Photocatalytic nucleophilic addition of alcohols to styrenes in Markovnikov and anti-Markovnikov orientation

Article abstract of DOI:10.3762/bjoc.11.62

The nucleophilic addition of methanol and other alcohols to 1,1-diphenylethylene (1) and styrene (6) into the Markovnikov- and anti-Markovnikov-type products was selectively achieved with 1-(N,N-dimethylamino)pyrene (Py) and 1,7-dicyanoperylene-3,4:9,10-tetracarboxylic acid bisimide (PDI) as photoredox catalysts. The regioselectivity was controlled by the photocatalyst. For the reductive mode towards the Markovnikov-type regioselectivity, Py was applied as photocatalyst and triethylamine as electron shuttle. This approach was also used for intramolecular additions. For the oxidative mode towards the anti-Markovnikov-type regioselectivety, PDI was applied together with Ph-SH as additive. Photocatalytic additions of a variety of alcohols gave the corresponding products in good to excellent yields. The proposed photocatalytic electron transfer mechanism was supported by detection of the PDI radical anion as key intermediate and by comparison of two intramolecular reactions with different electron density. Representative mesoflow reactor experiments allowed to significantly shorten the irradiation times and to use sunlight as "green"light source.

Full text of DOI:10.3762/bjoc.11.62

Photocatalytic nucleophilic addition of alcohols to styrenes in
Markovnikov and anti-Markovnikov orientation
Martin Weiser, Sergej Hermann, Alexander Penner and Hans-Achim Wagenknecht*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 568–575.
Institute of Organic Chemistry, Karlsruhe Institute of Technology
(KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
doi:10.3762/bjoc.11.62
Received: 09 January 2015
Accepted: 08 April 2015
Published: 27 April 2015
Email:
Hans-Achim Wagenknecht* - Wagenknecht@kit.edu
* Corresponding author
This article is part of the Thematic Series "Organic synthesis using
photoredox catalysis".
Keywords:
electron transfer; perylene bisimide; photocatalysis; photochemistry;
pyrene
Guest Editor: A. G. Griesbeck
© 2015 Weiser et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The nucleophilic addition of methanol and other alcohols to 1,1-diphenylethylene (1) and styrene (6) into the Markovnikov- and
anti-Markovnikov-type products was selectively achieved with 1-(N,N-dimethylamino)pyrene (Py) and 1,7-dicyanoperylene-
3,4:9,10-tetracarboxylic acid bisimide (PDI) as photoredox catalysts. The regioselectivity was controlled by the photocatalyst. For
the reductive mode towards the Markovnikov-type regioselectivity, Py was applied as photocatalyst and triethylamine as electron
shuttle. This approach was also used for intramolecular additions. For the oxidative mode towards the anti-Markovnikov-type
regioselectivety, PDI was applied together with Ph–SH as additive. Photocatalytic additions of a variety of alcohols gave the corres-
ponding products in good to excellent yields. The proposed photocatalytic electron transfer mechanism was supported by detection
of the PDI radical anion as key intermediate and by comparison of two intramolecular reactions with different electron density.
Representative mesoflow reactor experiments allowed to significantly shorten the irradiation times and to use sunlight as “green”
light source.
Introduction
Photocatalysts are organic or inorganic compounds that couple [7], were most often used as photocatalysts, whereas the poten-
the physical process of light absorption with a chemical reac- tial of organic compounds and dyes has not yet been fully
tion by means of time, space and energetics, in order to catalyse exploited [8]. The way towards a really complete organo-type
it. With respect to the “green” character of sunlight as unlim- photoredox catalysis has mainly been established for eosin Y as
ited natural light source and the availability of LEDs as cheap an important alternative for [Ru(bpy)3]2+ [9].
and reliable artificial light sources, the research field of
photoredox catalysis has tremendously grown over the past Photocatalytic nucleophilic additions of amines and alcohols to
decade [1-7]. Transition metal complexes, mainly [Ru(bpy)3]2+ olefins, especially styrenes, became an increasingly important
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task due to their potential and versatile applicability in chem- styrene derivatives) results from the two types of photoinduced
ical syntheses. Their non-photochemical counterparts require charge transfer initiated by the photoexcited catalyst
acids, bases or transition metal complexes as catalysts [10]. The (Scheme 1). If an electron-poor chromophore is applied, the
first examples of photochemical olefin aminations were first step that follows irradiation is an electron transfer leading
reported by Cookson et al. [11] and Kawanisi et al. [12] in the to one-electron oxidation of the substrate styrene and may
1960s/70s, and Lewis identified exciplex states as key inter- involve intermediates such as exciplexes. Nucleophilic attack
mediates [13,14]. The corresponding photohydration worked and loss of the proton of the alcohol yield a radical at the
only if the aromatic olefins as starting material were directly benzylic position that explains the anti-Markovnikov-type
excited by UV light [15,16]. The first approach towards a selectivity of this photocatalytic process. Back charge transfer
photocatalytic version of this type of reaction came from to the photocatalyst closes the photocatalytic cycle and subse-
Arnold, Maroulis et al. [17,18]. They demonstrated that elec- quent protonation yields the anti-Markovnikov-type addition
tron-rich naphthalenes are able to photoinitiate methanol addi- product. In contrast, an electron-rich chromophore photoin-
tions to olefins into the Markovnikov orientation and proposed duces an electron transfer onto the substrate. The corres-
an oxidative electron transfer mechanism for this process [17]. ponding radical anion is protonated rapidly to the neutral radical
Complementarily, electron-poor naphthalenes yielded the that is the key intermediate to explain the Markovnikov selec-
anti-Markovnikov-type addition of cyanide to styrene [18]. tivity of this route. Both steps, electron transfer and protonation,
Recently, we showed by a library of different chromophores could also occur in one proton-coupled electron transfer step.
that 1-(N,N-dimethyl-amino)pyrene (Py) can be applied as Back electron transfer to the photocatalyst finishes the photocat-
photocatalyst for the nucleophilic addition of methanol to alytic cycle of this process, and subsequent nucleophilic attack
styrene derivatives into the Markovnikov orientation [19]. Most accompanied by deprotonation gives the Markovnikov-type
recently, Nicewicz et al. published the hydrofunctionalization of addition product.
alkenes to the anti-Markovnikov products by photoredox catal-
ysis using 9-mesityl-10-methylacridinium [20,21]. Herein, we Reductive route: Markovnikov regioselectivity
want to present our complementary approach to perform For the reductive mode of photocatalysis towards the
inter- and intramolecular nucleophilic additions of alcohols to Markovnikov-oriented addition products, we recently applied
styrene derivatives by photocatalysis. The regioselectivity – Py as photocatalyst and 1,1-diphenylethylene (1) as test sub-
Markovnikov or anti-Markovnikov – can simply be controlled strate (Scheme 2). It was assumed that inefficient back electron
by the chosen photocatalyst, either Py or 1,7-dicyanoperylene- transfer was responsible for low yields of the MeOH addition
3,4:9,10-tetracarboxylic acid bisimide (PDI).
product 2 and rapid degradation of the photocatalyst Py. This
problem could be solved by adding triethylamine which served
as electron shuttle between back electron transfer that regener-
ates the photocatalyst and the final step of product formation.
Results and Discussion
Photocatalytic complementarity
The photocatalytic complementarity of the two different routes The substrate scope of this optimized photocatalytic conditions
(to the Markovnikov or anti-Markovnikov addition products of revealed that electron-poor α-phenylstyrenes and styrenes are
Scheme 1: Photooxidation of the substrate and reductive quenching of the photocatalyst (left) vs photoreduction of the substrate and oxidative
quenching of the photocatalyst (right) give two complementary photocatalytic cycles yielding either anti-Markovnikov-type or Markovnikov-type selec-
tivity for the nucleophilic addition of alcohols to styrene derivatives.
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Beilstein J. Org. Chem. 2015, 11, 568–575.
Scheme 2: Mechanism of the Markovnikov-type photocatalytic addition of methanol to 1,1-diphenylethylene (1) and other α-phenylstyrenes in the
presence of 10 mol % of Py (ET = electron transfer).
preferred which further supported the reductive electron transfer binding site on the photocatalyst that keeps the substrate in the
mechanism [19].
vicinity of Py as long as it is required for forward and back
electron transfer.
This photocatalysis was applied also for intramolecular addi-
Oxidative route: anti-Markovnikov regioselec-
tions. In the particular case of substrate 3, Et3N as electron
shuttle could not be used; it provided a competing nucleophile tivity
since the desired nucleophile could not be added in high excess. For the oxidative mode of photocatalysis towards the anti-
In order to shift the reaction more towards the intramolecular Markovnikov-oriented addition products, PDI (Scheme 4) was
alternative, the photoredox catalysis was performed in high applied as photocatalyst. Its absorption maximum in CH2Cl2 is
dilution (2 mM). The product 4 could be identified in 60% yield located at 525 nm that makes it an excellent candidate for
(Scheme 3).
photoirradiation by both sunlight and green light-emitting
diodes. Furthermore, based on Ered(PDI/PDI●−) = −0.28 V
(measured by cyclic voltammetry, vs SCE, see Supporting
Information File 1) and E00 = 2.35 eV (see Supporting Informa-
tion File 1), PDI is an electron deficient chromophore with an
excited state oxidation potential of 2.07 V. In combination with
the oxidation potential of 1.81 V (vs SCE) [22] for substrate 1
the driving force ΔG of initial oxidation was estimated by
Rehm–Weller to be around 250 meV. In general, irradiations
were carried out in quartz glass cuvettes at a constant tempera-
ture of 30 °C, using a 250 mW high-power LED (λ = 530 nm)
as light source while stirring.
Scheme 3: Intramolecular photocatalytic addition with substrate 3;
reaction conditions: 3 (2 mM), Py (2 mM), in MeCN, argon atmos-
phere, 3 h, 25 °C, 366 nm high-power LED, 3 and 4 identified and
quantified by GC–MS.
This example showed that the addition of Et3N as electron Preliminary experiments with substrate 1 revealed that forma-
shuttle was not required in all cases. A more detailed look on tion of benzophenone was nearly completely prevented by care-
the problem of inefficient back electron transfer indicated that fully degassing the reaction mixture. A previous report of Neun-
loss of polar attraction after rapid protonation of the substrate teufel and Arnold considered the electron transfer from the cata-
radical anion might lead to diffusion and separation of the lyst onto the substrate as key step [23]. In agreement with that
photocatalyst from the intermediate product-forming radical proposal, the Stern–Volmer plots (see Supporting Information
cation. If it was assumed that back electron transfer was a File 1) showed that fluorescence of PDI is significantly
strongly distance dependant process, the photocatalyst might quenched in the presence of 1. The critical step, however,
not be regenerated and hence removed from the catalytic cycle. seemed to be the back electron transfer that recovers the photo-
This scenario could potentially be improved by a substrate catalyst from the PDI radical anion after nucleophilic addition,
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Beilstein J. Org. Chem. 2015, 11, 568–575.
Scheme 4: Proposed mechanism of the anti-Markovnikov-type photocatalytic addition of methanol to 1,1-diphenylethylene (1) and styrene (6) yielding
the corresponding products 5 and 7 (ET = electron transfer).
since addition of Ph–SH as electron and proton shuttle helped to amounts of Ph–SH full conversion was achieved within six
significantly accelerated reactions [20,21]. In this respect, hours, whereas 40 mol % only reached 70% of conversion at
oxidative and reductive mode behaved similarily since both that time.
types of photocatalysis needed a suitable electron shuttle as
additive. Comparison of MeOH addition reactions to substrate 1 Nucleophilic addition of a variety of alcohols to substrate 1
in the presence of 0.4 and 1.0 equivalents of Ph–SH as additive gave the corresponding products in excellent yields (Table 1).
showed differences in conversion rates, especially during the Especially the conversion of 1 with benzyl alcohol was signifi-
first six hours of irradiation (Figure 1). With stoichiometric cantly slower, since longer irradiations were needed. Only the
addition of phenol failed completely. Since isopropanol and
tert-butanol as sterically demanding nucleophiles gave the
corresponding addition products in good yields, it was assumed
that the acidity of benzyl alcohol, and more significantly of
phenol, weakened the nucleophilicity for this type of reaction.
Styrene (6) has an oxidation potential of 1.94 V (vs SCE) [22]
and, hence, could also be oxidized by the chosen photocatalyst
PDI. The corresponding photocatalytic nucleophilic additions to
6 (Table 1) yielded less of each product, which was in agree-
ment with the higher oxidation potential (compared to 1). Here
again, the addition of phenol showed no significant amounts of
product formation.
We representatively demonstrated the dependency of the perfor-
mance of photocatalysis with substrate 1 on different PDI
concentrations (Figure 2). After three hours, the yields of
methanol addition product 5 differed only slightly, but on a
longer timescale (12 h and longer) the yields diverged as
expected. The reaction with 2 mol % of PDI was finished after
24 h, whereas the reaction with only 1 mol % reached full
conversion only after 12 additional hours of irradiation time.
Figure 1: Conversion of substrate 1 and formation of product 5
observed during photocatalysis with PDI in the presence of 0.4 equiv
(dashed lines) and 1.0 equiv (solid lines) of Ph–SH as additive; reac-
tion conditions: 1 (20 mM), Ph–SH (20 mM), PDI (0.5 mM), in CH2Cl2/
MeOH 3:1 (4 mL), argon atmosphere, 30 °C, 250 mW LED,
λ = 530 nm, 1 and 5 identified and quantified by GC–MS.
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Table 1: Photocatalytic nucleophilic addition of alcohols to 1 and 6a.
nucleophile
substrate 1
substrate 6
yields (%)b of 5 after 12 h
yields (%)b of 5 after 24 h
yields (%)b of 7 after 42 h
irradiation
irradiation
irradiation
methanol
ethanol
69
75
64
66
63
78
52
n.d.
100
100
100
100
100
98
32
21
24
21
19
19
8c
0
propanol
butanol
isopropanol
tert-butanol
benzyl alcohol
phenol
84
0
aReaction conditions: 1 or 6 (25 mM), Ph–SH (12.5 mM), PDI (0.5 mM), in CH2Cl2/alcohol 3:1 (4 mL), argon atmosphere, 30 °C, 250 mW LED,
λ = 530 nm, 1, 6 and products identified and quantified by GC–MS. bAveraged yield from at least two independent reactions. As no byproducts have
been detected conversion matches yield. cConversion = 24%.
The usage of just 0.2 and 0.5 mol % PDI increased the irradi- imately 4 min (Figure 3). The appearance of this intermediate
ation time at least to 36 h, and it was considered doubtful if strongly supported the proposed electron transfer mechanism of
prolonged irradiation would complete the reactions.
this type of photocatalysis (Scheme 4).
Figure 3: Spectra of PDI before (dotted black) and after excitation
(red), then every 2 min until ground state is reached after 30 min (solid
black). Reaction conditions: 1 (25 mM), Ph–SH (12.5 mM), PDI
(0.02 mM), in CH2Cl2/MeOH 3:1 (4 mL), argon atmosphere, 25 °C, ir-
radiation by 2 LEDs (250 mW), λ = 530 nm.
Figure 2: Conversion of substrate 1 (black dashed) and formation of
product 5 (red solid) observed during photocatalysis with different
amounts of PDI as photocatalyst; reaction conditions: 1 (25 mM),
Ph–SH (12.5 mM), PDI (0.05 (▼), 0.125 (▲), 0.25 (●), 0.50 (■) mM), in
CH2Cl2/alcohol 3:1 (4 mL), argon atmosphere, 30 °C, 250 mW LED,
λ = 530 nm, 1 and 5 identified and quantified by GC–MS.
Although the intramolecular additions of substrates 8 and 10 in
During these photocatalytic experiments, the colour of the solu- the presence of PDI as photocatalyst yielded the corresponding
tion changed from orange to blue after the first seconds of ir- products 9 and 11 only in moderate yields (Scheme 5), they
radiation and turned back to orange just when the reaction was additionally support the proposed photocatalytic mechanism
finished. If the irradiation of the photocatalytic sample was (Scheme 4). Comparison of product formation after 18 h
stopped it took about an hour until the blue color completely showed that the methoxy substituted product 9 was obtained in
disappeared and obviously the chromophore relaxed back to the approximately double yield compared to 11. Obviously, the
ground state. Spectroelectrochemistry measurements (see photooxidation of the electron-rich double bond in substrate 8
Supporting Information File 1) revealed that the blue colored by electron transfer occured faster than the one in substrate 10.
intermediate could be assigned to the radical anion of PDI as These results indicate that the initial charge transfer was the
photocatalyst whose half-lifetime was determined to be approx- rate-limiting step of this photocatalytic process.
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Beilstein J. Org. Chem. 2015, 11, 568–575.
Scheme 5: Intramolecular additions of substrates 8 and 10 to demonstrate the effect of different electron densities of the double bond. Reaction
conditions: 8 or 10 (25 mM), Ph–SH (12.5 mM), PDI (0.5 mM), in CH2Cl2 (4 mL), argon atmosphere, 30 °C, 250 mW LED, λ = 530 nm, 8–11 identi-
fied and quantified by GC–MS.
The photocatalytic capability of PDI was representatively
Table 2: Photocatalytic experiments with 1 in flow reactorsa.
compared to that of 9-mesityl-10-methylacridinium perchlorate
setup
yield of 5 (%)
(MesAcr) which was applied by Nicewicz et al. for similar
additions [20,21]. After 3 h irradiation at 448 nm by two LEDs
(250 mW) in the presence of MesAcr (otherwise identical
experimental conditions as those described in Table 1) product
5 was formed in 30% yield, whereas the corresponding reaction
with PDI as the photocatalyst yields 49% when PDI is irradi-
ated at 530 nm and 59% when irradiated at 470 nm. These irra-
diations were performed with the corresponding LEDs and
yields were identical with conversions.
mesoflow reactor 1b
mesoflow reactor 2d
sunlight w/o PDIf
76c
72e
0
aReaction conditions: 1 (25 mM), Ph–SH (12.5 mM), PDI (0.5 mM), in
CH2Cl2/MeOH 3:1, argon atmosphere, reactants identified and quanti-
fied by GC–MS. bSyringe pump with flow rate of 300 µL/h, 220 min,
30 °C, 4 × 250 mW LED, λ = 530 nm. cConversion = 88%. d4 mL, rt,
sunlight, 17/04/14, Karlsruhe, 11 a.m. until noon. eConversion = 100%.
f1 h, no conversion; 1 month, 51% conversion mainly to benzo-
phenone.
Finally, the nucleophilic addition of methanol to 1 using PDI as
photocatalyst was representatively executed in two mesoflow
reactors, since flow chemistry has significant advantages over controlled by the type of photoinduced charge transfer that was
batch chemistry, such as easier temperature control, larger initiated by the photoexcited catalyst. For the reductive mode
surface-to-volume ratio and more efficient photoirradiation. towards the Markovnikov orientation, Py was applied as photo-
Two setups were used to transfer the reaction to continuous- catalyst. It was previously elucidated that inefficient back elec-
flow systems. The first mesoflow reactor was equipped with tron transfer required the addition of Et3N as electron shuttle
four 250 mW high-power LEDs (λ = 530 nm), a syringe pump, that closed the photocatalytic cycle since back electron transfer
and temperature control to 30 °C. The second one was construc- occurred more efficiently. The photocatalytic process was used
tuted for exposure to sunlight and consisted of a PTFE tubing to also for intramolecular additions. For the oxidative mode
demonstrate applicability of this photocatalysis without need for towards the anti-Markovnikov-type regioselectivety, PDI was a
electricity. Mesoflow experiments were executed using either highly suitable photocatalyst based on its electrochemical and
sunlight, to give 72% yield over only 1 h, or four high-power optical properties. Photocatalytic additions of a variety of alco-
LEDs, to give 76% yield over 3 h (Table 2). As control that 1 hols to styrene derivatives gave the corresponding products in
was not excited directly by sunlight, a sample without PDI was good to excellent yields. Similar to the reductive mode, the
set into sunlight and, as expected, yielded no product.
oxidative nucleophilic addition needed the additive Ph–SH as
electron and proton shuttle. The proposed photocatalytic elec-
tron transfer mechanism was supported by the observation of
Conclusion
The photocatalytic complementarity of the two different routes the PDI radical anion as key intermediate and by comparison of
to either the Markovnikov- or anti-Markovnikov-type nucleo- two intramolecular reactions with different electron density.
philic alcohol addition to styrene derivatives was accomplished Representative mesoflow reactor experiments revealed that the
by Py and PDI as photoredox catalysts. The regioselectivity was irradiation times can be significantly shortened and sunlight can
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Beilstein J. Org. Chem. 2015, 11, 568–575.
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Supporting Information File 1
Spectral data: Cyclic voltammogram of PDI, determination
of E00 of PDI, Stern–Volmer plots of PDI in the presence
of substrate 1, spectroelectrochemistry of PDI, pictures of
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[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-62-S1.pdf]
Acknowledgements
Financial support by the DFG (grant Wa 1386/17-1 and GRK
1626) and KIT is gratefully acknowledged.
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