Combining organocatalysis with photoorganocatalysis: photocatalytic hydroacylation of asymmetric organocatalytic Michael addition products

Article abstract of DOI:10.1039/c8nj04274h

Organocatalysis and photoorganocatalysis are two areas of synthetic methodology that have found wide applications in organic synthesis. Herein, we report a combination of these two stategies, taking advantage of an organocatalytic Michael addition of α,α-disubstituted aldehydes to maleimides as the first step and a photocatalytic hydroacylation of diisopropyl azodicarboxylate as the second step. Employing an amino acid as the organocatalyst for the asymmetric organocatalytic part and an organic molecule as the photocatalyst, the combination of these two strategies led to the desired products. A number of alkyl- and aryl-substituted maleimides were successfully employed, while the protocol can be used on α,α-disubstituted aldehydes leading to products in moderate to high yields (44-84%) and excellent enantioselectivities (98-100% ee).

Full text of DOI:10.1039/c8nj04274h

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Combining organocatalysis with
photoorganocatalysis: photocatalytic
hydroacylation of asymmetric organocatalytic
Cite this: New J. Chem., 2018,
42, 18844
Michael addition products†
Andriana Schiza, Nikoleta Spiliopoulou, Adelajda Shahu and
Christoforos G. Kokotos
*
Organocatalysis and photoorganocatalysis are two areas of synthetic methodology that have found wide
applications in organic synthesis. Herein, we report a combination of these two stategies, taking
advantage of an organocatalytic Michael addition of a,a-disubstituted aldehydes to maleimides as the
first step and a photocatalytic hydroacylation of diisopropyl azodicarboxylate as the second step.
Employing an amino acid as the organocatalyst for the asymmetric organocatalytic part and an organic
molecule as the photocatalyst, the combination of these two strategies led to the desired products.
A number of alkyl- and aryl-substituted maleimides were successfully employed, while the protocol can
be used on a,a-disubstituted aldehydes leading to products in moderate to high yields (44–84%) and
excellent enantioselectivities (98–100% ee).
Received 21st August 2018,
Accepted 13th October 2018
DOI: 10.1039/c8nj04274h
rsc.li/njc
contributed in this area by introducing an organocatalytic
protocol employing low catalyst loading of b-phenylalanine,
Introduction
Organocatalysis, the use of small organic molecules to promote in combination with Cs2CO3, for this reaction leading to
asymmetric organic transformations, has become a comple- products in high yields and selectivities, that can be employed
mentary method for the synthesis of organic compounds with for further product manipulations (Scheme 1, A).5 In that study,
multiple purposes.1,2 During the last two decades of its rebirth,
organocatalysis has provided valuable solutions to unsolved
problems and complementary platforms of activation modes in
modern catalysis, establishing organocatalysis as one of the
three pillars of asymmetric catalysis, alongside transition-metal
catalysis and biocatalysis. A very well-studied reaction in organo-
catalysis is the Michael addition, since new C–C bonds can
be forged in a highly enantioselective fashion.3 Among the
different Michael reactions, the addition of a,a-disubstituted
aldehydes to maleimides represents an attractive transforma-
tion,4,5 since the obtained products, which are substituted
succinimides, are considered valuable synthetic targets and
precursors of biologically interesting compounds.6 Another
interesting feature of this transformation is that this reaction
leads to the formation of quaternary centers.7 This reaction is
typically catalyzed by bifunctional organocatalysts, bearing a
primary amino group and a thiourea.4 A few years ago, we have
Laboratory of Organic Chemistry, Department of Chemistry, National and
Kapodistrian University of Athens, Panepistimiopolis 15771, Athens, Greece.
E-mail: ckokotos@chem.uoa.gr; Fax: +30 210 7274761; Tel: +30 210 7274281
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Design plan.
c8nj04274h
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Table 1 Optimization of the reaction conditions for the combination of
the organocatalytic Michael addition of isobutyraldehyde with maleimide with
the photoorganocatalytic hydroacylation with diisopropyl azodicarboxylatea
the optimum amino acid catalyst was recognized, and the
substrate scope of the reaction was thoroughly examined, along
with an application in the one-pot synthesis of lactones.
Photochemistry constitutes also a very exciting field of
research, which has been known for more than a century.
However, for a long time, researchers were rather preoccupied
that the reactivity of radicals could not be controlled, let alone
manipulated in a designed manner. In 2008, photoredox catalysis,
the use of transition metal catalysts (mainly ruthenium or iridium)
along with cheap household or LED lamps as the irradiation
source, has emerged as a solution providing new activation
modes.8,9 Unfortunately, these types of metal complexes can be
expensive, and most of them are not commercially-available.
Photoorganocatalysis, the use of small organic molecules as photo-
catalysts, constitutes a cheaper and greener alternative.10 We have
recently introduced phenylglyoxylic acid as a potent photoorgano-
catalyst in a photoorganocatalytic protocol that is easy to operate,
employing cheap household lamps and a cheap, organic and
commercially available molecule as the photocatalyst.11 Initi-
ally, we introduced phenylglyoxylic acid as the photocatalyst for
a green and fast hydroacylation of dialkyl azodicarboxylates
(Scheme 1, B).11a We also took advantage of this methodology
to introduce visible-light induced one-pot procedures for the
synthesis of hydroxamic acids11b and amides11c from aldehydes.
Entry
Catalyst loading (mol%)
Solvent
Yieldb (%)
eec (%)
1
10
10
10
10
10
20
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Pet. ether
Pet. ether
22
54
36
38
53
42
94
86
96
98
88
95
2d
3e
4f
5
6e
DIAD: diisopropyl azodicarboxylate.a 1a (2.80 mmol), 2a (1.50 mmol),
b-Phe (0.075 mmol), KOH (0.075 mmol) in CH2Cl2 (2 mL) for 24 h, then
diisopropyl azodicarboxylate (3a) (1.00 mmol), PhCOCOOH (0.10 mmol)
b
in CH2Cl2 (5 mL) under household bulb irradiation for 24 h. Isolated
c
yield. Enantiomeric excess was determined by chiral HPLC analysis.
d
e
f
Reaction time 48 h. 2a : 3a ratio: 1 : 1.5. 2a : 3a ratio: 1 : 3.
The potential of these methodologies was highlighted in the We postulated that a prolonged reaction time could be needed for
synthesis of two commercial pharmaceuticals, vorinostat11b and the reaction to reach completion, and the reaction time was
moclobemide.11c
increased to 48 h, where indeed, the yield increased significantly
In this work, we present a novel and facile route for the (54%), albeit, at the expense of the enantioselectivity (86% ee)
combination of organocatalysis with photoorganocatalysis (Table 1, entry 2). This result suggests that upon prolonged
(Scheme 1, C). Our goal is to combine the highly versatile irradiation, the labile acidic proton of the product can slowly
asymmetric Michael reaction of a,a-disubstituted aldehydes epimerize, leading to erosion of the enantioselectivity. In order
to maleimides with photoorganocatalytic hydroacylation, in to address this issue, we studied the effect of the reagent ratio for
order to afford products of high molecular complexity in an the conversion and the enantioselectivity of the reaction (Table 1,
enantiopure fashion.
entries 3 and 4). Reversing the ratio of the reagents, a boost in the
yield was observed, along with a slight boost in the enantio-
selectivity (Table 1, entries 3 and 4 vs. 1). Since the yield remained
low, we decided to employ petroleum ether as the solvent for the
photocatalytic step, which proved to be the solvent of choice in our
Results and discussion
We began our optimization studies employing isobutyralde- previous studies.11a A significant increase in the yield of the
hyde (1a), N-phenyl maleimide (2a) and diisopropyl azodicarb- product was observed, with a slight decrease in the enantioselec-
oxylate (DIAD) (3a) in our previous optimized reaction conditions tivity (Table 1, entry 5). Increasing the catalyst loading of phenyl-
for both reactions (Table 1). For green chemistry and reproduci- glyoxylic acid did not alter the yield of the product, but altered the
bility purposes, we employed b-phenylalanine in combination with observed enantioselectivity (Table 1, entry 6). It seems that increas-
KOH for the asymmetric organocatalytic step and performed ing the catalyst loading pushes radicals towards the productive
simple extractions before performing the photocatalytic step. The pathway, not letting them participate in the enantioselectivity-
product was formed in a quantitative yield and high enantiomeric deteriorating pathways.
excess (499% ee). All racemic samples were produced using
We next turned our attention to screening different photo-
racemic phenylalanine as the catalyst. For the second step, CH2Cl2 catalysts for the second step (Table 2). Although in our previous
was kept as the solvent, having in mind to introduce a one-pot studies, phenylglyoxylic acid (5a) outperformed all other
process in the future, although our previous studies revealed that activated ketones employed as the photocatalyst,11a it was
dichloromethane is not a suitable solvent for this photocatalytic necessary to rescreen the photocatalyst, in order to match the
process.11a Previously, completion of the reaction was evident by organocatalytic step. Replacing phenylglyoxylic acid (5a) by
simple observation, since the color of the solution turned from known Hydrogen Atom Transfer (HAT) photocatalysts, like
yellow to colorless,11a however, this did not occur even after 24 h of acetophenone (5b) or benzophenone (5c), or other activated
irradiation (Table 1, entry 1). A low yield (22%) of the desired ketones, like 2,2,2-trifluoroacetophenone (5d), did not lead to
product was isolated, albeit in high enantiomeric excess (94% ee). higher yields, although the enantioselectivity was kept at high
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Table 2 Optimization of the reaction conditions for the combination of the
organocatalytic Michael addition of isobutyraldehyde with maleimide with the
photoorganocatalytic hydroacylation with diisopropyl azodicarboxylatea
petroleum ether further increased the reaction yield (Table 2,
entry 7). Diethyl ether or a combination of dichloromethane with
petroleum ether did not improve the reaction efficiency (Table 2,
entries 8 and 9). Finally, switching the organic solvent to water led to
a further improvement both in the yield and the enantioselectivity
(Table 2, entry 10). This can be attributed to the fact that water may
bring the reactants in close proximity in hydrophobic pockets or
maybe water helps the HAT process to occur faster.
In order to provide a one-pot protocol for this combination,
we attempted this reaction in a sequence (Scheme 2). Unfortu-
nately, if the intermediate extractions are omitted, a lower yield
was isolated, along with decreased enantioselectivity.
Once we have identified the optimum reaction conditions,
we turned our attention to probing the substrate scope of the
reaction (Scheme 3). There are many points of adding molecu-
lar complexity in the final product.11a–c,5 Initially, we began
with the moiety placed on the maleimide (R1). Aryl moieties
Entry
Catalyst
Solvent
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
5a
5b
5c
5d
5e
5f
5e
5e
5e
5e
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Pet. ether
Et2O
Pet. ether/CH2Cl2 1 : 1
H2O
36
12
16
Traces
67
65
77
52
64
96
96
98
—
99
87
99
96
98
499
84
DIAD: diisopropyl azodicarboxylate.a 1a (1.50 mmol), 2a (1.00 mmol),
b-Phe (0.05 mmol), KOH (0.05 mmol) in CH2Cl2 (2 mL) for 24 h, then
diisopropyl azodicarboxylate (3a) (1.50 mmol), catalyst (0.20 mmol) in
b
solvent (5 mL) under household bulb irradiation for 24 h. Isolated
c
yield. Enantiomeric excess was determined by chiral HPLC analysis.
levels (Table 2, entries 1–4). Moving to commercial photoinitiators
5e and 5f, the yield improved significantly (Table 2, entries 5 and 6).
Among these catalysts, benzoin methyl ether (5e) afforded the best
results. Since a new photocatalyst was proved to be the optimum to
match the organocatalytic step, we then performed a rapid solvent
screening (Table 2, entries 7–10). Replacing dichloromethane by
Scheme 3 Substrate scope for the combination of the organocatalytic
Michael addition of aldehydes to maleimides with the photoorganocata-
i
lytic hydroacylation with diisopropyl azodicarboxylate. Michael addition
Scheme 2 Attempted one-pot procedure for the combination of the organo-
catalytic Michael addition of isobutyraldehyde to maleimide with the photo-
organocatalytic hydroacylation with diisopropyl azodicarboxylate.
required 48 h. iiIn the photocatalytic reaction, a mixture of Pet ether : water
1 : 1 was employed.
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identify the appropriate photocatalyst and reaction conditions to
retain high enantioselectivities and yields. The desired products
were isolated in good to high yields and excellent enantio-
selectivities. This work constitutes a successful example of
combining asymmetric organocatalysis with photoorgano-
catalysis, that can be the basis for further developments
in the field.
Scheme 4 Expansion of the combination of the organocatalytic Michael
addition of isobutyraldehyde to nitrostyrene with the photoorganocatalytic
hydroacylation with diisopropyl azodicarboxylate.
Experimental section
bearing electron-withdrawing and electron-deficient groups General information
can be used successfully, leading to products 4a–c in high
Chromatographic purification of products was accomplished
using column chromatography on a Mercks Kieselgel 60 F254
230–400 mesh. Thin-layer chromatography (TLC) was per-
formed on aluminium backed silica plates (0.2 mm, 60 F254).
Visualization of the developed chromatogram was performed
by fluorescence quenching using ninhydrine. Mass spectra
(ESI) were recorded on a Finningans Surveyor MSQ LC-MS
spectrometer. HRMS spectra were recorded on Brukers maxis
Impact QTOF spectrometer. 1H and 13C NMR spectra were
recorded on a Varians mercury (200 MHz and 50 MHz,
respectively) and are internally referenced to residual solvent
signals. Data for 1H NMR are reported as follows: chemical shift
(d ppm), integration, multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br s = broad singlet),
coupling constant and assignment. Data for 13C are reported in
terms of chemical shift (d ppm). High Performance Liquid
Chromatography (HPLC) was used to determine enantiomeric
excesses and was performed on Agilent 1100 Series apparatus
using Chiralpak AD-H and OD-H columns. Optical rotations
were measured on a Perkin Elmer 343 polarometer. The dia-
stereomeric ratio of the crude reaction mixture was determined
by 1H-NMR. The configuration of the products has been
assigned by comparison to literature data. All new compounds
were assigned by analogy.
yields and excellent enantioselectivities. Moving from aryl- to alkyl-
substituted maleimides, benzyl-maleimide or alkyl-substituted
maleimides with cyclohexyl or methyl moieties reacted equally well,
leading to good to high yields and excellent enantioselectivities
(products 4d–f). We next focused on the aldehyde part of the
reaction. A variety of different a,a-disubstituted aldehydes can also
be employed with success. 2-Ethyl-butyraldehyde afforded 4g in
good yield and excellent ee, while cyclopentane-carboxaldehyde
and cyclohexane-carboxaldehyde led to 4h and 4i in similar good
yields and excellent enantioselectivities. When non-symmetrical
a,a-disubstituted aldehydes are employed, a mixture of diastereo-
mers can be foreseen.5 Indeed, when 2-methyl-butanal and
2-methyl-pentanal are employed, the desired products were
obtained in good yield, diastereoselectivity and enantioselectivity
(products 4j and 4k). Unfortunately, the use of 2-phenylpropanal did
not lead to the desired product. We postulated that once the
product of the organocatalytic step is formed,5 radical decomposi-
tion pathways are followed, due to the stable benzyl radicals that
can be formed. Also, the use of heptanal and 3-phenylpropanal
failed to deliver the desired product. In these cases, we postulate
that the acidic a-proton is not compatible with the second step,
leading to decomposition products. Another point of diversity of the
final product is the azodicarboxylate partner, but we chose not to
pursue, since it seems quite straightforward.
In order to further expand the scope of this combination we
decided to change the first step of the sequence (Scheme 4).
Instead of employing an organocatalytic asymmetric Michael
addition of a,a-disubstituted aldehydes with maleimides, we sub-
stituted the maleimide with nitrostyrene 6a. Indeed, the combi-
nation can be expanded to nitrostyrenes, leading to the desired
product 7a in good yield and excellent enantioselectivity.
General experimental procedure for the combination of
organocatalysis with photoorganocatalysis
In a dry flask, maleimide (1.00 mmol), L-b-phenylalanine
(0.05 mmol, 8 mg), and KOH (0.10 mmol, 6 mg) were dissolved
in CH2Cl2 (2 mL) and aldehyde (1.50 mmol) was added.
The mixture was stirred at room temperature for 24 h. After
completion of the reaction, the reaction mixture was diluted
with CH2Cl2 (5 mL) and washed with water (2 Â 5 mL). The
organic layer was dried over Na2SO4. Then, the solvent was
removed in vacuo. The crude product was transferred to a vial,
Conclusion
In conclusion, we have developed a green, cheap and easy-to- and benzoin methyl ether (5e) (0.20 mmol, 45 mg), diisopropyl
execute combination of organocatalysis with photoorganocatalysis. azodicarboxylate (DIAD) (3a) (1.50 mmol, 303 mg) and H2O
The first step constitutes a highly selective, low catalyst loading (1 mL) were added. The reaction mixture was stirred vigorously
asymmetric organocatalytic Michael addition of a,a-disubstituted under household bulb irradiation (2 Â 80 W household lamps)
aldehydes with maleimides, while the second step is a photo- for 24 h. The mixture was diluted with CH2Cl2 (5 mL) and
organocatalytic hydroacylation of dialkyl azodicarboxylates employ- washed with water (2 Â 5 mL). The organic layer was dried over
ing benzyl methyl ether as the photocatalyst, water as the solvent Na2SO4. Then, the solvent was removed in vacuo. Purification
and household bulb irradiation. In order to combine these two was performed by silica gel chromatography with petroleum
areas of catalysis, a new optimization was required, in order to ether/ethyl acetate.
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3 For reviews on organocatalytic Michael reactions, see:
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Experimental procedure for the synthesis of (S)-diisopropyl
1-(2,2-dimethyl-4-nitro-3-phenylbutano-yl)hydrazine-1,2-
dicarboxylate (7a)
In a dry flask, nitrostyrene (1.00 mmol, 149 mg), L-b-phenyl-
alanine (0.10 mmol, 17 mg), and KOH (0.10 mmol, 6 mg) were
dissolved in CH2Cl2 (2 mL) and isobutyraldehyde (1.50 mmol,
115 mg) was added. The reaction mixture was stirred at room
temperature for 48 h. After completion of the reaction, the
mixture was diluted with CH2Cl2 (5 mL) and washed with water
(2 Â 5 mL). The organic layer was dried over Na2SO4. Then, the
solvent was removed in vacuo. The crude product was trans-
ferred to a vial, and benzoin methyl ether (5e) (0.20 mmol,
45 mg), diisopropyl azodicarboxylate (DIAD) (3a) (1.50 mmol,
303 mg) and H2O (1 mL) were added. The reaction mixture was
stirred vigorously under household bulb irradiation (2 Â 80 W
household lamps) for 24 h. The reaction mixture was diluted
with CH2Cl2 (5 mL) and washed with water (2 Â 5 mL). The
organic layer was dried over Na2SO4. Then, the solvent was
removed in vacuo. The product was purified by silica gel
chromatography with petroleum ether/ethyl acetate.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
The authors would like to thank the Special Account for
Research (ELKE) and the Laboratory of Organic Chemistry of
the Department of Chemistry of the National and Kapodistrian
University of Athens for financial support. Also, the authors
would like to thank Dr Maroula Kokotou for her assistance in
acquiring HRMS data and COST Action C–H Activation in
Organic Synthesis (CHAOS) CA15106 is acknowledged for helpful
discussions.
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