A Structurally Robust Chiral Borate Ion: Molecular Design, Synthesis, and Asymmetric Catalysis

Article abstract of DOI:10.1002/anie.202001637

Catalysis by chiral weakly-coordinating anions (WCAs) remains underdeveloped due to the lack of a molecular design strategy for exploiting their characteristics, such as the non-nucleophilic nature. Here, we report the development of a chiral borate ion comprising an O,N,N,O-tetradentate backbone, which ensures hitherto unattainable structural robustness. Upon pairing with a proton, the hydrogen borate acts as an effective catalyst for the asymmetric Prins-type cyclization of vinyl ethers, providing access to structurally and stereochemically defined dihydropyrans. The key to selectivity control is the distinct ability of the borate ion to discriminate the prochiral faces of the acyclic oxonium ion intermediate and dictate the regiochemical outcome. We anticipate that this study paves the way for exploring the untapped potential of WCA catalysis for selective chemical synthesis.

Full text of DOI:10.1002/anie.202001637

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: A Structurally Robust Chiral Borate Ion: Molecular Design,
Synthesis, and Asymmetric Catalysis
Authors: Daisuke Uraguchi, Fumito Ueoka, Naoya Tanaka, Tomohito
Kizu, Wakana Takahashi, and Takashi Ooi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001637
Angew. Chem. 10.1002/ange.202001637
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10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
A Structurally Robust Chiral Borate Ion: Molecular Design,
Synthesis, and Asymmetric Catalysis
Daisuke Uraguchi, Fumito Ueoka, Naoya Tanaka, Tomohito Kizu, Wakana Takahashi, and Takashi
Ooi*
Abstract: Catalysis by chiral weakly-coordinating anions (WCAs)
remains underdeveloped due to the lack of a molecular design
strategy for exploiting their characteristics, such as the non-
nucleophilic nature. Here, we report the development of a chiral
borate ion comprising an O,N,N,O-tetradentate backbone, which
ensures hitherto unattainable structural robustness. Upon pairing with
a proton, the hydrogen borate acts as an effective catalyst for the
asymmetric Prins-type cyclization of vinyl ethers, providing access to
structurally and stereochemically defined dihydropyrans. The key to
selectivity control is the distinct ability of the borate ion to discriminate
the prochiral faces of the acyclic oxonium ion intermediate and dictate
the regiochemical outcome. We anticipate that this study paves the
way for exploring the untapped potential of WCA catalysis for
selective chemical synthesis.
while manifesting the current limitations. An alternative yet
distinct strategy for practicing ACDC is to exploit the intrinsic
features of weakly-coordinating anions (WCAs).[6] WCA can form
a stable ion pair with a cation without forming a covalent bond
owing to its non-nucleophilic nature; hence, it has traditionally
been employed as the counterion of highly electrophilic, labile
cations. Although it is implied by the name that WCA shows weak
interaction with the pairing cation, chiral WCAs can discriminate
the chirality of the cation as they have been utilized as reagents
for the separation of enantiomeric cationic compounds.[7] These
characteristics have encouraged the development of chiral WCAs
that are competent for catalytically controlling reactive cationic
intermediates, which in turn are attractive and advantageous for
expanding the potential of ACDC. However, the pursuit of this
approach has met with limited success.[8]
The borate ion such as tetraarylborate is a representative
WCA that is widely used as the counterion for a variety of cationic
species. In particular, tetraalkoxyborates (B(OR)4 ) constitute a
Introduction
−
venerable class of borates for developing chiral WCAs because
these structurally diverse ions can be easily prepared by the
condensation of boric acids (esters) with various chiral alcohols.
Echoing with their ready accessibility, several chiral B(OR)4− ions
have been elaborated and evaluated as stereocontrollers
following the pioneering contribution of Arndtsen, who used a
binaphthol-derived chiral borate for asymmetric copper
catalysis.[9] Nevertheless, the rational development of boron-
based chiral anions that can precisely control prochiral cations
remains elusive. This is probably attributed to the inherent
Prochiral cations occupy a uniquely important place in organic
chemistry as reagents and reactive intermediates. In the bond
formation of these cations with nucleophiles, precise
discrimination of the enantiotopic faces is often required for
obtaining stereochemically defined products. However, a reliable
strategy for implementing such a general mode of absolute
stereocontrol is underdeveloped. Ion pairing with a chiral anion is
one of the simplest and most powerful approaches for imparting
chirality to the cationic species, which is particularly true
considering the advent of asymmetric chiral anion-directed
catalysis (ACDC) and its rapid evolution over the last decade.[1]
The dominant strategy is based on the use of conjugate bases of
Brønsted acids, wherein selection and structural modification of
the chiral Brønsted acid itself or the organic scaffold that non-
covalently binds to an achiral anion can endow the anions with a
suitable chiral architecture and allow for tuning of their steric and
electronic properties. The widespread utility of this approach has
been demonstrated through the significant advancement in
asymmetric organocatalysis,[2] including anion phase-transfer[3]
and anion-binding catalysis[4], and transition-metal catalysis,[5]
−
structural instability of B(OR)4 resulting from the reversibility of
boron-oxygen (B-O) bond formation, allowing the exchange of the
oxy-ligand (OR), especially under acidic conditions.[10,11] In fact,
previously reported chiral borates, most of which adopt a bora-
spiro core consisting of two bidentate chiral backbones, are
known to be structurally fluxional in solution in the absence of
basic substrates such as imines or amines.[12] These profiles,
while crucial for the in situ assembly of thermodynamically
favorable borate catalysts, reflect the lack of a strategy for the
development of chiral borates with structural robustness and
sufficient diversity for eliciting the untapped potential as chiral
WCAs in the arena of asymmetric catalysis. Here, we describe a
solution to this problem; that is, the molecular design, synthesis,
and characterization of structurally robust chiral borates, which
act as highly effective chiral WCA catalysts for achieving a highly
regio- and enantioselective protonative Prins-type cyclization.
[*]
D. Uraguchi, F. Ueoka, N. Tanaka, T. Kizu, W. Takahashi, T. Ooi
Department of Molecular and Macromolecular Chemistry, Graduate
School of Engineering, Nagoya University, Nagoya 464-8603,
Japan.
E-mail: tooi@chembio.nagoya-u.ac.jp
T. Ooi
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Nagoya 464-8601, Japan.
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
The molecular design strategy was derived from our hypothesis
on the origin of structural instability associated with the existing
borate ions, which generally comprise bidentate chiral backbones
(Fig. 1a). The most negatively polarized oxygen atom is
susceptible to protonation, triggering the cleavage of the B-O
bond with concomitant rotation around the B-X axis to generate
the corresponding trivalent borane with a vacant orbital. The
facile nucleophilic attack of a Lewis base such as H2O to the
vacant orbital leads to decomposition through hydrolysis or the
formation of another borate derivative. With this consideration in
mind, we reasoned that the use of a tetradentate backbone would
provide a key structural element for rendering a borate ion stable,
as it bridges two X atoms to restrict the rotation around the B-X
axis. Furthermore, the introduction of sterically demanding
substituents to the appropriate positions could prevent the boron
center from undergoing nucleophilic attack, thus delivering kinetic
stabilization. The actual structure of borate 1 we designed is
shown in Fig. 1b. This borate ion features an O,N,N,O-
tetradentate backbone consisting of an axially chiral binaphthyl
framework with a rigid amide linkage to the oxygen-attached
carbon bearing geminal aromatic substituents, which allow not
only steric protection but also modification of the chiral
environment around the boron center. The synthesis of 1 was
implemented in a straightforward manner, starting from the
commercially available (R)-1,1’-binaphthyl-2,2’-diamine (Fig. 1b).
After initial acylation of the nitrogen atom with ethyl
chlorooxoacetate, the resulting bisamide 2 was exposed to an aryl
Grignard reagent to install the aromatic appendages, affording C2-
symmetric amide alcohol 3 in good yield. Subsequent treatment
of 3 with trimethyl borate in acetonitrile at 90 °C in the presence
of triethylamine (NEt3) furnished the requisite borate salt 1ꞏHNEt3.
As expected, triethylammonium borate 1ꞏHNEt3 had sufficient
structural robustness to be isolated by standard silica gel column
chromatography. The three-dimensional molecular architecture
of 1a (Ar = Ph) was unambiguously determined by single-crystal
X-ray diffraction analysis of its benzyldimethylammonium salt,
uncovering the unique boron-sharing 5:7:5-fused tricyclic core;
the two planes constructed by the NBO connection were nearly
perpendicular to each other (Fig. 1c and Fig. S2). From this core
structure, four phenyl groups spread outward to create a discrete
chiral environment over the boron center. Importantly, one of the
carbonyl oxygens of the amide moieties interacted with the pairing
benzyldimethylammonium cation via a weak hydrogen bond
(C=OꞏꞏꞏHN+ = 1.727 Å) so that the cation is positioned between a
phenyl group and a naphthyl group (Fig. 1c-iii).
The structural robustness of 1 prompted us to prepare the
corresponding hydrogen borate 1ꞏH by an ordinary ion-exchange
method for use as a chiral Brønsted acid. Thus, an ethanolic
solution of 1ꞏHNEt3 was allowed to pass through a cation-
exchange resin column (Amberlite IR-120 (H+ form)), and the
eluent was concentrated to afford 1ꞏH in essentially pure form.
Although the pKa value of the ionic acid could not be directly
(a)
(b)
O
O
OEt
O
OEt
 bidentate x 2
Cl
NH2
NH2
NH
ArMgBr
O
X
X
O
O
HO
X
X
H2O
H
H
B
B
hydrolysis
NaHCO3
CH2Cl2
THF
0 °C
NH
O
O
0 °C~RT
O
O
OEt
Ar
preventing rotation
Ar
 tetradentate
O
Ar
Ar
B(OMe)3
Et3N
X
X
O
O
X
X
OH
O
NH OH
N
O
O
B
B
B
HNEt3
steric protection
MeCN
90 °C
N
NH OH
Ar
G
G
Ar
Ar
O
O
Ar
1ꞏHNEt3
(c)
1a
1a
1a
iii) ꞏHNMe2Bn
i) front view of
ii) side view of
Figure 1. a) Hydrolysis of borates and design concept for structurally robust borates, b) Molecular structure and synthesis of chiral borate 1ꞏHNEt3. c) ORTEP
diagrams of chiral benzyldimethylammonium borate 1aꞏHNMe2Bn (Ar = Ph) (50% probability, Calculated hydrogen atoms are omitted for clarity).
This article is protected by copyright. All rights reserved.

10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
compared with that of non-ionic Brønsted acids, the calculated
pKa of 1aꞏH (9.9 in MeCN, cf.: 8.7 for H2SO4) revealed its high
acidity.[13,14] Furthermore, the self-consistent hydrogen-bond
acceptor parameter (β), introduced by Hunter, was estimated to
1aꞏH in chloroform (CHCl3) at −60 °C, which gave a mixture of
regioisomeric dihydropyrans 3a and 4a (3a/4a = 6:1) in 35% yield
(Fig. 2a, entry 1). The enantiomeric excess (ee) of the major
product 3a was determined to be 55% by chiral-stationary-phase
HPLC analysis. This promising result encouraged us to examine
the effect of the catalyst structure on reactivity and selectivity,
specifically, with respect to the steric and electronic properties of
the aromatic substituents. While replacement of the phenyl
groups with 4-tolyl groups (1b) subtly affected the reaction profile,
the use of 1c bearing 3,5-xylyl groups led to a marked
improvement in both stereoselectivity and catalytic efficiency
(entries 2 and 3). An additional intriguing and important
observation was that the regioselectivity was also significantly
improved in response to the catalyst modification, indicating that
the borate ion is responsible for not only the stereo-determining
step but also the product-determining deprotonation step (vide
infra). Eventually, the use of 1d with 3,5-di-n-butylphenyl groups
resulted in the exclusive production of 3a in 77% yield with 94%
ee (entry 4).[18] The remaining problem was the insufficient
chemical yield, which was likely due to the formation of hydration-
related side products and was solved by performing the reaction
in the presence of the desiccant Drierite (porous anhydrous
calcium sulfate) under otherwise identical conditions; 3a was
obtained in 95% yield without any detrimental effect on the
be
7.6,
which
is
comparable
to
that
of
bis(trifluoromethanesulfonyl)imidate (β = 7.3, pKa = 0.3 for
(CF3SO3)2NH),[15] indicating the weakly-coordinating character of
1a in pairing with a cation such as a proton.
As a reaction platform suitable for evaluating the catalytic
performance of 1ꞏH, we set a Prins-type cyclization of vinylic
ethers 2.[16]
This transformation involving the protonative
generation of an oxonium ion, followed by intramolecular carbon-
carbon bond formation and final deprotonation from the transient
carbocation, is of significant synthetic relevance as it offers
straightforward access to substituted dihydropyrans 3/4, which
are valuable six-membered oxygen heterocycles (Fig. 2a,
Scheme).
However, the available catalytic protocols are
scarce,[17] and precise control of the regio- and stereochemical
outcome remains a formidable challenge for realizing the full
potential of this reaction system. Our incipient expectation was
that the chiral hydrogen borate 1ꞏH would be sufficiently acidic to
generate the intermediary oxonium ion and that its prochiral faces
could be discriminated by the pairing 1 in the cyclization event.
As the initial attempt, vinyl ether 2a was treated with 5 mol% of
1
0
0
0
0
0
0
0.5
1
1.5
2
Figure 2. a) Catalyst optimization. The reaction was performed in 0.1 mmol scale with 2a and 5 mol% of 1ꞏH in CHCl3 (0.1 M) at –60 °C for 24 h. Isolated yield was
indicated. The regioisomeric ratio (rr) was assigned by 1H NMR (500 MHz) analysis of the crude aliquot. The enantiomeric excess (ee) of the major isomer was
determined by HPLC with DAICEL CHIRALCEL OJ3 as a chiral stationary phase and hexane/EtOH = 99:1 as eluent. Drierite = porous anhydrous CaSO4. b) and
c) Determination of the kinetic isotope effect (KIE) at the deprotonation stage. Conversions and isomeric ratios were determined by 1H NMR (500 MHz) analysis of
the crude aliquots containing trimethylsilylbenzene as an internal standard (average of three runs).
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10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
selectivity (entry 5). Moreover, 1 was easily recovered as a
triethylammonium salt (1dꞏHNEt3) during product purification after
quenching the reaction by the addition of NEt3.
internal standard. This observation led us to evaluate the primary
kinetic isotope effect (KIE) in these parallel reactions, and the KIE
for 2a/d2-2a was determined to be 1.37.[19] Judging from the
deuterium incorporation into the methyl group of the products (ca.
40% by 500 MHz 1H NMR analysis) and the magnitude of the KIE,
the rate difference presumably stems from the lower acidity of the
deuterated catalyst 1dꞏD,[20] implying that the actual KIE would be
negligible. We also evaluated the KIE in the intermolecular
competition reaction with an equimolar mixture of 2a and d2-2a
under otherwise similar conditions (Fig. 2c). While there was a
slight decrease in the overall reaction rate, the conversions of 2a
and d2-2a were nearly equal and almost equimolar amounts of
3a/4a and d1-3a/d2-4a were obtained (extent of deuterium
incorporation into the methyl groups was <10%), thereby
providing a KIE close to 1.0. These results suggested that the
deprotonation does not participate in the rate-limiting process and
that carbon-carbon bond formation is most likely the rate-
determining step. In addition, the constantly observed excellent
regioselectivity emphasized that the catalyst-controlled
deprotonation plays a critical role in dictating the reaction pathway
to give the dihydropyran product 3.
To gain insight into the origin of the catalyst-dependent
remarkable enhancement of regioselectivity in the final
deprotonation from the transient carbocation, we performed the
reaction with deuterated vinyl ether d2-2a (Fig. 2b). At the outset,
we sought to confirm the intrinsic regiochemical preference by the
use of (±)-10-camphorsulfonic acid as a simple catalyst (5 mol%),
which gave a mixture of d1-3a and d2-4a in a ratio of 0.8:1 (Fig.
S1) probably because of the preferable deprotonation over
dedeuteration as the opposite preference was observed in the
reaction of 2a under otherwise identical conditions (3a/4a = 1.7:1).
In sharp contrast, exposing d2-2a to the standard conditions with
deuterated catalyst 1dꞏD for 6 h afforded d1-3a exclusively (d1-
3a/d2-4a = >20:1), identical regioselectivity observed in the
separate reaction of 2a with 1dꞏH as a catalyst under similar
conditions (Fig. 2b). The selective formation of d1-3a through
dedeuteration in complete preference over deprotonation to give
d2-4a signified that the deprotonation (dedeuteration) step
proceeded in a catalyst-controlled manner rather than by
spontaneous release. Interestingly, the conversion of d2-2a was
lower than that of 2a, as revealed by 1H NMR analysis (500 MHz)
of the crude mixture containing trimethylsilylbenzene as the
Having identified the optimal borate structure and reaction
conditions, we investigated the substrate generality of the 1dꞏH-
catalyzed Prins-type cyclization of vinylic ethers 2 (Table 1). As
Table 1. Substrate generality[a]
entry
1
Ar’
4-MeOC6H4
4-MeC6H4
4-ClC6H4
4-FC6H4
3-MeC6H4
3-MeOC6H4
2-furyl
R1
H
R2
H
2
2b
2c
2d
2e
2f
time (h)
24
yield (%)b
74
rrc
drc
ee (%)d
88
Prod
3b
3c
3d
3e
3f
15:1
–
2
H
H
24
97
19:1
–
93
3
H
H
24
82
>20:1
>20:1
10:1
–
95
4
H
H
24
98
–
94
5
H
H
72
84
–
90
6
H
H
2g
2h
2i
48
73
17:1
–
–
88
3g
3h
3i
7
H
H
24
51
14:1
86
8
Ph
Me
Me
Me
Et
H
H
24
87
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
–
93
9
4-FC6H4
4-MeC6H4
Ph
H
2j
72
98
–
90
3j
10
11
12
13
14
15
H
2k
2l
24
89
–
89
3k
3l
H
48
79
–
94
Ph
Me
Me
Me
Et
2m
2n
2o
2p
24
92
>20:1
>20:1
>20:1
>20:1
94
3m
3n
3o
3p
4-ClC6H4
3-MeC6H4
Ph
H
24
74
93
H
24
96
90
H
24
92
91
[a] The reaction was performed in 0.1 mmol scale with 2 and 5 mol% of 1dꞏH in CHCl3 (0.1 M) at –60 °C. [b] Isolated yield was indicated. [c] The regioisomeric
ratio (rr) and diastereomeric ratio (dr) were assigned by 1H NMR (500 MHz) analysis of the crude aliquot. [d] The enantiomeric excess (ee) of the major isomer
was determined by HPLC equipped with a chiral column.
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10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
shown in Table 1, the reaction was tolerant to the incorporation of
aromatic substituents (Ar’) with different electronic attributes, and
the corresponding dihydropyrans 3 were obtained predominantly
with a high level of enantiocontrol, while slightly lower regio- and
stereoselectivities were observed in the cyclizations with 2 having
electron-rich (hetero)aryl groups (entries 1~7). In the reactions
with substituted vinyl ethers, only Z-configured isomers
underwent intramolecular bond formation to give 3 in good yield
with high stereoselectivity, probably because E-isomers were
resistant to the initial protonation by 1dꞏH (entries 8~11). Vinylic
ethers with a substituent at the terminal carbon of the nucleophilic
olefinic component were amenable to this protocol, and 2,3-
disubstituted dihydropyrans 3m~p were obtained with virtually
complete diastereoselectivity and excellent enantioselectivity
(entries 12~15). Notably, the olefin geometry was again important,
and the E-configuration was crucial for smooth ring closure.[21]
The stereochemically defined dihydropyran products 3
serve as versatile synthetic intermediates, as they can be
derivatized into various oxygen-containing six-membered
heterocycles in a straightforward manner with preservation of the
stereochemical integrity, thereby demonstrating the utility of the
catalytic asymmetric Prins-type cyclization protocol (Fig. 3). The
olefin component of 3 could be harnessed as a functional handle
to increase stereochemical and structural complexity. For
example, simple hydrogenation of 3m with palladium on charcoal
(Pd/C) afforded tetrahydropyran 5 possessing three consecutive
stereogenic centers in good yield with reasonably high
diastereoselectivity, and the hydroboration-oxidation sequence
allowed the introduction of a hydroxy group with a similar degree
of diastereocontrol. Alcohol 6 was subjected to single-crystal X-
ray diffraction analysis after conversion to ferrocenyl carboxylate,
and the absolute configuration of 3m was established as 2S,3S
at this stage (Fig. S3). Moreover, single-electron oxidation of 3m
under the photoredox conditions facilitated carbon-carbon bond
formation at the 5-position of the tetrahydropyran, although the
corresponding ketone
7 was obtained as a mixture of
diastereomers.[22] On the other hand, treatment of 3m with
pyridinium dichromate (PDC) promoted allylic oxidation to give
unsaturated lactone 8 in good yield.[23] The allylic methylene
protons are sufficiently acidic to be deprotonated by the n-butyl
lithium/tetramethylethylenediamine (TMEDA) system to generate
a carbanion that can be trapped by an electrophile such as a
Weinreb amide to provide acylated dihydropyran
9
in
stereochemically pure form.[24] Hydride abstraction at the allylic
position was also feasible by the action of the in situ generated
trityl cation, and the intermediary oxonium ion reacted with an
allylsilane to furnish tetrasubstituted dihydropyran 10 with
complete diastereoselectivity.[25]
Conclusion
We have introduced a molecular-design strategy for the rational
development of a structurally robust chiral borate ion based on
the use of an O,N,N,O-tetradentate backbone.
X-ray
crystallographic and theoretical analyses reveal the unique
structural features and weakly-coordinating character of this
borate ion. Furthermore, we have clearly demonstrated that the
corresponding chiral hydrogen borate acts as an effective catalyst
for the highly regio- and stereoselective Prins-type cyclization of
vinyl ethers, where the borate ion precisely discriminates the
prochiral oxonium ion intermediate and rigorously dictates the
final deprotonation pathway.
The structurally and
stereochemically defined dihydropyran products can serve as
versatile synthetic building blocks. We believe that the present
study shows a new direction for the development of chiral WCAs
capable of catalytically controlling prochiral cations for achieving
otherwise difficult stereoselective chemical transformations.
Ph
Ph
Me
Me
Pd/C, H2
PDC, NaOAc
AcOEt, RT
CH2Cl2, 40 °C
O
Me
O
O
Me
5
8
74%, drC2/C4 = 9:1
(94% ee)
79%, drC2/C3 = >20:1
(94% ee)
nBuLi, TMEDA
then
Ph
Ph
Ph
HO
Me
Me
Me
Me
Me
BH3ꞏSMe2
tBuCON(OMe)Me
H2C
tBu
THF, RT
Et2O, –78 °C~RT
O
6
O
3m
O
Me
then NaO2H
9
O
88%, drC2/C5 = 9:1
(94% ee)
drC2/C3 = >20:1
(94% ee)
63%, drC2/C6 = >20:1
(94% ee)
Ph
Me
Ph
Me
AcrBF4, PhSSPh
CH2=C(OMe)Me
Ph3CCl, AgBF4
CH2=CHCH2SiMe3
Ac
Blue LEDs
CH2Cl2, RT
O
7
Me
O
10
Me
(CF3)2CHOH
62%, drC2/C4 = 1.3:1 (CH2Cl)2, RT
(93% ee)
62%, drC2/C6 = >20:1
(94% ee)
Figure 3. Derivatization of dihydropyran 3m. Acr = N-phenyl-9-mesitylacridinium.
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10.1002/anie.202001637
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RESEARCH ARTICLE
[12] G. Hu, A. K. Gupta, L. Huang, W. Zhao, X. Yin, W. E. G. Osminski, R. H.
Huang, W. D. Wulff, J. A. Izzo,; M. J. Vetticatt, J. Am. Chem. Soc. 2017,
139, 10267-10285.
Acknowledgements
[13] A. Kütt, T. Rodima, J. Saame, E. Raamat, V. Mäemets, I. Kaljurand, I. A.
Koppel, R. Y. Garlyauskayte, Y. L. Yagupolskii, L. M. Yagupolskii, E.
Bernhardt, H. Willner, I. Leito, J. Org. Chem. 2011, 76, 391-395.
[14] K. Ohmatsu, T. Nakashima, M. Sato, T. Ooi, Nat. Commun. 2019, 10,
2706.
Financial support was provided by CREST-JST (JPMJCR13L2:
13418441, to TO), a Grant-in-Aid for Scientific Research on
Innovative Areas “Hybrid Catalysis” (No. 17H06444, to TO), and
Grants of JSPS for Scientific Research. N.T. and T.K.
acknowledges JSPS for financial support. Program for Leading
Graduate Schools "Integrative Graduate Education and Research
Program in Green Natural Sciences" in Nagoya University and
"Graduate Program of Transformative Chem-Bio Research" in
Nagoya University, supported by MEXT (WISE Program) are
acknowledged.
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10.1002/anie.202001637
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents (Please choose one layout)
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RESEARCH ARTICLE
A chiral borate ion comprising an
O,N,N,O-tetradentate backbone was
developed and the corresponding
hydrogen borate acted as a catalyst for
the asymmetric protonative Prins-type
cyclization through the precise control
of the prochiral oxonium ion
Daisuke Uraguchi, Fumito Ueoka,
Naoya Tanaka, Tomohito Kizu, Wakana
Takahashi, and Takashi Ooi*
Page No. – Page No.
A Structurally Robust Chiral Borate
Ion: Molecular Design, Synthesis, and
Asymmetric Catalysis
intermediate
and
the
final
deprotonation pathway.
This article is protected by copyright. All rights reserved.