Synthesis of Pyridylanthracenes and Their Reversible Reaction with Singlet Oxygen to Endoperoxides

Article abstract of DOI:10.1021/acs.joc.7b01765

The ortho, meta, and para isomers of 9,10-dipyridylanthracene 1 have been synthesized and converted into their endoperoxides 1-O₂ upon oxidation with singlet oxygen. The kinetics of this reaction can be controlled by the substitution pattern an

Full text of DOI:10.1021/acs.joc.7b01765

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Note
Synthesis of Pyridylanthracenes and their Reversible
Reaction with Singlet Oxygen to Endoperoxides
Werner Fudickar, and Torsten Linker
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01765 • Publication Date (Web): 31 Jul 2017
Downloaded from http://pubs.acs.org on August 1, 2017
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Synthesis of Pyridylanthracenes and their Reversible Reaction with Singlet
Oxygen to Endoperoxides
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Werner Fudickar and Torsten Linker*
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Department of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam,
Germany
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linker@uni-potsdam.de
ABSTRACT: The ortho, meta and para isomers of 9,10-dipyridylanthracene 1 have been synthesized and
converted into their endoperoxides 1−O2 upon oxidation with singlet oxygen. The kinetics of this reaction
can be controlled by the substitution pattern and the solvent: In highly polar solvents the meta isomer is
the most reactive, whereas the ortho isomer is oxidized fasted in non-polar solvents. Heating of the
endoperoxides afforded the parent anthracenes by release of singlet oxygen.
Singlet oxygen (1O2) is a powerful reagent used for the syntheses of organic peroxides from alkenes, such
1 3
–
as in the Schenck Ene reaction, the [2+2] and the [4+2] cycloaddition. Of particular interest is the
oxygenation of acenes to give aromatic endoperoxides (EPOs), as in some cases it can proceed reversibly
with the regeneration of the parent acene and oxygen, partly in its exited state.4,5 Acene
photooxygenations have been harnessed, for example, for lithographic applications,6 the design of
1
luminescent O2–response polymers,7 for oxygen storage,8 or for molecular rotors.9 It is therefore of
great importance to predict reactivities of acenes towards their oxygenation and the degree of the
reversibility of this reaction.
1
We have recently shown that the reactivity of substituted diarylanthracenes with O2 correlates in a
positive linear fashion with their HOMO energies and their redox potentials.10 Various substituents on
phenyl groups have been investigated. However, reversible photooxygenations of anthracenes with
heterocycles at the 9,10-position have not been reported until now. Pyridylanthracenes seem to be
especially attractive, since pyridine is inert towards reaction with O2,11 whereas thiophene or imidazole
1
are not suitable because of their oxidation lability.12 Additionally, the nitrogen atom of the pyridines
might be placed in the ortho-, meta-, or para-position, with the possibility to distinguish between
electronic and steric influences. Interestingly, pyridylanthracenes are known to possess important
luminescence properties,13 and have been used to construct coordination polymers,14 but their reaction
with 1O2 was hitherto unknown.
Herein, we report on the synthesis of three isomeric 9,10-dipyridylanthracenes 1 where the nitrogen
atom is placed in o-, m- or p-position (1o−p) and their photooxygenation to the corresponding EPOs. The
electronic effects of the pyridine substituents are quantified. Furthermore, the reversibility of this
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reaction is investigated, whether thermolysis affords either the starting species upon cleavage of the two
C−O bonds or causes decomposition as a result of an O−O bond cleavage (Scheme 1).
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Scheme 1. Oxygenation of pyridylanthracenes 1 and reconversion/decomposition of the EPOs.
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From our previous experience in the syntheses of diarylanthracenes we envisaged that the Suzuki-
Miyaura cross-coupling reaction between pyridineboronic acids 2 and 9,10-dibromoanthracene (3) is a
valuable method to provide the pyridylanthracenes 1.9 Therefore, the pyridineboronic acids 2 were
prepared by lithium–halogen exchange of the corresponding bromopyridines 4 followed by
transmetalation with triisopropylborate (Scheme 2).
Scheme 2. Synthesis of 9,10-dipyridylanthracenes 1o,m by Suzuki-Miyuara cross-coupling (method A).
While the meta and para pyridineboronic acids 2m and 2p could be prepared in moderate yields
(~70%),15 the ortho derivative could not be isolated. Upon inspection of literature reports, 2-
pyridineboronic acids are labile towards hydrolysis due to cleavage of the C−B bond and therefore, only
specifically modified synthetic protocols are suitable.16 For example, other borates than alkyl borates
were employed,17 and the intermittent boronic ester was isolated prior to hydrolysis requiring an
additional step.18 As this instability of the boronic acid 2o is reported to further cause diminished yields
in the following cross-coupling reaction,19 we switched to an alternative procedure to obtain 1o. Thus,
after in situ generation of 2-pyridyllithium (two equivalents) from the bromopyridine 4o using n–butyl
lithium (BuLi) at −78 °C, its reacꢀon with anthraquinone (5) lead to the intermediary coupling product,
9,10-dihydro-9,10-dipyridinyl-9,10-anthracenediol (6o), which was in turn reduced to the anthracene 1o
with potassium iodide in glacial acetic acid under reflux in moderate overall yield (Scheme 3, method B).
Owing to the good water solubility of all pyridylanthracenes 1 at pH<3, mono-coupled products and
starting materials could be easily removed by extractions at low pH into the organic phase, followed by
precipitation of the product which remained in the aqueous phase.
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Scheme 3. Synthesis of 9,10-o-dipyridylanthracene (1o) from 2-pyridyllithium and anthraquinone
(method B).
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Sensitized photooxygenations were carried out by irradiation of solutions of pyridylanthracenes 1 in the
presence of tetraphenylporphyrin as sensitizer under oxygen atmosphere. All three isomers delivered
exclusively 9,10-EPOs (1–O2) with nearly quantitative yields as characterized by NMR and mass
spectroscopy (see supporting information).
To evaluate the reactivities of the pyridylanthracenes 1, their rates of photooxygenations were
determined and compared with the rate of 9,10-diphenylanthracene (DPA) in dichloromethane (DCM) as
standard (Table 1, details of measurement and evaluation of experimental errors are given in the SI). As
expected, the reactivity relative to DPA is strongly reduced by a factor of ~3−4, owing to the electron
withdrawing character of the pyridine rings. The pyridine ring itself is chemically inert towards its
1
photooxygenation, and causes a slightly stronger physical deactivation of O2 as compared to benzene
(for example, t for 1O2 is 13−20 μs in pyridine and 20−30 μs in benzene).20 Thus, our pyridylanthracenes
½
1
are attractive substrates for reactions with O2. The absolute second order rate constant kr can be
derived from these data based on a literature value for DPA (krDPA=4.2x106 M-1s-1) and by assuming that
singlet oxygen is quenched only chemically by the substrate and physically by the solvent, thus,
neglecting the physical deactivation by the substrate.
Table 1. Rate constants, oxidation potentials, and calculated HOMO energies of the photooxygenation
of pyridylanthracenes 1 in DCM.
c
d
kobs
s-1
kr
EOx
EHOMO
acene
106 M-1s-1
1.01b
1.21b
0.80b
4.20
V
eV
1o
1m
1p
0.83±0.02a
1
+1.47
+1.43
+1.46
+1.38
−6.55
−6.61
−6.63
−6.67
0.66±0.03 a
3.45±0.11 a
DPA
aObserved rate relative to the meta isomer 1m. Experimental errors determined from four independent
b
c
photooxygenations. Absolute values based on literature data for DPA (4.2x106 M-1s-1). Measured by
cyclic voltammetry. dCalculated by Gaussian 09.
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The kinetic data summarized in Table 1 show a reactivity bias according to the position of the nitrogen
atom: The meta isomer 1m reacts faster than the other two isomers, where the nitrogen atoms are in
conjugation with the bridgehead carbon atoms C9 and C10. The redox potentials of the three
pyridylanthracene isomers were experimentally determined by cyclic voltammetry in DCM (see
supporting information Figure S1). In contrast to DPA, the pyridylanthracenes showed irreversible waves,
with one anodic peak according to the generation of the anthracene cation radical.21 The irreversibility
might be the result of a chemical reaction following the oxidation, that is, the attack of the nucleophilic
lone pair of the pyridine nitrogen at the cationic species (EC mechanism). However, the measured
potentials resemble the reactivity order, which is in agreement with our previous studies on the
reactivity of substituted arylanthracenes (Table 1).10 On the other hand, HOMO energies obtained from
density functional theory (DFT) calculations in DCM using a self-consistent reaction field method (see
supporting information) were not consistent with the reactivities, where the ortho isomer has the
highest HOMO energy level. This could be explained by a comparison of the optimized calculated
structures: In contrast to the other two isomers, 1o has a smaller acene-pyridine twist angle (70° for 1o,
~82° for 1m/1p) due to less steric hindrance, which affects its orbital energies (see Figure S2 supporting
information). Thus, the value of the HOMO energy of 1o cannot fit into the expected order.
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To prove whether the reactivity order is controlled by either electronic effects via conjugation or by
steric effects arising from the less space requiring nitrogen atom compared to the CH-group, we
investigated the photooxygenations of the three pyridylanthracenes in less polar solvents. Interestingly,
when performing the reaction in toluene, the order changed from kmeta>kortho>kpara to kortho>kmeta>kpara
.
This trend is even more remarkable in hexane (Table 2 and Figures S3 and S4 supporting information).
Even though the para isomer is still the least reactive as a result of conjugation, the strikingly high
reactivity of the ortho compound is owed to the reduced steric demand of the nitrogen atom. Moreover,
calculated HOMO energies in hexane (EHOMO/1o> EHOMO/1m>EHOMO/1p) fit nicely to this trend (see Table S1
supporting information). This confirms the surpassing of steric over electronic effects in non-polar
solvents, which was hitherto not discussed in photooxygenations of acenes.
Table 2. Relative reactivity of the three isomeric pyridylanthracenes compared to the reactivity of DPA
in various solvents.
solvent
toluene
hexane
0.55
0.25
0.19
DCM
0.24
0.28
0.18
acetonitrile
0.21
methanol
0.14
k1o/kDPA 0.56
k1m/kDPA 0.43
k1p/kDPA 0.35
0.24
0.20
0.22
0.14
1
To further prove the strong effect of solvents on the reactivity of pyridylanthracenes 1 with O2, we
investigated highly polar acetonitrile and methanol next. Indeed, now not only the meta isomer was the
most reactive as above reported for DCM, but also the ortho isomer reacts as slowly as the para isomer
(Table 2 and Figures S6 and S7 supporting information). This indicates that the sterical benefit from the
less space requiring nitrogen atom in ortho position is extinguished in polar solvents. The strongest effect
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and slowest reactivity for the ortho isomer was observed in methanol. This can be well explained by the
existence of hydrogen bonds between pyridine and methanol.22 Coordination of the nitrogen to the
solvent reduced the reactivity of 1o and 1p because of the partly positive charge at the nitrogen. Thus,
we could trigger the photooxygenation of pyridylanthracenes by change of the substitution pattern and
solvents.
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Finally, we investigated the thermolysis of our EPOs 1−O2, and therefore, heating was performed at 90 °C
in toluene (Table 3). The para and meta isomers both afforded quantitatively the parent forms 1m and
1p within 24 h, which could be isolated after chromatography in 90% yield. The chemical rate constants
of thermolysis are within the range of the rate of thermolysis of DPA−O2 (Table 3 and Figure S8). Both
1
EPOs also released O2 upon heating. The amount of this reactive species was quantified by trapping
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experiments and is in the range of O2 released by DPA−O2 (see NMR spectra, Figures S9−11, supporꢀng
information).23 In contrast, the EPO 1o−O2 showed no conversion at the same temperature even after 7
days. Eventually, treatment at 135 °C afforded a mixture of the parent form 1o in 62% yield and
tetracyclic compound 7 in 30% yield (Table 3, entry 1). The side product 7 is a result of the cleavage of
the O−O bond followed by rearrangement.5
This interesting behavior can be explained again by steric interactions. For the m-, p-isomers and DPA as
well, a C−H group is always in close proximity to the C−O bond. This steric hindrance weakens this bond,
leading to homolysis during heating and a clean back-reaction to the parent anthracenes. On the other
hand, for the o-isomer this unfavorable interaction is not operative and cleavage of the O−O bond can
compete. Thus, we found another remarkable effect of the three isomers on their ability to release 1O2.
Table 3. Thermolyses of EPOs 1−O2 and DPA−O2.
entry
EPO
1o−O2
1m−O2
1p−O2
DPA−O2
1 (%) 7 (%) 1O2 (%) k (sꢀ1)
1
2
3
4
62a
90b
92b
95b
30a
0
ꢀ
ꢀ
36
39
37
4,6x10ꢀ5
1,9x10ꢀ5
3.4x10ꢀ5
0
0
aThermolysis carried out at 135 °C in xylene. bAt 90 °C in toluene.
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In conclusion, we found that pyridylanthracenes 1 can be synthesized in only few steps and react
reversibly with singlet oxygen to the EPOs 1−O2, with kinetics depending on their substitution pattern.
Their reactivity, however, is lower and less diverse as compared to other arylanthracenes. In less polar
solvents, the ortho isomer reacts significantly faster owing to a reduced steric demand of the nitrogen
atom, whereas reactivity depends exclusively on conjugation effects in polar solvents. An unexpected
behavior is also observed for their retro-reactions, where the EPO 1o−O2 is significantly more stable than
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the two other isomers. Thus, we could control the reversible reaction with O2 by the substitution
pattern and the solvents. Therefore, pyridylanthracenes are important precursors for
photooxygenations, which offer additional interesting features as luminescent and metal coordinating
materials in future work.
Experimental Section
General Experimental Procedures. All reagents were purchased from Aldrich and used without further
purifications. Solvents were dried according to standard procedures. Column chromatography was
performed using Merck silica gel 60. TLC was performed on silica gel coated aluminum foils (0.25 mm
thick, 60 F254, Merck, Germany). 1H and 13C NMR spectra were recorded on a Bruker AC 300
spectrometer at 300 MHz and 75 MHz, respectively. Chemical shifts are referenced to TMS (0 ppm) and
1
the solvent signals of deuterochloroform (δ = 7.26 ppm for H and 77.0 ppm for 13C) and D2O (4.79 ppm
1
for H) were used as standards. The elemental analyses were performed with a Vario EL III elemental
analyzer. High resolution mass spectra were obtained using ESI-Q-TOF techniques.
General procedure for the synthesis of boronic acids 2m and 2p. The corresponding bromopyridine
(3.16 g, 20 mmol) and triisopropylborate (5.64 g, 30 mmol) were dissolved in absolute THF (40 mL) and
the solution was cooled to −78 °C. Under cooling BuLi (2M THF soluꢀon, 20 mmol) was added during 3h
by using a syringe pump. The reaction was left over night and allowed to warm to room temperature.
Hydrochloric acid (1 M aqueous solution, 40 mL) was carefully added followed by addition of chloroform
(100 mL). The organic phase was removed and the aqueous phase was neutralized using potassium
hydroxide (1 M aqueous solution). Water was evaporated to dryness and the residue was treated with
hot ethanol. After filtration, ethanol was evaporated giving the neat boronic acid.
1
3-Pyridylboronic acid (2m):17,24 white solid (1.74 g, 14.2 mmol, 71%); mp: >310 °C; H NMR (300 MHz,
D2O) δ=8.5 (s, 1H), 8,34 (d, J=5.2 Hz, 1H), 8.28 (d, J=7.6 Hz, 1H), 7.57 (dd, J=5.2 Hz, 7.6Hz, 1H); 13C NMR
(60 MHz, D2O) δ=148.0, 148.0, 143.9, 127.3.
4-Pyridylboronic acid (2p):25 white solid (1.81 g, 16.1 mmol, 74%); mp:>310 °C; 1H NMR (300 MHz, D2O/d-
TFA) δ=8.56 (d, J=5.67 Hz, 2H), 8.12 (d, J=5.67 Hz, 2H); 13C NMR (60 MHz, D2O/d-TFA) δ=139.1, 130.5.
Procedure for the synthesis of the anthracenediol 6o. A solution of 2-bromopyridine (1.58 g, 10 mmol)
in absolute THF (20 mL) was cooled to −78 °C and BuLi (2M in THF, 10 mmol) was added over a period of
15 min. After 30 min, while cooling was maintained, anthraquinone (832 mg, 4 mmol) was added in
several portions. The reaction mixture was allowed to warm to room temperature and left over night.
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Water (30 mL) was added carefully followed by addition of ethyl acetate (70 mL). The organic phase was
separated washed two times with water (30 mL) and dried over sodium sulfate. The crude product was
directly used for the next step with no further purification.
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9,10-Dihydro-9,10-dipyridinyl-9,10-anthracenediol (6o): brown solid (1.8 g yield of crude material); H
NMR (300 MHz, CDCl3) δ=8.56 (d, J=3.2 Hz, 2H), 7.83 (d, J=3.2 Hz, 2H), 7.64 (dd, J=3.1 Hz, 3.2 Hz, 2H),
7.37 (dd, J=3.3 Hz, 6.0 Hz, 4H), 7.23 (dd, J=3.3 Hz, 6.0 Hz, 4H), 7.18 (dd, J=3.1 Hz, 3.2 Hz, 2H), 5.95 (s, 2H,
OH); 13C NMR (60 MHz, CDCl3) δ=165.5, 147.6, 139.7, 137.1, 128.7, 128.4, 122.3, 73.2; HRMS (EI+): m/z:
calcd for C24H18O2N2: 366.1368 [M]; found 366.1375 [M].
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Procedure for the synthesis of the ortho-pyridylanthracene 1o. 9,10-dihydro-9,10-dipyridinyl-9,10-
anthracenediol (6o, 1.8 g, crude material) was dissolved in glacial acetic acid (100 mL). To this solution
were added potassium iodide (6g, 33 mmol) and NaH2PO2 (6g, 68 mmol) and heating at 110 °C was
maintained with stirring overnight. After cooling to room temperature, the solution was carefully
neutralized by addition of potassium hydroxide (2M aqueous solution) followed by extraction between
chloroform and water and drying over sodium sulfate. To the crude product was added hydrochloric acid
(1 M) until a pH of 1 was reached. During acidification a part of the residue dissolved in the water phase,
which gave a yellow color. The organic phase was removed and discarded. To the aqueous phase was
added potassium hydroxide (1M solution) until a pH of 8 was reached. The product was then isolated by
extraction into chloroform (2x 80 mL) and evaporation of the solvent. The procedure of acidification-
extraction-neutralization was repeated until the product was pure.
9,10-Bis(2-pyridyl)anthracene (1o); yellow solid (871 mg, 2.64 mmol, 66% over two steps); mp: 304−307
1
°C; H NMR (300 MHz, CDCl3) δ=8.98 (d, J=6.1Hz, 2H, Ar3), 7.96 (dd, J=7.6 Hz, 7.7 Hz, 2H, HAr5),
7.56−7.66 (m, 6H, H1, H4, H5, H8, HAr4), 7.51 (d, J=7.6 Hz, HAr6, 2H), 7.37 (dd, J=6.8 Hz, 3.8 Hz, 4H, H2,
H3, H6, H7); 13C NMR (60 MHz, CDCl3) δ=158.5 (s, CAr1), 150.0 (d, CAr3), 136.3 (d, CAr5), 136.1 (s, C-9, C-
10), 129.7 (s, C-11, C-12, C-13, C14), 126.9 (d, CAr4), 126.2 (d, C-1, C-4, C-5, C-8), 125.5 (d, C-2, C-3, C-6,
C-7), 122.4 (d, CAr6); IR (cm-1, ATR) ῦ=3032, 2590, 1540, 1438, 1401, 1390, 1196, 1063, 1031, 990, 882,
806, 770. Elemental analysis calcd (%) for C24H16N2 (332.39): C 86.74, H 4.81, N 8.43; found: C 86.44, H
4.90, N 8.48.
General procedure for the synthesis of 9,10-dipyridylanthracenes 1m and 1p by Suzuki coupling of the
corresponding boronic acid to 9,10-dibromoanthracene. 9,10 Dibromoanthracene (672 mg, 2mmol), the
boronic acid 2 (615 mg, 5 mmol), K2CO3 (2.72 g, 20 mmol) and tetrakis(triphenylphosphine)palladium(0)
(231 mg, 0.2 mmol) were dissolved in DMF (80 mL) and water (10 mL). Nitrogen was bubbled through
the solution for 10 min, after which the gas flow was stopped and a temperature of 100 °C was adjusted
under stirring. After keeping for 12h at this temperature, the solvents were removed under vacuum and
the residue was re-dissolved in a mixture of chloroform and water. The two phases were separated and
the organic layer was twice washed with water. Finally, to the last portion of water (80 mL) hydrochloric
acid (1 M) was added until a pH of 1 was reached. During acidification, a part of the residue dissolved in
the water phase, which gave a yellow color. The organic phase was removed and discarded. To the
aqueous phase was added potassium hydroxide (1M solution) until a pH of 8 was reached. The product
was then isolated by extraction into chloroform (2x 80 mL) and evaporation of the solvent. The
procedure of acidification-extraction-neutralization was repeated until the product was pure.
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9,10-Bis(3-pyridyl)anthracene (1m): yellow solid (451 mg, 1.35 mmol, 68 % yield); mp: 292−294 °C; H
NMR (300 MHz, CDCl3) δ=8.87 (d, J=3.97 Hz, 2H, HAr4), 8.77 (s, 2H, HAr2), 7.86 (m, 2H, HAr6), 7.57-7,71
(m, 6H, H1, H4, H5, H8, HAr5), 7.42 (dd, J=3.3 Hz, 6.8 Hz, 4H, H2, H3, H6, H7); 13C NMR (60 MHz, CDCl3)
δ=151.7 (d, CAr2), 149.11 (d, CAr4), 138.8 (d, CAr6), 134.6 (s, C11, C12, C13, C14), 133.7 (s, C9, C10),
130.1 (s, C11, C12, C13, C14), 126.4 (d, C1, C4, C5, C8), 125.7 (d, C2, C3, C6, C7), 123.4 (d, CAr5); IR (cm-1,
ATR): ῦ=3028, 1562, 1479, 1385, 1025, 940, 799, 765. Elemental analysis calcd (%) for C24H16N2 (332.39):
C 86.74, H 4.81, N 8.43; found: C 86.23, H 4.94, N 8.72.
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9,10-Bis(3-pyridyl)anthracene (1p):14b white solid (554 mg, 1.64 mmol, 82 % yield); mp: 363 °C; H NMR
(300 MHz, CDCl3) δ=8.91 (d, J=4.2 Hz, 4H), 7.64 (dd, J=3.3 Hz, 6.8 Hz), 7.47 (d, J=4.2 Hz, 4H), 7.42 (dd, 3.3
Hz, 6.8 Hz, 4H); 13C NMR (60 MHz, CDCl3) δ=150.0, 147.4, 134.7, 129.0, 126.4, 126.2, 125.9; IR (cm-1, ATR)
ῦ=3032, 1590, 1540, 1438, 1401, 1193, 806, 770. Elemental analysis calcd (%) for C24H16N2 (332.39): C
86.74, H 4.81, N 8.43; found: C 86.49, H 4.82, N 8.48.
Procedure for the Photooxygenations of Pyridylanthracenes 1o−m. In a 20 cm long Pyrex glass vial the
anthracene (160 mg, 0.5 mmol) and 5,10,15,20-tetraphenylporphyrin (5 mg, 8 μmol) were dissolved in
dichloromethane (10 mL). The solution was cooled to −20 °C and a slow steam of oxygen was
continuously passed through while the vial was irradiated with a sodium lamp (400 W) for 12 h. The
solvent was then evaporated and the product was isolated by column chromatography.
9,10-Bis(2-pyridyl)-9,10-dihydro-9,10-epidioxidoanthracene (1o-O2): white solid (171 mg, 0.48 mmol,
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95%); Rf = 0.30 (chloroform/ethyl acetate 95:5); mp: 199−205 °C; H NMR (300 MHz, CDCl3) δ=8.99 (d,
J=4.8 Hz, 2H, HAr3), 7.96 (dd, J=7.8 Hz, 7.9 Hz, 2H, HAr5), 7.84 (d, J=7.9 Hz, 2H, HAr6), 7.50 (dd, J=4.8 Hz,
7.8 Hz, 2H, HAr4), 7.21 (br, 8H, H1, H2, H3, H4, H5, H6, H7, H8); 13C NMR (60 MHz, CDCl3) δ=154.5 (s,
CAr1), 149.1 (d, CAr3), 140.3 (s, C11, C12, C13, C14), 137.7 (d, CAr5), 127.8 (d, C1, C4, C5, C8), 123.4 (d,
CAr4), 123.5 (d, C2, C3, C5, C8), 122.7 (d, CAr6), 84.5 (s, C9, C10); IR (cm-1, ATR) ῦ=3066, 1588, 1571,
1457, 1434, 1215, 1157, 1099, 1049, 997, 918, 751; HRMS (ESI+): m/z: calcd for C24H16N2O2: 364.1212
[M]; found 364.1220 [M].
9,10-Bis(3-pyridyl)-9,10-dihydro-9,10-epidioxidoanthracene (1m-O2): white solid (175 mg, 0.48 mmol,
97%); Rf = 0.33 (chloroform/ethyl acetate 95:5); mp: 285−288 °C; 1H NMR (300 MHz, CDCl3) δ=9.02 (s, 2H,
HAr2), 8.86 (d, J=4.8 Hz, 2H, HAr6), 8.09 (d, J=6.1 Hz, 2H, HAr4), 7.62 (dd, J=4.8 Hz, 6.1 Hz, 2H, HAr5),
7.30 (dd, J=4.9 Hz, 8.0 Hz, 4H, H1, H4, H5, H8), 7.15 (dd, J=4.9 Hz, 8.0 Hz, 4H, H2, H3, H6, H7); 13C NMR
(60 MHz, CDCl3) δ=149.7 (d, CAr2), 149.0 (d, CAr4), 139.2 (s, C11, C12, C13, C14), 134.7 (d, CAr4), 129.0
(s, CAr1), 128.9 (d, C1, C4, C5, C8), 125.2 (d, Ar5), 123.2 (d, C2, C3, C5, C8), 82.9 (s, C9, C10); IR (cm-1,
ATR) ῦ=3034, 1514, 1412, 1267, 1155, 1099, 1021, 997, 758; HRMS (ESI+): m/z: calcd for C24H16N2O2:
364.1212 [M]; found 364.1215 [M].
9,10-Bis(4-pyridyl)-9,10-dihydro-9,10-epidioxidoanthracene (1p-O2): white solid (173 mg, 0.48 mmol,
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96%); Rf = 0.25 (chloroform/ethyl acetate 95:5); mp: 358−360 °C; H NMR (300 MHz, CDCl3) δ=8.94 (d,
J=4.6 Hz, 4H, HAr3), 7.55 (d, J=4.6 Hz, 4H, HAr2), 7.30 (dd, J=3.2 Hz, 5.6 Hz, 4H, H1, H4, H5, H8), 7.16 (dd,
J=3.2 Hz, 5.6 Hz, 4H, H2, H3, H6, H7); 13C NMR (60 MHz, CDCl3) δ=150.1 (d, CAr3), 141.4 (s, CAr1), 138.7
(s, C11, , C12, C13, C14), 128.2 (d, C1, C4, C5, C8), 123.2 (d, C2, C3, C5, C8), 122.3 (d, CAr2), 83.4 (s, C9,
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C10); IR (cm-1, ATR): ῦ=3042, 1595, 1459, 1409, 1323, 1216, 918, 808, 757; HRMS (ESI+): m/z: calcd for
C24H16N2O2: 364.1212 [M]; found 364.1208 [M].
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Procedures for the Thermolyses of EPOs 1o−O2-1p−O2. The EPOs 1m−O2 and 1p−O2 (0.4 mmol) were
heated at 110 °C in refluxing toluene for 24 h and the EPO 1o−O2 was heated in chlorobenzene at 135 °C
for 48 h. The solvent was evaporated and the crude product was chromatographed using the same
eluent that was used for the starting material.
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4b,10a-Dihydro-4b,10a-di(2-pyridyl)-benzo[b]benzo[3,4]cyclobuta[1,2-e][1,4]dioxin (7o): yellow solid (42
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mg, 0.12 mmol, 30 %); Rf = 0.15 (chloroform/ethyl acetate 95:5); mp: 194–195 °C; H NMR (300 MHz,
CDCl3) δ=8.3 (d, J=4.3 Hz, 2H), 7.52 (m, 6H), 7.29 (m, 2H), 7.21 (m, 2H), 6.99 (m, 2H), 6.91 (m, 2H); 13C
NMR (60 MHz, CDCl3) δ=156.5, 148.9, 144.9, 144.5, 136.4, 132.0, 127.5, 124.3, 123.5, 123.2, 122.8,
118.9, 97.0; HRMS (ESI+): m/z: calcd for C24H16N2O2: 364.1212 [M]; found 364.1222 [M].
Associated Content
Supporting Information
NMR spectra of all compounds, determination of chemical rate constants of photooxygenations in
1
various solvents and thermolysis, cyclic voltammograms, theoretical calculations, description of the O2
trapping experiments. This material is available free of charge via the Internet at http://pubs.acs.org
ACKNOWLEDGMENT
We thank the University of Potsdam for generous financial support.
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