N-Arylphenothiazines as strong donors for photoredox catalysis – pushing the frontiers of nucleophilic addition of alcohols to alkenes

Article abstract of DOI:10.3762/bjoc.15.5

A new range of N-phenylphenothiazine derivatives was synthesized as potential photoredox catalysts to broaden the substrate scope for the nucleophilic addition of methanol to styrenes through photoredox catalysis. These N-phenylphenothiazines differ by their electron-donating and electron-withdrawing substituents at the phenyl group, covering both, σ and π-type groups, in order to modulate their absorbance and electrochemical characteristics. Among the synthesized compounds, alkylaminylated N-phenylpheno-thiazines were identified to be highly suitable for photoredox catalysis. The dialkylamino substituents of these N-phenylpheno-thiazines shift the estimated excited state reduction potential up to ?3.0 V (vs SCE). These highly reducing properties allow the addition of methanol to α-methylstyrene as less-activated substrate for this type of reaction. Without the help of an additive, the reaction conditions were optimized to achieve a quantitative yield for the Markovnivkov-type addition product after 20 h of irradiation.

Full text of DOI:10.3762/bjoc.15.5

N-Arylphenothiazines as strong donors for photoredox
catalysis – pushing the frontiers of nucleophilic
addition of alcohols to alkenes
Fabienne Speck, David Rombach and Hans-Achim Wagenknecht*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 52–59.
Institute of Organic Chemistry, Karlsruhe Institute of Technology
(KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
doi:10.3762/bjoc.15.5
Received: 13 July 2018
Email:
Accepted: 12 December 2018
Published: 04 January 2019
Hans-Achim Wagenknecht* - Wagenknecht@kit.edu
* Corresponding author
This article is part of the thematic issue "Photoredox catalysis for novel
organic reactions".
Keywords:
addition; phenothiazine; photochemistry; photoredox catalysis; redox
potential
Guest Editor: P. H. Seeberger
© 2019 Speck et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A new range of N-phenylphenothiazine derivatives was synthesized as potential photoredox catalysts to broaden the substrate scope
for the nucleophilic addition of methanol to styrenes through photoredox catalysis. These N-phenylphenothiazines differ by their
electron-donating and electron-withdrawing substituents at the phenyl group, covering both, σ and π-type groups, in order to modu-
late their absorbance and electrochemical characteristics. Among the synthesized compounds, alkylaminylated N-phenylpheno-
thiazines were identified to be highly suitable for photoredox catalysis. The dialkylamino substituents of these N-phenylpheno-
thiazines shift the estimated excited state reduction potential up to −3.0 V (vs SCE). These highly reducing properties allow the ad-
dition of methanol to α-methylstyrene as less-activated substrate for this type of reaction. Without the help of an additive, the reac-
tion conditions were optimized to achieve a quantitative yield for the Markovnivkov-type addition product after 20 h of irradiation.
Introduction
Visible-light photoredox catalysis has become a precious tool in reactions [1]. This complementarity allows for the development
modern synthetic organic chemistry and experiences a continu- of so far unknown transformations [2]. The photochemical reac-
ously growing interest in industrial applications. The access to tivity can be tuned by the absorption and excited state character-
electronically excited states of organic molecules allows istics of the photocatalyst. In this context, organic dyes repre-
unlocking new and sometimes complementary chemical reactiv- sent a perfectly suited class of photocatalysts as they can easily
ities that cannot be tackled by using thermally driven chemical be modified by the introduction of functional groups that allow
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Beilstein J. Org. Chem. 2019, 15, 52–59.
fine-tuning the optoelectronic properties of the molecules. During the last years, we investigated the photoredox chemistry
Photochemical methods have allowed to overcome some of the of new classes of catalysts like perylene bisimides, for their
current limitations in thermally driven chemistry and to substi- suitability in these types of processes [19] and evaluated the ad-
tute conventional energy demanding chemistry by highly sus- dition of methanol to alkenes as a simple model system. Due to
tainable photochemical methods [3-12].
the insufficient reduction potential of the photoredox catalyst,
the Markovnikov addition of alcohols through oxidative
Phenothiazines have become a precious class of organic mole- quenching is yet limited to highly activated, aromatic alkenes.
cules, not only due to their widespread use in medicinal chem- To the best of our knowledge no methods are known today that
istry [13] but also because of their fascinating electronic proper- allow the addition of alcohols to α-methyl-substituted styrenes
ties. Recently their use in photoredox catalysis allowed for the through photoredox catalysis. The currently available methods
development of some novel transformations, namely dehalo- are based on a two-step procedure involving an iodoalkoxyla-
genation [14] as well as the first pentafluorosulfanylation tion with NIS followed by the reduction of the formed alkyl
method starting from sulfur hexafluoride [2]. We are convinced iodide generating the product in moderate yields [20], or
that the value of phenothiazine derivatives in photoredox cataly- through the direct addition of MeOH catalyzed by either acidic
sis is still underestimated. While these compounds found wide- conditions or heated ion exchange resin [21,22]. These methods
spread use in ATRA (atom transfer radical addition) polymeri- are therefore not suitable for the alkoxylation of acid or
zation [15,16] the interest of using this class of catalysts only base-labile substrates. To overcome the current limitations of
gained limited interest during the last years. The advantage of reduction potentials of single electron transfer processes
using N-phenylphenothiazine catalysts in photoredox chemistry in photoredox catalysis we present herein a range of new
is attributed to their beneficial redox properties. Moreover, a N-phenylphenothiazine derivatives 1–11 as photoredox cata-
modification of the core is rather simple and allows fast access lysts. Three of them were identified to be highly suitable for the
to a wide variety of catalysts. Recently it was shown that the addition of methanol to alkenes affording the corresponding
radical cation of the photoredox catalyst can play a key role in Markovnikov products.
photoinduced oxidation chemistry [16]. This is rather unusual
due to the usually short lifetime of radical cations in solution at- Results and Discussion
tributed to their low-lying excited states. Normally, this is the Activated aromatic olefins, such as 1,1-diphenylethylene (12),
reason why photochemical processes can hardly compete with have reduction potentials Ered(S/S−·) in the range of −2.2 to
photophysical decay processes. However, a pre-coordination of −2.3 V [23,24], α- and β-methylstyrene (13a and 13b) have an
the substrate may facilitate electron transfer under non-diffu- Ered(S/S−·) of −2.5 to −2.7 V [25], and styrene (14) an
sional controlled conditions. Very recently, the fast (pico- Ered(S/S−·) of −2.6 V (Figure 1) [25,26]. For non-aromatic,
second) excited state dynamics of the radical cation of alkylated olefins, like 1-methylcyclohex-1-ene (15), the reduc-
N-phenylphenothiazine was investigated by Wasielewski et al. tion potentials are estimated to values of Ered(S/S−·) = −3.0 V
This radical cation had a high reduction potential of about [25]. In our initial photoredox catalyst screening [26], we iden-
+2.1 V (vs SCE) [17] allowing the reduction of poorly tified 1-(N,N-dimethylamino)pyrene (16) having an excited-
oxidizing agents. The combination of both properties in one state reduction potential E*ox(P+·/P*) of −2.4 V (determined by
system is a remarkable feature for chemical redox dynamics be- cyclic voltammetry and E00). Thus, we are able to photoreduce
tween −2.1 V up to +2.1 V. One of the key problems in photo- 1,1-diphenylethylene (12), but not yet α-methylstyrene (13),
chemistry, which was recently addressed by the development of and clearly not non-aromatic (alkylated) olefins, such as
the consecutive photoelectron transfer process (conPET) [5], is methylated cyclohexene 15, as basic structures. The absorption
the need to push the frontiers by accessing high reduction of N-phenylphenothiazine (1) disappears at around 390 nm.
potentials. While the classical photoredox concept is based on This feature requires the excitation of the molecule using
the photophysical properties of the excited photoredox catalyst, UV light sources and contradicts the use of visible light irradia-
the idea of the conPET concept mimics nature’s light collection tion due to vanishing extinction coefficients at the edge to the
system and consecutively collects the energy of two photons visible region. To reach for high excited state reduction poten-
stored in the excited state of the initially pre-promoted tials and excite at rather long irradiation wavelengths an ener-
photoredox catalyst’s radical ion. This was one of the features getically high lying electronic groundstate potential has to be
to use N-phenylphenothiazine for the photoactivation of SF6 for connected with a small S0–S1 gap for the development of
the pentafluorosulfanylation of styrenes [2]. This two photon strongly reducing photoredox catalysts. Thus, we first focused
concept can further be extended to the photoredox catalytically our strategy in catalyst development on the synthesis of some
generation of hydrated electrons as very powerful reductants highly electron-rich phenothiazines 2–5 as well as some elec-
(E = −2.8 V (vs NHE)) for organic reactions [18].
tron-deficient phenothiazines 6–9 to analyze the influences of
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Beilstein J. Org. Chem. 2019, 15, 52–59.
Figure 1: Reduction potentials (vs SCE) of common photoredox catalysts, pyrene 16 and phenothiazine 2, in comparison to addressable substrate
scope 12–15. Bottom: photoredox catalytic addition of MeOH to α-methylstyrene (13a).
modifications of the core and the aryl moiety (Figure 2). The N-phenylphenothiazine (1) shows an absorption maximum at
observed trends allowed us to come up with a set of strongly 320 nm. Substitution of the arene moiety results in a shift of the
reducing photoredox catalysts that operate under UV-A absorption maxima due to a change in the HOMO–LUMO gaps.
conditions close to the visible range. In order to extend the It turned out that the introduction of the π-donating dimethyl-
scope of available reduction potentials we expected that the amino substituent in 2 induces a hypsochromic shift of the
N-phenylphenothiazine core having installed additional elec- absorption maximum by 7 nm to 313 nm, while the mesityl
tron-donating groups, like NR2 in 2, reaches very low reduction group present in 5 as a σ-donor causes a bathochromic shift of
potentials in the range of E(P+·/P*) = −2.5 to −3.0 V which is in about 8 nm. The detailed structure of the alkyl group attached to
the range of solid sodium [27], that would be able to attack low- the amino part in the phenothiazines 2, 10 and 11 showed no
substituted styrenes like 13a and 13b.
significant change in the absorption maximum of the S1 transi-
tion (10 in comparison to 2), but replacement of the methyl
Firstly, the absorbance characteristics of the derivatives 1–9 groups by branched isobutyl groups in 11 resulted in a
were analyzed and compared (Figure 3). The parent compound hypsochromic shift of the bathochromic features of absorption.
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Beilstein J. Org. Chem. 2019, 15, 52–59.
Figure 2: Acceptor or donor-modified phenothiazines 1–11 as potential photoredox catalysts.
The nitro compound 6 turned out to show a distinct long wave- and 11 managed to undergo a second completely reversible oxi-
length absorption that is apart from the region of absorption of dation process. To exclude interference with water and oxygen
all other catalysts by a shift of about 40 nm which is probably the measurements were carried under strict exclusion of any
due to a charge transfer state. Interestingly, the spectrum of the contaminants. The first oxidation of the lead structure 1 was
methylpyridine derivative 9 showed a rather short absorption found to occur at E(1+·/1) = 0.75 V (vs SCE). The substitution
maximum at 302 nm.
of the arene moiety by one (see 7) or two fluorine substituents
(see 8) only leads to a shift in the reduction potential of about
All N-phenylphenothiazines 1–11 were additionally character- 0.06 V. This trend was expected due to the lower electron den-
ized by cyclic voltammetry (Figure 3 and Table 1) [28]. We sity of these two N-phenylphenothiazines at the arene moiety.
found the first oxidation half wave of the unsubstituted However, the effect by the pure σ-acceptor fluorine is not very
N-phenylphenothiazine (1) as a fully reversible process as it pronounced. In the case of the 4-NO2 substituted derivative 6
was described in literature recently [18]. The second oxidation the pronounced influence of the π-acceptor shifts the reduction
process of 1 is almost reversible but induces to some extent an potential to a value of up to E(6+·/6) = 0.89 V (vs SCE).
irreversible oxidation process that shows up as further reduc- Substitution of the N-aryl moiety by electron-donating substitu-
tion half wave in the cyclic voltammogram. This is true for ents shifts the potentials correspondingly towards lower
almost all synthesized derivatives 3–9. The radical dication is reduction potentials, as expected. By introducing the thioether
known to undergo disproportionation reactions [27], which substituent (see 4) to the arene the potential drops to about
potentially explain the results. Only the amino derivatives 2, 10 E(4+·/4) = 0.71 V (vs SCE). If the steric bulk is enhanced by a
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Beilstein J. Org. Chem. 2019, 15, 52–59.
Figure 3: Normalized UV–vis absorption spectra above 290 nm of N-phenylphenothiazines 1–11 (left) and representative cyclic voltammogram of 2
(right).
This can be explained by a twist of the arene moieties due to
Table 1: Reduction potentials Ered(X+·/X) of N-phenylphenothiazines
1–11 (determined by cyclic voltammetry using ferrocene as standard).
steric bulk causing an interruption of the delocalization. The
electron transfer is found to occur at E(5+·/5) = 0.67 V
(vs SCE). However, the introduction of the π-donating
dimethylamino substituent dramatically shifts the reduction
potential to up to E(2+·/2) = 0.57 V (vs SCE). We hypothesized
that the introduction of even more donating substituents could
reduce the reduction potential further. Therefore, we synthe-
sized the modified alkylated compounds 10 and 11, respective-
ly. Indeed, both compounds showed a lower potential with 11
having an Ered(11+·/11) = 0.49 V (vs SCE). This made us
curious about the excited state potential of the catalysts. Using
the Rehm–Weller equation (without considering the solvent
term) [29] we estimated the excited state potential of these cata-
lysts to be as low as Ered(X+·/X*) ≈ −2.9 to −3.0 V (vs SCE).
compound
E1(X+·/X)a E2(X+·/X) E1(X+·/X*)
E00b
1
2
0.75 V
0.57 V
0.73 V
0.71 V
0.67 V
0.89 V
0.75 V
0.77 V
0.80 V
0.53 V
0.49 V
1.50 Vc
1.00 V
1.49 V
–
−2.5 V
−2.5 V
−2.5 V
−3.0 V
−2.5 V
−2.1 V
−2.5 V
−2.6 V
−2.6 V
−2.9 V
−2.9 V
3.2 eV
3.1 eV
3.2 eV
3.7 eVd
3.1 eV
3.0 eV
3.3 eV
3.4 eV
3.4 eV
3.4 eV
3.4 eV
3
4
5
1.59 V
1.55 V
1.50 Vc
1.05 Vc
–
6
7
8
9
10
11
0.98 V
0.96 V
aConverted from the ferrocene scale to the scale vs SCE: +0.38 V [30].
bE00 was estimated by using the method of determination of the inter-
section of the normalized absorption and fluorescence. cIrreversible.
dFluorescence in the UV-A range, see Figure S27 (Supporting Infor-
mation File 1).
The proposed photoredox catalytic mechanism (Figure 4) for
the nucleophilic addition of methanol to olefins starts with pho-
toinduced electron transfer from the N-phenylphenothiazine (1)
as photocatalyst to 13a as substrate. The resulting substrate
mesityl substituent (see 5) the reduction potential interestingly radical anion 13a−· is instantaneously protonated to radical 13a·
is higher than in the parent compound 1 although there are elec- and back-electron transfer to the intermediate phenothiazine
tron-donating alkyl groups present in the molecular structure. radical cation 1+· yields the substrate cation 13a+. The latter is
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Beilstein J. Org. Chem. 2019, 15, 52–59.
Figure 4: Proposed mechanism for the photoredox-catalyzed addition of methanol to α-methylstyrene (13a). (ET = electron transfer,
BET = back-electron transfer).
attacked by methanol as nucleophile and finally deprotonation conclusion that only the dialkylamino derivatives 2, 10 and 11
gives rise to the product 17 (see Figure 4). The principal prob- come up with an estimated excited state reduction potential
lem of this type of photoredox catalytic cycle is that the back- capable of reducing α-methylstyrene (13a). The optoelectronic
electron transfer cannot compete with the initial electron properties and the excited state reduction potential of
transfer because both components, 1+· and 13−·, are formed only Ered(2+·/2*) = −2.5 V of dimethylamino compound 2 that is
in stationary low concentrations. In the past, we used electron close to the reduction potential of the substrate 13a encouraged
mediators as additives (triethylamine) [19,26] or peptides with us to approach the so far not yet observed addition of methanol
substrate-binding sites [31,32] to overcome this problem. For to this less activated substrate promoted by catalyst 2. After op-
the current work, we propose a radical ion pair in a solvent cage timizing our catalytic system with catalyst 2, we could
that undergoes an extremely fast proton transfer followed by the also confirm the expected reactivity of the branched dialkyl-
intracage back-electron transfer, since triethylamine is no longer amino-substituted derivatives 10 and 11 (entry 12 and 13,
needed (vide infra).
Table 2).
The evaluation of both the optical and electrochemical proper- The initial conditions included irradiation of substrate 13a in
ties of the prepared phenothiazine derivatives 1–11 leads to the the presence of the catalyst (5 mol %) in methanol and triethyl-
Table 2: Screening of reaction conditions for the methanol addition to α-methylstyrene (13a).a
entry
13a [mM]
catalyst
mol %
additive
solvent
yield
1
4
84.6
84.6
42.3
42,3
42.3
42.3
42.3
170
2
2
5
NEt3b
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOD
MeOD
MeOH
MeOH
31%
84%
5
–
–
5
2
10
10
10
–
quant.
–c
6
2
–
7
2
–
–d
8
–
NEt3
–
–d
9
2
10
10
10
10
78%e
quant.e
quant.e
quant.e
11
12
13
2
–
170
10
11
–
170
–
aConditions: 30 °C, 65 h, 365 nm LEDs. b10 equiv. cNo light. dNo catalyst. e20 h.
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Beilstein J. Org. Chem. 2019, 15, 52–59.
amine (10% (v/v)) as the additive according to our previously
reported photoredox catalysis with pyrene 16 [18]. Under these
conditions the product 17 was formed in a yield of 31%. It
turned out that omitting the additive as electron shuttle en-
hanced the catalytic efficiency and the yield increased up to
84%. Obviously, this is a major difference between the
photoredox catalysis with pyrene 16, where triethylamine was
absolutely crucial to obtaining good product yields, and
N-phenylphenothiazine 1. Having this electron shuttle (ca. 1 M)
in the reaction mixture efficiently leads to silent or non-silent
quenching of the reactive species due to the following modes of
quenching. While the back-electron transfer under generation of
the triethylamine radical cation unproductively consumes elec-
trons while oxidizing triethylamine, the hydrogen abstraction
pathway generates the reduced phenylethane, which is ob-
served in small concentrations in the reaction mixture. The anal-
ysis of the reaction mixture still showed some unreacted starting
material. Assuming the first electron transfer as the rate-deter-
mining step the substrate concentration was reduced to 42 mM
and the catalyst concentration was increased to 10 mol %. This
change in the reaction conditions led to a quantitative product
formation after 65 h. Finally, the rather long reaction times were
addressed by speeding up the reaction simply by raising the
concentration of all components to 170 mM. This reduced the
reaction time to 20 h irradiation producing the product 17 in
quantitative yield. However, a further increase of substrate con-
centration slowed down the reaction again by speeding up silent
electron transfer processes.
Supporting Information
Supporting Information File 1
Copies of 1H and 13C NMR spectra, mass spectra,
absorption and emission spectra and cyclic voltammetry
data of 1–11 and 17.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-5-S1.pdf]
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
(Wa 1386/16-2) and KIT is gratefully acknowledged. DR
thanks the GRK 1626 “Chemical photocatalysis” (funded by the
Deutsche Forschungsgemeinschaft) for participation in their
qualification program and the Landesgraduiertenförderung
Baden Württemberg for financial support.
ORCID® iDs
Fabienne Speck - https://orcid.org/0000-0003-2183-375X
David Rombach - https://orcid.org/0000-0002-6673-9567
Hans-Achim Wagenknecht - https://orcid.org/0000-0003-4849-2887
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