Mechanistic Studies on the Anodic Functionalization of Alkenes Catalyzed by Diselenides

Article abstract of DOI:10.1021/acscatal.8b01236

Herein, we present a detailed kinetic and thermodynamic analysis of the anodic allylic esterification of alkenes as well as the bulk application of the anodic amination and esterification of nonactivated alkenes catalyzed by diselenides. The electrochemical study led to a comprehensive picture of the coupled electrochemical and chemical reaction steps. Cyclic voltammetry measurements are consistent with a bimolecular step after initial electrochemical 1e^- oxidation of the diphenyl diselenide catalyst, 1a, and therefore we postulate a dimerization of the cation, which reacts very rapidly with the alkene, forming the addition product, i.e. the selenolactone 2a. Subsequent electrochemical oxidation of 2a occurs at a slightly higher potential than initial oxidation of 1a. The second oxidation is also followed by a bimolecular process and we hypothesize a dimerization of the cation, which finally eliminates 1a and protons in the rate-determining step, forming the product. Electrochemical analysis of various catalysts, i.e. nonsterically demanding diaryl diselenides with electron withdrawing and donating substituents, revealed that the oxidation potential of the catalyst and the intermediate can be readily tuned by the substituents, thus, prospectively allowing for a wide application of olefinic and nucleophilic substrates. The substituent pattern at the alkene has a smaller influence on the redox potential of the adduct. Controlled potential electrolysis experiments employing different nucleophiles proved that the reaction can be run electrochemically. The functionalization of unactivated alkenes with N- and O-nucleophiles was successfully demonstrated in several bulk electrolysis experiments, and the products were isolated in good yields.

Full text of DOI:10.1021/acscatal.8b01236

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Article
Mechanistic Studies on the Anodic Functionalisation
of Alkenes Catalysed by Diselenides
Mona Wilken, Stefan Ortgies, Alexander Breder, and Inke Siewert
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01236 • Publication Date (Web): 10 Oct 2018
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Mechanistic Studies on the Anodic Functionalisation of Alkenes Catalysed by Diselenides
Mona Wilken,[a] Stefan Ortgies,[b] Alexander Breder*,[b] Inke Siewert*[a]
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a) Universität Göttingen, Institut für Anorganische Chemie, Tammannstr. 4, 37077 Göttingen, Germany
b) Universität Göttingen, Institut für Organische und Biomolekulare Chemie, Tammannstr. 2, 37077 Göttingen, Germany
E-Mail: alexander.breder@chemie.uni-goettingen.de; inke.siewert@chemie.uni-goettingen.de
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Abstract: Herein, we present a detailed kinetic and thermodynamic analysis of the anodic allylic esterification of alkenes as well as
the bulk application of the anodic amination and esterification of non-activated alkenes catalysed by diselenides. The electrochemical
study led to a comprehensive picture of the coupled electrochemical and chemical reaction steps. Cyclic voltammetry measurements
are consistent with a bimolecular step after initial electrochemical 1e− oxidation of the diphenyl diselenide catalyst, 1a, and therefore
we postulate a dimerisation of the cation, which reacts very rapidly with the alkene forming the addition product, i.e. the selenolactone
2a. Subsequent electrochemical oxidation of 2a occurs at a slightly higher potential than initial oxidation of 1a. The second oxidation
is also followed by bimolecular process and we hypothesize a dimerisation of the cation, which finally eliminates 1a and protons in
the rate determining step, forming the product. Electrochemical analysis of various catalysts, i.e. non-sterically demanding diaryl
diselenides with electron withdrawing and donating substituents, revealed that the oxidation potential of the catalyst and the interme-
diate can be readily tuned by the substituents, thus, prospectively allowing for a wide application of olefinic and nucleophilic sub-
strates. The substituent pattern at the alkene has a smaller influence on the redox potential of the adduct. Controlled potential elec-
trolysis experiments employing different nucleophiles proved that the reaction can be run electrochemically. The functionalisation of
unactivated alkenes with N- and O-nucleophiles was successfully demonstrated in several bulk electrolysis experiments and the prod-
ucts were isolated in good yields.
Keywords: Electroorganic synthesis, selenium, oxidative allylic functionalisation, redox catalysis, mechanism
Introduction
Traditional selenocatalytic procedures for the oxidative
derivatization of alkenes predominantly necessitate the use
of strong and in part highly corrosive oxidants such as N-
haloamides and -imides, hypervalent iodine compounds,
and certain peroxide derivatives.[5] Despite the remarkable
achievements that have been made with these oxidants, their
overall utility was shown in many cases to be significantly
impaired by a low degree of redox economy,[6a] atom econ-
omy,[6b] carbon-efficiency,[6c] and – most importantly – a se-
verely restricted range of applicability due to limitations in
terms of chemoselectivity.[4e] Against this background, the
demand for new oxidation strategies that may offer a wider
scope of application becomes lucidly evident.[7]
The utilisation of simple, non-activated alkenes as readily
available and inexpensive building blocks for the oxidative
construction of (hetero)functionalised hydrocarbon archi-
tectures represents a strategically sound and highly sought-
after approach in the realm of chemical synthesis. To this
date, the main part of investigations toward the development
of catalytic alkene oxidations such as allylic functionalisa-
tions has centred on the use of transition metal catalysts.[1]
Due to the lack of selectivity in such processes, certain ole-
finic substrate classes such as internal di-, tri-, and tetrasub-
stituted alkenes still continue to cause severe challenges.[1d]
As a complementary approach that was frequently found to
be even suitable for multisubstituted internal alkenes, a
steadily increasing number of protocols involving catalysts
that are derived from p-block elements such as iodine, sul-
phur, and selenium have been reported.[2,3] With regard to
the latter element, organic (di)selenides were recurrently
shown to facilitate a diverse array of catalytic oxidation re-
actions including allylic alkene functionalisation .[3] Mech-
anistically, the oxidative allylic functionalisation of alkenes
catalysed by Lewis-acidic Se-species generally was pro-
posed to proceed through the oxidative and regiospecific
formation of seleniranium intermediates I,[4] which upon at-
tack by given nucleophiles furnish adducts 2 (Scheme 1).
Subsequent, oxidative elimination of the selenium residue
eventually generates the allylic functionalisation product
4a. Due to the mechanistic idiosyncrasies of Se-catalysed π-
bond activations, this methodology recurrently allowed for
the conversion of substrate classes that were found to be
unamenable for analogous transition metal-catalysed proto-
cols, thus, providing a complementary tool for the oxidative
functionalisation of simple alkenes.[4e]
Scheme 1. Proposed mechanism for the oxidative allylic
functionalisation of alkenes 3 catalysed by diselenides 1.
A promising approach would be the use of electric current,
i.e. free electrons and holes, as oxidant. Electrochemistry
represents a versatile method for generating reactive species
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under very mild conditions.[8] The implementation of an
of substrates, because their redox potentials should be read-
ily tuneable by the substituents and their synthesis is
straightforward.[16] Ideally, this would make our aspired
method highly adjustable to a wide scope of olefinic and
nucleophilic substrates. As a consequence of these consid-
erations, we disclose herein the results of our mechanistic
investigations on the anodic allylic alkene functionalisation
catalysed by 1 and the exemplification of the method in the
allylic amination and esterification of olefins. The study
provides the rational fundament for the design of anodic al-
kene functionalisations mediated by diselenide catalysts.
electrochemical redox regime in the context of Se-catalysed
alkene functionalisations would therefore entail significant
advantages compared to previous approaches.[5c,9] For ex-
ample, corresponding intermolecular processes that are
driven by customary chemical oxidants very frequently suf-
fer from insufficient chemoselectivity, since nucleophilic
byproducts derived from these oxidants (i.e., endogenous
nucleophiles) can superiorly compete with external nucleo-
philes that are actually intended to be coupled with the al-
kene.[4e] Consequently, an electrochemical approach, which
circumvents the transient generation of competing endoge-
nous nucleophiles, may provide an adequate solution to this
oxidant paradigm. Furthermore, the requisite redox poten-
tials can be accurately fine-tuned under electrochemical
conditions, which eliminates the risk of undesired side reac-
tions that may proceed at potentials higher than the actual
target reaction, but below the redox potential of the oxidant.
Along the same lines, the targeted electrochemical process
would become more energy-efficient as the applied poten-
tial matches the potential of the desired reaction. Further-
more, the stoichiometric production of multi-atomic waste
derived from customary chemical oxidants would be intrin-
sically obviated, which would overall render the oxidation
event more sustainable.[10,11] Against this background it
seems surprising that until today there is only a very limited
number of studies on record showcasing that selenium-me-
diated allylic functionalisation of alkenes can be driven
electrochemically. Torii et al. were the first to describe the
anodic synthesis of allylic alcohols and ethers from simple
alkenes using (PhSe)2, 1a, as a redox pre-catalyst.[12] The
authors postulated that phenylselanol (PhSe–OH) would
serve as the actual catalytically active species. Mellor et al.
later reported the stoichiometric acetamidoselenenylation of
non-functionalized alkenes through anodic oxidation of 1a
in acetonitrile (1.3 V vs. 0.1 M AgNO3/Ag).[13] In this con-
text, the authors suggested [PhSe]+ to be the crucial catalytic
species in these reactions.[14] The selenenium cation was be-
lieved to be formed via sequential oxidation of 1a and pro-
posed to react with the olefinic substrate to give an ami-
dation adduct analogous to species 2.[14] This mechanistic
hypothesis for stoichiometric reactions was later slightly re-
fined by Sasaki et al., who had speculated on the transient
formation of iranium ion I prior to the nucleophilic attack
of MeCN, and by Jouikov et al., who showed that the con-
version of adducts 2 to products 4a can proceed with second
order kinetics depending on the reaction medium.[15]
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Results and discussion
At the outset of our investigation, we aimed to get a de-
tailed understanding of the electrochemical and chemical re-
actions that are responsible for the conversion of the alkene
into the functionalised product.[17,18] As a model reaction we
chose the allylic esterification of hex-3-enoic acid, 3a,
forming 5-ethylfuran-2(3H)-one, 4a, catalysed by the un-
substituted diphenyl diselenide 1a (Scheme 2).[7f]
Scheme 2. Anodic allylic esterification of 3a.
Thus, 1a was analysed by cyclic voltammetry at first, as
the cationic product of this oxidation was proposed to be the
crucial species forming the intermediate 2a. The CV meas-
urements were conducted in dry and degassed acetonitrile
under a dinitrogen atmosphere employing a glassy carbon
(GC) electrode. All data are referenced vs. the Fc+/0 redox
couple. The CV of 1a in acetonitrile at 0.1 Vs−1 revealed
two anodic features, a shoulder at ~0.95 V and a peak at 1.04
V, and one corresponding cathodic feature at −0.43 V (Fig-
ure 1, top). This points to two subsequent oxidation pro-
cesses at very similar potentials. The cathodic feature is ab-
sent, if the CV is swept cathodically at first. The CV at 0.1
Vs−1 is similar to the one previously reported by Sasaki et
al.,[15a] though the authors did not observe the shoulder but
Despite these very indicative yet limited reports, no
comprehensive study on the anodic functionalisation of al-
kenes catalysed by diselenides, particularly concerning a
detailed mechanistic analysis in terms of reaction kinetics
and thermodynamics, has been conducted so far. We sur-
mised that a more in-depth understanding of the reaction
mechanism as well as a sound knowledge on the redox po-
tentials of critical reaction intermediates could aid in the de-
sign of highly customised electrochemical methods for a di-
verse array of allylic alkene derivatisations. Aromatic
diselenides seemed to be ideal candidates for a large variety
only one feature around 1 V.[14,15a] The second anodic fea-
ture and the cathodic feature at −0.43 V vanished upon in-
creasing scan rates (ν > 2 Vs−1), and a new cathodic feature
appeared around −0.15 V (ν = 10 Vs−1) (Figure 1, bottom).
This indicates minimum two chemical reactions following
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the oxidation of 1a, one fast reaction giving the redox active
The second oxidation process vanished with increasing
scan rates, which indicates that this oxidation process be-
longs to a redox active species that is formed in a rather slow
chemical reaction after initial oxidation of 1a. Furthermore,
this species is also formed in smaller amounts with increas-
ing concentrations of 1a ([1a] = 0.5-2 mM), as we concluded
from concentration dependent measurements. With increas-
ing concentrations of 1a the ratio of the second and first fea-
ture decreases, which indicates a second bimolecular route
after initial oxidation of 1a (Figure S 13).
Next, we wanted to investigate the impact of the pres-
ence of an alkene on the CV of 1a. The addition of a 10-fold
excess of 3a significantly altered the CV of 1a (Figure 2).
The second anodic feature of 1a (Epa,1 = 1.04 V) as well as
the cathodic features at −0.43 V were no longer present but
a new anodic feature appeared at 1.15 V (ν = 0.1 Vs−1). This
second oxidation process was assigned to the oxidation of
5-methyl-4-(phenylselanyl)dihydrofuran-2(3H)-one, 2a, as
was confirmed from independent measurements of 2a
(Scheme 2, Figure 2). The missing cathodic features with
regard to the CV of 1a indicate a rather fast and clean chem-
ical reaction of 3a and the oxidised species, producing 2a.
species with a reduction potential at −0.15 V and a slower
reaction forming the species showing a reduction process at
−0.43 V. The first oxidation process equals ~1 electron per
1a (Figure S 11). When we used a Pt working electrode, the
shape of the CV was very similar (Figure S 7, Figure S 8),
thus the CV does not depend on the electrode material. We
also studied the effect of the supporting electrolyte and the
solvent on the voltammogram, because Geiger and co-
workers observed significant differences in (ArS)2 by
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n
changing from nBu4NPF6/MeCN to Bu4NBArF/CH2Cl2.[19]
However, the oxidation chemistry of 1a was not affected by
the salt and therefore we used GC/MeCN/nBu4NPF6 for all
further measurements (Figure S 8-Figure S 10).
The peak current ipa,1 of the first anodic feature in-
creased linearly with the square root of the scan rate, which
is characteristic for diffusion controlled conditions (ν = 0.8
– 10 Vs−1, Figure S 11).[20] The large separation of the ca-
thodic and anodic wave over the whole scan rate range and
the anodic shift of the peak potential Epa,1 per ln(v) are in-
dicative for an irreversible electron transfer process or an
(ir)reversible electron transfer process followed by a fast
chemical reaction (EC sequence, E = electrochemical step,
C = chemical step, Figure S 12).[20] A following chemical
reaction, as already proposed previously, is very likely
given the reported low stability of [(PhSe)2]+.[14,15,21] Such-
like radical cations were only isolable with sterically de-
manding residues, e.g. bis(m-terphenyl).[22]
1a + 3a
1a
2a (i x4)
100
80
60
40
20
0
20
15
10
5
Epa,2
Epa,1
40
30
20
10
0
0.05 Vs-1
0.1 Vs-1
0.2 Vs-1
0.4 Vs-1
0
-20
0.4 0.6 0.8 1.0 1.2 1.4 1.6
E vs. Fc+/0 [V]
Figure 2. CV of 1a, 2a and 1a + 10 equiv. 3a in acetonitrile,
n
ν = 0.1 Vs−1, I = 0.1 M Bu4NPF6, [1a] = 1 mM, [2a] = 0.25
mM, the right y-axis belongs to the measurement of 2a.
-10
-1.0 -0.5 0.0
0.5
1.0
1.5
E vs. Fc+/0 [V]
The linear shift of the first anodic peak potential Epa,1
per ln(v) is indicative for minimum an EC mechanism, that
is the oxidation product derived from 1a reacts with 3 to
yield 2a (Figure S 14, Figure S 16). The current of the first
oxidation process equals ~1.8 electrons according to
Randles-Sevcik equation, indicating quantitative conver-
sion of 1a to 2a (Figure S 14). The current ratios of ipa,1 and
ipa,2 held constant over the whole sweep range of 0.02 Vs−1
to 10 Vs−1 (Figure S 17), which points to a very rapid for-
mation of 2a after initial oxidation of 1a. Therefore, the spe-
cies that was slowly formed after initial oxidation of 1a in
the absence of 3a (second anodic feature, Figure 1) does not
represent the kinetically relevant intermediate for the for-
mation of adduct 2a. This particular species (i) was not de-
tected in the CV of a mixture of 1a and 3a and (ii) is not
formed at high scan rates after oxidation of 1a, whereas ad-
duct 2a is formed quantitatively even at high scan rates. The
missing reverse wave of 2a and the linear shift of its anodic
peak potential Epa,2 per ln(v) is in line with an oxidation of
0.8 Vs-1
1.6 Vs-1
3 Vs-1
250
200
150
100
50
5 Vs-1
10 Vs-1
0
-50
-1.0 -0.5
0.0
0.5
1.0
1.5
E vs. Fc+/0 [V]
Figure 1. CV of 1a in acetonitrile at different scan rates, I
n
= 0.1 M Bu4NPF6, [1a] ~ 0.72 mM.
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adduct 2a followed by the elimination of a proton and 0.5
of the reaction of 1a and 0.66 equiv. NOBF4 to be
[(PhSe)3]+.
equiv. 1a, yielding the final product 4a (Figure S 14, Figure
S 16). Notably, both redox processes are under diffusion
control, since ipa,1 and ipa,2 increase linearly with the square
root of the scan rate (Figure S 15, [1a] = 0.25 or 1mM). In
other words, the CV is not recorded under catalytic condi-
tions and kcat is small in comparison to the time scale of the
CV. In line with this conclusion are the facts that ipa,2 re-
mains the same when the concentration of 3 is doubled and
that the current from the oxidation of 2a is not altered by the
presence of excesses of 3a (Figure 2, Figure S 18, Figure S
19).
Concentration dependent measurements of 1a and 3a
revealed that the first and second peak shift toward more
cathodic potentials with increasing concentrations of 1a
(Figure 3, Figure S 20). This finding is indicative for bimo-
lecular reactions proceeding both oxidations and thus, ex-
cludes [(SePh)2]+ as being the active species since the reac-
tion of 1a+ and 3a would be first order in 1a+.[20]
[(PhSe)3]+ exhibits a reduction process at −0.38 V (ν =
0.1 Vs−1, Figure 4). In contrast to the CV of 1a, this reduc-
tion process is already present when the potential is swept
cathodically at first and it gets more prominent when the
sweep direction is reversed (Figure S 28). The first two ox-
idation processes of [(PhSe)3]+ occur at the same potential
as in 1a (Epa,1 ~ 0.92 V, Epa,2 = 1.03 V; ν = 0.1 Vs−1), but in
contrast to 1a, the second process is not vanishing at high
scan rates (Figure S 29). That is, the species, which is oxi-
dised at 1.03 V, is not formed in-situ but is present already
in [(PhSe)3]+. The redox properties of [(PhSe)3]+ in the pres-
ence of 3a are very similar to the one of the mixture of 1a
and 3a and the reduction process in the reverse potential
scan is no longer present (Figure S 30). The first oxidation
process occurs at the same potential as in 1a and the second
one as in 2a. However, the current ratio of the first and the
second oxidation process increases from 150 to 180% with
respect to the CV of 1a and 3a. In other words, larger
amounts of adduct 2a are formed with regard to the number
of injected electrons in the first oxidation process. This in-
dicates that [(PhSe)3]+ reacts initially with 3a forming 1
equiv. 1a and 1 equiv. 2a. Subsequently, the in-situ gener-
ated 1a reacts with further equivalents of 3a upon electro-
chemical oxidation. However, the trimeric selenonium ion
[(PhSe)3]+ is most likely not the species being initially
formed during the oxidation of 1a in the presence of 3a. This
conclusion is based on the fact that the CV of [(PhSe)3]+ has
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[1a] = 0.05 M
[1a] = 0.2 M
[1a] = 0.5 M
500
400
300
200
100
0
-100
-200
two characteristic anodic features (cf. Epa,1 = 0.94 V; Epa,2
=
1.04 V, ν = 0.1 Vs−1) on the time scale at which adduct 2a
is formed, while at the same time scale, the CV of 1a shows
only the first feature.
0.5
1.0
1.5
E vs. Fc+/0 [V]
Figure 3. Concentration dependent measurements of 1a
and 5 mM 3a in acetonitrile, concentration normalised cur-
The monomeric selenenium ion [PhSe]+ was prepared
by treating PhSeBr with AgPF6 or AgBF4 in MeCN-d3. Sur-
prisingly, the (+)-ESI mass spectrum of this reaction mix-
ture also showed [(PhSe)3]+ as the only species (Figure S 6).
However, the NMR data and the redox chemistry of the re-
action product derived from 1a and NOBF4 and of PhSeBr
and AgPF6 differ largely. Therefore, we assume that in the
case of PhSeBr and AgPF6 serving as starting materials, the
trimeric cation [(PhSe)3]+ is produced under the reducing
conditions of the ESI MS experiment. The proton NMR
spectra of the reaction product of PhSeBr and AgPF6
showed distinctly shifted multiplets in comparison to the
n
rent; ν = 0.1 Vs−1, I = 0.1 M Bu4NPF6.
In order to obtain information on the structure of the se-
lenium intermediates that are presumably involved in the
conversion of 1a into 2a, we investigated the redox proper-
ties of [PhSe]PF6 and [(PhSe)3]BF4. Both cationic Se-spe-
cies were speculated to be oxidation products of 1a and to
serve as potent electrophiles towards 3a.[14,15a,7f] Both spe-
cies are rather unstable, but we had reasonable evidence that
they can be formed in-situ for a limited time period. The
chemical oxidation of 1a with substoichiometric amounts of
NOBF4 in MeCN-d3 lead to a new species, which showed
three distinctly shifted and differently split multiplets with
regard to 1a in the proton NMR spectra (Figure S 2). The
77Se NMR of the reaction mixture showed one singlet at 716
ppm (Figure S 3). A pronounced low field shift of the 77Se
resonance compared with 1a (cf. δ = 459 ppm)[23] was in line
with our expectation, and the shift was too low to count for
other species such as [(PhSe)2]+ or fluorinated selenium spe-
cies (Table S 1, Ref [22-26]). The (+)-ESI mass spectrum of
the reaction mixture showed one main species, which was
univocally assigned to [(PhSe)3]+ and a minor, second trinu-
clear species, which could not be identified (m/z = 419.0
u/e, Figure S 5). Thus, we tentatively assigned the product
spectra of 1a, PhSeBr, and [(PhSe)3]+ (Figure S 4). The 13
C
NMR of the reaction mixture showed two species in a ratio
of 10:1, which is similar to the ratio of the two main species
in the proton NMR spectra. 77Se NMR spectra of the reac-
tion mixture showed one slightly broadened signal at 850
ppm (Figure S 3). The signal exhibits a low field shift in
comparison to [(PhSe)3]+ (cf. δ = 716 ppm) and 1a (cf. δ =
459 ppm).[23] On the basis of our current analytical data, we
tentatively assign this initially formed species to
[PhSe]+.[27,28] The species, which we tentatively assign to
[PhSe]+, is stable for approximately 45 min, after 2 h it is
fully decomposed. Time dependent MS studies revealed full
conversion to
a
product with the composition of
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Scheme 3. Proposed reaction pathways after the electrochemical oxidation of 1a in the absence (light grey) and presence of 3a
(black). The green pathways describe minor pathways in the presence of 3a, as the formation of [PhSe]+ and [(PhSe)3]+ is a
rather slow process with regard to the formation of 2a. rds: rate-determining step.
C8H7NNaSe+, – a compound likely formed in a reaction of
[PhSe]+ or its very fast forming decomposition product
exhibits a reduction process at −0.43 V (ν = 0.1 Vs-1), which
is present at slow scan rates and is not affected by a preced-
ing anodic scan (Figure S 31). The same cathodic feature is
present in the CV of 1a at slow scan rates and in [(PhSe)3]+.
The first oxidation process of [PhSe]+ occurs at ~1.01 V,
which is at the same potential as the second anodic feature
in the CV of 1a at slow scan rates (Figure S 32, Figure 4).
Thus, we assign the species, which is formed in a slow pro-
ceeding reaction after initial oxidation of 1a, to be [PhSe]+
or its very fast forming decomposition product. The CV of
[PhSe]+ in the presence of 3a resembles that of 2a (Figure S
33), indicating that [PhSe]+ is capable of reacting with 3a to
furnish 2a. However, the rate of formation of [PhSe]+ sub-
sequent to the oxidation of 1a is much slower compared to
the rate of formation of 2a following the oxidation of 1a in
the presence of 3a. Thus, [PhSe]+ is rather unlikely to be a
kinetically relevant species to produce 2a after initial oxida-
tion of 1a under these conditions.
Taking all measurements together, we propose the fol-
lowing mechanistic scenario for the oxidation of 1a in the
absence and presence of 3a (Scheme 3). Having in mind that
adduct 2a is formed very rapidly – more rapidly than the
formation of [PhSe]+ – after initial one electron oxidation of
1a in a second order process with regard to 1a and that 1a+
did not show any reversibility even at high scan rates, we
propose a very rapid formation of tetrameric dication
[(PhSe)4]2+ upon initial oxidation of 1a (i.e., dimerisation of
1a+).[29] Such dimerisations of organoselenium cations have
precedents in the literature.[24,30] Dications with the generic
composition [R4E4]2+ (R = Et, Me, E = Se, Te) were isolated
and fully characterised after chemical one-electron-oxida-
tion of the corresponding dichalcogenides (R2E2). Such re-
activity is very similar to what is proposed herein after the
[PhSe]+ with MeCN.
[(PhSe)3]+
80
60
40
20
0
40
30
20
10
0
[PhSe]+
1a (i x2)
*
0.1 Vs-1
-20
-10
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
E vs. Fc+/0 [V]
500
250
200
150
100
50
[(PhSe)3]+
[PhSe]+
400
300
200
100
0
1a (i x2)
*
0
3 Vs-1
-100
-50
-100
-200
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
E vs. Fc+/0 [V]
Figure 4. Overlay of representative CVs of [(PhSe)3]BF4,
[PhSe]PF6, and 1a at two different scan rates, I = 0.1 M
nBu4NPF6; the right y-axis belong to the measurement of 1a.
The redox process marked with an asterisk likely belongs to
PhSeBr as reasoned from independent measurements of
PhSeBr under otherwise identical conditions.
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electrochemical oxidation of 1a. Notably, analogous tetram-
diselenides 1b-j, which are readily derived from the corre-
sponding set of aryl bromides and an aniline derivative ac-
cording to literature-known procedures (Table 1).[16]
In accordance with our expectations, the peak potentials
of the first oxidation process for each diselenide 1 vary over
a wide potential range (Table 2). More specifically, 1e (o-
MeO and p-OMe) exhibits the lowest peak potential of
about ~0.6 V and 1j (m-CF3 and m-CF3) the highest with
1.35 V (ν = 0.1 Vs−1, Table 2, Figure S 21, Figure S 22).
eric dications possessing non-stabilising aryl substituents
were not isolable.[24,30] This circumstance is in line with the
proposed formation of [PhSe]+ and [(PhSe)3]+ after electro-
chemical oxidation of 1a in the absence of 3a as well as the
formation of the trimeric species after substoichiometric
chemical oxidation of 1a. A dicationic tetramer could well
decompose yielding [PhSe]+ and [(PhSe)3]+, though the lat-
ter can also be formed in a reaction of in-situ formed
[PhSe]+ and 1a.[31] In the presence of 3a, however, the pro-
posed transient [(PhSe)4]2+ rapidly reacts with 2 equiv. of
3a, yielding 2 equiv. of 2a and 1 equiv. of 1a. Subsequent
oxidation of 2a leads to the corresponding cation 2a+, which
in turn dimerises to give [2a–2a]2+. A second order reaction
after electrochemical oxidation has been proposed previ-
ously for alkyl aryl selenoethers.[15c] The authors proposed
a disproportionation reaction. However, we assign the pri-
mary species, which is formed in a bimolecular step after
oxidation of 2a, to a dimeric cation, since the dimersiation
of a similar Se cation has been observed previously and a
hydrolysis product of [2f–2f]2+ has been observed recently
by in-situ ESI mass spectrometry during the chemical oxi-
dation of 2f with NOBF4.[7f,32] The rate determining step un-
der electrochemical conditions is the formation of 4a and 1a
upon oxidation of 2a.[33]
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Table 2. First anodic peak potentials Eap of 1 and 2 at 0.1
Vs−1. If not otherwise noted, 2 have been formed in-situ by
electrochemical oxidation of 1 in the presence of 3a. I = 0.1
n
M Bu4NPF6, [1] = 0.5-0.6 mM, [3a] = 5 mM, MeCN.
Eap /V
~0.95
0.87
0.80
1.13
~0.6
0.82
1.20
1.28
0.91
~1.35
Eap /V
1.15a
1.08
0.94
1.35
0.79
0.99
1.32a
1.40
1.11
1a
1b
1c
1d
1e
1f
1g
1h
1i
2a
2b
2c
2d
2e
2f
2g
2h
2i
b
-
1j
2j
a Determined by independent measurements of 2. b The ox-
idation of 2j is outside the potential window.
Table 1. Synthesis of diaryldiselenides 1b-j.
1.4
j
1.2
d
1.0
a
Epa of 1
i
0.8
b
Epa of 2
c
0.6
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
subsitutent parameter 
Figure 5. First anodic peak potentials Eap of 1 and 2 at 0.1
V s−1 vs. Hammett’s substituent parameter σ [34]
.
The peak potentials follow the trend of the substituent
constant σ (Figure 5),[34] meaning that electron-withdrawing
groups increase the oxidation potential of diselenides 1d,
1g, 1h, and 1j relative to 1a. Analogously, electron-donat-
ing groups (e.g., 1b, 1c, 1e, 1f and 1i) markedly lower the
oxidation potentials. The scan rate normalised peak currents
and the scan rate dependent shifts of Epa,1 show the expected
behaviour for an EC mechanism in all derivatives. All
selenides in this study show initial irreversible oxidation
owing the low steric demand at the aryl substituent.[22] 1b,
1e, and 1j each exhibit a second oxidation process near the
potential of the first oxidation. These second features vanish
at high scan rates – a behaviour analogous to 1a. 1g and 1h
did not show any further oxidation processes within the po-
tential window of the solvent, while 1c, 1f, and 1d exhibit
With the idea in mind to make our anodic, selenide-cat-
alysed alkene oxidation protocol tuneable in terms of redox
potentials, our efforts turned toward the investigation of ar-
omatic diselenides with electronically varying substitution
patterns. For this purpose we synthesised a series of
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further oxidation processes at considerably higher poten-
rivatives, and the missing reverse feature as well as the po-
tential shift of Epc,1 with ν is in line with an EC process (ν =
0.1-20 Vs−1, Figure S 26, Figure S 27).
In summary, we can conclude that the electronic nature
of the substituents within the carbon backbone of 5 has a
smaller influence on the redox potential than the substitu-
ents on the ArSe groups. This finding appears plausible,
since the selenium atom is the redox active centre and
should therefore experience stronger electronic effects ex-
erted by the substituents of the adjacent aryl groups.
tials. However, the ratios of the first and the second oxida-
tion peak in 1c, 1f, and 1d are scan rate independent in con-
trast to ratios in 1a. In 1i, the second oxidation process gets
more prominent with increasing scan rate, while the third
one vanishes. These disparities could point to slightly dif-
ferent reaction kinetics after initial oxidation of 1.
However, upon adding an excess of alkene 3a, the first
oxidation features of all diselenides 1 remain unchanged
and additional features vanish. As in the case of 1a, a second
oxidation process occurred in the CV of 3a in the respective
presence of 1b, 1c, 1d, 1e, 1f, 1h, and 1i (Figure S 23). 1j
cannot be oxidised any further within the potential window
of the solvent and 1g behaves slightly different, that is a pre-
wave occurs upon adding 3a. Thus, these two compounds
are excluded from the following discussion. The second fea-
tures, observed in the CV of 1 in the presence of 3a, belong
to the respective adducts 2. As was already observed for 2a,
the substituted analogues 2b-f, 2h, and 2i were formed very
rapidly, because the current ratios of the first (1) and second
(in-situ formed 2) oxidation peaks are very similar over the
whole scan rate range (0.1-20 Vs−1, Figure S 24). The peak
current normalised by the square root of the scan rate for
each diselenide 1 and in-situ formed adduct 2 is very simi-
lar, which is indicative of diffusion controlled conditions ra-
ther than catalytic conditions as in 1a.
The oxidation potentials of 2 vary over a wide range
(0.70-1.40 V). However, the peak potential differences be-
tween 1 and their respective 2 are relatively invariant for all
compound pairs, ranging from 0.12 V in 1g/2g to 0.22 V in
1d/2d (Table 2, Figure 5). The very similar behaviour of all
diselenides suggests a very similar mechanism independent
from the respective substitution pattern. The species that are
formed after initial oxidation of 1 – most likely tetrameric
dications – react very fast with 3a to form 2, which in turn
are further oxidised at slightly higher potentials than 1.
Next, we wanted to analyse the influence of electroni-
cally varied substituents embedded in the alkene-derived
carbon backbone of selenoacetoxylation adducts 5a-h on
the redox potential (Scheme 4). We surmised that the corre-
sponding redox potential would more strongly depend on
the substituents on the aryl group and less on the substitu-
ents incorporated in the alkene-derived carbon backbone, as
the former is spatially nearer to the redox active Se centre.
Accordingly, we synthesised adducts 5a-h through the con-
version of alkenes with PhSeBr and potassium acetate in a
10:1 mixture of acetic acid and acetic anhydride, furnishing
the target structures in 54-97% yield (Scheme 4). CV of 5a-
h revealed a first oxidation feature in a potential range sim-
ilar to the oxidation potential of 2a, i.e. 5g exhibits the low-
est peak potential at 1.03 V, while the peak potential of 1.22
V for 5c represents the upper limit within this series
(Scheme 4, ν = 0.1 Vs−1). The electron-withdrawing substit-
uents within 5c and 5f shifted the potentials anodically rel-
ative to 2a. An analogous trend toward lower oxidation po-
tentials was recorded for adducts possessing electron-donat-
ing groups (e.g., 5e, 5g, and 5h). Scan rate dependent data
revealed diffusion controlled oxidation processes for all de-
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Scheme 4. Synthesis and peak potentials of 5 at 0.1 Vs−1. I
n
= 0.1 M Bu4NPF6, [5] = 1 mM; a5f corresponds to syn-iso-
mer derived from Z-alkene.
Electrosynthesis
Finally, the question remained, if the catalysis can be
successfully conducted under electrochemical conditions.
Therefore, controlled potential electrolysis experiments
(CPE) were run. All CPE have been conducted in a one-
compartment cell under air with dry and degassed acetoni-
trile (Figure S 1). At first, the reaction of 1a and 3a in a 1 to
10 ratio was investigated. CPE at the half peak potential of
the first oxidation process of 1a and 3a was conducted in
order to confirm the electrosynthesis of 2a. After injection
of ~2 charge equivalents with regard to 1a, the yellow col-
our of 1a disappears and the CV lacks the first oxidation
wave (Figure S 35). The 1H NMR spectrum of the reaction
mixture showed 2a as the sole product after work-up, which
confirmed the electrosynthesis of 2a after initial oxidation
of 1a (Figure S 49).
Subsequently, we applied a potential in the CPE, which
matches the half peak potential of the second oxidation
wave, i.e. 1.10 V. The i vs. t curve showed that the current
drop is pronounced during the first 60 minutes of the elec-
trolysis and then rather slow (Figure S 37). Within the first
60 minutes the yellow colour of 1a disappeared. This be-
haviour is in line with the observation that the formation of
2a is rather fast and subsequent formation of 4a is the rate
limiting step in the electrocatalytic cycle. In a large-scale
experiment, 4a was isolated in 78% yield. Notably, no con-
version was observed in the absence of the Se-catalyst at the
same applied potential. In line with this, the charge transfer
at a set potential of 1.10 V was negligible.
Subsequently, we tested the applicability and tunability
of the method to a representative set of alkenes and nucleo-
philes (Table 3). Electrolysis of hex-3-enedioic acid, 3i,
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with 0.1 equiv. 1a at a potential of 1.10 V led to the for-
directing or activating groups, are known to be difficult to
perform regioselectively when resorting to palladium catal-
ysis (Table 4).[1d,36] In our case, however, products 6k and
6l were isolated as single regioisomers.[37]
mation of 2-(5-oxo-2,5-dihydrofuran-2-yl)acetic acid, 4i, in
isolated yield of 86% (Table 3 entry 2, Figure S 38, Figure
S 39).
4-(p-tolyl)but-3-enoic acid, 3j, is readily oxidised at an
onset potential of 1.1 V similar to the oxidation of 2a (Fig-
ure S 40). Indeed, the CV of 1a, and 3j showed that the ox-
idation of the respective selenofunctionalisation intermedi-
ate and the starting material overlap, which could lead to
undesired background reactivity during catalysis.[35] There-
fore, we switched to 1c as catalyst, since it has a lower oxi-
dation potential than 1a and thus, undesired oxidation of 3j
could be avoided. Indeed, the oxidation peaks of the inter-
mediate and the alkene were well separated in the CV and
electrolysis at an applied potential of 0.95 V led to the con-
version of 3j to 4j with an isolated yield of 58% (Table 3
entry 3, Figure S 40, Figure S 41, Figure S 42).
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Table 4. Representative comparison of regioselectivi-
ties observed in Pd- and Se-catalyzed acyloxylations of in-
a
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ternal, undirected alkenes. The number refers to conver-
sion into the depicted mixture of isomers.
Table 3. Overview of the electrosynthesis experiments, E vs.
Fc+/0, Conditions: 0.7-1 Mmol alkene, 0.1 equiv. cat, MeCN,
n
0.1 M NBu4PF6.
Finally, the electrocatalytic conversion of N-(2,2-dime-
thylhex-4-en-1-yl)-4-methylbenzenesulfonamide, 3l, form-
ing 4,4-dimethyl-1-tosyl-2-vinylpyrrolidine, 7 was investi-
gated. Amination reactions are of considerable importance
since the C–N bond motif can be found in numerous organic
compounds associated with biological, pharmaceutical, or
material scientific applications.[38] The CV of 1a and 3l
showed one oxidation feature at a peak potential of 1.06 V
(Figure S 47, ν = 0.1 Vs−1). Applying a potential of 1.00 V
to a solution of 0.1 equiv. of 1a and 3l leads to the conver-
sion of 3l to 7 as anticipated. The i vs. t curve showed that
the current drop is pronounced during the first 60 minutes
of the electrolysis and then rather slow, indicating a similar
reaction cascade as was observed for 1a and 3a (Figure S
48). The electrosynthesis yielded analytically pure 7 in
64%.
Conclusion
A detailed mechanistic analysis of the anodic allylic es-
terification of alkenes led to a comprehensive picture of the
proceeding electrochemical and chemical steps. Initial elec-
trochemical oxidation of the catalyst is followed by a bimo-
lecular reaction, likely the dimerisation of the cation
[(PhSe)2]+. The cationic tetramer reacts very rapidly with
the alkene forming the selenooxylation adduct 2. The adduct
is electrochemically oxidised at a slightly more anodic po-
tential than 1, and the cation reacts in a bimolecular reaction
step producing the dimer. The last chemical step, that is the
deprotonation and regeneration of the catalyst, is the rate
determining step of the catalysis.
The method is not restricted to internal O-nucleophiles
as demonstrated by the functionalisation of 1,6-diphenyl-
hex-3-ene, 3k with acetic acid and formic acid. Electrolysis
of 3k, with 0.1 equiv. 1a and 20 equiv. acetic acid and 30
equiv. formic acid at a set potential of 1.05 V for both cases,
led to the desired products 6k and 6l in yields of 59% and
40%, respectively (Figure S 43, Figure S 44, Figure S 45,
Figure S 46). Notably, such intermolecular allylic aceloxy-
lations, in which the alkene substrates do not possess any
The redox potential of the catalyst, 1, and the interme-
diate, 2, can be tuned readily by electron-withdrawing or
donating groups at the aryl residues. The potential of 2 is
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always slightly higher than the one of the respective
General procedure for the bulk electrolysis experiments: 1
(10 mol%) was dissolved in 30 mL MeCN with 0.1 M
NBu4NPF6 as electrolyte. After addition of the substrate
(and, if applicable, the nucleophile) the solution was elec-
trolysed at a set potential for several hours. The electrolysis
experiments were interrupted, when more than 105% of the
required charge for full conversion were injected (theoreti-
cally needed for full conversion: 212 C for 1 mmol of sub-
strate). The solvent was removed under reduced pressure
and the crude mixture was purified by column chromatog-
raphy on silica gel.
diselenide 1. The potentials of 1 and 2 increase with increas-
ing Hammett’s substituent parameters. On the other hand,
the redox potentials of the intermediates are rather insensi-
tive to the electronic nature of the substituent at the alkene.
This can be rationalised by smaller distance between the re-
dox active selenium centre and the substituent at the aryl
residue with regard to the distance between the Se atom and
the residue at the alkene.
We demonstrated in six proof-of-principle electrolysis
experiments that the anodic functionalisation of (unacti-
vated) alkenes can be used successfully in organic electro-
synthesis using different nucleophiles, namely an amine and
various acids. Internal and external O-nucleophils were
used and the functionalisation products were isolated in
good yields. Such C−O and C−N bond motifs are particular
important for various organic compounds applied in mate-
rial or pharmaceutical science. The electrochemical func-
tionalisation provides significant benefits with regard to
classical oxidation protocols, since the required potential to
drive the reaction can be adjusted easily. The tunability of
the catalyst’s redox potential makes the methods very ver-
satile with potential applications for a larger variety of nu-
cleophiles and alkenes including those, which are suscepti-
ble to oxidation as demonstrated by the reaction of the 3j
forming 4j. Our kinetic and thermodynamic study provides
a blueprint for such transformations.
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4a: 1a (31.3 mg, 100 μmol, 10 mol%), 3a (113 mg,
1.01 mmol, 1 equiv.), E = 1.10 V vs. Fc+/0 for 17 h, 341 C,
eluted with n-pentane/EtOAc (3:1) (88.1 mg, 784 µmol,
78%, colourless liquid); the analytical data matched those
reported in the literature.[7f]
4i: 1a (31.0 mg, 99.3 μmol, 10 mol%), 3i (145 mg,
1.00 mmol, 1 equiv.), E = 1.10 V vs. Fc+/0 for 16 h, 311 C
eluted with EtOAc/AcOH (100:1) (122 mg, 858 µmol,
86%, white solid); the analytical data matched those re-
ported in the literature.[7f]
4j: 1c (37.2 mg, 99.9 μmol, 10 mol%), 3j (177 mg,
1.00 mmol, 1 equiv.), E = 0.95 V vs. Fc+/0 for 15 h, 405 C
eluted with n-pentane/EtOAc (5:1) (101 mg, 580 µmol,
58%, yellow liquid); the analytical data matched those re-
ported in the literature.[35]
6k: 1a (31.4 mg, 101 μmol, 10 mol%), 3k (239 mg,
1.01 mmol, 1 equiv.), acetic acid (1.15 mL, 1.21 g,
20.1 mmol, 20 equiv.) E = 1.05 V vs. Fc+/0 for 68 h, 224 C,
diluted with DCM (50 mL) and washed with sat. aq. Na-
HCO3-soltn. (50 mL), eluted with n-pentane/EtOAc (20:1)
(175 mg, 594 µmol, 59%, colourless oil); TLC: Rf = 0.40
(n-pentane/EtOAc, 10:1); IR (neat): 휈̃ = 3451, 3061, 3025,
2926, 1732, 1602, 1494, 1453, 1369, 1231, 1018, 968, 743,
Experimental Section
Electrochemical Studies. All electrochemical measure-
ments were carried out using Gamry Reference 600, Gamry
Reference 600+, or Gamry Interface 1010B Potentiostats
using dried, degassed acetonitrile from an MBraun Solvent
Purification System. We utilised a common three electrode
setup for CV measurements consisting of a glassy carbon or
platinum working electrode (GC: CH Instruments, ALS Ja-
pan; A = 7.1 mm², Pt: ALS Japan; A = 2.0 mm²), a platinum
wire as a counter electrode, and a silver wire or diluted
Ag/AgNO3 electrode as pseudo reference electrode. All CV
data except those prior and after the bulk electrolysis exper-
iments were collected under nitrogen atmosphere. The data
were referenced vs. the Fc0/+ redox potential. The potentials
can be converted to the SCE scale by adding 0.40 V.[39] In
the [(PhSe)3]PF6 and [PhSe]PF6 measurements we utilised
a freshly prepared Ag+/AgNO3 electrode (~0.01 M AgNO3
in MeCN/nBuPF6) and referenced the potential of the elec-
trode vs. Fc+/0 at the end of each CV series, since both spe-
1
696 cm-1; H-NMR (300 MHz, CDCl3): δ (ppm) = 1.81-
2.15 (m, 5 H), 2.54-2.71 (m, 2 H), 3.33-3.45 (m, 2 H), 5.28
(tdd, J = 7.1, 6.0, 0.9 Hz, 1 H), 5.50 (ddt, J = 15.3, 7.1,
1.5 Hz, 1 H), 5.88 (dtd, J = 15.3, 6.8, 0.9 Hz, 1 H), 7.08-
7.37 (m, 10 H); 13C-NMR (126 MHz, CDCl3):
δ
(ppm) = 21.3, 31.6, 36.1, 38.6, 74.0, 125.8, 125.9, 126.1,
128.2, 128.3, 128.4, 128.4, 129.5, 132.6, 139.6, 141.2,
170.1; HR-MS (ESI): [C20H22NaO2]+ ([M + Na]+): obs.:
m/z = 317.1512, calcd.: m/z = 317.1512.
6l: 1a (31.3 mg, 100 μmol, 10 mol%), 3k (237 mg,
1.00 mmol, 1 equiv.), formic acid (1.13 mL, 1.38 g,
30.0 mmol, 30 equiv.) E = 1.05 V vs. Fc+/0 for 110 h, 244 C,
diluted with DCM (50 mL) and washed with sat. aq. Na-
HCO3-soltn. (50 mL), eluted with n-pentane/EtOAc (20:1)
(113 mg, 403 µmol, 40%, colourless oil); TLC: Rf = 0.41
(n-pentane/EtOAc, 20:1); IR (neat): 휈̃ = 3061, 3025, 2921,
1718, 1602, 1494, 1453, 1163, 1028, 1007, 970, 908, 739,
n
cies react with Fc. Bu4NPF6 was used as supporting elec-
trolyte, I = 0.1 M. Some of the experiments were conducted
in the glove box.
Bulk Electrolysis Experiments: The Electrolysis was
carried out in a custom-made cell in dry and degassed ace-
tonitrile under air with a GC-rod working electrode (diame-
ter 7 mm), a Pt-spiral counter electrode and an Ag/AgNO3
reference electrode (Figure S 1). The CV prior and after the
electrolysis were collected with a disk electrode (A = 7.1
mm²).
1
696 cm-1; H-NMR (400 MHz, CDCl3): δ (ppm) = 1.83-
2.17 (m, 2 H), 2.53-2.78 (m, 2 H), 3.31-3.47 (m, 2 H), 5.38
(m, 1 H), 5.51 (ddt, J = 15.2, 7.3, 1.5 Hz, 1 H), 5.93 (dtd,
J = 15.3, 6.8, 0.9 Hz, 1 H), 7.03-7.38 (m, 10 H), 8.08 (s,
1 H); 13C-NMR (101 MHz, CDCl3): δ (ppm) = 31.4, 36.0,
38.5, 74.1, 126.0, 126.2, 128.3, 128.4, 128.5, 128.5, 128.9,
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133.6, 139.4, 141.0, 190.4; HR-MS (ESI): [C19H20NaO2]+
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([M + Na]+): obs.: m/z = 303.1356, calcd.: m/z = 303.1356.
7: 1a (22.2 mg, 71.1 μmol, 10 mol%), 3l (200 mg, 0.71
mmol, 1 equiv.), E = 1.00 V vs. Fc+/0 for 22 h, 211 C, eluted
with n-pentane/EtOAc, 10:1, yield: 127 mg, 455 µmol,
64%, colourless oil); the analytical data matched those re-
ported in the literature.[40]
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Conflicts of interest
The authors declare no competing financial interest.
Supporting Information
Experimental details, synthesis and characterisation (NMR,
IR, HR-ESI-MS) of 1, 2, 3, 5; in-situ synthesis, NMR and
ESI-MS data of the selenium intermediates; additional CV
and electrosynthesis data; NMR and IR spectra of the
diselenides and selenolactones.
3.
For recent books on organoselenium chemistry, see:
a) Alberto, E. E.; Braga, A. L. In Selenium and Tellu-
rium Chemistry: From Small Molecules to Biomole-
cules and Materials; 1st ed.; Woollins, J. D.; Laitinen,
R. S. Eds.; Springer Verlag: Berlin Heidelberg, 2011;
p 251; b) Santi, C.; Santoro, S. In Organoselenium
Chemistry: Synthesis and Reactions; 1st ed.; Wirth,
T. Ed.; Wiley-VCH: New York, 2011; p 1; c) Beau-
lieu, P. L.; Deziél, R. In Organoselenium Chemistry:
A Practical Approach; Back, T. G. Ed., Oxford Uni-
versity Press: Oxford, 1999; p 35.
Acknowledgements
Dedicated to Prof. Dr. Dietmar Stalke on the occasion
of his 60th birthday. IS and AB thank the DFG (BR 4907/1
(AB), SI 1577/2 (IS)), the Fonds der Chemischen Industrie,
and the Dr. Otto Röhm Gedächtnisstiftung for funding. IS
and AB thank Göttingen University for support. We thank
Jia-Pei Du for conducting the in-situ mass spectrometry,
Katharina Rode for the synthesis of some diselenides, and
Ralf Schöne for collecting the 77Se-NMR spectra.
4.
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