Shedding light on organocatalysis - Light-assisted asymmetric ion-pair catalysis for the enantioselective hydrogenation of pyrylium ions

Article abstract of DOI:10.1002/chem.201300766

A new light-driven asymmetric ion-pair catalysis procedure for the metal-free enantioselective hydrogenation of in situ generated pyrylium ions from readily available chalcones was developed (see scheme). The photo-assisted Bronsted acid catalyzed procedure has broad scope and allows, for the first time, access to valuable 4H-chromenes in good yields and with excellent enantioselectivities. Copyright

Full text of DOI:10.1002/chem.201300766

COMMUNICATION
DOI: 10.1002/chem.201300766
Shedding Light on Organocatalysis—Light-Assisted Asymmetric Ion-Pair
Catalysis for the Enantioselective Hydrogenation of Pyrylium Ions
Chien-Chi Hsiao, Hsuan-Hung Liao, Erli Sugiono, Iuliana Atodiresei, and
Magnus Rueping*[a]
The use of dual catalytic methods, in which two or more
catalysts are employed in one-pot procedures, is becoming
an increasingly important tool for sustainable chemistry. In
this regard considerable progress has been achieved in the
development of asymmetric domino reactions[1] in which dif-
ferent metal, bio, or organo catalysts have been employed,
in a combined and synergistic fashion, to provide optically
active products from achiral substrates. However, the use of
benzopyrylium ions in asymmetric catalysis is rare[2] al-
though the corresponding chromanes are privileged structur-
al motifs of natural products and bioactive agents, which ex-
hibit antioxidant, antifungal, antiviral, cytotoxic, and anti-in-
flammatory activities.[3] Thus, it is desirable to develop im-
proved methodologies for the preparation of chiral chro-
manes, which could potentially lead to bioactive products
with enhanced pharmacological features. Based on our ex-
perience in Brønsted acid catalysis and the activation of al-
lylic alcohols and hemiaminals, we decided to examine the
use of chromene acetals 2 for the generation of reactive
pyrylium ion intermediates (Scheme 1). The Brønsted
acid[4,5] catalyzed protonation and subsequent elimination
was anticipated to result in the formation of an intermediary
chiral ion pair A consisting of a benzopyrylium ion and a
chiral acid anion.[6] The subsequent enantioselective nucleo-
philic addition at the 4-position of benzopyrylium ion would
give the desired 4H-chromenes. So far, no catalytic asym-
metric reaction with an intermediate of type A, comprising
a pyrylium ion and a chiral counter anion, has been report-
ed. Thus, the successful development of this Brønsted acid
catalyzed process would not only be the first example of
such a reaction, but more importantly would open new ave-
nues in asymmetric anion-pair catalysis.[7]
In order to validate our proposal, we examined the enan-
tioselective Brønsted acid catalyzed hydrogenation of ben-
zopyrylium ions[8] by employing Hantzsch ester[9] as the nu-
cleophile. Initial experiments were conducted with readily
available and generally stable chromene acetals 2 (R’’=Me
or Et, see Scheme 1). To our delight, the Brønsted acid cata-
lyzed reduction proceeded well and the desired 4H-chro-
menes were obtained with good yields. However, generally
better reactivities and significantly better selectivities were
obtained if 2H-chromen-2-ols (R’’=H) were applied. Un-
fortunately, 2H-chromen-2-ols are less stable and purifica-
tion can be difficult. Therefore, we decided to examine
enones 1 as precursors for the in situ generation of pyrylium
ions. Enones 1 are stable and easily prepared from commer-
cially available starting materials. Furthermore, they under-
go photocyclization to form the 2H-chromen-2-ols 2,[10]
which in the presence of the Brønsted acid could directly
yield the chiral ion pairs A, crucial intermediates for the
asymmetric hydrogenation.
From the outset we were aware that several problems
needed to be addressed in order to perform such an unpre-
cedented photocyclization–hydrogenation sequence: 1) the
Brønsted acid catalyzed transfer hydrogenation of enones 1
with Hantzsch dihydropyridine 5 as hydride source has been
reported.[11a] This reaction would lead to the undesired satu-
rated ketones, which undergo acid-catalyzed cyclization to
the chroman-2-ols. The subsequent acid-catalyzed water
elimination would result in our desired product 3. However,
the enantioselectivity determining step would be the initial
asymmetric reduction of the enone 1, which to date has not
been possible with chiral Brønsted acids; 2) the light driven
reduction of electron-deficient alkenes by photoexcited di-
hydropyridines, including Hantzsch ester has been stud-
Scheme 1. First asymmetric Brønsted acid catalyzed hydrogenation of
benzopyrylium ions.
[a] C.-C. Hsiao, H.-H. Liao, Dr. E. Sugiono, Dr. I. Atodiresei,+
Prof. Dr. M. Rueping
Institut fꢀr Organische Chemie, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-809-2665
E-mail: magnus.rueping@rwth-aachen.de
[+] Determination of the absolute configuration.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300766.
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ied.[11b] This reaction may also occur through a photoinduced
electron-transfer mechanism[11c] and would result in the ra-
ACHTUNGTRENNUNGceACHTUNGTRENNUNGmic saturated ketones, which would ultimately give rac-3.
red. However, 1a was smoothly reacted to the desired 2H-
chromen-2-ols 2 when the photocyclization was performed
in toluene, setting the stage for exploring the photo assisted
Brønsted acid catalyzed transfer hydrogenation. Initial reac-
tions were carried out with different dihydropyridines 5a–e
and BINOL-derived phosphoric acid diester 4a, which had
proven to be a good catalyst in organocatalytic hydrogena-
tions of N-heterocycles. To our delight, not only was the
product 3a formed, but enantioselectivities of up to 18%
enantiomeric excess (ee) were obtained if dihydropyridine
5d was applied (Table 1, entry 4).
Furthermore, the chroman-2-ol could also be reduced by the
photoactivated Hantzsch ester yielding rac-3; 3) the photo-
oxidation and photocatalytic production of hydrogen from
Hantzsch esters 5 has been well established and could lead
to the undesired decomposition of the hydride donor;
4) next to their use as antioxidants, benzopyrylium cations
have been shown to be excellent electron acceptors, which
are involved in charge-transfer reactions. They can also act
as a source of free radicals, which could lead to undesired
side reactions.[11d]
In order to increase the enantioselectivity, several BINOL
derived phosphoric acids and the corresponding triflyl-
Given the above considerations we performed several test
reactions (Table 1). First, we started with the Brønsted acid
catalyzed reduction of chalcone 1a (R and R’=Ph) employ-
ing the Hantzsch ester 5 as the hydride source. Pleasingly,
no reduction was observed when the reaction was per-
formed in aromatic solvents at room temperature. Addition-
ally, no direct acid-catalyzed cyclization of 1a to 2a occur-
ACHTUNGTRENaNUGN mides were evaluated. From all catalysts tested, 4 f was the
best and yielded 97% of the desired chiral 4H-chromene 3a
with 73% ee (Table 1, entry 10). Interestingly, performing
the reaction with 2,4-diphenyl-2H-chromen-2-ol (2a) result-
ed in reduced enantiomeric excess (Table 1, entry 11). All
N-triflylphosphoramides resulted in lower enantioselection.
In order to ascertain that the light-driven reduction by pho-
toexcited dihydropyridines had not occurred we performed
a reaction without catalyst under otherwise identical condi-
tions. Fortunately, no considerable product formation was
observed (Table 1, entry 13). However, at higher light inten-
sity and with prolonged reaction times this reaction occurs
leading to considerable loss in enantioselectivity. The pres-
ence of oxygen results in the photodecomposition of the di-
hydropyridine. In order to further improve the enantioselec-
tivity, the reaction was performed at lower temperatures.
Pleasingly, decreasing the reaction temperature to ꢀ208C
improved the enantioselectivity significantly and the product
was isolated in 69% yield and with an excellent enantiomer-
ic excess of 94% ee (Table 1, entry 14).
Table 1. Optimization of the Brønsted acid catalyzed photocyclization re-
duction reaction.[a]
Further optimization, including the addition of molecular
sieves for trapping any water produced, performing the re-
action in different solvents[12] or using lower temperatures
did not improve the results. With the optimal reaction con-
ditions in hand the scope of the asymmetric Brønsted acid
catalyzed photocyclization–reduction sequence was explored
(Table 2).
In general a range of chalcone derivatives 1 with various
substitution patterns could be applied and the 4H-chromene
derivatives 3a–o were isolated in good to excellent yields
and enantioselectivities (Table 2, entries 1–15). It is worth
mentioning that the newly developed protocol is not only
the first example of a catalytic asymmetric reduction of
pyrylium ions, but also the first example in which 4H-chro-
menes are obtained in organocatalytic enantioselective fash-
ion.
In order to shed light on the reaction mechanism of this
new light driven Brønsted acid catalyzed hydrogenation of
benzopyrylium ions, we performed a series of experiments.
In addition to the overall transformation (1!3), we also ex-
amined the individual cyclization (1!2) and hydrogenation
(2!3) steps of the reaction sequence as our experiments in-
dicated a more complex mechanistic scenario (Figure 1).
Under the optimal reaction conditions 1) the phosphoric
T
[8C]
4
5
Yield
[%][d]
ee
[%][e]
1
2
3
4
5
6
7
8
9
10
11[b]
12
13
14
15[b]
16[c]
17
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
ꢀ20
ꢀ20
ꢀ20
ꢀ45
4a
4a
4a
4a
4a
4b
4c
4d
4e
4 f
4g
4 f
–
5a
5b
5c
5d
5e
5d
5d
5d
5d
5d
5d
5d
5d
5d
5d
5d
5d
57
55
41
70
67
50
83
77
87
97
91
92
3
9
17
8
18
racemic
8
racemic
racemic
22
73
21
34
–
94
55
80
94
4 f
4 f
4 f
4 f
69
84
89
42
[a] Reactions were performed with chalcone 1a at 0.06m concentration,
dihydropyridines 5 (1.3 equiv), and 4 (5 mol%). The solution was irradi-
ated for 1 h at room temperature or for 12 h at ꢀ208C. Irradiation was
carried out in a Rayonet reactor at 300 nm or with a TQ 150 high pres-
sure mercury lamp, lꢁ300 nm. [b] Reactions performed with preformed
2,4-diphenyl-2H-chromen-2-ol. [c] Addition of 3 ꢂ MS. [d] Yield of iso-
lated product after column chromatography. [e] Enantiomeric excess was
determined by HPLC on a chiral stationary phase.
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Enantioselective Hydrogenation of Pyrylium Ions
COMMUNICATION
Table 2. Scope of the new photocyclization–reduction cascade.[a]
acid is not sufficiently active to catalyze the cyclization (1!
2); 2) as expected the cyclization proceeds under irradiation;
3) irradiation in the presence of the Brønsted acid leads to
significant rate enhancement, clearly demonstrating its in-
volvement in the cyclization event; 4) as also evident from
the overall reaction (Table 1, entry 13) no reduction takes
place under irradiation conditions alone; 5) the Brønsted
acid is crucial for the benzopyrylium-ion formation,[13,14]
which allows the subsequent hydrogenation to proceed; 6)
surprisingly, the Brønsted acid catalyzed hydrogenation is
enhanced by light irradiation. Although it is known that
light can positively influence metal-catalyzed transforma-
tions this effect has, to our knowledge, never been reported
for asymmetric Brønsted acid catalysis.
An unexpected consequence of the above is that both
light and the Brønsted acid are involved in both of the indi-
vidual steps of the cyclization–reduction sequence. Further-
more, we were unable to detect the chiral benzopyrylium/
phosphate ion pair in any of the experiments we performed.
This indicates that it is the formation of the ion pair that is
the rate-determining step in the cyclization–hydrogenation
sequence.
Based on the above experiments we propose that the first
step of the catalytic cycle involves a light induced and
Brønsted acid catalyzed photoisomerization of 1 to 1’. The
subsequent Brønsted acid catalyzed cyclization with elimina-
tion of water leads to the highly reactive chiral ion pair A
consisting of a pyrylium cation and the chiral phosphate
anion. Hydride transfer from the dihydropyridine 5 provides
the chromene 3 and the regenerated Brønsted acid catalyst
(Scheme 2). Although it is apparent that the Brønsted acid
3
R
Ar1
Ar2
Yield
[%][b]
ee
[%][c]
1
2
3
3a
3b
3c
H
H
H
Ph
Ph
Ph
Ph
4-MePh
4-tBuPh
69
74
98
94
92
90
4
3d
H
Ph
61
92
5
6
7
8
9
10
11
12
13
3e
3 f
3g
3h
3i
3j
3k
3l
H
H
H
OMe
Me
H
H
H
Ph
Ph
Ph
Ph
4-FPh
4-ClPh
3-BrPh
Ph
Ph
Ph
Ph
4-ClPh
4-ClPh
56
75
50
95
76
72
90
76
82
91
89
88
91
80
86
92
87
81
Ph
3-MePh
4-MePh
4-MePh
4-PentPh
3m
H
14
15
3n
3o
H
H
4-PentPh
3-MePh
80
93
83
80
[a] Reactions were performed with chalcone 1 at 0.06m concentration in
toluene, dihydropyridine 5d (1.3 equiv), and 4 f (5 mol%). The solution
was irradiated with a TQ 150 high pressure mercury lamp for 12–48 h
under argon. [b] Yield of isolated product after column chromatography.
[c] Enantiomeric excess was determined by HPLC on a chiral stationary
phase.
Scheme 2. First asymmetric Brønsted acid catalyzed hydrogenation of
benzopyrilium ions.
is engaged in each step of the catalytic cycle, we currently
have no proof for the additional effect played by light in the
reduction step and whether irradiation accelerates the water
elimination, the hydride transfer, or both elimination and
hydride transfer.
In summary, we have developed a new light driven[15]
asymmetric ion-pair catalysis procedure for performing an
enantioselective hydrogenation of pyrylium ions. The newly
developed dual and combined photo-assisted Brønsted acid
catalyzed procedure has broad scope and allows, for the first
Figure 1. Influence of light and Brønsted acid on the individual steps of
the reaction sequence.
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M. Rueping et al.
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with excellent enantioselectivities. The reaction sequence
consists of a dual light and Brønsted acid mediated isomeri-
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ates. The subsequent Brønsted acid catalyzed elimination of
water leads to an unprecedented intermediary chiral ion
pair consisting of a benzopyrylium ion and a chiral phos-
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tioenriched 4H-chromenes.[16]
Detailed mechanistic investigations demonstrate an unex-
pected synergistic effect as both light and the Brønsted acid
catalyst are involved in both the cyclization and transfer hy-
drogenation events. In addition, the enantiodifferentiating
fluorescence quenching has not been observed for BINOL-
phosphoric acid derivatives, which make their use in light
driven catalysis promising.[17] From a synthetic view, the
present approach is particularly attractive as it utilises readi-
ly available chalcones and avoids the preparation of rather
sensitive and unstable chroman-2-ol intermediates. Thus, we
are confident that the concept of asymmetric pyrylium ion
pair catalysis, in which the chiral information is efficiently
transferred from the Brønsted acid anion to the product,
will find broad application.
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Acknowledgements
We thank the DFG for financial support. C.-C. H acknowledges DAAD
and H.-H. Liao acknowledges Taiwan Ministry of Education for doctoral
fellowships. We thank Dr. Michael Muratore for providing chiral catalyst.
Keywords: asymmetric catalysis · benzopyrane · chiral ion
pair · oxocarbenium ion · reduction
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Enantioselective Hydrogenation of Pyrylium Ions
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2008, 120, 3687–3690; Angew. Chem. Int. Ed. 2008, 47, 3631–3634.
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[12] The reactions can be performed in chlorinated solvent or acetoni-
trile; however, reduced reactivities and selectivities were observed.
[13] For an enormous rate acceleration in organocatalytic acetalizations,
see: a) M. Kotke, P. R. Schreiner, Tetrahedron 2006, 62, 434–439;
b) Kotke, Mike (Ph. D. thesis 2009, http://geb.uni-giessen.de/geb/
volltexte/2010/7835/); c) Z. Zhang, P. R. Schreiner, Chem. Soc. Rev.
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Received: December 3, 2013
Revised: February 26, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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M. Rueping et al.
Organocatalysis
C.-C. Hsiao, H.-H. Liao, E. Sugiono,
I. Atodiresei, M. Rueping* . &&&& — &&&&
Shedding Light on Organocatalysis—
Light-Assisted Asymmetric Ion-Pair
Catalysis for the Enantioselective
Hydrogenation of Pyrylium Ions
A new light-driven asymmetric ion-
pair catalysis procedure for the metal-
free enantioselective hydrogenation of
in situ generated pyrylium ions from
readily available chalcones was devel-
oped (see scheme). The photo-assisted
Brønsted acid catalyzed procedure has
broad scope and allows, for the first
time, access to valuable 4H-chromenes
in good yields and with excellent enan-
tioselectivities.
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www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!