Phenazine Radical Cations as Efficient Homogeneous and Heterogeneous Catalysts for the Cross-Dehydrogenative Aza-Henry Reaction

Article abstract of DOI:10.1002/hlca.202000184

The redox activity of molecular phenazine catalysts has been previously exploited for aerobic oxidative amine homo- and cross-coupling reactions. In this contribution, we have extended the reaction scope of this novel type of organocatalyst and used them in the cross-dehydrogenative aza-Henry coupling of isoquinolines with nitromethane under aerobic conditions. Additionally, we have designed and prepared a novel porous organic polymer by cross-linking of tetrakis(4-bromophenyl)silane and dihydrophenazine through Pd-catalyzed Buchwald-Hartwig cross-coupling. This new type of heterogeneous catalyst, apart from being robust and easily reusable, also showed outstanding catalytic activities and improved selectivity compared to its molecular counterpart. A plausible reaction mechanism was proposed based on spectroscopic and kinetic measurements.

Full text of DOI:10.1002/hlca.202000184

HELVETICA
chimica acta
Accepted Article
Title: Phenazine Radical Cations as efficient Homogeneous and
Heterogeneous Catalysts for the Cross-Dehydrogenative Aza-
Henry Reaction
Authors: Esteban Mejía, Felix Unglaube, Paul Hünemörder, Xuewen
Guo, Zixu Chen, and Dengxu Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Helv. Chim. Acta 10.1002/hlca.202000184
Link to VoR: https://doi.org/10.1002/hlca.202000184
www.helv.wiley.com

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
Phenazine Radical Cations as efficient Homogeneous and Heterogeneous
Catalysts for the Cross-Dehydrogenative Aza-Henry Reaction
Felix Unglaube,a Paul Hünemörder,a Xuewen Guo,a ZixuChen,b DengxuWangb and EstebanMejía*,a
a Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany. Esteban.Mejia@Catalysis.de
b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University,
Jinan 250100, China
Dedicated to Prof. Antonio Togni on the occasion of his 65th birthday.
The redox activity of molecular phenazine catalysts has been previously exploited for aerobic oxidative amine homo- and cross-coupling reactions. In this
contribution we have extended the reaction scope of this novel type of organocatalyst and used them in the cross-dehydrogenative aza-Henry coupling of
isoquinolines with nitrometane under aerobic conditions. Additionally, we have designed and prepared a novel porous organic polymer by cross-linking of
tetrakis(4-bromophenyl)silane and dihydrophenazine via Pd-catalyzed Buchwald-Hartwig cross-coupling. This new type of heterogeneous catalyst, apart
from being robust and easily reusable, also showed outstanding catalytic activities and improved selectivity compared to its molecular counterpart. A
plausible reaction mechanism was proposed based on spectroscopic and kinetic measurements.
Keywords: Phenazines • Porous Organic Polymers • Aerobic Oxidation • Aza-Henry Reaction •Cross-dehydrogenative coupling
Sonogashira,[59] and Buchwald-Hartwig cross-couplings,[60] and Yamamoto
Introduction
polymerization,[61] among others.
The development of metal-free catalysts has been the focus of many
research efforts during the last decades, due to the environmental[1-4] and
toxicologic[5] concerns associated with the widespread use of noble metal-
based catalysts and the increasing urge towards sustainable chemical
processes.[6-14] Among the many alternatives, carbon-based materials have
attracted great interest not only due to their low cost, but also thanks to
their robustness and recyclability (making them ideal candidates for
industrial applications), along with the tunability of their surface physico-
chemical properties.[15-21] These materials have been used as catalysts in
several organic reactions including oxidations and reductions,[22-25] Friedel-
Crafts alkylations,[26] and hydrocarbon dehydrogenations.[27]
Recently, our group reported a novel synthetic methodology for the
synthesis of the elusive 1,4-functional tetramethylpyrazines (Scheme 1, A)
and used them as catalysts for the aerobic oxidative homocoupling of
primary amines, thanks to the reversible redox behavior of these
compounds and the remarkablestability of their radical-cationic form.[62] We
explored further the exploitation of stable radical cations as oxidation
catalysts with the development of a series of 5,10-disubstitued phenazines
(Scheme 1, B) which showed remarkable activities for the oxidative homo
and cross coupling of amines.[63] Interestingly, even though the strong redox
properties of phenazines and the stability of their open-shell derivatives are
relatively well-known, there are very few additional reports on their use as
redox catalysts. Notable examples are the reports from the Miyake group on
The catalytic activity of heterogeneous organocatalysts depends on two
main factors, the porosity (since a high surface area and pore volume
enhances substrate adsorption and transport),[15-21, 28] and the presence of
active sites (which can adsorb/coordinate and activate the reactants and
stabilize reaction intermediates). A common approach to obtain such sites
at the catalyst surface is the doping with heteroatoms, specially nitrogen,[29-
their use as catalysts for photoinduced atom transfer radical
65]
polymerization,[64,
and the use the non-functionalized 5,10-
dihydrophenazine (in stoichiometric amounts) as oxidant and proton-
transfer reagent for aldehyde esterification.[66] Inspired by our previous
works on the preparation of hybrid porous polymers based on polyhedral
oligomeric silsesquioxane (POSS),[42] and the aforementioned pyrazine and
phenazine radical-cation oxidation catalysts,[63] we designed and
synthesized a novel type of covalently linked porous polymers exhibiting
tetrakis(phenyl)silane moieties as structural elements cross-linked by
phenazine groups (Scheme 1, C).
39]
either by chemical modification of a pre-existing material, or by
calcination of pre-organized precursors such as organic polymers,[40] metal-
organic frameworks (MOFs),[41] and covalently linked porous polymers
(CPP).[42] The latter material class, also known as (micro)porous organic
polymers (POPs or MOPs) or covalent organic frameworks (COFs), have
gained a lot of attention on their own right, due to their tunable properties
and their myriad of potential applications,[43-45] including gas capture and
separation,[46-51] and catalysis.[52-55] The synthesis of these modular materials
has been achieved via cross-linking of functional building blocks using
different methodologies including Friedel-Crafts arylation,[42, 56, 57] Suzuki,[58]
1
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
Scheme 2. A) General synthesis of the molecular phenazine pre-catalysts. B)
Preparation of the catalytic active species via oxidation.
The covalently linked porous polymer (CPP) containing immobilized
phenazine moieties (2) was prepared by cross-linking of tetrakis(4-
bromophenyl)silane and dihydrophenazine via Pd-catalyzed Buchwald-
Hartwig cross-coupling in toluene at 110°C for 48 h (Scheme 3). After the
cross-linking, the crude product was recovered by filtration, washed, and
extracted with several solvents, and dried under vacuum to yield the final
material. The received red powder is insoluble in common solvents, such as
water, methanol, chloroform, THF, and acetone, thereby indicating their
high cross-linking degree of the network.
Scheme 1. Development of metal-free radical cation precursors to be used as catalysts
in aerobic oxidative coupling reactions
Moreover, we extended the reaction scope of this type of catalysts and used
successfully both the molecular derivatives (1a-f) and the CCP (2) for C-C
bond formations via cross-dehydrogenative Aza-Henry reactions. This
represents, to the best of our knowledge, the first example of a metal-free
heterogeneous system for cross-dehydrogenative coupling reactions.
During the preparation of this manuscript, a similar type of microporous
polymer containing phenazine groups was independently reported by Guo
and co-workers, which showed remarkableactivity on the oxidative coupling
of amines.[67]
Results and Discussion
Catalyst Synthesis and Characterization
Homogenous phenazine pre-catalyst (1a-f) were prepared according to the
reported procedure.[63] The active 7-π-electron radical-cationic state of the
phenazines was achieved by oxidation with stoichiometric amounts of
NOBF4 (Scheme 2). [63]
Scheme 3. Synthesis of covalently linked porous polymer (CPP) containing immobilized
phenazine moieties (2) via Buchwald-Hartwig cross-coupling.
2
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
To confirm its structure, the CPP 2 was fully characterized by solid-state 13C,
29Si Cross Polarization/Magic Angle Spinning Nuclear Magnetic Resonance
(CP/MAS NMR) and Fourier-transform infrared spectroscopy (FTIR). In the
solid-state 13C NMR spectrum (Figure 1a), the peaks ranging from 105 to 150
ppm are attributed to the sp2 phenylene carbon atoms from the silicon-
centered unit and phenazine unit. In the 29Si NMR spectrum (Figure 1b), one
signal at -18 ppm was observed and can be attributed to the central Si atom
in the framework (the chemical shift of the starting material was -13.52 in
CDCl3),[68] indicating no occurrence of Si-C cleavage during the
polymerization
a)
b)
c)
d)
Figure 1. Solid-state 13C NMR (a) and 29Si NMR (b) spectra of 2. The signals with the
e)
asterisks are attributed to the spinning side bands.
The porosity of catalyst 2 was investigated by nitrogen adsorption-
desorption experiment at 77 K. The material gives rise to a combination of
type I and IV isotherms (Figure 2a),[69] which exhibits a sharp uptake at low
relative pressures and then a gradually increasing uptake at higher relative
pressureswith ahysteresis. The Brunauer–Emmett–Teller (BET) surfacearea
(SBET) and the total pore volume (Vtotal) of the catalyst are 377 m2 g-1 and 0.27
cm3 g-1. The micropore surface area (Smicro) and the micropore volume (Vmicro
)
Figure 2. Solid state characterization of 2. a) Nitrogen adsorption-desorption isotherm;
b) pore size distribution curve; c) TGA curves under N2 atmosphere with a heating rate
of 10°C min−1; d) FE-SEM image; e) Powder XRD pattern.
were calculated to be 253 m2 g-1 and 0.10 cm3 g-1 by using the t-plot method.
Thus, the Vmicro/Vtotal ratio, representing the contribution of microporosity in
the network, is 0.37, suggesting that the network is predominantly
mesoporous. The pore size distribution (PSD) was evaluated by nonlocal
density functional theory (NL-DFT). The material possesses a relatively
uniform micropores with an average diameter centered at ca. 1.23 nm and
a broad mesopore (Figure 2b). Thermogravimetric analysis (TGA) reveals
that the catalyst shows high thermal stability with Td (5% mass loss) at
approximately 350°C, comparable to other silicon-centered porous
materials (Figure 2c).[42] Field emission scanning electron microscopy (FE-
SEM) of 2 shows that the solid consists of a nanostructured interlinking
irregular particles of lamellar shape (Figure 2, d). In contrast to the
conventional amorphous porous polymers, the X-ray diffraction (XRD)
pattern of 2 shows two broad reflexes centred at 12.7° and 19.4°, equivalent
to d-spacing distances of 6.2 and 4.4 Å, respectively (Figure 2e). This finding
indicates that the obtained CPP have locally molecular homogeneity and
long-range order although the degree of the order is not very high. Indeed,
those crystalline or partially crystalline porous organic polymers are very
rare, to the best of our knowledge only one example from 2010 is reported
in literature.[70]
The presence of unpaired electrons at the material’s surface was assessed
by electron paramagnetic resonance (EPR), just as it was previously
investigated for its molecular counterparts.[63] A very strong signal was
observed at 3524 Gs (g value of 2.003), which is in the typical range for an
organic radical (Figure 3).[71, 72] Importantly, this strong signal was observed
both in the solid sample and in suspension in methyl tert-butyl ether (MTBE)
and was not affected by bubbling of oxygen in the sample (Figure 3). This
important observation suggests that 2, as prepared, already possess open-
shell phenazine moieties, without additional oxidative treatment. It can be
thus assumed that 2 was readily oxidized by atmospheric oxygen during its
work-up. In contrast to what was observed for molecular phenazines
(measured and simulated in a previous work),[63] no hyperfine splitting was
observed. Only marginal shoulders are visible and do not lead to structural
information about the radical. The broad signals suggest that the radical
moieties in the catalyst are relatively close to each other, resulting in an
interaction of the paramagnetic moments.
3
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
as it has been thoroughly exploited in cross-dehydrogenative couplings
(CDC),[73] a powerful and elegant methodology for furnishing C-C bonds from
two different C-H groups.[74-76]
Figure 3: Normalized EPR signals from pure 2 (powder), 2 in solution in MTB with
oxygen (MTBE + O2), and 2 in MTBE under inert atmosphere (MTBE)
In the FTIR spectroscopy of 2 (Figure 4, upper trace), the characteristic Si-C
stretching vibration was observed at 1481 cm-1. Compared to the vibration
spectrum of dihydrophenazine (1a, Figure 4, lower trace) The disappearance
of absorption band (N-H stretching) at ca. 3501 cm-1, confirms the successful
cross-coupling between the tretraphenylsilane monomer and 1a. Parts of
the infrared vibrations of 1a can also be found in the IR spectrum of 2, e.g.
the bands at 1481, 1387, 1261, 1038, 912, 762, 735 and 679 cm-1, which can
be attributed to the phenyl moieties of the phenazine molecule. Other
bands such as those at 3064, 3051 and 3014 cm-1 as well as 1099 cm-1 can
be associated with the increased number of phenyl rings in the polymer and
to Si-Ph vibrations, respectively (see also Figure S6).
Scheme 4. Generation and reactivity of hydroisoquinoline-2-ium intermediates in a
cross-dehydrogenative coupling reaction
Hence, we selected the cross-dehydrogenative aza-Henry (nitro-Mannich)
reaction between hydroisoquinolines and nitromethane as model system to
test our hypothesis. This reaction has been widely used in organic synthesis
since the introduced nitro synthon can be converted into 1,2-diamines[77, 78]
or α-aminocarbonyls (Nef reaction).[79] There are many catalytic approaches
to achieve this transformation, mostly using noble metals like Ru,[80] and
Ir,[81] and more recently under photochemical conditions using metal
83]
complexes.[82, Metal-free photocatalytic systems, which often use Eosin
as photosensitizer,[84] are relatively rare.[85] Beside the catalytic systems
there is a larger number of reports using stoichiometric amounts of oxidants
[87]
like DDQ,[86] or H2O2. However, metal-free systems able to catalyze aza-
Henry reactions under thermal conditions are still under-developed,
displaying very slow kinetics or requiring stoichiometric amounts of
additives which leads to significant amounts of waste.[88-90]
Thus, initial experiments on the cross-dehydrohgenative aza-Henry reaction
using the preactivated molecular phenazine radical cations ([1]BF4 and the
heterogeneous counterpart 2) as catalysts, hydroisoquinoline 3a as pre-
electrophile and nitromethane as both pre-nucleophile and solvent, were
performed under atmospheric oxygen pressure at elevated temperature
(Table 1). The possible formation of the 3,4-dihydroisoquinolone side
product 5a will not be addressed at this point; the chemoselectivity of this
reaction is described further below (Table 2).
Figure 4. IR spectra of pure 2 as powder (upper trace) and 1a dissolved in
dichloromethane (lower trace)
To ensure that any detected catalytic activity is solely caused by the catalysts,
inductively coupled plasma / optical emission spectrometry (ICP-OES)
analysis was performed to 3a to assess the presence of traces of copper
coming from the substrate synthesis, (since copper itself is well able to
catalyze the reaction).[91] No copper was detected by this method, so if there
are any traces of this metal, should be in amounts lower than 0.005 mol%
(relative to 3a). However, since there is a small product being formed in the
absence of phenazine catalyst (Table 1, entry 8), we performed the reaction
with the addition of 1 mol% CuI to the mixture, leading to a only slightly
Catalytic Experiments
The ability of phenazine radical cations to furnish iminium ions from amines
via single-electron transfer (SET) under aerobic conditions has been
previously observed and confirmed in our previous investigations.[63] Thus,
we figured, it should be possible to trap different carbon nucleophiles with
these reactive intermediates under the proper conditions (Scheme 4), just
4
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
increased conversion compared to the reaction without any catalyst (Table
1, entry 10). Consequently, the catalytic activity can be attributed to the
phenazine catalysts 1.
heterogeneous catalyst (up to eight time) with only a small initial apparent
activity decrease after which it plateaued at 74% (Figure S11).
Table 1. Aerobic cross-dehydrogenative aza-Henry reaction catalyzed by molecular
phenazine radical cations
The conditions were optimized to improve the reaction yield. The loading of
the catalyst 1 was varied from 0.1 to 50 mol% relative to the substrate 3a
(Figure S7). The lowest loading (0.1 mol%) showed no effect on the reaction,
while it reached a maximum of 86 % with 5 mol% catalyst loading and
dropped rapidly with higher loadings. Therefore, 5 mol% was chosen as
catalyst loading for all further experiments with 1. Loadings higher than 20
mol% showed no catalytic effect, suggesting that the catalyst is deactivated
through intramolecular interactions between catalyst molecules. The
reversible dimerization of the curved π-radical cations, as described by
Hiroto and Shinokubo upon increasing its concentration could explained the
observed deactivation.[92] The optimal catalyst loading for the phenazine-
containing CPP (2) wasalso investigated, varyingit from5 to 25 mg per mmol
of substrate (mmolsubs). The yield increased with rising catalyst loading form
25 % up to a plateau at 78 % when 20 mg/mmolsubs were used (Figure S8).
A temperature screening was also performed, for which catalyst 1b was
selected (Figure S9). It was observed that the conversion of 3a is possible at
room temperature (20 % yield), while the yield increased continuously until
90°C, where it reached a plateau at around 90 % yield. Therefore, 90°C was
chosen as reaction temperature for the rest of the experiments. A solvent
screening was not conducted since nitromethane serves as both substrate
and solvent. The influence of a decreased oxygen partial pressure was
investigated by using air instead of pure oxygen as oxidation agent, resulting
in a considerable drop in the catalyst activity Table 1, entry 10).
Entry[a]
Catalyst
3a consumption [%][b]
1
1a
1b
1c
1d
1e
1f
2
3
2
90
43
51
40
56
80
15
21
26
3
4
5
6
7
8
-
9[c]
10[d]
CuI
1b
[a] Reaction conditions (if notstated differently): 90°C, 1 bar oxygen, neat, 15 h, 5
mol% of catalyst [1]BF4 or 20 mg/mmolsubs.of catalyst 2. [b] Determined by GC-MS
[c] 1 mol% of CuI [d] Using air as oxidant.
In line with our previous observations,[63] pre-oxidation of the homogeneous
phenazine catalysts with NOBF4 was necessary to achieve reproducible
catalytic activity, hence for this investigation also the homogeneous
catalysts were used as the corresponding BF4 salt. ([1]BF4). Interestingly, in
contrast to its molecular analogues, the heterogeneous catalyst 2 does not
require pre-oxidation. Hence previous treatment with either NOBF4, H2O2 or
O2 does not lead to any improvement in catalytic activity and was therefore
not applied for the subsequent experiments.
A substrate screening was done to investigate the influence of both the
electrophile and nucleophile on the reaction yield and selectivity. For this
purpose, the molecular pre-catalyst 1b and the CPP 2 were chosen (Table 2).
Apart from the expected aza-Henry coupling product (4), almost in every
case the corresponding 3,4- dihydroisoquinolone (5) was obtained, in some
cases, to our surprise, as the major product (the identity of the isoquinolone
product was confirmed by single crystal X-ray diffractometry; Figure S3 and
S4). These amides are common minor side products in oxidation of benzylic
amines and isoquinolines and its selective formation can be achieved under
the right reaction conditions.[63, 93, 94] What came as a surprise was the strong
selectivity dependence on the electronic properties of the substrate. For
instance, when the molecular catalyst [1b]BF4 was used with the electron
rich p-methoxy tetrahydroisoquinoline 3c, the almost exclusive formation of
the isoquinolone 5c was achieved (95 %, Table 2, entry 5). In contrast, using
the same catalyst with the electron poor p-trifluoromethyl containing
substrate 3d, the desired aza-Henry coupling product 4d is obtained as the
major product (75%, Table 2, entry 7). Interestingly, when the
heterogeneous catalyst 2 was used, the preferred reaction product was
always the aza-Henry adduct 5, in approximately 2:1 ratio (4c to 5c) with the
methoxy-substituted substrate substrate (Table 2, entry 6). and in
quantitative amounts with the trifluoromethylated analogue (Table 2, entry
8).
All the tested catalyst (except from the underivatized phenazine 1a, which
shows an inhibitory effect, resulting in conversions even lower that the
uncatalyzed reaction) showed good to excellent conversions, being 1b the
most active (Table 1, entry 2). The catalytic activity of the 1c to 1f increased
along with the electron density at the N-Phenyl rings (Table 1, entry 3-6).
Their overall lower catalytic activity compared to 1b can be explained
through their lower ionization energy. In our previous work, the ionization
energies of these molecules was determined with DFT calculations.[63] For
instance, the lower ionization barrier for radical formation of 1f (129.8 kcal
mol-1) compared to 1b (139.9 kcal mol-1) suggests a more stable radical state
and therefore, lower reactivity under catalytic conditions.
To our delight, the newly synthesized covalently linked porous polymer 2
displayed a good catalytic activity under these conditions (80%, Table 1,
entry 7). Moreover, it was possible the recover and re-use the
5
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
groups.[95] Interestingly, when using catalyst 2, a clear σ free-energy
relationship was not observed, suggesting that polar interactions do not play
any role in the rate determining step in the heterogeneous system.
Table 2. Substrate conversion with different catalysts applied to the reaction
Entry[a]
Substrate (R) Catalyst
Yield of 4 [%][b] Yield of 5 [%][b]
1
2
3
4
5
6
7
8
3a (H)
1b
2
73
78
19
81
4
25
0
3b (CH3)
3c (OCH3)
3d (CF3)
1b
2
80
24
95
21
21
0
1b
2
55
75
99
Figure 5. Hammett σ free-energy relationship for the homogeneous cross-
dehydrogenative aza-Henry coupling of tetrahydroisoquinolines with nitrometane
under aerobic conditions.
1b
2
[a] Reaction conditions: 90°C, 1 bar oxygen, 15 h, 5 mol% of catalyst [1b]BF4 or 20
mg/mmolsubs. of catalyst 2; [b] Determined by GC
Based on our previous fluorescence, UV-vis and EPR analysis of phenazine
catalyzed oxidation reactions,[63] together with the kinetic observation
described above, a general reaction mechanism can be proposed (Scheme
5). In an initial stage, the phenazine pre-catalyst needs to be oxidized. Hence,
since the O2 concentration in the reaction mixture is low, in the case of the
molecular catalysts 1, a better performance is achieved by pre-treatment
with NOBF4. Conversely, since CPP 2 already attains its activated radical-
cationic stage by reaction with atmospheric oxygen in the solid state (as
confirmed by EPR), a pre-oxidation is not necessary and can directly be used
in catalysis. Either way, oxidation of 1 shall produce the corresponding
phenazine radical-cation and, when oxygen enters the cycle, a reduced
Mechanistic Investigations
Kinetic plots for the aza-Henry reaction (substrate concentration vs. time,
Figures S12 to S17) for substrates bearing different substituents in para
position of the 2-phenyl group (3a-d) using pre-catalyst 1b showed a pseudo
zero order dependency every case, indicating that the reaction rate is highly
limited by the concentration of one reactant, presumably by the diffusion of
oxygen, as previously observed in oxidative amine coupling reactions with
the same catalysts.[63] Consumption rates (k) were obtained from the slope
of the linear function of the substrate concentration against time (Table 3).
•-
oxygen species, most likely a superoxide radical anion, O2 (alas, an
undisputable proof of its intermediacy remains elusive). Subsequently, SET
with the isoquinoline substrate 3 regenerates the closed-shell phenazine 1
and yields the amine radical cation 6. After the formation of 6, hydrogen
atom abstraction by O2.- forms radical 7 and the hydroperoxyl radical HOO..
These two species might subsequently react either by direct recombination
or SET to produce an equilibrating mixture of the iminium ion 8 and
hydroperoxide 9.[96-98] The intermediacy of the positively charged species 8
is supported by the observed Hammett relationships (see above). The
iminium intermediate 8 can thus undergo two different pathways: it reacts
electrophilically with nitromethane to generate the expected aza-Henry
adduct 4 or it is nucleophilic attacked by adventitious water in the solution
to produce a hemiaminal (10), which can be oxidized to form the
isoquinolone product (5).[93, 99] Addition of molecular sieves to the reaction
mixture inhibited the formation of 5, thus confirming this hypothesis.
We hypothesize that in the homogeneous system the nucleophilic attack to
the cationic intermediate 6 is the slower step, thus allowing the side reaction
with water and leading to the (sometimes preferential) formation of 5. On
the contrary, for the heterogeneous system, since the iminium intermediate
Table 3. Hammett Constants (σp) and Kinetic Constants (k) of Substrate consumption
from 3a-d with pre-catalyst 1b
Entry[a]
Substrate (R) σp
k [L mol-1 s-1
]
Lg (k)
1
3
5
7
3a (H)
0
-0.028
-0.029
-0.034
-0.021
-1.553
-1.524
-1.469
-1.686
3b (CH3)
3c (OCH3)
3d (CF3)
-0.17
-0.27
0.54
[a] Reaction conditions: 90°C, 1 bar oxygen, 15 h, 5 mol% of catalyst [1b]BF4
When using the homogeneous catalyst ([1b]BF4), data clearly showed that
the stronger the electron donating capability of the substituent in para
position of the phenyl ring, the faster the substrate consumption in the
reaction was. This trend was further observed in the corresponding
Hammett-plot (Figure 5), attesting the intermediacy of a positively charged
species (iminium) that is stabilized by the influence of electron-donating
6
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
is not involved in the rate determining step, its rapid coupling with
nitromethane impairs is collateral reaction with water and results in the
preferential formation of the aza-Henry product 4. Moreover, mass-
transport effects at the surface of the heterogeneous catalysts might play
an important role on the reaction selectivity, but further mechanistic
investigations are necessary to assess this possibility.
porous material was fully characterized, and its catalytic potential was
demonstrated in the aza-Henry coupling reaction. Interestingly, the
selectivity of the reaction towards the expected aza-Henry product (vs. the
corresponding isoquinolone) depends heavily on the electronic situation of
the substrate and on the nature of the catalyst. Thus, with the
heterogeneous polymeric phenazine catalysts 2, the aza-Henry product (4)
was obtained preferentially, disregarding the electronic properties of the
substrate. A plausible reaction mechanism was proposed, considering
previous spectroscopic observations along with additional kinetic
experiments. Importantly, these organocatalysts are among the few that
can catalyze aza-Henry reactions under thermal conditions. Moreover, CPP
2 is, to best of our knowledge, the first metal-free heterogeneous system
reported for cross-dehydrogenative coupling reactions.
ExperimentalSection
General Information
All reactions were performed under Schlenk, air/moisture-free conditions.
All solvents were dried, degassed and stored in septum-sealed flasks over
molecular sieves under Argon atmosphere. Solvents for catalyst preparation
were distilled over Na, CaH2 or molecular sieves following standardized
procedures. All chemicals were purchased from commercial sources (unless
stated otherwise). Most of chemicals were used without further purification.
The purity of purchased products was confirmed by either NMR
spectroscopy or GC/MS analysis.
NMR spectra in liquid phase were recorded in CDCl3 or Toluene-D8 using
Bruker Avance 300 spectrometer with QNP probe head (1H: 300 MHz, 13C:
75 MHz) or Bruker Avance 400 spectrometer (1H: 400 MHz, 13C: 100 MHz).
The calibration of the spectra was carried out on referenced with residual
solvent shifts and were reported as parts per million (ppm) relative to SiMe4.
Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t),
quartet (q), doublet-doublet (dd), doublet-triplet (dt), quintet (quint), sextet
(sext), septet (sept), multiplet (m), and broad (br). Samples were generally
measured at room temperature (297 K). Deuterated solvents were degassed
and distilled if necessary.
Solid-state 13C cross-polarization/ magic-angle-spinning (CP/MAS) NMR and
29Si MAS NMR data were collected on Bruker AVANCE-500 NMR
Spectrometer operating at a magnetic field strength of 9.4 T. The resonance
frequencies at this field strength were 125 and 99 MHz for 13C NMR and 29Si
NMR, respectively. A chemagnetics 5 mm triple-resonance MAS probe was
used to acquire 13C and 29Si NMR spectra. 29Si MAS NMR spectrum with high
power proton decoupling was recorded using a π/2 pulse length of 5 μs, a
recycle delay of 120 s and a spinning rate of 5 kHz.
Scheme 5: Proposed reaction mechanism for phenazine catalyzed cross-
dehydrogenative aza-Henry reaction in the presence of atmospheric oxygen
Conclusions
Molecular phenazine catalysts have previously shown outstanding activities
as catalysts for aerobic oxidative amine coupling and cross-coupling
reactions. In this contribution we expanded the reaction scope of this novel
type of metal-free oxidation catalyst and demonstrated their applicability in
cross-dehydrogenative aza-Henry coupling of different isoquinolines with
nitrometane under aerobic conditions. Moreover, we have designed and
synthesized a porous covalent polymer containing catalytically active
phenazine moieties cross-linked by tetraphenylsilane moieties. This novel
ESI-TOF spectra were obtained on Waters Q-TOF micro mass spectrometer.
The samples were dissolved in Methanol-Water (9:1) mixture with 0.1 %
HCOOH to achieve sufficient ionization. All chromatograms were obtained
on GC analysis was performed on Hewlett-Packard 6890 Series. Samples
were dissolved in tetrahydrofuran (THF) and sampled into the GC column.
7
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
GC/MS analysis was performed on Agilent 6890/5973 GC-MS or on Agilent
7890/5977 GC-MS. Samples were dissolved in tetrahydrofuran (THF).
IR spectra were recorded on a Nicolet Avatar 370 (Thermo Electron) FTIR
spectrometer equipped with a smart endurance ATR accessory.
Thermogravimetric analysis (TGA) was performed under N2 using a TA
SDTQ600 at a temperature range of room temperature to 800°C with a
heating rate of 10°C min-1. To eliminate the effect of possible adsorption of
moisture from atmosphere, all the samples were dried at 100 °C under
vacuum for 10 h before measurements.
extracted tree times with 20 ml diethyl ether. The combined organic phases
werewashed with brine (100 g sodium chloride per 1L water) in a separating
funnel. The solution was dried over magnesium sulfate and filtrated. Silica
was added to the organicphase and the diethyl ether was removedby rotary
evaporation. After performing thin layer chromatography (TLC) the product
was purified by column chromatography on silica gel (n-heptane/ethyl
acetate 10:1). The desired product was collected with a retention time Rf =
0.7. The desired product was a white solid, mass 1.6 g (yield: 51%). The
obtained spectra match literature reports:[101, 102] 1H NMR (300 MHz, CDCl3)
δ 7.28 – 7.19 (m, 2H), 7.16 – 7.07 (m, 4H), 6.97 – 6.89 (m, 2H), 6.81 – 6.73
(m, 1H), 4.35 (s, 2H), 3.51 (t, J = 5.9 Hz, 2H), 2.93 (t, J = 5.7 Hz, 2H) 13C NMR
(75 MHz, CDCl3) δ 150.69, 135.02, 134.61, 129.34, 128.66, 126.68, 126.46,
126.16, 118.79, 115.28, 50.87, 46.66, 29.26. 07.
Field-emission scanning electron microscopy (FE-SEM) experiment was
performed by using HITACHI S4800 Spectrometer.
The EPR spectra were recorded at 300 K with a Bruker EMX CW-micro
spectrometer equipped with an ER 4119HS-WI high-sensitivity optical
resonator. g values have been calculated from the resonance field B0 and
the resonance frequency ν using the resonance condition hν = gβB0.
The molecular phenazine catalysts (1a-f) and their corresponding
tetrafluoroborate salts ([1]BF4) were obtained following the previously
reported procedures.[63]
2-(p-tolyl)-1,2,3,4-tetrahydroisoquinoline (3b): Following the general
procedure, copper (I) iodide (0.203 g, 1 mmol), potassium phosphate (4.26
g, 20 mmol) and iodotoulene (2.18 g, 10 mmol) were added together with
2-propanol (10 mL), ethylene glycol (1.2 mL, 20 mmol), 1,2,3,4-tetrahydro-
isoquinoline (2 mL, 15 mmol). The pure product was obtained by column
chromatography on silica gel using a mixture of pentane and ethyl acetate
(10:1), Rf = 0.6. The desired product was a yellowish oil, mass 2.5 g (yield
75 %). The obtained spectra match literature reports:[102] 1H NMR (400 MHz,
CDCl3) δ 7.66 – 7.60 (m, 1H), 7.29 – 7.19 (m, 2H), 4.43 (s, 1H), 3.58 (t, J = 5.9
Hz, 1H), 3.05 (t, J = 5.7 Hz, 1H), 2.38 (s, 1H). 13C NMR (101 MHz, CDCl3) δ
148.63, 137.22, 131.21, 129.73, 128.60, 126.55, 126.26, 125.95, 115.83,
51.44, 47.25, 29.14, 21.04.
Synthesis of Covalent Porous Polymer 2
A three-necked flask was charged with tetrakis(4-bromophenyl)silane (1.96
g 3 mmol), palladium(II) acetate (68.39 mg, 0.3 mmol), potassium carbonate
(4.19 g, 30.36 mmol) and tri-tert-butylphosphine (226.6 mg, 1.12 mmol) in
toluene (40 mL) under argon atmosphere. The resultant mixture was
bubbled with argon under stirring for at least 0.5 h at room temperature and
then dihydrophenazine (1.09 g, 6 mmol) was added. Then the mixture was
stirred at 110°C for 48 h. After the reaction, the precipitate was filtered,
washed with THF, chloroform, water, methanol, and acetone to remove
unconsumed monomers, inorganic salts, catalyst and other residues.
Further purification was carried out by Soxhlet extraction with THF for 24 h
and chloroform for 24 h. Finally, the product was dried in a vacuum at 80°C
to obtain a brown solid with the yield of 91%. Elemental analysis calcd (%)
for C48H2N4Si: C 83.24, H 4.62, N 8.08; found: C 78.14, H 5.01, N 7.95.
2-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline (3c): Following the
general procedure, copper (I) iodide (0.203 g, 1 mmol), potassium
phosphate (4.27 g, 20 mmol) and iodoanisole (2.34 g, 10 mmol) were added
together with 2-propanol (10 mL), ethylene glycol (1.2 mL, 20 mmol),
1,2,3,4-tetrahydro-isoquinoline (2 mL, 15 mmol). The pure product was
obtained by column chromatography on silica gel using a mixture of n-
pentane and ethyl acetate (10:1), Rf = 0.8. The desired product was a white
solid, mass 2.4 g (yield: 68%). Single crystals of 3c were obtained by slow
evaporation of a saturated diethyl ether solution at room temperature. The
obtained spectra match literature reports:[102] 1H NMR (400 MHz, CDCl3) δ
7.21 – 7.10 (m, 1H), 7.02 – 6.97 (m, 1H), 6.90 – 6.85 (m, 1H), 4.31 (s, 1H),
3.79 (s, 1H), 3.46 (t, J = 5.9 Hz, 1H), 6.91 – 6.85 (m, 1H), 7.59 – 7.53 (m, 1H),
3.00(t, J = 5.9Hz 1H), 7.02– 6.96(m, 1H). 13C NMR (101 MHz, CDCl3) δ 153.61,
145.48, 138.32, 128.82, 126.64, 126.03, 118.15, 116.48, 55.77, 55.44, 52.80,
29.24.
Substrate Synthesis
Dihydroisoquinoline derivatives (3a-d) were prepared according to the
reported procedure,[100]
given exemplarily
for
2-phenyl-1,2-
dihydroisoquinoline 3a. All substrates were synthesized following the
general procedure.
2-phenyl-1,2-dihydroisoquinoline (3a), General procedure: Copper (I)
iodide (0.213 g, 1 mmol) and potassium phosphate (4.25 g, 20 mmol) were
added into a 50 ml three-neck round-bottom flask with a Dimroth condenser,
a Schlenk valve and a Teflon® magnetic stirrer. The flask was evacuated tree
times for 20 min and flushed with argon. Subsequently, 2-propanol (10 ml),
ethylene glycol (1.2 ml, 20 mmol), 1,2,3,4-tetrahydro-isoquinoline (2 ml, 15
mmol) and iodobenzene (1.12 ml, 10 mmol) were added successively with a
syringe at room temperature. The resulting orange mixture was heated at
90°C in an oil bath and stirred at this temperature for 24 h. After cooling to
room temperature, 20 mL water was added, and the organic layer was
2-(4-(trifluoromethyl)phenyl)-1,2,3,4-tetrahydroisoquinoline
(3d):
Following the general procedure, copper (I) iodide (0.230 g, 1 mmol),
potassium phosphate (4.26 g, 20 mmol) and 4-iodo-triflourotoluene (2.72 g,
10 mmol) were added together with 2-propanol (10 mL), ethylene glycol (1.2
mL, 20 mmol), 1,2,3,4-tetrahydro-isoquinoline (2 mL, 15 mmol). The pure
product was obtained by column chromatography on silica gel using a
mixture of pentane and ethyl acetate (10:1), Rf = 0.4. The desired product
was a white solid, mass 3.5 g (yield: 83%). Single crystals of 3d were obtained
by slow evaporation of a saturated diethyl ether solution at room
8
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
temperature. The obtained spectra match literature reports:[102] 1H NMR
(300 MHz, CDCl3) δ 7.53 (d, J = 8.6 Hz 1H), 7.27 – 7.15 (m, 2H), 6.95 (d, J =
8.7 Hz 2H), 4.50 (s, 1H), 3.64 (t, J = 5.9 Hz, 2H), 3.01 (t, J = 5.9 Hz, 3H). 13C
NMR (75 MHz, CDCl3) δ 152.27, 138.17, 135.05, 133.95, 128.47, 126.84,
126.65, 126.48, 113.07, 77.58, 77.16, 76.74, 49.56, 45.35, 32.04, 29.11,
22.85, 14.26. 19F NMR (282 MHz, CDCl3) δ -61.03.
2-(4-methoxyphenyl)-1-(nitromethyl)-1,2,3,4-tetrahydroisoquinoline (4c):
Following the general procedure, using 2-(4-methoxyphenyl)-1,2,3,4-
tetrahydroisoquinoline (200 mg, 0.9 mmol). The pure product was obtained
by column chromatography on silica gelusing amixture of pentane and ethyl
acetate (10:1), Rf = 0.8. The desired product was a yellowish liquid. For yields
see Table 2. The obtained spectra match literature reports:[103] 1H NMR (400
MHz, CD3CN) δ 7.27 – 7.20 (m, 3H), 7.18 – 7.14 (m, 1H), 6.95 – 6.89 (m, 2H),
6.83 – 6.78 (m, 2H), 5.41 (dd, J = 9.6, 5.3 Hz, 1H), 4.89 (dd, J = 12.4, 9.6 Hz,
1H), 4.73 (dd, J = 12.4, 5.2 Hz, 1H), 3.70 (s, 3H), 3.62 – 3.52 (m, 2H), 2.94 –
2.84 (m, 1H), 2.64 (m, 1H).13C NMR (101 MHz, CD3CN) δ 154.77, 144.21,
136.78, 133.72, 130.40, 128.66, 128.29, 127.26, 119.71, 118.34, 115.49,
115.45, 79.64, 59.19, 56.03, 43.27, 25.74, 1.94, 1.73, 1.52, 1.47, 1.32, 1.26,
1.11, 0.91, 0.70. The side product 2-(4-methoxyphenyl)-3,4-
dihydroisoquinolin-1(2H)-one (5c) was analyzed via GC/MS. (see Supporting
Information).
Catalysis
The general procedure for the catalytic aza-Henry reaction is given
exemplarily for the 1-(nitromethyl)-2-phenyl-1,2,3,4-tetrahydroisoquinoline
4a. The other products were synthesized following the general procedure.
1-(nitromethyl)-2-phenyl-1,2,3,4-tetrahydroisoquinoline (4a), General
procedure: 2-phenyl-1,2-dihydroisoquinoline (0.95 mmol, 200 mg), 0.048
-
mmol of catalyst [1][BF4 ] (5 mol%) or 20 mg of 5,10-di(4-methoxyphenyl)-
5,10-dihydrophenazine 2 and nitromethane (37 mmol, 0.5 ml, 39 eq) were
added together in a Schlenk flask. The solution was flushed tree times with
oxygen. A balloon with oxygen (~ 1 bar) was attached to the Schlenk valve.
The reaction was conducted for 15 h at 90°C in an oil bath. After performing
TLC with the cold reaction mixture, the product was purified by column
chromatography on silica gel (heptane/ethyl acetate 10:1). The product was
collected with a retention time of Rf = 0.8. The desired product was a
greenish liquid. For yields see Table 2. The obtained spectra match literature
reports:[103] 1H NMR (300 MHz, CDCl3) δ 7.29 – 7.11 (m, 6H), 6.99 (d, 2H), 6.86
(t, J = 7.2 Hz, 1H), 5.56 (t, J = 7.2 Hz, 1H), 4.88 (dd, J = 11.8, 7.8 Hz, 1H), 4.57
(dd, J = 11.8, 6.6 Hz, 1H), 3.73 – 3.54 (m, 2H), 3.16 – 2.98 (m, 1H), 2.80 (dt, J
= 16.3, 4.8 Hz, 1H). 13C NMR (75 MHz, CDCl3) δ 148.57, 135.42, 133.08,
129.65, 129.33, 128.27, 127.15, 126.85, 119.59, 115.26, 78.94, 58.34, 42.24,
26.62. (ESI-TOF): m/z = 268.12057, calc. for C16H16O2N2 [M+H]+ m/z =
268.12063. The side product 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one
(5a) was analyzed via GC/MS. The EI-MS spectrum matches literature
reports[104] (see Supporting Information). Single crystals of 5a were obtained
by slow evaporation of a saturated diethyl ether solution at room
temperature. Then, one single crystal was removed from the mother liquor
and used as seed in a fresh diethyl ether solution and allow to grow for 14
days at room temperature. The molecular structure of 5a was confirmed by
single-crystal X-ray diffraction (see Supporting Information).
1-(nitromethyl)-2-(4-(trifluoromethyl)phenyl)-1,2,3,4-
tetrahydroisoquinoline (4d): Following the general procedure using 2-(4-
(trifluoromethyl)phenyl)-1,2,3,4-tetrahydroisoquinoline (200 mg, 0.9 mmol.
The pure product was obtained by column chromatography on silica gel
using a mixture of pentane and ethyl acetate (10:1), Rf = 0.8. The product
was a yellowish liquid. For yields see Table 2. The obtained spectra match
literature reports:[105] 1H NMR (400 MHz, CDCl3) δ 7.57 (m, 2H), 7.37 – 7.25
(m, 3H), 7.20 (dd, J = 7.5, 1.5 Hz, 1H), 7.07 (m, 2H), 5.68 (t, J = 7.2 Hz, 1H),
4.92 (dd, J = 12.0, 7.7 Hz, 1H), 4.65 (dd, J = 12.0, 6.7 Hz, 1H), 3.78 – 3.68 (m,
2H), 3.17 (ddd, J = 16.4, 7.6, 5.7 Hz, 1H), 2.92 (dt, J = 16.3, 5.4 Hz, 1H). 13C
NMR (101 MHz, CDCl3) δ 150.61, 134.99, 132.44, 129.28, 128.59, 127.10,
127.04, 126.94, 126.90, 113.41, 78.50, 77.48, 77.16, 76.84, 57.86, 41.84,
26.62.19F NMR (282 MHz, CDCl3) δ -61.36. The side product 2-(4-
(trifluoromethyl)phenyl)-3,4-dihydroisoquinolin-1(2H)-one
analyzed via GC/MS. (see Supporting Information).
(5d)
was
SupplementaryMaterial
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/MS-number. X-ray crystallographic CIF data were
deposited in Cambridge Crystallographic Data Centre and they are available
free of charge with the number [CCDC 1938159 (3d), CCDC 1952847 (3c) and
CCDC 1952651 (5a)].
1-(nitromethyl)-2-(p-tolyl)-1,2,3,4-tetrahydroisoquinoline (4b): Following
the general procedure, using 2-(p-tolyl)-1,2,3,4-tetrahydroisoquinoline (200
mg, 0.9 mmol). The pure product was obtained by column chromatography
on silica gel using a mixture of pentane and ethyl acetate (10:1), Rf = 0.8. The
desired product was a yellowish liquid. For yields, see Table 2. The product
was analyzed by GC/MS and EI-MS. The EI-MS spectrum is given in the
supporting Information. The obtained NMR spectra match literature
reports:[103] 1H NMR (300 MHz, CDCl3) δ 7.19 – 7.03 (m, 2H), 7.02 – 6.97 (m,
1H), 6.84 – 6.78 (m, 1H), 5.42 (t, J = 8.0, 6.4 Hz, 1H), 4.78 (dd, J = 11.9, 8.0
Hz, 1H), 4.49 (dd, J = 11.8, 6.4 Hz, 0H), 3.62 –3.46 (m, 1H), 2.99 (ddd, J = 15.4,
9.0, 6.0 Hz, 1H), 2.69 (dt, J = 16.4, 4.6 Hz, 1H), 2.26 (s, 3H). The side product
2-(p-tolyl)-3,4-dihydroisoquinolin-1(2H)-one (5b) was analyzed via GC/MS.
(see Supporting Information).
Acknowledgements
We would like to warmly thank Dr. Anke Spannenberg for the X-Ray
diffraction measurements, Dr. Nils Rockstroh for the IR spectra and Dr Dirk
Hollmann for his support with the EPR experiments. Prof. Udo Kragl is also
gratefully acknowledged for his constant support.
Author ContributionStatement
FU, PH and XG prepared the molecular catalysts and conducted all the
catalytic experiments. ZC and DW carried out the synthesis and
9
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
[23]
[24]
[25]
Y. Lin, B. Li, Z. Feng, Y. A. Kim, M. Endo, D. S. Su, ‘Efficient Metal-
Free Catalytic Reaction Pathway for Selective Oxidation of
Substituted Phenols’, ACS Catal. 2015, 5, 5921-5926.
J. Qin, S. Wang, H. Ren, Y. Hou, X. Wang, ‘Photocatalytic
reduction of CO2 by graphitic carbon nitride polymers derived
from urea and barbituric acid’, Appl. Catal., B 2015, 179, 1-8.
S. Yang, L. Peng, P. Huang, X. Wang, Y. Sun, C. Cao, W. Song,
‘Nitrogen, Phosphorus, and Sulfur Co-Doped Hollow Carbon
Shell as Superior Metal-Free Catalyst for Selective Oxidation of
Aromatic Alkanes’, Angew. Chem., Int. Ed. 2016, 55, 4016-4020.
F. Hu, M. Patel, F. Luo, C. Flach, R. Mendelsohn, E. Garfunkel, H.
He, M. Szostak, ‘Graphene-Catalyzed Direct Friedel–Crafts
Alkylation Reactions: Mechanism, Selectivity, and Synthetic
Utility’, J. Am. Chem. Soc. 2015, 137, 14473-14480.
J. Gläsel, J. Diao, Z. Feng, M. Hilgart, T. Wolker, D. S. Su, B. J. M.
Etzold, ‘Mesoporous and Graphitic Carbide-Derived Carbons as
Selective and Stable Catalysts for the Dehydrogenation
Reaction’, Chem. Mater. 2015, 27, 5719-5725.
characterization of the polymeric catalyst. EM conceived the project and
supervised the research. FU, DW and EM wrote the manuscript.
References
[1]
M. Saurat, S. Bringezu, ‘Platinum group metal flows of Europe,
part 1: Global supply, use in industry, and shifting of
environmental impacts’, J. Ind. Ecol. 2008, 12, 754-767.
M. Saurat, S. Bringezu, ‘Platinum Group Metal Flows of Europe,
Part II: Exploring the Technological and Institutional Potential
for Reducing Environmental Impacts’, J. Ind. Ecol. 2009, 13, 406-
421.
[2]
[26]
[27]
[3]
S. J. Fallon, J. C. White, M. T. McCulloch, ‘Porites corals as
recorders of mining and environmental impacts: Misima Island,
Papua New Guinea’, Geochim. Cosmochim. Acta 2002, 66, 45-
62.
[4]
[5]
[6]
[7]
[8]
T. Norgate, N. Haque, ‘Energy and greenhouse gas impacts of
mining and mineral processing operations’, J. Cleaner Prod.
2010, 18, 266-274.
[28]
[29]
M.-M. Titirici, R. J. White, N. Brun, V. L. Budarin, D. S. Su, F. del
Monte, J. H. Clark, M. J. MacLachlan, ‘Sustainable carbon
materials’, Chem. Soc. Rev. 2015, 44, 250-290.
K. S. Egorova, V. P. Ananikov, ‘Which metals are green for
catalysis? Comparison of the toxicities of Ni, Cu, Fe, Pd, Pt, Rh,
and Au salts’, Angew. Chem., Int. Ed. 2016, 55, 12150-12162.
D. S. Su, J. Zhang, B. Frank, A. Thomas, X. C. Wang, J.
Paraknowitsch, R. Schlogl, ‘Metal-Free Heterogeneous Catalysis
for Sustainable Chemistry’, ChemSusChem 2010, 3, 169-180.
S. Ahmad, E. Guillen, L. Kavan, M. Gratzel, M. K. Nazeeruddin,
‘Metal free sensitizer and catalyst for dye sensitized solar cells’,
Energy Environ. Sci. 2013, 6, 3439-3466.
M. Fevre, J. Pinaud, Y. Gnanou, J. Vignolle, D. Taton, ‘N-
Heterocyclic carbenes (NHCs) as organocatalysts and structural
components in metal-free polymer synthesis’, Chem. Soc. Rev.
2013, 42, 2142-2172.
S. Majeed, J. M. Zhao, L. Zhang, S. Anjum, Z. Y. Liu, G. B. Xu,
‘Synthesis and electrochemical applications of nitrogen-doped
carbon nanomaterials’, Nanotechnol. Rev. 2013, 2, 615-635.
W. Kicinski, M. Szala, M. Bystrzejewski, ‘Sulfur-doped porous
carbons: Synthesis and applications’, Carbon 2014, 68, 1-32.
X. K. Kong, C. L. Chen, Q. W. Chen, ‘Doped graphene for metal-
free catalysis’, Chem. Soc. Rev. 2014, 43, 2841-2857.
X. W. Wang, G. Z. Sun, P. Routh, D. H. Kim, W. Huang, P. Chen,
‘Heteroatom-doped graphene materials: syntheses, properties
and applications’, Chem. Soc. Rev. 2014, 43, 7067-7098.
J. J. Duan, S. Chen, M. Jaroniec, S. Z. Qiao, ‘Heteroatom-Doped
Graphene-Based Materials for Energy-Relevant Electrocatalytic
Processes’, ACS Catal. 2015, 5, 5207-5234.
S. Agnoli, M. Favaro, ‘Doping graphene with boron: a review of
synthesis methods, physicochemical characterization, and
emerging applications’, J. Mater. Chem. A 2016, 4, 5002-5025.
M. A. Patel, F. Luo, M. R. Khoshi, E. Rabie, Q. Zhang, C. R. Flach,
R. Mendelsohn, E. Garfunkel, M. Szostak, H. He, ‘P-Doped
Porous Carbon as Metal Free Catalysts for Selective Aerobic
Oxidation with an Unexpected Mechanism’, ACS Nano 2016, 10,
2305-2315.
Y. Gao, G. Hu, J. Zhong, Z. Shi, Y. Zhu, D. S. Su, J. Wang, X. Bao,
D. Ma, ‘Nitrogen-Doped sp2-Hybridized Carbon as a Superior
Catalyst for Selective Oxidation’, Angew. Chem., Int. Ed. 2013,
52, 2109-2113.
Q. L. Wei, X. Tong, G. X. Zhang, J. L. Qiao, Q. J. Gong, S. H. Sun,
‘Nitrogen-Doped Carbon Nanotube and Graphene Materials for
Oxygen Reduction Reactions’, Catalysts 2015, 5, 1574-1602.
O. Y. Podyacheva, Z. R. Ismagilov, ‘Nitrogen-doped carbon
nanomaterials: To the mechanism of growth, electrical
conductivity and application in catalysis’, Catal. Today 2015,
249, 12-22.
M. M. Li, F. Xu, H. R. Li, Y. Wang, ‘Nitrogen-doped porous
carbon materials: promising catalysts or catalyst supports for
heterogeneous hydrogenation and oxidation’, Catal. Sci.
Technol. 2016, 6, 3670-3693.
M. Saleh, J. N. Tiwari, K. C. Kemp, M. Yousuf, K. S. Kim, ‘Highly
Selective and Stable Carbon Dioxide Uptake in Polyindole-
Derived Microporous Carbon Materials’, Environ. Sci. Technol.
2013, 47, 5467-5473.
K. Shen, X. Chen, J. Chen, Y. Li, ‘Development of MOF-Derived
Carbon-Based Nanomaterials for Efficient Catalysis’, ACS Catal.
2016, 6, 5887-5903.
D. Wang, E. Mejia, ‘POSS-Based Nitrogen-Doped Hierarchically
Porous Carbon as Metal-Free Oxidation Catalyst’,
[30]
[31]
[32]
[33]
[34]
[35]
[9]
S. Wertz, A. Studer, ‘Nitroxide-catalyzed transition-metal-free
aerobic oxidation processes’, Green Chem. 2013, 15, 3116-3134.
X. X. Zou, Y. Zhang, ‘Noble metal-free hydrogen evolution
catalysts for water splitting’, Chem. Soc. Rev. 2015, 44, 5148-
5180.
[10]
[11]
[12]
[13]
[14]
Y. Nie, L. Li, Z. D. Wei, ‘Recent advancements in Pt and Pt-free
catalysts for oxygen reduction reaction’, Chem. Soc. Rev. 2015,
44, 2168-2201.
M. H. Shao, Q. W. Chang, J. P. Dodelet, R. Chenitz, ‘Recent
Advances in Electrocatalysts for Oxygen Reduction Reaction’,
Chem. Rev. 2016, 116, 3594-3657.
Y. Xu, M. Kraft, R. Xu, ‘Metal-free carbonaceous electrocatalysts
and photocatalysts for water splitting’, Chem. Soc. Rev. 2016,
45, 3039-3052.
M. Zhou, H. L. Wang, S. J. Guo, ‘Towards high-efficiency
nanoelectrocatalysts for oxygen reduction through engineering
advanced carbon nanomaterials’, Chem. Soc. Rev. 2016, 45,
1273-1307.
[36]
[37]
[38]
[15]
[16]
C. C. Huang, C. Li, G. Q. Shi, ‘Graphene based catalysts’, Energy
Environ. Sci. 2012, 5, 8848-8868.
D. S. Su, S. Perathoner, G. Centi, ‘Nanocarbons for the
Development of Advanced Catalysts’, Chem. Rev. 2013, 113,
5782-5816.
[39]
[40]
[17]
[18]
S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia,
‘Carbocatalysis by Graphene-Based Materials’, Chem. Rev. 2014,
114, 6179-6212.
P. Tang, G. Hu, M. Li, D. Ma, ‘Graphene-Based Metal-Free
Catalysts for Catalytic Reactions in the Liquid Phase’, ACS Catal.
2016, 6, 6948-6958.
[41]
[42]
[43]
[44]
[19]
[20]
[21]
[22]
D. Su, G. Wen, S. Wu, F. Peng, R. Schloegl, ‘Carbocatalysts in
Liquid Phase’, Angew. Chem., Int. Ed. 2016, 56, 936-964.
X. Liu, L. Dai, ‘Carbon-based metal-free catalysts’, Nat. Rev.
Mater. 2016, 1, 16064.
X. B. Fan, G. L. Zhang, F. B. Zhang, ‘Multiple roles of graphene in
heterogeneous catalysis’, Chem. Soc. Rev. 2015, 44, 3023-3035.
B. Chen, L. Wang, S. Gao, ‘Recent Advances in Aerobic Oxidation
of Alcohols and Amines to Imines’, ACS Catal. 2015, 5, 5851-
5876.
ChemistrySelect 2017, 2, 3381-3387.
N. Chaoui, M. Trunk, R. Dawson, J. Schmidt, A. Thomas, ‘Trends
and challenges for microporous polymers’, Chem. Soc. Rev.
2017, 46, 3302-3321.
T. L. Church, A. B. Jasso-Salcedo, F. Bjoernerbaeck, N. Hedin,
‘Sustainability of microporous polymers and their applications’,
Sci. China: Chem. 2017, 60, 1033-1055.
10
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
[45]
Q. Li, S. Razzaque, S. Jin, B. Tan, ‘Morphology design of
[65]
[66]
J. P. Cole, C. R. Federico, C.-H. Lim, G. M. Miyake, ‘Photoinduced
Organocatalyzed Atom Transfer Radical Polymerization Using
Low ppm Catalyst Loading’, Macromolecules 2019, 52, 747-754.
S. Chun, Y. K. Chung, ‘Transition-Metal-Free Poly (thiazolium)
Iodide/1, 8-Diazabicyclo [5.4. 0] undec-7-ene/Phenazine-
Catalyzed Esterification of Aldehydes with Alcohols’, Organic
letters 2017, 19, 3787-3790.
J. Lin, Z. Guo, H. Zhan, ‘A robust phenazine-containing organic
polymer as catalyst for amine oxidative coupling reactions’, J.
Catal. 2020, 385, 338-344.
M. Wander, P. J. C. Hausoul, L. A. J. M. Sliedregt, B. J. van Steen,
G. van Koten, R. J. M. Klein Gebbink, ‘Synthesis of Polyaryl Rigid-
Core Carbosilane Dendrimers for Supported Organic Synthesis’,
Organometallics 2009, 28, 4406-4415.
S. Lowell, J. E. Shields, in Powder Surface Area and Porosity,
Springer Netherlands, Dordrecht, 1984, pp. 11-13.
W. Chaikittisilp, A. Sugawara, A. Shimojima, T. Okubo,
‘Microporous hybrid polymer with a certain crystallinity built
from functionalized cubic siloxane cages as a singular building
unit’, Chem. Mater. 2010, 22, 4841-4843.
J. L. Grant, V. J. Kramer, R. Ding, L. D. Kispert, ‘Carotenoid cation
radicals: electrochemical, optical, and EPR study’, J. Am. Chem.
Soc. 1988, 110, 2151-2157.
T. Li, G. Tan, D. Shao, J. Li, Z. Zhang, Y. Song, Y. Sui, S. Chen, Y.
Fang, X. Wang, ‘Magnetic Bistability in a Discrete Organic
Radical’, J. Am. Chem. Soc. 2016, 138, 10092-10095.
A. A. Almasalma, E. Mejía, in Heterocycles via Cross
Dehydrogenative Coupling: Synthesis and Functionalization
(Eds.: A. Srivastava, C. K. Jana), Springer Singapore, Singapore,
2019, pp. 329-356.
microporous organic polymers and their potential applications:
an overview’, Sci. China: Chem. 2017, 60, 1056-1066.
M. M. Abdelnaby, N. A. A. Qasem, B. A. Al-Maythalony, K. E.
Cordova, O. C. S. Al Hamouz, ‘A Microporous Organic Copolymer
for Selective CO2 Capture under Humid Conditions’, ACS
Sustainable Chem. Eng. 2019, 7, 13941-13948.
G. Chang, Z. Shang, T. Yu, L. Yang, ‘Rational design of a novel
indole-based microporous organic polymer: enhanced carbon
dioxide uptake via local dipole-π interactions’, J. Mater. Chem. A
2016, 4, 2517-2523.
W. Li, A. Zhang, H. Gao, M. Chen, A. Liu, H. Bai, L. Li, ‘Massive
preparation of pitch-based organic microporous polymers for
gas storage’, Chem. Commun. 2016, 52, 2780-2783.
H. Lim, M. C. Cha, J. Y. Chang, ‘Preparation of Microporous
Polymers Based on 1,3,5-Triazine Units Showing High CO2
Adsorption Capacity’, Macromol. Chem. Phys. 2012, 213, 1385-
1390.
Y. Yuan, F. Sun, L. Li, P. Cui, G. Zhu, ‘Porous aromatic
frameworks with anion-templated pore apertures serving as
polymeric sieves’, Nat. Commun. 2014, 5, 4260.
L. Zou, J. Yuan, Y. Yuan, J. Gu, G. Li, L. Zhang, Y. Liu, ‘A Zn(II)
metal-organic framework constructed by a mixed-ligand
strategy for CO2 capture and gas separation’, CrystEngComm
2019, 21, 3289-3294.
V. R. Battula, H. Singh, S. Kumar, I. Bala, S. K. Pal, K. Kailasam,
‘Natural Sunlight Driven Oxidative Homocoupling of Amines by
a Truxene-Based Conjugated Microporous Polymer’, ACS Catal.
2018, 8, 6751-6759.
S. Chun, Y. K. Chung, ‘Transition-Metal-Free Poly(thiazolium)
Iodide/1,8-Diazabicyclo[5.4.0]undec-7-ene/Phenazine-Catalyzed
Esterification of Aldehydes with Alcohols’, Org. Lett. 2017, 19,
3787-3790.
Z. Liu, Q. Su, P. Ju, X. Li, G. Li, Q. Wu, B. Yang, ‘A hydrophilic
covalent organic framework for photocatalytic oxidation of
benzylamine in water’, Chem. Commun. 2020, 56, 766-769.
V. M. Suresh, S. Bonakala, H. S. Atreya, S. Balasubramanian, T.K.
Maji, ‘Amide Functionalized Microporous Organic Polymer (Am-
MOP) for Selective CO2 Sorption and Catalysis’, ACS Appl. Mater.
Interfaces 2014, 6, 4630-4637.
X. Yang, L. Tan, L. Xia, C. D. Wood, B. Tan, ‘Hierarchical Porous
Polystyrene Monoliths from PolyHIPE’, Macromol. Rapid
Commun. 2015, 36, 1553-1558.
Q. Yu, D. Tan, T. Huang, T. Zhao, L. Li, ‘Preparation of asphalt-
based microporous organic polymers catalyzed by heteropoly
acids’, Green Chem. 2018, 20, 4746-4751.
L. Chen, Y. Yang, Z.-Q. Guo, D.-L. Jiang, ‘Highly Efficient
Activation of Molecular Oxygen with Nanoporous
Metalloporphyrin Frameworks in Heterogeneous Systems’, Adv.
Mater. 2011, 23, 3149-3154.
K. Zhang, B. Tieke, F. Vilela, P. J. Skabara, ‘Conjugated
Microporous Networks on the Basis of 2,3,5,6-Tetraarylated
Diketopyrrolo[3,4-c]pyrrole’, Macromol. Rapid Commun. 2011,
32, 825-830.
Y. Liao, J. Weber, C. F. J. Faul, ‘Conjugated microporous
polytriphenylamine networks’, Chem. Commun. 2014, 50, 8002-
8005.
J. Schmidt, M. Werner, A. Thomas, ‘Conjugated Microporous
Polymer Networks via Yamamoto Polymerization’,
Macromolecules 2009, 42, 4426-4429.
R. Brišar, D. Hollmann, E. Mejía, ‘Pyrazine Radical Cations as a
Catalyst for the Aerobic Oxidation of Amines’, Eur. J. Org. Chem.
2017, 2017, 5391-5398.
R. Brišar, F. Unglaube, D. Hollmann, H. Jiao, E. Mejía, ‘Aerobic
Oxidative Homo-and Cross-Coupling of Amines Catalyzed by
Phenazine Radical Cations’, J. Org. Chem. 2018, 83, 13481-
13490.
J. C. Theriot, C.-H. Lim, H. Yang, M. D. Ryan, C. B. Musgrave, G.
M. Miyake, ‘Organocatalyzed atom transfer radical
polymerization driven by visible light’, Science 2016, 352, 1082-
1086.
[46]
[47]
[67]
[68]
[48]
[49]
[69]
[70]
[50]
[51]
[71]
[72]
[73]
[52]
[53]
[74]
[75]
[76]
[77]
C.-J. Li, ‘Cross-Dehydrogenative Coupling (CDC): Exploring C−C
Bond Formations beyond Functional Group Transformations’,
Acc. Chem. Res. 2009, 42, 335-344.
[54]
[55]
C. Zhang, C. Tang, N. Jiao, ‘Recent advances in copper-catalyzed
dehydrogenative functionalization via a single electron transfer
(SET) process’, Chem. Soc. Rev. 2012, 41, 3464-3484.
S. A. Girard, T. Knauber, C.-J. Li, ‘The Cross-Dehydrogenative
Coupling of Csp3-H Bonds: A Versatile Strategy for C-C Bond
Formations’, Angew. Chem., Int. Ed. 2014, 53, 74-100.
J. C. Anderson, A. Noble, D. A. Tocher, ‘Reductive Nitro-Mannich
Route for the Synthesis of 1, 2-Diamine Containing Indolines
and Tetrahydroquinolines’, J. Org. Chem. 2012, 77, 6703-6727.
A. Noble, J. C. Anderson, ‘Nitro-mannich reaction’, Chem. Rev.
2013, 113, 2887-2939.
N. Ono, The Nitro Group in Organic Synthesis, Wiley-VCH., New
York, 2001.
H. Bartling, A. Eisenhofer, B. Kꢀnig, R. M. Gschwind, ‘The
Photocatalyzed Aza-Henry Reaction of N-
Aryltetrahydroisoquinolines: Comprehensive Mechanism, H•-
versus H+-Abstraction, and Background Reactions’, J. Am. Chem.
Soc. 2016, 138, 11860-11871.
A. W. Gregory, A. Chambers, A. Hawkins, P. Jakubec, D. J. Dixon,
‘Iridium‐Catalyzed Reductive Nitro‐Mannich Cyclization’,
Chem. - Eur. J. 2015, 21, 111-114.
P. Kumar, S. Varma, S. L. Jain, ‘A TiO2 immobilized Ru (II)
polyazine complex: a visible-light active photoredox catalyst for
oxidative cyanation of tertiary amines’, J. Mater. Chem. A 2014,
2, 4514-4519.
C. Li, R. Dickson, N. Rockstroh, J. Rabeah, D. B. Cordes, A.
Slawin, P. Hünemörder, A. Spannenberg, M. Buehl, E. Mejía, E.
Zysman-Colman, P. Kamer, ‘Ligand electronic fine-tuning and its
repercussion on the photocatalytic activity and mechanistic
pathways of the copper-photocatalysed aza-Henry Reaction’,
Catal. Sci. Technol. 2020.
D. P. K. Hari, Burkhard, ‘Catalyzed Visible Light Oxidative CC and
CP bond Formation ’, Org. Lett. 2011, 13, 3852-3855.
Q. Liu, Y. N. Li, H. H. Zhang, B. Chen, C. H. Tung, L. Z. Wu,
‘Reactivity and Mechanistic Insight into Visible‐Light‐
Induced Aerobic Cross‐Dehydrogenative Coupling Reaction by
Organophotocatalysts’, Chem. - Eur. J. 2012, 18, 620-627.
A. S.-K. Tsang, M. H. Todd, ‘Facile synthesis of vicinal diamines
via oxidation of N-phenyltetrahydroisoquinolines with DDQ’,
Tetrahedron Lett. 2009, 50, 1199-1202.
[56]
[57]
[58]
[78]
[79]
[80]
[59]
[81]
[82]
[60]
[61]
[62]
[63]
[83]
[84]
[85]
[64]
[86]
11
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
[87]
T. Nobuta, N. Tada, A. Fujiya, A. Kariya, T. Miura, A. Itoh,
isoquinoleines). Formation d'une isoquinoleine, d'un
‘Molecular iodine catalyzed cross-dehydrogenative coupling
reaction between two sp3 C–H bonds using hydrogen peroxide’,
Org. Lett. 2013, 15, 574-577.
H. Ueda, K. Yoshida, H. Tokuyama, ‘Acetic acid promoted metal-
free aerobic carbon–carbon bond forming reactions at α-
position of tertiary amines’, Org. Lett. 2014, 16, 4194-4197.
D. Kundu, R. K. Debnath, A. Majee, A. Hajra, ‘Zwitterionic-type
molten salt-catalyzed syn-selective aza-Henry reaction: solvent-
free one-pot synthesis of β-nitroamines’, Tetrahedron Lett.
2009, 50, 6998-7000.
isocarbostyryle ou d'une indoline selon les conditions’,
Tetrahedron 1982, 38, 3471-3478.
[105]
H. E. Ho, Y. Ishikawa, N. Asao, Y. Yamamoto, T. Jin, ‘Highly
efficient heterogeneous aerobic cross-dehydrogenative
coupling via C–H functionalization of tertiary amines using a
nanoporous gold skeleton catalyst’, Chem. Commun. 2015, 51,
12764-12767.
[88]
[89]
[90]
[91]
A. Tanoue, W.-J. Yoo, S. Kobayashi, ‘Sulfuryl chloride as an
efficient initiator for the metal-free aerobic cross-
dehydrogenative coupling reaction of tertiary amines’, Org.
Lett. 2014, 16, 2346-2349.
Z. Li, C.-J. Li, ‘Highly Efficient Copper-Catalyzed Nitro-Mannich
Type Reaction: Cross-Dehydrogenative-Coupling between sp3
C−H Bond and sp3 C−H Bond’, J. Am. Chem. Soc. 2005, 127,
3672-3673.
[92]
[93]
H. Yokoi, S. Hiroto, H. Shinokubo, ‘Reversible σ-Bond Formation
in Bowl-Shaped π-Radical Cations: The Effects of Curved and
Planar Structures’, J. Am. Chem. Soc. 2018, 140, 4649-4655.
J. L. Clark, J. E. Hill, I. D. Rettig, J. J. Beres, R. Ziniuk, T. Y.
Ohulchanskyy, T. M. McCormick, M. R. Detty, ‘Importance of
Singlet Oxygen in Photocatalytic Reactions of 2-Aryl-1, 2, 3, 4-
tetrahydroisoquinolines Using Chalcogenorosamine
Photocatalysts’, Organometallics 2019, 38, 2431-2442.
L. Zhang, W. Wang, A. Wang, Y. Cui, X. Yang, Y. Huang, X. Liu, W.
Liu, J.-Y. Son, H. Oji, T. Zhang, ‘Aerobic oxidative coupling of
alcohols and amines over Au–Pd/resin in water: Au/Pd molar
ratios switch the reaction pathways to amides or imines’, Green
Chem. 2013, 15, 2680-2684.
[94]
[95]
[96]
C. Hansch, A. Leo, R. Taft, ‘A survey of Hammett substituent
constants and resonance and field parameters’, Chem. Rev.
1991, 91, 165-195.
E. Boess, D. Sureshkumar, A. Sud, C. Wirtz, C. Fares, M.
Klussmann, ‘Mechanistic Studies on a Cu-Catalyzed Aerobic
Oxidative Coupling Reaction with N-Phenyl
Tetrahydroisoquinoline: Structure of Intermediates and the Role
of Methanol As a Solvent’, J. Am. Chem. Soc. 2011, 133, 8106-
8109.
E. Boess, C. Schmitz, M. Klussmann, ‘A Comparative Mechanistic
Study of Cu-Catalyzed Oxidative Coupling Reactions with N-
Phenyltetrahydroisoquinoline’, J. Am. Chem. Soc. 2012, 134,
5317-5325.
M. O. Ratnikov, M. P. Doyle, ‘Mechanistic Investigation of
Oxidative Mannich Reaction with tert-Butyl Hydroperoxide. The
Role of Transition Metal Salt’, J. Am. Chem. Soc. 2013, 135,
1549-1557.
L. Zhang, W. Wang, A. Wang, Y. Cui, X. Yang, Y. Huang, X. Liu, W.
Liu, J.-Y. Son, H. Oji, T. Zhang, ‘Aerobic oxidative coupling of
alcohols and amines over Au-Pd/resin in water: Au/Pd molar
ratios switch the reaction pathways to amides or imines’, Green
Chem. 2013, 15, 2680-2684.
[97]
[98]
[99]
[100]
[101]
J. Zhang, B. Tiwari, C. Xing, X. Chen, Y. R. Chi, ‘Enantioselective
Oxidative Cross‐Dehydrogenative Coupling of Tertiary Amines
to Aldehydes’, Angew. Chem., Int. Ed. 2012, 51, 3649-3652.
L. Huang, J. Zhao, ‘C 60-Bodipy dyad triplet photosensitizers as
organic photocatalysts for photocatalytic tandem oxidation/[3+
2] cycloaddition reactions to prepare pyrrolo [2, 1-a]
isoquinoline’, Chem. Commun. 2013, 49, 3751-3753.
Q. Xia, H. Tian, J. Dong, Y. Qu, L. Li, H. Song, Y. Liu, Q. Wang,
‘N‐Arylamines Coupled with Aldehydes, Ketones, and Imines
by Means of Photocatalytic Proton‐Coupled Electron
Transfer’, Chem. - Eur. J. 2018, 24, 9269-9273.
X. Meng, Y. Wang, B. Chen, G. Chen, Z. Jing, P. Zhao, ‘OMS-
2/H2O2/Dimethyl Carbonate: An Environmentally-Friendly
Heterogeneous Catalytic System for the Oxidative Synthesis of
Benzoxazoles at Room Temperature’, Org. Process Res. Dev.
2017, 21, 2018-2024.
[102]
[103]
[104]
M. Cherest, X. Lusinchi, ‘Reaction des nitrones avec les
chlorures d'acides: action de chlorures d'aryl-sulfonyles et du
chlorure de benzoyle sur des n-oxy (aryl-1 dihydro-3, 4
12
This article is protected by copyright. All rights reserved.

10.1002/hlca.202000184
Helvetica Chimica Acta
HELVETICA
Entry for the Table of Contents
((Insert TOC Graphic here; max. width: 17.5 cm; max. height: 7.0 cm))
Twitter
Organoredox radical-cation-based catalysts achieved their next evolutionary step: molecular and polymeric phenazines showed excellent
performances in cross-dehydrogenative aza-Henry couplings
13
This article is protected by copyright. All rights reserved.