One-Pot Sequential Photoredox Chemistry and Asymmetric Transfer Hydrogenation with a Single Catalyst

Article abstract of DOI:10.1002/ejoc.201701652

A sequential process is reported, in which different photoredox reactions are placed in sequence with asymmetric transfer hydrogenations of aryl ketones to provide a diverse set of chiral alcohols with enantioselectivities of up to 99 % ee. The method relies on a single chiral-at-metal catalyst which is added at the beginning of the two step sequence and only a final purification of the reaction product is required.

Full text of DOI:10.1002/ejoc.201701652

DOI: 10.1002/ejoc.201701652
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Photoredox Catalysis | Very Important Paper |
One-Pot Sequential Photoredox Chemistry and Asymmetric
Transfer Hydrogenation with a Single Catalyst
Xiao Zhang,[a] Jie Qin,[a] Xiaoqiang Huang,[a] and Eric Meggers*[a]
Abstract: A sequential process is reported, in which different method relies on a single chiral-at-metal catalyst which is
photoredox reactions are placed in sequence with asymmetric
transfer hydrogenations of aryl ketones to provide a diverse set
of chiral alcohols with enantioselectivities of up to 99 % ee. The
added at the beginning of the two step sequence and only a
final purification of the reaction product is required.
Introduction
schemes in which a single catalyst mediates two mechanisti-
cally distinct reactions in a sequential fashion and we reported
a one-pot sequential asymmetric hydrogenation followed by a
photoredox reaction (Figure 1a).[6,7] Herein, we complement our
previous methodology with a one-pot sequential catalysis
protocol in which a photoredox reaction[8–10] (one-electron-
process) is executed first, followed by an asymmetric transfer
hydrogenation[11] (two-electron-process) to provide diverse chi-
ral alcohols with formation of a new C–C bond and establishing
one or more chiral centers requiring only a single catalyst and
a single final workup and purification step (Figure 1b).
Structural and stereochemical complexity is often associated
with superior biological properties regarding target affinity and
binding selectivity,[1] and has therefore become an important
concept for drug development.[2] Developing synthetic meth-
odology to build such molecular complexity in a straightfor-
ward and economical fashion is therefore an important objec-
tive in organic synthesis.[3] This has been addressed with the
concepts of multicomponent, cascade, domino, and tandem
processes, including (concurrent) tandem and sequential cataly-
sis.[4,5] We recently became interested in developing reaction
Results and Discussion
Initial Experiments
Previously, we introduced a novel class of asymmetric transition
metal catalysts in which the chirality of the complexes origi-
nates solely from a stereogenic metal center.[12] Such chiral-at-
metal complexes have demonstrated to be versatile catalysts
for a large number of reaction types including visible-light-
induced transformations.[13] For example, the bis-cyclometal-
ated iridium(III) complex IrS (see Figure 1 for structure)[14] was
demonstrated to catalyze enolate reactions, Michael additions,
cycloadditions, asymmetric hydrogenations, asymmetric trans-
fer hydrogenations, and a variety of different photoredox reac-
tions. In previous work we demonstrated that IrS is capable of
sequentially catalyzing asymmetric hydrogenations of ketones
followed by photoredox reactions in a one-pot two-step reac-
tion sequence by adding a single batch of the catalyst at the
beginning of the process. We now wanted to investigate if it
would be feasible to reverse the order of reactions, means exe-
cuting the photoredox process first, followed by an asymmetric
ketone reduction. This would provide additional flexibility in
the design of synthetic procedures and provide access to other
classes of products. Despite extensive experiments, we were not
successful to use asymmetric hydrogenations in such a de-
scribed Scheme most likely due to the sensitivity of the asym-
metric hydrogenation to certain reagents. However, gratifyingly,
Figure 1. Previous work and this study regarding combining photoredox
chemistry and asymmetric (transfer) hydrogenation in a sequential fashion
with a single catalyst. AH = asymmetric hydrogenation, ATH = asymmetric
transfer hydrogenation.
[a] Fachbereich Chemie, Philipps-Universität Marburg,
Hans-Meerwein-Straße 4, 35043 Marburg, Germany
*E-mail: meggers@chemie.uni-marburg.de
https://www.uni-marburg.de/fb15/ag-meggers
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701652.
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we found that the previously reported Λ-IrS-catalyzed asym- phenacyl bromides, followed by a trapping of these electron-
metric transfer hydrogenation (ATH) of aryl ketones[15] is robust deficient radicals with electron-rich silyl ketene acetals and
enough to be placed in sequence following an Λ-IrS-catalyzed/ thereby providing the ketone intermediate I which is then
mediated photoredox reaction. In all discussed processes, the
photoredox reaction introduces an aryl ketone functionality
which is then enantioselectively reduced in the following ATH
reaction. Note that the requirement for aryl ketones stems from
an overall low asymmetric induction in the IrS-catalyzed ATH
of alkyl ketones.[15] Since the IrS-catalyzed ATH reaction re-
quires the presence of a pyrazole coligand,[15] 3,5-dimethyl-
pyrazole (dmp) was added in most protocols right at the begin-
ning of the two-step reaction protocol.
enantioselectively reduced to the corresponding alcohol (Fig-
ure 2).[14a,16] Accordingly, we subjected phenacyl bromides 1
together with the silyl ketene acetal 2 in the presence of 2 mol-
% Λ-IrS and dmp (40 mol-%) to irradiation with a compact
fluorescence lamp (21 W) for 18 hours. Afterwards, the solvent
was simply changed from MeCN to THF/H2O (1:1), ammonium
formate (9 equiv.) was added for the subsequent ATH reaction,
and the reaction mixture was slightly warmed to 40 °C for 14 h.
To our delight, the chiral alcohols 3a–c were isolated in 73–
85 % yields and with high enantioselectivities of 93–95 % ee. It
is worth noting that a decreased enantioselectivity (86 % ee)
was obtained for 3a when blue LEDs were used instead, thereby
revealing the importance of the light source (see SI, p. S3). Inter-
estingly, the lactone (3d) was obtained in situ without any loss
of enantioselectivity (95 % ee) by treating the product mixture
with aqueous HCl.
Iridium-Catalyzed Phenacyl Radical Addition to Electron-
Rich Silyl Ketene Acetal and Sequential Asymmetric
Transfer Hydrogenation
The first investigated strategy relies on the generation of
phenacyl radicals via the photoredox-mediated reduction of
Figure 2. Iridium-catalyzed phenacyl radical addition to electron-rich silyl ketene acetal and sequential asymmetric transfer hydrogenation.
Figure 3. Iridium-catalyzed CF3 radical addition to electron-rich silyl enol ether and sequential asymmetric transfer hydrogenation.
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Iridium-Catalyzed CF3 Radical Addition to Electron-Rich
Silyl Enol Ethers and Sequential Asymmetric Transfer
Hydrogenation
oxidatively initiated photoredox processes in sequence with an
ATH reaction, we chose the reaction sequence shown in Fig-
ure 4. Accordingly, aryl vinyl ketones 7 were treated with N-
methyl-9,10-dihydroacridine (8) under photoredox conditions
and in the presence of trifluoroacetic acid (TFA)[21] followed by
ATH to provide the chiral alcohols 9, including a newly formed
C–C bond in the ꢀ-position to the chiral alcohol, in 71–79 %
yields and with 98–99 % ee (3 examples).[22] Herein, the acridine
is apparently converted into the corresponding γ-amino alkyl
radical via a sequence of single electron oxidation by the pho-
toexcited iridium catalyst and subsequent deprotonation, fol-
lowed by trapping of this electron-rich radical with the vinyl
ketone substrate and thereby forming the ketone intermediate
III, which is subsequently reduced highly enantioselectively to
the corresponding alcohol.
In order to construct more CF3-containing chiral alcohols that
might be attractive building blocks for the synthesis of bio-
active compounds,[17] we chose silyl enol ethers as suitable pre-
cursors to introduce carbonyl motifs given their well-estab-
lished capacity to accept electron-deficient radicals.[18–20] We
were delighted to find that different silyl enol ethers 4 reacted
with Togni's reagent (5) under photoredox conditions to pro-
vide the intermediate ketones II, which were then subjected to
ATH conditions providing chiral trifluoromethyl alcohols 6a–d
in 76–81 % yields and with 91–94 % ee (Figure 3). It is worth
mentioning that increasing the amount of dmp and the utiliza-
tion of a compact fluorescence lamp are crucial for achieving
high ee values (see SI, p. S5).
Two Catalytic Asymmetric Reactions in Sequence with
Λ-IrS
Iridium-Catalyzed Radical Conjugate Addition and
Sequential Asymmetric Transfer Hydrogenation
As a proof-of-principle demonstration, our final efforts were
In all photoredox reactions presented so far, Λ-IrS/dmp served dedicated to investigate the possibility to link two catalytic
as a photoredox mediator/catalyst for a reductive substrate acti- asymmetric reactions catalyzed both by Λ-IrS (Figure 5). This
vation in otherwise redox neutral reactions. In order to demon- would demonstrate the merit of our modular, sequential strat-
strate that Λ-IrS/dmp is also capable of catalyzing or mediating egy applied to products bearing two stereocenters. As shown
Figure 4. Iridium-catalyzed radical conjugate addition and sequential asymmetric transfer hydrogenation.
Figure 5. Iridium-catalyzed asymmetric photoredox chemistry and asymmetric transfer hydrogenation.
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charged with phenacyl bromides 1 (0.20 mmol), Λ-IrS (3.8 mg,
2 mol-%), and 3,5-dimethyl pyrazole (7.7 mg, 40 mol-%). The tube
in Figure 5, 2-acyl imidazole 10 was first reacted with phenacyl
bromide 1a in the presence of Λ-IrS (4 mol-%) and a weak base
under the activation by visible light to generate the intermedi-
ate product IV as reported recently,[14a] which was not isolated,
but upon solvent exchange and the addition of ammonium for-
mate and the ligand dmp, subjected to asymmetric transfer
hydrogenation to provide compound 11 in 80 % yield, 96 % ee,
and 4.5:1 dr. It is worth noting that the second carbonyl group
is not affected in this chemoselective reduction which might be
was purged with nitrogen, then CH3CN (0.5 mL, 0.4
M) was added
via syringe, followed by silyl ketene acetal 2 (112.8 mg, 3.0 equiv.).
The reaction mixture was degassed via freeze-pump-thaw for three
cycles. After the mixture was thoroughly degassed, the vial was
sealed and positioned approximately 5 cm from a compact fluores-
cent lamp (21 W). The reaction was stirred at room temperature for
the indicated time (monitored by TLC) under nitrogen atmosphere.
Afterwards, the solvent was removed, and HCO2NH4 (113.4 mg,
due to the higher steric crowding around the 2-acyl imidazole 1.80 mmol, 9.0 equiv.) and THF/H2O (0.1 mL/0.1 mL, 1.0
M) were
added to the residual crude material. The reaction solution was
stirred at 40 °C until complete disappearance of the starting mate-
rial (detected by TLC), then cooled down to room temperature, con-
centrated under reduced pressure, and purified by flash chromatog-
raphy on silica gel (n-hexane/ethyl acetate = 2:1) to afford the prod-
uct 3.
moiety. Intriguingly, in this process, a single charge of Λ-IrS
was added at the beginning of the reaction, which catalyzed
two mechanistically distinct asymmetric processes in a sequen-
tial fashion, and thereby implementing one new C–C bond and
two new stereocenters.
Methyl (R)-4-(4-Bromophenyl)-4-hydroxybutanoate (3a):[23] Ac-
cording to the general procedure, the reaction between 2-bromo-
1-(4-bromophenyl)ethan-1-one 1a (55.6 mg, 0.20 mmol) and silyl
ketene acetal 2 (112.8 mg, 3.0 equiv.) gave 46.2 mg (85 %) of 3a as
a yellow oil. Enantiomeric excess was established by HPLC analysis
using a Chiralcel OD-H column, ee = 95 % [HPLC: OD-H, 220 nm, n-
hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr (minor) =
Conclusions
We have introduced a sequential protocol, in which a single
chiral-at-metal complex mediates a photoredox reaction in se-
quence with the catalysis of a subsequent asymmetric transfer
hydrogenation to provide chiral alcohol building blocks with
enantiomeric excess reaching up to 99 % ee. A single batch of
catalyst is added at the beginning of the sequence and only a
single purification step is required. Although this new protocol
has clear limitations in scope, it nicely complements our previ-
ous work on sequential asymmetric hydrogenation/photoredox
processes.
1
9.87 min, tr (major) = 11.62 min]. [α]2D2 = +20.4 (c = 1.0, CH2Cl2). H
NMR (300 MHz, CDCl3): δ = 7.40 (d, J = 8.4 Hz, 2 H), 7.15 (d, J =
8.1 Hz, 2 H), 4.68–4.63 (m, 1 H), 3.60 (s, 3 H), 2.38–2.33 (m, 3 H),
1.99–1.92 (m, 2 H) ppm.
Methyl (R)-4-(4-Chlorophenyl)-4-hydroxybutanoate (3b):[24] Ac-
cording to the general procedure, the reaction between 2-bromo-
1-(4-chlorophenyl)ethan-1-one 1b (46.6 mg, 0.20 mmol) and silyl
ketene acetal 2 (112.8 mg, 3.0 equiv.) gave 38.0 mg (83 %) of 3b as
a yellow solid. Enantiomeric excess was established by HPLC analy-
sis using a Chiralcel OD-H column, ee = 95 % [HPLC: OD-H, 220 nm,
n-hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr (minor) =
Experimental Section
General: All reactions were carried out under an atmosphere of
nitrogen with magnetic stirring. Catalytic reactions were performed
in a Schlenk tube (10 mL). A 21 W compact fluorescent lamp (CFL),
6 W and 24 W Blue LEDs served as light sources. The catalyst Λ-
IrS[14] was synthesized according to our published procedures.
HPLC grade of solvents and deionized water were used without
further purification. Reagents that were purchased from commercial
suppliers were used without further purification. Flash column chro-
1
9.13 min, tr (major) = 10.81 min]. [α]2D2 = +28.5 (c = 1.0, CH2Cl2). H
NMR (300 MHz, CDCl3): δ = 7.26–7.19 (m, 4 H), 4.672–4.665 (m, 1
H), 3.60 (s, 3 H), 2.38–2.33 (m, 3 H), 1.99–1.93 (m, 2 H) ppm.
Methyl (R)-4-Hydroxy-4-(3-nitrophenyl)butanoate (3c): Accord-
ing to the general procedure, the reaction between 2-bromo-1-(3-
nitrophenyl)ethan-1-one 1c (48.8 mg, 0.20 mmol) and silyl ketene
acetal 2 (112.8 mg, 3.0 equiv.) gave 35.0 mg (73 %) of 3c as a yellow
oil. Enantiomeric excess was established by HPLC analysis using a
Chiralcel OD-H column, ee = 93 % [HPLC: OD-H, 220 nm, n-hexane/
2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr (minor) =
13.77 min, tr (major) = 15.32 min]. [α]2D2 = +24.3 (c = 1.0, CH2Cl2).
1H NMR (300 MHz, CDCl3): δ = 8.24 (s, 1 H), 8.13 (d, J = 8.1 Hz, 1 H),
7.70 (d, J = 7.5 Hz, 1 H), 7.52 (t, J = 7.8 Hz, 1 H), 4.93–4.89 (m, 1 H),
3.69 (s, 3 H), 2.80 (s, 1 H), 2.58–2.40 (m, 2 H), 2.18–2.01 (m, 2 H)
ppm.
Lactone 3d:[25] Followed the general procedure for the reaction
between 2-bromo-1-(4-chlorophenyl)ethan-1-one 1b (46.6 mg,
0.20 mmol) and silyl ketene acetal 2 (112.8 mg, 3.0 equiv.), 10 mL
of saturated HCl aqueous solution was added to quench the reac-
tion. The mixture was extracted with EtOAc and concentrated to
afford the crude residue, which was further dissolved in MeOH
matography was performed with silica gel 60
M from Macherey–
Nagel (irregular shaped, 230–400 mesh, pH 6.8, pore volume:
0.81 mL g–1, mean pore size: 66 Å, specific surface: 492 m2 g–1, par-
ticle size distribution: 0.5 % < 25 μm and 1.7 % > 71 μm, water
1
content: 1.6 %). H NMR, 19F NMR and proton decoupled 13C NMR
spectra were recorded on Bruker Avance 300 (300 MHz), or Bruker
AM (500 MHz) spectrometers at ambient temperature. NMR stan-
dards were used as follows: 1H NMR spectroscopy: δ = 7.26 ppm
(CDCl3). 19F NMR spectroscopy: δ = 0 ppm (CFCl3). 13C NMR spectro-
scopy: δ = 77.0 ppm (CDCl3). IR spectra were recorded on a Bruker
Alpha FT-IR spectrophotometer. High-resolution mass spectra were
recorded on a Bruker En Apex Ultra 7.0 TFT-MS instrument using
ESI/EI/APCI/FD technique. Chiral HPLC chromatography was
performed with an Agilent 1200, Agilent 1260 HPLC system or
Shimadzu Lc-2030c HPLC system. Optical rotations were measured
on a Krüss P8000-T polarimeter with [α]2D2 values reported in de-
grees with concentrations reported in g/100 mL.
(0.2 mL). To the corresponding solution, a solution of HCl (1 M) was
added. The mixture was stirred at 40 °C for 24 h, then the solvent
was carefully evaporated under reduced pressure, water was added,
General Procedure for Iridium-Catalyzed Phenacyl Radical Ad- and the solution was extracted with EtOAc. The organic layers were
dition to Electron-rich Silyl Ketene Acetal and Sequential Asym-
combined, dried with Na2SO4, and purified by flash chromatogra-
metric Transfer Hydrogenation: A dried 10 mL Schlenk tube was
phy on silica gel (n-hexane/dichloromethane/ethyl acetate = 8:2:1)
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to afford 27.8 mg (71 %) of 3d as a white solid. Enantiomeric excess
was established by HPLC analysis using a Chiralpak AS-H column,
ee = 95 % [HPLC: AS-H, 210 nm, n-hexane/2-propanol = 75:25, flow
2-yl)vinyl]oxy}trimethylsilane 4d (69.6 mg, 0.30 mmol) and Togni's
reagent 5 (63.2 mg, 0.20 mmol) gave 35.0 mg (76 %) of 6d as a
yellow oil. Enantiomeric excess was established by HPLC analysis
using a Chiralpak AS-H column, ee = 91 % [HPLC: AS-H, 220 nm, n-
rate 1 mL/min, 35 °C, tr (major) = 11.99 min, tr (minor) = 14.36 min].
1
[α]2D2 = +27.4 (c = 1.0, CH2Cl2). H NMR (300 MHz, CDCl3): δ = 7.32– hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr (major) =
1
7.28 (m, 2 H), 7.22–7.19 (m, 2 H), 5.43–5.39 (m, 1 H), 2.65–2.54 (m,
3 H), 2.16–2.00 (m, 1 H) ppm.
6.46 min, tr (minor) = 7.46 min]. [α]2D2 = +25.6 (c = 1.0, CH2Cl2). H
NMR (300 MHz, CDCl3): δ = 7.49 (d, J = 7.5 Hz, 1 H), 7.39 (d, J =
8.1 Hz, 1 H), 7.25–7.14 (m, 2 H), 6.63 (s, 1 H), 5.16 (br. s, 1 H), 2.77–
2.64 (m, 2 H), 2.28 (br. s, 1 H) ppm.
General Procedure for Iridium-Catalyzed CF3 Radical Addition
to Electron-rich Silyl Enol Ethers and Sequential Asymmetric
Transfer Hydrogenation: A dried 10 mL Schlenk tube was charged
with Togni's reagent 5 (63.2 mg, 0.20 mmol), Λ-IrS (3.8 mg, 2 mol-
General Procedure for Iridium-Catalyzed Radical Conjugate Ad-
dition and Sequential Asymmetric Transfer Hydrogenation: A
%) and 3,5-dimethyl pyrazole (57.6 mg, 3.0 equiv.). The tube was dried 10 mL Schlenk tube was charged with 10-methyl-9,10-di-
purged with nitrogen, then CH3CN (0.5 mL, 0.4
M) was added via
hydroacridine 8 (39.0 mg, 0.20 mmol), Λ-IrS (3.8 mg, 2 mol-%) and
syringe, followed by silyl enol ethers 4 (0.30 mmol) and 2,6-lutidine
3,5-dimethyl pyrazole (7.7 mg, 40 mol-%). The tube was purged
(42.8 mg, 0.40 mmol). The reaction mixture was degassed via with nitrogen, then CH3CN (0.8 mL, 0.25
M) was added via syringe,
freeze-pump-thaw for three cycles. After the mixture was thor-
oughly degassed, the vial was sealed and positioned approximately
5 cm from a 21 W compact fluorescent lamp. The reaction was
stirred at room temperature for the indicated time (monitored by
followed by enones 7 (0.40 mmol), and trifluoroacetic acid (22.8 mg,
0.20 mmol). The reaction mixture was degassed via freeze-pump-
thaw for three cycles. After the mixture was thoroughly degassed,
the vial was sealed and positioned approximately 3 cm from blue
TLC) under nitrogen atmosphere. Afterwards, the solvent was re- LEDs (6 W). The reaction was stirred at room temperature for the
moved, HCO2NH4 (113.4 mg, 1.80 mmol, 9.0 equiv.) and THF/H2O
indicated time (monitored by TLC) under nitrogen atmosphere.
(0.1 mL/0.1 mL, 1.0 ) were added to the residual crude material. Afterwards, the solvent was removed, HCO2NH4 (113.4 mg,
The reaction solution was stirred at 60 °C until complete disappear- 1.80 mmol, 9.0 equiv.) and THF/H2O (0.1 mL/0.1 mL, 1.0
) were
added to the residual crude material. The reaction solution was
M
M
ance of the starting material (detected by TLC), then cooled down
to room temperature, concentrated under reduced pressure, and
purified by flash chromatography on silica gel (n-hexane/ethyl acet-
ate = 12:1) to afford the products 6.
stirred at 40 °C until complete disappearance of the starting mate-
rial (detected by TLC), then cooled down to room temperature, con-
centrated under reduced pressure and purified by flash chromatog-
raphy on silica gel (n-hexane/ethyl acetate = 12:1) to afford the
products 9.
(R)-1-([1,1′-Biphenyl]-4-yl)-3,3,3-trifluoropropan-1-ol
(6a):[26]
According to the general procedure, the reaction between ({1-[(1,1′-
biphenyl)-4-yl]vinyl}oxy)trimethylsilane 4a (80.4 mg, 0.30 mmol)
and Togni's reagent 5 (63.2 mg, 0.20 mmol) gave 43.0 mg (81 %) of
6a as a yellow oil. Enantiomeric excess was established by HPLC
analysis using a Chiralpak AD-H column, ee = 93 % [HPLC: AD-H,
254 nm, n-hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr
(minor) = 7.49 min, tr (major) = 8.30 min]. [α]2D2 = +31.0 (c = 1.0,
CH2Cl2). 1H NMR (300 MHz, CDCl3): δ = 7.50–7.23 (m, 9 H), 5.02–
4.97 (m, 1 H), 2.64–2.29 (m, 2 H), 2.18 (s, 1 H) ppm.
(R)-3-(10-Methyl-9,10-dihydroacridin-9-yl)-1-phenylpropan-1-ol
(9a): According to the general procedure, the reaction between 1-
phenylprop-2-en-1-one 7a (52.8 mg, 0.40 mmol) and 10-methyl-
9,10-dihydroacridine 8 (39.0 mg, 0.20 mmol) gave 50.7 mg (77 %)
of 9a as a white solid. Enantiomeric excess was established by HPLC
analysis using a Chiralcel OD-H column, ee = 99 % [HPLC: OD-H,
254 nm, n-hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr
(major) = 14.03 min, tr (minor) = 29.75 min]. [α]2D2 = +7.0 (c = 1.0,
CH2Cl2). 1H NMR (300 MHz, CDCl3): δ = 7.25–7.02 (m, 9 H), 6.87–
6.80 (m, 4 H), 4.40 (t, J = 6.0 Hz, 1 H), 3.73 (t, J = 6.6 Hz, 1 H), 3.27
(s, 3 H), 1.71–1.56 (m, 4 H), 1.47–1.39 (m, 1 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 144.6, 142.4, 128.4, 128.2, 128.1, 127.6, 127.5,
127.4, 126.85, 126.81, 125.9, 120.5, 112.1, 112.0, 74.6, 44.0, 36.0, 33.6,
(R)-3,3,3-Trifluoro-1-(4-methoxyphenyl)propan-1-ol (6b): Ac-
cording to the general procedure, the reaction between {[1-(4-
methoxyphenyl)vinyl]oxy}trimethylsilane 4b (66.6 mg, 0.30 mmol)
and Togni's reagent 5 (63.2 mg, 0.20 mmol) gave 34.0 mg (77 %) of
6b as a yellow oil. Enantiomeric excess was established by HPLC
analysis using a Chiralcel OJ-H column, ee = 94 % [HPLC: OJ-H,
220 nm, n-hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr
(minor) = 11.42 min, tr (major) = 12.13 min]. [α]2D2 = +19.4 (c = 1.0,
CH2Cl2). 1H NMR (300 MHz, CDCl3): δ = 7.24–7.19 (m, 2 H), 6.86–
6.81 (m, 2 H), 4.97–4.95 (m, 1 H), 3.74 (s, 3 H), 2.66–2.47 (m, 1 H),
2.45–2.27 (m, 1 H), 1.98 (s, 1 H) ppm.
32.9 ppm. IR (film): ν = 3336, 3029, 2945, 2904, 2869, 2812, 1591,
˜
1465, 1336, 1267, 1200, 1127, 1061, 1035, 928, 878, 841, 732, 697,
640, 574, 534, 500, 416 cm–1. HRMS (APCI, m/z) calcd. for C23H22
N
[M + H – H2O]+: 312.1747, found 312.1739.
(R)-1-(4-Fluorophenyl)-3-(10-methyl-9,10-dihydroacridin-9-yl)-
propan-1-ol (9b): According to the general procedure, the reaction
between 1-(4-fluorophenyl)prop-2-en-1-one 7b (60.0 mg,
0.40 mmol) and 10-methyl-9,10-dihydroacridine 8 (39.0 mg,
0.20 mmol) gave 55.0 mg (79 %) of 9b as a white solid. Enantio-
meric excess was established by HPLC analysis using a Chiralcel OD-
H column, ee = 98 % [HPLC: OD-H, 254 nm, n-hexane/2-propanol =
90:10, flow rate 1 mL/min, 25 °C, tr (major) = 14.26 min, tr (minor) =
29.11 min]. [α]2D2 = +33.8 (c = 1.0, CH2Cl2). 1H NMR (300 MHz, CDCl3):
δ = 7.14–7.01 (m, 6 H), 6.88–6.79 (m, 6 H), 4.35 (t, J = 6.0 Hz, 1 H),
3.72 (t, J = 6.6 Hz, 1 H), 3.25 (s, 3 H), 1.67–1.53 (m, 4 H), 1.42–1.34
(m, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 163.0, 161.1, 142.3,
140.22, 140.20, 128.12, 128.10, 127.6, 127.49, 127.46, 127.39, 126.89,
126.86, 120.5, 115.2, 115.1, 112.08, 112.05, 73.9, 44.0, 36.1, 33.4, 32.9
(R)-1-(4-Bromophenyl)-3,3,3-trifluoropropan-1-ol (6c): Accord-
ing to the general procedure, the reaction between {[1-(4-
bromophenyl)vinyl]oxy}trimethylsilane 4c (81.0 mg, 0.30 mmol) and
Togni's reagent 5 (63.2 mg, 0.20 mmol) gave 43.0 mg (80 %) of 6c
as a yellow oil. Enantiomeric excess was established by HPLC analy-
sis using a Chiralcel OJ-H column, ee = 93 % [HPLC: OJ-H, 220 nm,
n-hexane/2-propanol = 90:10, flow rate 1 mL/min, 25 °C, tr (minor) =
1
8.34 min, tr (major) = 9.13 min]. [α]2D2 = +32.8 (c = 1.0, CH2Cl2). H
NMR (300 MHz, CDCl3): δ = 7.44 (d, J = 8.4 Hz, 2 H), 7.18 (d, J =
8.1 Hz, 2 H), 5.00–4.97 (m, 1 H), 2.62–2.44 (m, 1 H), 2.42–2.30 (m, 1
H), 2.13 (d, J = 2.1 Hz, 1 H) ppm.
(R)-1-(Benzofuran-2-yl)-3,3,3-trifluoropropan-1-ol (6d): Accord-
ing to the general procedure, the reaction between {[1-(benzofuran-
ppm. 19F NMR (282 MHz, CDCl3): δ = –115.32 ppm. IR (film): ν =
˜
3334, 3286, 3068, 3037, 2952, 2915, 2880, 1594, 1505, 1470, 1341,
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1274, 1217, 1153, 1130, 1068, 1041, 929, 882, 828, 746, 649, 537,
499, 411 cm–1. HRMS (APCI, m/z) calcd. for C23H21FN [M + H – H2O]+:
330.1653, found 330.1653.
Keywords: Sequential catalysis · One-pot reaction · Chiral-
at-metal · Photoredox · Asymmetric transfer hydrogenation
(R)-1-(4-Chlorophenyl)-3-(10-methyl-9,10-dihydroacridin-9-yl)-
propan-1-ol (9c): According to the general procedure, the reaction
between 1-(4-chlorophenyl)prop-2-en-1-one 7c (66.4 mg,
0.40 mmol) and 10-methyl-9,10-dihydroacridine 8 (39.0 mg,
0.20 mmol) gave 51.6 mg (71 %) of 9c as a white solid. Enantiomeric
excess was established by HPLC analysis using a Chiralcel OD-H
column, ee = 98 % [HPLC: OD-H, 254 nm, n-hexane/2-propanol =
90:10, flow rate 1 mL/min, 25 °C, tr (major) = 14.21 min, tr (minor) =
27.66 min]. [α]2D2 = +4.2 (c = 1.0, CH2Cl2). 1H NMR (300 MHz, CDCl3):
δ = 7.17–7.10 (m, 4 H), 7.05–7.02 (m, 4 H), 6.87–6.81 (m, 4 H), 4.38
(t, J = 5.7 Hz, 1 H), 3.73 (t, J = 6.0 Hz, 1 H), 3.27 (s, 3 H), 1.65–1.56
(m, 4 H), 1.45–1.36 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
143.0, 142.4, 133.0, 128.5, 128.1, 127.45, 127.40, 127.3, 126.93,
126.90, 120.5, 112.11, 112.09, 73.8, 44.0, 36.0, 33.4, 32.9 ppm. IR
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(film): ν = 3417, 3370, 3058, 3030, 2907, 2837, 2012, 1590, 1469,
˜
1338, 1269, 1209, 1133, 1085, 1061, 1008, 921, 882, 826, 746, 578,
541, 500, 463, 418 cm–1. HRMS (APCI, m/z) calcd. for C23H21ClN [M
+ H – H2O]+: 346.1357, found 346.1356.
Iridium-Catalyzed Asymmetric Photoredox Chemistry and
Asymmetric Transfer Hydrogenation: A dried 10 mL Schlenk tube
was charged with 2-bromo-1-(4-bromophenyl)ethan-1-one 1a
(27.8 mg, 0.10 mmol), 2-acyl imidazole 10 (34.6 mg, 0.20 mmol),
Na2HPO4 (15.6 mg, 0.11 mmol) and Λ-IrS (3.8 mg, 4 mol-%). The
tube was purged with nitrogen, then MeOH/THF (0.4 mL/0.1 mL,
0.2
M) was added via syringe. The reaction mixture was degassed
via freeze-pump-thaw for three cycles. After the mixture was thor-
oughly degassed, the vial was sealed and positioned approximately
5 cm from a 21 W compact fluorescent lamp. The reaction mixture
was stirred at 40 °C for the indicated time (monitored by TLC) under
nitrogen atmosphere. Afterwards, the solvent was removed, 3,5-di-
methyl pyrazole (7.7 mg, 80 mol-%), HCO2NH4 (56.8 mg, 0.9 mmol,
9.0 equiv.) and THF/H2O (0.10 mL/0.10 mL, 0.5
M) were added to
the residual crude material. The reaction mixture was stirred at
60 °C until complete disappearance of the starting material (de-
tected by TLC), then cooled down to room temperature, concen-
trated under reduced pressure and purified by flash chromatogra-
phy on silica gel (n-hexane/ethyl acetate = 2:1) to afford 33.0 mg
of 11 (80 % yield, 4.5:1 dr) as a yellow oil. The enantioselectivity for
the major product was 96 % ee. Enantiomeric excess was estab-
lished by HPLC analysis using a Chiralpak IG column, 96 % ee [HPLC:
IG, 220 nm, n-hexane/2-propanol = 90:10, flow rate 1.0 mL/min,
25 °C, tr (minor) = 19.39 min, tr (major) = 22.21 min]. [α]2D2 = +30.9
(c = 1.0, CH2Cl2). The product was present as a 4.5:1 mixture of
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1
diastereoisomers. H NMR (300 MHz, CDCl3): δ = 7.32 (t, J = 8.4 Hz,
3 H), 7.17–6.97 (m, 6 H), 6.94–6.86 (m, 1 H), 5.66–5.54 (m, 1 H), 4.51–
4.47 (m, 1 H), 3.86 and 3.81 (s, 3 H), 2.57–2.31 (m, 4 H), 1.97–1.88
(m, 1 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 193.5, 143.7, 143.4,
137.6, 136.3, 131.2, 130.4, 129.2, 127.6, 127.53, 127.46, 127.40, 126.9,
126.1, 120.8, 70.6, 44.9, 44.2, 36.1, 20.1 ppm. IR (film): ν = 3390,
˜
3109, 3055, 3020, 2954, 2921, 2866, 1669, 1593, 1482, 1401, 1289,
1265, 1232, 1156, 1067, 1006, 962, 908, 868, 825, 770, 735, 694, 662,
625, 597, 559, 520, 456 cm–1. HRMS (APCI, m/z) calcd. for
C21H21BrN2O2H [M + H]+: 413.0859, found 413.0852.
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Acknowledgments
We are grateful for the financial support from the Deutsche
Forschungsgemeinschaft (ME 1805/13-1) and the Alexander
von Humboldt Foundation (X. Z.).
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Received: November 28, 2017
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