Asymmetric Photocatalysis by Intramolecular Hydrogen-Atom Transfer in Photoexcited Catalyst–Substrate Complex

Article abstract of DOI:10.1002/anie.201905647

3-(2-Formylphenyl)-1-pyrazol-1-yl-propenones undergo an asymmetric photorearrangement to benzo[d]cyclopropa[b]pyranones with up to >99 % ee, which is catalyzed by a bis-cyclometalated rhodium catalyst in the presence of visible light. Mechanistic experiments and DFT calculations support a mechanism in which a photoexcited catalyst/substrate complex triggers an intramolecular hydrogen-atom transfer followed by a highly stereocontrolled hetero-Diels–Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible-light π→π* excitation of the catalyst-bound enone substrate, 2) facilitating the hydrogen-atom transfer, and 3) providing the asymmetric induction for the hetero-Diels–Alder reaction.

Full text of DOI:10.1002/anie.201905647

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International Edition: DOI: 10.1002/anie.201905647
German Edition: DOI: 10.1002/ange.201905647
Photochemistry
Asymmetric Photocatalysis by Intramolecular Hydrogen-Atom
Transfer in Photoexcited Catalyst–Substrate Complex
Chenhao Zhang+, Shuming Chen+, Chen-Xi Ye, Klaus Harms, Lilu Zhang, K. N. Houk,* and
Eric Meggers*
Abstract:
3-(2-Formylphenyl)-1-pyrazol-1-yl-propenones
undergo an asymmetric photorearrangement to benzo-
[d]cyclopropa[b]pyranones with up to > 99% ee, which is
catalyzed by a bis-cyclometalated rhodium catalyst in the
presence of visible light. Mechanistic experiments and DFT
calculations support a mechanism in which a photoexcited
catalyst/substrate complex triggers an intramolecular hydro-
gen-atom transfer followed by a highly stereocontrolled hetero-
Diels–Alder reaction. In this reaction scheme, the rhodium
catalyst fulfills multiple functions by 1) enabling visible-light
p!p* excitation of the catalyst-bound enone substrate,
2) facilitating the hydrogen-atom transfer, and 3) providing
the asymmetric induction for the hetero-Diels–Alder reaction.
P
hotochemistry provides new synthetic and mechanistic
opportunities for asymmetric catalysis.[1] For example, visible
light has been used as a convenient and mild source of energy
to generate free radicals, which are then interfaced with an
asymmetric catalysis cycle.[2] In such systems, the photo-
chemistry is not part of the catalytic cycle and often serves as
an inducer of a thermal chain process. This role is distinct
from mechanistic scenarios in which the photoexcited state is
directly embedded within an asymmetric catalysis cycle,
thereby opening untapped opportunities for exploiting the
special reactivity of photoexcited states in the context of
asymmetric catalysis.[3]
Figure 1. Photoinduced asymmetric catalysis cycle in which visible-
light-photoexcited catalyst/substrate complexes are key intermediates.
ble of undergoing stereocontrolled chemical transformations,
either through an initial outersphere electron transfer or by
engaging in direct stereocontrolled bond-forming reactions
(Figure 1).[5] The realization of new reaction modes of
photoexcited catalyst/substrate complexes is highly desirable
for developing visible-light-activated asymmetric conversions
that are otherwise elusive.
Our group recently demonstrated that bis-cyclometalated
iridium(III) and especially rhodium(III) complexes are
exquisite catalysts for realizing visible-light-activated asym-
metric catalysis.[4,5] Upon binding to a substrate and under-
going selective visible-light activation of the formed catalyst/
substrate complexes, such photoexcited assemblies are capa-
Here we report a unique example of a visible-light-
activated catalyst/substrate complex engaging in a hydrogen-
atom transfer (HAT) reaction to generate a catalyst-bound
reactive intermediate, which is then primed to undergo
a highly stereocontrolled intramolecular hetero-Diels–Alder
reaction.
We previously found that upon binding to bis-cyclometa-
lated rhodium catalysts, a,b-unsaturated N-acyl pyrazoles can
undergo p!p* excitation under visible light. We further
demonstrated that these ligand-centered excited states can
perform stereocontrolled cycloaddition reactions.[3d,f,6] At the
onset of this study we were speculating that such photoexcited
catalyst-bound enones might instead be capable of engaging
in HAT reactions to generate catalyst-bound reactive inter-
mediates followed by stereocontrolled transformations. We
were inspired by a reaction reported by Xia and co-workers in
which (E)-ethyl 3-(2-formylphenyl)acrylate (1a), upon pho-
tolysis with a 500 W high-pressure mercury lamp, underwent
an interesting rearrangement to the cyclopropane 2a
(Table 1, entry 1), and this reaction was proposed to proceed
through an intramolecular HAT from the aldehyde to the
triplet excited state of the enone.[7] As major drawbacks, the
reaction required intense UV light and provided the product
[*] C. Zhang,[+] C.-X. Ye, Dr. K. Harms, Dr. L. Zhang, Prof. Dr. E. Meggers
Fachbereich Chemie, Philipps-Universität Marburg
Hans-Meerwein-Straß e 4, 35043 Marburg (Germany)
E-mail: meggers@chemie.uni-marburg.de
Dr. S. Chen,[+] Prof. Dr. K. N. Houk
Department of Chemistry and Biochemistry, University of California
Los Angeles, CA 90095-1569 (USA)
E-mail: houk@chem.ucla.edu
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201905647.
ꢀ 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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Table 1: Initial experiments and optimization of reaction conditions.[a]
similar results (entry 8), but MeCN only provided traces of
the product (entry 9). Interestingly, the presence of air only
slightly affects yield and enantioselectivity, indicating that
free-radical chemistry is not operational in this photoreaction
(entry 10). Finally, CFL as a light source provided similar
results (entry 11). Control experiments furthermore con-
firmed that the reaction requires the rhodium catalyst and
light for product formation (entries 12 and 13).
With the optimized reaction conditions in hand, we
screened substrates for this reaction. Figure 2 reveals that
this asymmetric visible-light-activated photorearrangement
tolerates methyl substituents at any position of the phenyl
moiety (2e–h), bulky substituents (2i, 2j), electron-with-
drawing substituents (2k–q), electron-donating substituents
(2r–u), and a thioether (2v). For all these reactions, yields of
50–93% and enantioselectivities of 87– > 99% ee were
observed. Lower yield and lower enantioselectivity were
obtained with a thiophene derivative (cyclopropane 2w), but
a methyl group in the b-position of the a,b-unsaturated N-acyl
pyrazole provided the corresponding rearrangement product
Entry Substrate Cat (mol%) Light[b]
Solvent
t
Yield ee
[h] [%][c] [%][d]
1[e]
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1d
1d
1d
1d
1d
none
4
4
4
2
1
0.5
4
UV light
benzene
5
89
–
38
97
98
95
95
blue LEDs CH2Cl2 16 60
blue LEDs CH2Cl2 16 74
blue LEDs CH2Cl2 16 83
blue LEDs CH2Cl2 24 71
blue LEDs CH2Cl2 24 70
blue LEDs CH2Cl2 48 46[f] 95
blue LEDs acetone 16 72
blue LEDs MeCN
blue LEDs CH2Cl2 20 75
97
–
96
96
–
–
4
4
16 trace
10[g] 1d
11
12
13
1d
1d
1d
4
CFL
blue LEDs CH2Cl2 48
dark CH2Cl2 48
CH2Cl2 36 86
none
4
0
0
[a] Standard reaction conditions for entries 2–8: Substrates 1b–d
(0.1 mmol) in the indicated solvent (2.0 mL) with D-RhS (0.5–4 mol%)
photolyzed for the indicated time under an atmosphere of N2. [b] 24 W
blue LEDs with emission maximum at 450 nm or 23 W CFL. [c] Yields of
isolated products. [d] Enantioselectivities determined by HPLC on chiral
stationary phase. [e] Taken from Ref. [7]. Irradiation with a 500 W high-
pressure mercury lamp in combination with a Pyrex filter. [f] The
conversion was 67%. [g] Under an atmosphere of air.
as a racemic mixture. We speculated that by replacing the
ester with an N-acyl pyrazole moiety to generate a binding
site for the rhodium catalyst, the rearrangement could be
rendered visible-light-activated and asymmetric. Accordingly,
when we submitted N-acyl 3-(4-methoxyphenyl)pyrazole
(1b) to photolysis with blue LEDs (lmax = 450 nm) in the
presence of catalytic amounts of the chiral-at-metal rhodium
catalysts D-RhS (4 mol%), 2b was obtained in an encourag-
ing 60% yield but with a disappointing 38% ee (entry 2).
However, it turned out that this rearrangement is very
sensitive to the nature of the pyrazole substituent. Using
instead the N-acyl 3,5-dimethylpyrazole 1c provided the
cyclopropane in 74% yield and 97% ee as a single diastereo-
mer (entry 3). The best results were achieved with the N-acyl
3-methylpyrazole 1d, which provided the rearrangement
product in 83% yield with 98% ee upon photolysis in
CH2Cl2 at room temperature for 16 hours (entry 4). Reducing
the catalyst loading fourfold to 1.0 mol% only gradually
affected the yield and enantioselectivity (entries 5 and 6).
Even at a catalyst loading of merely 0.5 mol%, 46% of the
pyranone 4d with 95% ee was isolated after a longer
irradiation time of 48 hours, thus demonstrating the efficiency
of this asymmetric photorearrangement (entry 7). The reac-
tion can be performed in other solvents, such as acetone, with
Figure 2. Substrate scope. Standard conditions (entry 4 of Table 1)
were applied if not indicated otherwise. Modified reaction conditions
were used for obtaining products 2m, 2s, and 2u. [a] Catalyst loading
of 8 mol% was used to increase the ee value. [b] Reduced substrate
concentration of 0.005m was used to improve the yield. [c] Photolysis
with weaker blue LEDs (3 W, 420 nm) for 10 h afforded an improved
yield. [d] 1,2-Dichloroethane was used as the solvent which provided
a higher ee value.
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2x, containing an all-carbon quaternary center, in 67% yield
and excellent 99% ee. For practical reasons, it is worth noting
that this new asymmetric photoreaction can be scaled up. On
a scale of 4.5 mmol (1.08 g), the reaction 1d!2d afforded an
improved yield of 90% with a slightly increased enantiomeric
excess of 99% and the rhodium catalyst was reisolated in
86% yield by capture with an auxiliary ligand in a procedure
reported recently.[8]
The proposed mechanism is shown in Figure 3. The
catalytic cycle starts with the rhodium catalyst binding to
the N-acyl pyrazole substrate in a bidentate fashion (int. I).
This catalyst/substrate complex constitutes the photoactive
species. Upon absorption of visible light, the photoexcited
Figure 4. Mechanistic experiments. UV/Vis-absorption spectra mea-
sured in CH2Cl2 (0.05 mm).
Density-functional theory (DFT) calculations were per-
formed to evaluate the energetic feasibility of the proposed
mechanistic pathway (Figure 5a). The intramolecular HAT
reaction of the Rh-bound substrate was found to have a free-
energy barrier of 8.0 kcalmolÀ1, compared to 9.7 kcalmolÀ1
for the free substrate (see Figure S2 in the Supporting
Information). After intersystem crossing back to the singlet
state to furnish the ketene IV, the intramolecular hetero-
Diels–Alder reaction leading to the major enantiomer
proceeds through hDA-TS-1 with a barrier of 5.4 kcalmolÀ1
(Figure 5b). In contrast, hDA-TS-2, the hetero-Diels–Alder
transition state leading to the minor enantiomer, has a much
higher barrier of 16.6 kcalmolÀ1. The 11.2 kcalmolÀ1 differ-
ence in free energy between hDA-TS-1 and hDA-TS-2 can be
attributed to 1) stabilization from a p–p stacking interaction
between the aryl group on the substrate and the ligand
framework in hDA-TS-1, and 2) severe steric clashes between
the substrate and the tert-butyl group on the ligand in hDA-
TS-2.
Even though the direction of enantioselectivity predicted
by our calculations is in good agreement with the experimen-
tally observed asymmetric induction, the calculated DDG°
value of 11.2 kcalmolÀ1 far exceeds the experimental value of
2.2–2.4 kcalmolÀ1 (corresponding to 98% ee). Further calcu-
lations revealed that two possible triplet excited-state struc-
tures exist for II, one with the substrate alkene carbon centers
possessing most of the spin density (II-1 in Figure 5c), and the
other with the majority of its spin density localized on Rh (II-
2 in Figure 5c). Though the two triplet excited species lie
Figure 3. Proposed mechanism.
catalyst/substrate complex II is formed. In previous work we
demonstrated that such rhodium-bound a,b-unsaturated N-
acyl pyrazoles populate a ligand-centered p!p* photoex-
cited state with the spin density localized mainly at the
olefinic double bond.[3d,f] The close vicinity of the aldehyde
moiety now permits HAT to the a-position to generate the
diradical intermediate III.[9,10] An intersystem crossing to the
singlet-state ketene IV sets the stage for an intramolecular
asymmetric hetero-Diels–Alder reaction[11] to form the rho-
dium bound cyclopropane product VI. Product release and
recoordination of new substrate then initiates a new catalytic
cycle.
Several control experiments back up this mechanism.
Absorption spectra shown in Figure 4a reveal that the
catalyst/substrate complex absorbs visible light significantly
more efficiently compared to the catalyst (RhS), whereas the
free substrate does not absorb significantly in the visible-light
region. Thus, under the reaction conditions, the catalyst/
substrate complex is the main light absorbing species. The
HAT reaction from the photoexcited catalyst/substrate com-
plex II was verified by a deuterium isotope-labelling experi-
ment. The reaction of [D]-1d, with a deuterated aldehydic
position, yielded [D]-2d, in which the deuterium ended up in
the expected position of the cyclopropane moiety (78%
deuteration; Figure 4b).[12]
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substrate does not only facilitate a visible-light-induced
substrate-centered p!p* excitation, which permits the intra-
molecular HAT, but also enables a highly stereocontrolled
intramolecular oxo-Diels–Alder reaction through a very
congested transition state. Despite its mechanistic appeal,
the photochemistry introduced here provides access to non-
racemic, complex tricyclic architectures which are attractive
building blocks for further derivatizations.[14] Further catalytic
schemes which exploit the unique reactivities of photoexcited
catalyst/substrate complexes in the context of asymmetric
catalysis are ongoing in our laboratory.
Acknowledgements
E.M. gratefully acknowledges funding from the Deutsche
Forschungsgemeinschaft (ME 1805/17-1). K.N.H. is grateful
to the National Science Foundation (Grant CHE-1764328) for
financial support. Calculations were performed on the Hoff-
man2 cluster at the University of California, Los Angeles, and
the Extreme Science and Engineering Discovery Environ-
ment (XSEDE), which is supported by the National Science
Foundation (Grant OCI-1053575). C.H.Z. is grateful for
Ph.D. fellowship from the China Scholarship Council (CSC).
Conflict of interest
The authors declare no conflict of interest.
Figure 5. Computational study. a) Computed free-energy diagram of
the photoinduced HAT/hetero-Diels–Alder cascade reaction catalyzed
by D-RhS. b) Calculated transition states for the intramolecular hetero-
Diels–Alder reaction leading to major and minor product enantiomers.
Energies are in kcalmolÀ1, and interatomic distances are in Šngstrçms.
Hydrogen atoms are omitted for clarity. c) Two different calculated
triplet excited states, II-1 and II-2, for the Rh-bound substrate.
Interatomic distances denoted in Šngstrçms. Hydrogen atoms are
omitted for clarity. Spin densities on atoms are shown with red italic
numbers.
Keywords: asymmetric catalysis ·Diels–Alder ·
hydrogen atom transfer ·photochemistry ·rhodium
How to cite: Angew. Chem. Int. Ed. 2019, 58, 14462–14466
Angew. Chem. 2019, 131, 14604–14608
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In conclusion, we report herein a unique example of
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Manuscript received: May 7, 2019
Revised manuscript received: July 19, 2019
Version of record online: August 30, 2019
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