Condition Screening for Sustainable Catalysis in Single-Electron Steps by Cyclic Voltammetry: Additives and Solvents

Article abstract of DOI:10.1002/cssc.201900344

Cyclic voltammetry-based screening method for Cp₂TiX-catalyzed reactions is extended to the screening of solvents other than tetrahydrofuran for bulk electrolysis of the catalyst and radical arylation. It was found that CH₃CN can be

Full text of DOI:10.1002/cssc.201900344

Accepted Article
Title: Condition Screening for Sustainable Catalysis in Single-Electron
Steps by Cyclic Voltammetry: Additives and Solvents
Authors: Andreas Gansäuer, Theresa Liedtke, Tobias Hilche, and
Sven Klare
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Condition Screening for Sustainable Catalysis in Single-Electron
Steps by Cyclic Voltammetry: Additives and Solvents
Theresa Liedtke,[+] Tobias Hilche,[+] Sven Klare,[+] and Andreas Gansäuer*
Abstract: Our cyclic voltammetry-based screening method for
Cp2TiX-catalyzed reactions is extended to the screening of other
solvents for bulk electrolysis of the catalyst and radical arylation. It
was found that CH3CN is a solvent that can be used for both
processes without additives. Furthermore, in THF, squaramide L2 is
more efficient than our previously best supramolecular halide binder,
Schreiner’s thiourea L1. The results extend the usefulness of our time
and resource efficient screening method for the reaction design in
catalysis in single-electron steps.
complex (Eq) to the anionic [Cp2TiX2]- species which can undergo
a reversible ligand dissociation (Cr) to form Cp2TiX, the
catalytically active species.[4] For efficient catalysis, a Cr-
equilibrium shifted to Cp2TiX is critical. In THF, this goal was
achieved with the aid of thiourea-derived additives, especially
Schreiner’s thiourea L1.[9] Thioureas are well known H-bond
donors and CV reveals that they act as reversible abstractors of
X- from [Cp2TiX2]- via H-bonding.[10] This abstraction does not only
increase the concentration of Cp2TiX. However, it also enables
bulk electrolysis of precatalyst Cp2TiX2 in THF, presumably by
preventing a deactivation of the electrode. Without additives, no
bulk electrolysis of Cp2TiX2 in THF was possible.
Introduction
Herein, we further validate the adaptability of our CV-screening
method by identifying reaction conditions for an ‘additive-free’
electrochemical activation of Cp2TiCl2 in other solvents than THF
and a new additive for the bulk electrolysis of Cp2TiX2 in THF and
CH3CN. The solutions obtained are tested in radical arylations.[6]
The results on the identification of solvents will be discussed first.
Understanding the mechanism of a catalytic reaction and its key
steps is an essential prerequisite for increasing product formation
and developing more sustainable reaction conditions. We recently
reported a cyclic voltammetry-based screening method for
catalysis in single-electron steps[1,2] that enabled us to develop
highly sustainable conditions for the arylation of epoxide-derived
radicals by the exact knowledge and characterization of the
catalytically active titanocene-species.[3] Our concept is based on
the ‘before’ analysis of the nature and relative proportions of the
catalytically active titanocene-species in solution[4] at various
potential reaction conditions. The technique used to this end is
cyclic voltammetry (CV).[5] Moreover, our CV analysis represents
a minimum waste screening technique for the titanocene-
catalyzed arylation reaction of epoxides[6] and potentially of other
titanocene-catalyzed reactions.[7]
Results and Discussion
Solvent Screening and Arylation after Bulk Electrolysis in
CH3CN
Solvent Screening
For the solvent screening two issues are essential. First, the
solvent employed must not solvate Cl- well enough to shift the Cr-
equilibrium completely to the side of Cp2TiCl. In this case no
resting state is available to the catalyst. Second, the generation
of Cp2Ti+ in the presence of a protic solvent, such as H2O or
CH3OH, is to be avoided, because complexes of Cp2Ti+ with these
solvents have been shown to be excellent hydrogen atom
donors[11] that can intercept radicals and prevent the radical
arylation. It should be noted that the titanocene catalyzed 5-exo
cyclizations is about one order of magnitude faster than the
arylation.[12]
Our approach relies on the precise knowledge of the EqCr-
mechanism[8] of the generation of the active catalyst Cp2TiCl that
is essential for a catalyst design and reaction condition screening
by CV (Scheme 1).
Scheme 1. EqCr-
Mechanism of the electrochemical reduction of titanocene complexes.
Therefore, we have chosen the following aprotic solvents: CH3CN
(ε = 37.5), DMF (ε = 37.0), acetone (ε = 20.7), pyridine (ε = 12.4),
and CH2Cl2 (ε = 8.9) ranging from ‘polar’ to ‘unpolar’.[13] The cyclic
voltammograms of the reduction of Cp2TiCl2 in these solvents and
in THF[3] are shown in Figure 1.
In particular, we have gained insight into the feasibility of the
quasi-reversible electrochemical reduction of the titanocene
As screening criterion[3] for an attractive solvent for bulk-
electrolysis and for the radical arylation, we chose the presence
of the oxidation waves due to [Cp2TiCl2]- and Cp2TiCl. For the
success of the bulk-electrolysis and the radical arylation it is
important that the latter wave has a higher intensity than the
former. This was found to be essential for preventing electrode
deactivation and reflects an EqCr-mechanism with the Cr-
Dr. T. Liedtke,[+] T. Hilche,[+] S. Klare,[+] Prof. Dr. A. Gansäuer
Kekulé-Institut für Organische Chemie und Biochemie
Universität Bonn, Gerhard-Domagk-Straße 1, 53121 Bonn (Deutschland)
E-mail: andreas.gansaeuer@uni-bonn.de
[+] These authors contributed equally.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cssc.201900344
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equilibrium shifted towards the catalytically active Cp2TiCl. This
requirement has led to the discovery of the conditions for bulk-
electrolysis in THF by adding Schreiner’s thiourea L1.[3]
and minor amounts of its resting state are formed during the CV
experiment. This meets our criterion for the CV-screening.
Bulk-Electrolysis and Radical Arylation
The screening has identified pyridine and CH3CN as potentially
attractive solvents for the bulk-electrolysis of Cp2TiCl2. However,
when exposed to bulk-electrolysis conditions, extensive
decomposition of the Cp2TiCl2 derived species was observed in
pyridine. It is beyond the scope of this study to provide a
mechanistic interpretation of this observation. The use of CH3CN
resulted in a successful reduction. This can also be judged by the
color of the solution obtained (see Supporting Information). At first
glance, it may seem counterintuitive that the dipolar aprotic
solvent CH3CN is capable of solvating chloride. However, it has
been reported that H2O and CH3CN can have similar interactions
with Cl-.[15] Therefore, the shift of the Cr-equilibrium is indeed
perfectly reasonable.
10
[Cp2TiCl2]–
[Cp2TiCl]
0
-10
2 mM Cp2TiCl2 / CH3CN
2 mM Cp2TiCl2 / DMF
[Cp2TiCl2]
2 mM Cp2TiCl2 / acetone
-20
-2.0
-1.5
-1.0
-0.5
0.0
0.5
E / V vs. Fc+/Fc
[Cp2TiCl2]–
10
0
[Cp2TiCl]
Our results of the radical arylation with the electrochemically
reduced solutions of Cp2TiCl2 in CH3CN are summarized in Table
1.
Table 1. Catalytic radical arylation by electrochemically reduced Cp2TiCl2 in
CH3CN.
-10
2 mM Cp2TiCl2 / THF
2 mM Cp2TiCl2 / pyridine
2 mM Cp2TiCl2 / CH2Cl2
[Cp2TiCl2]
-20
-2.0
-1.5
-1.0
-0.5
0.0
0.5
E / V vs. Fc+/Fc
Figure 1. Cyclic Voltammograms of Cp2TiCl2 in different organic solvents with
0.2 M Bu4NPF6 at a scan rate of 2 Vs-1.
Of the solvents with low permittivity (ε < 21) only pyridine showed
a voltammogram fulfilling our criterion. In CH2Cl2, and acetone
only minor amounts of Cp2TiCl were detected. The situation is
different for CH3CN and DMF, the solvents with a permittivity ε of
about 37.
In DMF, three waves were observed that we attribute to
[Cp2TiCl2]-, Cp2TiCl•DMF, and Cp2TiCl. The assignment of
Cp2TiCl•DMF is based on the redox-potential that lies between
those of the other two species and is in agreement with an earlier
study of titanocenes with a pending hemilabile amide ligand.[14]
Moreover, Daasbjerg has shown that the even better donor HMPA
also coordinates to Cp2TiCl.[4c] Thus, while the catalyst’s resting
state and the active catalyst are formed, DMF complexes the
catalyst strongly and will inhibit substrate binding.
Acetonitrile (CH3CN), on the other hand, seems a highly attractive
solvent for bulk-electrolysis and radical arylation. On the oxidative
sweep, the voltammogram in CH3CN shows two
The reaction leads to the formation of the desired products in
good to high yields. For P1 the catalyst loading can be reduced to
2 mol%. With respect to sustainability it is worth noting that no
additives for the stabilization of the catalytic system are needed.
waves that are due to [Cp2TiCl2]- and Cp2TiCl with the wave
pertaining to Cp2TiCl having a substantially higher intensity. This
assessment was further supported by the addition of NBu4Cl
(TBAC). In this case, the wave originating from the oxidation of
[Cp2TiCl2]- increases in intensity whereas the second wave
(oxidation of Cp2TiCl) is depleted. Thus, only the active catalyst
However, it should be noted that CH3CN is a relatively toxic
solvent because it can be metabolized to HCN.
An advantage of using CH3CN over our previously described
system in THF[3] is that Cp2TiCl2 can be employed as catalyst
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10.1002/cssc.201900344
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precursor. It is cheaper than Cp2TiBr2 required for high yielding
reactions in THF.
be carried out in the presence of 0.5 or 1 eq. of L2 and requires a
potential of -1.3 V (vs. Ag/Ag+). With L1, at least 1 eq. of the
additive and -1.4 V are required. The results of some arylation
reactions are summarized in Table 2.
Effect of Additives on the EqCr-Mechanism: CV-Screening
and Arylation after Bulk-Electrolysis
The results shown above demonstrate that unpolar solvents are
not suitable for bulk-electrolysis of Cp2TiX2 (X = Cl, Br). In our
initial work, we have focused on the use of ureas and thioureas to
abstract chloride or bromide from [Cp2TiX2]- in THF to allow bulk-
electrolysis and to obtain active solutions for the radical arylation
at the same time. Here, we investigate, if squaramides that have
been used with great success in organocatalysis[16] and in halide
binding and transport[17] are also useful additives for allowing bulk-
electrolysis of Cp2TiX2 and radical arylation in THF and how they
affect the composition of the Cp2TiX2 solutions reduced in CH3CN.
4
[Cp2TiCl] · L2
2
0
Halide Binding through
Squaramides
Hydrogen Bonding with
-2
2 mM Cp2TiCl2
2 mM Cp2TiCl2 / 1 mM L2
While Schreiner’s thiourea L1 serves the desired purpose very
well with Cp2TiBr2 as precatalyst it is less efficient with Cp2TiCl2.
Therefore, it is interesting to investigate the effect of other
additives, in particular those that might lead to an improved
activation of Cp2TiCl2. To this end, better halide binders than
thioureas seemed suitable.
-4
2 mM Cp2TiCl2 / 2 mM L2
2 mM Cp2TiCl2 / 10 mM L2
[Cp2TiCl2] · L2
-2.0
-1.5
-1.0
-0.5
0.0
E / V vs. Fc+/Fc [0.2 V s–1]
10
5
[Cp2TiCl] · L2
In the comparison of ureas and thioureas, we found that the
efficiency of halide abstraction depended on the acidity of the
additive. L2[18] has a pKa value similar to L1 (L1: 8.5, L2: 8.4) in
dimethylsulfoxide (DMSO)[19] and is therefore an attractive
0
additive for screening. Gratifyingly, L2’s chloride binding is higher
-5
2 mM Cp2TiCl2
Cl
by a factor of 16 as judged by its chloride binding constant (Ka
,
2 mM Cp2TiCl2 / 1 mM L2
2 mM Cp2TiCl2 / 2 mM L2
2 mM Cp2TiCl2 / 10 mM L2
NBu4Cl in DMSO/0.5% H2O at 298 K) due to its more rigid
coordination geometry.[17] For our purposes this allows to
additionally understand the geometrical effects of complexation of
Cp2TiCl2 and halide abstraction from [Cp2TiCl2]-•L2. To study
these issues, we applied our CV-screening method (Figure 2).
-10
[Cp2TiCl2] · L2
-15
-2.0
-1.5
-1.0
-0.5
0.0
E / V vs. Fc+/Fc [2 V s–1]
Figure 2. Cyclic Voltammograms of Cp2TiCl2 in THF in the presence of L2 with
0.2 M Bu4NPF6 at a scan rate of 0.2 and 2 Vs-1.
The voltammograms show that the effects of L1 and L2 are
markedly different. First, the complex Cp2TiCl2•L2 is more
activated towards reduction than Cp2TiCl2•L1 (ΔE = 160 mV for
½
The arylation reactions with the Cp2TiCl2 solutions reduced in the
presence of L2 give higher yields than those generated in the
presence of L1. With Cp2TiBr2, L2 is slightly inferior to L1,
however. This is in line with the better halide binding ability of L2.
The use of L2 results in a higher concentration of the active
catalyst Cp2TiCl. At the same time, a sufficiently high amount of
the resting state is present and, so, a stable and active catalytic
system is formed to avoid catalyst deactivation. In the presence
of L1 the concentration of Cp2TiCl is too low. With Cp2TiBr2, as
precatalyst, the situation is reversed. With L1 the optimal balance
of Cp2TiBr and [Cp2TiBr2]-•L1 is obtained. With L2, the
concentration of [Cp2TiBr2]- is too low to allow for a stable catalyst.
From the perspective of practicability and sustainability, L2 is
therefore the superior additive because it allows the use of the
cheaper and more readily accessible catalyst Cp2TiCl2. The use
of the more expensive Cp2TiBr2 is not necessary.
Cp2TiCl2 vs. Cp2TiCl2•L2 compared to ΔE = 110 mV for Cp2TiCl2
½
vs. Cp2TiCl2•L1).[3] This is a clear indication of a stronger
complexation of Cp2TiCl2. The complexation by L2 is fully
reversible because addition of TBAC results in the regeneration
of Cp2TiCl2 (see Supporting Information). Second, in the CVs no
complex between [Cp2TiCl2]- and L2 can be observed whereas
this is the case for L1. Once again, this underlines the improved
chloride abstraction ability of L2. Finally, L2 forms a complex with
Cp2TiCl whereas L1 does not. Cp2TiCl•L2 has a lower reducing
power than Cp2TiCl (ΔE = 160 mV for Cp2TiCl vs. Cp2TiCl•L2).
½
The complex is not caused by follow-up reactions during the CV
experiment because the intensity of its oxidation wave increases
with higher sweep rate.[4]
Besides revealing the important information about the different
complexes formed, the CVs also suggest that a bulk-electrolysis
of Cp2TiCl2 in the presence of L2 should be viable and the
solutions obtained should be active catalysts for the radical
arylation. This is indeed the case. The bulk-electrolysis can even
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Table 2: Catalytic radical arylation by electrochemically reduced
Cp2TiCl2 and Cp2TiBr2 in THF in the presence of L2.
4
2
[Cp2Ti]+
[Cp2TiCl]
[Cp2TiCl2]–
0
-2
-4
-6
-8
2 mM Cp2TiCl2
2 mM Cp2TiCl2 / 1 mM L1
2 mM Cp2TiCl2 / 2 mM L1
2 mM Cp2TiCl2 / 6 mM L1
[Cp2TiCl2] · L1
-1.5
-1.0
-0.5
0.0
0.5
E / V vs. Fc+/Fc [0.2 V s–1]
[Cp2Ti]+
[Cp2TiCl]
10
0
[Cp2TiCl2]–
-10
-20
2 mM Cp2TiCl2
2 mM Cp2TiCl2 / 1 mM L1
2 mM Cp2TiCl2 / 2 mM L1
2 mM Cp2TiCl2 / 6 mM L1
[Cp2TiCl2] · L1
-1.5
-1.0
-0.5
0.0
0.5
Finally, we investigated the influence of L1 and L2 on the
reduction of Cp2TiCl2 in CH3CN. In these cases, the interesting
problem arises how the additive will affect the already favorable
Cr-equilibrium. The CVs of investigations with both ligands are
shown in Figure 3.
E / V vs. Fc+/Fc [2 V s–1]
4
2
[Cp2Ti]+
[Cp2TiCl]
Compared to the situation in CH3CN without additives (Figure 1),
the CVs show a third oxidation wave at higher potential than
Cp2TiCl that is assigned to the oxidation of Cp2Ti+.[18] With 1 eq.
of L1 and L2, the intensity of this wave is reduced at higher sweep
rate (2 Vs-1). This indicates the formation of the cation in a follow-
up reaction during the CV experiment (parent-child reaction).[4]
With 3 eq. of L2, the ratio of intensities remains almost constant
indicating that Cp2Ti+ may indeed be an original component of the
solution. This change should not affect the bulk-electrolysis and,
indeed, it could be carried out without problems. However, the
solutions in the presence of 1 or 3 eq. of the ligands have a
distinctly different appearance. With 3 eq. the solution is dark blue
which indicates the formation of Cp2Ti+.[19] This was verified by a
CV-analysis of the solutions obtained by bulk-electrolysis (Figure
4). It is clear that with 3 eq. of L1 or L2 the cation is formed almost
exclusively, whereas in the presence of 1 eq. of the ligands a
mixture of Cp2TiCl and Cp2Ti+ is obtained.
0
[Cp2TiCl2]–
-2
-4
-6
2 mM Cp2TiCl2
2 mM Cp2TiCl2 / 1 mM L2
2 mM Cp2TiCl2 / 2 mM L2
2 mM Cp2TiCl2 / 6 mM L2
[Cp2TiCl2] · L2
-1.5
-1.0
-0.5
0.0
0.5
1.0
E / V vs. Fc+/Fc [0.2 V s–1]
10
5
[Cp2Ti]+
[Cp2TiCl]
[Cp2TiCl2]–
0
-5
2 mM Cp2TiCl2
-10
-15
-20
2 mM Cp2TiCl2 / 1 mM L2
2 mM Cp2TiCl2 / 2 mM L2
2 mM Cp2TiCl2 / 6 mM L2
[Cp2TiCl2] · L2
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
E / V vs. Fc+/Fc [2 V s–1]
Figure 3. Cyclic Voltammograms of Cp2TiCl2 in CH3CN in the presence of L1
and L2 with 0.2 M Bu4NPF6 at a scan rate of 0.2 and 2 Vs-1.
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Previous studies have shown that Cp2Ti+ in CH3CN is a less active
catalyst than Cp2TiCl based systems.[6d] This is because epoxide
opening is slower for more electron deficient catalysts.
[Cp2Ti]+
In CH3CN this step is even further slowed down by the
complexation of Cp2Ti+ by CH3CN[20,6d] that efficiently competes
with the epoxide substrates. However, Cp2Ti+ is an attractive
catalyst for reactions with epoxides containing electron deficient
arenes. The electrochemically reduced solutions of Cp2TiCl2 in
CH3CN with and without L2 react accordingly (Table 3).
[Cp2TiCl]
4
[Cp2TiCl2]–
0
2 mM EL-Cp2TiCl2
Table 3: Comparison of catalytic radical arylation by electrochemically reduced
Cp2TiCl2 in CH3CN with and without L2.
2 mM EL-Cp2TiCl2 / 2 mM L1
2 mM EL-Cp2TiCl2 / 6 mM L1
[Cp2TiCl2]
-1.5
-4
-1.0
-0.5
0.0
0.5
E / V vs. Fc+/Fc [0.2 V s–1]
20
[Cp2Ti]+
[Cp2TiCl]
10
0
[Cp2TiCl2]–
2 mM EL-Cp2TiCl2
2 mM EL-Cp2TiCl2 / 2 mM L1
2 mM EL-Cp2TiCl2 / 6 mM L1
-10
[Cp2TiCl2]
-1.5
-1.0
-0.5
0.0
0.5
E / V vs. Fc+/Fc [2 V s–1]
8
4
[Cp2Ti]+
[Cp2TiCl]
[Cp2TiCl2]–
0
[Cp2TiCl]+
2 mM EL-Cp2TiCl2
-4
2 mM EL-Cp2TiCl2 / 2 mM L2
2 mM EL-Cp2TiCl2 / 6 mM L2
The reactivity of the catalytic systems with added L2 are reduced
compared to the system without additives. However, P3 that
requires Cp2Ti+ for its formation can be obtained in the presence
of L2 albeit the yield is low.
[Cp2TiCl2]
-1.5
-1.0
-0.5
0.0
0.5
1.0
E / V vs. Fc+/Fc [0.2 V s–1]
[Cp2Ti]+
20
10
0
Conclusions
[Cp2TiCl]
[Cp2TiCl2]–
In summary, we have demonstrated that our CV-screening
method for the identification of conditions for the bulk-electrolysis
have been successfully extended to other solvents and
supramolecular chloride binders. CH3CN is a suitable solvent for
bulk-electrolysis of Cp2TiX2 and radical arylation. Moreover,
squaramide L2 is a better ligand for chloride and bromide
abstraction than our previously best ligand Schreiner’s thioureas
L1. In the presence of L2 even Cp2TiCl2 is an excellent and highly
sustainable catalyst for the arylation.
[Cp2TiCl]+
2 mM EL-Cp2TiCl2
-10
-20
2 mM EL-Cp2TiCl2 / 2 mM L2
2 mM EL-Cp2TiCl2 / 6 mM L2
[Cp2TiCl2]
-1.5
-1.0
-0.5
0.0
0.5
1.0
E / V vs. Fc+/Fc [2 V s–1]
Acknowledgements
Figure 4: Cyclic Voltammograms of the solutions of Cp2TiCl2 in CH3CN in the
presence of L1 and L2 prepared by bulk-electrolysis with 0.2 M Bu4NPF6 at a
scan rate of 0.2 and 2 Vs-1.
We thank the Deutsche Forschungsgemeinschaft (Ga 619/12-1),
the ‘Evangelisches Studienwerk Villigst’ (fellowship to T. L.) and
the ‘Jürgen Manchot Stiftung’ (fellowships to T.H. and S.K.).
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Ed. 2017, 56, 6103-6106; Angew. Chem. 2017, 129, 6199-6202; j) Y.-Q.
Zhang, E. Vogelsang, Z.-W. Qu, S. Grimme, A. Gansäuer, Angew. Chem.
Int. Ed. 2017, 56, 12654-12657; Angew. Chem. 2017, 129, 12828-12831;
k) X. Wu, W. Hao, K.-Y. Ye, B. Jiang, G. Pombar, Z. Song, S. Lin, J. Am.
Chem. Soc. 2018, 140, 14836-14843.; l) L. H. Leijendekker, D. Weweler,
T. M. Leuther, D. Kratzert, J. Streuff, Chem. Eur. J.
10.1002/chem.201805909.
Keywords: arylation • cyclic voltammetry • epoxide •
squaramide • titanium
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This article is protected by copyright. All rights reserved.

10.1002/cssc.201900344
ChemSusChem
FULL PAPER
FULL PAPER
T. Liedtke,[+] T. Hilche,[+] S. Klare,[+] and
A. Gansäuer*
Page No. – Page No.
Condition Screening for Sustainable
Catalysis in Single-Electron Steps by
Cyclic Voltammetry: Additives and
Solvents
Text for Table of Contents
Catalysis predicted by screening with of solvents and additves with cyclic
voltammetry! Ti-X bond activation in CH3CN or in THF with squaramide L enables
bulk electrolysis of Cp2TiCl2 and sustainable catalysis.
This article is protected by copyright. All rights reserved.