Asymmetric Pnictogen-Bonding Catalysis: Transfer Hydrogenation by a Chiral Antimony(V) Cation/Anion Pair

Article abstract of DOI:10.1021/jacs.1c02808

Pnictogen-bonding catalysis based on σ-hole interactions has recently attracted the attention of synthetic chemists. As a proof-of-concept for asymmetric pnictogen-bonding catalysis, we report herein an enantioselective transfer hydrogenation of benzoxazi

Full text of DOI:10.1021/jacs.1c02808

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Asymmetric Pnictogen-Bonding Catalysis: Transfer Hydrogenation
by a Chiral Antimony(V) Cation/Anion Pair
Jian Zhang, Jun Wei, Wei-Yi Ding, Shaoyu Li, Shao-Hua Xiang, and Bin Tan*
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ABSTRACT: Pnictogen-bonding catalysis based on σ-hole interactions has recently attracted the attention of synthetic chemists. As
a proof-of-concept for asymmetric pnictogen-bonding catalysis, we report herein an enantioselective transfer hydrogenation of
benzoxazines catalyzed by a novel chiral antimony cation/anion pair. The chiral pnictogen catalyst library could be rapidly accessed
from triarylstibine with readily available mandelic acid analogues, and the catalyst displays remarkable efficiency and enantiocontrol
potency even at 0.05 mol % loading. Moreover, the properties of the catalyst and the mechanistic insights have been investigated by
1
nonlinear effect studies, H NMR, LC-MS, and control experiments.
pnictogen bond (PnB) refers to an attractive noncovalent
hydrogenation and Ritter-like reaction efficiently.42 Later,
Ainteraction established between a region of negative both stibine A (R = H) and stiborane B (Ar = 3,4,5-F3-
C6H2) were found to afford anti-Baldwin selectivity in
electrostatic potential on an acceptor and a region of positive
electrostatic potential on a group V (15) atom.1−7 The latter is
also called the σ-hole8−13 and exists usually at the opposite side
of σ-bonds connecting electron-withdrawing substituents.
Noncovalent interactions of this class include halogen
bonds,8,14−16 chalcogen bonds,17−21 and tetrel bonds,22−25
which are named after the donor atom’s group. These
interactions are frequently encountered in crystal struc-
tures26−28 and applied regularly in supramolecular assem-
bly29−32 as well as anion recognition.33−35 By contrast,
applications in synthetic chemistry have been less studied;
only a few organocatalytic protocols have been reported on the
implementation of σ-hole interactions throughout the last two
decades, with most methods designed around a halogen
bond.36−39
brevetoxin-type polyether cyclizations by Matile and co-
workers.40 Gabbai et al. also demonstrated that stibonium
̈
cations could be more active species than neutral stiborane in
transfer hydrogenation of quinolines, and dicationic systems43
C were superior activators (Figure 1).44 Matile’s work in
epoxide-opening polyether cyclizations has verified that
pnictogen-bonding catalysis could provide different reactivity
from Lewis acid catalysis, potentially opening up new
mechanistic possibilities.40,45 These achievements are certainly
stimulating; however, utilization of pnictogen-bonding catalyst
(PnB-cat) for enantioselective transformation has remained
elusive.
Antimony with deeper σ-holes is expected to be a promising
candidate among group V elements. The strength of pnictogen
bonding will be highly tunable through variations of
substituents41,46,47 and valence states.42 As σ-holes are usually
embedded in the cavity created by substituents, the steric
profile of PnB donors could therefore be tailored through the
modulation of substituents. There are also ample coordination
sites for the incorporation of chiral components on multivalent
pnictogen donors. These beneficial opportunities notwith-
standing, asymmetric catalysis facilitated by σ-hole interactions
has limited literature precedent.48 Finding a balance between
reactivity and stability of σ-hole donors could be a significant
challenge. Successful stibine catalysts often carry electron-
withdrawing substituents (A) to offer appropriate σ-holes, and
electron-deficient ligands such as tetrachlorocatecholate are
Pnictogen-bonding catalysis40 was introduced only recently
and has been defined as the noncovalent, supramolecular
counterpart of classical covalent Lewis acid catalysis by Matile
et al. In 2018, they revealed the ability of electron-deficient
stibine A (Figure 1, R = F) to process Reissert-type
substitution and an SN1-type reaction via halogen abstrac-
tion.41 Gabbai et al. showed that stiborane B (Figure 1, +V
̈
state) displayed enhanced activity compared with the stibine
precursor (+III state) and could promote the transfer
Received: March 15, 2021
Published: April 27, 2021
Figure 1. Reported pnictogen-bonding catalysts.
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© 2021 American Chemical Society
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crucial to stabilizing stiboranes (B) in the +V state.42,49,50
Introducing strong electron-withdrawing aryl substituents onto
the antimony cation could indeed enhance the catalytic activity
greatly, but the tendency to undergo reduction arises at the
same time.51 Besides, π-acidic properties of electron-deficient
substituents on PnB donors may complicate the design of
chiral pnictogen-bonding catalyst.42
In keeping with our ongoing research in asymmetric
organocatalysis, the untapped potential of pnictogen-bonding
catalysis motivated our pursuit of a chiral PnB-cat. We
considered obtaining a chiral PnB-cat by replacing the
tetrachlorocatecholate ligand of catalyst B with a non-
conjugated O,O-ligand (Figure 2a), and mandelic acid was
enantioselectivity (Figure S1). Besides, unusual rate accel-
eration appeared at the middle stage of the reaction (Figure
2c). These observations led to speculation that the trace
amount of impurities present could have promoted the
reaction. After ruling out various factors, the actual active
species was eventually identified as chiral antimony(V) cation/
anion pair 4a, which could be prepared from 3a on heating at
80 °C (Figure 2b). This ion pair promoted the reduction of 5a
efficiently in 2 h, and the product 7a was isolated in 99% yield
with 80% ee (Table 1, entry 1).
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
cat
solvent
time
yield (%)
ee (%)
1
2
3
4
5
6
7
8
(R,R)-4a
(R,R)-4b
(R,R)-4c
(S,S)-4d
(S,S)-4e
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
(R,R)-4b
toluene
toluene
toluene
toluene
toluene
mesitylene
c-hexane
n-hexane
Et2O
EtOAc
MeOH
MeCN
CH2Cl2
CCl4
2 h
2 h
8 h
8 h
4 h
2 h
3 h
8 h
5 d
5 d
5 d
5 d
5 d
0.5 h
3 h
4 h
12 h
12 h
99
99
99
99
99
99
99
99
99
70
61
trace
99
99
99
99
99
99
80
82
78
−81
−81
84
82
81
76
73
9
10
11
12
13
14
15
16
17
18
2
73
86
91
92
94
94
d
CCl4
CCl4
CCl4
CCl4
e
f
g
a
Conditions: 5a (0.2 mmol), 6 (0.24 mmol), catalyst (2.5 mol %),
b
c
d
room temperature. Isolated yield. Determined by HPLC analysis. 0
°C. −10 °C. −20 °C. −20 °C and 0.05 mol % catalyst.
e
f
g
Subsequently, analogues 4b−4e with differing mandelic acid
ligands were prepared. Screening of these compounds (Table
1, entries 1−5) revealed that 4a−4e were all robust catalysts
for asymmetric reduction of benzoxazine (99% yield, 78−82%
ee), and 4b gave a marginally better result in terms of
enantioselectivity (82% ee). Further optimization studies thus
continued with 4b. Reactions in nonpolar solvents (Table 1,
entries 6−8) generally gave excellent yields (99%) and good
ee’s (81−84%). Those in oxygen-containing solvents such as
Et2O, EtOAc, and MeOH (Table 1, entries 9−11) required a
longer time to achieve good yields (61−99%). An attendant
decrease of enantioselectivity was observed in Et2O (76% ee)
and EtOAc (73% ee). Polar solvents (Table 1, entries 11 and
12) exerted a negative influence on this chemistry:
enantioselectivity diminished completely in MeOH, and the
reaction was inhibited when conducted in MeCN. In CCl4 the
fastest transformation occurred (Table 1, entry 14) and gave
the highest ee (86%) among all tested solvents (Table 1,
entries 5−14). The enantioselectivity further improved with
lower temperature (Table 1, entries 15−17), and 7a was
produced in 99% yield with 94% ee at −20 °C. Strikingly, the
reaction went equally well (99% yield, 94% ee) with only 0.05
Figure 2. Synthesis of chiral PnB-catalyst and initial trial in transfer
hydrogenation.
selected as the ligand. Mandelic acid 2a reacted readily with
triarylstibine 1 under oxidative conditions,52 affording 3a in
nearly quantitative yield (Figure 2b). Different from previous
work on tetrameric antimony(V) α-hydroxy carboxylato
complexes that consist of racemic ligands,53 optically pure
(R)-3a was stable in monomeric form. By applying 3a as a
catalyst in the reduction of benzoxazine 5a with Hantzsch ester
6, we observed that the catalytic performance of crude 3a
decreased after purification, but without an obvious decrease in
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mol % (R,R)-4b (Table 1, entry 18), which stood as the lowest
loading that could be applied for this reaction to date.54−62
With the optimal conditions in hand, we set out to explore
the generality of reaction by examining the substrate scope
(Table 2). Benzoxazines with electron-donating (7b, 7c) or
weak negative nonlinear relationship between ee of the catalyst
and ee of the product (Figure 3a). This phenomenon indicated
a
Table 2. Substrates Scope
a
Conditions: 5b−5z (0.2 mmol), 6 (0.24 mmol), (R,R)-4b (0.05 mol
b
%), −20 °C. 2 mol % (R,R)-4b, room temperature.
electron-withdrawing substituents (7d−7h) at the 6-position
gave corresponding products in good yields (93−99%) and
enantioselectivities (93−98% ee), although an extended
reaction time was required for 7h. The bulkier substituents
such as phenyl and tBu were tolerated, as documented by the
preparation of 7i and 7j (99% yield, 95% ee). Besides, the 7-
substituted (7k−7m) and multisubstituted (7n) products were
obtained in high yield (96−99%) and good ee (92−97%). The
benzoxazine bearing an extended π system also afforded
naphthalene derivative 7o in 99% yield and 95% ee. Substrates
that bore diversely substituted aryl groups at the 3-position
were smoothly transformed into products (7p−7u) in nearly
quantitative yield and good enantiopurity (83−92%), on
varying but generally convenient time scales. While 2,2-
dimethyl-substituted benzoxazine gave corresponding product
(7v) in 94% ee, the five-membered analogue (7w) showed
inferior enantioselectivity (85% ee). As a comparison, 2-alkyl-
substituted substrate underwent a sluggish transformation
under standard conditions, affording 7x in low yield (8%) and
enantioselectivity (3% ee). Our catalyst also promoted the
reduction of the less active 2-phenyl-quinoline and benzox-
azinone at room temperature, providing target products (7y
and 7z) with good efficiency (87−99% yield) and selectivity
(89% ee). The absolute configuration of 7f was determined by
X-ray crystallographic analysis, and those of other products in
Table 2 were assigned by analogy.
Figure 3. Nonlinear effect studies and ligand exchange phenomenon.
(a) The ee (%) of the catalyst was calculated as [4b(R,R) − 4b(S,S)]/
[4b(R1,R) + 4b(S,S)], and reactions were conducted in CCl4 at −20 °C.
(b) H NMR spectrum (fragment) of 4a(R,R), 4b(R,R), 4a(R,R)/4b(R,R)
mixture (2:1), and 4a(R,R)/4b(S,S) mixture (1:1); the integral ratio of
peaks at 5.01 (ppm) and 4.64 (ppm) was 1:1. (c) Ion signal in the
negative mode of the 4a/4b mixture. Peaks between 833 and 838
validated the formation of hybrid anions (A3 and A5 ). (d) Ligand
exchange process; for clarity, the substituents of the antimony cation
were omitted.
−
−
the presence of ligand exchange (Figure 3c) and/or catalyst
aggregation in this reaction. Indeed, two new peaks at 4.64 and
1
5.01 ppm appeared in the H NMR spectrum of the 4a(R,R)
/
−
4b(R,R) mixture (Figure 3b), and a hybrid anion (A3 ) was
confirmed by LC-MS of the 4a(R,R)/4b(R,R) mixture (Figure 3d,
top). According to the equilibrium constant K (0.86)
To gain mechanistic insights into the chiral antimony(V)
cation/anion pair catalyzed asymmetric transfer hydrogena-
tion, nonlinear effect studies were conducted. There was a
1
determined from H NMR spectroscopy (Figure S4), a small
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free energy difference (ΔG = 0.37 kJ/mol) before and after
obvious improvement of catalytic activity appeared after an ion
exchange to [Sb(ArtBu)4]+[B(ArF)4]− 9 (Figure 4a). This has
validated the catalytic activity of the stibonium cation. The
bulkier and soft nature of the antimony counteranion may
account for the higher activity of our PnB-cat 4b compared to
9. Tetraaryl stibonium mandelate 10 was also inactive for
catalysis probably due to the tight association of the ion pair.
ligand exchange (Figure 3c, top) was deduced. The hybrid
−
anion (A5 ) of the 4a(R,R)/b(S,S) mixture was also detected in
the LC-MS analysis (Figure 3d, bottom). However, the ratio of
diastereoisomers could not be precisely determined from the
1H NMR spectrum of the 4a(R,R)/4b(S,S) mixture (Figure 3b),
as the signal of 4(R,S) was too weak for identification. It was
thus concluded that diastereoisomers 4(R,S) are unstable in the
nonpolar solvent. Even at a low concentration, the heterochiral
catalyst 4(R,S) exhibited greater reactivity than the homochiral
catalyst 4a(R,R) or 4b(S,S) according to the ML2 model
developed by Kagan and co-workers.63,64
̈
Gabbai et al. proved that stibonium cations could promote
transfer hydrogenation between 2-phenyl quinoline and the
Hantzsch ester, while phosphonium cations which serve as
counterions via electrostatic attraction in ion-pairing catal-
ysis68−70 do not.44 These results indicate that the deeper σ-
hole of the stibonium cation may account for the high
performance of the novel chiral PnB-cat.
Next, time-course and control experiments were conducted
(Figure 4). A sluggish background reaction was found in the
In summary, we have developed the first asymmetric
pnictogen-bonding catalysis enabled by the chiral antimony
ion pair. The cheap and abundant chiral pool material,
mandelic acid, has been incorporated as the ligand for the
antimony anion to induce asymmetry. The high performance
of the newly developed PnB-catalyst was proven in
enantioselective transfer hydrogenation reaction with a low
catalyst loading. A ligand exchange phenomenon was observed,
and control experiments revealed that pnictogen bonding
combined with the Lewis basic property of the antimony anion
could have led to the high catalytic activity of these PnB-
catalysts. We believe that this novel chiral antimony(V)
cation/anion pair will open an avenue for other trans-
formations by employing asymmetric pnictogen-bonding
catalysis.
ASSOCIATED CONTENT
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* Supporting Information
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https://pubs.acs.org/doi/10.1021/jacs.1c02808.
Coordinate files; experimental procedures, character-
ization data for new compounds, NMR spectra, and
HPLC traces (PDF)
Accession Codes
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
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Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Figure 4. (a) Time-course experiments for transfer hydrogenation in
the presence of various catalysts at room temperature (based on H
Corresponding Author
1
Bin Tan − Shenzhen Grubbs Institute, Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China; orcid.org/0000-0001-8219-
9970; Email: tanb@sustech.edu.cn
NMR yield). (b) Control experiments.
absence of any catalyst (Figure 4a). To exclude the possibility
that mandelic acid, which might be released from ligand
exchange, has acted as the catalytically active species,65−67 the
control reaction in the presence of (R)-2-chloromandelic acid
2b was conducted. Catalytic activity of 2b was indeed
exhibited, but the reaction rate was apparently lower than
when PnB-cat 4b was used (Figure 4a). Moreover, there was a
striking difference in the product enantioselectivity of reactions
catalyzed by 2b (Figure 4b, 12% ee) and 4b (Table 1, entry 2,
82% ee). While no catalytic activity was observed when
tetraaryl stibonium bromide 8 was intended as the catalyst,
Authors
Jian Zhang − School of Chemistry and Chemical Engineering,
Harbin Institute of Technology, Harbin 150001, China;
Shenzhen Grubbs Institute, Department of Chemistry,
Southern University of Science and Technology, Shenzhen
518055, China
Jun Wei − Shenzhen Grubbs Institute, Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China
6385
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pubs.acs.org/JACS
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Wei-Yi Ding − Shenzhen Grubbs Institute, Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China
Shaoyu Li − Shenzhen Grubbs Institute, Department of
Chemistry and Academy for Advanced Interdisciplinary
Studies, Southern University of Science and Technology,
Shenzhen 518055, China; orcid.org/0000-0001-5565-
0148
Shao-Hua Xiang − Shenzhen Grubbs Institute, Department of
Chemistry and Academy for Advanced Interdisciplinary
Studies, Southern University of Science and Technology,
Shenzhen 518055, China; orcid.org/0000-0003-4262-
5626
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Complete contact information is available at:
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (No. 21825105), Guangdong
Provincial Key Laboratory of Catalysis (No.
2020B121201002), Guangdong Innovative Program
( 2 0 1 9 B T 0 2 Y 3 3 5 ) , S h e n z h e n S p e c i a l F u n d s
(JCYJ20180305123508258), Shenzhen Nobel Prize Scientists
Laboratory Project (C17213101), and SUSTech Special Fund
for the Construction of High-Level Universities (No.
G02216402).
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6387
https://doi.org/10.1021/jacs.1c02808
J. Am. Chem. Soc. 2021, 143, 6382−6387