Copper(II) Triflate As a Reusable Catalyst for the Synthesis of trans-4,5-Diamino-cyclopent-2-enones in Water

Article abstract of DOI:10.1021/acs.joc.7b02931

trans-4,5-Diamino-cyclopent-2-enones (CP) are usually prepared by Lewis acid-catalyzed condensation of furfural and a secondary amine in an organic solvent. The reaction proceeds through the formation of a Stenhouse salt (SS) intermediate followed by an electrocyclization reaction to afford the desired CP. Herein, we described the use of Cu(OTf)₂ as a very efficient catalyst for the synthesis of CP in water at room temperature. Furthermore, the mild reaction conditions, catalyst reusability, and outstanding functional group tolerance suggest that this CP platform can be further used in chemical biology.

Full text of DOI:10.1021/acs.joc.7b02931

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Note
Copper(II) triflate as a reusable catalyst for the synthesis
of trans-4,5-diamino-cyclopent-2-enones in water
Rafael F. A. Gomes, Nuno R. Esteves, Jaime A. S. Coelho, and Carlos A. M. Afonso
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02931 • Publication Date (Web): 05 Feb 2018
Downloaded from http://pubs.acs.org on February 5, 2018
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The Journal of Organic Chemistry
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Copper(II) triflate as a reusable catalyst for the synthesis of
trans-4,5-diamino-cyclopent-2-enones in water
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Rafael F. A. Gomes, Nuno R. Esteves, Jaime A. S. Coelho,* Carlos A. M. Afonso*
Research Institute for Medicines (iMed.ULisboa), Faculty of Pharmacy, Universidade de Lisboa, Av. Prof. Gama
Pinto, 1649-003, Lisboa, Portugal
ABSTRACT: Trans-4,5-diamino-cyclopent-2-enones (CP) are usually prepared by Lewis acid-catalyzed condensation of
furfural and a secondary amine in an organic solvent. The reaction proceeds through the formation of a Stenhouse salt
(
SS) intermediate followed by an electrocyclization reaction to afford the desired CP. Herein, we described the use of
Cu(OTf) as a very efficient catalyst for the synthesis of CP in water at room temperature. Furthermore, the mild reaction
2
conditions, catalyst reusability and outstanding functional group tolerance suggests that this CP platform can be further
used in chemical biology.
The ever increasing population growth, and the
concomitant industrial development have led to an
increase demand for chemicals and energy, which is
cyclopent-2-enones in water under microwave irradiation (60
°C, 5 min).
14
1
leading to a depletion of the oil reservoirs. In this
Scheme 1. Selected methodologies for the synthesis of trans-4,5-
dimorpholino-cyclopent-2-enone (1).
context, new sustainable sources for fuels and bulk
chemicals are of the highest interest, and biomass is the
2
most attractive alternative. Furfural is easily obtained
from xylose and is included in the U.S. Department of
3
Energy top 10+4 list of biobased materials. The selective
transformation of furfural to trans-4,5-diamino-
cyclopent-2-enones promoted by dysprosium in dry
acetonitrile was reported by Batey and co-workers in
4
2007. The product is formed through a Nazarov type
5
electrocyclization of a Stenhouse salt intermediate. In
addition, the same group also reported the total synthesis
of natural product Agelastatin
A from trans-4,5-
6
bis(diallylamino)-cyclopent-2-enone. Driven by the
pioneering work of Batey in this field, several alternative
reaction conditions for the selective synthesis of trans-
4
,5-diamino-cyclopent-2-enones were developed (Scheme
1
), such as the use of ionic liquid 1-methylimidazolium
7
8
tetrafluoroborate, tosylamine as a metal free procedure,
aluminium(III)
clusters,
more sustainable alternative . In addition, our team
reported the use of erbium(III) chloride immobilized on
silica as a reusable catalyst for the same transformation.
9
III
III
chloride,
Dy /Ni
heteronuclear
1
0,11
Recently, we have been interested in the preparation of
trans-4,5-diamino-cyclopent-2-enones in aqueous media
both for the appealing environmentally friendly
characteristics, and for the potential application of the
and erbium(III) chloride in ethyl lactate as a
1
2
1
3
cyclopentenone
platform
to
chemical
biology.
During the conclusion of the current work, Nardi and
co-workers reported the preparation of trans-4,5-diamino-
Conjugation of small molecules with biomolecules is a
very challenging field that has attracted the interest of
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1
5,16
several research groups.
to develop new strategies for chemo and site selective
introduction of non-natural functionalities in
In this context, it is important
Table 1. Optimization of the reaction conditions for the
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synthesis
of
trans-4,5-dimorpholino-cyclopent-2-
a
enone
biomolecules because it allows the difficult process of
introducing non-natural amino acids on the protein
1
7–19
skeleton to be avoided.
Such reactions have to meet
the following requirements: 1) be compatible with
aqueous media, 2) have fast kinetics and complete
conversion, 3) be orthogonal. We envisioned that the
selective formation of cyclopentenones bearing secondary
amines, not often found in biological media, could be an
easy and cheap procedure to insert both an external
secondary amine and an enone that can be further
functionalized.
b
Entry
Catalyst
Yield (%)
traces
1
none
0
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Dy(OTf) (10 mol %)
53
3
c
Lewis acid (10 mol %)
up to 42
d
e
Cu(OTf) (10 mol %)
quant. (quant.)
2
c
The reaction of furfural with morpholine in water
resulted in the formation of traces amount of the desired
product 1 after 5 minutes (Table 1, entry 1). Based on that,
we started this study by testing several Lewis acids
catalysts under the same reaction conditions. To our
CuX (10 mol %)
up to 46
up to 32
Brønsted acid (10 mol %)
Cu(OTf) (1 mol %)
quant. (10 min)
2
f
8
g
Cu(OTf) (0.1 mol %)
93 (8 h)
2
h
delight, 10 mol% of Dy(OTf) afforded 1 in 53% yield
9
Cu(OTf) (10 mol %)
quant.
3
2
(
Table 1, entry 2). Other Lewis acids, such as Sc(OTf)3,
[a] Reaction conditions: furfural (0.2 mmol), morpholine (2.2 equiv),
catalyst (10 mol %), H2O (0.2 mL), rt, 5 min. [b] Isolated yield. [c] See SI
for complete list. [d] The reaction time is 1 minute. [e] The reaction
concentration is 0.1 M (2 mL of H2O). [f] Reaction performed using 10
gram of furfural. The reaction was stirred for 1h in an ice bath, after
which was allowed to warm until room temperature. [g] The product was
isolated as a 6.7:1 mixture of 1 and the corresponding 2,4-isomer. [h] The
reaction solvent is acetonitrile (2 mL).
GdCl .6H O, ErCl , ZnCl or AlCl , were not able to
3
2
3
2
3
catalyse the reaction (up to 42% yield, Table 1, entry 3).
Remarkably, Cu(OTf) afforded 1 in outstanding isolated
2
yield (quantitative, Table 1, entry 4). It is noteworthy that
full conversion was observed in most cases. Remarkably, the
use of 10 mol% of Cu(OTf) results in full conversion of
2
the starting materials in less than 1 min. Next, we
investigated the influence of the copper ligand by
evaluating several different copper salts under the same
reaction conditions. The tested salts afforded up to 50%
yield of 1 in opposite to an outstanding yield obtained
with the trifluoromethanesulfonate salt (Table 1, entry 5).
Furthermore, the Brønsted acids p-toluenosulfonic acid
and triflic acid afforded low yield of 1 (25% and 32% yield,
respectively, Table 1, entry 6). Further reduction of
To further understand the low yield of 1 in the reaction
catalysed by other Lewis acids, we first note that full
conversion was obtained in all cases. For example, the
reaction of furfural with morpholine in water catalysed by
GdCl3 afforded the desired product in 10% yield (Scheme
2A). A negligible decomposition (Scheme 2B) of the product
in the presence of GdCl3 in water point towards an
unselective catalysis, in which GdCl3, as well as the other
Lewis acids tested, promote the undesired side reactions of
furfural or intermediates.
Cu(OTf)2 loading to
1 mol% did not affect the
performance and 1 was obtained in excellent yield after 10
minutes reaction time (quantitative, Table 1, entry 7). In
addition, the combined scale up to 10 g of furfural and
reduction of catalyst load to 0.1 mol% afforded the
product in 93% yield after 8h as a 6.7:1 mixture of 1 and
the thermodynamically more stable regioisomer 2,4-
Scheme 2. Stability study of cyclopentenone 1 in the presence of LA
in aqueous medium.
4
dimorpholino-cyclopent-2-enone (Table 1, entry 8). Full
conversion to the 2,4 regioisomer was observed after 24h.
Finally, similar efficiency of the catalyst was observed in
an organic solvent (acetonitrile, quantitative yield of 1,
Table 1, entry 9).
With the optimized conditions in hands, the amine scope
was evaluated (Scheme 3). Several secondary amines,
including cyclic amines and anilines were well tolerated and
afforded the corresponding product in excellent yield (up to
>99% yield) in a very short reaction time (1 minute).
Diallylamine also reacted with furfural to give the
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corresponding product 4 in 72% yield (80% conversion) after
molecules bearing different functional groups (Scheme 4).
Both aldehydes and ketones can potentially inhibit the
reaction by reaction of the amine substrate with these
electrophiles. Surprisingly, the yield of 1 was not significantly
affected in the presence of salicylaldehyde, cinnamaldehyde
or cyclohexenone. Thiols, commonly found as detoxification
platforms due to their nucleophilic and antioxidant
character, were also tested. In this case the concern was the
known ability of thiols to undergo 1,4-addition, however, 1
was isolated in 98% yield under our reaction conditions.
Carboxylic acids, commonly found in the C-terminal region
of proteins and in several biomolecules were also tested
because of their potential ability to protonate amines. Gladly,
the reaction proceeded smoothly in the presence of 2
equivalents of octanoic acid and 1 was isolated in 96%.
Primary amines, frequently found in proteins (e.g. as lysine
regions), and one of the most used sites for bioconjugation,
were of great concern because of the potential competition
with the secondary amine substrate. Furthermore, the
reaction with primary amines give the corresponding imine
in nearly quantitative yield as observed in the amine
1
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4 h reaction. When pyrrolidine or N-methyl-piperazine were
employed, if the mixture was allowed to react for longer
periods of time isomerization to the more stable 2,4-
diamino-cyclopent-2-enones was observed. The more
4
challenging primary amines
such as aniline and
benzylamine did not offered the desired product and the
intermediate imine was isolated in these cases.
Scheme 3. Reaction scope for the formation of trans-4,5-diamino-
cyclopent-2-enones under the optimized conditions
1
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substrate scope study. In fact, the Cu(OTf) -catalyzed
2
reaction of furfural with morpholine in the presence of 2
equivalents of benzylamine offered the imine when
performed in acetonitrile (result not shown). However, the
use 4.4 equivalents of morpholine and water as the solvent
allowed the isolation of the desired product 1 in very good
yield (93% yield, Scheme 4). Similarly, the reaction in the
presence of lysine required an excess of morpholine in order
to achieve very good yield of 1. Several other amino acids
such as tryptophan, serine, cysteine, sodium glutamate and
tyrosine were tested with no significant inhibition of the
reaction. In addition, the presence of glucose in the reaction
medium was also well tolerated with no decrease in yield or
apparent reaction rate. Finally, the reaction efficiency was
also studied in the presence of selected macromolecules. The
presence of bovine serum albumin (BSA), chosen as a model
protein, did not affected the reaction yield, affording 94% of
1 in the presence of 200% w/w of BSA (relative to furfural).
However, we observed a decrease to 82% yield of 1 (100%
yield based on recovered furfural, result not shown) in the
presence of 200% w/w of DNA from salmon. Remarkably,
extension of the reaction time to 15 minutes afforded the
product in 97% yield. Based on that we propose that the
apparent decrease of the reaction rate is due to the trapping
of the catalyst by the DNA rather than undesired reactions of
the morpholine. No other products were identified during
the competition assays.
[
a] Reaction conditions: furfural (1.0 mmol), amine (2.2 equiv), Cu(OTf)
mol %), H O (1 mL), rt, 1 min. The average yield of 3 experiments are shown [b]
Reaction time is 4 h.
2
(10
2
In general, the synthesis of cyclopentenones was a very
efficient reaction under the conditions reported herein and
water was the only by-product generated. Based on that, we
envisioned that the ability to easily extract the product from
the aqueous reaction medium using an organic solvent
(
diethyl ether) would allow reuse of catalyst. Remarkably,
Cu(OTf) was reused 4 times without considerable loss of
2
catalytic activity. Furthermore, the yield of 1 remained
greater than 50% after 4 times. The results obtained during 7
cycles are summarized in Figure 1.
Figure 1. Catalyst reuse on the synthesis of 1 in aqueous media.
Reaction conditions: furfural (1.0 mmol), morpholine (2.2 equiv),
Cu(OTf) (10 mol %), H O (1 mL), rt, 1 min. Isolated yield by diethyl
2 2
ether extraction. All the experiments were performed in triplicate.
We next sought to study the functional group tolerance of
the reaction by performing the Cu(OTf) -catalyzed reaction
2
of furfural with morpholine in the presence of selected
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To a solution of a selected catalyst (10 mol%) in water (0.2
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Scheme 4. Cu(OTf)2-catalyzed reaction of furfural with morpholine
in the presence of an external additive.
mL) was added morpholine (40 µL, 0.45 mmol, 2.2 equiv)
and furfural (20 mg, 0.2 mmol). The reaction was allowed to
stir vigorously at room temperature for 5 minutes. Then the
reaction mixture was diluted with water (0.8 mL) and
extracted with diethyl ether (3 × 1 mL). The organic phase
was dried with MgSO and the solvent was evaporated under
4
1
reduced pressure. The crude mixture was analysed by H-
NMR.
Stability of trans-4,5-dimorpholino-cyclopent-2-enone 1
To a solution of GdCl ·6H O (7 mg, 0.019 mmol, 10 mol%) in
0
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water (0.1 mL) was added an aqueous solution of trans-4,5-
dimorpholino-cyclopent-2-enone (50 mg, 0.2 mmol in 0.1 mL
water). The mixture was allowed to stir vigorously at room
temperature for 15 minutes. The solution was diluted with
water (0.8 mL) and extracted with diethyl ether (3 × 1 mL).
The solvent was evaporated under reduced pressure and the
1
crude mixture was analysed by H-NMR.
General procedure for the catalyst reuse in the
preparation of trans-4,5-dimorpholino-cyclopent-2-
enone 1
To a solution of Cu(OTf)2 (40 mg, 10 mol%) in water (1 mL)
was added morpholine (225 µL, 2.29 mmol, 2.2 equiv) and
furfural (100 mg, 1.04 mmol). The reaction was allowed to stir
vigorously at room temperature for 1 minutes. Diethyl ether
was added to the reaction mixture (2 mL) and after 1 min of
vigorous stirring the phases were separated. The process was
repeated one more time and the collected organic phases
were dried with MgSO4 and evaporated under reduced
pressure to yield the pure trans-4,5-dimorpholino-cyclopent-
2-enone. The remaining aqueous layer was loaded with more
morpholine (200 µL, 2.29 mmol, 2.2 equiv.) and furfural (100
mg, 1.04 mmol) and the process repeated for 7 cycles.
[a] The values correspond to the isolated yield of 1. [b] Yield
1
determined by H NMR yield after extraction. [c] Reaction performed
with 4.4 equivalents of morpholine. [d] Reaction time is 15 minutes.
In conclusion, we described a fast methodology for the
synthesis of trans-4,5-diamino-cyclopent-2-enones (CP) in
General procedure for the preparation of trans-4,5-
dimorpholino-cyclopent-2-enone in the presence of
small molecules competitors
water at room temperature using Cu(OTf) as catalyst. This
2
protocol is distinguished by its operational simplicity, mild
reaction conditions, great efficiency, high tolerance to the
presence of external molecules with several functional-
groups, and amenability to gram-scale synthesis. We
anticipate that these characteristics will allow for further
applications in bioorthogonal transformations.
To a solution of Cu(OTf)2 (8 mg, 10 mol%) in water (0.2 mL)
was added the corresponding competitor (2 equiv),
morpholine (40 µL, 0.45 mmol, 2.2 equiv) and furfural (20
mg, 0.2 mmol). The reaction was allowed to stir vigorously at
room temperature for 5 minutes. Then the reaction mixture
was diluted with water (0.8 mL) and extracted with diethyl
ether (3 × 1 mL). The organic phase was dried with MgSO4
and the solvent was evaporated under reduced pressure. The
1
EXPERIMENTAL SECTION
crude mixture was analysed by H-NMR.
Procedure
for
the
preparation
of
trans-4,5-
General Remarks
dimorpholino-cyclopent-2-enone in the presence of BSA
All solvents were distilled prior use. Furfural was distilled
and stored at 4°C. Unless otherwise stated, all reagents were
used as received from commercial suppliers. Reaction
progress was monitored by thin layer chromatography (TLC)
performed on aluminium plates coated with silica gel F254
To a solution of Cu(OTf)2 (8 mg, 10 mol%) in water (0.2 mL)
was added BSA (40 mg), morpholine (40 µL, 0.45 mmol, 2.2
equiv) and furfural (20 mg, 0.2 mmol). The reaction was
allowed to stir vigorously at room temperature for 5 minutes.
Then the reaction mixture was diluted with cold acetone (2
mL) and left at -20°C for 1 h. The sample was centrifuged and
the supernatant was evaporated under reduced pressure.
Water (1 mL) was added to the crude mixture and the
reaction was extracted with diethyl ether (3 × 2 mL). The
1
13
with 0.2 mm thickness. H and C NMR spectra were
acquired on Bruker MX300 spectrometer.
General procedure for the synthesis of trans-4,5-
dimorpholino-cyclopent-2-enone 1 in water
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collected organic phases were dried with MgSO4 and the
trans-4,5-bispyrrolidine-cyclopent-2-enone (3) The product
1
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solvent was evaporated under reduced pressure to afford the
final product in 94% yield.
was isolated in 97% yield (222 mg) as a yellow oil. The
spectral data is in agreement with the literature.
7
Procedure
for
the
preparation
of
trans-4,5-
trans-4,5-bis(diallylamine)-cyclopent-2-enone (4) The product
dimorpholino-cyclopent-2-enone in the presence of
DNA
was isolated in 72% yield (204 mg) as a brown oil. The
4
spectral data is in agreement with the literature.
To a solution of Cu(OTf)2 (8 mg, 10 mol%) in water (0.2 mL)
was added DNA (40 mg), morpholine (40 µL, 0.45 mmol, 2.2
equiv) and furfural (20 mg, 0.2 mmol). The reaction mixture
was allowed to stir vigorously at room temperature for 5
minutes. Then the reaction mixture was diluted with cold
ethanol (2 mL) and left at -20°C for 1 h. The sample was
centrifugated to remove the precipitated DNA and the
supernatant was evaporated under reduced pressure. Water
trans-4,5-bispiperidino-cyclopent-2-enone (5) The product
was isolated in quantitative yield (258 mg) as a yellow oil.
7
The spectral data is in agreement with the literature.
1
1
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trans-4,5-bis(N-methylpiperazine)-cyclopent-2-enone (6) The
product was isolated in quantitative yield (290 mg) as an
1
orange oil. H NMR (300 MHz, CDCl
) δ 7.60 (dd, J = 6.2, 2.2
3
Hz, 1H) 6.22 (dd, J = 6.2, 1.8 Hz, 1H) 3.89 (q, J = 2.1 Hz, 1H)
(
1 mL) was added to the crude mixture and the reaction was
3.35 (d, J = 3.0 Hz, 1H) 2.76-2.79 (m, 2H) 2.61-2.69 (m, 6H)
13
extracted with diethyl ether (3 × 2 mL). The combined
2.47 (broad s, 8H) 2.29 (s, 6H). C NMR (100 MHz, CDCl
3
) δ
organic phases were dried with MgSO and the solvent was
46.1, 49.2, 49.4, 55.4, 65.7, 67.9, 135.3, 161.4, 206.0. MS (ESI-
4
+
evaporated under reduced pressure. The crude mixture was
MS) m/z calcd for C15H26N4O [M + H] 279.2179, found
1
analysed by H-NMR.
279.2177.
Procedure
for
the
preparation
of
trans-4,5-
trans-4,5-bis(N-methylaniline)-cyclopent-2-enone (7) The
dimorpholino-cyclopent-2-enone in a 10-gram scale
product was isolated in quantitative yield (304 mg) as a red
4
oil. The spectral data is in agreement with the literature.
To a round bottom flask loaded with a Cu(OTf)2 (4 mg, 0.1
mol%) and water (100 mL) were added morpholine (20 mL,
trans-4,5-bis(dihydroquinoline)-cyclopent-2-enone (8) The
product was isolated in quantitative yield (358mg) as a
yellow solid. The spectral data is in agreement with the
229 mmol, 2.2 equiv) and furfural (10 g, 104 mmol) in an ice
bath. The reaction was allowed to stir vigorously for 1 h. Then
the ice bath was removed and the reaction was allowed to
stir at room temperature. Aliquots of 0.1 mL were taken at 3
h and 8 h, extracted with CDCl3 and analysed by 1H-NMR.
After 8 h, 50 mL of the homogenous reaction mixture was
collected from the flask, diluted with water (200 mL) and
extracted with MTBE (3 × 200 mL). The combined organic
phases were dried with MgSO4 and the solvent was
evaporated under reduced pressure to give a crude product
4
literature.
trans-4,5-bis(6-methoxy-dihydroquinoline)cyclopent-2-enone
(9) The product was isolated in quantitative yield (421mg) as
1
a orange oil H NMR (300 MHz, CDCl ) δ 7.65 (dd, J = 6.3, 2.0
3
Hz, 1H) 6.57-6.60 (m, 2H) 6.42-6.48 (m, 4H) 6.00 (d, J =8.8
Hz, 1H) 5.25 (m, 1H) 4.24 (d, J = 3.6 Hz, 1H) 3.7 (s, 6H) 3.08-
1
3
3
.26 (m, 4H) 2.65-2.77 (m, 4H) 1.87-1.97 (m, 4H). C NMR
(
100 MHz, CDCl ) δ 22.6, 22.8, 27.3, 28.3, 42.4, 45.6, 55.7, 60.6,
(
12.2 g, 93%) as a 6.7:1 mixture of isomers 1 and 2,4-
3
6
8.8, 112.0, 112.2, 112.4, 113.2, 115.5, 115.7, 125.2, 125.3, 134.3, 138.0,
dimorpholino-cyclopent-2-enone. The remaining 50 mL of
the reaction mixture were stirred for an additional 16 h after
which similar work-up protocol was performed to yield the
regioisomer 2,4-dimorpholino-cyclopent-2-enone in 87%
yield (11.4 g).
1
38.8, 151.6, 151.8, 162.3, 202.8. MS (ESI-MS) m/z calcd for
+
C25H29N2O3 [M + H] 405.2173, found 405.2189.
1-(furan-2-yl)-N-phenylmethanimine (10) The product was
isolated in quantitative yield (178 mg) and 93% purity as a
brown oil. The spectral data is in agreement with the
General procedure for the preparation of trans-4,5-
diamino-cyclopent-2-enones
20
literature.
N-benzyl-1-(furan-2-yl)methanimine (11) The product was
To a solution of Cu(OTf)2 (40 mg, 10 mol%) in water (1 mL)
was added amine (2.29 mmol, 2.2 equiv.) and furfural (100
mg, 1.04 mmol). The reaction was allowed to stir vigorously
at room temperature for 1 minutes. Then the reaction
mixture was diluted with water (9 mL) and extracted with
diethyl ether (3 × 10 mL). The combined organic phases were
dried with MgSO4 and the solvent was evaporated under
reduced pressure to yield the pure cyclopent-2-enones as
isolated in quantitative yield (192 mg) as a brown oil. The
21
spectral data is in agreement with the literature.
ASSOCIATED CONTENT
Supporting Information.
1
13
H and C NMR spectra for compounds 1-10.
The supporting information is available free of charge via the
Internet at http://pubs.acs.org.”
1
shown by H-NMR analysis.
trans-4,5-bismorpholino-cyclopent-2-enone (1) The product
AUTHOR INFORMATION
Corresponding Author
was isolated in quantitative yield (262 mg) as a yellow solid.
4
The spectral data is in agreement with the literature.
*
E-mail: jaimeacoelho@ff.ulisboa.pt
trans-4,5-bis(dibenzylamine)-cyclopent-2-enone
(2)
The
* E-mail: carlosafonso@ff.ulisboa.pt
product was isolated in 98% yield (482 mg) as a yellow solid.
4
The spectral data is in agreement with the literature.
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6361.
ACKNOWLEDGMENT
We thank the Fundação para a Ciência e Tecnologia
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Griffiths, K.; Kumar, P.; Mattock, J. D.; Abdul-Sada, A.;
Pitak, M. B.; Coles, S. J.; Navarro, O.; Vargas, A.; Kostakis,
G. E. Inorg. Chem. 2016, 55 (14), 6988–6994.
Procopio, A.; Costanzo, P.; Curini, M.; Nardi, M.; Oliverio,
M.; Sindona, G. ACS Sustain. Chem. Eng. 2013, 1 (5), 541–
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(
SFRH/BPD/100433/2014, PD/BD/128316/2017, PTDC/QEQ-
QOR/3644/2014 and UID/DTP/04138/2013), COMPETE
Programme (SAICTPAC/0019/2015) and European Research
Area Net-work; ERANet LAC (ref. ELAC2014/BEE-0341) for
financial support.
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