Visible light photooxidation of nitrate: The dawn of a nocturnal radical

Article abstract of DOI:10.1039/c5cc01580d

Highly oxidizing nitrate radicals (NO₃^?) are easily accessed from readily available nitrate salts by visible light photoredox catalysis using a purely organic dye as the catalyst and oxygen as the terminal oxidant. The interaction of the excited catalyst and nitrate anions was studied by spectroscopic methods to elucidate the mechanism, and the method was applied to the NO₃^? induced oxidation of alkynes and alcohols.

Full text of DOI:10.1039/c5cc01580d

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DOI: 10.1039/C5CC01580D
COMMUNICATION
Visible light photooxidation of nitrate: The dawn of a
nocturnal radical
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
b
a
T. Hering, T. Slanina, A. Hancock, U. Wille* and B. König* ,
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
•
14, 20
8
Highly oxidizing nitrate radicals (NO
3
) are easily accessed 350 nm)
However, the use of toxic gases, high electrode
from readily available nitrate salts by visible light potentials, or UV irradiation are so far limiting the applications
photoredox catalysis using a purely organic dye as the and lead to undesired side reactions.
catalyst and oxygen as the terminal oxidant. The inter- We were pleased to observe that, upon excitation of the organic
action of the excited catalyst and nitrate anions was phototcatalyst 9-mesityl-10-methylacridinium perchlorate (1)
•
studied by spectroscopic methods to elucidate the mech- with blue light, oxidation of nitrate anions to NO , readily
3
•
•
anism, and the method was applied to the NO3 induced occurs (Scheme 1), thus providing a convenient access to NO3
oxidation of alkynes and alcohols.
on a preparative scale. 9-Mesityl-10-methylacridinium per-
chlorate ( ) was chosen, because it is known to have a strong
oxidizing capacity in the excited state.
) is the most important nocturnal free knowledge, this is the first visible light mediated generation of
1
2
1, 22
To the best of our
•
The nitrate radical (NO
3
radical oxidant in the troposphere and thus accounts for the nitrate radicals.
1
majority of the oxidative reactions at night-time. In the
atmosphere NO3 oxidizes a broad scope of volatile organic
•
singlet states (S)
strongly oxidizing (2.18 V - 2.08 V)
nanosecond lifetime
2
, 3
4, 5
1
6
species including alkenes,
alcohols,
terpenes, esters,
E
1
and sulfides. It is a highly reactive and chemically versatile
centered radical with an oxidation potential of +2.00 V (vs.
SCE in MeCN). Apart from electron transfer (ET),
O
-
LES
CTS
7
NO3-
*
8
9, 10
•
chemical reactions
NO3
1
, 11
N
also reacts by addition to

systems
Overall, the reactivity of NO3 with
organic molecules can be seen in between that of hydroxyl
and by hydrogen atom
ISC
N
8
, 12, 13
•
abstraction (HAT).
NO3
triplet states (CTT and LET)
+1.88 V - 1.45 V, longer lifetimes
oxidation potential too low
•
•- 14
blue light
455 nm)
radicals (OH ) and sulfate radical anions (SO ).
4
(
Despite its high chemical versatility, it is surprising that only
•
limited synthetic applications of NO3 are available so far.
Shono reported the addition of electrochemically generated
N
•
11
•
O2
NO to alkenes. The reaction of NO with cyclic alkynes and
3
3
Acr - Mes
max = 520 nm
O2
alkynones was employed to obtain cis-fused bicyclic ketones in
N
1
5, 16
self-terminating oxidative radical cyclizations.
This concept
+
Acr -Mes (1)
was later extended to alkyne ethers yielding tetrasubstituted
1
7, 18
•
tetrahydrofurans.
One reason for the limited use of NO as
•
3
3
Scheme 1 Proposed mechanism of visible light mediated generation of NO via
+
-
a reagent in organic transformations is its rather difficult photocatalytic oxidation by Acr -Mes (1). The electron transfer from NO
accessibility. Common methods for NO3 generation on
preparative scale in solution are the reaction of nitrogen dioxide
3
occurs from
S
S
•
the short-lived singlet state (LE or CT ) with sufficient oxidative capacity to generate
•
•
T
the reduced catalyst Acr -Mes and NO
3
, the longer lived transient triplet species (CT or
. The reduced catalyst Acr -Mes is regenerated by
T
-
•
LE ) is not reactive towards NO
3
1
,
19
11
23, 25, 26
and ozone,
electrooxidation of nitrate anions
or the oxygen. (All oxidation potentials are given vs. SCE in MeCN or PhCN).
photolysis of (NH ) Ce(NO ) (CAN) with UV light (

=
4
2
3 6
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•
In order to elucidate the mechanism of the NO formation, we of light or catalyst no reaction occurred (entries 7, 9). However,
3
•
monitored the generation of the reduced catalyst Acr -Mes in small amounts of diketone
3 were formed in the direct reaction
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the presence of LiNO upon continuous irradiation of a 5 µM of
solution of Acr -Mes (1) in MeCN with 455 nm light under (entry 8).
2
with the excited catalyst in the absence of nitrate ions
DOI: 10.1039/C5CC01580D
3
+
anaerobic conditions. The differential absorption spectrum
•
a
3
Table 1 Oxidation of diphenylacetylene 2 by NO .
•
shows the appearance of Acr -Mes with a maximum at 520 nm
2
1, 23
Mes
after irradiation for 120 s and 240 s. (see supporting
information, Figure S6) This observation suggests a direct
1
–
N
O
O
ClO4-
oxidation of NO3 by the excited catalyst and demonstrates that
5 mol%
Ph
–
Ph
Ph + LiNO3
2 eq.
+
Ph
Ph
Ph
NO3 can act as an electron donor to the excited catalyst. The
455 nm LED,
•
O
reduced catalyst Acr -Mes is stable under argon, however, the
2
2 h, air, MeCN
3
4
signal vanishes completely after aeration of the reaction
•
Yield b
+4 (%)
mixture due to reoxidation of Acr -Mes to the ground state
Entry
Conditions
5 mol% 1, air
3
+
24
catalyst Acr -Mes by oxygen (see Scheme 1). The negative
signal at < 460 nm in the differential absorption spectrum is
caused mainly by the decrease of the ground state absorption of

1
2
50 (30+20)
55 (31+24)
5 mol% 1, O
2
+
•
Acr -Mes as a result of the formation of Acr -Mes and partial
+
†
photobleaching of Acr -Mes. The long-lived triplet state with
3
4
NaNO3
41 (27+15)
38 (24+14)
a microsecond lifetime is generally discussed as the reactive
10 mol% 1
2
5, 26
state in most oxidative reactions.
The exact nature of this
T
state is controversial and could be both a CT state with an
oxidation potential of +1.88 V vs. SCE, as reported by
5
6
7
8
9
(NH
4
)
2
S
2
O
8
, N
2
atmosphere
46 (27+19)
DCM
52 (32+20)
2
5
T
Fukuzumi
or a locally excited triplet state, LE , with an
without light
0
oxidation potential of +1.45 V vs. SCE as reported by
2
6
-
Verhoeven, However, neither would have the oxidative
without NO
3
13 (3 only)
0
–
capacity to oxidize NO . Recent detailed mechanistic
3
without 1
investigations by the group of Nicewicz revealed that for
substrates with oxidation potentials exceeding +1.88 V (vs.
a) Reactions were carried out using diphenylacetylene (2, 0.25 mmol) and the
respective amount of 9-mesityl-10-methylacridinium perchlorate (1) in 1 mL
of MeCN unless otherwise noted with an irradiation time of 2 h. b)
SCE), a reaction should occur out of the short-lived excited
S
singlet state (mainly CT ), which has an estimated oxidation Quantitative GC yields using acetophenone as internal standard.
2
3
potentials of 2.08 V (Scheme 1). Since both singlet states are
2
3
According to computational studies, the mechanism for the
fluorescent (ɸF ̴ 8%), whereas the triplet states do not emit, we
•
NO3 induced oxidation of diphenylacetylene, diketone
benzophenone ( ) are formed through competing pathways in
the initial vinyl radical adduct (Scheme 2). While diketone
results from a 5-endo cyclization, followed by loss of NO , the
key-step in the formation of benzophenone is
fragmentation with elimination of NO , and subsequent Wolff-
3 and
performed fluorescence quenching experiments to explore the
–
4
nature of the reactive state involved in NO3 oxidation. A clear
5
3
quenching of the fluorescence by LiNO3 confirms that
•
-
oxidation of NO occurs from the singlet excited state of
1 (see
3
(4
)
-
supporting information, Figure S6). Moreover, laser flash
photolysis experiments confirmed that no interaction of the
•
2
–
rearrangement of the carbene intermediate
oxidative decarboxylation.
7
followed by
long lived triplet state and NO3 can be observed. (Figure S8 in
2
7
the supporting information) Based on these findings, we
suggest that the reaction proceeds via a singlet excited state as
depicted in Scheme 1.
Having demonstrated the pathway for photocatalytic NO
generation, we selected the well-studied reaction of NO3 with
•
3
•
diphenylacetylene (2) yielding benzil (3) and benzophenone (4)
to explore the synthetic application of this new method and to
compare it with the previously reported methods. The results
are compiled in Table 1. Under photocatalytic conditions using
• 27
3
Scheme 2 Proposed mechanism for the oxidation of aromatic alkynes by NO .
+
5
mol% of Acr -Mes
(
1
), 0.25 mmol of alkyne
and ketone were obtained after 2 h of
irradiation with blue light (= 455 nm) with yields comparable
2 and 2 eq. of
LiNO , diketone
3
4
•
3
Next, we applied the photocatalytic NO3 formation to the
synthesis of tetrasubstituted tetrahydrofurans, which proceeds
2
7
to previous methods.
When oxygen was replaced by
•
via a self-terminating radical cascade that is initiated by NO
addition to the triple bond in alkyne
described previously using either anodic oxidation of lithium
3
ammonium persulfate as the electron acceptor in a degassed
system, the yield and product ratio was not changed
significantly (entry 5). This shows that potential interfering
reactions by singlet oxygen could be excluded. In the absence
9
. The reaction was
17, 18
nitrate or CAN photolysis.
The starting material 9 (Scheme
2
| J. Name., 2012, 00, 1‐3
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ARTICLE
using
3
) contains an aliphatic alkyne, which is more difficult to irradiation with blue light in the presence of LiNO
oxidize compared to and thus decreases the background 5mol% of in acetonitrile. No reaction was observed in the
reaction that is caused by direct oxidation of by the absence of nitrate, which clearly confirms the role of NO3 in
photocatalyst. The reaction of 9b with 2 eq. of LiNO and 5 this reaction. Stepwise reduction of the amount of LiNO from
3
2
1
View Article Online
•
9
DOI: 10.1039/C5CC01580D
3
3
•
mol%
67% based on conversion), with 45% of the starting material can act as mediator in this reaction (Scheme 5). An
being recovered. Methyl ether 9a gave lower yields and an acidification of the solution due to formation of nitric acid was
incomplete conversion, which can be rationalized by a non- observed, but no apparent influence on the reaction or the
1 gave the anticipated product 10b in a yield of 37% 2 eq. to 20 mol% did not affect the outcome, showing that NO3
(
9
b
•
‡
§
regioselective addition of NO3 to both ends of the alkyne, in stability of the catalyst was found.
accordance with previous reports. The low conversion (and
resulting low product yield) is likely due to the fact that NO3
•
leads to degradation of catalyst 1. This effect could also be
observed in UV/Vis measurements of the reaction mixture,
which showed severe photobleaching of the ground state during
•
irradiation (see Figure S7 in the Supporting Information). It is Scheme 5 Experimental conditions and results for the NO3 mediated oxidation of
alcohols.
likely that the observed degradation proceeds via oxidation of
the methyl groups on the mesityl moiety of the catalyst, which
8
The scope of this method was explored towards other non-
activated alcohols and electron deficient benzyl alcohols. All
reactions were carried out by two sequential additions of
is a known degradation pathway that leads to loss of catalytic
2
8
activity. The problem of low conversion could be partly
overcome through slow addition of the catalyst via syringe
pump.
5
mol% of 1 in order to counteract the loss of catalytic activity
caused by degradation of the catalyst. The reactions proceed
with good selectivity (see Table 2, entries 1, 2, 4), but the
conversion was incomplete and unreacted starting material was
recovered. Aliphatic (entries 1, 2, 3) and benzylic alcohols
(
entries 4,6) were converted.
•
3
Table 2 Experimental conditions and results for the NO mediated oxidation
a
Scheme 3 Self-terminating radical oxidative cyclization to tetrasubstituted of alcohols.
1
7, 18
tetrahydrofurans 10
.
yield
recovered
starting
material (%)
Entry
1
alcohol
Product
product
•
b
b
Apart from addition to
hydrogen abstraction,

systems, NO
3
also reacts through
(%)
8
,12,13
which was explored in the catalytic
•
oxidation of non-activated alcohols. In this reaction, NO3 acts
as a redox mediator, which is regenerated during the catalytic
cycle, according to the mechanism in Scheme 4. Initial HAT
leads to the regeneration of
NO3 as nitric acid and formation of radical 12. The latter is
subsequently oxidized by either NO or oxygen to give cationic
intermediate 13, which deprotonates to yield ketone 14. The
mechanism is similar to the indirect anodic oxidation of
alcohols by nitrate. Donaldson and Styler reported the
enhanced gas phase oxidation of propanol under UV irradiation
45 (79)
42 (95)
44
56
•
29
from the alcohol carbon by NO3
–
2
3
•
3
40 (40)
--
3
0
c
using TiO2 co-embedded with KNO . The finding was
4
55 (100)
45
3
•
explained by formation of NO3 and its ability to abstract
hydrogen atoms from the alcohol carbon atom.
3
1
d
d
d
5
--
--
6
16 (20)
17
Scheme 4 General mechanism of the nitrate mediated alcohol oxidation via initial a) Reactions carried out using 0.25 mmol of the alcohol 11, 1 eq. of LiNO
3
hydrogen abstraction followed by oxidation and loss of a proton.
and 10 mol% of 1 (two subsequent additions of 5 mol%) in 1 mL of MeCN
with an irradiation time of 6 h. b) Isolated yields, in brackets yield based on
3
conversion. c) Background reaction without LiNO is 9%. d) Decomposition
of substrate 11e.
The reaction was explored using tert-butyl cyclohexanol (11a
)
and the results are compiled in Scheme 5. To our delight,
oxidation into the corresponding ketone 14a occurred upon
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In the oxidation of isomenthol (11b) (entry 2) the configuration 6. S. Langer, E. Ljungstrom and I. Wangberg, J. Chem. Soc., Faraday
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View Article Online
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acid/base reaction of pyridine with nitric acid that is generated 8. E. Baciocchi, T. D. Giacco, S. M. Murgia and G. V. Sebastiani, J.
¶
•
during this reaction by the H-abstraction by NO or a possible
Chem. Soc., Chem. Commun., 1987, 1246-1248.
3
direct oxidation of the nitrogen of pyridine by the photocatalyst 9. H. Suzuki and T. Mori, J. Chem. Soc., Perkin Trans. 2, 1996, 677-
•
32
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683.
3
1
1
1
0. E. Baciocchi, I. Del Giacco, C. Rol and G. V. Sebastiani,
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Kashimura, Synthesis, 1994, 895-897.
In conclusion, we described a new and simple access to highly
reactive nitrate radicals using visible light photocatalysis with
an organic dye as the photoredox catalyst. This method avoids
the use of toxic compounds, or high electrochemical potentials
and is, to the best of our knowledge, the first method yielding
2. Andrey A. Fokin, Sergey A. Peleshanko, Pavel A. Gunchenko,
Dmitriy V. Gusev and Peter R. Schreiner, Eur. J. Org. Chem., 2000,
3
357-3362.
1
3. M. Mella, M. Freccero, T. Soldi, E. Fasani and A. Albini, J. Org.
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•
NO3 in a catalytic process using in visible light. We verified
the formation of nitrate radicals by observation of the reduced
1
1
1
1
1
1
4. U. Wille, Chem. Eur. J., 2002, 8, 340-347.
•
catalyst Acr -Mes and showed that the mechanism is
5. U. Wille, J. Am. Chem. Soc., 2001, 124, 14-15.
proceeding via the singlet excited state of the catalyst. By
investigating the addition to aromatic alkynes, a previously well
studied model reaction of NO , we showed that the photo-
catalytic procedure is as efficient as the previously employed
methods.
6. U. Wille, Chem. Rev., 2012, 113, 813-853.
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2
2
0. D. C. E. Sigmund and U. Wille, Chem. Commun., 2008, 2121-2123.
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Acknowledgements
Financial
support
by
the
Deutsche
Forschungs-
gemeinschaft (DFG), the GRK 1626 and the Australian
Research Council is acknowledged. TH thanks the Fonds der
Deutschen Chemischen Industrie for a fellowship.
2
2
2
2
2
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1
Notes and references
a
Institut für Organische Chemie, Universität Regensburg,
2
Universitätsstrasse 31, D-93053 Regensburg, Germany.
,
b
School of Chemistry and BIO21 Molecular Science and Biotechnology
1
Institute, The University of Melbourne, 30 Flemington Road, Parkville,
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Ramesdonk, M. M. Groeneveld, H. Zhang and J. W. Verhoeven, J.
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VIC 3010, Australia.
+
†
After aeration the ground state absorption of Acr -Mes cannot be fully
recovered (see Supporting Information).
‡
§
For the mechanism of this reaction see SI.
The addition of different bases (LiNO
27. U. Wille and J. Andropof, Aust. J. Chem., 2007, 60, 420-428.
3
, LiOAc, pyridine, lutidine) did 28. A. C. Benniston, K. J. Elliott, R. W. Harrington and W. Clegg, Eur.
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J. Org. Chem., 2009, 253-258.
¶
based on the assumption that both the initial hydrogen abstraction and 29. S. Langer and E. Ljungstrom, J. Chem. Soc., Faraday Trans., 1995,
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