Molecular oxygen as a redox catalyst in intramolecular photocycloadditions of coumarins

Article abstract of DOI:10.1002/anie.201201222

Oxygen as a catalyst! While oxygen is usually excluded in nonoxidative photochemical reactions, the photocycloaddition of 3-(alk-4-en-1-yl)-substituted coumarins greatly benefits from the presence of O₂, which was shown to act as a redox catalyst, preferentially in combination with the common antioxidant 3,5-di-tert-butyl-4-hydroxytoluene (BHT). Copyright

Full text of DOI:10.1002/anie.201201222

Angewandte
Chemie
DOI: 10.1002/anie.201201222
Photochemistry with Visible Light
Molecular Oxygen as a Redox Catalyst in Intramolecular
Photocycloadditions of Coumarins**
Darius Paul Kranz, Axel Georg Griesbeck,* Ronald Alle, Raul Perez-Ruiz,
Jçrg Martin Neudçrfl, Klaus Meerholz, and Hans-Gꢀnther Schmalz*
Nature uses sunlight as a source of energy for various
chemical transformations by exploiting suitable chromo-
phores or light-absorbing photocatalysts. Taking natural
processes as a role model, various new photochemical trans-
formations are currently being developed especially targeting
applications in photovoltaics, solar energy storage, and water
splitting.[1] Moreover, photocatalysis enjoys increasing use in
organic synthesis,[2] most prominently in [2+2] cycloadditions
of enones,[3] in photoredox-mediated enantioselective orga-
nocatalysis,[4] and in the photoreductive generation of radicals
from alkyl bromides and chlorides.[5] Most common are metal-
based visible-light photocatalysts such as the readily acces-
sible complex [Ru(bpy)3Cl2] (bpy = 2,2’-bipyridyl).[6]
However, after a full day of irradiation only very little
conversion of 1 to a new (isomeric) product (rac-2) could be
detected by GC–MS analysis (Table 1, entry 1). By chance, we
Table 1: Intramolecular [2+2] photocycloaddition of the 3-substituted
coumarin derivative 1 under different conditions demonstrating an
accelerating effect of oxygen.
Entry
Conditions[a]
t [days]
1/rac-2[b]
Photochemical [2+2] cycloadditions are of importance
because cyclobutanes represent valuable synthetic intermedi-
ates and are also found as substructures in many bioactive
natural products.[7] The intramolecular [2+2] cycloaddition of
4-substituted coumarins was extensively studied by Bach and
co-workers, who even succeeded in performing such reactions
in an enantioselective fashion using chiral templates or Lewis
acids.[8]
Recently, we became interested in the intramolecular
[2+2] cycloaddition of coumarins bearing an unsaturated
alkyl substituent in the 3-position (such as 1), and we realized
that virtually no examples for this type of transformation have
been reported in the literature.[9] Herein we describe the
results of a study which has led to the discovery that such
transformations can be performed efficiently using visible
light in the presence of oxygen as a redox catalyst. Moreover,
the unique role of molecular oxygen as a promoter of these
(nonoxidative) photochemical transformations was proven by
spectroscopic and electrochemical methods.
1
2
3
4
5
6
degassed under Ar
under Ar
under air
1
4
4
4
4
4
95:5
59:41
10:90
5:95
5:95
55:45
under O2
under O2 + Rose B[c]
under Ar + Rose B[c]
[a] A solution of 1 was irradiated with a 150 W HQI lamp (white light) at
room temperature. [b] The ratio 1/rac-2 (conversion) was determined by
1H NMR spectroscopy. [c] Reaction performed in the presence of
1 mol% of Rose Bengal (Rose B.).
found that the formation of rac-2 was significantly faster when
a non-degassed solution of 1 (in the same solvent) was
employed (Table 1, entry 2). To probe whether oxygen might
promote the photocycloaddition, a third experiment (Table 1,
entry 3) was conducted using a solution of 1 flushed with air.
And indeed, after 4 days 90% conversion of 1 to rac-2 was
observed, which could even be improved by using pure
oxygen gas (Table 1, entry 4). The very clean reaction
afforded the product rac-2 as a single diastereomer (NMR
analysis) the structure of which was unambiguously proven by
X-ray crystal structure analysis (Figure 1). Two additional
experiments (Table 1, entries 5 and 6) showed that the
addition of 1 mol% of Rose Bengal (as a common singlet-
oxygen sensitizer) had at best very little effect on the reaction
outcome both in the presence and absence of oxygen gas.
In a second series of experiments the irradiation of 1 with
white light was investigated in the presence of oxygen and
different additives using a standard irradiation time of 24 h.
The results shown in Table 2 indicated that the presence of
Rose Bengal or other common sensitizers such as TPP
(tetraphenylporphyrin), or electron acceptors such as DCA
(9,10-dicyanoanthracene), DNB (1,4-dinitrobenzene), or
para-benzoquinone (p-BQ) did not result in any increase of
product formation. However, the addition of 5 mol% of the
Our study started with an attempt to synthesize the
cyclobutane rac-2 by irradiating a degassed dichloromethane
solution of the coumarin 1[10] using a 150 W HQI lamp.
[*] Dipl.-Chem. D. P. Kranz, Prof. Dr. A. G. Griesbeck, Dr. R. Alle,
Dr. J. M. Neudçrfl, Prof. Dr. K. Meerholz, Prof. Dr. H.-G. Schmalz
Department fꢀr Chemie, Universitꢁt zu Kçln
Greinstrasse 4, 50939 Kçln (Germany)
E-mail: griesbeck@uni-koeln.de
schmalz@uni-koeln.de
Dr. R. Perez-Ruiz
Departamento de Quimica
Universidad Politecnica de Valencia (Spain)
[**] This work was supported by the Universitꢁt zu Kçln and the Fonds
der Chemischen Industrie. We thank Dr. S. Neufeind and
Dipl.-Chem. A. M. Heinsch fꢀr stimulating discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201222.
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Scheme 1. Irradiation of 1 in the presence of oxygen and 2,3-dimethyl-
but-2-ene. The formation of 3 indicates the presence of 1O2.
Figure 1. Structures of the [2+2] cycloadducts rac-2 (left) and rac-10
(right) in the crystalline state. C gray, H white, O red.
Table 2: Outcome of the irradiation of 1 under various conditions (for
the reaction shown in Table 1).
Entry
Conditions[a]
Additive[b]
1/rac-2[c]
1
2
3
4
5
6
7
8
O2
O2
Ar
Ar
Ar
O2
Ar
O2
Ar
Ar
O2
O2
O2
–
21:79
20:80
91:9
83:17
90:10
66:34
91:9
66:34
83:17
87:13
66:34
14:86
54:46
Rose B
TPP
TPP/acetone
DCA
DCA
p-BQ
Scheme 2. Various substrates (upper row) used in this study and the
expected photocycloaddition products (lower row).
p-BQ
9
1,4-DNB
DABCO[d]
DABCO[d]
BHT[d]
Besides the 3-substituted coumarins 1, 4–7, the symmetric
diolefin 8 was employed (for comparison purposes as this
compound had been studied before by Yoon et al. in a [Ru-
(bpy)3Cl2]-catalyzed photocycloaddition).[3c] All substrates
were irradiated in CH2Cl2 under three different standard
conditions, that is, (A) under an atmosphere of argon, (B) in
the presence of oxygen, and (C) in the presence of oxygen and
5 mol% of BHT. The resulting solutions were analyzed by
GC–MS and NMR spectroscopy. The outcomes of the various
experiments are summarized in Table 3.
10
11
12
13
BHT+DABCO[d]
[a] A solution of 1 in CH2Cl2 was irradiated for 24 h with a 150 W HQI
lamp at room temperature either in the presence of oxygen (1 atm) or
under argon (after ultrasound-assisted deoxygenation of the solvent).
[b] Unless otherwise indicated 1 mol% of additive was used. [c] Ratios
were determined by 1H NMR spectroscopy and represent average values
of several independent experiments. [d] 5 mol% of additive was used.
DABCO=1, 4-diazabicyclo[2.2.2]octane.
In contrast to 1, which smoothly afforded the photoadduct
rac-2 under the preoptimized conditions (C) in 81% yield, the
related substrate 4 bearing two geminal methyl substituents at
the cyclohexene ring did not react at all, probably as
a consequence of steric crowding. The cyclohexenone 5,
however, provided the photoproduct rac-10 in a clean trans-
formation. Again, the rate was strongly accelerated by oxygen
(especially in the presence of BHT). After prolonged
irradiation (to achieve full conversion) rac-10 could be
isolated in 90% yield as a pure diastereomer, the structure
of which was confirmed by X-ray crystallography
(Figure 1).[13] Irradiation of the methoxy-substituted cou-
marin 6 resulted in complete conversion after 12 h to give the
cycloadduct rac-11 in 85% yield. However, the presence of
oxygen (and BHT) did not have much effect in this case. This
was demonstrated by determining the conversion under the
different conditions after 4 h.
“antioxidant” BHT (3,5-di-tert-butyl-4-hydroxytoluene) to
the oxygen-saturated reaction mixture resulted in a signifi-
cantly improved (86 Æ 10%) conversion (Table 2, entry 12).
Noteworthy, BHT did not lead to any increase in product
formation in the absence of oxygen. We also performed an
experiment using DABCO as a 1O2-quenching additive,[11]
which markedly inhibited product formation in the presence
of oxygen, also after addition of BHT (Table 2, entries 11 and
13).
The existence of 1O2 in the reaction system was also
supported by the outcome of an experiment performed in the
presence of 2,3-dimethylbut-2-ene (Scheme 1). In this case,
the conversion of 1 after 24 h was only 25% while significant
amounts of the peroxide 3 were formed, probably as a result
of the Schenck–Alder ene reaction of 2,3-dimethylbut-2-ene
In contrast to 1, 5, and 6, the coumarin 7 with a simple
pent-4-en-1-yl side chain reacted only very slowly. Never-
theless, the product rac-12 was obtained in good yield after
several days of irradiation under the proven conditions (C).
The photocycloaddition of substrate 8 proceeded slowly (but
1
with O2.[12]
We next tested the effect of oxygen (alone or in
combination with BHT) on the outcome of [2+2] cyclo-
additions employing a set of additional substrates (Scheme 2).
2
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Table 3: Results of the irradiation of various substrates under three
different sets of standard conditions.
Entry
Substr.
Cond.[a]
t
[h]
Conv.
[%][b]
Product
(isolated)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
1
1
4
5
5
5
6
6
6
7
7
7
8
8
8
A
B
C
24
24
24
24
24
24
24
4
5
79
86
0
rac-2
rac-2
rac-2 (81%)
--
rac-10
A, B, C
A
B
C
A
B
C
A
B
C
A
B
C
15
31
61
44
34
36
22
59
84
21
32
37
rac-10
rac-10 (90%)[c]
rac-11 (85%)[c]
rac-11
4
4
rac-11
rac-12
rac-12
rac-12 (69%)
13
Figure 2. Laser flash photolysis of 1 (10À3 m) at 355 nm in acetonitrile
96
96
96
24
24
24
&
*
under nitrogen ( ) and under air ( ) recorded 0.4 ms after the laser
pulse. Inset: decay trace at 380 nm under nitrogen.
13
13 (36%)
[a] The substrate (0.2 mmol in 1 mL of CH2Cl2) was irradiated (150 W
HQI lamp) at room temperature either in the presence of O2 (1 atm) or
under Ar after ultrasound-assisted deoxygenation of the solvent.
[b] Determined by 1H NMR spectroscopy. [c] Yield obtained after pro-
longed irradiation to achieve full conversion of the substrate.
cleanly) under argon to afford the product 13 (21% con-
version after 24 h). Again, the conversion rate was signifi-
cantly higher in the presence of oxygen but associated with
the formation of several by-products, unless BHT was added.
Using conditions (C) the expected product 13 could be
isolated in 36% yield after 24 h. A control experiment
performed parallel using [Ru(bpy)3Cl2] (5 mol%) as a photo-
catalyst (as described by Yoon et al.)[3c] afforded 13 in
a comparable yield of 41%.
Having thus demonstrated the rate-accelerating effect of
oxygen in the photocycloaddition of different substrates, we
next turned our attention to the mechanistic rationalization of
this unique phenomenon. To gain insight into the behavior of
the primary species formed by photoexcitation of the
coumarin substrates, we first investigated compounds 1 and
5 by means of time-resolved transient absorption spectrosco-
py (laser flash photolysis).[14] The triplet of the parent
coumarin generated by triplet–triplet (TT) energy transfer
from benzophenone showed a TT absorption at 400 nm and
a lifetime of 0.9 ms in acetonitrile under nitrogen. The direct
excitation of 1 and 5 led to triplets (TT absorption at 380 nm)
with a lifetime of 9 and 4 ms, respectively, both showing
monoexponential decay (the data for 1 are displayed in
Figure 2).
Figure 3. Cyclic voltammogram (black, left scale) and its semideriva-
tive (gray, right scale) of 1 in MeCN with 0.1m TBAPF6 using
a platinum disk electrode (r=0.25 mm). Inset: pure MeCN/TBAPF6 in
equilibrium with ambient air at a glassy carbon (gray, r=1 mm) or
a platinum electrode (black, r=1.5 mm). T=293 K; v=0.1 VsÀ1
.
argon shows an anodic wave at 1.48 V (oxidation of 1) and
a cathodic one at À2.32 V (reduction of 1) both indicating
irreversible processes (Figure 3). Under the same conditions
(without argon) half-wave potentials of À1.26 V and À1.32 V
(depending on the material of the electrode) were determined
À
for the reduction of O2. These values for E(O2 /O2) are in
accordance with the results reported by Vasudevan and
Wendt.[16]
The free energy change for a photoinduced electron
transfer from the phototransient derived from 1 to oxygen can
now be estimated using the equation DG0 = Eox(1)ÀE(O2 /
À
Thus, the transients observed for 1 and 5 lived even longer
than that of the parent coumarin and too long to act as
“reactive” triplets. Under air, however, no transients were
observed 0.4 ms after the laser pulse for the parent coumarin
as well as for 1 (Figure 2) and 5. From the TT sensitization
experiments and literature data[15] the triplet energy of the
coumarins 1 and 5 was estimated as 2.6–2.7 eV.
O2)ÀET(1).[17] As a result, triplet-state quenching of the
coumarin substrates by oxygen (with formation of the super-
oxide radical anion) should be slightly exergonic (or at least
isoenergetic) and therefore possible. A clearly exergonic
electron transfer could also occur from the corresponding
singlet states (DG0 > À1 eV with ES(1) = 3.8 eV; as calculated
from the redox potential differences of 1).
The cyclic voltammogram of 1 in acetonitrile with
tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte (0.1m) at room temperature under
Based on these data and considerations, we propose an
overall mechanism for the oxygen-promoted photocycloaddi-
tion of these coumarin derivatives, in which oxygen plays
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possibility that related effects may also play a role in aerobic
photochemical reactions in nature. In any case, the developed
(metal-free) oxygen-mediated photocycloaddition of 3-sub-
stituted coumarins represents a useful method for organic
synthesis. We demonstrated this by performing the synthesis
of rac-2 and rac-10 on a gram scale using a standard 150 W
HQI lamp (Scheme 4).
Received: February 14, 2012
Published online: && &&, &&&&
Keywords: [2+2] cycloaddition · cyclobutanes ·
electron transfer · oxygen · photocatalysis
.
Scheme 3. Proposed mechanism for the oxygen-mediated coumarin
photocycloaddition.
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a key role as a redox catalyst (Scheme 3). This mechanism
consists of the following five steps:
1) The triplet transient A is formed from 1 through light
absorption and intersystem crossing (ISC).
2) Intermediate A reacts with oxygen (3O2) under single-
electron transfer (SET).
[2] Selected reviews: a) J. M. R. Narayanam, C. R. J. Stephenson,
Chem. Soc. Rev. 2011, 40, 102 – 113; b) T. P. Yoon, M. A. Ischay,
J. Du, Nat. Chem. 2010, 2, 527 – 532; c) K. Zeitler, Angew. Chem.
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Q. Zou, J.-R. Chen, X.-P. Liu, L.-Q. Lu, R. L. Davis, K. A.
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[3] a) M. A. Ischay, Z. Lu, T. P. Yoon, J. Am. Chem. Soc. 2010, 132,
8572 – 8574; b) J. Du, T. P. Yoon, J. Am. Chem. Soc. 2009, 131,
14604 – 14605; c) M. A. Ischay, M. E. Anzovino, J. Du, T. P.
Yoon, J. Am. Chem. Soc. 2008, 130, 12886 – 12887; for a related
[3+2] photocycloaddition, see: d) Z. Lu, M. Shen, T. P. Yoon, J.
Am. Chem. Soc. 2011, 133, 1162 – 1164.
[4] a) A. McNally, C. K. Prier, D. W. C. MacMillan, Science 2011,
334, 1114 – 1117; b) H. Shih, M. N. Vander Wal, R. L. Grange,
D. W. C. MacMillan, J. Am. Chem. Soc. 2010, 132, 13600 – 13603;
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Stephenson, Angew. Chem. 2011, 123, 9829 – 9833; Angew.
Chem. Int. Ed. 2011, 50, 9655 – 9659; b) M. R. Narayanam,
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8756 – 8757.
3) The resulting radical cation B then undergoes a 5-exo-trig
radical cyclization to form the distal radical cation C.
4) C is reduced by the superoxide radical anion (formed in
step 2) to give the biradical D.
5) The isolated cycloaddition product rac-2 finally arises
from radical combination under formation of the four-
membered ring.
This mechanism is in accordance with all the experimental
facts. The different behavior of the methoxy-substituted
coumarin 6, the reaction of which is not accelerated by
oxygen, is probably due to a more favorable intramolecular
electron transfer. Similar to 1 and 5, the triplet of 6 has
a lifetime of 5 ms, but the oxidation potential is shifted by
0.5 V to lower potentials (see the Supporting Information).
The formation of singlet oxygen under the reaction conditions
3
may result from the (nonproductive) reaction of A with O2
1
(to give 1 and O2) or, possibly, from the oxidation of the
superoxide radical anion by intermediate C.[18] The beneficial
effect of the antioxidant BHT in the reaction mixture remains
unclear; it may be attributed to the prevention of radical side
reactions.
In summary, we have discovered a rather unique effect of
oxygen on a nonoxidative photochemical transformation.[19]
The proposed mechanism, in which oxygen serves as a redox
catalyst, was supported by transient absorption spectroscopy
and electrochemical measurements. As light and oxygen are
both abundant, the results presented here suggest the
[6] a) K. Kalyanasundaram, Coord. Chem. Rev. 1982, 46, 159 – 244;
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[7] a) V. M. Dembitsky, J. Nat. Med. 2008, 62, 1 – 33; b) J. Iriondo-
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[8] H. Guo, E. Herdtweck, T. Bach, Angew. Chem. 2010, 122, 7948 –
7951; Angew. Chem. Int. Ed. 2010, 49, 7782 – 7785.
[9] The only two known examples of intramolecular photocycload-
ditions involving 3-substituted coumarins concern substrates, in
which the unsaturated side chain was attached through an ether
or an amide unit (leading to heterocyclic products): a) M. S.
Shepard, E. M. Carreira, Tetrahedron 1997, 53, 16253 – 16276;
Scheme 4. Oxygen-promoted coumarin photocycloadditions performed
on a preparative scale.
4
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b) M. Sakamoto, M. Kato, E. Oda, S. Kobaru, T. Mino, T. Fujita,
an Oriel photomultiplier tube (PMT) system made up of a 77348
Tetrahedron 2006, 62, 3028 – 3032.
side-on tube, 70680 housing, and a 70705 power supply. The
signal was registered with a TDS-640A Tektronix oscilloscope.
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[16] D. Vasudevan, H. Wendt, J. Electroanal. Chem. 1995, 392, 69 –
74.
[10] Compounds
1 was prepared through hydroboration of 3-
allylcoumarin and subsequent Suzuki coupling (for details see
the Supporting Information).
[11] C. Quannes, T. Wilson, J. Am. Chem. Soc. 1968, 90, 6527 – 6528.
[12] a) G. O. Schenck, Naturwissenschaften 1948, 35, 28 – 29; b) A. G.
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[13] CCDC 866494 (rac-2) and CCDC 866495 (rac-10) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] The LFP system employed is based on a Nd:YAG SL404G-10
Spectron Laser Systems (lexc = 355 nm, ca. 10 ns pulse width, ca.
15 mJ per pulse) with a pulsed Lo255 Oriel Xenon lamp as
detecting light source. The observation wavelength was selected
with a 77200 Oriel monochromator, and the signal amplified by
[17] A. Weller, Z. Phys. Chem. 1982, 133, 93 – 98.
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[19] The use of oxygen as a cooxidant in a photocatalytic Diels–Alder
reaction using a Ru-based photocatalyst was recently reported:
a) S. Lin, M. A. Ischay, C. G. Fry, T. P. Yoon, J. Am. Chem. Soc.
2011, 133, 19350 – 19353; for an early report on electron transfer
from a phototransient to molecular oxygen, see: b) M. Kojima, S.
Sakuragi, K. Tokumaru, Bull. Chem. Soc. Jpn. 1989, 62, 3863 –
3868.
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Communications
Photochemistry with Visible Light
D. P. Kranz, A. G. Griesbeck,* R. Alle,
R. Perez-Ruiz, J. M. Neudçrfl,
K. Meerholz,
H.-G. Schmalz*
&&&& — &&&&
Oxygen as a catalyst! While oxygen is
usually excluded in nonoxidative photo-
chemical reactions, the photocycloaddi-
tion of 3-(alk-4-en-1-yl)-substituted cou-
marins greatly benefits from the presence
of O2, which was shown to act as a redox
catalyst, preferentially in combination
with the common antioxidant 3,5-di-tert-
butyl-4-hydroxytoluene (BHT).
Molecular Oxygen as a Redox Catalyst in
Intramolecular Photocycloadditions of
Coumarins
6
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