Photocatalyzed Generation of Nitrosocarbonyl Intermediates Under Solar Light Irradiation

Article abstract of DOI:10.1002/ejoc.201900596

Tetrabutylammonium decatungstate (TBADT) has emerged as a valuable photocatalyst for sunlight-induced organic reactions. In this communication we wish to present its use for the photocatalytic oxidation of hydroxamic acids to generate fleeting nitrosocarb

Full text of DOI:10.1002/ejoc.201900596

A Journal of
Accepted Article
Title: Photocatalyzed Generation of Nitrosocarbonyl Intermediates
under Solar Light Irradiation
Authors: Paolo Quadrelli, Teresa Basile, Luca Capaldo, and Davide
Ravelli
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900596
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900596
Supported by

European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
Photocatalyzed Generation of Nitrosocarbonyl Intermediates
under Solar Light Irradiation
[
a]
Teresa Basile, Luca Capaldo, Davide Ravelli,* and Paolo Quadrelli*
Dedicated to the memory of Prof. Pierluigi Caramella, for 27 years “maestro” of cycloaddition chemistry.
Abstract: Tetrabutylammonium decatungstate (TBADT) has
emerged as a valuable photocatalyst for sunlight-induced organic
reactions. In this communication we wish to present its use for the
photocatalytic oxidation of hydroxamic acids to generate fleeting
nitrosocarbonyl intermediates. The reaction required a fine tuning of
the reaction conditions, especially in terms of solvent and additives,
since an aqueous acetonitrile solution containing a mild base
ensuing activation of organic molecules via an electron transfer
[
10]
step. An ever-growing number of pioneering works carried out
by different research groups determined in recent years the
renaissance of this approach in organic synthesis, especially in
[
11]
the wake of the 12 principles of Green Chemistry.
Thus, given the importance of nitroso compounds as dienophiles
and enophiles, suitable for the construction of valuable C–O and
C–N bonds through hetero-DA (HDA) and ene reactions,
photoredox catalysis has been adopted as well for the
development of mild and green methods for their generation. As
(
3
NaHCO ) gave the best results. The obtained highly reactive
nitrosocarbonyl species were then efficiently trapped with suitable
reactive dienes, such as cyclopentadiene and 1,3-cyclohexadiene, to
afford the desired Hetero Diels-Alder (HDA) cycloadducts in good
yields. Dedicated photophysical experiments helped to elucidate the
mechanistic scenario.
an example, Ru(bpy)
in the presence of O
or hydroxycarbamates
3
Cl
2
has been used as photoredox catalyst
[
12]
2
(or air) to convert arylhydroxylamines 1
[
7]
3
to nitroso and nitrosocarbonyl
intermediates, respectively (Scheme 1).
G. Masson, ref. 12
Introduction
PG
PG
HN
Ru(bpy) Cl •6H O
Blue LED, rt
HN
3
2
2
OH
Nitrosocarbonyl intermediates (R-CONO) are highly reactive
species that can be conveniently exploited on the way towards
different synthetic targets. Nowadays, the chemical community
is becoming increasingly interested in these fleeting and
valuable derivatives, whose impact in the current organic
synthesis is determined by the possibility to realize a number of
C–N and C–O bond-forming processes, as implicated in ene,
HN
Ar
+
O
N
2,6-lutidine, O2
2,2,2-trifluoroethanol
Ar
1
2 (47-95%)
J. Read de Alaniz, ref. 7
Ru(bpy)
3 2 2
Cl •6H O
2
2
x26 W white light, rt
Cbz
OH
Cbz
N
H
+
N
O
[
1]
Diels-Alder (DA) and aldol reactions.
Different starting materials can be used for their generation such
2,6-lutidine, Air
,2,2-trifluoroethanol
3
4 (87%)
[
2]
[3]
as hydroxamic acids, N-hydroxy-carbamates and -ureas,
[
4]
[5]
nitrile oxides and 1,2,4-oxadiazole 4-oxides. As for the
generation strategy, both thermal and photochemical conditions
can be exploited, also including catalytic processes. When
required, oxidative or aerobic conditions can be adopted as
This work
n
TBADT
Solar light
via:
O
O
n
O
OH
O
N
O
6
R
N
H
R
O
2
R
N
5
[
6]
well. Among these protocols, different research groups have
R = Alk, Ar
[
7]
explored the aptitude of transition metals
with hydrogen
[
8]
[9]
peroxide, metal oxides
and peroxides with TEMPO,
to
Scheme 1. Photoredox catalytic approaches for nitroso and nitrosocarbonyl
convert hydroxamic acids or, more in general, hydroxylamine
derivatives into nitroso reactive species. At any rate, the
multitude of activation modes showcases the versatility of these
intermediates in synthesis, where excellent chemical yields and
mild reaction conditions are often required to prepare complex
molecules.
generation. PG: Protecting Group.
In view of the above and pushed by our interest in the
development of novel approaches for the generation of
nitrosocarbonyl intermediates, we explored the competence of a
An emerging tool in organic chemistry is represented by
photoredox catalysis, a family of processes where a purposely
added catalyst is responsible for light absorption and the
well-known photocatalyst, namely the decatungstate anion
4– [13]
[
10
W O32] , in the generation of such species. The latter were
subsequently trapped in-situ by cyclic dienes in
a [4+2]
cycloaddition process. The choice of this photocatalyst is related
to its remarkable properties in the excited state. In fact, upon
absorption of a UV-B photon, the highly oxidizing excited state
T. Basile, Dr. L. Capaldo, Prof. D. Ravelli, Prof. P. Quadrelli
Department of Chemistry
wO is ultimately formed, which shows
a
redox potential
University of Pavia
Viale Taramelli 12, 27100 – Pavia (Italy)
E-mail: paolo.quadrelli@unipv.it; davide.ravelli@unipv.it
5–
[14]
E(wO/[W10
O32] ) around +2.4 V vs SCE. Remarkably, such a
mighty excited state can be smoothly generated upon solar light
irradiation as well, fully profiting of the potentialities of the so-
Supporting information for this article is given via a link at the end of
the document.
[15]
called “window-ledge” chemistry.
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European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
Table 1. Optimization of reaction conditions for the HDA cycloaddition
between benzohydroxamic acid 5a and 1,3-cyclopentadiene (6) or 1,3-
Results and Discussion
[
a]
cyclohexadiene (7).
We commenced our investigation by choosing a model reaction.
In particular, the feasibility of the HDA reaction was tested by
studying the conversion of benzohydroxamic acid 5a (0.026 M)
to the cycloadduct 8a, in the presence of cyclopentadiene (6, 2
equiv.). The reaction was carried out with tetrabutylammonium
decatungstate (TBADT, 2 mol%) as the photoredox catalyst and
the solution was irradiated in a solar light simulator equipped
TBADT (2 mol%)
hν (Solarbox),
O
O
n
n
OH
+
N
Ph
Ph
N
Conditions
O
H
6
, n = 1
7, n = 2
8a, n = 1
9a, n = 2
5a
Entry
Diene
Conditions
Yield (%)
–
2
1
2
3
6
6
6
MeCN, N2, 24 h
8a, n.d.
with a Xenon lamp (light intensity: 500 W·m ).
When -sparged solution of 5a was irradiated, HDA
8a, 13[b]
8a, 14
MeCN, air, 24 h
a
N
2
cycloadduct 8a was not formed (Table 1, entry 1), despite
complete conversion (>95%) of benzohydroxamic acid was
observed. However, when adopting an air-equilibrated
acetonitrile solution of 5a, the HDA product 8a was obtained in
MeCN, air, 24 h[c]
4
5
6
6
6
6
MeCN, air, 1 + 2 h
8a, 30
8a, 32
8a, 43
MeCN, O2, 1 + 2 h
MeCN/H2O 5:1, O2, 1 + 2 h
1
3% yield based on HPLC analysis (entry 2). The crude reaction
mixture was mainly composed of dimers and/or polymers of 6,
along with fragments of ammonium salts deriving from the
catalyst. An increased amount of the photocatalyst (up to 4
mol%) did not ameliorate the reaction yield (entry 3). Due to the
large amount of decomposition products and tar observed, we
hypothesized that prolonged irradiation could have a detrimental
effect on the reaction. Indeed, when the reaction mixture was
irradiated for 1 hour and then left under stirring at room
temperature in the dark for two additional hours, cycloadduct 8a
was obtained in 30% yield, thus confirming our hypothesis (entry
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
7
6
6
6
6
8a, 49
8a, 44
8a, n.d.
8a, 6
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
8[d]
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
9[e]
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
0[f]
1
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
1[g]
1
6
8a, 50
4
). The use of an oxygen-saturated solution did not improve
further the reaction yield (entry 5). Conversely, when an
aqueous reaction medium (MeCN/H O 5:1, entry 6) was used,
compound 8a was obtained in 43% yield. Finally, the addition of
a mild base, i.e. NaHCO (acting as an insoluble additive, 0.5
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
1
1
2
3
7
7
7
7
9a, 39
9a, 55
9a, 41
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (1 equiv)
2
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (0.5 equiv)
14[g]
15[g]
3
equiv.), to buffer the acidity developed during the process was
investigated. HDA cycloadduct 8a was formed in an improved
yield of 49% (entry 7). Addition of 1 equiv. bicarbonate under the
same conditions afforded the desired product in comparable
yield (48% data not shown). On the other hand, when the
reaction was performed by irradiating in the UV-A range with a
MeCN/H2O 5:1, O2, 1 + 2 h
NaHCO3 (1 equiv)
9
a, 63
[
a]
Reaction conditions: 5a (0.26 mmol), 6 or 7 (0.52 mmol) and TBADT ((n-
Bu N) 32], 2 mol%) in 10 mL of the chosen reaction medium. Reaction
yields were determined via HPLC analysis.
4
4
[W10O
[b]
Massive polymerization of 6
occurred. TBADT, 4 mol%. Reaction carried out by irradiation with a 18W
[c]
[d]
[
e]
3
65 nm LED (18W power), a slightly diminished yield (44%,
entry 8) was observed. In all cases, the observed conversion of
a was > 95%. Furthermore, blank experiments confirmed that
LED with emission centered at 365 nm. The reaction vessel was wrapped in
[f]
aluminium foil (blank experiment). Reaction carried out in the absence of
[
g]
TBADT (blank experiment).
5a was replaced by the corresponding
5
potassium salt 5'a. n.d.: not detected.
light and photocatalyst were both necessary for the reaction to
occur (entries 9, 10). Interestingly, when replacing
benzohydroxamic acid with the corresponding potassium salt
(
5'a) under optimized reaction conditions, essentially the same
yield of 8a was consistently observed (entry 11).
Afterwards, we extended our optimization to 1,3-cyclohexadiene
(
7) in the role of the diene. Encouragingly, when reaction
conditions reported in entry 7 were adopted, HDA cycloadduct
a was obtained in 39% yield (entry 12). Notably, a further
increase in the amount of NaHCO present in the reaction vessel
from 0.5 to 1 equiv.) led to an improved formation of 9a (55%,
9
3
(
entry 13), with an almost negligible formation of tar.
Finally, 5a could be conveniently substituted with 5'a, affording
9
a in 41% with half equiv. bicarbonate (entry 14) and pleasantly
leading to 9a in > 60% yield with 1 equiv. base (entry 15).
Moreover, by tracking the reaction time profile for the formation
of 7a and 8a under the optimized conditions, we confirmed that
1
hour was the time required for the reaction to occur (Figure 1).
Figure 1. Reaction time profiles for the formation of products 8a and 9a.
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European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
Table 2. TBADT-Mediated [4+2] Cycloaddition Between 1,3-Cycloalkenes (6
and 7) and Substituted Hydroxamic Acids 5a-j.
decompose leaving the final stable products of these processes,
[
a]
i.e. compounds 10 and 11.
TBADT (2 mol%),
NaHCO (0.5 equiv)
O
O
3
n
OH
n
O
OH
+
10
N
O
NH
R
N
Ph
TBADT (2 mol%),
NaHCO (0.5 equiv)
MeCN/H
2
O 5:1, O
2
H
O
SolarBox, 1 + 2 h
n
3
5
a-h
6, n = 1
, n = 2
8a-h, n = 1
9a-h, n = 2
a)
+
MeCN/H
2
O 5:1, O
2
MeO
OMe
7
SolarBox, 1 + 2 h
5
i
6, n = 1
7, n = 2
(
8i or 9i not formed)
OH
O
O
OMe
O
n
n
n
O
Br
O
N
O
N
Ph
N
OH
NH
H
N
H
N
O
11
TBADT (2 mol%),
NaHCO (0.5 equiv)
8
9
a, 49%
a, 55%
8b, 30%
9b, 34%
8c, 55%
9c, 48%
n
3
b)
+
O
O
2
N
NO
2
MeCN/H
2 2
O 5:1, O
O
O
O
n
n
n
n
SolarBox, 1 + 2 h
5
j
6, n = 1
7, n = 2
(8j or 9j not formed)
O
O
O
N
NO
2
N
N
O
O
Br
8
9
d, 65%
d, 56%
8e, 44%
9e, 37%
8f, 49%
9f, 46%
Scheme 2. HDA cycloaddition between substituted hydroxamic acid 5i,j and
cyclopentadiene or 1,3-cyclohexadiene.
O
O
n
O
O
N
H
3
C
N
n-Pr
As reported in Scheme 3, we also performed dedicated control
experiments to have some insights into the mechanism, In
8
9
g, 84%
g, 95%
8h, 33%
9h, 53%
particular,
di-
(5''a)
and
mono-methylated
(5'''a)
[
a]
benzohydroxamic acids were subjected to the reaction under
optimized conditions. However, either in the presence of 6 or 7,
no product formation was observed, with recovery of the
unreacted starting materials.
Reaction conditions: 5a-h (0.26 mmol), 6 or 7 (0.52 mmol) and TBADT ((n-
Bu N) 32], 2 mol%) in the presence of NaHCO (0.5 equiv.) in 10 mL of
MeCN/H O 5:1. Solutions were bubbled with oxygen (3 min), capped and then
irradiated under simulated solar light (SolarBox equipped with a Xe lamp; 500
4
4
[W10
O
3
2
–
2
W
m ). The obtained products were isolated via flash column
chromatography.
TBADT (2 mol%),
O
With the optimized conditions in hand, we started investigating
the scope of the reaction by employing different hydroxamic acid
derivatives.
NaHCO3 (0.5 equiv)
n
O
a)
+
no reaction
N
MeCN/H O 5:1, O
2
2
SolarBox, 1 + 2 h
5
''a
6, n = 1
As shown in Table 2, reactions were performed on
a
7, n = 2
representative set of hydroxamic acids bearing aromatic (5a-d;
[
16]
brominated 5c,d were synthesized by literature protocols),
TBADT (2 mol%),
NaHCO3 (0.5 equiv)
O
[
17]
heteroaromatic (5e-f, synthesized by literature protocols) and
aliphatic (5g-h) substituents. Reactions conducted on
proceeded smoothly affording the cyclopentadiene derivatives
a-h generally in good yields, ranging from 30 to 84%. Even
n
O
b)
+
no reaction
N
H
5
MeCN/H2O 5:1, O2
SolarBox, 1 + 2 h
5
'''a
6, n = 1
8
7, n = 2
better results were obtained by using 1,3-cyclohexadiene 7 as
trapping agent, and products 9a-h were isolated in yields
ranging from 34 to 95%. Notably, while compounds 8b,d,g and
Scheme 3. Control experiments.
[
18-20]
9
b,d,g are known in the literature,
all the other products
were here prepared for the first time and fully characterized (see
experimental details and full characterization in the SI).
To shine light on the initial photocatalytic process, we performed
dedicated laser flash photolysis (LFP) experiments. These were
intended to study the evolution in time of the different species
deriving from the decatungstate anion in the presence of
increasing amounts of acetohydroxamic acid (5g). As already
reported in the literature, upon absorption of a UV-photon, the
decatungstate anion ultimately populates a reactive excited state
named wO (with a lifetime around 50-60 ns in neat acetonitrile)
that can interact with organic substrates present in solution to
In view of evaluating the scope of the protocol and to have
additional insight on potential electronic effects, we further
investigated the reactions of two substituted benzohydroxamic
acids, bearing respectively electron-donating (-OMe, 5i) and
electron-withdrawing (-NO
reactions were performed under identical conditions as in Table
, but the expected products 8i,j and 9i,j were not isolated.
However, the reaction mixtures revealed the presence of ketone
0 and the urea 11, whose structures are closely related to the
2
, 5j) groups (Scheme 2). The
2
5
–
give the mono-reduced form [W10
O
32] . Notably, both species,
32] , are deeply blue colored and their
evolution in time can be easily tracked by monitoring the
5
–
1
viz. wO and [W10
O
[
21]
[22]
dimerization and decomposition, with rearrangement, of the
[
1]
[
14]
original nitrosocarbonyl intermediates
11i,j. These results
absorption of the solution at 780 nm.
Thus, upon laser
somewhat confirm the generation of the expected intermediates,
that do not survive for long and enough to be trapped by the
dienes in solution. On the other hand, they dimerize and
excitation at 355 nm of decatungstate solution in
a
acetonitrile/water 5:1, it was possible to observe the decay of the
wO species, which showed a lifetime of 57 ns. Upon addition of
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European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
increasing amounts of the quencher 5g in the range from 0 to
oxidizing species wO, whose redox potential (~ + 2.4 V vs SCE)
-
3
9
·10 M (see SI), the lifetime decreased more and more, finally
is sufficient to trigger the mono-electronic oxidation of
•
+
reaching a minimum value around 38 ns. On a longer timescale,
a persistent and flat signal, attributed to the mono-reduced form
hydroxamic acids 5 (as an example, E(5a /5a) = + 1.35 V vs
[
24]
•+
SCE), leading to the corresponding radical cations 5 and the
5
–
5–
[
W
10
O
32] , is apparent and its intensity increases along with the
reduced photocatalyst [W10O32] . The latter is then restored by
amount of 5g present in solution (Figure 2). Additionally, a
the oxygen dissolved in solution, which is an essential reaction
partner as demonstrated by the process performed under inert
Stern-Volmer plot allowed to determine the bimolecular
8
–1 –1
•+
quenching constant (k
Q
) to be 4.6·10 M ·s . This value
atmosphere (see Table 1). On the other hand, 5 can undergo
indicates a very efficient quenching of wO by 5g. On the other
hand, literature reports indicate that alkenes can likewise
quench wO, ultimately leading to the corresponding oxidized
deprotonation, aided by the inorganic base present in the
system, and the desired nitrosocarbonyl intermediates 11 are
obtained upon reaction with superoxide anion (in turn formed in
the photocatalyst restoration step). The final HDA adducts 8 or 9
result from the [4+2] cycloaddition of 11 with 6 or 7. Indeed, the
reaction worked satisfactorily also when adopting 5'a as the
substrate and a similar reaction pathway as that described for
the neutral species 5a can be proposed. It is worth noting that
we can safely exclude the in situ 5'a from 5a promoted by
[
23]
derivatives when oxygen is present in the reaction mixture.
Indeed, we cannot exclude that this pathway is operative also
under our conditions, nevertheless it is not productive, since we
never observed oxidation products deriving from 6 or 7.
NaHCO
3
, based on the acidity values available in the literature
[
24,25]
(
pK for 5a = 8.80).
A
Conclusions
Among the available alternatives, decatungstate salts have
emerged as valuable photocatalysts for sunlight-induced organic
reactions through
a
hydrogen atom transfer (HAT)
[
13,15]
mechanism.
Indeed, the present study adds to the few
[
26]
instances, wherein TBADT can be exploited as a photoredox
catalyst. Thus, we have here demonstrated the possibility to
oxidize hydroxamic acids for the in situ generation of fleeting
nitrosocarbonyl intermediates. As previously reported for
[
26]
analogous TBADT-photoredox catalyzed processes,
the
reaction required a fine tuning of reaction conditions, mainly in
terms of solvent and additives. Decatungstate is able to promote
the oxidation of hydroxamic acids to nitrosocarbonyl
intermediates in a very short time, with only 1h irradiation and
reaction completion is guaranteed upon leaving the reaction in
the dark for a residual time. Trapping was efficiently obtained
with reactive dienes, such as cyclopentadiene and 1,3-
cyclohexadiane, to afford the desired HDA cycloadducts in fair to
good yields. Optimism for future progress lies in the extension of
the present methodology to the challenging ene reaction of
nitrosocarbonyl intermediates and applications to more complex
molecules.
Figure 2. Nanosecond decays at 780 nm of the wO transient in the presence
of increasing concentrations of acetohydroxamic acid 5g in MeCN/H
2
O 5:1. (a)
-
3
-3
-3
-3
0
9
M; (b) 1.87·10 M; (c) 3.75·10 M; (d) 5.62·10 M; (e) 7.50·10 M; (f)
.37·10 M. Insert: Stern-Volmer plot of the recorded decay.
-3
O
OH
R
N
H
5
wO
hν
O
Photocatalytic
Cycle
OH
R
N
H
W10O32]4-
[W10O32]5-
[
5
[4+2]
Cycloaddition
Experimental Section
O
O
n
O2
O
O
R
N
R
N
n
Typical Procedure for HDA Cycloaddition Reactions. In a cylindrical
flask (Ø 4cm x h 3 cm) 0.26 mmol of the chosen hydroxamic acid were
dissolved in the acetonitrile (10 mL) or acetonitrile/water mixture (10+2
mL) along with 2% TBADT and 2 equivalents of suitable dienes (freshly
distilled cyclopentadiene or 1,3-cyclohexadiene). When required, suitable
H2O2
11
8
9
(n =1)
(n = 2)
O2
6
7
(n =1)
(n = 2)
Scheme 4. Plausible reaction mechanism.
amounts of NaHCO
3
were also added. The mixture was saturated with
O
2
gas for three minutes in the cases reported. The sealed flask was
-
2
then irradiated in a Solarbox 1500 (Xe lamp, 500 Wm ) for a proper
reaction time and eventually left under stirring in the dark when
requested. The mixture was filtered and the clear solution passed
through LC-SAX SPE tubes (1 mL) for capturing cations and anions. The
resulting solution was diluted to 25 mL in acetonitrile and analyzed in
On the basis of the above investigations, a plausible reaction
mechanism can be suggested, as shown in Scheme 4. Thus,
upon irradiation with solar light, TBADT populates the highly
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European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
1
HPLC for the reaction yield determination. H NMR spectra were also
recorded to corroborate the structure assignments.
Weinheim, 2018; c) F. Strieth-Kalthoff, M. J. James, M. Teders, L.
Pitzer, F. Glorius, Chem. Soc. Rev. 2018, 47, 7190–7202; d) J. Twilton,
C. Le, P. Zhang, M. H. Shaw, R. W. Evans, D. W. C. MacMillan, Nat.
Rev. Chem. 2017, 1, 0052.
For all the newly synthesized compounds the full characterization was
conducted and the relative data are reported in the SI.
[
[
[
11] a) D. M. Schulz, T. P. Yoon, Science 2014, 343, 1239176; b) D. Ravelli,
D. Dondi, M. Fagnoni, A. Albini, Chem. Soc. Rev. 2009, 38, 1999–2011.
12] V. Santacroce, R. Duboc, M. Malacria, G. Maestri, G. Masson, Eur. J.
Org. Chem. 2017, 2095–2098.
Acknowledgements
13] a) D. Ravelli, M. Fagnoni, T. Fukuyama, T. Nishikawa, I. Ryu, Acs
Catalysis 2017, 8, 701–713; b) M. D. Tzirakis, I. N. Lykakis, M.
Orfanopoulos, Chem. Soc. Rev. 2009, 38, 2609−2621.
Support by the University of Pavia and MIUR (PRIN 2011; CUP
F11J12000210001) is gratefully acknowledged. We also thank
[
14] a) V. De Waele, O. Poizat, M. Fagnoni, A. Bagno, D. Ravelli, ACS Catal.
“
VIPCAT – Value Added Innovative Protocols for Catalytic
2
016, 6, 7174–7182 and references therein; b) I. Texier, J. A. Delaire,
Transformations” project (CUP: E46D17000110009) for valuable
financial support.
C. Giannotti, Phys. Chem. Chem. Phys. 2000, 2, 1205–1212; c) C.
Tanielian, Coord. Chem. Rev. 1998, 178–180, 1165–1181.
[
15] D. Ravelli, S. Protti, M. Fagnoni, Acc. Chem. Res. 2016, 49, 2232–2242.
Keywords: Nitrosocarbonyl intermediates • Hydroxamic acids •
Decatungstate • Photoredox catalysis • Hetero Diels-Alder
cycloaddition
[16] Organic Syntheses, Coll. Vol. 2, p.67 (1943); Vol. 19, p.15 (1939).
[
[
[
[
[
[
[
17] S. R. Sandler, W. Karo, Organic Functional Group Preparations,
Academic Press, Eds.; New York: 1972, Vol III, pp.406–432.
18] P. Quadrelli, M. Mella, A. Gamba Invernizzi, P. Caramella, Tetrahedron
1
999, 55, 10497–10510.
19] C. P. Frazier, A. Bugarin, J. R. Engelking, J. Read de Alaniz, Org. Lett.
012, 14, 3620–3623.
20] L. H. Dao, J. M. Dust, D. Mackay, K. N. Watson, Can. J. Chem. 1979,
7, 1712–1719.
21] P. Quadrelli, G. Campari, M. Mella, P. Caramella, Tetrahedron Lett.
000, 41, 2019–2022.
22] P. Quadrelli, M. Mella, P. Caramella, Tetrahedron Lett. 1999, 40, 797–
00.
23] A. Molinari, R. Amadelli, V. Carassiti, A. Maldotti, Eur. J. Inorg. Chem.
000, 2000, 91–96. b) C. Tanielian, R. Seghrouchni, C. Schweitzer, J.
Phys. Chem. 2003, 107, 1102–1111. c) I. N. Lykakis, M.
[
1]
M. G. Memeo, P. Quadrelli, Chem. Rev. 2017, 117, 2108–2200 and
references therein.
2
[
[
[
2]
3]
4]
G. W. Kirby, Chem. Soc. Rev. 1977, 6, 1–24.
B. S. Bodnar, M. J. Miller, Angew. Chem. Int. Ed. 2011, 50, 5630–5647.
P. Quadrelli, M. Mella, A. Gamba Invernizzi, P. Caramella, Tetrahedron
5
1
999, 55, 10497–10510.
2
[
[
[
5]
6]
7]
P. Quadrelli, P. Caramella, Curr. Org. Chem. 2007, 11, 959–986.
M. G. Memeo, P. Quadrelli, ARKIVOC 2013, (i), 418–423.
C. P. Frazier, L. I. Palmer, A. V. Samoshin, J. Read de Alaniz,
Tetrahedron Lett. 2015, 56, 3353–3357.
8
2
[
8]
a) M. Baidya, K. A. Griffin, H. Yamamoto, J. Am. Chem. Soc. 2012, 134,
A
1
1
3
8
8566–18569. b) T. Kano, F. Shirozu, K. Maruoka, Org. Lett. 2014, 16,
530–1532. c) B. Maji, M. Baidya, H. Yamamoto, Chem. Sci. 2014, 5,
941–3945. d) B. Maji, H. Yamamoto, Angew. Chem. Int. Ed. 2014, 53,
714–8717.
Orfanopoulos, Synlett 2004, 2131–2134.
[
[
[
24] M. Masui, Y. Kaiho, T. Ueshima, S. Ozaki, Chem. Pharm. Bull. 1982,
3
0, 3225–3230.
25] O. Exner, W. Simon, Collect. Czech. Chem. Commun. 1965, 30, 4078–
094.
26] a) S. Montanaro, D. Ravelli, D. Merli, M. Fagnoni, A. Albini, Org. Lett.
012, 14, 4218–4221; b) L. Capaldo, L. Buzzetti, D. Merli, M. Fagnoni,
[
[
9]
T. Kano, F. Shirozu, K. Maruoka, J. Am. Chem. Soc. 2013, 135,
8036–18039.
4
1
10] a) Photoorganocatalysis in organic synthesis, Eds.: M. Fagnoni, S.
Protti, D. Ravelli, World Scientific Publishing Europe Ltd, 2019, DOI:
2
D. Ravelli, J. Org. Chem. 2016, 81, 7102–7109; c) L. Capaldo, R.
Riccardi, D. Ravelli, M. Fagnoni, ACS Catal. 2018, 8, 304–309.
1
0.1142/q0180; b) Visible Light Photocatalysis in Organic Chemistry,
Eds.: C. R. J. Stephenson, T. P. Yoon, D. W. C. MacMillan, Wiley-VCH,
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European Journal of Organic Chemistry
10.1002/ejoc.201900596
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Photocatalysis, Nitrosocarbonyl
Teresa Basile, Luca Capaldo, Davide
[
a]
Ravelli,* and Paolo Quadrelli*
Page No. – Page No.
Photocatalyzed
Generation
of
Tetrabutylammonium decatungstate (TBADT) promoted the oxidation of
hydroxamic acids to nitrosocarbonyl intermediates. The reaction took place under
aerobic conditions upon irradiation with solar light. The thus generated
nitrosocarbonyls were efficiently trapped with reactive dienes to afford the
corresponding Hetero Diels-Alder (HDA) cycloadducts in good yields.
Nitrosocarbonyl Intermediates under
Solar Light Irradiation
This article is protected by copyright. All rights reserved.