Semi-heterogeneous Dual Nickel/Photocatalysis using Carbon Nitrides: Esterification of Carboxylic Acids with Aryl Halides

Article abstract of DOI:10.1002/anie.201902785

Cross-coupling reactions mediated by dual nickel/photocatalysis are synthetically attractive but rely mainly on expensive, non-recyclable noble-metal complexes as photocatalysts. Heterogeneous semiconductors, which are commonly used for artificial photosynthesis and wastewater treatment, are a sustainable alternative. Graphitic carbon nitrides, a class of metal-free polymers that can be easily prepared from bulk chemicals, are heterogeneous semiconductors with high potential for photocatalytic organic transformations. Here, we demonstrate that graphitic carbon nitrides in combination with nickel catalysis can induce selective C?O cross-couplings of carboxylic acids with aryl halides, yielding the respective aryl esters in excellent yield and selectivity. The heterogeneous organic photocatalyst exhibits a broad substrate scope, is able to harvest green light, and can be recycled multiple times. In situ FTIR was used to track the reaction progress to study this transformation at different irradiation wavelengths and reaction scales.

Full text of DOI:10.1002/anie.201902785

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Semi-Heterogeneous Dual Nickel/Photo-catalysis using Carbon
Nitrides: Esterification of Carboxylic Acids with Aryl Halides
Authors: Bartholomäus Pieber, Jamal A. Malik, Cristian Cavedon,
Sebastian Gisbertz, Aleksandr Savateev, Daniel Cruz, Tobias
Heil, Guigang Zhang, and Peter H. Seeberger
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902785
Angew. Chem. 10.1002/ange.201902785
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Semi-Heterogeneous Dual Nickel/Photocatalysis using Carbon
Nitrides: Esterification of Carboxylic Acids with Aryl Halides
Bartholomäus Pieber,[a]* Jamal A. Malik,[a] Cristian Cavedon,[a] Sebastian Gisbertz,[a] Aleksandr
Savateev,[b] Daniel Cruz,[b] Tobias Heil,[b] Guigang Zhang[b] and Peter H. Seeberger[a]
Abstract: Cross-coupling reactions via dual nickel/photocatalysis
are synthetically attractive but rely mainly on expensive, non-
recyclable noble metal complexes as photocatalysts. Heterogeneous
semiconductors, commonly used for artificial photosynthesis and
visible area (band gap <2.7 eV; onset of absorption: <450-460
nm). g-CN polymers are easy to synthesize from commodity
chemicals, and exhibit a high thermal and chemical stability. The
band gap and position of the valence and conduction band
depend on several factors such as the C/N ratio, the
polymerization degree, or the crystallinity, all of which can be
tailored via the synthetic approach.[9a]
Here, we describe the application of g-CN materials in dual
nickel/photocatalysis in a semi-heterogeneous catalytic system
(Scheme 1).
wastewater treatment, are
a sustainable alternative. Graphitic
carbon nitrides, a class of metal-free polymers that can be easily
prepared from bulk chemicals, are heterogeneous semiconductors
with high potential for photocatalytic organic transformations. Here,
we demonstrate that graphitic carbon nitrides in combination with
nickel catalysis can induce selective C–O cross-couplings of
carboxylic acids with aryl halides, yielding the respective aryl esters
in excellent yield and selectivity. The organic semiconductor exhibits
a broad substrate scope, is able to harvest green light, and can be
recycled multiple times. In situ FTIR was used to track the reaction
progress to study this transformation at different irradiation
wavelengths and reaction scales.
Transition-metal catalyzed cross-coupling reactions are key
transformations in modern organic synthesis.[1] Numerous
elegant and robust methods relying primarily on palladium
catalysts to form carbon–carbon[2] and carbon–heteroatom[3]
bonds exist. High economic and environmental cost of precious
metal catalysts renders the catalytic systems not viable in the
long-term. Nickel, an attractive alternative to rare metals, is
capable of catalyzing many of the same transformations as
palladium.[4] The combination of nickel and photoredox catalysis
opened up new avenues for cross-coupling chemistry.[5] Still,
ruthenium or iridium photocatalysts (PC) are required to turn
over the nickel species. Photoactive complexes with earth
abundant metals[6] or organic dyes[7] are less efficient or prone to
degradation.
Heterogeneous semiconductors are promising alternatives given
their ease of preparation and straightforward recycling strategies
by filtration or centrifugation.[8] Graphitic carbon nitrides (g-CN),
a class of metal-free polymers, are among the most potent
materials for heterogeneous photocatalysis.[9] Unlike the most
widely studied semiconductor TiO2 (band gap ~3.2 eV; onset of
absorption: 380-390 nm), g-CN materials absorb light in the
Scheme 1. Homogeneous versus semi-heterogeneous dual Ni/photocatalysis.
Carbon nitride materials can catalyze the esterification between
N-(tert-Butoxycarbonyl)-proline (Boc-Pro-OH) and methyl 4-
iodobenzoate using white LED (RGB) irradiation (Table 1).
Mesoporous graphitic carbon nitride (mpg-CN),[10] a modified
carbon nitride derived from a cyanuric acid/melamide/barbituric
acid complex (CMB0.05-CN),[11]
a
sulfur-doped material
(CNS600),[12] and the strongly oxidizing potassium poly(heptazine
imide) (K-PHI),[13] gave 12-15% of ester 1. A carbon nitride
derivative prepared via co-condensation of urea and oxamide
followed by post-calcination in a molten salt (CN-OA-m)[14]
showed the highest activity in the photocatalyst screening,[15]
presumably due to its enhanced optical absorption in the visible
region compared to most other known CN materials (Supporting
Information, Figure S1). The absence of any detectable amounts
of the corresponding decarboxylative C–C coupling product 2[16]
indicates a selective photosensitization rather than single-
electron transfer process.[17] This is in stark contrast to the usual
reactivity of carbon nitride materials which are reported to follow
single electron transfer (photoredox) pathways.[9b]
[a]
Dr. B. Pieber, Dr. J. A. Malik, C. Cavedon, S. Gisbertz, Prof. Dr. P.
H. Seeberger
Department of Biomolecular Systems
Max-Planck-Institute of Colloids and Interfaces
Am Mühlenberg 1, 14476 Potsdam, Germany
E-mail: bartholomaeus.pieber@mpikg.mpg.de
[b]
Dr. A. Savateev, D. Cruz, Dr. T. Heil, Dr. G. Zhang
Department of Colloid Chemistry
A
systematic evaluation of all other reaction parameters
indicated that a cocktail consisting of CN-OA-m (3.33 mg mL-1),
NiCl2·glyme, 4,4-di-tert-butyl-2,2-dipyridyl (dtbbpy), and N-tert-
butylisopropylamine (BIPA) in dimethyl sulfoxide (DMSO) is
Max-Planck-Institute of Colloids and Interfaces
Am Mühlenberg 1, 14476 Potsdam, Germany
Supporting information for this article is given via a link at the end of
the document
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Table 1. Screening of potential carbon nitride semiconductors. [a]
Table 2. Optimized conditions and control experiments[a]
Entry
CN catalyst
CN-OA-m
CNS600
1 [%][b]
22
2 [%][b]
n.d.[c]
n.d.
Entry
Conditions
1
3
4
[%][b]
[%][b]
[%][b]
1
2
3
4
5
1
2
3
4
5
6
7
8
9
as shown
Ir(ppy)3 (1mol%) instead of CN-OA-m
Ni(OAc)·4H2O instead of NiCl2·glyme[c]
4-Methyl bromobenzoate as substrate
No CN-OA-m
96
97
trace
trace
trace
5
2
1
18
mpg-CN
K-PHI
15
n.d.
85
4
14
n.d.
68
2
CMB0.05-CN
12
n.d.
n.d.[c]
n.d.
10
n.d.
2
n.d.
1
[a] Reaction conditions: methyl 4-iodobenzoate (0.3 mmol), Boc-Pro-OH (0.45
mmol), NiCl2·glyme (10 mol%), dtbbpy (10 mol%), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU, 0.45 mmol), CN catalyst (10 mg),
DMSO (anhydrous, 3 mL), white LEDs at 40 °C for 14 h. [b] Determined by
1H-NMR using 1,3,5-trimethoxybenzene as internal standard. [c] not detected.
No NiCl2·glyme
No dtbbpy
3
2
No light
n.d.
3
n.d.
trace
n.d.
n.d.
No base
particularly suited, obtaining the desired ester 1 in 96% after 14
h irradiation (Table 2, Entry 1).[15] The only side products were
small amounts of the dehalogenated methylbenzoate 3 and the
corresponding phenol 4, which either originates from
etherification with residual water[18] or ester hydrolysis. The
selectivity is identical to the homogeneous protocol using
Ir(ppy)3 (Entry 2). Inexpensive Ni(OAc)2 ·4H2O shows similar
catalytic activity (Entry 3), but an additional side product (methyl
4-acetoxybenzoate, 6%) resulted from the esterification of the
aryl iodide with the acetate anion of Ni(OAc)2·4H2O. The method
also selectively converts the corresponding bromide to the
desired product 1, albeit with lower efficiency (Entry 4). Control
experiments showed that the reaction does not occur in the
absence of carbon nitride, nickel catalyst, or light; and just small
amounts of the desired product were observed in absence of
ligand (10%) or base (3%).
The synthetic versatility of the catalytic system was evaluated
using both nickel catalysts (Scheme 2).[15] The anhydrous
NiCl2·glyme gave best results with 3 equiv of BIPA whereas
inexpensive Ni(OAc)2·4H2O necessitated a higher excess of the
base (Supporting Information, Table S7 & Figure S6). Both
systems work well with electron-deficient aryl iodides (1, 5-15)
whereas substrates lacking an electron-withdrawing group show
low reactivity (27, 28). A broad range of functional groups
including esters (1, 10, 13), nitriles (5, 14), ketones (6, 15),
aldehydes (8), and boronic acid pinacol esters (9) were tolerated
under the conditions applied. Para-substituted aryl iodides (1, 5,
6) react significantly faster than their meta analogues (13-15).
Although substituents in the ortho-position are tolerated (10), low
reactivity for 2-iodobenzonitrile (26) was observed. Carbonyl
groups in the 2-position (31, 33) do not give the corresponding
ester product. We assume that, after the initial oxidative addition
of the aryl halide, coordination of the carbonyl oxygen to the
metal center[19] hinders association of the carboxylate
[a] Reaction conditions: methyl 4-iodobenzoate (0.3 mmol), Boc-Pro-OH (0.45
mmol), NiCl2·glyme (10 mol%), dtbbpy (10 mol%), BIPA (0.9 mmol), CN-OA-m
(10 mg), DMSO (anhydrous, 3 mL), white LEDs at 40 °C for 14 h. [b]
Determined by 1H-NMR using 1,3,5-trimethoxybenzene as internal standard.
[c] 5 equiv BIPA were used. [d] not detected.
nucleophile. 1-Bromo-4-iodobenzene gave the desired product
11 without any detectable amount of the 1,4-diester, as the iodo-
group reacts significantly faster and the resulting esterification
product deactivates the aryl bromide towards
a second
esterification. With respect to the carboxylic acid coupling
partner, a wide array of substrates can be efficiently coupled,
including aliphatic (16, 19, 21, 22), olefinic (17), and benzylic (20,
24) carboxylic acids, as well as benzoic acid derivatives (18, 23).
The esterification of Biotin (25) shows the potential of the
presented methodology for conjugation purposes. Artesunate
(35), acetylated sialic acid (36) as well as the unprotected amine
34 did not give the desired ester under these reaction conditions.
In all cases, NiCl2·glyme resulted in a significantly higher
selectivity than Ni(OAc)2·4H2O partly due to coupling of the
acetate anion with the aryl iodide (up to 22%) and partly from the
formation of higher quantities of the dehalogenated and phenol
side products (Supporting Information, Table S14 & S15).
A major advantage of carbon nitride catalysis is the potential to
reuse the heterogeneous material.[20] With dual nickel/carbon
nitride catalysis, however, deposition of Ni on the
semiconducting material is possible and may alter its
photocatalytic properties.[9a] As such, we sought to determine
whether the CN-OA-m material is recyclable in our catalytic
system (Figure 1). CN-OA-m was recovered after each reaction
by centrifugation, washed, and used in the next reaction by
adding fresh NiCl2·glyme and dtbbpy. The material proved to be
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Scheme 2. Scope of the semi-heterogeneous esterification of carboxylic acids with aryl iodides.
reusable without any loss of the catalytic activity over three
cycles. Thereafter, a reduction in the yield of 1 from 96 to ~80%
was noticed. This may result from a reduced availability of active
catalytic sites for the photocatalytic step due to deposited Ni
species,[15] but could be also rationalized by the loss of small
amounts of the photocatalyst during the series of experiments.
Analysis of the heterogeneous catalyst before and after a
reaction using X-ray powder diffraction (XRD) showed the same
characteristic peaks at 8° and 28° for CN-OA-m without any
obvious Ni species in case of the recovered semiconductor
(Figure 2, A). Similarly, identical FTIR (Figure 2, B) and UV/Vis
spectra (Figure 2, C) were observed for the recovered and fresh
CN-OA-m catalyst, proofing that the photocatalytic properties
are not changing. The morphology of CN-OA-m shows a porous
texture that was not altered during the catalytic transformation
(Figure 2, E & F). As highly dispersed Ni species are potentially
undetectable by XRD,[21] X-ray photoelectron spectroscopy
(XPS) detected Ni in the recovered material (Figure 2, D). The
high resolution XPS Ni2 p3/2 spectrum of the recovered CN-OA-
m material shows two main deconvoluted peaks located at
853.7(±0.02) eV and 852.5 (±0.02) eV that can be assigned to
the binding energy of Ni2+ and Ni0 species, respectively
(Supporting Information, Figure S13). Nickel deposition was
further confirmed by ICP-OES analysis showing a Ni
Figure 1. Reusability of CN-OA-m in the dual nickel/photocatalytic
esterification of methyl 4-iodobenzoate with Boc-Pro-OH.
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Figure 2. Analysis of new and recovered CN-OA-m by powder XRD (A), FTIR (B), UV/Vis (C) and XPS (D) spectroscopy as well as SEM analysis (E & F).
HAADF-STEM image (G) of nickel particles (bright spots) attached to the recovered CN-OA-m.
concentration of 1.4 % w/w suggesting that 5-8% of the
homogeneous nickel catalyst was deposited on the organic
semiconductor.[15] Scanning transmission electron microscopy
(STEM) was used to visualize nickel particles on the surface of
the recovered CN-OA-m (Figure 2, G).
3B, grey), showcasing that the heterogeneous catalyst has not
only the same selectivity (Table 2) but also comparable
efficiency with the state-of-the-art homogeneous noble metal
photocatalysts. Although slower than the aryl iodide, the bromide
(Figure 3B, purple) furnished the product in 88% yield within 48
To determine whether the immobilized nickel is catalytically
active in the model reaction, the esterification was carried out
with the recovered CN-OA-m material from one and five reaction
cycles in the absence of additional NiCl2·glyme. Both
experiments resulted in no more than ~6% ester 1, clearly
indicating that the adsorbed, low-valent Ni species do not serve
as effective catalytic species.[15]
h (Supporting Information, Figure S9). Scale-up of the
esterification from 0.3 mmol to 3.0 mmol (Figure 3B, orange) is
facile, affording the desired ester 1 in 96% isolated yield on
gram scale in roughly six instead of three hours using this setup.
As this scaled-up reaction has the same concentration of all
components, the longer reaction time is mostly due to Beer–
Lambert limitations inherent in using a larger reaction vessel
(Supporting Information, Figure S8).
To study the scalability of the photocatalytic system and to
determine if the photocatalytic system can harvest energy from
varying light sources, a real-time monitoring strategy was
developed.[15] Although NMR is often an ideal choice for kinetic
analysis and has been used to track photocatalytic
transformations,[22] the presence of a heterogeneous component
that broadens peaks, and the potential involvement of
paramagnetic nickel species precluded its use. In situ FTIR
analysis eliminates any interference from the heterogeneous
photocatalyst,[23] and provides high-fidelity reaction tracking
under normal reaction conditions. Observing the reaction under
optimized reaction conditions, an incipient peak at ~1764 cm-1
(Figure 3, A), separable from the other two carbonyl stretches in
the reaction mixture, was observed. This C=O stretch from the
product’s internal ester is heavily blue-shifted from the
corresponding acid starting material. This peak served as a
competent measure of reaction progress (Supporting
Information, Figure S8).
The in situ method was also employed to assess the reaction’s
progress with different light settings from the same LED strips
(Figure 3, C). Blue light (~400 – 500 nm) catalyzes the reaction
to completion, albeit slower, as the energy from light above 500
nm is not available (Supporting Information, Figure S3). Red
light (600 – 650 nm), and a control experiment in the dark,
afforded no meaningful amount of product. Encouragingly, green
light (460 – 600 nm) begets partial completion in an overnight
reaction, affirming that the heterogeneous photocatalyst is able
to use a wide spectrum of visible light (Supporting Information,
Figure S1). After eight days, the reaction gave 72%
(Supporting Information, Figure S12).
1
In conclusion, it was shown that a homogeneous nickel catalyst
can be combined with a heterogeneous, metal-free carbon
nitride semiconductor for dual catalysis. The insoluble
photocatalyst can be recycled multiple times. Two nickel
catalysts were studied in detail for a broad range of substrates,
with NiCl2·glyme showing a significantly higher selectivity than
Ni(OAc)2·4H2O, which nonetheless resulted in moderate to good
yields of the desired esters. The organic semiconductor is able
After a brief induction period, the catalysis reaches its maximum
rate quickly and proceeds until completion (Figure 3B, black). A
highly similar kinetic profile was observed using Ir(ppy)3 (Figure
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to harvest a broad range of the visible light spectrum (up to ~600
nm) as shown by in situ FTIR analysis. The FTIR reaction
monitoring strategy was used, to the best of our knowledge, for
the first time to study photocatalytic transformations and is
essential to analyze reaction progress kinetics of photocatalysis
to better understand the underlying mechanisms. The
inexpensive heterogeneous materials are an efficient and
sustainable alternative to noble-metal complexes in
photocatalysis.
Acknowledgements
We gratefully acknowledge the Max-Planck Society for generous
financial support. B.P. acknowledges financial support by a
Liebig Fellowship of the German Chemical Industry Fund (Fonds
der Chemischen Industrie, FCI). S.G. and B.P. acknowledge the
International Max Planck Research School on Multiscale Bio-
Systems for financial support. We thank our colleagues Markus
Antonietti, Martin Oschatz, Tobias Schmidt, Cliff Janiszewski,
Rona Pitschke, Heike Runge, Katharina Otte (all MPIKG), Mat-
thew Mower (Janssen Pharmaceutical), and Dario Cambié
(Eindhoven University of Technology) for scientific, technical and
analytical support.
Keywords: graphitic carbon nitride • photocatalysis •
heterogeneous catalysis • metallaphotoredox • dual catalysis
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Figure 3. In situ reaction tracking of dual catalytic esterification. A, The
internal ester peak at ~1764 cm-1 serves as a measure of reaction progress. B,
Small-scale (0.3 mmol substrate) with the aryl iodide (black) and bromide
(purple) as well as a gram-scale reaction (3.0 mmol methyl 4-iodobenzoate,
orange) were easily tracked in situ. A comparison with Ir(ppy)3 as PC shows a
similar kinetic profile (grey). C, White light (black) and blue light (blue) catalyze
the reactions quickly, while green light (green) also shows activity.
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B. Pieber,* J. A. Malik, C. Cavedon, S.
Gisbertz, A. Savateev, D. Cruz, T. Heil,
G. Zhang, P. H. Seeberger
Page No. – Page No.
Semi-Heterogeneous Dual
Nickel/Photocatalysis using Carbon
Nitrides: Esterification of Carboxylic
Acids with Aryl Halides
A graphitic carbon nitride in combination with nickel catalysis can induce selective
C–O cross-couplings. The organic semiconductor exhibits a broad substrate scope,
is able to harvest green light, and can be recycled multiple times.
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