Reversible photooxygenation of alkynylanthracenes: Chemical generation of singlet oxygen under very mild conditions

Article abstract of DOI:10.1002/chem.201102230

In the dark and very fast: The generation of singlet oxygen ( ¹O₂) from endoperoxides, which are readily available by photooxygenation of the corresponding anthracenes, proceeds within minutes in the dark (see scheme), a rate hitherto unknown for other anthracenes or naphthalenes. This provides an efficient chemical source of singlet oxygen under very mild conditions.

Full text of DOI:10.1002/chem.201102230

COMMUNICATION
DOI: 10.1002/chem.201102230
Reversible Photooxygenation of Alkynylanthracenes: Chemical Generation
of Singlet Oxygen under Very Mild Conditions
Werner Fudickar and Torsten Linker*[a]
Dedicated to Professor Erich Kleinpeter on the occasion of his 65th birthday
The oxidation of organic compounds with singlet oxygen
(1O2) is important in stereoselective synthesis,[1] medicine,[2]
industrial processes[3] and material sciences.[4] Under the
conditions used for conventional light-induced photooxyge-
nation, electron transfer from excited states might compete
and result in undesired side reactions.[5] Therefore, “dark”
1
oxygenations by chemical sources of O2 offer an attractive
alternative, especially in biological systems[6] or when pre-
1
Scheme 1. Generation of EPOs from acenes and release of O2.
cise control over the amount of the donor is required.[7] The
reaction of H2O2 with hypohalides, lanthanides, or transition
1
metals is a powerful method to produce O2 in high quanti-
ties but typically requires biphasic or emulsion conditions.[8]
1
In contrast, thermal generation of O2 from endoperoxides
(EPOs) of acenes can be conducted in organic media. The
most important substrates in this field are 1,4-dimethyl-
A
(DMN)[9]
and
9,10-diphenylanthracene
(DPA).[10] However, a drawback of their application as O2
carriers is that a strong donor is slowly formed from the
acene, whereas EPOs that are generated more quickly are
1
1
too stable to act as O2 sources. The formation and the fate
of the EPO are both linked to the number of fused rings;
whereas naphthalenes react slowly to form the correspond-
1
ing EPO and favor a rapid reconversion by release of O2,
anthracenes react faster to form an EPO but their dissocia-
tion only starts at high temperatures at which reconversion
competes with decomposition (Scheme 1).[11]
Recently, we found that remote substituents of
9,10-arylanthracenes affected the reactivity of EPO forma-
tion and cycloreversion, but not in opposite directions as ex-
pected.[12] This finding prompted us to incorporate substitu-
ents at the bridgehead positions to hopefully give access to
new and efficient chemical sources of singlet oxygen.
Scheme 2. Reversible photooxygenation of alkynylanthracenes 1.
EPOs of naphthalenes, arylanthracenes, or heterocyclic com-
pounds) operate at higher temperatures,[13] our new class of
acenes is important for applications at low temperatures.
Additionally, kinetic results point at different mechanisms
of forward and reverse reactions.
Herein, we describe the first photooxygenation of
alkynyl-substituted anthracenes 1 to give EPOs that release
1O2 in high yield under very mild conditions (Scheme 2). Be-
1
cause the majority of common chemical sources of O2 (e.g.,
Alkynylanthracenes 1a–c were prepared by Sonogashira
coupling of commercially available aryl acetylenes and 9,10-
dibromoanthracene,[14] whereas derivatives 1d–f were syn-
thesized by coupling of aryl bromides and bis-9,10-(tri-
[a] Dr. W. Fudickar, Prof. Dr. T. Linker
Department of Chemistry, University of Potsdam
Karl-Liebknecht-Strasse 24–25
14476 Potsdam (Germany)
AHCTUNGTRENNUNG
methylsilylethynyl)anthracene.[15,16] EPOs 1-O2 were ob-
Fax : (+49)331-9775056
E-mail: linker@uni-potsdam.de
tained by photooxygenation under standard conditions.[16]
Although compounds 1a–c were consumed quantitatively
within a few hours, derivatives 1e–f required longer irradia-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102230.
Chem. Eur. J. 2011, 17, 13661 – 13664
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13661

Table 1. Singlet oxygen production measured by the disappearance of ru-
brene (EPO/rubrene 1:4) and half-life values for reconversion of EPOs
1-O2 at 258C.
tion times and starting material still remained in the solu-
tions after irradiation for 1 day. EPOs 1-O2 could be isolated
by chromatography at À208C in moderate to good yields,
and were stored in a freezer for several days. The structures
1
Yield of O2 [%]
t1/2 EPO [min]
1
of 9,10-EPOs were confirmed by H and 13C NMR spectros-
1a-O2
1b-O2
1c-O2
1d-O2
1e-O2
DMN-O2
DPA-O2
57Æ5
53Æ5
67Æ6
48Æ5
43Æ4
49Æ5
30–35[a]
23
31
33
15
18
350
copy and mass spectroscopy.[16]
To our surprise, we found that the parent species rapidly
regenerated with no formation of rearrangement products if
the EPOs were left at room temperature. It is worth noting
600 (at 868C)[a]
1
that the reversible binding of O2 to anthracenes has only
[a] Measured at elevated temperatures, see ref. [18].
been reported for substrates with aryl substitution at the
9,10-position. Anthracenes that have hydrogen atoms or
alkyl groups do not reversibly bind oxygen and their EPOs
decompose on warming.[17] Furthermore, a noticeable recon-
version of 9,10-aryl substituted EPOs requires temperatures
>808C.[18] To the best of our knowledge, the reaction of sin-
glet oxygen with anthracenes functionalized with triple
bonds is hitherto unknown. To verify the formation of O2
upon cleavage of EPOs 1-O2, trapping experiments were
carried out. When the EPOs
found in the literature,[18] even for unsubstituted naphtha-
lene.[20] This exceptional feature provides ideal conditions
1
for the use of alkynylanthracene EPOs as O2 donors.[21]
For the evaluation of time profiles of 1O2 generation,
donor 1b-O2 was added to solutions of rubrene as the ac-
ceptor (1b-O2/rubrene 40:1 and 130:1) at room temperature
(Figure 1). For comparison with a common 1O2 donor, an
1
were left for 24 h in the dark at
room temperature and in the
presence of the 1O2 acceptor
2,3-dimethyl-2-butene (2; ratio
EPO/2 1:4), the O2–ene reac-
tion product was identified
Scheme 3. Trapping of 1O2 re-
leased by singlet-oxygen
donors 1-O2.
1
(hydroperoxide 3; Scheme 3).
To quantify the fraction of released singlet-state oxygen,
we chose rubrene (4) as an acceptor because the disappear-
ance of 4 can be measured quantitatively by UV/Vis spec-
troscopy (Scheme 4, Table 1). Importantly, these experi-
ments were carried out at room temperature. However,
1
physical quenching of O2 by the substrate plays a more sig-
nificant role at room temperature, causing a loss of released
1O2.[19] Therefore, we used DMN-O2 as a reference that can
1
produce O2 nearly quantitatively at 408C.[18] At room tem-
1
perature, the production of O2 by EPOs of 1a–e is in the
range of 50%, which is comparable to the measured 1O2
production of the EPO of DMN (Table 1).
Figure 1. Consumption of rubrene (5ꢁ10 (&) and 1.5ꢁ10À5 m (~) in
CHCl3) measured by the extinction at l=530 nm upon addition of
2ꢁ10À3 m 1b-O2 (c) and DMN-O2 (a) in CHCl3.
analogous experiment was carried out with identical
amounts of DMN-O2 (same ratios). At lower concentrations
1
of rubrene (130:1), the rapid release of O2 by 1b-O2 caused
complete consumption of rubrene within the first few min-
AHCTUNGTERGNUNuN tes, whereas the reaction with DMN-O2 required around
Scheme 4. Reaction of rubrene with 1O2 released by endoperoxides 1-O2.
8 h. At higher rubrene concentrations, the reaction stopped
after about 1 h after the addition of 1b-O2, whereas it con-
tinued with DMN-O2. In consequence, EPOs of alkynyl-
The half-life (t1/2) values for the reconversion of EPOs 1-
O2 to the parent form by the release of oxygen were deter-
mined from the reappearance of the starting material by
UV/Vis spectroscopy (Table 1). The most striking feature of
these compounds is revealed by comparison of the rate of
reconversion with that of other EPOs; the values are much
lower than for all anthracene and naphthalene derivatives
AHCTUNGTREGaNNUN nthracenes are superior for short exposure times, for exam-
ple, in sensitive biological systems[22] and if the acceptor
reacts fast.[23]
Encouraged by these findings, we turned our attention to-
wards the reactivity of the forward reaction to evaluate the
1
suitability of alkynylanthracenes 1 as carriers of O2. Time
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Chem. Eur. J. 2011, 17, 13661 – 13664

Photooxygenation of Alkynylanthracenes
COMMUNICATION
profiles for the disappearance of alkynylanthracenes 1, an-
thracene (ANT), DPA, and DMN upon photooxygenation
were recorded under identical conditions (Figure 2).[16]
1
Figure 3. Comparison of the rates of EPO formation (&) and the O2-re-
leasing retro-reaction (&) for alkynylanthracene 1b, DMN, and DPA at
room temperature after 120 min.
Figure 2. Absorbance vs. time profiles of lmax for alkynylanthracenes
1a–f, ANT, DPA, and DMN under continuous irradiation in the presence
of the sensitizer tetraphenylporphyrin (TPP).
(Scheme 5).[12] Both aryl and alkynyl groups provide suffi-
cient radical stabilization energy (RSE) to favor the forma-
The decay curves of alkynylanthracenes 1, anthracene,
and DMN are in the same range, whereas the reactivity of
DPA is significantly higher. Inspection of the slopes within
the series of substituted alkynylanthracenes discloses a
strong dependence of the reactivity on the substituent. For
example, derivative 1b, which has electron-donating me-
thoxy groups, reacts faster than derivative 1 f, which has an
electron-withdrawing cyano group.
À
tion of carbon radical 5, which results from a C O cleavage
1
The requirements for a good chemical source of O2 are
the fast generation of the donor form and a high propensity
1
to release O2. These features are depicted by a comparison
of 1b-O2, DMN, and DPA in Figure 3. We determined the
conversion from the acene to the EPO after 120 min and
1
the degree of reconversion by release of O2 after 120 min.
It becomes clear that DPA is rapidly converted into an
1
EPO, but the release of O2 at room temperature is not de-
tectable. In contrast, the reconversion of DMN-O2 is notice-
able at room temperature, but formation of the EPO pro-
ceeded very slowly. Conversely, alkynylanthracene 1b be-
haved exceptionally well; the transformation to the EPO
was almost quantitative after 120 min and the retro-reaction
of EPO 1b-O2 was the fastest of the three donors. In sum-
mary, 1b-O2 is a very good O2 source that is also generated
rapidly.
Although the reactivity of the forward reaction of
alkynylACHTUNGTRENNUNGanthracenes is tunable by the choice of the para sub-
stituent, no substituent effects apply for the reverse reaction.
This observation can be explained by two different mecha-
nistic pathways, namely a concerted one for the forward and
a radical pathway for the reverse reaction.[24] The striking
difference in reactivity of the reverse reaction between aryl
and alkynyl substituents can be rationalized by consider-
ation of a stepwise process with radical intermediates
Scheme 5. Proposed pathways for the thermolysis of EPOs.
(path A).[16,25] For R =H and alkyl groups, an O O cleavage
À
to form intermediate 6 is favored (path B), followed by de-
composition.[17] To further support this hypothesis, we per-
formed theoretical calculations at the HF/6-31G* level.[16]
Local minima were found for intermediates 5 (R=Ph,
alkynyl), whereas diradicals 6 gave no energetic minima and
directly rearranged. Additionally, our isodesmic calculations
showed that alkynyl groups have a smaller RSE than aryl
groups,[16] in good agreement with the literature.[26] This ex-
plains the short lifetime of alkynyl-substituted diradical 5
(compared with R=Ph) and its faster release and higher
1
1
yields of O2.
In conclusion, we have shown that alkynylanthracenes 1
belong to a rare class of anthracenes that reversibly react to
Chem. Eur. J. 2011, 17, 13661 – 13664
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13663

T. Linker and W. Fudickar
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Acknowledgements
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Keywords: anthracenes
reversibility · singlet oxygen
·
oxidation
·
peroxides
·
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Received: July 20, 2011
Published online: November 3, 2011
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