Bifunctional photocatalysts for enantioselective aerobic oxidation of β-ketoesters

Article abstract of DOI:10.1021/jacs.6b11418

A novel visible-light-responsive chiral ligand has been developed by grafting a triplet state photosensitizer to chiral bisoxazoline ligands. Complexation of this ligand with Ni(acac)₂ results in a powerful catalyst for the asymmetric oxidation reaction of β-ketoesters, which uses oxygen or air as the green oxidant and visible light or sunlight as the ideal driving force. Using this protocol, Products containing the α-hydroxy-β-dicarbonyl motif are produced in high yields and with excellent enantiopurities.

Full text of DOI:10.1021/jacs.6b11418

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Communication
Bifunctional Photocatalysts for Enantioselective
Aerobic Oxidation of #-Ketoesters
Wei Ding, Liang-Qiu Lu, Quan-Quan Zhou, Yi Wei, Jia-Rong Chen, and Wen-Jing Xiao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11418 • Publication Date (Web): 21 Dec 2016
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Bifunctional Photocatalysts for Enantioselective Aerobic Oxi-
dation of β-Ketoesters
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Wei Ding,§ Liang-Qiu Lu,§ Quan-Quan Zhou, Yi Wei, Jia-Rong Chen, and Wen-Jing Xiao*
CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide and Chemical Biology, Ministry of Education; College
of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan 430079, China.
Supporting Information Placeholder
hybrid would efficiently coordinate the various metals and that a
library of chiral bifunctional photocatalysts could be established for
asymmetric visible light photocatalysis.
ABSTRACT: A novel visible light-responsive chiral ligand has
been developed by grafting a triplet state photosensitizer to chiral
bisoxazoline ligands. Complexation of this ligand with Ni(acac)2
results in a powerful catalyst for the asymmetric oxidation reaction
of β‑ketoesters, which uses oxygen or air as the green oxidant and
visible light or sunlight as the ideal driving force. Using this proto-
col, products containing the α-hydroxy-β-dicarbonyl motif are pro-
duced in high yields and with excellent enantiopurities.
New strategies in asymmetric catalysis hold the potential to rev-
olutionize the production of chiral materials, pharmaceuticals,
agro-, and fine chemicals. For this reason, seeking efficient cata-
lysts to promote stereoselective chemical transformations is a fun-
damentally important task in contemporary chemical synthesis.1
Over the past decade, asymmetric photocatalysis induced by visible
light has flourished as a powerful tool for the sustainable, highly
efficient and selective production of enantioenriched com-
pounds.2,3 However, asymmetric visible light photocatalysis has
mainly relied on the use of two separate catalysts, an achiral pho-
tocatalyst and a chiral co-catalyst, in a single organic transfor-
mation.4,5 A single chiral photocatalyst that is able to induce a ste-
reoselective photochemical reaction would be advantageous but is
challenging,6-8 because it must possess the following two functions:
harvest visible light or sunlight to activate the reagents and control
the stereoselectivity during the chemical bond formation. In this
area, Bach demonstrated the first example of highly enantioselec-
tive photochemical reactions where a chiral hydrogen bonding-
based thioxanthone was used as a bifunctional visible light photo-
sensitizer to promote an intramolecular asymmetric [2+2] cycload-
dition.6b Soon after, Meggers elegantly developed a chiral iridium
complex as bifunctional photocatalysts to achieve asymmetric
transformations of 2-acyl imidazoles with excellent enantiocon-
trol.7a Despite these pioneering work, the design and invention of
new bifunctional chiral photocatalysts bearing cheap metals for ef-
ficient asymmetric photochemical transformations is highly desir-
able to enable new reactivity.8e,f
Figure 1. Blueprint for bifunctional photocatalysts. ET: energy
transfer. HAT: hydrogen abstraction transfer. X: O, S…
Oxidative hydroxylation is an important process in Nature. For
example, Cytochrome P-450, an important monooxygenase, can
activate dioxygen under mild conditions and allow the living cells
to introduce the hydroxyl group with high efficiencies and perfect
selectivities.12 In contrast to Nature’s biosynthesis, the chemical
synthesis most often relies on the use of strong oxidants. In this
field, given many bioactive molecules containing the α-hydroxy-β-
dicarbonyl motif (e.g., natural product Hamigeran A and insecti-
cide Indoxacarb),13 ongoing efforts have been dedicated to the di-
rect asymmetric oxidative hydroxylation of β-keto esters.14 The use
of oxygen gas as a green oxidant to perform this transformation
under mild conditions would be ideal. In this work, we chose the
visible light-induced asymmetric aerobic photooxygenation of β-
keto esters to assess our approach for bifunctional photocatalyst de-
velopment and applications. As illustrated in Figure 2, we
Given that bisoxazolines (BOXs) are privileged ligands in asym-
metric catalysis9 and that diarylketones (e.g., xanthone, thioxan-
thone and 9-fluorenone) can activate many functional groups or re-
agents under irradiation with ultraviolet (UV)/visible light,6,10 we
considered whether new visible light-responsive ligands could be
invented by grafting a thioxanthone motif onto chiral BOX ligands
(Figure 1). Tang and Gade have elegantly demonstrated that the
introduction of an additional group in the bridge carbon would im-
prove the stereo-control capacities of chiral BOX ligands in the
asymmetric catalysis. Thus, as a critical element of this design, we
recognized that the BOX component in this ligand/photosensitizer
Figure 2. Work hypothesis: application of bifunctional photocata-
lysts in asymmetric photooxygenation of β-keto esters.
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Table 1. Optimization of Reaction Conditiona
speculated that the thioxanthone component could function as an
efficient energy-transfer photocatalyst,15 with its ground state ab-
sorbing visible light and its triplet state then sensitizing the unreac-
tive triplet state of molecule oxygen (3O2) to generate the reactive
singlet state of molecular oxygen (1O2).16 The chiral metal complex
derived from the BOX component can serve as an efficient catalyst
to activate β-keto carbonyls and then control the enantioselectivity
of the C-O bond-formation step of the reaction.
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2
3
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5
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8
Lewis
acid
T
yield
ee
To start this research, as shown in Scheme 1, chiral ligands 3
were synthesized from readily available reagents 1 and 2 through
an esterification operation. The UV/Vis spectra (Figure S1) re-
vealed that chiral ligands 3 have a maximum absorb of visible light
at ca. 400 nm wavelengths. Then, the aerobic photooxygenation of
isopropyl ester 4a was investigated with a series of bifunctional
chiral photocatalysts which were in situ generated from 3a and
entry
3
solvent
(oC) (%)b
(%)c
9
1
2
Cu(OTf)2 3a
Zn(OTf)2 3a
DCM
DCM
-20
-20
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3
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5
Mg(OTf)2 3a
Ni(OTf)2 3a
Sc(OTf)3 3a
Ni(acac)2 3a
DCM
DCM
DCM
DCM
-20
-20
-20
-20
-20
-10
-10
-10
-10
-10
-10
-10
-15
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0
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0
Scheme 1. Synthesis of New Chiral Ligands
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64
-70
-27
-68
-64
73
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95
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15d
16d,e
Ni(acac)2 3a toluene
Ni(acac)2 3a toluene
Ni(acac)2 3b toluene
Ni(acac)2 3c toluene
Ni(acac)2 3d toluene
Ni(acac)2 3e toluene
Ni(acac)2 3f toluene
Ni(acac)2 3g toluene
Ni(acac)2 3f toluene
Ni(acac)2 3f toluene
aUnless otherwise noted, reactions are conducted with 4a (0.1
mmol), Lewis acid/3 (10 mol%) and 4Å MS (20 mg) in 2 mL of
solvent under visible light at O2 for 24 h. bAll yields are isolated
yields cEnantiomeric excess determined by chiral HPLC analy-
sis. dWithout 4Å MS. eSubstrate 4b was used.
various metal precursors (Table 1, entries 1-6). Preliminary studies
revealed that the use of Ni(acac)2 gave the hydroxylation product
5a in high yield and with modest enantioselectivity (entry 6, 88%
yield and 33% ee) under irradiation with visible light. Surveying
the reaction media for this photoreaction indicated that toluene sub-
stantially improved the stereochemical control (entries 7-8). Chiral
ligands 3a-g were examined in the presence of Ni(acac)2 (entries 8-
14), and gratifyingly, biphenyl-substituted ligand 3f was deter-
mined to be the optimal ligand choice for enantiocontrol (entry 13,
80% yield and 73% ee). The substituents on the spacer, such as Me
or i-Pr, show only minimal impact on the reaction efficiency (entry
13 vs 14). Varying the ester moiety in the substrate from isopropyl
to adamantyl groups gave superior levels of enantioselectivity and
reaction efficiency (entry 16, 97% yield and 95% ee). Control ex-
periments indicated that visible light, Lewis acid, chiral ligand and
oxygen gas were essential for this transformation (Table S3).17
yields and 90-92% ee). Substrates with other steric ester groups and
amide groups were also examined and could be successfully trans-
formed into hydroxylation products with satisfactory yields and en-
antioselectivities (5q-s, 82-95% yields and 88-90% ee). Six- and
seven-membered bicyclic β-keto esters and the aliphatic cyclic β-
keto esters could be effectively employed as substrates for this
transformation, producing the hydroxylation products 5t-v in 85-
95% yields and 85-93% ee. In contrast, when cyclic 1,3-diketones
and acyclic -keto esters were examined with this asymmetric pho-
tooxygenation reaction, no conversion was observed at this stage.
Further experiments were conducted to demonstrate the utility of
this bifunctional catalyst system. For example, the photooxygena-
tion reaction of β-keto ester 4b proceeded well when air was di-
rectly used as the oxidant (Scheme 2a, 5b, 85% yield and 92% ee).
Notably, a shorter reaction time was observed by directly applying
sunlight as the light source (Scheme 2b, 5u, 90% yield and 92% ee).
Also, we were gratified to find that the reaction efficiency could be
further improved by using a continuous-flow reactor, achieving the
photooxygenation reaction of substrate 4l in 30 min (Scheme 2c, 5l,
81% yield and 88% ee). Moreover, a structurally complex β-keto
ester derived from the pharmaceutical agent, estrone 3-methyl
ether,18 was subjected to our standard conditions. In this case, the
hydroxyl functional group was readily incorporated with excellent
stereocontrol albeit in moderate yield (Scheme 2d, 5w, 53%
yield, >95:5 dr), providing a new protocol for the late-stage modi-
fication of complex molecules.19 Furthermore, synthetic transfor-
mations of chiral hydroxylation products are presented in Scheme
2e. As expected, reduction or reductive amination of 5b gives anti-
1,2-diol 6a or 1,2-amino alcohol 6b with excellent stereoselectivi-
ties, which are useful synthetic blocks in asymmetric synthesis.20
Experiments were conducted under the optimized conditions to
probe the substrate scope of this asymmetric photooxygenation re-
action. As summarized in Table 2, a wide range of β-keto esters
were tolerated, and generally, high levels of stereoselectivity and
reaction efficiency were observed. In the case of 1-indanone-de-
rived substrates, incorporation of either electron-donating or elec-
tron-accepting substituent on the benzene ring had little impact on
the reaction, and the corresponding products were obtained in 78-
97% yields and 90-95% ee (5b-f). The steric modification of the
benzene ring could be accomplished with little influence on the re-
action outcomes, affording hydroxylation products in 82-98%
yields and 90-94% ee (5g-l). Some functional groups, e.g., alkynyl,
vinyl and heteroaryl motif, such as pyridine and thiophene, that are
prone to oxidation were inert under the reaction conditions; thus,
the reaction displayed excellent chemoselectivity (5m-p, 90-96%
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Table 2. Substrate Scopea
emission spectra of photocatalyst 3f-Ni complex with those of lig-
1
2
3
4
5
6
7
8
and 3f showed that no variation of the maximum absorption and
emission wavelengths was observed. This result indicated that the
coordination of Ni(acac)2 with the 3f did not change photochemical
behaviour of the thioxanthone moiety in 3f (Figure S1). The fluo-
rescence quenching experiments of photocatalyst 3f-Ni were per-
formed with the addition of substrate 4b,21 and the results showed
that substrate 4b itself cannot quench the fluorescence of photosen-
sitizer, even at the concentration of 1 mol/L. Thus, the energy or
electron transfer between photocatalyst 3f-Ni* and substrate 4b
might be ruled out in our cases (Figure S3d). In addition, the results
of cyclic voltammetry (CV) measurements and the radical control
experiments (see section 7.3 and 7.4 in SI) suggested that the radi-
cal pathway might not be dominant in the current reaction.22 By
contrast, when the singlet oxygen quencher23 (DABCO) was added
to the reaction of 4b, a greatly reduced product yield was observed.
Additionally, replacing toluene with D8-toluene resulted in an ac-
celerated reaction (toluene, 12 h, 97% yield; D8-toluene, 7 h, 95%
yield) because of the significantly longer lifetime of singlet oxygen
in deuterated solvents.24 These results suggested the presence of
photoexcited singlet oxygen in this reaction system (See section 7.5
in SI). Thus, the thioxanthone component in 3f-Ni most likely func-
tions as a triple-state sensitizer for the activation of molecular oxy-
gen, although the indirect way involving the triplet−triplet energy
transfer between the Ni(II) and the excited state of the photosensi-
tizer cannot be ruled out at the current stage (Figure S1 and Section
7.3 in SI).25 Note that both the isolated yield and the ee of the prod-
uct decreased significantly when the photooxygenation reaction
was performed with chiral BOX ligand 3h and photosensitizer Me-
PS separately (Section 7.6 in SI). This result indicated that the in-
troduction of a thioxanthone not only provides a visible light pho-
tosensitizer, but also improves the stereo-controlling capacity of
chiral BOX component through dynamic steric effect.11 Further-
more, a linear relationship between the enantiopurity of chiral lig-
and 3f and product 5b suggested that the 1:1 complex of ligand 3f
and Ni(acac)2 is the catalytically active species. Control experi-
ments reveal that the acac anion is important for the high reaction
efficiency and enantioselectivity (Section 7.9 in SI). According to
these results and previous studies,26 a possible stereo-induction
model is proposed in Figure 3 based on the assumption that the 1Ad
group was located away from the chiral ligand to avoid possible
steric constraints. The Si-face of enol-formed β-keto ester 4b was
blocked by the back phenyl groups. Therefore, the attack of oxi-
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aUnless otherwise noted, reactions were performed under the stand-
ard condition as indicated in Table 1, entry 16. All yields are iso-
lated yields and ee values were determined by chiral HPLC. bReac-
c
tion conducted at 25 oC. Toluene/n-hexane (1:3) used as solvent.
d3a used as the chiral ligand.
1
dants, activated O2 or peroxide 7b,14c,g from the Re-face of 4b
seems relatively favourable, resulting in the hydroxylation product
5b with the R-configuration.
Scheme 2. Demonstration of Synthetic Utility
Figure 3. Proposed asymmetric induction model.
In conclusion, we have successfully developed a novel family of
visible light-responsive chiral ligands for asymmetric photocataly-
sis. Complexation of these ligands in situ with different metal cat-
alyst precursors will result in a broad spectrum of chiral bifunc-
tional photocatalysts. Moreover, the application of these catalysts
accomplished a visible light-induced metal-photocatalytic asym-
metric aerobic oxidation reaction of β-ketoesters and β-ketoamides,
affording -hydroxy-β-dicarbonyl products in high yields and en-
To better understand this photochemical oxidation reaction, the
basic properties of the bifunctional photocatalysts were initially
probed. Comparison of the UV-Vis absorption and fluorescence
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13045. (c) Wang, C.; Qin, J.; Shen, X.; Riedel, R.; Harms, K.; Meggers, E.
Angew. Chem. Int. Ed. 2016, 55, 685.
antioselectivities. We believe that this novel bifunctional photo-
catalyst will be transferable to other transformations, providing
new opportunities for visible light-driven asymmetric photochemi-
cal synthesis.
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(8) The interesting but special cases of photosensitizer-free visible-light-
induced asymmetric transformations, in which transient reaction inter-
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures, and characterization data for all the prod-
ucts. The Supporting Information is available free of charge on the
ACS Publications website.
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AUTHOR INFORMATION
Corresponding Author
wxiao@mail.ccnu.edu.cn
Author Contributions
§These authors contributed equally.
Notes
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Org. Chem. 2004, 69, 8165. (d) Ishimaru, T.; Shibata, N.; Nagai, J.; Naka-
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Lu, M.; Zhu, D.; Lu, Y.; Zeng, X.; Tan, B.; Xu, Z.; Zhong, G. J. Am. Chem.
Soc. 2009, 131, 4562. (f) Li, J.; Chen, G.; Wang, Z.; Zhang, R.; Zhang, X.;
Ding, K. Chem. Sci. 2011, 2, 1141. (g) Lian, M.; Li, Z.; Cai, Y.; Meng, Q.;
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H.; Song, Y.; Qu, J. Org. Lett. 2013, 15, 3106.
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gew. Chem. Int. Ed. 2012, 51, 10329. (c) Xuan, J.; Xia, X.-D.; Zeng, T.-T.;
Feng, Z.-J.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem. Int. Ed. 2014,
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(16) (a) Cordova, A.; Sunden, H.; Engqvist, M.; Ibrahem, I.; Casas, J. J.
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(17) See the Supporting Information for details.
(18) Paquette, L. A.; Dahnke, K.; Doyon, J.; He, W.; Wyant, K.; Frie-
drich, D. J. Org. Chem. 1991, 56, 6199.
(19) The structure of product 5w (CCDC 1468752) was confirmed by X-
ray crystallographic analysis.
(20) (a) Ager, D. J.; Prakash, I.; Schaad, D. R. Chem. Rev. 1996, 96, 835.
(b) Bergmeier, S. C. Tetrahedron 2000, 56, 2561. (c) Notz, W.; List, B. J.
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The authors declare no competing financial interests.
ACKNOWLEDGMENT
We gratefully acknowledge the National Natural Science Founda-
tion of China (NO. 21232003, 21472057, 21472058 and 21572074)
for financial support.
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