Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis Superacids with a Silyl, Germyl, or Stannyl Cationic Center

Article abstract of DOI:10.1002/asia.202100873

The synthesis and isolation of atrane-type molecules 1E⁺ (E=Si, Ge, or Sn) having a cationic group 14 elemental center are reported. The cations 1E⁺ act as hard and soft Lewis superacids, which readily interact with various hard and soft Lewis basic substrates. The rigid atrane framework stabilizes the localized positive charge on the elemental center and assists the formation of the well-defined highly coordinated states of 1E⁺. The cations were applied to the hydrodefluorination, Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduction as Lewis acid catalysts. Most notably, [1Si][ClO₄] exhibits unique chemoselectivity that depends on a solvent in the competitive reaction of silyl enol ether with a mixture of benzaldehyde dimethyl acetal and benzaldehyde. Our findings indicate the potential of hard and soft Lewis superacids in organic synthesis.

Full text of DOI:10.1002/asia.202100873

doi.org/10.1002/asia.202100873
Communication
Synthesis and Catalytic Activity of Atrane-type Hard and Soft Lewis
Superacids with a Silyl, Germyl, or Stannyl Cationic Center
Daiki Tanaka,[a] Akihito Konishi,*[a, b, c] and Makoto Yasuda*[a, c]
as LSAs.[3] On the other hand, hydride ion affinity (HIA)[7] is used
Abstract: The synthesis and isolation of atrane-type mole-
to evaluate soft Lewis acidity. Lewis acids that exceed the HIA
cules 1E+ (E=Si, Ge, or Sn) having a cationic group 14
of B(C6F5)3 are considered to be soft LSAs.[3] Lewis acids that
elemental center are reported. The cations 1E+ act as hard
and soft Lewis superacids, which readily interact with
meet these two criteria are categorized into hard and soft LSAs.
Significant progress in this field has been achieved in recent
various hard and soft Lewis basic substrates. The rigid
years (Figure 1A). The Greb group reported bis(catecholato)
atrane framework stabilizes the localized positive charge on
silanes as the first neutral silicon-based hard LSAs[8–10] The same
the elemental center and assists the formation of the well-
group also successfully synthesized bis(perchlorocatecholato)
defined highly coordinated states of 1E+. The cations were
germane as a hard and soft LSA having a germanium
applied to the hydrodefluorination, Friedel-Crafts reaction,
alkyne cyclization, and carbonyl reduction as Lewis acid
catalysts. Most notably, [1Si][ClO4] exhibits unique chemo-
center.[11,12] Driess et al. demonstrated facile access to pristine
LSA, Si(OTf)4, in which the OTf substituents work as excellent
leaving groups and modulate reactivity towards hard and soft
selectivity that depends on a solvent in the competitive
Lewis bases.[13] In these systems that activated a variety of
reaction of silyl enol ether with a mixture of benzaldehyde
substrates, the hard and soft Lewis acidities result from the
dimethyl acetal and benzaldehyde. Our findings indicate
formation of the highly coordinated Si and Ge centers. The
the potential of hard and soft Lewis superacids in organic
importance of the highly coordinated group 14 centers[14–16] in
synthesis.
organic synthesis has been extensively investigated, resulting in
chemo-,[17] stereo-,[18,19] and regioselective[20] synthetic ap-
proaches. Despite the high synthetic utility of highly coordi-
Lewis acids play an important role in all areas of chemistry.[1]
Control of Lewis acidity determined by the type of central
element and ligand is a central issue in making the most
efficient and widespread use of an ideal Lewis acid.[2] Although
a wide range of affinity for various substrates, from hard to soft
Lewis basic substrates, have been sought, the coexistence of
hard and soft Lewis acidities in the same central element is
mostly difficult. The recent advances in hard and soft Lewis
superacids (LSAs)[3,4] using heavier group 14 elements (Si, Ge,
and Sn) have the potential to be a reasonable approach to solve
this problem. As the rational design guide for hard and soft
LSAs, the values of two ion affinities are frequently employed.
Fluoride ion affinity (FIA)[5,6] is an ideal method to measure the
scale of the hardness of Lewis acids. Lewis acids that have larger
FIA values than monomeric SbF5 in the gas phase are classified
nated states, precise control of the coordination number of
LSAs has scarcely been considered. There is still room for the
active use of highly coordinated group 14 centers in the
investigation of more efficient organic syntheses catalyzed by
hard and soft LSAs.
In order to tame the high coordination states in LSAs, we
designed atrane-type molecules 1E+ having a cationic group 14
center (Figure 1B). The high reactivity of the heavier group 14
cations,[21] which readily cleave various bonds such as CÀ F[22]
and SiÀ H,[23] could serve the potential development of efficient
[a] D. Tanaka, Dr. A. Konishi, Prof. Dr. M. Yasuda
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
E-mail: a-koni@chem.eng.osaka-u.ac.jp
yasuda@chem.eng.osaka-u.ac.jp
[b] Dr. A. Konishi
Center for Atomic and Molecular Technologies
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
[c] Dr. A. Konishi, Prof. Dr. M. Yasuda
Innovative Catalysis Science Division, Institute for Open and Transdiscipli-
nary Research Initiatives (ICS-OTRI)
Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
Figure 1. (A) Previously studied hard and soft LSAs having neutral group 14
centers. (B) Molecular structure of atrane-type LSAs having cationic group
14 centers is presented herein.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202100873
Chem Asian J. 2021, 16, 1–7
1
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
hard and soft LSAs. However, due to their poor stability against
external environments, such as temperature and moisture,
thermodynamic[24,25] or kinetic[26–29] stabilization is required for
sufficient stability, which has hampered the catalytic utility
except for silyl cations.[30] Our design employs the atrane-type
framework[31] and brings three benefits for the establishments
of heavier group 14 cation-based LSAs: 1) stabilization of the
cationic center by the intramolecularly transannular NÀ E (E=Si,
Ge, or Sn) bond, 2) orientation of the coordinated site of the
cationic center by blocking one of the two apical vacant orbitals
of the NÀ E bond, and 3) facile tuning of steric and electronic
factors by changing the aromatic ligands. Although few works
to isolate cationic atrane-type molecules having a group 14
center were reported,[32–34] their use as hard and soft catalytic
LSAs remains unexplored. Herein, we report the synthesis and
catalytic activity of the atrane-type molecules 1E+ (E=Si, Ge, or
Sn) with a cationic group 14 center (Figure 1B). The molecules
1E+ behave as hard and soft LSAs and display activity in a
variety of Lewis acid-catalyzed transformations.
suitable for the synthesis of 1Ge+ and 1Sn+, because insepa-
rable side products were generated. When other silver salts,
AgBF4, AgPF6, and AgSbF6 were applied to the synthesis of 1Si+,
the fluorinated atrane-type molecule 1SiF was quantitatively
obtained, indicating that the in situ generated cation 1Si+
abstracted a fluoride ion from the counterions because of its
high FIA (see below).[8,12] The 29Si NMR signals for 1Si+ ([1Si
(CH3CN)][SbCl6], À 53.9 ppm; [1Si(CH3CN)][ClO4], À 53.8 ppm, in
CD3CN) are close to that of neutral 1SiCl (À 52.3 ppm), implying
a strong electron donation from the nitrogen atom via the
transannular NÀ Si bond and the coordination of a CH3CN (or
CD3CN) molecule. The 119Sn NMR signal of [1Sn(CH3CN)][SbCl6]
(À 59.3 ppm in CD3CN) appeared at the range reported for five
coordinated stannyl cations (À 26.8[37] to À 76.3[25] ppm), indicat-
ing that the Sn center of [1Sn(CH3CN)][SbCl6] assumes a highly
coordinated state in solution.
The molecular geometries of [1E(solvent)]+ were confirmed
by X-ray crystallographic analysis. The ortep drawings are
depicted in Figure 2. Recrystallization from a CH3CN solution
gave a single crystal of [1E][SbCl6] as a CH3CN-adduct ([1E-
(CH3CN)]+ [E=Si, Ge, or Sn]). Through the recrystallization of
[1Si][ClO4] from a CH3CN solution, we surprisingly obtained a
water adduct species ([1Si(H2O)][ClO4]) in a crystalline form.[38]
When the recrystallization was performed even in a nitrogen-
filled glovebox, the water-adduct was obtained. We believe that
even trace amounts of moisture in the glovebox may be easily
captured by the cationic species. Despite several attempts, the
isolation of [1Si(H2O)][ClO4] remained in trace amounts, hamper-
ing other spectroscopic characterizations except for the X-ray
analysis. Some geometric parameters are summarized in
Table 1. All cations show an approximate C3 symmetry and the
The synthetic route for 1E+ is summarized in Scheme 1.
Preparation of tribenzylamine derivative 3 from commercially
available o-bromobenzyl bromide 2[35] and subsequent trilithia-
tion followed by treatment with ECl4 (E=Si, Ge, or Sn) afforded
the atrane-type molecules 1ECl in high yields. The treatment of
[36]
1ECl with SbCl5 in CH3CN gave the desired cationic atrane
molecules 1E+ as CH3CN-adducts ([1E(CH3CN)][SbCl6]). For the
synthesis of 1Si+, the addition of AgClO4 into 1SiCl was also
applicable, to yield [1Si][ClO4]. The use of AgClO4 was not
1
2
°
average of the dihedral LÀ EÀ C À C angles (ϕ) are small (~15 ).
This structural feature indicates that the positive charge
communicates with the aromatic skeletons less and strongly
localizes on the group 14 center. The group 14 center assumes
a nearly trigonal-bipyramidal structure with a five coordination.
The relatively short bond length of the N1À E bond ([1Si
(CH3CN)][SbCl6], 2.016 Å; [1Ge(CH3CN)][SbCl6], 2.091 Å; [1Sn
(CH3CN)][SbCl6], 2.267 Å; [1Si(H2O)][ClO4], 2.110 Å) suggests the
efficient transannular interaction at the apical position. In the
structures [1E][SbCl6], the average of the CipÀ EÀ Cip angle
decreases in order from 1Si+ to 1Ge+ to 1Sn+ ([1Si
°
°
(CH3CN)][SbCl6], 119.49 ; [1Ge(CH3CN)][SbCl6], 119.24 ; [1Sn
°
(CH3CN)][SbCl6], 117.59 ), which is induced by the difference in
the elemental size. The larger elemental center leads to a longer
N1À E bond and lower planarity around the center.
In order to assess the characters of 1E+ with no adducts of
solvents, we considered the Lewis acidity of 1E+ by quantum
Scheme 1. Synthetic route for atrane-type molecules 1E+ bearing a cationic
group 14 center.
Table 1. Summary of selected geometric parameters of atrane-type molecules 1E+.
[1Si(CH3CN)][SbCl6]
[1Ge(CH3CN)][SbCl6]
[1Sn(CH3CN)][SbCl6]
[1Si(H2O)][ClO4]
○[a]
ϕ(CoÀ CipÀ EÀ L)/
N1À E/Å
15.16
2.016(4)
119.49(15)
14.81
2.091(2)
119.24(11)
12.06
2.267(4)
117.59(17)
14.05
2.110(14)
118.43(7)
○[a]
<(CipÀ EÀ Cip)/
[a] Mean values.
Chem Asian J. 2021, 16, 1–7
www.chemasianj.org
2
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
Figure 2. X-ray structures of atrane-type molecules 1E+. (A) Side and (B) top views. For top views, an adducted molecule, CH3CN or H2O, is omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level.
chemical calculations. The values of FIA and HIA were estimated
according to the reported method[13] (Table 2 and see Support-
ing Information). The gas phase FIAs of 1E+ were obviously
larger than that of SbF5, meeting the criteria for hard LSAs.
Furthermore, the gas phase HIAs of 1E+ sharply exceeded the
HIA of B(C6F5)3, rendering these new compounds also soft LSAs.
The orders of FIA (1Si+ >1Ge+ �1Sn+) and HIA (1Si+ <1Ge+ <
1Sn+) are reversed, and this is reflected by the difference in
atomic sizes (Si<Ge<Sn). Where 1Si+, with the smallest
elemental center exhibits the largest FIA (hardest Lewis acidity),
1Sn+ with the largest elemental center shows the largest HIA
values (softest Lewis acidity). Using both criteria, 1Ge+ is
located between 1Si+ and 1Sn+. It should be noted that both
the gas phase FIAs and HIAs of 1E+ are much larger (>200 kJ/
mol) than those of the previously reported neutral Lewis acids.
The distinct duality of hardness and softness is derived from the
presence of a positive charge on the group 14 center of 1E+.
The localized positive charge of 1E+ should be significantly
affected by solvation. The effect of solvation in CH2Cl2 on the
FIAs/HIAs were also evaluated (parenthesis values in Table 2).
The large damping of FIAs/HIAs of 1E+ was revealed, and the
corrected FIAs/HIAs of 1E+ are reduced to a level comparable
to the reported neutral Lewis acids. Similar damping upon
solvation was also reported in other hard and soft LSAs.[13]
The experimental evaluation of the Lewis acidity of atrane-
type cationic group 14 species was also performed. Subjecting
[1Si][ClO4] in CDCl3 to Et3P=O provided an estimate of Lewis
acidity (Gutmann-Beckett method).[39,40] After the addition of
phosphine oxide to the solution of the silyl cation [1Si][ClO4],
the acceptor number (AN) was determined by the 31P NMR
chemical shift difference (Δδ31P) between the free phosphine
oxide and the formed adduct of 1Si+ with Et3P=O.[41] The
estimated AN of [1Si][ClO4] (AN=85.3) is in the same range as
the reported Lewis acids, B(C6F5)3 (AN=81.8),[42] BF3 (AN=
84.0),[43] and smaller than that of the silyl cation [Et3Si][B(C6F5)4]
(AN=105.2).[44] Unfortunately, the evaluation of Lewis acidity
for hexachloro antimonate salts ([1Si(CH3CN)][SbCl6], [1Ge
(CH3CN)][SbCl6], and [1Sn(CH3CN)][SbCl6]) was unsuccessful
because those cations gradually decomposed after the addition
of phosphine oxide. We also attempted to compare the C�N
bond stretching modes of [1E(CH3CN)][SbCl6], but the undesired
intermolecular interactions of the coordinated CH3CN with
lattice solvents and/or counterion hampered the quantitative
evaluations. Alternatively, the DFT calculation provides insight
into the LUMO orbitals of 1E+. The calculated LUMOs of 1E+ at
the B3LYP/6-31G** for C, H, and N and LANL2DZ for Si, Ge, and
Sn are shown in Figure 3. The vacant p-type orbital is mainly
localized on the group 14 center, correlating with the structural
features observed in the crystallographic analysis. The LUMO
Table 2. DFT-calculated ion affinities (fluoride: FIA, hydride: HIA) for
selected Lewis acids (LA) in kJmolÀ 1 [a]
.
[b]
[b]
Lewis acid
FIA (FIAsol
)
HIA (HIAsol)
LA/LAÀ FÀ
LA/LAÀ HÀ
1Si+
762 (303)
710 (252)
713 (255)
507[8][c]
754 (264)
764 (273)
783 (289)
–
1Ge+
1Sn+
Si(catCl)2
495 (283)[13]
521 (305)
500 (338)
443 (227)
491 (251)[13]
547 (306)
–
[13]
Si(OTf)4
SbF5
B(C6F5)3
[13]
[13]
514 (268)
[a] PW6B95-D3BJ/def2-QZVPP//B3LYP-D3BJ/def2-SVP. [b] B3LYP-D3BJ/
def2-SVP, gas phase geometries with CPCM in CH2Cl2. [c] DLPNO-CCSD(T)/
cc-aug-pVQZ//PW6B95-D3(BJ)/def2-TZVPP.[8]
Chem Asian J. 2021, 16, 1–7
www.chemasianj.org
3
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
stabilized by the coordination of solvents as the observation
from the NMR and X-ray analyses.
The utility in Lewis acid catalyst for our hard and soft LSAs
1E+ was probed. The harmony of extreme hardness and
softness of 1E+ readily activates a wide range of substrates as a
LSA catalyst. The investigated catalytic activities of the atrane-
type cations 1E+ and 1E(CH3CN)+ are summarized in Table 3.
First, the hydrodefluorination of an aliphatic CÀ F bond was
catalytically achieved for 1-adamantyl fluoride 4 in CH2Cl2
(reaction A in Table 3). [1Si][ClO4] afforded the product 5 in the
Figure 3. LUMO orbitals and WBIs of 1E+ calculated by the B3LYP/6-31G**
for C, H and N and LANL2DZ for Si, Ge and Sn.
À
highest yield (92%) compared to the other cation salts of SbCl6 .
level decreases (1Si+, À 0.15 eV; 1Ge+, À 0.17 eV; 1Sn+,
À 0.20 eV) with increasing elemental size and N1À E bond length.
This trend can be explained by the strength of the interaction
between the group 14 center and the nitrogen atom. For
quantitative information on the bonding N1À E interaction, a
natural bond orbital (NBO) analysis was conducted. The NBO
charges and Wiberg bond indices (WBI) obtained (Figure 3 and
Table S5) reveal that the positive charge mainly localizes on the
group 14 center rather than the N1 atom and the bond length
of the N1À E bond increases in the order from Si to Ge to Sn.
These results provide strong support that the cations are readily
When [1E(CH3CN)][SbCl6] was employed, GC-MS monitoring
indicated the formation of adamantyl chloride as a side product.
À
Under the reaction conditions, SbCl6 would work as
a
chlorination source, which lowers the catalytic activities of
[1E(CH3CN)][SbCl6]. The fluorination of the solvent or the
chlorination of the catalyst were not observed.
In contrast, the Friedel-Crafts dimerization of 1,1-diphenyl-
ethylene 6 was catalyzed effectively by [1E(CH3CN)][SbCl6],
whereas [1Si][ClO4] did not afford the product 7 (reaction B). In
the catalytic cycle, [1E(CH3CN)][SbCl6] can reproduce the active
cationic species 1E+ more effectively than [1Si][ClO4], presum-
ably because of the weaker coordination of SbCl6À compared to
Table 3. Catalytic applications of our atrane-type molecules 1E+ and 1E(CH3CN)+.
reaction
catalyst
[1Si(CH3CN)][SbCl6]
[1Ge(CH3CN)][SbCl6]
[1Sn(CH3CN)][SbCl6]
[1Si][ClO4]
A yield of 5 [%]
B yield of 7 [%]
C yield of 9 [%]
D yield of 11 [%]
44
82
46
89
25
59
44
91
19
43
77
99
92
trace
85
89
Chem Asian J. 2021, 16, 1–7
www.chemasianj.org
4
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
À
that of ClO4 . To confirm the affinity toward alkyne substrates,
Table 5. Competitive reactions of silyl enol ether 14 with a mixture of
benzaldehyde dimethyl acetal 15 and benzaldehyde 16 catalyzed by
[1Si][ClO4].
the cyclization of propargylamide 8 was conducted (reaction C).
All cations 1E+ and 1E(CH3CN)+ gave the cyclic compound 9
even though substrate 8 has a coordinating amide group. Hard
Lewis acids are readily coordinated by harder Lewis basic
moieties, which inhibits the interaction with softer Lewis basic
moieties. Notably, the rigid structures of 1E+ around the group
14 center given by the atrane framework help to activate the
alkyne moiety of 8 without any deactivation by a strong
interaction with the amide moiety. [1Sn(CH3CN)][SbCl6] exhib-
ited the highest catalytic activity because of the softness of the
Sn center in comparison to the Si and Ge centers.[45]
Furthermore, all cations were further applicable to the cycliza-
tion reaction of the other alkyne 10 for the synthesis of
isocoumarin 11 even though substrate 10 has an acidic proton
(reaction D). There are no reported heavier group 14 species to
catalyze alkyne cyclization reactions. Our results demonstrate
the catalytic utility of 1E+ having a cationic group 14 center
stabilized by the atrane framework.
entry catalyst
solvent yield of 17
yield of 18
[%]
Ratio of 17/
18
[%]
1
2
3
4
5
6
7
8
9
[1Si][ClO4] THF
toluene 11
Me3SiClO4 THF
94
1
89
4
62
30
77
59
88
11
18
99/1
11/89
95/5
25/75
68/32
16/84
40/60
6/94
–
83
toluene 21
THF
toluene 15
THF
Me3SiOTf
B(C6F5)3
64
40
6
0
toluene
THF
The effect of the atrane structure of 1E+ on its catalytic
activity was clearly represented in the carbonyl reduction of 4-
(trifluoromethyl)benzaldehyde 12 with triethylsilane (Table 4).
In the catalytic system of [1Si][ClO4], product 13 was easily
obtained without any cleavage of the CÀ F bond (entry 1). The
other atrane-type cations [1E(CH3CN)][SbCl6] also gave product
13 in high yield (Table S4). On the other hand, the activation of
Et3SiH with [Ph3C][B(C6F5)4],[22,46–48] in which some cationic silyl
species are in-situ generated but their cationic characters are
far from a ‘true silyl cation’,[49] showed inferior catalytic activity
BF3 ·OEt2
10
toluene
0
–
[50]
Lewis acids, Me3SiClO4
and Me3SiOTf, exhibited a similar
solvent dependency to that of [1Si][ClO4], but the selectivity
towards 17/18 was lower (entries 3–6). The B-based Lewis acids,
B(C6F5)3 and BF3 ·OEt2, showed no solvent dependence (en-
tries 7–10). The observed selectivity of 1Si+ would be derived
from the difference in the interaction of the Si+ center with the
substrate. For the activation of the acetal 15 with 1Si+, a
cationic oxonium-type intermediate is generated by the elimi-
nation of one methoxy group. The process, assisted by 1Si+,
would be accelerated in a high polar solvent, resulting in the
highly selective formation of 17 in THF. In contrast, in a low
polar solvent, the interaction of the carbonyl group of 16 with
1Si+ would take precedence over the activation of 15. In
addition to the coexistence of the hardness and softness of
Lewis acidity, the localized cationic nature of 1Si+ allowed for
sensitive recognition of the substrates depending on the
polarity of the solvent. The observed result is the first example
of a switch in the selectivity in the competitive reaction
between acetal and aldehyde. Notably, this chemoselectivity is
the result of a concentrated effect of the structural and
electronic features of our hard and soft LSAs.
In conclusion, we successfully synthesized the atrane-type
group 14 cations 1E+ (E=Si, Ge, or Sn) in only three steps. The
atrane structure of 1E+ stabilized a localized positive charge on
the group 14 center, introducing hard and soft Lewis super-
acidity. The robustness of 1E+ allowed handling under standard
experimental conditions, which served as a versatile Lewis acid
catalyst. Various reactions, including hydrodefluorination, the
Friedel-Crafts reaction, alkyne cyclization, and carbonyl reduc-
tion, were efficiently catalyzed by 1E+. The rigid atrane frame-
work assisted the formation of the highly coordinated states,
resulting in the catalytic activities of 1E+. The high sensitivity of
the localized positive charge of the 1Si+ center to solvent
polarity demonstrated unique chemoselectivity.
(entry 2).
A number of unidentified side products were
generated and confirmed by NMR measurement. The atrane
framework of 1E+ dramatically improves the catalytic activities
of the cationic group 14 center.
Judging from the features of 1E+ concerning Lewis acidity
and reactivity, [1Si][ClO4], which has a more stable counter
anion under the reaction conditions, is the most tolerant salt.
Thus, we applied the silyl cation [1Si][ClO4] to a competitive
reaction of silyl enol ether 14 with a mixture of benzaldehyde
dimethyl acetal 15 and benzaldehyde 16 (Table 5). Interestingly,
a significant solvent dependency was observed. In THF, acetal
15 reacted with 14 prior to aldehyde 16 in a highly selective
manner (17/18=99/1, entry 1). On the other hand, upon
changing the solvent to toluene, the selectivity was completely
reversed, giving the product 18 (17/18=11/89, entry 2). Other
Lewis acids were also investigated (entries 3–10). The Si-based
Table 4. Difference of catalytic activity between [1Si][ClO4] and Et3SiH/
[Ph3C][B(C6F5)4].
entry
catalyst
yield of 13 [%]
1
2
[1Si][ClO4]
[Ph3C][B(C6F5)4]
100
48
Chem Asian J. 2021, 16, 1–7
www.chemasianj.org
5
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
[17] M. Yasuda, K. Hayashi, Y. Katoh, I. Shibata, A. Baba, J. Am. Chem. Soc.
1998, 120, 715–721.
[18] S. E. Denmark, T. Wynn, J. Am. Chem. Soc. 2001, 123, 6199–6200.
[19] Y. Minami, K. Nishida, A. Konishi, M. Yasuda, Chem. Asian J. 2020, 15,
1852–1857.
The design concept that employs the atrane-type frame-
work to stabilize a highly reactive cationic metal provides a new
strategy to propose highly reactive species as stable catalysts
with hard and soft Lewis superacidities. Further study expand-
ing the catalytic utility of hard and soft LSAs having a cationic
metal center are ongoing in our group.
[20] M. Yasuda, T. Oh-Hata, I. Shibata, A. Baba, H. Matsuda, J. Chem. Soc.
Perkin Trans. 1 1993, 859–865.
[21] H. F. T. Klare, L. Albers, L. Süsse, S. Keess, T. Müller, M. Oestreich, Chem.
Rev. 2021, 121, 5889–5985.
[22] V. J. Scott, R. Çelenligil-Çetin, O. V. Ozerov, J. Am. Chem. Soc. 2005, 127,
2852–2853.
Acknowledgements
[23] R. Panisch, M. Bolte, T. Müller, J. Am. Chem. Soc. 2006, 128, 9676–9682.
[24] H. Schmidt, S. Keitemeyer, B. Neumann, H.-G. Stammler, W. W. Schoeller,
P. Jutzi, Organometallics 1998, 17, 2149–2151.
This work was supported by JSPS KAKENHI, Grant Numbers
JP20K05464, and by JST CREST, Grant Number JPMJCR20R3,
Japan. We also thank Dr. N. Kanehisa (Osaka University) for
valuable advice regarding X-ray crystallography. We also acknowl-
edge the Analytical Instrumentation Facility, Graduate School of
Engineering, Osaka University.
[25] A. Růžička, R. Jambor, I. Císařová, J. Holeček, Chem. Eur. J. 2003, 9, 2411–
2418.
[26] K.-C. Kim, C. A. Reed, D. W. Elliott, L. J. Mueller, F. Tham, L. Lin, J. B.
Lambert, Science 2002, 297, 825–827.
[27] C. Schenk, C. Drost, A. Schnepf, Dalton Trans. 2009, 2, 773–776.
[28] A. Sekiguchi, T. Fukawa, V. Y. Lee, M. Nakamoto, M. Ichinohe, Angew.
Chem. Int. Ed. 2003, 42, 1143–1145; Angew. Chem. 2003, 115, 1175–
1177.
[29] A. Sekiguchi, T. Fukawa, V. Y. Lee, M. Nakamoto, J. Am. Chem. Soc. 2003,
125, 9250–9251.
[30] J. C. L. Walker, H. F. T. Klare, M. Oestreich, Nat. Chem. Rev. 2020, 4, 54–
62.
Conflict of Interest
[31] J. G. Verkade, Coord. Chem. Rev. 1994, 137, 233–295.
[32] A. Tzschach, K. Jurkschat, Pure Appl. Chem. 1986, 58, 639–646.
[33] A. Kavoosi, E. Fillion, Angew. Chem. Int. Ed. 2015, 54, 5488–5492; Angew.
Chem. 2015, 127, 5578–5582.
The authors declare no conflict of interest.
Keywords: Lewis acid catalyst · hard and soft Lewis superacid ·
group 14 element · cationic metal center · chemoselectivity
[34] E. Fillion, A. Kavoosi, K. Nguyen, C. Ieritano, Chem. Commun. 2016, 52,
12813–12816.
[35] Q. Chen, C. E. Buss, V. G. Young, S. Fox, J. Chem. Crystallogr. 2005, 35,
177–181.
[36] P. N. Billinger, P. P. K. Claire, H. Collins, G. R. Willey, Inorg. Chim. Acta
[1] A. Corma, H. García, Chem. Rev. 2003, 103, 4307–4366.
[2] H. Yamamoto, Ed., Lewis Acids in Organic Synthesis, Wiley, Weinheim,
Germany, 2000.
1988, 149, 63–67.
[37] B. Kašná, R. Jambor, L. Dostál, A. Růžička, I. Císařová, J. Holeček,
Organometallics 2004, 23, 5300–5307.
[38] Z. Xie, R. Bau, C. A. Reed, J. Chem. Soc. Chem. Commun. 1994, 2519.
[39] U. Mayer, V. Gutmann, W. Gerger, Monatsh. Chem. 1975, 106, 1235–
1257.
[3] L. Greb, Chem. A Eur. J. 2018, 24, 17881–17896.
[4] L. O. Müller, D. Himmel, J. Stauffer, G. Steinfeld, J. Slattery, G. Santiso-
Quiñones, V. Brecht, I. Krossing, Angew. Chem. Int. Ed. 2008, 47, 7659–
7663; Angew. Chem. 2008, 120, 7772–7776.
[5] T. E. Mallouk, G. L. Rosenthal, G. Mueller, R. Brusasco, N. Bartlett, Inorg.
Chem. 1984, 23, 3167–3173.
[6] K. O. Christe, D. A. Dixon, D. McLemore, W. W. Wilson, J. A. Sheehy, J. A.
Boatz, J. Fluorine Chem. 2000, 101, 151–153.
[7] H. Böhrer, N. Trapp, D. Himmel, M. Schleep, I. Krossing, Dalton Trans.
2015, 44, 7489–7499.
[40] M. A. Beckett, G. C. Strickland, J. R. Holland, K. Sukumar Varma, Polymer
1996, 37, 4629–4631.
[41] I. B. Sivaev, V. I. Bregadze, Coord. Chem. Rev. 2014, 270–271, 75–88.
[42] M. A. Beckett, D. S. Brassington, S. J. Coles, M. B. Hursthouse, Inorg.
Chem. Commun. 2000, 3, 530–533.
[43] E. L. Myers, C. P. Butts, V. K. Aggarwal, Chem. Commun. 2006, 4434–
4436.
[8] R. Maskey, M. Schädler, C. Legler, L. Greb, Angew. Chem. Int. Ed. 2018,
57, 1717–1720; Angew. Chem. 2018, 130, 1733–1736.
[9] A. L. Liberman-Martin, R. G. Bergman, T. D. Tilley, J. Am. Chem. Soc.
2015, 137, 5328–5331.
[10] D. Hartmann, M. Schädler, L. Greb, Chem. Sci. 2019, 10, 7379–7388.
[11] D. Roth, H. Wadepohl, L. Greb, Angew. Chem. Int. Ed. 2020, 59, 20930–
20934; Angew. Chem. 2020, 132, 21116–21120.
[12] T. Thorwart, D. Roth, L. Greb, Chem. A Eur. J. 2021, 27, 10422–10427.
[13] A. Hermannsdorfer, M. Driess, Angew. Chem. Int. Ed. 2021, 60, 13656–
13660.
[44] H. Großekappenberg, M. Reißmann, M. Schmidtmann, T. Müller, Organo-
metallics 2015, 34, 4952–4958.
[45] R. Vivas-Reyes, A. Aria, Ecletica Quim. 2008, 33, 69–76.
[46] J. B. Lambert, Y. Zhao, H. Wu, J. Org. Chem. 1999, 64, 2729–2736.
[47] K. Hara, R. Akiyama, M. Sawamura, Org. Lett. 2005, 7, 5621–5623.
[48] M. Kira, T. Hino, H. Sakurai, Chem. Lett. 1992, 21, 555–558.
[49] M. Nava, C. A. Reed, Organometallics 2011, 30, 4798–4800.
[50] T. Saitoh, S. Yoshida, J. Ichikawa, J. Org. Chem. 2006, 71, 6414–6419.
[14] A. Konishi, Y. Minami, T. Hosoi, K. Chiba, M. Yasuda, Chem. Eur. J. 2016,
22, 12688–12691.
[15] M. Kira, M. Kobayashi, H. Sakurai, Tetrahedron Lett. 1987, 28, 4081–4084.
[16] M. Braun, Modern Enolate Chemistry: From Preparation to Applications
in Asymmertric Synthesis, Wiley-VCH, 2016.
Manuscript received: July 29, 2021
Revised manuscript received: August 15, 2021
Version of record online: ■■■, ■■■■
Chem Asian J. 2021, 16, 1–7
www.chemasianj.org
6
© 2021 Wiley-VCH GmbH
��
These are not the final page numbers!

Communication
COMMUNICATION
Atrane-type molecules 1E+ with a
cationic group 14 center are synthe-
sized and characterized; all act as
hard and soft Lewis superacids (LSAs)
with high catalytic activity. The
framework stabilizes the localized
positive charge, facilitating highly co-
ordinated states of 1E+. Hard and
soft LSAs have potential roles in
organic synthesis.
D. Tanaka, Dr. A. Konishi*, Prof. Dr. M.
Yasuda*
1 – 7
Synthesis and Catalytic Activity of
Atrane-type Hard and Soft Lewis
Superacids with a Silyl, Germyl, or
Stannyl Cationic Center