A simple ketone as an efficient metal-free catalyst for visible-light-mediated Diels-Alder and aza-Diels-Alder reactions

Article abstract of DOI:10.1039/c9gc00485h

Diels-Alder reactions are highly effective between electron-rich dienes and electron-poor dienophiles. However, these reactions with electron-rich dienophiles are limited and require forcing conditions. Based on this, an efficient metal-free homogeneous s

Full text of DOI:10.1039/c9gc00485h

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DOI: 10.1039/C9GC00485H
ARTICLE
Simple Ketone as an Efficient Metal-Free Catalyst for Visible-Light-
Mediated Diels-Alder and Aza-Diels-Alder Reactions
Jiri Kollmann§, Yu Zhang§, Waldemar Schilling, Tong Zhang, Daniel Riemer, Shoubhik Das*
Received 00th January 20xx,
Accepted 00th January 20xx
Diels-Alder reactions are highly effective between electron-rich dienes and electron-poor dienophiles. However, these
reactions with electron-rich dienophiles are limited and require forcing conditions. Based on this, an efficient metal-free
homogeneous system has been developed for the Diels-Alder reactions between electron-rich dienophiles and dienes under
visible-light conditions. Additionally, this catalyst has shown excellent reactivity for the aza-Diels-Alder reactions. This simple
catalyst is commercially available, non-toxic, cheap and has shown excellent reactivity which is comparable and in some
cases even better than the reported metal-based catalysts. Finally, the mechanism of this reaction has been proposed based
on the experimental evidence.
DOI: 10.1039/x0xx00000x
turn over. In fact, they also investigated metal-free photocatalysts,
however their efforts were not successful.7a After then, Shores group
and Ferreira group developed Cr complexes under UV light
Introduction
Diels-Alder (D-A) reactions are the fundamental strategy for the
syntheses of versatile cyclohexene derivatives with the formation of
new stereogenic centers.1 These reactions have been well-
documented in organic syntheses, in phar-maceutical industry and in
biochemistry.2 It should be noted that D-A reactions are highly
effective between electron-rich dienes and electron-poor
dienophiles, which has been explained based on the frontier
molecular orbital (FMO) theory.3a To enhance the conversion as well
as the stereoselectivity of thermally-initiated organocatalytic Diels-
Alder reactions, different strategies have been developed. Evans
group established oxazolidinones as chiral organic auxiliaries which
sterically hindered the attack of the diene on one face of the
dienophile.3b MacMillan and co-workers developed imidazolidinones
as highly enantioselective chiral organocatalysts which form iminium
species with enones. The iminium species lower the LUMO of the
dienophile and sterically block one face from being attacked by the
diene.3c However, electronically mismatched coupling between
dienes and dienophiles is highly challenging and require strict
conditions.1-2 This electronically mismatched D-A reactions can be
achieved by the formation of radical cation intermediates from
dienophiles via one electron oxidation by using stoichiometric
oxidants such as aminium salts or by photoinitated electron transfer
(PET) with photosensitizers.4,5
irradiation.8 However, the Cr catalyst required longer reaction time
and excess of diene (10 equiv.) to achieve full conversion.8b
Previous work:
R2
RuL3(PF6)2
Radi
cal
R1
chain
Visible Light
R3
R2
R1
R3
R2
[Cr3](BF4)3
R1
en
Dioxyg
R3
UV light
This work:
R2
R3
R2
O
R1
R2
R1
R3
Fluorenone
OMe
9-Fluorenone
Commercially available
112 Eur/kg
R2
O
Radical chain propagation
N
R3
N
TMS
R3
O
Figure 1 Homogeneous catalysts for photocatalytic D-A reactions of
electron-rich dienophiles.
In contrast, visible-light-mediated radical cation generation from the
electron-rich dienophile is more sustainable and does not require
any stoichiometric oxidant (Figure 1).6 Based on this, Yoon and co-
workers developed a Ru complex which showed excellent reactivity
towards electron-rich dienophiles under visible-light conditions.7 In
this case the radical cation was generated by the Ru catalyst with the
Parallel to the D-A reactions, aza-Diels-Alder reactions are also
efficient and powerful approaches for the syntheses of N-
heterocycles in a straightforward way.9 With the same concept of the
generation of radical cation intermediates from electron-rich
dienophiles, iminium ions can be generated from imines with the aid
of visible light.6 Based on this, Zhang, Arai and Ohkuma groups
developed visible-light-promoted aza-Diels-Alder reactions with the
Ru and Cr catalysts.10 It should be noted that in all the cases of visible-
light-mediated D-A or aza-D-A reactions, only transition metal based
photocatalysts exist. In contrast, organic dyes are commercially
available, cheaper in price, non-toxic in nature and are an interesting
avenue to explore.11 Based on all the above information and to
Institut für Organische und Biomolekulare Chemie, Georg-August-Universität
Göttingen, Tammannstraße 2, 37077 Göttingen, Germany
Email: Shoubhik.Das@chemie.uni-goettingen.de
§ J. K. and Y. Z. contributed equally.
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
aid of the visible-light and air/oxygen was utilized for the catalyst
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Journal Name
develop our own interest in metal-free catalysis, herein we describe the desired product.14 However, with our cheap aVnidewmArteictlaelO-fnrleinee
an efficient metal-free homogeneous catalyst for visible-light- fluorenone catalyst, these cyclohexene derDivOaIt:iv10e.s10c3a9n/Cb9eGaCc0h0i4e8v5eHd
mediated Diels-Alder and aza-Diels-Alder reactions.12
with excellent yields in only 2 h. Additionally, 9-vinylcarbazole was
also capable of acting as a dienophile and resulted 11c with 2,3-
dimethyl-1,3-butadiene as a diene.
Results and discussion
At the outset of our reaction, we focused on the organic dyes which
have high oxidation potential (e.g. in case of fluorenone +1.7 V vs
SCE)11a to generate radical cation from trans-anethole (1a) (+1.1 V vs
SCE).7a We started the optimization using trans-anethole (1a) as a
dienophile and 2,3-dimethyl-1,3-butadiene (1b) as diene under air
(Table S1; see supporting information). Eight different organic dyes
were applied using nitromethane (MeNO2) as solvent and MgSO4 as
desiccant under the irradiation of 12 W blue LED for 4 h. Among
them, fluorenone showed 81% yield of the corresponding Diels-Alder
product. Subsequently, other aprotic polar solvents such as THF,
DMF, DMSO and ACN were investigated but did not show any
improvement of the yield. Nitromethane was the best solvent in this
reaction as aprotic polar solvents with higher dipole moment at
comparable polarities can solvate the formed radical cation
intermediate which enhances the product formation.13
Inspired by the excellent reactivity of our catalyst in [4+2]
cycloaddition reactions, we sought to apply these reaction conditions
to aza-D-A reactions. Our main target was to synthesize important N-
heterocycles in a straightforward way. For this purpose, we aimed to
synthesize 2,3-dihydropyridin-4(1H)-ones derivatives as these have
wide applications for the treatment of emergent infectious
diseases.15 Additionally, these compounds have been utilized for the
syntheses of piperidine containing natural products and bioactive
molecule synthesis.16 In general, syntheses of 2,3-dihydropyridin-
4(1H)-ones derivatives require either expensive metal based
catalysts or longer reaction time.17 Gratifyingly, fluorenone showed
an excellent yield within 2 h when (E)-1-(4-methoxyphenyl)-N-
phenylmethanimine (1aa) as a dienophile and Danishefsky's diene
(1ab) were applied to synthesize 2-(4-methoxyphenyl)-1-phenyl-2,3-
dihydropyridin-4(1H)-one (1ac) as the product (Table S2). Different
ratios of diene and dienophile were also investigated and to our
delight, ratio 2:1 proceeded smoothly with the formation of 96% of
the desired product in 2 h.
Scheme 1 Scope of the fluorenone-catalyzed Diels-Alder reactions
under visible-light irradiationa-c
.
R4
air
R1
R3
R1
R2
R3
R5
R4
R5
1b-11b
fluorenone (3 mol %)
+
With these optimized reaction conditions, we applied fluorenone
catalyst to other imines to obtain diverse 2,3-dihydropyridin-4(1H)-
ones derivatives (Scheme 2). Various electron-rich imines showed
high reactivities with the formation of desired products in 2 h up to
96% yield. The catalyst showed high tolerance towards alkyl, aryl and
heteroaryl substituents (Scheme 2; entries 2ac−10ac). Additionally,
six new 2,3-dihydropyridin-4(1H)-ones derivatives have been
synthesized whose biological activities should be further investigated
considering the promising lead compounds against human infectious
diseases (Scheme 2; entries 3ac–4ac, 7ac−10ac).16 Expediently, with
this fluorenone catalyst, we have been able to synthesize previously
reported bioactive derivatives which were evaluated in vitro against
wide spectrum of viruses (Scheme 3; entries 1ac, 11ac−12ac).15a The
scope of the fluorenone-catalyzed aza-Diels-Alder reactions was not
only limited to the synthesis of 2,3-dihydropyridin-4(1H)-one
derivatives but also applied to synthesize other important
heterocycles (Scheme 4). For example, when the Danishefsky's diene
was replaced by 1-methoxy-1,3-butadiene, it generated 1,2,3,6-
MeNO2, r.t.
blue LED
R2
1a-11a
1c-11c
O
O
O
O
O
O
b
c
c
c
c
1c
2c
6c
3c
7c
4c
, 6h, 81% , 74%
, 23h, 82%
, 23h, 88%
, 28h, 64%
O
O
O
O
O
O
O
O
O
O
O
c
c
c
c
5c
8c
, 2h, 94%
, 25h, 80%
, 44h, 69%
, 44h, 77%
O
O
O
N
N
O
O
c
c
c
9c
10c
11c
, 24h, 50%
, 2h, 95%
, 8h, 86%
[a] Reaction conditions: air, 12 W blue LED, dienophiles (0.5 mmol, 1 eq.),
dienes (3 eq.), photocatalyst (3 mol%), 5 mL solvent, room temperature. [b] tetrahydropyridine derivative (Scheme 4; entry 13ac). In addition to
Yield was determined by NMR yield using iodoform as an internal standard.
[c] Isolated yields.
this, 2,3-dimethyl-1,3-butadiene reacted with 1,4-benzoquinone to
generate
3,4-dimethyl-1-oxaspiro[5.5]undeca-3,7,10-trien-9-one
(Scheme 4; entry 14ac).
With these optimized reaction conditions in hand, we extended the
scope of this metal-free system to other electron-rich dienophiles
and dienes (Scheme 1; entries 1c–11c). The formation of [4+2]
cycloadducts proceeded smoothly using 2,3-dimethyl-1,3-butadiene
as a diene and other electron-rich dienophiles such as myrcene,
isoprene etc. (Scheme 1; entries 2c–4c). In addition to these, 1,2-
dimethoxy-4-propenylbenzene was also effectively explored as a
dienophile with a series of dienes (Scheme 1; entries 5c–7c).
Furthermore, 2,5-furandione was also suitable in our system and
generated corresponding cyclohexene derivatives in excellent yields
in 2 h (Scheme 1; entries 8c–9c). It should be noted that for 2,5-
furandione, expensive metal-based catalysts are required to obtain
Scheme
2 Scope of the fluorenone-catalyzed aza-Diels-Alder
reactions under visible light irradiationa-c
.
2 | J. Name., 2012, 00, 1-3
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ARTICLE
OMe
air
OMe
air
R2
O
R2
O
R2
R2
View Article Online
fluorenone (3 mol %)
fluorenone (3 mol %)
+
+
DOI: 10.1039/C9GC00485H
N
R1
N
MeNO2, r.t.
blue LED
MeNO2, r.t.
blue LED
R1
N
N
R1
R1
TMS
TMS
O
O
1ab
O
1aa-10aa
1ac-10ac
11aa-12aa 1ab
11ac-12ac
O
O
O
N
O
O
O
O
O
O
O
O
N
O
N
N
N
N
O2N
O
11ac
, 8h, 83%
12ac
1ac
, 8h, 80%
, 2h, 94%
b
c
c
c
1ac
, 2h, 96% , 80 %
2ac
3ac
, 2h, 91%
, 2h, 89%
Reaction conditions: air, 12 W blue LED, imines (0.10 mmol, 1 eq.), diene (2
eq.), photocatalyst (3 mol%), 5 mL solvent, room temperature.
O
O
O
N
N
Scheme 4 Fluorenone-catalyzed hetero-Diels-Alder reactions under
visible light irradiation.
N
O
c
c
c
6ac
, 8h, 86%
5ac
, 2h, 86%
4ac
, 2h, 87%
O
O
O2N
O2N
air
fluorenone (3 mol %)
O
O
O
MeNO2, r.t.
blue LED
N
N
N
N
N
O
O
c
c
c
7ac
, 2h, 90%
8ac
13aa
13ab
, 2h, 93%
13ac
, 4 h, 82 %
O
9ac
, 2h, 94%
O
O
air
O
fluorenone (3 mol %)
N
MeNO2, r.t.
blue LED
O
c
O
10ac
, 2h, 92%
[a] Reaction conditions: air, 12 W blue LED, imines (0.10 mmol, 1 eq.), diene
(2 eq.), photocatalyst (3 mol%), 5 mL solvent, room temperature. [b] Yield was
determined by NMR yield using iodoform as an internal standard. [c] Isolated
yields.
14aa
1b
14ac
, 23 h, 81 %
Reaction conditions: air, 12 W blue LED, imine or 1,4-benzoquinone (0.10
mmol, 1 eq.), dienes (2 eq.), photocatalyst (3 mol%), 5 mL solvent, room
temperature.
Considering the broad scope of this catalyst, we sought to enquire
the role of the catalyst, air, and light source in this reaction. Control
experiments showed no product formation in the absence of light or
photocatalyst (see supporting information). Notably, 60% yield was
achieved under nitrogen, which clearly indicates that there is a chain
propagation mechanism similar to the earlier report of Ru complex.7a
The singlet oxygen in the air acts as an indispensable electron
mediator which further increases the yield. Furthermore, the effect
of different quenchers was investigated to recognize the reactive
oxygen species from air and possible intermediates (Table 1).12c-e
When 2,6-di-tert-butyl-4-methylphenol (BHT) or 2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO) were added to the reaction
mixture, the reaction was completely inhibited, which proved a
radical pathway. Furthermore, addition of CuCl2 to the reaction
mixture showed lower yields, which showed the involvement of
single electron processes in this photocatalytic system. The addition
of sodium azide also showed a decreased yield, indicating the
involvement of singlet oxygen, or activated oxygen species.
Combining all the mechanistic experiments, we have been able to
propose the mechanism for visible-light-mediated D-A and aza-D-A
reactions in Figure 2. At first, the photocatalyst was excited to the
photo-excited state by the irradiation of visible light and underwent
a single electron transfer (SET) with trans-anethole (1a). In fact the
redox potential value of the catalyst (+1.7 V vs SCE)11a is enough to
oxidize 1a (+1.1 V vs SCE).14d The resulting radical cation 1a·+ reacted
with the diene via [4+2] cycloaddition to afford radical cation product
1c·+. This product 1c·+ accepted one electron mainly from another
equivalent of 1a with a chain propagation step to form the final
product (1c).7a Meanwhile, oxygen will also be involved partly in
electron donating. Here the role of air/oxygen was to turn over the
reduced photocatalyst to the original state. The reported value for
the reduction potential of the excited-state fluorenone resides at –
0.61 V vs SCE,11d which is sufficient for the reduction of molecular
·-
oxygen to its superoxide radical form (O2/O2 ) with the reduction
potential residing at –0.56 V vs SCE.19
To acquire further information about the reaction mechanism, Stern-
Volmer quenching experiments were carried out (see supporting
information). The excited state of the photocatalyst was quenched
by trans-anethole.18 As shown in Figure S3, a clear decrease of light
emission was observed with the increasing concentration of trans-
anethole (1a) and no change was observed with different
concentrations of 2,3-dimethyl-1,3-butadiene.
Table
1 Quenching experiments for fluorenone catalyzed
cycloaddition reactions.
Scheme 3 Syntheses of bioactive derivatives using fluorenone
catalyst.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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O
O
air
fluorenone
+
Scheme 5 (a) Examples of bioactive cyclobutane lignan natural
DOI: 10.1039/C9GC00485H
products. (b) Our work on [2+2] cycloaddition reactions.
View Article Online
CH3NO2 5 mL
r.t., blue LED
1.5 mmol
0.5 mmol
1a
1b
1c
Reaction conditions: air, 12 W blue LED, 0.5 mmol dienophiles (1 eq.), 1.5
Quencher
BHT
Equivalents
Yield [%]
Scavenger for
radical
O
O
1.0
1.0
1.0
1.0
0
0
1a|||||
fluorenone*
TEMPO
CuCl2
radical
1c|||||
1a
73
63
single electron
O2
fluorenone
O2
SET
(main)
NaN3
singlet oxygen
mmol dienes (3 eq.), photocatalyst (3 mol%),
temperature.
5
mL solvent, room
O2
fluorenone
O2
1a
O
O
1b
1a|||||
1c|||||
O
O
O2
O2
SET
[2+2] cycloaddition
(a) Examples of bioactive cyclobutane lignan natural products
O
O
HO
OH
O
O
O
O
di-O-methylendiandrin A
endiandrin A
(b) Our work on [2+2] cycloaddition reactions
O
O
O
air
fluorenone (3 mol %)
MeNO2, r.t.
blue LED
1d
, 6h, 76%
O
O
air
fluorenone (3 mol %)
+
MeNO2, r.t.
blue LED
O
O
2d
, 23 h, 72%
Figure 2 Proposed reaction mechanism for the fluorenone catalyzed
Diels-Alder reactions.
Reaction conditions: air, 12 W blue LED, dienophiles (0.5 mmol, 1 eq.), dienes
(3 eq.), photocatalyst (3 mol%), 5 mL solvent, room temperature.
The [2+2] cycloaddition was also proceeded well via SET with product
1a·+ in the absence of diene.20 In fact, cyclobutane derivatives have
wide applications in natural product syntheses and many of them
already have become promising candidates as anticancer, antiviral
and antifungal drugs.20b Additionally, lignan and neolignan type
compounds also contain a cyclobutane motif (Scheme 5a).20b Based
on all this information, we applied fluorenone catalyst to promote
[2+2] cycloaddition reactions to obtain cyclobutane derivatives
(Scheme 5b; entries 1d–2d). To our delight, the catalyst not only
showed the activity for homodimerization of alkenes (1d) but also
showed an excellent reactivity towards heterodimerization of alkene
(2d).
Conclusions
In summary, we have developed fluorenone as a metal-free catalyst
for the visible-light-mediated cycloaddition reactions. This catalyst is
very cheap and commerically available and has shown excellent
substrates scope for [4+2] and [2+2] cycloaddition reactions with
electron-rich dienophiles. In addition, we have been able to
synthesize bioactive molecules via aza-Diels Alder reactions. We
believe this protocol can be further applied and extended for the
syntheses of pharmaceuticals and natural products. Additionally,
detailed mechanistic studies revealed the role of the catalyst and
oxygen and led to the proposed mechanism of this reaction.
4 | J. Name., 2012, 00, 1-3
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ACS Catal., 2018, 8, 5425. (e) D. Riemer, W. Schilling, A.
Conflicts of interest
View Article Online
Goetz, Y. Zhang, S. Gehrke, I. Tkach, O. Holloczki and S.
Das, ACS Catal., 2018, 8, 11679; (Df)OMI:.1M0.1a0j3e9k/aCn9dGCA0.0J.4V85oHn
Wangelin, Angew. Chem. Int. Ed., 2015, 54, 2270.
12 (a) D. Riemer, B. Mandaviya, W. Schilling, A. C. Götz, T.
Kühl, M. Finger and S. Das, ACS Catal., 2018, 8, 3030; (b)
D. Riemer, P. Hirapara and S. Das, ChemSusChem., 2016,
9, 1916; (c) P. Hirapara, D. Riemer, N. Hazra, J. Gajera, M.
Finger and S. Das, Green Chem., 2017, 19, 5356.
There are no conflicts to declare.
Acknowledgements
We thank Fonds der Chemischen Industrie (FCI, Liebig-Fellowship to
S.D.) and Chinese Scholarship Council (CSC funding to Y.Z.) for the
financial support. We are highly thankful to Prof. Dr. Lutz Ackermann
for his support behind our work.
13 V. Santacroce, R. Duboc, M. Malacria, G. Maestri, G.
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