On the mechanism of photocatalytic reactions with eosin y

Article abstract of DOI:10.3762/bjoc.10.97

A combined spectroscopic, synthetic, and apparative study has allowed a more detailed mechanistic rationalization of several recently reported eosin Y-catalyzed aromatic substitutions at arenediazonium salts. The operation of rapid acid-base equilibria, direct photolysis pathways, and radical chain reactions has been discussed on the basis of pH, solvent polarity, lamp type, absorption properties, and quantum yields. Determination of the latter proved to be an especially valuable tool for the distinction between radical chain and photocatalytic reactions. 2014 Majek et al; licensee Beilstein-Institut.

Full text of DOI:10.3762/bjoc.10.97

On the mechanism of photocatalytic reactions
with eosin Y
Michal Majek*1, Fabiana Filace2 and Axel Jacobi von Wangelin*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 981–989.
1Institute of Organic Chemistry, University of Regensburg,
Universitaetsstr. 31, 93040 Regensburg, Germany and 2Institute of
Organic Chemistry, University of Alcalá, Alcalá de Henares, Madrid,
Spain
doi:10.3762/bjoc.10.97
Received: 30 January 2014
Accepted: 10 April 2014
Published: 30 April 2014
Email:
Michal Majek* - michal.majek@ur.de; Axel Jacobi von Wangelin* -
axel.jacobi@ur.de
This article is part of the Thematic Series "Organic synthesis using
photoredox catalysis".
* Corresponding author
Guest Editor: A. G. Griesbeck
Keywords:
© 2014 Majek et al; licensee Beilstein-Institut.
License and terms: see end of document.
mechanism; organocatalysis; photocatalysis; photoredox catalysis;
quantum yields; visible light
Abstract
A combined spectroscopic, synthetic, and apparative study has allowed a more detailed mechanistic rationalization of several
recently reported eosin Y-catalyzed aromatic substitutions at arenediazonium salts. The operation of rapid acid–base equilibria,
direct photolysis pathways, and radical chain reactions has been discussed on the basis of pH, solvent polarity, lamp type, absorp-
tion properties, and quantum yields. Determination of the latter proved to be an especially valuable tool for the distinction between
radical chain and photocatalytic reactions.
Introduction
The ability of natural systems to harness solar energy for the strated to catalyze light-driven organic reactions. The use of the
genesis of matter has been fascinating mankind since time pyridyl-based complexes [Ru(bpy)3]2+, [Ir(ppy)3] and
immemorial and has stimulated numerous reproduction attempts [Ir(ppy2)(dtbbpy)]+ for the mediation of redox processes has
in the context of chemical synthesis over the last two centuries. certainly attracted the most interest, incipiently in photocat-
The vast majority of photochemical reactions known until the alytic reactions of activated organic electrophiles [2-8].
1980s exploited stoichiometric amounts of a photoactive molec-
ular entity to drive a chemical transformation [1]. Only recently, Despite the numerous examples of efficient catalytic
a steadily growing number of homogeneous transition metal photoredox transformations with organometallic dyes known to
complexes which are redox-active and show absorption date, their high price, toxicity profile, and problematic re-
in the visible range of the solar spectrum have been demon- cyclability might limit their more general use especially on
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larger scales. However, the recent pursuit of environmentally reductants in aqueous media [12-14]. This renaissance of arene
more benign photoactive catalysts has focused on much cheaper diazonium chemistry has recently led to various applications of
metal-free dyes. Several commercially available fluorescein and eosin Y to visible light-driven syntheses of biaryls [15], stil-
xanthene dyes have been successfully applied to photoredox benes [16], benzothiophenes [17], α-phenylketones [18],
reactions, including radical substitutions at α-amino, β-carbon- phenanthrenes [19], arylsulfides [20] and arylboron pinacolates
yl, and aryl moieties [9,10]. Among them, eosin Y, the [21] (Scheme 1).
2’,4’,5’,7’-tetrabromo derivative of fluorescein, has been most
widely employed. The redox potential of the EY+/EY* pair of Numerous mechanistic studies have been performed at reactions
1.1 V (vs. SCE) is experimentally not available as both of the with organometallic photocatalysts [3-8], whereas much less
compounds are short-lived intermediates. However, the redox attention has been directed at eosin Y-catalyzed reactions. The
potential can be obtained indirectly via analysis of the thermo- reductive quenching pathway of eosin Y, which operates in the
dynamic cycle involving the energy of the triplet state eosin Y* photooxidation of isoquinolines [9], has been studied in a single
(T1) (derived from fluorescence measurements) and the energy report [22]. To the best of our knowledge, related data have not
of the radical cation eosin Y+• (derived from cyclovoltam- been collected for the much more widely used oxidative
metric experiments, for more details see Supporting Informa- quenching. Most literature protocols were interpreted in analogy
tion File 1). Much effort has been directed at the oxidative to the related [Ru(bpy)3Cl2]-catalyzed reactions and similar
quenching of eosin Y* (T1) with suitable electrophiles in order mechanisms were proposed (Scheme 2) [15-21]. These general-
to generate aryl radicals by a light-driven single-electron ly commence with the SET between excited eosin Y and the
transfer (SET) process (i.e., one-electron reduction of Ar–X, see arenediazonium salt I to give aryl radical II. Nucleophilic attack
Scheme 1). Due to their easy reducibility, arenediazonium salts onto reactive II generates the more stable complex III which is
are especially attractive precursors which constitute versatile prone to back-electron transfer upon oxidative formation of the
alternatives to haloarene-based strategies. They are readily cationic species IV. Terminal elimination of X+ (mostly a
available by diazotization of anilines, no toxic metals are proton) gives the substitution product V. Two general pathways
required; the bond cleavage generates gaseous N2 which of back-electron transfer can be followed: Path A involves one-
escapes the reaction mixture. Photoredox reactions with arene- electron reduction of the radical cation state of the catalyst.
diazonium salts are often more selective than traditional Radical chain propagation (B) can occur when the SET occurs
methods such as copper(II)-mediated Meerwein arylations [11] with another molecule of the starting material I. Some attempts
or protocols employing stoichiometric iron(II) or titanium(III) have been made to differentiate between radical chain and
Scheme 1: Oxidative quenching of eosin Y with arenediazonium salts and reactions of the resultant aryl radicals.
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Scheme 2: Proposed general reaction mechanism of eosin Y-catalyzed substitutions with arene diazonium salts.
Results and Discussion
Effect of pH on the efficiency of eosin
Y-mediated photocatalysis
photocatalysis mechanisms by monitoring the reaction progress
after the light sources have been switched off [20]. Clearly,
such experiments neglect the existence of short radical
chains and thus provide no conclusive evidence of the mecha-
nism.
For better comparison, we have employed identical reagent
concentrations and reaction conditions (solvents) as reported in
the original papers [15-21]. Dry DMSO was mostly used as
solvent, with the exception of the phenanthrene [19] and aryl-
boron pinacolate [21] syntheses which were run in acetonitrile.
Much to our surprise, the solutions of eosin Y and arenediazo-
nium salts in acetonitrile were yellow (instead of the usual red
colour; see Figure 1, conditions as in [21]) and did not exhibit
intense fluorescence (Figure 2, conditions as in [21]). We have
suspected this to be a consequence of facile acid–base reactions
of the catalyst eosin Y. Indeed, immediate colour change was
effected by addition of base (tetra-n-butylammonium
hydroxide, TBAOH), and strong fluorescence occurred in the
green spectrum. This dramatic pH effect on the spectroscopic
properties of eosin Y solutions prompted us to further investi-
gate this behaviour.
In general, the thermodynamic feasibility of a redox process is
determined by the difference of redox potentials of the two half
reactions. The redox potential of most arenediazonium salts
(redox pair I/II, Scheme 2) is close to 0 V vs. SCE [23,24]. The
redox potentials of the short-lived adducts III/IV are unknown
and experimentally not (easily) available. However, it is likely
that their potential is greater than 0 V in many cases which
makes the radical species III a sufficiently strong reductant for
arenediazonium salts of type I. After all, the unambiguous
determination of the underlying reaction mechanisms is not
possible without further spectroscopic, kinetic, and theoretical
experiments. The studied systems are mostly too complex for
transient absorption spectroscopy. We have so far failed to
obtain any insight from photo-CIDNP NMR experiments.
Therefore, we have decided to evaluate the efficiency of the The organic dye eosin Y can exist in four different structures in
radiation events of several recent literature protocols by solution: the spirocyclic form EY1, the neutral EY2, the
recording the quantum yields Φ [15-17,19-21]. Furthermore, we monoanionic EY3, and the dianionic form EY4 (Scheme 3).
have learned along the way that the reactions appear to be Eosin Y contains two relatively acidic protons (pKa 2.0, 3.8 in
strongly dependent on the pH of the solution and the type of water) [25] which can be easily abstracted to give dianionic
lamp used for irradiation. Here, we present a detailed study on EY4. Lack of clarity exists in many publications on photoredox
the direct consequences of these three major factors for the catalysis with regard to the nature of the employed dye. The
outcome and mechanism of several recently reported eosin authors either report the use of “eosin Y, spirit soluble” – which
Y-catalyzed aromatic substitution protocols starting from arene- can be EY1 or EY2 according to the Sigma–Aldrich catalogue
diazonium salts [15-21].
[26], or claim the use of the free acid EY2. The presence of
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Beilstein J. Org. Chem. 2014, 10, 981–989.
Figure 1: UV–vis spectra of the photoborylation reaction mixture (RM).
Scheme 3: Acid–base behaviour of eosin Y.
photoredox catalysis. In such cases, catalytic activity as
observed in recent publications by König et al. is dependent on
the operation of acid–base equilibration (e.g., in DMSO solu-
tion) so that the employed eosin Y is converted in situ to the
active species EY3 or EY4 [15-18]. Efforts to reproduce the
photoredox synthesis of arylboron pinacolates in acetonitrile
have so far failed in our hands when irradiating at 535 nm [21].
No product formation was observed, and the UV–vis spectra
showed no appreciable absorption at this wavelength. However,
the addition of minor quantities of base (TBAOH) to the reac-
Figure 2: Fluorescence spectra of the photoborylation reaction mix-
ture (RM). Ex. = excitation wavelength.
stoichiometric amounts of bases, e.g., in eosin Y-catalyzed tion mixture resulted in strong absorption at 535 nm and good
photo-oxidative transformations with amines [9], results in the photocatalytic activity under irradiation. The substitution of
quantitative generation of the dianionic form EY4 under the acetonitrile with DMSO in the same reaction gave strong
reaction conditions. On the other hand, the SET-generation of absorption at 535 nm and allowed the photocatalytic synthesis
aryl radicals from arenediazonium salts by photocatalysis of the desired arylborinic ester in good yields in the absence of
proceeds in the absence of base. The non-aqueous conditions extra base. This discrepancy is most likely associated with the
should not provide a significant buffering capacity. Here, the different properties of DMSO and acetonitrile as bases. DMSO
presence of only minute amount of impurities in the solvents or is a stronger base than acetonitrile which enhances the acidity of
starting materials is sufficient to push the acid–base equilibria most Brønsted acids in DMSO [28]. These observations lead to
of the catalytic amounts of eosin Y in either direction.
the conclusion that photoredox reactions catalyzed by eosin Y
(or similar organic dyes) cannot be discussed without strict
The spirocyclic form EY1 contains an interrupted conjugation specification of the employed form of the dye and the reaction
of the fluorone ring system and thus would be photocatalyti- conditions. Conclusive mechanistic proposals of visible-light-
cally inactive under visible light irradiation. The neutral form driven reactions with these dyes also need to address the opera-
EY2 exhibits only weak fluorescence when irradiated with tion of acid–base equilibria and the pH dependence of absorp-
visible light (Figure 2). Studies on related alkylated eosin tion properties.
derivatives suggest that this fluorescent state is very short lived
Effect of the light source on the eosin
and is therefore also not appropriate for photoredox catalysis
[27]. The charged forms EY3 and EY4 are catalytically active, Y-mediated photocatalysis
which in turn also means that solutions containing commercial Another reaction parameter which lacks clarity and consistency
“eosin Y, spirit soluble” would per se not trigger efficient among the literature reports is the source of irradiation. Several
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groups including ourselves have used commercial narrow-band Y, 54% yield of the borylation product were obtained by direct
LEDs with a maximum intensity at 535 nm (green light) [15- photolysis (Scheme 4). These observations are in full accord
18,20]. Other reactions were irradiated with white light from with a report of direct reaction of thermally generated aryl
broad-band compact fluorescent lamps (CFL) [19,21]. The cations (from arenediazonium salts) with bispinacolato diboron
determination of quantum yields requires the use of narrow- to give the corresponding arylborinic esters [31]. It is thus very
band light sources due to the variation of the optical density of likely that direct light-triggered heterolysis of the starting ma-
the samples with the wavelength. Therefore, we have studied terial accounts for substantial amounts of product formation
the impact of different irradiation types on the course of the under conditions which were believed to proceed through
photocatalytic reaction. Although the type of CFLs was not photocatalytic SET.
specified in the literature [19,21], the majority of commercial
CFLs cover similar spectral ranges with the individual UV edge
being significantly below 400 nm and with substantial radiation
power in the region of 400–500 nm. A similar wavelength
distribution is seen in the spectrum of commercial white-
colored LEDs (see Supporting Information File 1 for spectrum).
The activation energy of thermal heterolysis of arenediazonium
ions is approx. 115 kJ/mol (1.19 eV) [29]. The energy of a
photon at the edge of the visible spectrum (400 nm) is 3.1 eV so
that such photons carry sufficient energy to heterolyze diazo-
nium ions to give highly electrophilic aryl cations. Moreover,
arenediazonium salts are known to form weak charge-transfer
complexes with solvent molecules whose absorption tails into
Figure 3: UV–vis spectrum of p-bromobenzenediazonium tetrafluoro-
borate (pBrPhN2) and bispinacolato diboron (B2pin2) in acetonitrile.
the visible range [30].
These observations support the notion that the use of broad-
band visible irradiation indeed can have a profound effect on A similar behaviour was found in our study of the phenan-
the outcome of a photocatalytic reaction. We have therefore threne synthesis [19]. A solution of o-biphenyldiazonium tetra-
examined if direct absorption of the arenediazonium ions can fluoroborate in acetonitrile showed strong absorption between
trigger a productive pathway under irradiation with broad-band 400–500 nm with all light at the UV–vis edge at 400 nm being
light sources even in the absence of the photocatalyst. As completely absorbed. This again indicates that direct photo-
representative examples we chose the recently reported cleavage of the C–N bond might be operating. The cyclization
syntheses of arylborinic esters [21] and phenanthrenes [19]. The reaction with ethyl propiolate according to the literature
absorption spectrum of a mixture of para-bromobenzenediazo- protocol [19] but in the absence of eosin Y did not afford any
nium tetrafluoroborate (p-BrC6H4N2BF4) and bispinacolato phenanthrene product (Scheme 5). Instead, a Ritter-type reac-
diboron (B2pin2) in acetonitrile shows a significant shoulder of tion proceeded to give the corresponding acetanilide after
a UV-absorption band tailing into the visible part of the spec- aqueous work-up which is consistent with the original report by
trum (Figure 3) [30]. At the UV–vis edge (400 nm), the Deronzier from 1984 [32]. The significant overlap of the
absorbance of the system is still more than 0.1 which translates absorption spectra of the arenediazonium salt recorded before
into 21% of all light being absorbed at this wavelength. When and after the addition of eosin Y (Figure 4) suggests that direct
we performed the borylation reaction according to the literature photolysis of the C–N bond could account for the erosion of
report [21] but without the addition of the photocatalyst eosin product yield in the catalytic process due to competitive hetero-
Scheme 4: Eosin Y-catalyzed and dye-free photolytic borylation.
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Beilstein J. Org. Chem. 2014, 10, 981–989.
Scheme 5: Eosin Y-catalyzed and dye-free reactions with ethyl propiolate.
lysis of the substrate and subsequent ionic Ritter reaction. This approaches unity as the efficiency of the photocatalytic step
notion was supported by our experiments performed in DMSO increases. In reality, quantum yields can exceed unity in cases
where a higher portion of the photocatalyst resides in the active where the products of the photocatalytic reaction induce
EY3 and EY4 states. Following an otherwise identical protocol, (radical) chain reactions. Therefore, the determination of
irradiation at 535 nm resulted in the formation of the phenan- quantum yields Φ provides a meaningful answer to mechanistic
threne in 71% yield (cf. literature yield of 53%)[19].
ambiguities. For the eosin Y-catalyzed reactions with arenedia-
zonium salts, conclusive answers to the distinction between
photocatalytic and radical chain mechanisms can be derived
directly from Φ.
A quantification of the photon flux is rather problematic.
Recently, devices became available which use solar cells for the
direct measurement of photon fluxes [33]. However, chemical
actinometry constitutes a prevalent indirect method of photon
flux measurement [34]. Several effective chemical actinome-
ters are known for UV spectral studies whereas similar experi-
ments in the visible range are much more limited by the avail-
ability of chemical actinometers. Even more challenging are
photon flux determinations above 500 nm which marks the
spectral cut-off of the commonly used Hatchard–Parker ferriox-
alate. None of the chemical actinometers that operate in this
region are commercially available. We therefore decided to
prepare potassium Reineckate, a robust actinometer for the
>500 nm region, according to a literature method [34,35].
Figure 4: UV–vis spectra of ortho-biphenyldiazonium tetrafluoroborate
(biPhN2) in acetonitrile.
Quantum yields of eosin Y-catalyzed
reactions
The determination of quantum yields of light-driven reactions
provides valuable insight into the efficiency of the radiative Quantum yields Φ were measured for all aforementioned
processes and thus can be used for the mechanistic under- visible-light-driven reactions in the same solvent, DMSO. Ir-
standing of such processes. The magnitude of quantum yields Φ radiation was performed with a green LED (3.8 W) at 535 nm.
also describes how much energy is wasted into thermal dissipa- All other reaction conditions were adopted from the individual
tion in such systems, which is an especially critical parameter literature reports [15-21]. For more details on the actinometry
for the evaluation of sustainability of a photocatalytic process. experiments and quantum yield determinations, see Supporting
The quantum yield is defined as the efficiency of a photochem- Information File 1. The observed quantum yields Φ of the
ical reaction in the studied system:
studied reactions varied by almost two orders of magnitude,
between 4.7 and 0.075. This already indicates the operation of
different mechanisms in these aromatic substitution reactions
with arene diazonium salts. The redox potentials of most substi-
Φ = (rate of substrate conversion)/(absorbed photon flux)
Theoretically, if a simple photocatalytic process is considered, tuted arene diazonium salts cluster with very little deviation
the quantum yield would be in the range 0 < Φ ≤ 1. It around 0.0 V vs. SCE (±0.2 V) [23,24] so that the observed
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Beilstein J. Org. Chem. 2014, 10, 981–989.
differences in Φ can be largely attributed to different mecha- (Scheme 2, A). Finally, the benzothiophene synthesis [17] and
nistic pathways. Our experiments (Scheme 6) afforded quantum photothiolation [19] exhibited relatively low quantum yields
yields of Φ > 1 for the heterobiaryl coupling [15] and the Heck- which could be a consequence of non-productive processes that
type olefination with styrene [16], respectively. This indicates are responsible for the significant loss of energy. The presence
that in addition to a photocatalytic path (Scheme 2, A) radical- of electron-rich alkylthiolate moieties in both systems could in
chain propagation is operating under the reaction conditions principle effect reversible redox processes with the catalyst
(Scheme 2, B). This contrasts with the other reactions where the eosin Y which might account for the erosion of Φ.
quantum yields range between 0.6 and 0.075. In the original
paper from Deronzier [32], where similar reactions under catal- Conclusion
ysis of Ru(bpy)32+ were studied, Φ values of 0.46–0.78 were In summary, we have investigated the impact of several reac-
reported. Our actinometric experiments of the eosin Y-catalyzed tion parameters on the outcome and mechanism of photocat-
phenanthrene synthesis [19] and photoborylation [21] gave alytic aromatic substitution reactions of arenediazonium salts in
similar quantum yields of 0.35 and 0.60, respectively. This the presence of eosin Y. However, the significance of these data
suggests that the photocatalytic pathway is indeed populated certainly extends to other light-driven reactions that lie beyond
Scheme 6: Quantum yield determinations of selected visible-light-driven aromatic substitutions.
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Beilstein J. Org. Chem. 2014, 10, 981–989.
7. Majek, M.; Jacobi von Wangelin, A. Angew. Chem., Int. Ed. 2013, 52,
5919–5921. doi:10.1002/anie.201301843
the focus of this study. Eosin Y (and many other organic photo-
catalysts) undergo rapid acid–base equilibria which signifi-
cantly alter the photophysical properties. It is therefore of
pivotal importance to ascertain the actual nature of the
employed dye under the reaction conditions. Experimental
details should always be given that specify the employed dye,
the presence of acids and bases as well as the purity of the
reagents, solvents, and additives. The use of broad-spectrum
lamps in photocatalytic reactions with arenediazonium salts is
strongly discouraged as they promote heterolytic C–N bond
cleavage toward highly reactive aryl cation species. The stan-
dard reaction conditions of many literature reports involve
concentrations which are orders of magnitude higher than those
suitable for absorption spectroscopy studies. This means that
even very inefficient transitions (at tailings of absorption
maxima) can indeed trigger productive processes and therefore
need to be addressed in mechanistic rationalizations. Quantum
yield determinations with potassium Reineckate have now
allowed the distinction between photocatalytic and radical-chain
mechanisms. However, the operation of the prevalent pathway
is likely dictated by the stability of the relevant catalytic inter-
mediates.
8. Reckenthäler, M.; Griesbeck, A. G. Adv. Synth. Catal. 2013, 355,
2727–2744. doi:10.1002/adsc.201300751
9. Hari, D. P.; König, B. Org. Lett. 2011, 13, 3852–3855.
doi:10.1021/ol201376v
10.Condie, A. G.; González-Gómez, J. C.; Stephenson, C. R. J.
J. Am. Chem. Soc. 2010, 132, 1464–1465. doi:10.1021/ja909145y
11.Rondestvedt, C. S. In Organic Reactions; Baldwin, J. E.; Bittman, R.;
Boswell, G. A.; Heck, R. F.; Hirschmann, R. F.; Kende, A. S.;
Leimgruber, W.; Marshall, J. A.; McKusick, B. C.; Meinwald, J.;
Trost, B. M.; Weinstein, B., Eds.; Wiley: New York, 1976; pp 225–259.
12.Wetzel, A.; Ehrhardt, V.; Heinrich, M. R. Angew. Chem., Int. Ed. 2008,
47, 9130–9133. doi:10.1002/anie.200803785
13.Heinrich, M. R.; Wetzel, A.; Kirschstein, M. Org. Lett. 2007, 9,
3833–3835. doi:10.1021/ol701622d
14.Heinrich, M. R. Chem.–Eur. J. 2009, 15, 820–833.
doi:10.1002/chem.200801306
15.Hari, D. P.; Schroll, P.; König, B. J. Am. Chem. Soc. 2012, 134,
2958–2961. doi:10.1021/ja212099r
16.Schroll, P.; Hari, D. P.; König, B. ChemistryOpen 2012, 1, 130–133.
doi:10.1002/open.201200011
17.Hari, D. P.; Hering, T.; König, B. Org. Lett. 2012, 14, 5334–5337.
doi:10.1021/ol302517n
18.Hering, T.; Hari, D. P.; König, B. J. Org. Chem. 2012, 77,
10347–10352. doi:10.1021/jo301984p
19.Xiao, T.; Dong, X.; Tang, Y.; Zhou, L. Adv. Synth. Catal. 2012, 354,
3195–3199. doi:10.1002/adsc.201200569
Supporting Information
20.Majek, M.; Jacobi von Wangelin, A. Chem. Commun. 2013, 49,
5507–5509. doi:10.1039/c3cc41867g
Supporting Information File 1
Experimental and analytical data.
21.Yu, J.; Zhang, L.; Yan, G. Adv. Synth. Catal. 2012, 354, 2625–2628.
doi:10.1002/adsc.201200416
22.Liu, Q.; Li, Y.-N.; Zhang, H.-H.; Chen, B.; Tung, C.-H.; Wu, L.-Z.
Chem.–Eur. J. 2012, 18, 620–627. doi:10.1002/chem.201102299
23.Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.;
Pinson, J.; Savéant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207.
doi:10.1021/ja963354s
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-97-S1.pdf]
Acknowledgements
24.Combellas, C.; Jiang, D.-e.; Kanoufi, F.; Pinson, J.; Podvorica, F. I.
Langmuir 2009, 25, 286–293. doi:10.1021/la8025792
25.Baristela, V. R.; Pellosi, D. S.; de Souza, F. D.; da Costa, W. F.;
de Oliveira Santin, S. M.; de Souza, V. R.; Caetano, W.;
Moisés de Oliveira, H. P.; Scarminio, I. S.; Hioka, N.
Spectrochim. Acta, Part A 2011, 79, 889–897.
We gratefully acknowledge generous intellectual and financial
support from the Graduate College on “Chemical Photocatal-
ysis” of the Deutsche Forschungsgemeinschaft (DFG, fellow-
ship to M.M.). We thank Prof. Burkhard König and Dr. Ilya
Shenderovich (both UR) for technical support.
doi:10.1016/j.saa.2011.03.027
26.Aldrich Catalog. Handbook of Fine Chemicals; Aldrich Chemicals:
Milwaukee, WI, 2012.
References
1. Griesbeck, A. G.; Oelgemöller, M.; Ghetti, F., Eds. CRC Handbook of
Organic Photochemistry and Photobiology, 3rd ed.; CRC Press: Boca
Raton, 2012.
27.del Valle, J. C.; Catalán, J.; Amant-Guerri, F.
J. Photochem. Photobiol., A 1993, 72, 49–53.
doi:10.1016/1010-6030(93)85084-L
28.Himmel, D.; Goll, S. K.; Leito, I.; Krossing, I. Angew. Chem., Int. Ed.
2010, 49, 6885–6888. doi:10.1002/anie.201000252
29.Crossley, M. L.; Kienle, R. H.; Benbrook, C. H. J. Am. Chem. Soc.
1940, 62, 1400–1404. doi:10.1021/ja01863a019
2. Nguyen, J. D.; D’Amato, E. M.; Narayanam, J. M. R.;
Stephenson, C. R. J. Nat. Chem. 2012, 4, 854–859.
doi:10.1038/nchem.1452
3. Teplý, F. Collect. Czech. Chem. Commun. 2011, 76, 859–917.
doi:10.1135/cccc2011078
30.Hirose, Y.; Wahl, G. H.; Zollinger, H. Helv. Chim. Acta 1976, 59,
1427–1437. doi:10.1002/hlca.19760590504
4. Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40,
102–113. doi:10.1039/b913880n
31.Zhu, C.; Yamane, M. Org. Lett. 2012, 14, 4560–4563.
doi:10.1021/ol302024m
5. Zeitler, K. Angew. Chem., Int. Ed. 2009, 48, 9785–9789.
doi:10.1002/anie.200904056
32.Cano-Yelo, H.; Deronzier, A. J. Chem. Soc., Perkin Trans. 2 1984,
1093–1098. doi:10.1039/p29840001093
6. Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012, 51, 6828–6838.
doi:10.1002/anie.201200223
988

Beilstein J. Org. Chem. 2014, 10, 981–989.
33.Megerle, U.; Lechner, R.; König, B.; Riedle, E.
Photochem. Photobiol. Sci. 2010, 9, 1400–1406.
doi:10.1039/c0pp00195c
34.Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. Pure Appl. Chem. 2004, 76,
2105–2146. doi:10.1351/pac200476122105
35.Wegner, E. E.; Adamson, A. W. J. Am. Chem. Soc. 1966, 88, 394–404.
doi:10.1021/ja00955a003
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The definitive version of this article is the electronic one
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doi:10.3762/bjoc.10.97
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