Enantioselective Visible-Light-Mediated Formation of 3-Cyclopropylquinolones by Triplet-Sensitized Deracemization

Article abstract of DOI:10.1002/anie.201814193

3-Allyl-substituted quinolones undergo a triplet-sensitized di-π-methane rearrangement reaction to the corresponding 3-cyclopropylquinolones upon irradiation with visible light (λ=420 nm). A chiral hydrogen-bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88–96 % yield, 32–55 % ee). Surprisingly, it was found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of a deracemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.

Full text of DOI:10.1002/anie.201814193

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Accepted Article
Title: Enantioselective Visible Light-mediated Formation of 3-
Cyclopropylquinolones via Triplet-sensitized Deracemization
Authors: Andreas Tröster, Andreas Bauer, Christian Jandl, and
Thorsten Bach
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814193
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10.1002/anie.201814193
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Photochemistry
Enantioselective Visible Light-Mediated Formation of 3-Cyclopropyl-
quinolones via Triplet-Sensitized Deracemization
Andreas Tröster, Andreas Bauer, Christian Jandl, and Thorsten Bach*
Abstract. 3-Allyl-substituted quinolones undergo
sensitized di- -methane rearrangement reaction to the
corresponding 3-cyclopropylquinolones upon irradiation with
visible light ( = 420 nm). A chiral hydrogen bonding sensitizer
a
triplet-
insufficient enantioface differentiation could be overcome and the
first, highly enantioselective, triplet-sensitized reaction was dis-
closed in 2009.[8] A modified version of the xanthone catalyst used
in this and related studies[9] was presented in 2014 and was based
on a thioxanthone as sensitizing unit.[10] The compound which is
available in both enantiomeric forms 4 (Figure 1)[11] and ent-4 has
the advantage to operate with visible light and holds promise to be
a versatile catalyst for several photochemical reactions.[12,13]
π
λ
(10 mol%) was shown to promote the reaction enantioselectively
(88-96% yield, 32-55% ee). Surprisingly, it was found that the
enantiodifferentiation does not occur at the state of initial product
formation but that it is the result of a deracemization event. The
individual parameters that control the distribution of enantiomers
in the photostationary state have been identified.
Triplet sensitization represents an efficient and elegant way to
promote a molecule from its ground state to a triplet state, in most
cases to the lowest lying triplet state T1. The mechanism of the
process has been elucidated[1] and apart from an energetic driving
force it is crucial that the sensitizer and the substrate are in close
spatial proximity to allow for an efficient sensitization. The idea to
employ triplet energy transfer by a chiral sensitizer and to perform
photochemical reactions enantioselectively was first disclosed in
the 1960s. Pioneering studies were directed to the formation of
chiral trans-1,2-diphenylcyclopropane (1) from its racemate (rac-1).
After initial work by Hammond with a sensitizer that was later
shown to operate on the singlet hypersurface,[2] Ouannès et al.
found in 1973 that chiral ketone 2 induced a notable but small
enantioselectivity (3% ee) in the formation of 1 (Scheme 1).[3] The
efficiency of the reaction suffers from the simultaneous formation
of the achiral cyclopropane 3.
Figure 1. Structure and absolute configuration of thioxanthone 4 as
proven by single-crystal X-ray diffraction (anomalous dispersion).
In the current study, we attempted to use catalyst 4 in an
enantioselective[14] di-π-methane rearrangement reaction[15] of 3-
allyl-substituted quinolones. Although the enantioselectivities of
the reaction remained moderate the mode of action turned out to be
remarkable. It was found that the enantioenriched cyclopropanes
are formed in a deracemization process, which resembles the
seminal work on chiral cyclopropanes[2,3] (cf. Scheme 1) and which
likely proceeds via a 1,3-diradical intermediate. The results of our
preliminary experiments are summarized in this communication.
Initial experiments were performed with quinolone 5a which was
readily obtained from the respective methyl (2-chloro-7-benzyl-
oxyquinolin-3-yl)acetate by double α-methylation, reduction to the
aldehyde, methylenation and final hydrolysis of the 2-
chloroquinolone (see Supporting Information for further details).
The compound was expected to behave in light-induced reactions
similar to previously studied alkenyl-substituted quinolones.[10] The
short gem-dimethyl substituted methylene linker would, however,
not allow for an intramolecular [2+2] photocycloaddition and a di-
π-methane rearrangement reaction was anticipated. Indeed the
reaction could be initiated by direct excitation at λ = 300 nm and λ
= 350 nm (Table 1, entries 1, 2) leading to racemic product rac-6a
in high yield. Since the absorption of quinolone 5a ceases above λ
≥ 360 nm, there was no reaction upon exposure to visible light (λ =
420 nm, entry 3). Catalyst 4 induced the desired photochemical
reaction at longer wavelength and the reaction was complete after
30 min at ambient temperature (85% yield, 33% ee, entry 4). The
enantioselectivity improved to 55% ee at −25 °C (entry 5) but did
not increase further in a solvent mixture of trifluorotoluene and
hexafluoro-meta-xylene (HFX)[9b] at −65 °C (entry 6). The use of a
different light source (LED = light emitting diode) with an identical
emission maximum as the previously used fluorescent lamps
resulted in a longer reaction time and a diminished enantio-
selectivity (entry 7). The polar solvent acetonitrile precluded an
enantioselective reaction. The reaction was slow (37% conversion
after 5 h) and the product was almost racemic (entry 8).
Scheme 1. Isomerization and deracemization of 1,2-diphenylcyclo-
propane (rac-1) by triplet energy transfer via chiral ketone 2.[3]
In the 1980s and 1990s, little attention was paid to the topic of
enantioselective triplet-sensitized reactions.[4,5] The few reported
studies were discouraging and enantioselectivities did not exceed
20% ee.[6] With the advent of efficient hydrogen bonding templates
and catalysts[7] it became evident, however, that the issue of
[∗]
Dr. A. Tröster, Dr. A. Bauer, Dr. C. Jandl, Prof. Dr. T. Bach
Department Chemie and Catalysis Research Center (CRC)
Technische Universität München
Lichtenbergstr. 4, D-85747 Garching
Fax: +49 (0)89 289 13315
E-mail: thorsten.bach@ch.tum.de
Homepage: http://www.oc1.ch.tum.de/home_en/
Supporting information and the ORCID identification
number(s) for the author(s) of this article can be found
under: http://dx.doi.org./10.1002/....
1
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10.1002/anie.201814193
Angewandte Chemie International Edition
trifluorotoluene (ET = 263 kJ mol−1)[12b] they indicated that triplet
energy transfer was possible to both substrate and product. In line
with this observation, it was shown that product 6a was not
configurationally stable under the irradiation conditions. The
enantiomerically enriched material obtained by chiral HPLC
purification (99% ee) underwent rapid racemization (Scheme 2)
upon irradiation at λ = 420 nm in the presence of achiral 9-
thioxanthenone (7). The latter catalyst is reported to have a triplet
energy of ET = 272 kJ mol−1.[17]
Table 1: Optimization of the reaction conditions for the
enantioselective formation of cyclopropane 6a from 3-allylquinolone
5a.
λ
t
[h]
yield[b] ee[c]
entry[a]
sens.
T [°C]
solvent
[nm]
[%]
[%]
1
2
3
4
5
6
7
8
300
350
420
420
420
420
420[f]
420



4
0.75
0.75
4
20
20
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
90
88

85



33
20
0.5
1.5
3
20
4
−25
90[d] 55
−65 PhCF3/HFX[e]
91
85
55
43
3
Scheme 2. Racemization of enantiomerically pure cyclopropane 6a
upon irradiation at λ = 420 nm in the presence of thioxanthone (7).
4
4
3.5
5
−25
−25
PhCF3
MeCN
33[g]
4
A second observation that supported the hypothesis of an
enantiodifferentiation after initial cyclopropane formation relates to
the development of the enantioselectivity over time. Upon
irradiation of substrate 5a (λ = 420 nm) in the presence of catalytic
quantities (10 mol%) of thioxanthone 4 (Table 1, entry 5), the
enantioselectivity increased from very low values to the recorded
value (55% ee) within 1 h and remained constant thereafter (Figure
3A). Almost the same enantioselectivity was achieved when
irradiating the racemic cyclopropane rac-6a under the same
conditions (Figure 3B). A photostationary state was reached after
30 min in which enantiomer 6a prevailed (50% ee).
[a]
The reactions were performed under the indicated conditions (c = 2.5
mM) in a merry-go-round reactor[16] employing 16 fluorescent lamps (8 W
each) as the irradiation source. [b] Yield of isolated product. [c] The enantiomeric
excess (ee) was calculated from the enantiomeric ratio, which was determined
[d]
by HPLC analysis using
a
chiral stationary phase.
The reaction was
repeated on a larger scale (50 mg) and delivered the same results after
[e]
irradiation for 75 min.
Trifluorotoluene and hexafluoro-meta-xylene (HFX)
[f]
were employed as the reaction solvent in a 1:2 ratio. The irradiation was
conducted with a 420 nm LED (2 W). [g] The reaction was stopped after 37%
conversion, the yield based on recovered starting material was 90%.
The absolute configuration of the major enantiomer 6a obtained
from the enantioselective reaction of 5a (entry 5) was determined
after further purification and derivatization. To this end, the
enantiopurity was increased by semipreparative chiral HPLC to
>99% ee. The benzyl group was removed by ether cleavage (BBr3
in CH2Cl2, 77%) and the resulting alcohol was acylated with para-
bromobenzoyl chloride (NEt3 in CH2Cl2, 58%). Product 6b so
obtained gave crystals which allowed for the determination of its
absolute configuration by X-ray diffraction (Figure 2).[11]
Figure 3. Rate profile for the formation of enantiomerically enriched
compound 6a by triplet sensitization via 4 (10 mol%, λ = 420 nm,
−25 °C in PhCF3): A) Formation from precursor 5a and B) formation
by deracemization of racemic cyclopropane rac-6a.
Figure 2. Absolute configuration of cyclopropane 6b as determined
by single-crystal X-ray diffraction based on anomalous dispersion.
Based on the accumulated data and supported by analogy to the
recent discovery of an efficient allene deracemization[12b,c]
mediated by thioxanthone ent-4, it seems reasonable to assume that
Based on the suggested mechanism of
a di-π-methane
rearrangement[15] the determined absolute configuration was
counterintuitive. In previous work it had been established that the
formation of diradical intermediates in the environment of
thioxanthone 4 was followed by a stereoselective ring closure that
is dictated by the absolute configuration of the sensitizer.[8-10]
Consequently, C−C bond formation was expected to lead to the
opposite enantiomer ent-6a but not to 6a. Two key observations
confirmed the notion that the enantioselectivity was not determined
during the initial cyclopropane formation. Firstly, the triplet
energies ET of substrate 5a (ET = 271 kJ mol−1) and product 6a (ET
= 270 kJ mol−1) were almost identical. Although the values are
slightly higher than the triplet energy of thioxanthone 4 in
the observed enantioselectivity is the consequence of
a
photochemical cyclopropane deracemization.[18] As mentioned in
the introduction, processes of this type are known but the highest
enantioselectivity achieved in the formation of 1,2-diphenylcyclo-
propane has not exceeded 20% ee.[19] For
a successful
photochemical deracemization to occur, an equilibrium must be
established in the photostationary state with different rate constants
for the formation of 6a and ent-6a. In the present case, a major
reason for the different rates seems to be the fact that the rates of
sensitization are different, i.e. the minor enantiomer is
preferentially sensitized as compared to the major enantiomer. This
hypothesis is supported by the association behavior (in benzene-d6
at 25 °C) of 6a and ent-6a towards sensitizer 4, with ent-6a
2
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10.1002/anie.201814193
Angewandte Chemie International Edition
Table 2: Enantioselective formation of various 3-cyclo-
propylquinolones 6 by sensitized irradiation of 3-allylquinolones 5.
exhibiting an almost 10-fold higher association constant Ka than 6a
(Scheme 3). Intramolecular sensitization within complex 4·ent-6a
is faster than any intermolecular sensitization and also outcompetes
sensitization within 4·6a due to its higher association constant.
Interestingly, preliminary DFT calculations[20] revealed that – in
strong contrast to the allene deracemization[12b] – the distances
between the two chromophores (thioxanthone carbonyl group and
quinolone double bond) within 4·ent-6a and 4·6a are not
significantly different. A kinetic preference during the sensitization
event is therefore not to be expected. On the contrary, the 1,3-
diradical 8 which is a likely intermediate in this process[21,22] is
expected to decay preferentially to product ent-6a due to the
geometric constraints in complex 4·8.[8-10] In other words, the
enantioselective cyclopropane formation within 4·8 does not match
the intrinsic preference of the sensitization event. The latter process
favours formation of 6a while the former process shows a
preference for ent-6a thus reducing the enantioselectivity of the
deracemization process.
t[a]
[h]
yield[b]
[%]
ee
[%]
entry substrate
X
R, R
product
1
2
3
4
5
6
7
8
9
5a
5c
5d
5e
5f
BnO
MeO
Me, Me
Me, Me
6a
6c
6d
6e
6f
1
1.5
1
91
96
95
88
94
91
88
88
88
55
53
44
40
32
47
45
47
37
MeO -(CH2)4-
MeS Me, Me
MeO2S Me, Me
2
1.5
1
5g
5h
5i
TfO
H
Me, Me
Me, Me
Me, Me
Me, Me
6g
6h
6i
1
Me
Ph
2
5j
6j
3
[a]
Reaction time required for full conversion. The reactions were performed
in a merry-go-round reactor[16] employing 16 fluorescent lamps (8 W each) with
an emission maximum at λ = 420 nm as the irradiation source (c = 2.5 mM). [b]
Yield of isolated product.
In conclusion we could show that cyclopropanes undergo a
triplet-sensitized deracemization in the presence of a chiral
thioxanthone sensitizer. The process is unique and it can only be
performed photochemically while thermal deracemization reactions
are entropically impossible. The enantioselectivities achieved in the
deracemization reaction are high compared to previous results
obtained for this process. Still, the key design elements which have
been corroborated in this study should allow to further improve the
enantioselectivity and work along these lines is ongoing in our
laboratory.
Scheme 3. Mechanistic picture for the preferred formation of
enantiomer 6a in the photocatalytic deracemization mediated by
chiral thioxanthone 4 (Ka = association constant, determined by NMR
titration[23] at 25 °C in benzene-d6).
Acknowledgement
Financial support by the Deutsche Forschungsgemeinschaft
(DFG) is gratefully acknowledged (Ba1372/20, GRK 1626). A. T.
thanks the research training group (Graduiertenkolleg) 1626
After the origin of the enantioselectivity was established we
studied additional quinolones 5 in the enantioselective derace-
mization reaction. The optimized conditions of Table 1 were
applied and the reaction was performed with a series of 3-allyl-
substituted quinolones (Table 2). As previously shown for substrate
5a (Figure 3A) the reaction was very efficient and full conversion
was reached at λ = 420 nm in all cases within three hours or less.
“Chemical Photocatalysis” for
a scholarship. We thank O.
Ackermann for his help with the HPLC analyses and Dr. T. S.
Chung for luminescence measurements. We thank Dr. P. Altmann
and Dr. J. D. Jolliffe for their assistance with the X-ray data
collection. Prof. S. M. Huber (Ruhr-Universität Bochum) is
acknowledged for preliminary DFT calculations.
The compatibility with several functional groups
X was
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
demonstrated and yields were consistently high (88-96%). The
influence of the functional groups on the enantioselectivity likely
reflects slight differences in the binding properties. The lowest
selectivity was observed with the electron withdrawing sulfonyl
group (X = MeO2S, entry 5) and with the phenyl group (X = Ph,
entry 9). Electronrich quinolones exhibited the highest
enantioselectivity (entries 1-3, 6, 8). The triflate (TfO) group (entry
6) invites a substitution by other groups at position C7 of the
quinolone and it was shown during the synthesis of several starting
materials (5h-5j) that cross-coupling reactions are feasible (see
Supporting Information). It is yet not completely understood why
substrate 5d (entry 3) exhibits a lower enantioselectivity than 5c
(entry 2) although one would intuitively consider a spirocyclic
cyclopentyl group bulkier than two methyl groups.
Keywords: chiral resolution · enantioselectivity · hydrogen bonds ·
photochemistry · sensitizers · small ring systems
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10.1002/anie.201814193
Angewandte Chemie International Edition
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4
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Angewandte Chemie International Edition
Entry of the Table of Contents
Photochemistry
A. Tröster, A. Bauer, C. Jandl, T. Bach*
__________ Page – Page
Enantioselective Visible Light-Mediated
Formation of 3-Cyclopropylquinolones
via Triplet-Sensitized Deracemization
Mirror mirror on the wall, where has the other enantiomer gone? Catalyst 2 estab-
lishes an equilibrium between the two enantiomers 1 and ent-1 in which one enan-
tiomer prevails (up to 55% ee). The deracemization is responsible for the enantio-
selective formation of 3-cyclopropylquinolones from 3-allyl-substituted quinolones.
5
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