, furans, and 1,4-dicarbonyl building blocks.8
In this panorama, recently, we entered this challenging
competition for 1O2 generation easy methods by proposing
the catalytic use of oxidized graphitic carbon nitride (g-C3N4)
for 1O2 generation under photochemical conditions. Some
HDA and ene reactions were conducted on selected dienes
and alkenes showing a general ability of oxidized g-C3N4 to
promote chemoselective and unselective oxidative processes
with strong dependence on substrate nature (Fig. 1). The
results offer a good picture of the ability of this type of
catalyst to be employed in organic reactions to prepare
valuable synthons that can be used in several value-added
preparations.15 Tuning of the oxidative properties of suitably
modified g-C3N4 has
a pivotal role in determining a
sustainable change in the approach to 1O2 generation
methods, allowing for greener ways to perform organic
reactions.
sustainable and greener O2 methodology in organic synthesis
taking advantage of easily tunable catalysts.
1
g-C3N4,
a
polymeric semiconductor, consisting of
Results and discussion
6-membered rings with sp2-nature bonds between the C and
N atoms, is prepared by bulk and hard template pyrolysis of
melamine (MLM) or, alternatively, from other carbon/
nitrogen containing sources (e.g. urea). The large possibilities
for its functionalization and controlling its catalytic activity
make g-C3N4 a very promising material for the development
of new solar-driven ecologically-friendly reactions.16 It must
be noted that g-C3N4 is the most stable allotropic form of
carbon nitride and possesses unique electronic properties as
well as catalytic and photocatalytic activity.17
We
have
prepared
g-C3N4/PEA2MX4
(PEA
=
phenylethylammonium; M = Pb and Sn; X = Br and Cl)
composites at 5 wt% metal halide perovskite loadings (Table 1,
entries
1–3)
and
g-C3N4/DMASnBr3
(DMA
=
dimethylammonium; Table 1, entry 4) by adapting known
methodologies.18,19 As mentioned above, such composites have
been selected based on their favorable photocatalytic activity
shown in hydrogen generation and organic pollutant
degradation.18–20 Such preliminary evidence suggests their
possible exploitation in other catalysis fields as pursued in the
present work. Details on the preparation and characterization of
the g-C3N4/metal halide perovskite composites have been
already reported by our group.18,19
Upon pursuing our interest in finding new and greener
methods for 1O2 generation, as well as sustainable applications
in the synthesis of valuable organic intermediates, we wish to
report here our recent results concerning 1O2 generation
Table 1 reports all the formulas of the catalysts with the
corresponding labeling. To optimize the reaction conditions,
and in particular, to select the best performing catalyst from
the list in Table 1, we conducted a typical 1O2 HDA
Table 1 List of the catalysts with their formula and literature references
for preparation
Entry
Catalysta
Formulab,c
Ref.
1
2
3
4
C1
C2
C3
C4
g-C3N4/PEA2PbCl4 5%
g-C3N4/PEA2SnBr4 5%
g-C3N4/PEA2PbBr4 5%
g-C3N4/DMASnBr3 5%
18
18
18
20
Fig. 1 1O2 generation from oxidized g-C3N4 and application in HDA
cycloaddition reaction (upon permission from ACS Sustainable Chem.
Eng. 2019, 7, 8176–8182).
a
b
PEA, phenylethylammonium. Percentage of perovskite in g-C3N4
(5%). c DMA, dimethylammonium.
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Table
1,3-cyclohexadiene
2
Product
2
yields in photocatalyzed HDA reactions with
cycloaddition reaction with 1,3-cyclohexadiene using
acetonitrile as a solvent, at room temperature for 24 h
(Scheme 1). The conditions were the same as those used in
previous experiments catalyzed by oxidized g-C3N4 and
applied here for the sake of comparison.15
Entry
Catalysta
Product 2 (%)
1
2
3
4
C1
C2
C3
C4
43
26
16
63
The choice of 1,3-cyclohexadiene is due to its reduced
reactivity with respect to cyclopentadiene; this helps in
limiting the potentially expected polymerization of the diene
as well as side reactions under the experimental conditions.
Irradiation was conducted under simulated solar light for 24
hours, saturating the acetonitrile phase with pure oxygen,
and leaving the reaction under an oxygen atmosphere (using
a balloon as a reservoir connected to the photochemical
vessel). Table 2 reports the data relative to the different
catalysts used and the yield values for the reaction at hand.
In all the reactions, and as similarly done in the previous
work,15 we verified that the 1,3-cyclohexadiene conversions
were quantitative by GC-MS analyses that served also for the
correct detection of the reaction products and structure
attribution, here and in the other reported experiments.
Independent replicated experiments were performed to verify
the obtained results.
a
The amount of catalyst is 13% w/w with respect to
1,3-cyclohexadiene.
the reaction mixtures is reported in Table 3 as found by GC-
MS analyses and comparison with standard references. The
experimental conditions clearly activate
cyclopentadiene dimerization that is responsible for the
reaction outcome as shown in Scheme 2.
The primary DA cycloadduct 3 was neither isolated nor
detected in the final reaction mixtures from independent
experiments. Compound 4 that derives from the peroxide
bond reductive cleavage was not detected too. All the
reported products are known in the cited literature and the
structures are consistent with the reported data.21–29
a
very fast
Intermediate 1 represents the primary HDA cycloadduct
and was never detected in all the performed experiments.
4-Hydroxycyclohex-2-en-1-one (2) was the only reaction
product for the yields reported in Table 2. This hydroxyl
ketone is a known compound derived from the reductive
cleavage of the peroxide O–O bond and simultaneous
oxidation of one of the hydroxy functionalities, both being
pathways promoted by the catalyst; the structure was
confirmed by comparison with that of authentic samples
(primary standard compounds were purchased) as well as
NMR characterization.15
Catalysts C1–3 gave modest to good yields (Table 2, entries
1–3) of compound 2 and the best result was obtained with
catalyst C4 (Table 2, entry 4) that provided 63% yield of 2.
Catalyst C4 ameliorated the results obtained with oxidized g-
C3N4 alone (55% yield of compound 2).15
Based on the above-illustrated results, we also investigated
the behavior of cyclopentadiene to probe the scope of the
reaction with a highly reactive diene, testing all the prepared
catalysts. Scheme 2 shows the structures of the products
obtained from the reaction of photogenerated 1O2 with
freshly distilled cyclopentadiene. The reaction conversion is
complete although affected by an important amount of
cyclopentadiene polymerization material. The composition of
Catalysts of type C1–3 showed somewhat a similar
behavior with a neat preference for compound 5d that is the
bis-epoxy derivative of the cyclopentadiene dimer.
Comparable amounts of the ketones 5a and 5e were observed
(13–21%) as oxidation products of the cyclopentadiene dimer.
Minor amounts of the reduction product 5c were also
observed in the range of 5–13% yield.
Ending with catalyst C4, the mono-epoxy derivative of the
cyclopentadiene dimer 5f was obtained in 43% yield, along
with up to 10% of ketones 5a and b. In all the experiments
reported above, the catalyst was simply filtered at the end of
each reaction and disposed. Freshly prepared catalysts were
employed for new reactions and repetitions.
Oxidation of alkenes and ene reactions represent another
important task where 1O2 plays a pivotal role in olefin
functionalization, extremely useful for the synthesis of allylic
alcohols and unsaturated carbonyl compounds and, in
general, value-added compounds.
The investigations in this field were conducted on a
variety of alkenes, acyclic and cyclic, ranging from the highly
CC
double
bond
substituted
ones,
such
as
tetramethylethylene (TME) and trimethylethylene (tme), to
Scheme 1 HDA cycloaddition reaction of 1,3-cyclohexadiene and 1O2
used as the benchmark reaction to test the efficiency of all the
catalysts.
Scheme 2 HDA cycloaddition reaction of cyclopentadiene and 1O2.
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Table 3 Product distribution in the photocatalyzed HDA reactions with cyclopentadiene
Entry
Catalysta
4 (%)
5a (%)
5b (%)
5c (%)
5d (%)
5e (%)
5f (%)
1
2
3
7
C1
C2
C3
C4
—
—
—
—
13
13
16
9
—
—
—
10
5
13
7
30
34
27
—
21
15
13
—
—
—
—
43
—
a
The amount of catalyst is 13% w/w with respect to cyclopentadiene.
the monosubstituted ones (e.g. 1-octene). The studies were
extended to some aromatic alkenes, such as allylbenzene,
indene and dihydroanthracene. In all these cases, the
reactions were performed by using the catalyst that gave the
best performances in HDA cycloadditions (catalyst C4) under
the typical set-up experimental conditions above reported.
Scheme 3 shows the results corresponding to the first
from the presence of bromide from the catalyst structure.
Disubstituted alkenes, such as cis-2-hexene or trans-4-nonene,
just to cite some of the tested olefins, did not give any
positive result. In contrast, the monosubstituted 1-octene
underwent oxidative pathways to afford in fair yields the diol
12a and the ketoacid 12b. The structures were confirmed
upon comparison with known data reported in the cited
literature and the used appropriate standard reference
compounds.35–37
Representative aromatic unsaturated compounds were also
tested in photocatalyzed 1O2 reactions and the results are
shown in Scheme 5. Indene (13) is prone to be oxidized with
our catalytic method but the primary ene adduct 14a was not
detected in the crude reaction mixture. The diol 14b was
obtained in the reactions with catalyst C4 in fair yields as a
mixture of cis and trans isomers while ketone 14c, a product
of the direct oxidation of the sp2 carbon atom, was also
obtained. Dihydroanthracene (15) afforded anthraquinone
16a in excellent yields (71%). The reaction is accompanied by
the formation of anthracen-9(10H)-one (16b) (9% yield) and of
anthracene (16c) from complete reduction of the starting
material in 20% yield. Finally, we also investigated the
1
series of reactions of O2 conducted in the presence of cyclic
olefins. In addition, we tested acyclic alkenes, from mono- to
tetra-substituted ones with different degrees of substitution
on the CC double bond as reported in Scheme 4.
Cyclic alkenes (from 5- to 8-membered rings) gave interesting
and various results that can be classified within allylic oxidation
reactions such as in the cases of products 6a and b, 7a and b, 8b
and 9b (Scheme 3). The product yields are fair; good
performances are observed for cyclohexene where a variety of
oxidized compounds were obtained with poor chemoselectivity.
Some epoxydated products are often obtained in particular for
6-, 7- and 8-membered rings. Remarkably, compound 9a was
obtained in nearly 50% yield. Cyclooctene also affords
compound 9c derived from partial reductive processes. Lastly,
the structures of brominated compounds 6b and 7e and f can be
attributed on the basis of the presence of bromide anions in the
catalysts. All the detected products are known compounds
reported in the cited literature.30–34
oxidation of allylbenzene (17).
A poorly chemoselective
process results in the yields of the obtained products 18a–f.
The primary ene adduct 18a was obtained in 3% yield using
catalyst C4. Cinnamaldehyde (18b) and the corresponding
alcohol 18c are the major products along with the hydroxyl-
ketone 18d. Minor amounts of 18e and f complete the mix of
the products. All the compounds reported in Scheme 5 are
known in the literature38–42 and some of them were already
reported in our previous work.15
On the other hand acyclic alkenes were quite unprolific
(Scheme 4); the ene adduct 10, quantitatively prepared in
previous studies,15 could not be obtained here and
compound 11 was obtained in modest yields and “suffers”
In order to summarize, g-C3N4/PEA2MX4 composites and
g-C3N4/DMASnBr3 composites (see Table 1) were prepared
and used as photocatalysts in HDA, ene and general
oxidation reactions of alkenes. HDA cycloaddition reactions
with 1,3-cyclohexadiene and cyclopentadiene were used to
Scheme
3
Oxidation products from cyclic alkenes in 1O2
Scheme
photocatalyzed reactions. ND, not detected.
4
Oxidation products from acyclic alkenes in 1O2
photocatalyzed reactions. ND, not detected.
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Scheme 6 Photooxidation reactions of type I and type II pathways.
can be photogenerated by suitable triplet state sensitizers
and oxygen suitably trapped to give the oxidized products
according to the type I photoreaction. An example is offered
by the absence of any 1O2 ene reactions in some of the
examined reactions; the presence of other non-1O2 products
such as in the indene case suggests a process promoted by a
type I reaction, derived from oxygen radicals and oxygen
radical ions.44–46 On the other hand, the sensitizer promotes
Scheme
5
Oxidation products from aromatic compounds in 1O2
photocatalyzed reactions. ND, not detected.
define the best catalysts to conduct photooxidative processes
with a variety of substrates. Catalyst C4 was found to give the
best results with HDA in the presence of 1,3-cyclohexadiene,
affording compound 2 up to 63% yield, ameliorating our
previous results obtained with simple oxidized g-C3N4 as the
catalyst.15
1
the O2 formation which undergoes oxidation reactions with
various substrates (type II). To discern between these two
types of mechanisms, new studies are needed (chemical trap
investigations) and can be reasonably planned.
In spite of the new catalysts employed, cyclopentadiene
continues to confirm its extreme reactivity towards
dimerization; this spontaneous tendency also accompanied
by polymerization under the experimental conditions offers
however the possibility to obtain a variety of oxidized
compounds of the dimer itself.
Looking at Table 2, it is possible to observe that the best
performing composite, providing the highest reaction yield, is
the one containing DMASnBr3 (C4). This general trend agrees
with the superior photocatalytic activity observed during
hydrogen photogeneration for this system relative to C1–C3,
suggesting a direct correlation between the general activity level
and the further use in HAD reactions.18,19 On the other hand,
among the 2D-based systems, the best activity is found for C1
(PEA2PbCl4) which has a band-gap centered at around 3.6 eV,
i.e. in a region where the redox potentials may even directly
promote oxidation reactions. C2 and C3 have analogous band-
gaps of around 2.7–2.8 eV and provide a direct synergy with
carbon nitride (band-gap of about 2.75 eV) in terms of charge
carrier transfer.18,19 However, as we determined through
Interesting results were obtained with cyclic alkenes of
different ring sizes. The oxidation reactions are not
chemoselective but α,β-unsaturated ketones are often the
main products of the reactions along with the epoxides. Less
promisingly, the results of acyclic alkenes do not offer a
picture of the applicability of the methodology for this type
of molecule, presumably not structurally fit to interact
positively with the catalysts in order to give valuable oxidized
compounds. Finally, aromatic compounds such as indene,
dihydroanthracene and allylbenzene were easily oxidized by
the selected catalysts. Specifically, catalyst C4 did not give
quantitatively anthraquinone 16a as in our previous work15
but still, 16a remains the major product (up to 71%). Indene
is mainly oxidized to the corresponding diol 14b and
allylbenzene offers a wide selection of oxidized compounds.
We have conducted photocatalyzed reactions with in situ
1O2 generation in the presence of a variety of olefinic
compounds. The catalysts used in the present work are
substantially different from the oxidized g-C3N4 previously
employed and offer a large matter of discussion. There are
analogies and strong differences. In close similarity, catalyst
C4 (but all the catalysts in Table 1 behave similarly) displays
a low chemoselectivity in the presence of some substrates or
even in the absence of any reaction.
computational modeling, such
a positive band-alignment
between the two semiconductors, namely g-C3N4 and the
perovskite, has the maximum efficiency for C4, as demonstrated
by the reaction yields. Let us also remember that the charge
carrier lifetimes are generally longer for 3D perovskites also due
to the lower binding energies, resulting in higher photocatalytic
efficiencies.20 The selectivity reported in Table 3 suggests a
correlation between the type of perovskite used in the
heterojunction formation (2D or 3D) and the product selectivity
of the reaction. In this case, irrespective of the relative yields,
2D-based systems, both containing Br or Cl and Pb or Sn,
behave similarly. In this respect, and considering the above
reported arguments, it seems that the increase in catalytic
activity of C4 directly correlates with the product selectivity.
Conclusions
These outcomes lead to the introduction of the possibility
to differentiate between photooxidation reactions of type I
and type II as shown in Scheme 6.43 Radicals or radical ions
In conclusion, we can affirm that the g-C3N4/PEA2MX4 and g-
C3N4/DMASnBr3 composites are interesting and valuable
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photocatalysts for the in situ generation of 1O2 to perform
HDA, ene and oxidation reactions with suitable dienes and
alkenes. The methodology has been reasonably standardized
and made applicable to a variety of olefinic substrates. The
scope of the method has been finely illustrated by the results
in all the tested reactions, which allowed for obtaining of
desymmetrized hydroxy-ketone derivatives, unsaturated
ketones and epoxides. Notable limitations were also observed
especially in the case of the alkene oxidation as well as the
poor chemoselectivity somewhere observed. The tuned-up
oxidative properties of the suitably modified g-C3N4 catalysts
offer a remarkable improvement in determining a change in
the approach to singlet oxygen generation methods that may
open other ways to perform organic reactions through
greener and sustainable methodologies.
Finally, we highlight that this paper reports the first
application of MHP-based composites for in situ generation of
1O2 and that the experimental protocol applied here can be used
as a platform to further expand the knowledge and applicability
of MHPs to this class of organic reactions. As is well known,
perovskites offer a rich variety of tuning strategies which may be
explored to improve reaction yields and selectivities.
injection volume of
1
μL was employed. The injector
temperature was set at 250 °C and it was operated in split
mode, with
split flow of 10 mL min−1. The oven
a
temperature was programmed from 60 °C (isothermal for 4
min) to 220 °C (isothermal for 10 min) at a rate of 10 °C
min−1. The mass transfer line temperature was set at 250 °C.
The total GC running time was 30 min.
All mass spectra were acquired with an electron ionization
system (EI, electron impact mode) with an ionization energy
of 70 eV and a source temperature of 250 °C, with spectral
acquisition in full scan mode over a mass range of 35–450
Da. The chromatogram acquisition, detection of mass
spectral peaks and their waveform processing were
performed using Xcalibur MS software version 2.1 (Thermo
Scientific Inc.). Assignment of chemical structures to
chromatographic peaks was based on the comparison with
the databases from the GC-MS NIST Mass Spectral Library
(NIST 08) and Wiley Registry of Mass Spectral Data (8th
Edition) as well as for the standard samples. The percentage
content of each component was directly computed from the
peak areas in the GC/MS chromatogram.
Author contributions
Experimental
These authors made equal contributions to this work.
For all the HDA, ene and oxidation reactions, the following
experimental procedure was used.
Conflicts of interest
There are no conflicts to declare.
Reaction method
1.4 mmol of diene or alkene substrates were dissolved in 20
mL pure acetonitrile in the presence of 15 mg of the catalyst
(typically 13% w/w) in a 35 mL cylindrical Pyrex reaction
vessel and the mixture was saturated with oxygen gas by
bubbling for 15 minutes. The sealed vessel was then
irradiated under vigorous stirring in a Solar Box Co.Fo.Me.
Gra. 1500 e (500 W m−2) for 24 hours. Further saturations
with oxygen (a couple) were done during the reaction time.
After this period of time, the suspension was filtered off to
obtain a clear solution that was evaporated at moderate
reduced pressure heating with a water bath at 30 °C. The
residues were submitted to GC-MS analyses to determine the
structures of the products and the corresponding yields. The
Acknowledgements
Financial support from the University of Pavia, MIUR (PRIN
2015, CUP: F12F16001350005) is gratefully acknowledged. We
also thank the “VIPCAT – Value Added Innovative Protocols for
Catalytic Transformations” project (CUP: E46D17000110009) for
valuable financial support. Thanks are also given to the project
“Scent of Lombardy” (CUP: E31B19000700007) for financial
support.
Notes and references
1 A. A. Frimer, Singlet Oxygen, CRC Press, Boca Raton, FL,
1985, Vol. I.
instrument used is
a Thermo Scientific DSQII single
quadrupole GC/MS system (TraceDSQII mass spectrometer,
Trace GC Ultra gas chromatograph, TriPlus Autosampler –
Thermo Scientific®, San Jose, CA, USA).
For the product characterization of known compounds,
reference standards were purchased from different chemical
suppliers to be submitted to GC-MS analyses for the sake of
MS spectra comparison. Being known compounds, we also
took advantage of the suitable MS spectra libraries available
at the CGS of the University of Pavia (vide infra).
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