Photooxygenation of oxygen-substituted naphthalenes

Article abstract of DOI:10.1002/poc.3734

The reaction of oxygen-substituted naphthalenes with singlet oxygen (¹O₂) has been investigated, and labile endoperoxides have been isolated and characterized at –78°C for the first time. Low-temperature kinetics by UV spectroscopy revealed that alkoxy and silyloxy substituents remarkably increase the rate of photooxygenations compared to 1,4-dimethylnaphthalene, whereas acyloxy-substituted acenes are inert towards ¹O₂. The reactivities nicely correlate with HOMO energies and free activation energies, which we determined by density functional theory calculations. The lability of the isolated endoperoxides is due to their very fast back reaction to the corresponding naphthalenes even at –20°C under release of ¹O₂, making them to superior sources of this reactive species under very mild conditions. Finally, a carbohydrate-substituted naphthalene has been synthesized, which reacts reversibly with ¹O₂ and might be applied for enantioselective oxidations in future work.

Full text of DOI:10.1002/poc.3734

Received: 30 April 2017
DOI: 10.1002/poc.3734
Revised: 31 May 2017
Accepted: 5 June 2017
S P E C I A L I S S U E A R T I C L E
Photooxygenation of oxygen‐substituted naphthalenes
Marcel Bauch | Angela Krtitschka | Torsten Linker
Chemistry Department, University of
Abstract
Potsdam, Golm, Germany
1
The reaction of oxygen‐substituted naphthalenes with singlet oxygen ( O ) has been
investigated, and labile endoperoxides have been isolated and characterized at
2
Correspondence
Torsten Linker, Chemistry Department,
–78°C for the first time. Low‐temperature kinetics by UV spectroscopy revealed that
University of Potsdam, Karl‐Liebknecht‐Str.
2
4‐25, D‐14476 Golm, Germany.
alkoxy and silyloxy substituents remarkably increase the rate of photooxygenations
Email: linker@uni‐potsdam.de
compared to 1,4‐dimethylnaphthalene, whereas acyloxy‐substituted acenes are inert
1
towards O . The reactivities nicely correlate with HOMO energies and free activa-
Funding information
2
University of Potsdam
tion energies, which we determined by density functional theory calculations. The
lability of the isolated endoperoxides is due to their very fast back reaction to the
1
corresponding naphthalenes even at –20°C under release of O , making them to
2
superior sources of this reactive species under very mild conditions. Finally, a
carbohydrate‐substituted naphthalene has been synthesized, which reacts reversibly
1
with O and might be applied for enantioselective oxidations in future work.
2
KEYWORDS
kinetics, labile peroxides, low‐temperature experiments, naphthalenes, singlet oxygen
1
| INTRODUCTION
1a are smoothly oxidized to stable EPOs 2a, which reconvert
to the parent acene only at temperatures >80°C. More
recently, our group found a remarkable influence of triple
1
Singlet oxygen ( O ) is the first excited state of molecular
2
oxygen, can be easily generated in the presence of a
sensitizer by irradiation with visible light, and reacts with
bonds (R = alkynyl, 1b) on the thermolysis, with release of
[
7]
oxygen already at room temperature.
Thomas et al
[
1]
various organic compounds (photooxygenation).
The
described interesting properties of various other alkyne‐
[
8]
most important oxidations with this reactive species are
substituted acenes. In all of these studies, the HOMO ener-
gies play a crucial role for the rates of the photooxygenation
and the thermolysis, which we could quantify by using
[2]
the Schenck Ene reaction of alkenes and the [4 + 2]
[3]
cycloaddition to 1,3‐dienes. In the pioneering work of
[
9]
Adam, first diastereoselective versions of both reaction
p‐substituted anthracenes 1a.
[4]
types have been described.
During these studies, a
In contrast to anthracenes, naphthalenes react much
remarkable effect of hydroxy and ether groups on the
selectivities has been discovered. Our group investigated
the influence of carboxylic acids on photooxygenations
slower with singlet oxygen, and unsubstituted naphthalene
1
[10]
(R = H, 1c) is inert towards O . However, the pioneering
2
work of Wasserman demonstrated that methyl groups
[5]
more than 20 years ago.
Another intriguing feature of O is its reversible reaction
(R = Me, 1d) accelerate the photooxygenations remarkably,
1
[11]
and EPO 2d can be isolated at 0°C.
Additional methyl
2
[
6]
with acenes 1 to endoperoxides (EPOs) 2 (Scheme 1).
groups in other positions of the naphthalene further increase
their stability. Very recently, we described the influence of
esters and amides in the 2‐position of naphthalenes on the
During the back reaction, oxygen might be formed in its
singlet or triplet state. Especially aryl‐substituted anthracenes
1
[12]
kinetics of the reversible reaction with O2
and applied
such compounds for an intramolecular transfer of singlet
oxygen for the first time.
Dedicated to Prof. Waldemar Adam on the occasion of his 80th birthday and
in honor of his pioneering work on labile peroxides.
[
13]
J Phys Org Chem. 2017;e3734.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/poc.3734

2
of 11
BAUCH ET AL.
elemental analyzer. Optical rotary values were measured on
a P‐2000 Digital Polarimeter from Jasco.
2.2 | Synthesis of starting materials
2
.2.1 | 1,4‐Dimethoxynaphthalene (1e)
[
16]
According to literature,
1,4‐dihydroxynaphthalene (1f,
1
.602 g, 10.0 mmol) and K CO (4.146 g, 30.0 mmol) were
2 3
SCHEME 1 Reversible reaction of acenes 1 with singlet oxygen to
endoperoxides 2
suspended in acetone (100 mL) and stirred at rt for
5 minutes. Dimethylsulfate (2.37 mL, 25.0 mmol) was
1
added, and the suspension was heated to reflux for 1 hour.
After cooling, the solvent was evaporated and the residue
was dissolved in dichloromethane (100 mL) and filtered. The
organic phase was washed consecutively with water
To further improve the reactivity of naphthalenes in
photooxygenations, the introduction of oxygen substituents
(R = OX, 1e‐m) seems to be attractive, since donors should
(
2 × 100 mL) and brine (100 mL). After drying over MgSO4,
increase the HOMO energies. On the other hand, the
resulting EPOs 2e‐m might be quite labile, because of their
peroxy ketal structure. Indeed, 1,4‐dimethoxynaphthalene
the solvent was removed in vacuo and the crude product was
purified by column chromatography (PE/CHCl3 4:1) to
afford 1e in 1.674 g (89%); a similar approach from literature
(
1e) has been photooxygenated in only few examples; the
[
17]
[14]
gave 95% yield.
White solid; mp 84°C to 85°C (Lit
corresponding EPO 2e could not be isolated.
More
[
18]
86°C‐87°C);
R = 0.29 (PE/CHCl 4:1); H NMR (600 MHz, CD Cl ):
recently, Aubry described the reaction of water‐soluble
1
1
[15]
naphthyl ethers with O , but again, the EPO decomposed.
f
3
2
2
2
δ = 3.96 (s, 6H; OMe), 6.73 (s, 2H; 2‐H, 3‐H), 7.49 to 7.52
m, 2H; 6‐H, 7‐H), 8.17 to 8.20 ppm (m, 2H; 5‐H, 8‐H);
Therefore, we became interested in the photooxygenation of
other oxygen‐substituted naphthalenes (R = OX). Herein,
we describe our results on the influence of such substituents
on the reversible reaction with singlet oxygen by a combina-
tion of kinetic studies and density functional theory (DFT)
calculations. Furthermore, we were able to isolate and char-
acterize such labile EPOs for the first time.
(
1
3
C NMR (125 MHz, CD Cl ): δ = 56.0 (q; OMe), 103.6
2
2
(d; C‐2, C‐3), 122.0 (d; C‐5, C‐8), 126.1 (d; C‐6, C‐7),
1
26.6 (s; C‐4a, C‐8a), 149.7 ppm (s; C‐1, C‐4); IR (film):
eν = 3072 (w), 3012 (w), 2957 (w), 2839 (w), 1780 (w),
1
1
1
630 (m), 1595 (m), 1471 (s), 1447 (s), 1388 (s), 1241 (m),
−
1
087 cm (s); HRMS (GC‐TOF): m/z calcd for C H O :
1
2 12 2
+
88.0837 [M ]; found: 188.0830; anal calcd (%) for
2
| EXPERIMENTAL SECTION
.1 | General
C H O (188.23): C 76.57, H 6.43; found: C 76.54, H 6.77.
12 12 2
2
2
.2.2 | 1,4‐Diethoxynaphthalene (1g)
All compounds were used as purchased without further
purification. Melting points were determined on a Mel‐Temp
from Electrothermal. Thin‐layer chromatography (TLC) was
performed on TLC Silica gel 60 F254 aluminum sheets from
Merck. Masses were weighted with a Sartorius AX 224
[
16]
In analogy to literature,
1,4‐dihydroxynaphthalene (1f,
1.602 g, 10.0 mmol) and K CO (4.146 g, 30.0 mmol) were
2
3
suspended in acetone (100 mL) and stirred at rt for
15 minutes. Diethylsulfate (3.27 mL, 25.0 mmol) was added,
and the suspension was heated to reflux for 1 hour.
After cooling, the solvent was evaporated and the residue
was dissolved in dichloromethane (100 mL) and filtered. The
organic phase was washed consecutively with water
(2 × 100 mL) and brine (100 mL). After drying over MgSO4,
the solvent was removed in vacuo and the crude product was
purified by column chromatography (PE/CHCl3 5:1) to
afford 1g in 1.818 g (84%).
(
(
Δ = 0.1 mg) for reactions and with a Metler Toledo MX5
1
Δ = 1 μg) for analytic scale concentrations. H NMR and
1
3
C NMR spectra were measured on Bruker Avance 500
(500, 100 MHz) and Bruker Avance 600 (600, 125 MHz)
NMR spectrometers in deuterated dichloromethane. The
solvent residual signal was used as internal reference (5.32
and 53.84 ppm). The IR spectra were recorded on a Nicolet
6700 FTIR spectrometer from Thermo Electron Corporation
[
19]
in neat. The UV/Vis spectra were measured on an ATI
Unicam UV/Vis UV3 spectrometer or an Analytik Jena
Specord S600 spectrometer with a solvent‐filled reference
cuvette. High‐resolution mass spectra (HRMS) were
recorded on a GC‐MS Trace DSQ II mass spectrometer.
The elemental analyses were performed with a Vario EL III
Yellow solid; mp 85°C to 87°C (Lit
88°C‐90°C);
1
R = 0.27 (PE/CHCl 5:1); H NMR (600 MHz, CD Cl ):
f
3
2
2
δ = 1.52 (t, J = 7.0 Hz, 6H; Me), 4.15 (q, J = 7.0 Hz, 4H;
O‐CH ), 6.71 (s, 2H; 2‐H, 3‐H), 7.48 to 7.51 (m, 2H; 6‐H,
2
13
7‐H), 8.21 to 8.24 ppm (m, 2H; 5‐H, 8‐H); C NMR
(125 MHz, CD Cl ): δ = 15.2 (q; Me), 64.5 (t; O‐CH ),
2
2
2

BAUCH ET AL.
3 of 11
1
1
=
04.8 (d; C‐2, C‐3), 122.1 (d; C‐5, C‐8), 126.0 (d; C‐6, C‐7),
26.8 (s; C‐4a, C‐8a), 149.0 ppm (s; C‐1, C‐4); IR (film): eν
δ = 2.46 (s, 6H; OAc), 7.25 (s, 2H; 2‐H, 3‐H), 7.58 to 7.61
(m, 2H; 6‐H, 7‐H), 7.89 to 7.92 ppm (m, 2H; 5‐H, 8‐H);
13
3059 (w), 2979 (w), 2928 (w), 1626 (w), 1594 (m), 1459
C NMR (125 MHz, CD Cl ): δ = 21.2 (q; OAc), 118.1
2 2
(m), 1427 (w), 1380 (m), 1271 (s), 1236 (s), 1157 (m),
(d; C‐2, C‐3), 122.0 (d; C‐5, C‐8), 127.4 (d; C‐6, C‐7),
128.0 (s; C‐4a, C‐8a), 144.8 (s; C‐1, C‐4), 169.8 ppm (s;
OAc); IR (film): eν = 3083 (w), 3049 (w), 3014 (w), 1752
(s), 1605 (m), 1464 (w), 1369 (m), 1205 (s), 1155 (m),
(m); HRMS (GC‐TOF): m/z calcd for
+
−1
1116 (s), 1085 cm (s); HRMS (GC‐TOF): m/z calcd for
+
C H O : 216.1150 [M ]; found: 216.1156; anal calcd (%)
for C H O (216.28): C 77.75, H 7.46; found: C 77.76, H
7
1
4 16 2
14 16 2
−
1
.22.
1061 cm
C H O : 244.0736 [M ]; found: 244.0743; anal calcd
14 12 4
(
6
%) for C H O (244.25): C 68.85, H 4.95; found: C
8.78, H 5.02.
14
12
4
2
.2.3 | 1,4‐Diisopropoxynaphthalene (1h)
[20]
In analogy to literature,
1,4‐dihydroxynaphthalene (1f,
1
.602 g, 10.0 mmol) and K CO (4.146 g, 30.0 mmol) were
2
3
2
.2.5 | 1,4‐Dibenzoyloxynaphthalene (1j)
suspended in acetonitrile (100 mL) and stirred at rt for
1
[
23]
5 minutes. 2‐Bromopropane (3.27 mL, 25.0 mmol) was
In analogy to literature,
1,4‐dihydroxynaphthalene (1f,
added, and the suspension was heated to reflux for 6 hours.
After cooling, the solvent was evaporated and the residue
was dissolved in dichloromethane (100 mL) and filtered. The
organic phase was washed consecutively with water
1.602 g, 10.0 mmol) was dissolved in pyridine (25 mL) and
cooled to 0°C. Benzoylchloride (3.46 mL, 30.0 mmol) was
added within 15 minutes, and the solution was stirred at rt
for 16 hours.
(
2 × 100 mL) and brine (100 mL). After drying over MgSO4,
The solution was poured into HCl (2.5 M, 50 mL) and
extracted with toluene (3 × 50 mL). The organic phase was
washed consecutively with a saturated solution of NaHCO3
in water (2 × 100 mL), water (100 mL), and brine
the solvent was removed in vacuo and the crude product was
purified by column chromatography (PE/CHCl3 5:1) to
afford 1h in 1.935 g (79%).
Yellow solid; mp 57°C to 59°C; R = 0.28 (PE/CHCl
(100 mL). After drying over MgSO , the solvent was
f
3
4
1
5
:1); H NMR (600 MHz, CD Cl ): δ = 1.41 (d,
removed in vacuo and the crude product was purified by
column chromatography (PE/CHCl3 2:1) to afford 1j in
3.153 g (86%).
2
2
J = 6.1 Hz, 6H; Me), 4.64 (sept, J = 6.1 Hz, 2H; O‐CH),
6
8
.78 (s, 2H; 2‐H, 3‐H), 7.45 to 7.48 (m, 2H; 6‐H, 7‐H),
13
[24]
.18 to 8.21 ppm (m, 2H; 5‐H, 8‐H); C NMR (125 MHz,
White solid; mp 167°C to 169°C (Lit 169°C‐169.5°C);
1
CD Cl ): δ = 22.4 (q; Me), 71.5 (d; O‐CH), 107.7 (d; C‐2,
R = 0.18 (CHCl /PE 2:1); H NMR (600 MHz, CD Cl ):
2
2
f
3
2
2
C‐3), 122.4 (d; C‐5, C‐8), 125.8 (d; C‐6, C‐7), 128.1 (s;
C‐4a, C‐8a), 147.8 ppm (s; C‐1, C‐4); IR (film): eν = 3072
δ = 7.46 (s, 2H; 2‐H, 3‐H), 7.57 to 7.60 (m, 2H; 6‐H, 7‐H),
7.61 to 7.64 (m, 4H; 3‐PhH, 5‐PhH), 7.73 to 7.76 (m, 2H;
4‐PhH), 8.00 to 8.03 (m, 2H; 5‐H, 8‐H), 8.36 to 8.38 ppm
(
1
w), 2976 (m), 2929 (m), 2871 (w), 1625 (w), 1590 (m),
458 (m), 1370 (s), 1267 (m), 1233 (m), 1114 (s), 1076 cm
−1
13
(m, 4H; 2‐PhH, 6‐PhH); C NMR (125 MHz, CD Cl ):
2
2
(
[
(
m); HRMS (GC‐TOF): m/z calcd for C H O : 244.1463
δ = 118.4 (d; C‐2, C‐3), 122.1 (d; C‐5, C‐8), 127.6 (d; C‐6,
C‐7), 128.2 (s; C‐4a, C‐8a), 129.2 (d; PhC‐3, PhC‐5), 129.6
(s; PhC‐1), 130.6 (d; PhC‐2, PhC‐6), 134.3 (d; PhC‐4),
1
6 20 2
+
M ]; found: 244.1453; anal calcd (%) for C H O
1
6 20 2
244.33): C 78.65, H 8.25; found: C 78.66, H 8.20.
1
=
45.1 (s; C‐1, C‐4), 165.5 ppm (s; C = O); IR (film): eν
3067 (w), 1735 (s), 1560 (m), 1491 (w), 1450 (m), 1389
2
.2.4 | 1,4‐Diacetoxynaphthalene (1i)
−
1
(m), 1250 (s), 1219 (s), 1156 (m), 1071 cm (s); HRMS
[21]
+
According to literature,
1,4‐naphthoquinone (4, 1.582 g,
(GC‐TOF): m/z calcd for C H O : 244.0736 [M ]; found:
24 16 4
1
2
0.0 mmol) was grounded with zinc dust (1.438 g,
2.0 mmol) and suspended in acetic anhydride (10 mL).
244.0743; anal calcd (%) for C H O (368.39): C 78.25,
H 4.38; found: C 78.08, H 4.37.
24 16 4
Sodium acetate (0.287 g, 3.5 mmol) was added, and the
reaction mixture was stirred for 3 hours at 90°C.
After cooling, the reaction mixture was poured into ice
water (25 mL) and extracted with dichloromethane
2
.2.6 | 1,4‐Bis(trimethylsiloxy)‐naphthalene
(
1k)
[
25]
(3 × 25 mL). The organic phase was washed consecutively
In analogy to literature,
1,4‐dihydroxynaphthalene (1f,
with water (2 × 100 mL) and brine (100 mL). After drying
1.602 g, 10.0 mmol), NEt3 (4.16 mL, 30.0 mmol) and
DMAP (0.122 g, 1.0 mmol) were dissolved in dry
dimethylformamide (50 mL) under nitrogen. The solution
was cooled to 0°C, trimethylsilyl chloride (3.17 mL,
25.0 mmol) was added within 15 minutes, and the solution
was stirred at rt for 3 hours.
over MgSO , the solvent was removed in vacuo and the crude
4
product was purified by column chromatography (CHCl /PE
3
[
21]
2
:1) to afford 1i in 2.317 g (95%, Lit
96%).
[22]
Pale yellow solid; mp 128°C to 129°C (Lit
128°C);
R = 0.22 (CHCl /PE 2:1); H NMR (600 MHz, CD Cl ):
1
f
3
2
2

4
of 11
BAUCH ET AL.
The reaction mixture was filtered over celite; the filtrate
was diluted with dichloromethane (100 mL) and washed
consecutively with water (2 × 100 mL) and brine (100 mL).
within 30 minutes, and the solution was stirred at rt for
12 hours.
The reaction mixture was filtered over celite and the
filtrate was washed consecutively with a saturated solution
After drying over MgSO , the solvent was removed in vacuo.
4
Because of the hydrolytic sensitivity, the crude product was
used without further purification.
of NaHCO in water (2 × 100 mL), water (100 mL), and
3
brine (100 mL). After drying over MgSO , the solvent was
4
1
Green oil; Rf = 0.66 (PE/EtOAc 3:1); H NMR
removed in vacuo and the crude product was purified by
column chromatography (PE/EtOAc 1:1) to afford 1m in
3.001 g (73%); a similar approach from literature gave only
(
600 MHz, CD Cl ): δ = 0.34 (s, 18H; Me), 6.77 (s, 2H;
2 2
2
‐H, 3‐H), 7.47 to 7.50 (m, 2H; 6‐H, 7‐H), 8.08 to
13
[28]
8.11 ppm (m, 2H; 5‐H, 8‐H); C NMR (125 MHz, CD Cl ):
31% yield.
2
2
[
29]
δ = 0.41 (q; Me), 113.0 (d; C‐2, C‐3), 122.9 (d; C‐5, C‐8),
25.8 (d; C‐6, C‐7), 129.1 (s; C‐4a, C‐8a), 145.9 ppm (s;
C‐1, C‐4).
White solid; mp 169°C to 171°C (Lit
178°C‐183°C);
= −67° (c = 1.00 in
CHCl ); H NMR (600 MHz, CD Cl ): δ = 2.04 (s, 3H;
24
1
R = 0.18 (PE/EtOAc 1:1); [α]
f
D
1
3
2
2
3
3
4
5
‐OAc), 2.04 (s, 3H; 4‐OAc), 2.05 (s, 3H; 2‐OAc), 2.06 (s,
H; 6‐OAc), 3.94 (ddd, J = 10.0, 5.0, 2.5 Hz, 1H; 5‐H),
.20 (dd, J = 12.3, 2.5 Hz, 1H; 6a‐H), 4.32 (dd, J = 12.3,
.0 Hz, 1H; 6b‐H), 5.21 (dd, J = 10.0, 9.4 Hz, 1H; 4‐H),
2
.2.7 | 1,4‐Bis(triisopropylsiloxy)‐naphthalene
(
1l)
[25]
In analogy to literature,
1,4‐dihydroxynaphthalene (1f,
5.21 (d, J = 7.9 Hz, 1H; 1‐H), 5.36 (dd, J = 9.7, 9.4 Hz,
1H; 3‐H), 5.43 (dd, J = 9.7, 7.9 Hz, 1H; 2‐H), 7.02 (s, 2H;
2‐ArH, 3‐ArH), 7.53 to 7.56 (m, 2H; 6‐ArH, 7‐ArH), 8.02
to 8.05 ppm (m, 2H; 5‐ArH, 8‐ArH); C NMR (125 MHz,
CD Cl ): δ = 20.8 (q; OAc‐2, OAc‐3), 20.9 (q; OAc‐6),
1.602 g, 10.0 mmol), NEt3 (4.16 mL, 30.0 mmol) and
DMAP (0.122 g, 1.0 mmol) were dissolved in dry
dimethylformamide (50 mL) under nitrogen. The solution
was cooled to 0°C, triisopropyldimethylsilyl chloride
13
2
2
(
6.43 mL, 25.0 mmol) was added within 15 minutes, and
20.9 (q; OAc‐4), 62.2 (t; C‐6), 68.7 (d; C‐4), 71.4 (d; C‐2),
72.5 (d; C‐5), 72.8 (d; C‐3), 100.3 (d; C‐1), 109.7 (d;
ArC‐2), 121.8 (d; ArC‐5, ArC‐8), 126.9 (s; ArC‐4a, ArC‐8a),
127.1 (d; ArC‐6, ArC‐7), 149.0 (s; ArC‐1, ArC‐4), 169.8
(s; OAc‐4), 169.9 (s; OAc‐2), 170.3 (s; OAc‐3),
170.7 ppm (s; OAc‐6); IR (film): eν = 3024 (w), 2960 (w),
1746 (s), 1599 (w), 1371 (m), 1212 (s), 1122 (m),
the solution was stirred at rt for 3 hours.
The reaction mixture was filtered over celite; the filtrate
was diluted with dichloromethane (100 mL) and washed
consecutively with water (2 × 100 mL) and brine (100 mL).
After drying over MgSO , the solvent was removed in vacuo
and the crude product was purified by column chromatogra-
4
−
1
phy (PE) to afford 1l in 3.701 g (78%).
1035 cm
(s); HRMS (ESI‐Q‐TOF): m/z calcd for
+
1
White solid; mp 43°C to 44°C; R = 0.27 (PE); H NMR
C H NaO : 843.2324 [M + Na ]; found: 843.2296; anal
38 44 20
f
(
(
600 MHz, CD Cl ): δ = 1.15 (d, J = 7.6 Hz, 36H; Me), 1.39
sept, J = 7.6 Hz, 6H; Si‐CH), 6.74 (s, 2H; 2‐H, 3‐H), 7.46 to
calcd (%) for C H O (820.75): C 55.61, H 5.40; found:
C 55.50, H 5.55.
2
2
38 44 20
7
.49 (m, 2H; 6‐H, 7‐H), 8.19 to 8.22 ppm (m, 2H; 5‐H, 8‐H);
1
3
C NMR (125 MHz, CD Cl ): δ = 13.4 (d; Si‐CH), 18.3 (q;
2
2
2
.3 | Photooxygenations on NMR scale
Me), 112.0 (d; C‐2, C‐3), 123.0 (d; C‐5, C‐8), 125.7 (d; C‐6,
C‐7), 128.9 (s; C‐4a, C‐8a), 146.2 ppm (s; C‐1, C‐4); IR
The naphthalene 1 (0.15 mmol) was dissolved in deuterated
dichloromethane (3 mL), tetraphenylporphin (TPP) (0.9 mg,
0.0015 mmol) was added. The solution was cooled to −78°C
and irradiated with a sodium lamp (400 W) under passing
gaseous oxygen through the solution. The crude product
was submitted to NMR spectroscopy under careful cooling
and measured at −78°C.
(
film): eν = 3073 (w), 2943 (m), 2866 (m), 2723 (w), 1622
(w), 1589 (m), 1459 (s), 1385 (s), 1268 (s), 1230 (m),
−
1
1
080 cm
(m); HRMS (GC‐TOF): m/z calcd for
+
C H O Si : 472.3193 [M ]; found: 472.3195; anal calcd
2
8
48
2
2
(
%) for C H O Si (472.86): C 71.12, H 10.23; found: C
28 48 2 2
71.02, H 10.27.
2
.2.8 | 1,4‐Bis‐(1‐O‐2,3,4,6‐tetra‐O‐acetyl‐β‐D‐
2.3.1 | 1,4‐Dimethyl‐1,4‐dihydro‐1,4‐
glucopyranosyl)‐naphthalene (1m)
epidioxidonaphthalene (2d)
[26]
In analogy to literature,
4 Å molecular sieve, 1,4‐
Reaction of naphthalene 1d (23 mg, 0.15 mmol) for
dihydroxynaphthalene (1f, 0.801 g, 5.0 mmol) and 2,3,4,6‐
tetra‐O‐acetyl‐α‐D‐glucopyranosyltrichloroacetimidate
240 minutes afforded the title compound with a conversion
[
27]
[11]
of 99%.
1
(4.927 g, 10.0 mmol) were dissolved in dry dichloromethane
H NMR (500 MHz, CD Cl ): δ = 1.80 (s, 6H; Me), 6.69
2
2
(
100 mL) under nitrogen and cooled to −15°C. Boron
(s, 2H; 2‐H, 3‐H), 7.24 to 7.27 (m, 2H; 6‐H, 7‐H), 7.28 to
13
trifluoride diethyl etherate (0.62 mL, 5.0 mmol) was added
7.31 ppm (m, 2H; 5‐H, 8‐H); C NMR (100 MHz, CD Cl ):
2
2

BAUCH ET AL.
5 of 11
δ = 15.6 (q; Me), 78.2 (s; C‐1, C‐4), 119.8 (d; C‐5, C‐8),
2.3.6 | 1,4‐Dibenzoyloxy‐1,4‐dihydro‐1,4‐
126.4 (d; C‐6, C‐7), 138.5 (d; C‐2, C‐3), 140.3 ppm (s;
epidioxidonaphthalene (2j)
C‐4a, C‐8a).
Photooxygenation of naphthalene 1j (55 mg, 0.15 mmol)
gave even after 600 minutes no conversion.
2
.3.2 | 1,4‐Dimethoxy‐1,4‐dihydro‐1,4‐
epidioxidonaphthalene (2e)
2.3.7 | 1,4‐Bis(trimethylsiloxy)‐1,4‐dihydro‐
1
,4‐epidioxidonaphthalene (2k)
Reaction of naphthalene 1e (28 mg, 0.15 mmol) for
3
0 minutes afforded the title compound with a conversion
Reaction of naphthalene 1k (46 mg, 0.15 mmol) for
120 minutes afforded 1,4‐naphthoquinone (4), as detected
in the crude NMR.
of 95%.
1
H NMR (500 MHz, CD Cl ): δ = 3.75 (s, 6H; OMe),
2
2
7
7
.10 (s, 2H; 2‐H, 3‐H), 7.25 to 7.29 (m, 2H; 6‐H, 7‐H),
13
.36 to 7.39 ppm (m, 2H; 5‐H, 8‐H); C NMR (100 MHz,
2
.3.8 | 1,4‐Bis(triisopropylsiloxy)‐1,4‐dihydro‐
,4‐epidioxidonaphthaline (2l)
CD Cl ): δ = 53.7 (q; OMe), 102.2 (s; C‐1, C‐4), 119.0 (d;
2
2
1
C‐5, C‐8), 126.9 (d; C‐6, C‐7), 133.5 (d; C‐2, C‐3),
37.3 ppm (s; C‐4a, C‐8a).
1
Reaction of naphthalene (1l, 71 mg, 0.15 mmol) for
20 minutes afforded the title compound with a conversion
of 99%.
1
1
H NMR (500 MHz, CD Cl ): δ = 1.04 (d, J = 7.2 Hz,
2
.3.3 | 1,4‐Diethoxy‐1,4‐dihydro‐1,4‐
2
2
1
8H; Me), 1.05 (d, J = 7.2 Hz, 18H; Me), 1.18 (sept,
epidioxidonaphthalene (2g)
J = 7.2 Hz, 6H; Si‐CH), 6.66 (s, 2H; 2‐H, 3‐H), 7.25 to
Reaction of naphthalene 1g (32 mg, 0.15 mmol) for
3
of 95%.
7
8
1
1
.28 (m, 2H; 6‐H, 7‐H), 7.46 to 7.49 ppm (m, 2H; 5‐H,
‐H); C NMR (100 MHz, CD Cl ): δ = 12.5 (d; Si‐CH),
7.4 (q; Me), 100.0 (s; C‐1, C‐4), 118.9 (d; C‐5, C‐8),
26.6 (d; C‐6, C‐7), 138.0 (s; C‐2, C‐3), 139.2 ppm (s;
0 minutes afforded the title compound with a conversion
13
2
2
1
H NMR (500 MHz, CD Cl ): δ = 3.33 (t, 6H; Me), 3.96
2
2
to 4.08 (q, 4H, O‐CH ) 7.04 (s, 2H; 2‐H, 3‐H), 7.25 to 7.28
2
C‐4a, C‐8a); HRMS (ESI‐Q‐TOF): m/z calcd for
C H O Si: 505.3169 [M + H ]; found: 505.3145.
(
m, 2H; 6‐H, 7‐H), 7.39 to 7.42 ppm (m, 2H; 5‐H, 8‐H);
+
28 49 4
1
3
C NMR (100 MHz, CD Cl ): δ = 15.4 (q; Me), 62.0 (t;
2
2
O‐CH ), 102.0 (s; C‐1, C‐4), 119.0 (d; C‐5, C‐8), 126.8 (d;
C‐6, C‐7), 134.2 (d; C‐2, C‐3), 137.5 ppm (s; C‐4a, C‐8a).
2
2.3.9 | 1,4‐Bis‐(1‐O‐2,3,4,6‐tetra‐O‐acetyl‐β‐D‐
glucopyranosyl)‐1,4‐dihydro‐1,4‐
epidioxidonaphthalene (2m)
2
.3.4 | 1,4‐Diisopropoxy‐1,4‐dihydro‐1,4‐
Reaction of naphthalene (1m, 123 mg, 0.15 mmol) for
epidioxidonaphthalene (2h)
2
40 minutes afforded the title compound with a conversion
of 99%.
Reaction of naphthalene 1h (37 mg, 0.15 mmol) for
1
H NMR (500 MHz, CD Cl ): δ = 1.93 (s, 3H; OAc),
2
2
3
0 minutes afforded the title compound with a conversion
1
.99 (s, 3H; OAc), 1.99 (s, 3H; OAc), 2.00 (s, 6H; OAc),
2.00 (s, 3H; OAc), 2.05 (s, 3H; OAc), 2.06 (s, 3H; OAc),
.84 (ddd, J = 10.0, 4.7, 2.1 Hz, 1H; 5‐H), 3.88 (ddd,
J = 10.1, 3.8, 2.4 Hz, 1H; 5’‐H), 4.01 (dd, J = 12.5,
.4 Hz, 1H; 6a’‐H), 4.06 (dd, J = 12.5, 2.1 Hz, 1H; 6a‐H),
4.28 (m, 2H; 6b‐H, 6b′‐H), 5.11 to 5.31 (m, 8H; 1‐H, 2‐H,
‐H, 4‐H); 6.94 (d, J = 9.3 Hz, 1H; ArH‐2/3), 7.16 (d,
of 99%.
1
H NMR (500 MHz, CD Cl ): δ = 1.28 (d, J = 6.1 Hz,
2
2
3
3
H; Me), 1.32 (d, J = 6.1 Hz, 3H; Me), 4.43 (sept,
J = 6.1 Hz, 2H; O‐CH), 6.99 (s, 2H; 2‐H, 3‐H), 7.23 to
2
7
8
.27 (m, 2H; 6‐H, 7‐H), 7.38 to 7.41 ppm (m, 2H; 5‐H,
13
‐H); C NMR (100 MHz, CD Cl ): δ = 23.4 (q; Me), 24.7
2
2
3
(
q; Me), 70.8 (d; O‐CH), 102.7 (s; C‐1, C‐4), 119.0 (d; C‐5,
C‐8), 126.7 (d; C‐6, C‐7), 135.2 (d; C‐2, C‐3), 137.7 ppm
s; C‐4a, C‐8a).
J = 9.3 Hz, 1H; ArH‐2/3), 7.28 to 7.33 (m, 2H; ArH‐6,
ArH‐7), 7.35 to 7.39 (m, 1H; ArH‐5/8), 7.43 to 7.46 (m,
(
13
1
H; ArH‐5/8); C NMR (100 MHz, CD Cl ): δ = 20.6 (q;
2
2
2
x OAc), 20.6 (q; 3 x OAc), 20.7 (q; 2 x OAc), 20.7 (q;
OAc), 61.0 (t; C‐6/6′), 61.1 (t; C‐6/6′), 66.5 (d; ), 66.8
d; ), 69.4 (d; ), 70.1 (d; ), 70.9 (d; ), 71.2 (d; ), 71.5 (d; ),
1.6 (d; ), 96.5 (d; C‐1), 101.7 (s; C‐1/4), 102.8 (s; C‐1/4),
119.2 (d; C‐5/8), 119.4 (d; C‐5/8), 127.5 (d; C‐6/7), 127.6 (d;
C‐6/7), 133.3 (s; C‐4a/8a), 135.5 (d; C‐2/3), 135.9 (s; C‐4a/
2
.3.5 | 1,4‐Diacetoxy‐1,4‐dihydro‐1,4‐
(
7
epidioxidonaphthalene (2i)
Photooxygenation of naphthalene 1i (37 mg, 0.15 mmol)
gave even after 600 minutes no conversion.

6
of 11
BAUCH ET AL.
8a), 136.0 (d; C‐2/3), 169.1 (s; OAc), 169.4 (s; OAc), 169.4
dimethylnaphthalene (1d) as a reference system was also
(s; OAc), 169.5 (s; OAc), 170.0 (s; OAc), 170.2 (s; OAc),
determined in cyclohexane at 10°C (for details see Data S1).
1
70.6 (s; OAc), 170.6 (s; OAc); HRMS (ESI‐Q‐TOF): m/z
+
calcd for C H O : 853.2402 [M + H ]; found: 853.2427.
38 45 22
3
| RESULTS AND DISCUSSION
We started our investigations with 1,4‐dimethoxynaphthalene
2
.4 | Kinetic measurements by UV
(
1e), which is commercially available or can be easily
spectroscopy
[
16]
synthesized on a large scale.
Furthermore, it has been
The exact mass of naphthalene 1 (0.05 mmol, Metler Toledo
MX5, Δ = 1 μg) was dissolved in a solution of TPP in dichlo-
photooxygenated previously, but no endoperoxides 2e were
[
14]
detected and only rearrangement products isolated.
We
–5
1
romethane (10 M, 50 mL) to obtain an accurate concentra-
tion of 10− M substrate and stored in the dark at −5°C. From
this stem solution, 9 mL were put into a test tube with a
magnetic stirrer and a septum, cooled to −78°C in the dark
and flushed with oxygen for 5 minutes through a cannula.
Then the test tube was irradiated with a sodium lamp
generated O by irradiation of the sensitizer TPP with a
2
3
sodium lamp (400 W) in the presence of molecular oxygen
in deuterochloroform at −20°C (Experimental), a procedure,
[
5,9,12,13]
which was very convenient in our previous studies.
However, 1,4‐dimethoxynaphthalene (X = Me, 1e) gave no
conversion after 60 minutes and only 30% conversion after
240 minutes (Table 1, entries 1 and 2).
(400 W) under passing gaseous oxygen through the solution.
Periodically, an aliquot of 0.5 mL was taken from the solution
and stored in the dark at −78°C.
Furthermore, no EPO 2e could be detected in the NMR of
the crude reaction mixture, and only cleavage and rearrange-
ment to hydroperoxide 3e was observed (Data S1, Scheme 2).
This decomposition product was found in previous
In an UV/VIS spectrometer with diode array detector
(Analytik Jena Specord S600), each aliquot was diluted by
a tenth in a precooled cuvette (−40°C), filled with dichloro-
photooxygenations of 1,4‐dimethoxynaphthalene (1e) as
−
4
[14]
methane to obtain a concentration of 10
M. The
well.
We explain the formation of intermediate 3e by an
photooxygenation was monitored by the decrease of the
absorption maximum of the naphthalenes 1 (290‐320 nm).
The kinetics were determined at least 3 times for each
substance. In an additional experiment, the k value for 1,4‐
acid‐catalyzed cleavage of the peroxy ketal group in EPO
2e (X = Me) in the presence of traces of water, since
deuterochloroform is always slightly acidic. Subsequent reac-
tion under acidic conditions affords 1,4‐naphthoquinone (4)
a
TABLE 1 Optimization of the photooxygenation of naphthalenes 1e and 1f
Entry
Naph
1e
X
Solvent
Temp
−20°C
−20°C
−20°C
−20°C
−20°C
−20°C
−20°C
−78°C
−78°C
−78°C
Time
60 min
240 min
150 min
240 min
480 min
600 min
90 min
5 min
Conv
<5%
30%
2 (%)
1
2
3
4
5
6
7
8
9
Me
Me
Me
Me
Me
Me
H
CDCl
CDCl
3
–
b
1e
3
c
–
–
d
1e
CDCl
3
>97%
<5%
<5%
<5%
>97%
25%
1e
CD
CD
CD
CD
CD
CD
CD
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
2
2
–
–
–
1e
1e
d
1f
–
1e
Me
Me
Me
25%
40%
95%
1e
10 min
30 min
40%
1
0
1e
95%
a
Procedure: Naphthalene 1 (0.15 mmol) and TPP (0.9 mg, 1 mol%) in the appropriate solvent (3.0 mL) was irradiated with a sodium lamp (400 W) under passing gaseous
oxygen through the solution. Conversions and yields were determined by NMR of the crude product.
b
Formation of hydroperoxide 3e.
c
Addition of diluted hydrochloric acid (5 mol%).
d
Formation of 1,4‐naphthoquinone (4).

BAUCH ET AL.
7 of 11
photooxygenation further. Best conditions were found in
deuterodichloromethane at −78°C and direct NMR mea-
surement of the crude product at this low temperature,
which requires careful handling of the samples (Experimen-
tal, Data S1). Thus, 1,4‐dimethoxynaphthalene (1e) reacts
1
cleanly with O , and only the corresponding EPO 2e
2
besides starting material was detected by NMR. Conver-
sions can be influenced by the reaction times (Table 1,
entries 8‐10), already after 30 minutes > 95% conversion
was achieved, and only EPO 2e could be detected in the
NMR of the crude product. Importantly, we were able to
isolate and characterize such a labile endoperoxide for
SCHEME 2 Proposed mechanism for the cleavage of EPOs 2e and
the first time. That the reaction of oxygen‐substituted
2
f to 1,4‐naphthoquinone (4)
1
naphthalenes with O is completely reversible was easily
2
demonstrated by keeping the NMR tube of the
abovementioned experiment (entry 10) for only 2 minutes
at −20°C. Now the NMR shows exclusively 1,4‐
dimethoxynaphthalene (1e) and no side products. Impor-
tantly, the back reaction releases oxygen mainly in its
singlet state, as demonstrated by a trapping experiment
as the final product (Scheme 2). Such a mechanism is in
accordance to our very recent studies on the stability of EPOs
derived from anthracenes.
[30]
To provide further evidence for this hypothesis, we added
mol% of diluted hydrochloric acid before the
5
photooxygenation (Data S1). Indeed, now, full conversion
was achieved after only 150 minutes, and 1,4‐
naphthoquinone (4) was detected as the main product
with tetramethylethylene (Data S1). Therefore, such labile
1
EPOs can be used as O sources under very mild condi-
2
tions. Although we developed other singlet oxygen donors
[
12,13]
(
Table 1, entry 3,). To suppress the undesired cleavage of
EPO 2e, we conducted the photooxygenation of
,4‐dimethoxynaphthalene (1e) in less acidic
deuterodichloromethane under careful exclusion of water
Experimental). Thus, no 1,4‐naphthoquinone (4) could be
detected by TLC or NMR. However, after 4, 8, or even
0 hours, no conversion of the starting material was observed
very recently,
it was hitherto not possible to release
this reactive species at such low temperatures from organic
compounds.
1
After careful optimization of the reaction conditions, we
became interested in the photooxygenation of various other
oxygen‐substituted naphthalenes (1g‐m), which can be
easily synthesized (Experimental). We wanted to gain
information on their reactivity, kinetics, and stability of
the corresponding EPOs and therefore, compared them with
well‐known 1,4‐dimethylnaphthalene (1d) as standard
(Table 2). which was investigated by Wasserman in
(
1
at −20°C (Table 1, entries 4‐6). This result is surprising,
since in deuterochloroform, 30% conversion was achieved
after 4 hours (entry 2). An explanation might be the shorter
1
lifetime
and
slower
reactivity
of
O2
in
[
31]
[11]
4
deuterodichloromethane compared to deuterochloroform.
However, the reversibility of the photooxygenation of acenes
detail.
(M
Although the rate constant of k = 1.2 × 10
−
1
−1
s ) for its photooxygenation was determined in
[6]
10a
has to be taken into account as well. Thus, EPO 2e might
be formed under the reaction conditions, but since no further
cleavage to hydroperoxide 3e and 1,4‐naphthoquinone (4) in
this solvent can compete, it reacts back to the starting
material even at such low temperature.
cyclohexane at 10°C,
we found full conversion under
our reaction conditions at very low temperature after
240 minutes as well (entry 1). The same reaction with
1,4‐dimethoxynaphthalene (1e) at −78°C proceeded much
faster and afforded EPO 2e already after 30 minutes (entry
2). To obtain more accurate relative rate constants, we per-
formed kinetic measurements by UV spectroscopy at low
temperature and low concentrations, and thus, we could
determine the conversions conveniently by the decrease of
the naphthalene absorption bands at 290 to 320 nm (Data
S1). 1,4‐Dimethylnaphthalene (1d) was used as standard
To distinguish between these 2 possibilities, we investi-
gated the photooxygenation of 1,4‐dihydroxynapthalene
(
1f) in deuterodichloromethane next (entry 7). Indeed,
now the very labile EPO 2f is formed, which due to its
peroxy hemiketal structure is cleaved without acid and
water (X = H, Scheme 2), and 1,4‐naphthoquinone (4)
can be isolated quantitatively after 90 minutes (Data S1).
(entry 1), and the relative rate constants for the reaction
1
1
This proves that O2 reacts with oxygen‐substituted
of all naphthalenes 1e–m with O were easily obtained,
2
8b
naphthalenes, but the corresponding EPOs are thermally
very labile, if no cleavage to other products can compete.
To have a chance to isolate and characterize such labile
species, we had to lower the temperature during the
a method used in other photooxygenations as well. Thus,
krel for 1,4‐dimethoxynaphthalene (1e) was found to be 66
(entry 2), remarkably faster than alkyl‐substituted naphtha-
lene 1d. The same kinetic protocol was used for 1,4‐

8
of 11
BAUCH ET AL.
a
TABLE 2 Photooxygenation of naphthalenes 1d‐1m
b
c
d
♯d
d
Entry
Naph
1d
1e
X
OX = Me
Me
Time (min)
k
rel
HOMO (eV)
ΔG (kcal/mol)
ΔG (kcal/mol)
1
2
3
4
5
6
7
8
9
240
30
1.0 ± 0.03
67 ± 4
61 ± 4
66 ± 5
<0.01
−5.68
−5.17
−5.14
−5.06
−5.97
−5.99
−5.32
−5.14
−5.62
19.48
14.81
14.57
13.07
22.65
23.71
–
–0.86
1.45
1.29
–0.90
3.57
3.87
–
1g
Et
30
1h
1i
iPr
30
e
Ac
600
e
1j
Bz
600
<0.01
f
1k
1l
SiMe
3
120
–
SiiPr
3
120
240
35 ± 2
1.0 ± 0.02
15.82
19.35
0.10
2.23
1m
Glu
a
Procedure: Naphthalene 1 (0.15 mmol) and TPP (0.9 mg, 1 mol%) in deuterodicholoromethane (3.0 mL) was irradiated with a sodium lamp (400 W) at −78°C under
passing gaseous oxygen through the solution.
b
Time until complete conversion to EPO 2, determined by NMR of the crude product.
c
Relative rate constants, determined by UV spectroscopy (Experimental, Data S1).
d
Determined by DFT calculations with B3LYP/6‐31G* (Data S1).
e
No conversion after 10 hours.
f
Decomposition to 1,4‐naphtoquinone (4).
diethoxy‐ (1g) and 1,4‐diisopropoxynaphthalene (1h),
giving similar reaction times and relative rate constants
naphthalenes 1e‐h exhibit remarkably higher HOMO
energies than dimethylnaphthalene (1d) (entries 1‐4), in
accordance to our observed reactivity. To provide further
evidence for this hypothesis and to determine the limits of
functional groups, we photooxygenated 1,4‐diacetoxy‐ (1i)
and 1,4‐dibenzoyloxynaphthalene (1j) next (entries 5 and
6). Now, no conversion could be achieved even after
10 hours, which indicates that these groups are no longer
(Table 2, entries 3‐4).
To compare the relative rate constants with absolute
values, we applied our UV spectroscopy protocol for 1,4‐
dimethylnaphthalene (1d) in cyclohexane at 10°C (Data
1
S1). Under such conditions, the rate with O has been
2
4
−1 −1 10a
determined as k = 1.2 × 10 (M s ) and we found krel
to be 0.21, which can be explained by the longer lifetime of
singlet oxygen in dichloromethane compared to cyclohex-
donors and the importance of electronic effects on the
1
reactivity of naphthalenes with O . The calculated HOMO
2
[31]
ane.
Thus, although we did not determine absolute rate
energies of 1i and 1j are in good accordance to this observa-
tion, which are even lower than for 1,4‐dimethylnaphthalene
(1d) (Table 2).
constants, oxygen‐substituted naphthalenes 1e‐h react with
1
6
−1 −1
O in the range of k = 3 × 10 (M s ) and at least
2
6
0 times faster than 1,4‐dimethylnaphthalene (1d), in good
To extend the photooxygenation of oxygen‐substituted
naphthalenes to other functional groups, we turned our atten-
tion to silyl‐protected systems 1k and 1l, which can be easily
synthesized from 1,4‐dihydroxynapthalene (1f) (Experimen-
tal). Furthermore, their calculated HOMO energies are in the
same range than for 1,4‐dimethoxynapthalene (1e), due to
accordance to studies of Aubry with water soluble
substrates.
characterize such labile EPOs 2e‐h for the first time
including HRMS (Experimental) and demonstrated their
[
15]
Furthermore, we were able to isolate and
clean back reactions to naphthalenes 1 under release of
1
O even at −20°C.
the electron‐donating character of the silyl groups, and we
2
1
To find a mechanistic rationale for the unusual reactivity
of oxygen‐substituted naphthalenes and the lability of the
corresponding EPOs, we conducted DFT calculations on
the B3LYP/6‐31G* level (Data S1). Firstly, we determined
the HOMO energies of the starting materials (Table 2),
which nicely correlate with the reactivity of acenes
expected a fast reaction with O . Indeed, both substrates
2
gave full conversion after 120 minutes (entries 7 and 8).
However, the very labile trimethyl silyl group was cleaved
under the reaction conditions, affording 1,4‐naphthoquinone
(4) according to our mechanistic proposal in Scheme 2. The
more stable triisopropyl silyl derivative 1l reacted cleanly
1
[8,9]
1
with electrophilic O .
Indeed, the oxygen‐substituted
with O , and the corresponding EPO 2 l was isolated
2
2

BAUCH ET AL.
9 of 11
quantitatively after 120 minutes in pure form (entry 8,
Experimental). The somewhat lower reactivity and longer
reaction time can be explained by the higher steric demand
compared to alkoxy EPOs 2e‐h. Important was again the
low‐temperature NMR at −78°C to obtain clean spectra
shifting the equilibrium to the EPOs. At −78°C the back
reaction is prevented and the labile peroxides can be isolated
1
and characterized. Even at −20°C, the EPOs cleave off O2
and form back the parent naphthalenes, making them to
superior sources of this reactive species under very mild
conditions.
(Data S1), since like other EPOs, derivative 2l reacts
quantitatively back to the parent naphthalene under release
Finally, because of our interest in carbohydrate chemis-
1
[32]
of O even at −20°C.
try,
we turned our attention to an oxygen‐substituted
2
To better understand the unusual reactivity of oxygen‐
substituted naphthalenes and the lability of the corresponding
EPOs, we calculated free activation and reaction energies by
DFT (Table 2, Data S1). Interestingly, the activation barriers
of alkoxy‐ 1e‐h and silyloxy‐substituted naphthalenes 1k and
naphthalene 1m, decorated with a glucose derivative
(Table 2, entry 9, and Scheme 3). We easily synthesized
this compound from 1,4‐dihydroxynapthalene (1f), by the
[
33]
Schmidt imidate method (Experimental).
Indeed, the
photooxygenation proceeded smoothly under our mild
reaction conditions and afforded EPO 2m after 240 minutes
quantitatively (entry 9). Thus, naphthalene 1m is less
reactive than alkoxy‐ or silyloxy‐substituted derivatives,
which can be explained by the acetal functional group of
the carbohydrate. Indeed, its HOMO energy, free activation
energy, and kinetics (Table 2, entry 9) are very similar to
1,4‐dimethylnaphthalene (1d) (entry 1) and not to the
electron‐rich systems. To further prove the same reactivity
of both naphthalenes 1d and 1m, we performed a competi-
tion experiment by low‐temperature NMR. Thus, a 1:1
mixture of the starting materials was photooxygenated at
−78°C and was monitored at different times (Data S1).
Indeed, the conversions for both systems are within the
error limit the same.
1l are much lower than for 1,4‐dimethylnaphthalene (1d),
whereas acyloxy derivatives 1i and 1j require even higher
activation energies (Figure 1), nicely correlating with the
observed reactivities.
Even more important are the free reaction energies.
Thus, in contrast to the slightly exergonic formation of
EPO 2d from 1,4‐dimethylnaphthalene (1d), the
photooxygenation of oxygen‐substituted naphthalenes pro-
ceeds endergonic. Only the isopropyl derivatives 1h and 1l
react slightly exergonic or neutral, which can be explained
by the higher steric demand of this substituent, in
accordance to studies of Wasserman on more stable EPOs
1
1b
of sterically hindered methylnaphthalenes.
That the
photooxygenation of oxygen‐substituted naphthalenes can
be forced to complete conversion can be explained by the
On the other hand, in contrast to 1,4‐dimethylnaphthalene
(1d), the free reaction energy of the photooxygenation
of the carbohydrate derivative 1m is with ΔG = +2.23
(kcal/mol) endergonic and therefore the EPO 2m is less
1
excess of O , continuously generated photochemically
2
during the reaction. On the other hand, such positive free
reaction energies in combination with low free activation
♯
energies ΔG explain the lability of the formed EPOs and
stable than 2e. Indeed, it reacts back to the parent naphtha-
1
their fast back reaction even at −20°C (Figure 1). Overall,
lene 2m already at −20°C under release of O . Interesting
2
the photooxygenation of oxygen‐substituted naphthalenes is
are the attached carbohydrates in close proximity to the
peroxide bridge (Scheme 3). The pioneering work of Adam
has demonstrated that naphthalenes with an adjacent
stereocenter are photooxygenated with very high
1
only possible at very low temperatures with excess of O ,
2
4b
diastereoslectivity. Thus, our new EPO 2m represents
1
not only a O2 source under very mild conditions; this
reactive species might be transferred enantioselectively to
prochiral substrates, which we will investigate in future
studies.
FIGURE 1 Calculated free activation and reaction energies for the
photooxygenation of oxygen‐substituted naphthalenes 1
SCHEME
3
Reversible reaction of carbohydrate‐substituted
naphthalene 1m with singlet oxygen to endoperoxide 2m.

1
0 of 11
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| CONCLUSION
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1
smoothly with O . However, reaction conditions have to be
2
–
carefully adjusted, and the corresponding labile EPOs can
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1
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2
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[
[
[
−
20°C. The reactivity of oxygen‐substituted naphthalenes
6
in photooxygenations nicely correlates with their HOMO
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[
[
[
2
ACKNOWLEDGEMENTS
9
We thank the University of Potsdam for generous financial
support. We thank Dr. Fudickar for helpful discussions and
Prof. Möller for providing the NMR facilities.
6
[
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How to cite this article: Bauch M, Krtitschka A,
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