Chiral Calcium Phosphate Catalyzed Enantioselective Amination of 3-Aryl-2-benzofuranones

Article abstract of DOI:10.1021/acs.orglett.0c03059

A 4-tert-butyl-phenyl substituted (R)-[H8]-BINOL chiral calcium phosphate catalyzed enantioselective amination of 3-aryl-2-benzofuranones with dibenzyl azodicarboxylate is described. The catalyst loading of the reaction is 1 mol %. This transformation is facile and has a high degree atom economy, which gave products with good yields and high enantioselectivities (79% to 99%). This reaction has excellent ee and a broad substrate scope with mild reaction conditions.

Full text of DOI:10.1021/acs.orglett.0c03059

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Letter
Chiral Calcium Phosphate Catalyzed Enantioselective Amination of
3‑Aryl-2-benzofuranones
Ruihan Liu,§ Suvratha Krishnamurthy,§ Zhenwei Wu, K. S. Satyanarayana Tummalapalli,
and Jon C. Antilla*
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ABSTRACT: A 4-tert-butyl-phenyl substituted (R)-[H8]-BINOL
chiral calcium phosphate catalyzed enantioselective amination of 3-
aryl-2-benzofuranones with dibenzyl azodicarboxylate is described.
The catalyst loading of the reaction is 1 mol %. This transformation
is facile and has a high degree atom economy, which gave products
with good yields and high enantioselectivities (79% to 99%). This
reaction has excellent ee and a broad substrate scope with mild
reaction conditions.
nantioselective C−N bond formation at the α-position of
Ecarbonyl compounds is an important transformation.
Chiral α-amino carbonyl compounds are present in numerous
natural products. There are several novel enantioselective
amination reactions that include aldehydes,1 ketones,2 1,3-
dicarbonyls,3 2-oxindoles,4 and α-cyanoacetates,5 which could
produce chiral α-amino carbonyl compounds.
Asymmetric amination at the α-position of 2-benzofur-
anones generates chiral 3-amino-2-benzofuranones which are
present in medicinally valuable products like (−) fumimycin6
(antibacterial), sorbicillactone A7 (antileukemic), hopeahainol
A8 (acetylcholinesterase inhibitor), and phalarine9 (Figure 1).
Over the past decades, there are only two examples of
asymmetric amination of 2-benzofuranones;10 however, they
exhibit a narrow substrate scope, require long reaction times
and need a comparatively higher catalyst loading. Therefore,
developing an efficient method for the amination of 3-aryl-2-
benzofuranones is desirable. Furthermore, in the field of
phosphoric acid catalysis, the activation of 2-benzofuranones
appears to be only disclosed by Ooi and co-workers11 using
transition-metal Pd catalysis, by taking advantage of the
structural modularity of ion-paired chiral ligands. The
development of highly efficient CPA-activation of 2-benzofur-
anones is a challenge to organic chemists. Here we reported
the amination of 3-aryl-2-benzofuranones using a chiral
calcium phosphate.
Since chiral phosphoric acids (CPAs) were introduced to
asymmetric catalysis in 2004,12 they have been applied in
various enantioselective reactions.13 Successful efforts were
also made to increase the acid strength (low pKa) in order to
enable additional catalysis to be achieved.14 In 2010, a
significant breakthrough in this field was made by Ishihara
and co-workers. He found that calcium phosphate could be
used as a catalyst to achieve a highly enantioselective Mannich-
type reaction of aldimines with 1,3-dicarbonyl compounds.15
This report paved the way for the extensive use of chiral metal
phosphates as a catalyst for additional asymmetric organic
transformations.16
Received: September 13, 2020
Figure 1. Some important medicinal chiral 2-benzofuranones.
https://dx.doi.org/10.1021/acs.orglett.0c03059
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
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Letter
Calcium phosphates are excellent asymmetric catalysts
among the various alkali metal phosphates and alkaline-earth
metal phosphates. In the past decade, chiral calcium phosphate
developed rapidly,17 successfully catalyzing benzoyloxylation17a
and chlorination17b,c of 3-aryloxindoles, hetero-Diels−Alder
reactions,17d,e Mannich reactions of cyclic 1,3-diketones,17f
imide addition of imines,17g amination of enamides,17h and
chloroamination of enecarbamates.17i
Table 1. Condition Optimization for Amination of 3-
Phenyl-2-benzofuranone 1a with Dibenzyl Azodicarboxylate
a
2
In 2011, our group reported a highly enantioselective
catalytic benzoyloxylation of 3-aryloxindoles catalyzed by
calcium phosphate.17a In the same year, our group disclosed
a highly enantioselective chlorination of 3-substituted
oxindoles with N-chlorosuccinimide using calcium VAPOL as
catalyst.17b Based on this previous work, a calcium VAPOL-
catalyzed one-pot double asymmetric cascade reaction was
realized to provide chiral chlorinated oxindoles and geminal
diamines simultaneously, which afforded two structurally
complex and diverse chiral products with high levels of
stereocontrol.17c
We initiated this study of catalytic enantioselective
amination of 3-phenyl-2-benzofuranone 1a with dibenzyl
azodicarboxylate 2 as the amination reagent. First, we screened
CPAs as catalysts for the amination. The reaction smoothly
afforded product 3a in good yield at ambient temperature.
When we used the vaulted biaryl (S)-VAPOL P1 and (S)-
VANOL P2 phosphoric acids as catalysts, we found that the
enantioselectivities were poor (Table 1, entries 1 and 2).
Utilization of (R)-TRIP P3, a typically successful BINOL
based catalyst, resulted in no improvement in the enantiose-
lectivity (Table 1, entry 3). We were pleased to see that there
was an appreciable increase in the enantioinduction when
using 9-anthracenyl CPA P4 (Table 1, entry 4). Varying the
alkyl group on the diazo compound had no improvement in
the selectivity. The best results were obtained with the benzyl
substituted diazo compound. Lowering the temperature had no
effect on the enantioselectivity. Next we screened chiral
calcium phosphate as the catalyst. The calcium salt of 9-anthryl
CPA P5 exhibited very poor enantioselectivity in comparison
to its acid (Table 1, entry 5). We speculated that varying the
type of sterically hindering groups could find meaningful
results. When using a 4-tert-butyl-phenyl substituted calcium
phosphate P6 to catalyze the reaction, we observed good
enantioselectivity (Table 1, entry 6). When we replaced the
(R)-BINOL backbone with the (R)-[H8]-BINOL skeleton, we
found significant improvement (Table 1, entry 7). The
enantioselectivity of the reaction was indeed improved to
79%. In the next stage, 4-tert-butyl-phenyl substituted (R)-
[H8]-BINOL calcium phosphate P7 was used to catalyze the
amination of 1a. Further attempts to improve the enantiose-
lectivities by changing to alternative solvents such as
chlorobenzene, trifluoromethylbenzene, dichloromethane,
ether, tetrahydrofuran, and 1,4-dioxane were carried out
(Table 1, entries 8−13), and the results indicated that solvent
effect played an important role in enantioselectivities, with
trifluoromethylbenzene being superior. Further optimization
found that a low catalyst loading (1 mol %) could still provide
high yield and enantioselectivity in the reaction (Table 1, entry
14).
b
c
entry
catalyst
solvent
toluene
time (min)
ee (%)
yield (%)
d
1
P1
P2
P3
P4
P5
P6
P7
P7
P7
P7
P7
P7
P7
P7
10
10
10
10
30
30
30
30
30
30
30
30
30
30
10
6
3
80
0
56
79
96
99
90
50
16
10
99
92
96
93
92
96
92
93
95
99
96
99
90
87
99
2
3
4
toluene
toluene
toluene
toluene
toluene
toluene
Cl−Ph
CF3−Ph
CH2Cl2
ether
e
5
6
7
8
9
10
11
12
13
THF
1,4-dioxane
CF3−Ph
f
14
a
Reaction conditions: 1a (0.025 mmol), 2 (0.03 mmol), catalyst,
solvent (1.1 mL), and 4 Å MS (25 mg) were stirred under argon at
room temperature. Established by HPLC. Isolated yield. Entries
b
c
d
e
1−4: 10 mol % catalyst was used. Entries 5−13: 5 mol % catalyst was
f
used. Entry 14: 1 mol % catalyst was used.
underwent the amination to give the products in excellent
yields. The presence of electron-donating groups on the 2-
benzofuranone core have little effect on the yield and
enantioselectivity (ee was up to 90%) (3b−3e). Similarly
mono-/disubstituted electron-withdrawing groups like fluoro,
chloro, and bromo on the 2-benzofuranone gave comparable
results (3f−3j) to the electron-donating groups.
The effects of substituents on the phenyl group at the α-
position were next investigated. Initially, we tested electron-
donating groups like methyl and methoxy on the phenyl group
and found the enantioselectivities improved further compared
to electron-donating substituents on 2-benzofuranone (3k−
3m). Introduction of electron-withdrawing groups (3n−3q)
on the phenyl group at the α-position greatly enhanced the
Having established the optimized reaction conditions, we
next explored the substrate scope of the reaction. We were
pleased to find that consistently excellent enantioselectivities
were achieved, and the results are summarized in Scheme 1. As
illustrated, a wide range of 3-aryl-2-benzofuranones smoothly
B
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a b c
, ,
Scheme 1. Substrate Scope of Group on the 2-
Benzofuranone Core
Scheme 2. Substrate Scope of 3-Aryl-2-benzofuranones
a b c
, ,
a
The reaction condition: 1a−j (0.025 mmol), 2 (0.03 mmol), and P7
(0.3 mg, 1 mol %) were stirred in trifluoromethylbenzene (1.1 mL)
with 4 Å MS (25 mg) as an additive under argon gas for 30 min.
b
c
Isolated yields. Determined by HPLC.
enantioselectivity (ee was up to 99%) in comparison to other
substituents on the 2-benzofuranone.
However, an ortho fluoro substituent on the phenyl group at
the α-position decreased the enantioselectivity (88% ee) in
comparison to the para fluoro substituted counterpart (3q vs
3n). We explored the use of two electron-withdrawing or
electron-donating groups, among which the difluoro sub-
stitution being a good substrate (92% ee, 3r), but two electron-
donating groups (methyls) had a adverse impact on the
enantioselectivity (79% ee, 3s). Introducing both fluoro and
methyl groups had no adverse effect on the enantioselectivity
(88% ee, 3t). We were pleased to find that the reaction also
works for 3-phenylnapthofuranone (3u), giving the product in
high yield, albeit with lower enantioselectivity in comparison to
the 3-phenylbenzofuranone (3u vs 3a). Substrate introducing a
β-naphthyl ring versus a phenyl at the α-position had an
adverse effect on the enantioselectivity (65% ee) and showed
that only the smaller phenyl ring at the α-position (3v) can be
tolerated in our method.
a
Reaction conditions: 1k−w (0.025 mmol), 2 (0.03 mmol), and P7
(0.3 mg, 1 mol %) were stirred in trifluoromethylbenzene (1.1 mL)
with 4 Å MS (25 mg) as an additive under argon gas for 30 min.
b
c
Isolated yields. Determined by HPLC.
conditions, which gave the product 3a in 98% yield and 94%
enantioselectivity (Scheme 3).
Scheme 3. Gram-Scale Reaction
To further probe the limits of the reaction we used 3-
methyl-2-benzofuranone as a substrate for the amination with
the conditions in Scheme 2. Unfortunately, though we found a
90% conversion, the product was only obtained with a 20% ee
(3w). The presence of the phenyl group at the α-carbon is a
necessary requirement for this method’s overall success. We
also tried the reaction in the large-scale treatment of 1a (1.05
g, 5 mmol) and 2 (1.78 g, 6 mmol) under optimized
The absolute configuration of the product 3a was
determined to be (R), by comparison of the optical rotation
with known compounds in reported literature.11b Although a
detailed mechanism is not yet established, our previous studies
employing chiral calcium phosphates as catalyst gave us insight
into a possible reaction mechanism.17 As shown in Figure 2, we
believed the chiral calcium phosphate complex binds to
C
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Letter
Author Contributions
§R.L. and S.K. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by financial support from the
National 1000 Talent Plan of China and from Tianjin
University start-up funding.
Figure 2. Proposed transition state for enantioselective amination.
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In conclusion, we successfully developed a 4-tert-butyl
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The Supporting Information is available free of charge at
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General condition, experimental procedures, character-
ization, copies of NMR spectra and HPLC traces (PDF)
AUTHOR INFORMATION
Corresponding Author
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Jon C. Antilla − School of Pharmaceutical Sciences and
Technology, Tianjin University, Tianjin 300072, P. R. China;
School of Sciences, Zhejiang Sci-Tech University, Hangzhou
310018, P. R. China; orcid.org/0000-0002-8471-3387;
Email: jantilla@tju.edu.cn
Authors
Ruihan Liu − School of Pharmaceutical Sciences and Technology,
Tianjin University, Tianjin 300072, P. R. China
Suvratha Krishnamurthy − School of Pharmaceutical Sciences
and Technology, Tianjin University, Tianjin 300072, P. R.
China
Zhenwei Wu − School of Pharmaceutical Sciences and
Technology, Tianjin University, Tianjin 300072, P. R. China
K. S. Satyanarayana Tummalapalli − School of Pharmaceutical
Sciences and Technology, Tianjin University, Tianjin 300072,
P. R. China
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