Mizoroki-Heck type reactions and synthesis of 1,4-dicarbonyl compounds by heterogeneous organic semiconductor photocatalysis

Article abstract of DOI:10.1039/d0gc03792c

We report the synthesis of 1,4-dicarbonyl compounds and substituted alkenes (Mizoroki-Heck type coupling) starting from secondary and tertiary alkyl halides and vinyl acetate or styrene derivatives using visible-light photocatalysis. The protocol uses mesoporous graphitic carbon nitride (mpg-CN) as a heterogeneous organic semiconductor photocatalyst and Ni(ii) salts as Lewis acid catalysts. Detailed post-characterization of the heterogeneous material has been carried out to support the proposed catalytic cycle. Apart from high functional-group tolerance, mild reaction conditions, scalability as well as easy recovery and reuse of the mpg-CN photocatalyst provide a practical solution to these widespread transformations in terms of sustainability and efficiency and this methodology is recommended for applications in academic and industrial synthesis.

Full text of DOI:10.1039/d0gc03792c

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Mizoroki–Heck type reactions and synthesis of
1,4-dicarbonyl compounds by heterogeneous
organic semiconductor photocatalysis†
Cite this: Green Chem., 2021, 23,
2017
b
Jagadish Khamrai, a Saikat Das, a Aleksandr Savateev, b Markus Antonietti
a
and Burkhard König
*
We report the synthesis of 1,4-dicarbonyl compounds and substituted alkenes (Mizoroki–Heck type
coupling) starting from secondary and tertiary alkyl halides and vinyl acetate or styrene derivatives using
visible-light photocatalysis. The protocol uses mesoporous graphitic carbon nitride (mpg-CN) as a
heterogeneous organic semiconductor photocatalyst and Ni(II) salts as Lewis acid catalysts. Detailed post-
characterization of the heterogeneous material has been carried out to support the proposed catalytic
cycle. Apart from high functional-group tolerance, mild reaction conditions, scalability as well as easy
recovery and reuse of the mpg-CN photocatalyst provide a practical solution to these widespread trans-
formations in terms of sustainability and efficiency and this methodology is recommended for appli-
cations in academic and industrial synthesis.
Received 9th November 2020,
Accepted 18th January 2021
DOI: 10.1039/d0gc03792c
rsc.li/greenchem
α-photoalkylation of β-ketocarbonyls by merging ruthenium-
based homogeneous photoredox catalysis and primary amine
Introduction
Highly substituted 1,4-dicarbonyl motifs are found in a wide catalysis has also been reported (Scheme 1a).13–15 However,
variety of biologically active natural products and pharma- most of these methods suffer from the use of expensive tran-
ceutical agents,1 and the development of novel methods for
their synthesis continues to be an active research area.2
Classical methods for the synthesis of such compounds
involve the Michael type addition of acyl anion synthons to
α,β-unsaturated carbonyl compounds or nucleophilic substi-
tution between enolates and α-haloketones.3 However, these
reactions lack general applicability due to limited functional
group tolerance caused by the harsh reaction conditions and
reactive intermediates.4 Lately, alternative routes such as oxi-
dative cross-coupling of enolates,5 enamines,7 and enol
silanes6 have been reported, but most of them suffer from the
use of stoichiometric amounts of toxic oxidants as well as com-
peting homocoupling reactions.8 Very recently, the application
of visible light photocatalysis has offered the synthesis of such
compounds under milder reaction conditions. Among them,
visible light induced generation of acyl radicals followed by
Giese-type addition to α,β-unsaturated carbonyl compounds is
an effective synthetic route to 1,4-dicarbonyl compounds.9–12
Along these lines, direct construction of 1,4-dicarbonyls via
aFakultät für Chemie und Pharmazie, Universität Regensburg, 93040 Regensburg,
Germany. E-mail: burkhard.koenig@ur.de
bDepartment of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces,
Research Campus Golm, 14424 Potsdam, Germany
Scheme 1 Mizoroki–Heck type coupling and synthesis of the 1,4-
dicarbonyl compound using organic semiconductor visible light photo-
redox catalysis.
†Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc03792c
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sition metal photocatalysts, which limits their practical appli- materials, and the polymeric material is stable towards reactive
cation specially in larger scale synthesis. On the other hand, radicals or nucleophiles. It possesses a suitable bandgap
Mizoroki–Heck reaction is one of the most useful cross-coup- between the valence band maxima (VBM) and conduction
ling reactions in organic synthesis which is mostly catalyzed band minima (CBM),23 which allows the use of photoexcited
by expensive transition metals, such as palladium.16,17
mpg-CN for controlled oxidation and reduction of many sub-
Many applications have been found in visible light photoca- strates. Very recently, the use of carbon nitride-based hetero-
talysis complementing classical methods for the preparation geneous photocatalysts in several synthetic transformations
of substituted olefins, for example addition of alkyl radicals has been demonstrated by Seeberger and Reisner groups.25,26
from alkyl halides18,19 or N-hydroxyphthalimide ester21 precur- However, intermolecular addition of radicals to olefins using a
sors to styrene derivatives using photoexcited palladium(0) heterogeneous photocatalyst has not been thoroughly explored
complexes20 under visible light irradiation (Scheme 1b). until today. Herein we report the use of an mpg-CN organic
However, most of these methods are constrained by the use of semiconductor as a sustainable heterogeneous photocatalyst
expensive metal catalysts and molecular photocatalysts. We to promote the reaction between alkyl halides and vinyl acetate
describe a divergent, practicable and sustainable route for the or styrene derivatives under visible light for the synthesis of
synthesis of 1,4-dicarbonyl and Mizoroki–Heck type of cross 1,4-dicarbonyl compounds and the products of Mizoroki–Heck
coupling products using heterogeneous organic semi- type reactions, respectively (Scheme 1c).
conductor photocatalysis namely the use of mesoporous gra-
phitic carbon nitride (mpg-CN).
We recently reported mpg-CN as a purely organic semi-
Results and discussion
conductor photocatalyst capable of performing many organic
transformations under diverse reaction conditions.22,24 We explored the reactivity of mpg-CN as a heterogeneous
mpg-CN can easily be synthesized using inexpensive starting organic semiconductor first by the synthesis of 1,4-dicarbonyl
Table 1 Optimization of reaction conditions and control reactionsa
Entry
Photocatalyst
Additive
Yieldb
1
2
3
4
mpg-CN
mpg-CN
mpg-CN
—
—
29%
79%
NiBr2 (5 mol%)
NiBr2 (5 mol%)
NiBr2 (5 mol%)
0% (in the dark)
1% (without mpg-CN)
5
6
7
8
mpg-CN
mpg-CN
Recovered mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
mpg-CN
Na-PHI
NiBr2 (1.25 mol%)
NiBr2·glyme (1.25 mol%)
—
43%
86% (83%)c
74%
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
Cu(OTf)2 (1.25 mol%)
In(OTf)3 (1.25 mol%)
Sc(OTf)3 (1.25 mol%)
Zn(OTf)2 (1.25 mol%)
AlCl3 (5 mol%)
Y(OTf)3 (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
NiBr2·glyme (1.25 mol%)
59% (under air)
49%d
70%e
83%f
37%g
72%h
38%
43%
40%
42%
38%
30%
31%
38%
44%
22%
69%
59%
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
K-PHIi
CN-ATZ-NaK
K-PHIj
Mn-PHI
H-PHI
a The reaction was performed using 0.2 mmol of 1-(p-tolyl)vinyl acetate, 1.25 equiv. of 2,6-lutidine as a base and 1 ml of DMF as a solvent under
455 nm blue LED irradiation for 24 h. b GC yields were determined using 1,3,5-trimethoxybenzene as an internal standard. c Isolated yield.
d Without 2,6-lutidine. e DMSO was used as the solvent. f DMA was used as the solvent. g Dioxane was used as the solvent. h ACN was used as the
solvent. i K-PHI (prepared from 5-aminotetrazole in the LiCl/KCl eutectic mixture using mechanochemical pretreatment of reagents). j K-PHI (pre-
pared from 5-aminotetrazole in the LiCl/KCl eutectic mixture using 0.5 wt% of K-PHI nanoparticles as nucleation seeds).
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compounds. For this purpose, we chose vinyl acetate, easily
synthesized from stable and commercially available starting
materials and different alkyl bromides. We began our investi-
gation with 1-(p-tolyl)vinyl acetate (1a) and α-bromo-
γ-butyrolactone (1.5 equivalent) (2a) as model substrates and
mpg-CN as a heterogeneous photocatalyst in the presence of
2,6-lutidine as a base. When the reaction mixture in DMF was
irradiated using a 455 15 nm (Imax = 1000 mA, 1.12 W) blue
LED for 24 h under nitrogen, the corresponding 1,4-dicarbonyl
product 3a was obtained in 29% GC yield (Table 1, entry 1).
The use of a catalytic amount of NiBr2 increased the product
yield to 79% (entry 2). Control reactions in the absence of light
or mpg-CN confirmed their essential roles in the photo-
catalytic reaction (entries 3 and 4). While 1.25 mol% of NiBr2
decreased the yield to 43% (entry 5), the use of 1.25 mol%
NiBr2·glyme provided the best result and the desired product
was obtained in 83% isolated yield (entry 6, GC yield: 86%).
When the reaction was performed using recovered mpg-CN
without adding any Ni source, compound 3a was formed in
74% yield (entry 7), which indicates that the amount of Ni de-
posited on the surface of the mpg-CN is sufficient to accelerate
the reaction in the 2nd cycle.
In the 3rd cycle, 46% product formation was observed
without adding Ni, indicating that the Lewis acid is necessary
to drive the reaction. The reaction becomes sluggish when per-
formed in the presence of air (entry 8). Moreover, the reaction
performed in the absence of 2,6-lutidine provides a dimin-
ished product yield27 (entry 9) along with the formation of
4-methyl acetophenone as a major side product. This is due to
the generation of HBr during the reaction, which deprotects
the acetate group leading to the formation of keto carbonyl
compounds. Other commonly used solvents such as DMSO,
dioxane, and ACN resulted in lower yield of the desired
Scheme 2 Synthesis of 1,4-dicarbonyl compounds using various vinyl
acetate derivatives and α-bromo-γ-butyrolactone in the presence of
mpg-CN as a heterogeneous photocatalyst. Standard conditions: Vinyl
acetate (0.2 mmol, 1.0 equiv.), α-bromo-γ-butyrolactone (49.5 mg,
0.3 mmol, 1.5 equiv.), mpg-CN (10.0 mg), NiBr2·glyme (0.7 mg,
0.0025 mmol, 0.0125 equiv.), 2,6-lutidine (29 µL, 0.25 mmol, 1.25 equiv.)
in DMF (1 mL), under a nitrogen atmosphere for 24 h. Similar conditions
were used for gram scale reactions on a 6 mmol scale; for more details,
see the ESI.†
product compared to DMF except DMA (entries 10–13). Other This reaction is not only useful for the enol-acetates substi-
Lewis acids as additives failed to improve the yield and the for- tuted with an aromatic ring, but also for the aliphatic enol-
mation of 4-methyl acetophenone as a major side product was acetate. Successful examples of this type 3h and 3i were
observed (entries 14–19). Finally, the use of other modified obtained by the reaction among dihydronaphthalene acetate,
carbon nitrides such as Na-PHI,28a K-PHI,28b CN-ATZ-NaK,28c isopropenyl acetate and α-bromo-γ-butyrolactone, respectively.
K-PHI,28d Mn-PHI,28e and H-PHI28e did not increase the
After screening several enol-acetates we switched our focus
product yield compared to mpg-CN (entries 20–25).
to the exploration of the utilization of different alkyl bromides
With the optimized reaction conditions in hand, we (Scheme 3). We chose the 1-(p-tolyl)vinyl acetate (1a) as a
explored the scope of this reaction using different vinyl acet- model substrate to study the reaction of other alkyl bromides.
ates as substrates as shown in Scheme 2.
Treatment of α-bromo-γ-butyrolactone containing a methyl
Notably, the side product 3a′ was formed in 12% isolated substituent with 1a under the standard reaction conditions
yield along with the desired compound 3a. Unsubstituted vinyl afforded the desired product 3j in 70% isolated yield. Methyl-
acetate gave the desired product 3b in 66% isolated yield. 2-bromo-2-methyl propanoate also provided the corresponding
Enol-acetate functionalized with a highly electron rich aro- compound 3k in 52% isolated yield, while diethyl-2-bromo-2-
matic ring delivered product 3c in 79% isolated yield. methylmalonate converted to compound 3l in 62% isolated
Moreover, the halogen substituent on the aromatic-ring did yield. Here it is noteworthy to mention that particularly for
not alter the outcome of the reaction. Iodo and fluoro substitu- this bromide substrate a considerable amount of background
ents at the para-position of the aromatic ring provided the reaction was observed in GC when the reaction was performed
desired products 3d and 3e in 87% and 62% isolated yields, without a catalyst but in the presence of light. Ethyl-2-bromo-
respectively. Electron withdrawing substituents on the aro- 2-phenylacetate gave compound 3m in low yield along with the
matic ring are tolerated well as shown by a cyano substituent recovered starting material. The synthesis of 1,4-dicarbonyl
at meta and para positions, which gave the corresponding pro- compounds bearing all-carbon quaternary stereocenters is
ducts 3f and 3g in 67% and 64% isolated yields, respectively. another commendable feature of the reaction. For example,
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Scheme 4 Mizoroki–Heck type reactions of olefins and alkyl bromides
using mpg-CN as a heterogeneous photocatalyst. Standard conditions:
4-methoxystyrene (26.8 mg, 0.2 mmol, 1.0 equiv.), alkyl bromide
(0.4 mmol, 2.0 equiv.), mpg-CN (10.0 mg), NiBr2·glyme (0.7 mg,
0.0025 mmol, 0.0125 equiv.), DMF (1 mL), under a nitrogen atmosphere
for 48 h. Similar conditions were used for gram scale reactions on a
6 mmol scale; for more details, see the ESI.†
ratio, while in the absence of a base the desired compound 5a
was obtained in 77% isolated yield as a single regioisomer.
The reaction conditions for Mizoroki–Heck type coupling
required only mixing of styrene, alkyl bromide (2.0 equiv.), and
1.25 mol% NiBr2·glyme in the presence of mpg-CN and
irradiation under nitrogen for 48 h. Variation of the alkyl bro-
mides afforded the corresponding products 5b–5j in moderate
to good isolated yields. The reaction of α-bromo-γ-butyrolactone
with 4-acetoxystyrene and 4-fluorostyrene gave the expected pro-
ducts (5k–5l) demonstrating the applicability of the protocol.
This reaction remained effective for a gram-scale protocol as
shown by the preparation of 5g and 5h in Scheme 4.
Scheme 3 Synthesis of 1,4-dicarbonyl compounds using various alkyl
bromides in the presence of mpg-CN as a heterogeneous photocatalyst.
Standard conditions: 1-(p-tolyl)vinyl acetate (35.2 mg, 0.2 mmol, 1.0
equiv.), alkyl bromide (0.3 mmol, 1.5 equiv.), mpg-CN (10.0 mg),
NiBr2·glyme (0.7 mg, 0.0025 mmol, 0.0125 equiv.), and 2,6-lutidine
(29 µL, 0.25 mmol, 1.25 equiv.) in DMF (1 mL), under a nitrogen atmo-
sphere for 24 h.
bromo-compounds bearing a cyclobutane or cyclohexane ring
Apart from operational simplicity, one of the main advan-
delivered 1,4-dicarbonyl products 3n and 3p. Similarly, the all- tages of our protocol is the use of the heterogeneous mpg-CN
carbon quaternary stereocenters also formed when 3-bromo-3- photocatalyst, which can be easily recovered from the reaction
methyldihydrofuran-2(3H)-one was treated with 1a to provide mixture, even from gram-scale reactions, via simple filtration
an inseparable mixture of 3q and 3q′ combined in 68% iso- or centrifugation. The heterogeneous nature and the remark-
lated yield. Interestingly, 2-bromo-propionitrile also takes part able stability of mpg-CN under the photochemical reaction
in the reaction and gives the desired compound 3r bearing a conditions allow the reuse of the material several times
keto and a cyano group, useful for follow-up chemistry.
without the loss of the photocatalyst reactivity or decrease of
Interestingly, addition of the alkyl radicals to styrene deriva- the yield of the desired product. As shown in Fig. 1 the photo-
tives instead of vinyl acetate produced Mizoroki–Heck type catalyst can be recycled at least six times, and the rates of the
coupling products under the previously optimized reaction photochemical reactions remain the same over four catalytic
conditions.
Under standard reaction conditions the reaction between
cycles.
After the photocatalytic experiments, mpg-CN was washed
4-methoxystyrene and α-bromo-γ-butyrolactone (2a) yielded an with different solvents and water, and characterized by the
E : Z mixture of the desired compound 5a in an almost 1 : 1 same set of techniques used for fresh mpg-CN (see the ESI†
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Fig. 1 Catalyst recycling (for six catalytic cycles) and assessment of the reaction rates (for four catalytic cycles). Standard conditions: 1-(p-tolyl)
vinyl acetate (35.2 mg, 0.2 mmol, 1.0 equiv.), α-bromo-γ-butyrolactone (49.5 mg, 0.3 mmol, 1.5 equiv.), mpg-CN (10.0 mg), NiBr2·glyme (0.7 mg,
0.0025 mmol, 0.0125 equiv.), and 2,6-lutidine (29 µL, 0.25 mmol, 1.25 equiv.) in DMF (1 mL), under a nitrogen atmosphere. GC yields were deter-
mined using 1,3,5-trimethoxybenzene as an internal standard.
for details). Thus, the position and intensity of all peaks mission electron microscopy (TEM) images confirmed that the
observed in the attenuated total reflectance Fourier-transform mesoporous structure of mpg-CN is retained after the photo-
infrared (ATR FT-IR) spectrum of mpg-CN revealed that the catalytic synthesis (Fig. S12d†). High resolution TEM
bulk chemical structure of the photocatalyst had not changed (HR-TEM, Fig. S12d†) and high angle annular darkfield scan-
(Fig. S2†). Energy Dispersive X-Ray Analysis (EDX) and X-ray ning transmission electron microscopy (HAADF-STEM)
photoelectron spectroscopy (XPS) revealed slightly enhanced (Fig. S12e†) also revealed the absence of Ni(0) agglomerates.
oxygen content (Table S1, S2 and Fig. S3b†) that we explained HAADF-STEM in high resolution confirmed the presence of
by partial hydrolysis of terminal amino-groups on the surface single heavy atom clusters that could be ascribed to Ni
of mpg-CN during work-up washing with water. Inductively (Fig. S12e†) further supporting our hypothesis that Ni atoms
coupled plasma optical emission spectrometry (ICP-OES) are stabilized by chelation by the mpg-CN framework.
revealed that mpg-CN after the photocatalytic tests contained However, the observed single atom clusters could alternatively
0.0268 0.00033 wt% of Ni (Table S3,† entry 2). The detection represent Si derived from the template. The sensitivity of the
limit of nickel by ICP-OES was determined to be >0.0013
technique is too low to support or refute this hypothesis. It
0.00001 wt% (Table S3,† entry 1). In agreement with the should be noted that the deposition of Ni black (1.4–12.6 wt%)
results of ICP-OES, Ni 2p3/2 XPS showed a distinct signal of on the carbon nitride photocatalyst has been observed by
nickel (Fig. S13b†). Due to its low intensity, a precise assign- Pieber et al. in dual nickel/photoredox C–N coupling.26a,29 We
ment of oxidation states is not possible. However, considering explain the difference in nickel content in the recovered photo-
the presence of the complementary signal in high-resolution catalyst found in this work (∼0.03 wt%) and earlier reports (up
Br 3d XPS (Fig. S14b†), it likely arises from Ni(II) coordinated to 13 wt%) mainly by the different structures of the carbon
by the lone pairs of nitrogen rather than agglomerated Ni(0) nitride photocatalysts, covalent mpg-CN (used in this work)
particles. Such Ni(II) species could be considered a precatalyst. and ionic CN-OA-m30 (used by Pieber et al.). Ionic carbon
Taking into account the loading of NiBr2·glyme (0.77 mg) and nitrides contain N-metal moieties in their structure that can
mpg-CN (10 mg) in a typical experiment, <1% of nickel was undergo ion exchange.31,28e In the context of dual Ni/photo-
retained in the mpg-CN after catalyst recovery. Given that our redox catalysis, at the first step, covalent carbon nitrides form
synthetic protocol does not require explicitly added ligands, a chelating 16 electron nickel precatalyst complex (Fig. S15a†),
the mpg-CN framework due to abundant nitrogen atoms, while ionic carbon nitrides coordinate nickel atoms via trans-
might indeed act as a polydentate chelating ligand stabilizing metalation and form a more reactive 14 electron Ni(II)-amide
the Ni(II) precatalyst. In agreement with the hypothesis, Ni in complex (Fig. S15b†).
the recovered mpg-CN is represented by atomically dispersed
Therefore, the difference in the electronic configurations of
species rather than Ni(0) agglomerates. The powder X-Ray the Ni(II) complexes coordinated by carbon nitrides defines
diffraction (PXRD) pattern that is similar to the PXRD pattern their reactivity and tendency to form nickel black. Although
of fresh mpg-CN and does not show peaks that could be the exact mechanistic picture of this transformation remains
assigned to Ni(0) (Fig. S8†). Furthermore, analysis of trans- to be elucidated, we depict a working hypothesis in Fig. 2
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the mpg-CN photocatalyst overcomes some of its shortcomings
in terms of sustainability and efficiency and the method is rec-
ommended for application in academia and industry with
great environmental benefits.
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
This work was supported by the German Science Foundation
(DFG, KO 1537/18-1). This project has received funding from
the European Research Council (ERC) under the European
Union’s Horizon 2020 Research and Innovation Programme
(grant agreement no. 741623). We thank Dr Rudolf Vasold for
GC-MS measurements and Regina Hoheisel for CV measure-
ments. The authors also thank Miss Jiamei Liu at the
Instrument Analysis Center of Xi’an Jiaotong University for her
assistance with XPS analysis, Bolortuya Badamdorj for acquir-
ing TEM images and Dr. Nadezda V. Tarakina for fruitful dis-
cussion of TEM data interpretation.
Fig. 2 Plausible mechanism for the reaction involving
semiconductor.
a mpg-CN
based on the experimental results. The exact role of the nickel
(II) salt is not clear; we speculate that the nickel salt acts as a
Lewis acid to coordinate with the ester group of the alkyl
bromide and thereby lowers the reduction potential of the
alkyl bromide. Alternatively, it may be possible that it could
activate the double bond by coordinating with the olefin as
product formation was observed in the presence of other Lewis
acids. However, reactions involving non-activated alkyl bro-
mides did not yield any product, which ruled out a typical
cross-coupling mechanism. Based on these observations we
believe that mpg-CN under photochemical illumination gener-
ates two-dimensional surface redox centers as electron–hole
pairs. The photogenerated electron effectively reduces the alkyl
bromide and generates the alkyl radical. The alkyl radical adds
to the double bond of vinyl acetate and the resulting radical is
oxidized by the photogenerated hole delivering the carbo-
cation. Successive loss of the acetyl group presumably sup-
ported by nucleophiles present in the reaction medium such
as bromide anions or the solvent yields the corresponding 1,4-
dicarbonyl compounds, whereas the proton loss in the case of
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