+: A Masked Potent Boron Lewis Acid

Article abstract of DOI:10.1021/acs.organomet.9b00671

The chemistry of the boron cation has been revitalized in the past decade due to its newfound application in stoichiometric and catalytic organic reactions. To extend the frontier of boron cation catalysis, we came to discover that a mesityl-substituted η⁵-Cp*-coordinated boron cation could serve as a powerful Lewis acid for organic catalytic transformations. The boron cation [Cp*B-Mes][B(C₆F₅)₄] ([1][B(C₆F₅)₄]) stabilized in a boronium-like electronic structure binds to Et₃PO readily and displays an acceptor number exceeding that of B(C₆F₅)₃ on the Gutmann-Beckett acidity scale. The steric and electronic stabilization exerted by the electron-donating Cp? renders the highly Lewis acidic boron cation an easy-to-handle catalyst for hydrodeoxygenation of aryl ketones at ambient temperature. The exceptional catalytic performance of [1]⁺ implies that the incorporation of a coordinatively flexible substituent at boron is critical in bringing catalytic activity and stability to boron cation catalysts.

Full text of DOI:10.1021/acs.organomet.9b00671

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
[η5‑Cp*B-Mes]+: A Masked Potent Boron Lewis Acid
Hsi-Ching Tseng,†,∥ Chao-Tang Shen,†,∥ Kentaro Matsumoto,‡ Ding-Nan Shih,† Yi-Hung Liu,†
Shie-Ming Peng,† Shigehiro Yamaguchi,‡ Ya-Fan Lin,§ and Ching-Wen Chiu*,†
†Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
‡Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
§Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, 100 Shi-Chuan first Road, Kaohsiung 80708,
Taiwan
S
* Supporting Information
ABSTRACT: The chemistry of the boron cation has been
revitalized in the past decade due to its newfound application in
stoichiometric and catalytic organic reactions. To extend the
frontier of boron cation catalysis, we came to discover that a
mesityl-substituted η5-Cp*-coordinated boron cation could serve
as a powerful Lewis acid for organic catalytic transformations. The
boron cation [Cp*B-Mes][B(C6F5)4] ([1][B(C6F5)4]) stabilized
in a boronium-like electronic structure binds to Et3PO readily and
displays an acceptor number exceeding that of B(C6F5)3 on the
Gutmann−Beckett acidity scale. The steric and electronic
stabilization exerted by the electron-donating Cp* renders the
highly Lewis acidic boron cation an easy-to-handle catalyst for
hydrodeoxygenation of aryl ketones at ambient temperature. The exceptional catalytic performance of [1]+ implies that the
incorporation of a coordinatively flexible substituent at boron is critical in bringing catalytic activity and stability to boron cation
catalysts.
bond cleavage upon reaction and, thus, has never been applied
in catalytic reactions.5 On the opposite side of the spectrum,
the boron center of the tetracoordinate boronium cation
([R2BL2]+) is electronically saturated and undergoes reduc-
tion6 or nucleophilic addition at the coordinating ligands.7
Therefore, the catalytic application of the boron cation is
limited to the tricoordinate borenium cation ([R2BL]+).
In addition to the aforementioned typical boron cations, the
chemistry of a unique class of boron cations featuring a η5-Cp*
substituent has been much less explored.8 With a general
formula of [η5-Cp*BR]+, the boron center is hypercoordinated,
and an octet configuration is achieved at the boron (Chart 1).
With an electronic configuration akin to that of a boronium
cation, [η5-Cp*BR]+ is expected to be highly stable in
comparison to the tricoordinate and dicoordinate boron
cations. However, unlike the prototypical boronium cation
that exhibits ligand-based reactions, the boron center of [η5-
Cp*BR]+ remains Lewis acidic and binds Lewis bases with a
concomitant hapticity change of Cp* from η5 to η1.9 While the
flexible coordination of Cp* has been widely applied in
transition-metal catalysis, the incorporation of Cp* in main-
group-element catalysts has been much less explored.10 Thus,
we decided to investigate the Lewis acidity and catalytic
activity of a Cp*-substituted boron cation. In this study, we
INTRODUCTION
■
The applications of the boron cation in catalytic and
stoichiometric organic transformations have been invigorated
in the past decade.1 Typical boron cations can be classified into
three distinct classes on the basis of the coordination number
at the boron (Chart 1).2 The linear dicoordinate borinium
Chart 1. Classification of Boron Cations
cation with a formula of [R2B]+ possessing two empty p
orbitals is the most electron deficient species in the series and
generally requires at least one π-donating amino substituent to
be isolable.3 With the help of a weakly coordinating anion, a
two-carbon substituted variant ([Mes2B]+) was finally achieved
in 2014.4 The highly Lewis acidic [Mes2B]+ undergoes B−C
Received: October 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00671
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have prepared a Cp*-stabilized boron cation ([1][B(C6F5)4])
and demonstrated that the Lewis acidity of [1]+ could exceed
that of B(C6F5)3 on the Gutmann−Beckett acidity scale.
Finally, the application of [1]+ in catalytic hydrosilylation/
hydrodeoxygenation of ketones was explored as well.
The use of a weakly coordinating anion is critical to prevent
an undesired cation−anion interaction, which is consistent
with the high Lewis acidity of [1]+. Attempts to isolate
[1][OTf] were unsuccessful due to the coordination of triflate
at boron (Scheme 2). The extent of the B-OTf coordination
RESULTS AND DISCUSSION
Scheme 2. Equilibrium between Cp*B(Mes)-OTf and
[1][OTf]
■
The synthesis of [η5-Cp*B-Mes][B(C6F5)4] ([1][B(C6F5)4) is
straightforward (Scheme 1). The reaction of Cp*BCl2 with
Scheme 1. Synthesis and Reactivity of [1][B(C6F5)4]
was found to be dependent on the solvent polarity. In nonpolar
C6D6, [1][OTf] exists as a triflate-coordinated neutral borane,
Cp*B(Mes)-OTf, with a broad 11B NMR resonance detected
at 55 ppm. When the solvent was changed to CD2Cl2, a
[1][OTf]:Cp*B(Mes)-OTf ratio of 2:1 was recognized from
the 1H NMR spectrum of the solution (Figure S1). The
cation−anion interaction could be significantly suppressed in
CD3NO2 as a result of favorable solvation of the ionic complex
in polar solvents (Figure S2).
[1][B(C6F5)4] shows no reaction toward CO2 and diphenyl
acetylene in DCM and is also inert toward H2 (4 atm in
DCM). The η5-Cp*-coordinated boron cation even shows
appreciable tolerance toward air and moisture. Unlike
B(C6F5)3, which cannot be handled in the presence of
moisture, a solid sample of [1][B(C6F5)4] shows no noticeable
decomposition after 24 days of storage outside of a glovebox
(Figures S8 and S9). [1][B(C6F5)4] in the solution state is
more vulnerable toward hydrolysis. Addition of 3 equiv of
water to a CD2Cl2 solution of [1][B(C6F5)4] leads to the
decomposition of [1]+ to yield [Cp*H2][B(C6F5)4]11 and an
unidentified boron fragment (Scheme 1). Surprisingly, while
the complete decomposition of [1]+ with 3 equiv of water
requires 36 h of reaction time (Figure S11), [1]+ forms a 1:1
adduct with Et3PO immediately in DCM (Scheme 1). The
adduct features a broad 31P NMR signal at 97.6 ppm, which
equals an acceptor number of 104.5 on the Gutmann−Beckett
acidity scale.12 Although the measured acidity of [1]+ is
inferior to that of chelate-restrained borinium cation (31P
NMR δ 106.9 ppm),13 the observed 31P chemical shift is
shifted notably downfield in comparison to that of B(C6F5)3
(31P NMR δ 78.1 ppm) and aryl-substituted silyliums (31P
NMR δ 85.4−91.3 ppm),14 confirming the strong Lewis acidity
of [1]+. The η5 to η1 hepticity change upon base coordination
was further corroborated by a structural characterization of [1-
OPEt3][B(C6F5)4] (Figure 1b).
To address the stabilizing effect of Cp*, the energy
difference between [η5-Cp*B-Mes]+ and [η2-Cp*B-Mes]+
was calculated by the DFT method at the M06-2x/6-311g**
level of theory using a PCM solvent model in DCM (Figure
S12). All attempts to locate the stationary point of the η1-Cp*
isomer were unsuccessful, potentially due to the shallow energy
barrier to the more stable [η2-Cp*B-Mes]+ structure. While the
η2-coordination mode of Cp* is rare for transition metals, such
a geometry has been identified in Cp* complexes of boron and
silicon.8k,15 As indicated by the DFT calculation, the η5-
coordinated boron cation is considerably more stable than its
η2 counterpart by 31.9 kcal/mol, suggesting that the η5−η2
excess mesityllithium in toluene yields the corresponding
chloroborane (Cp*(Mes)BCl), which is then subject to a
metathesis reaction with Li[B(C6F5)4] in dichloromethane
(DCM) to afford [1][B(C6F5)4] in 50% overall yield. The
detection of the characteristic 11B NMR signal at −42 ppm
suggests the formation of the η5-Cp*B structure in the
solution. Single-crystal X-ray diffraction analysis on a colorless
crystal of [1][B(C6F5)4] confirmed the molecular connectivity
of the boron cation (Figure 1a).
Figure 1. Molecular structures of [1]+ (a) and [1-OPEt3]+ (b).
Hydrogen atoms and counteranions are omitted for clarity. Thermal
ellipsoids are set at 50% probability.
The observed average B−C(Cp*) bond distance of 178.3
pm is noticeably longer than that of [η5-Cp*B−Br]+ (average
168.3 pm)8d as a result of steric repulsion between the mesityl
and Cp* groups. In addition, the B−Cipso(mesityl) bond of
155.9(3) pm is also markedly longer than that determined for
[Mes2B]+ (average 145.9 pm),4 indicating the presence of a
weaker p−π interaction of the B−Cipso(mesityl) bond in [1]+
due to the coordination of Cp*.
B
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Scheme 3. Et3SiH Reduction of 2b Catalyzed by [1]+ or in
Situ Generated [Et3Si]+
hepticity change would be difficult to achieve with thermal
activation. Fortunately, the [C5B] cage can be readily opened
by introducing an appropriate Lewis base. The complexation of
[η5-Cp*B-Mes]+ and Et3PO proceeds 14.4 kcal/mol downhill
in energy, which is consistent with the facile formation of [1-
OPEt3]+ upon mixing [1]+ and Et3PO in CD2Cl2.
To test the catalytic application of [1]+, we have examined
the hydrosilylation of benzophenone (2a) with Et3SiH in
DCM. Surprisingly, the hydrodeoxygenation product, diphe-
nylmethane (3a), was obtained instead of the expected silyl
ether (4a). Although catalytic hydrodeoxygenation of carbonyl
compounds can be accomplished by main-group Lewis acids,
such as silylium cations,16 fluorophosphonium cations,17 and
B(C6F5)3,18 such a reaction has never been reported for boron
cations. With a loading of 1 mol %, [1][B(C6F5)4] is able to
convert 2a to 3a nearly quantitatively with 2.5 equiv of Et3SiH
in less than 10 min at room temperature (Table 1). Under the
from Et3SiH was further verified by NMR studies, in which no
detectable Cp*(Mes)B−H was observed in the 1:1 mixture of
[1][B(C6F5)4] and Et3SiH in CD2Cl2.
To test the catalytic application of [1][B(C6F5)4] in ketone
reduction, a series of aromatic and aliphatic ketones were
examined. In general, 1 mol % of [1][B(C6F5)4] was added
into a DCM mixture of ketone and 2.5 equiv of Et3SiH at room
Table 1. Catalytic Reduction of 2a with Et3SiH
1
temperature, and the reaction was monitored with H NMR
spectroscopy. As shown in Table 2, [1]+ is capable of
Table 2. Catalytic Reduction of Ketones with Et3SiH
amt of silane
(equiv)
product
(yield (%))
a
catalyst (amt (equiv))
time
b
[1]+ (0.01)
1.1
2.5
2.5
2.5
2.5
2.5
<10 min
<10 min
<10 min
16 h
16 h
<10 min
3a (50)
[1]+ (0.01)
3a (>99)
4a (>99)
no reaction
3a (4)
B(C6F5)3 (0.01)
[9BBN-IPr]+ (0.02)
HOTf (0.1)
HOTf (0.1) + [1]+
(0.01)
a
e
substrate
2a (R1 = R2 = Ph)
3:4
isolated
98:0
0:99
91:0
99:0
0:99
98:0
0:0
99:0
99:0
0:99
0:0
3a (44)
4b (93)
3c (69)
3d (54)
4e (98)
3f (53)
3a (>99)
2b (R1 = C6F5, R2 = CH3)
2c (R1 = R2 = 4-F-C6H4)
2d (R1 = Ph, R2 = C6F5)
a
1
The yield was determined by H NMR with naphthalene as the
b
b
internal standard. 50% of 2a remained.
2e (R1 = R2 = C6F5)
c
2f (R1 = Ph, R2 = 4-MeO-C6H4)
2g (R1 = Ph, R2 = 4-Me2N-C6H4)
2h (R1 = Me, R2 = Ph)
2i (R1 = Me, R2 = 4-Br-C6H4)
2j (R1 = Me, R2 = 4-NO2-C6H4)
2k (R1 = Me, R2 = 4-NH2-C6H4)
2l (R1 = CF3, R2 = Ph)
2m (R1 = Me, R2 = t-Bu)
2n (R1 = R2 = Et)
same conditions, silyl ether 4a was obtained when B(C6F5)3 (1
mol %) was utilized, and no reaction was observed with 2 mol
% of the N-heterocyclic carbene dialkylborenium cation [9-
BBN-IPr][B(C6F5)4].19 In addition, lowering the silane
content does not affect the product selectivity. Since [1]+
undergoes hydrolysis to yield [Cp*H2]+,11 a strong Brønsted-
Lowry acid that could potentially be the active catalyst of the
reaction, we also examined the reaction in the presence of 10
mol % of triflic acid. The detection of 3a in low yield suggests
that triflic acid is not capable of catalyzing the hydro-
deoxygenation of 2a. Addition of 1 mol % of [1][B(C6F5)4]
to the HOTf-containing mixture transformed the remaining 2a
into 3a immediately, confirming that the observed reaction is
catalyzed by [1]+.
f
3h (−)
3i (47)
4j (78)
d
0:98
0:93
0:97
4l (92)
4m (67)
4n (93)
a
1
The yield was determined by H NMR with naphthalene as the
b
c
d
internal standard. 24 h of reaction time. 3 mol % of catalyst. 18 h
of reaction time. The isolated yield of 3 is considerably lower than
the corresponding NMR yield due to the difficulty in separating 3 and
(Et3Si)2O. Yields are given in parentheses in percent. 3h is volatile
e
f
and cannot be isolated from the reaction mixture.
To examine the possibility that the observed hydro-
deoxygenation of 2a could be catalyzed by the in situ
generated [Et3Si]+, we carried out the hydrosilylation of the
deactivated acetophenone 2b (Scheme 3). In the presence of 1
mol % of [1]+ and 2.5 equiv of Et3SiH, silyl ether 4b was
obtained in virtually quantitative yield. However, when the
reaction was carried out with 1 mol % of [Ph3C][B(C6F5)4], a
completely different set of products was obtained. Instead of
4b, ethylpentafluorobenzene (3b) and pentafluorostyrene (5)
were obtained in about a 5:1 ratio (Figure S22). The observed
stark contrast in product selectivity ruled out silylium as the
active catalyst for the observed hydrosilylation/hydrodeoxyge-
nation of ketones. The inability of [1]+ to abstract hydride
catalyzing the deoxygenation of benzophenone and acetophe-
none derivatives in high yield with excellent selectivity. For
highly deactivated aryl ketones (2b,e,j) and aliphatic ketones
(2m,n), silyl ether derivatives (4) were obtained. However, no
catalytic transformation of the amino-containing derivatives
(2g,k) could be accomplished, probably due to the
coordination of the carbonyl group at the boron center that
quenches the reactivity of the boron cation. To demonstrate
the potential of [1][B(C6F5)4] serving as an easy-to-handle
Lewis acid catalyst, we have also examined the hydro-
deoxygenation of 2a under hydrous conditions. In the presence
of saturated water content (0.2% w/w in DCM at 30 °C), a
C
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General Procedure of [1][B(C6F5)4]-Catalyzed Ketone Re-
duction with Et3SiH. Solid catalyst (1 mol %) was added to a DCM
solution that contained 1 equiv of the substrate and 2.5 equiv of
HSiEt3. For NMR-scale reactions, the yield was determined by
addition of 50 μL of a 0.8 M naphthalene CD2Cl2 solution. Two
workup procedures were developed for product isolation. For the
deoxygenation reaction, the product was separated from (Et3Si)2O by
hexane/acetonitrile extraction. The isolated yield is significantly lower
than the corresponding NMR yield due to the high solubility of the
product in hexane. The isolation of silyl ether product can be readily
accomplished by removal of excess Et3SiH in vacuo, followed by
extraction with pentane. The detailed isolation procedure and
characterization of product can be found in the Supporting
Information.
maximum conversion of 30% was obtained with 1 mol % of
[1][B(C6F5)4] after 48 h of reaction at room temperature.
However, the reaction could be significantly accelerated by
raising the reaction temperature to 40 °C. Under heated
conditions, reduction of 2a,c,d,h,i to the corresponding alkane
derivatives was accomplished in 1 h in hydrous DCM (Table
S3), showing that [1][B(C6F5)4] can serve as a robust potent
Lewis acid for organic synthesis.
CONCLUSIONS
■
In summary, a boron cation featuring an η5-coordinated Cp*
and an apical mesityl group was prepared and isolated, and its
application in catalytic ketone reduction was examined. The
[Cp*B-R]+ cation may be viewed as a masked strong Lewis
acid, which displays boronium-like stability and borinium-like
acidity. Thus, the reported boron cation can be a convenient
Lewis acid for organic synthesis. The cation can be stored
under ambient conditions and also catalyzes deoxygenation of
aryl ketones in a wet solvent. Since the apical substituent of
[Cp*B-R]+ can be readily replaced, boron cation catalysts
having diverse stereoelectronic properties are expected.
Currently, we are exploring the catalytic activity of [1][B-
(C6F5)4] in Lewis acid catalyzed silylation and borylation. The
incorporation of different substituents at the apical position of
[Cp*B-R]+ to tune the reactivity and selectivity of the catalyst
is also under investigation.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00671.
Synthesis Cp*B(Mes)-OTf, hydrolysis of [1][B-
(C6F5)4], details on [1][B(C6F5)4]-catalyzed ketone
reduction and characterization of products, and NMR
spectra (PDF)
Optimized geometries of [η5-Cp*B-Mes]+, [η2-Cp*B-
Mes]+, and [η1-Cp*B(Mes)-OPEt3]+ (XYZ)
Accession Codes
CCDC 1880820 and 1947146 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
EXPERIMENTAL SECTION
■
General Procedure. All experiments were carried out under an
atmosphere of nitrogen using standard Schlenk techniques or inside a
glovebox. Diethyl ether, toluene, and dichloromethane (DCM) were
dried with a molecular sieve loaded solvent purification system.
CD2Cl2 was dried over P2O5 and distilled under reduced pressure.
Liquid ketones were dried over anhydrous CaSO4 and distilled under
reduced pressure. NMR spectra were recorded on a Bruker AVIII-400
FT-NMR spectrometer (1H, 400.2 MHz; 11B, 128.4 MHz; 13C, 100.6
MHz; 19F, 377 MHz). Chemical shifts (δ) are given in ppm and are
referenced to the signals of the residual solvent (1H, 13C) or external
BF3·OEt2 (11B) and fluorobenzene (19F). Elemental analyses were
performed on a Heraeus varioIII-NCH elemental analyzer.
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.-W.C.: cwchiu@ntu.edu.tw.
■
ORCID
Shigehiro Yamaguchi: 0000-0003-0072-8969
Ching-Wen Chiu: 0000-0001-7201-0943
Synthesis of [1][B(C6F5)4]. A solution of Cp*BCl2 (1.00 g, 4.61
mmol) in toluene (20 mL) was added slowly to a toluene suspension
of MesLi (757 mg, 6.00 mmol) at room temperature and the mixture
stirred for 3 h. Afterward, excess MesLi and LiCl were filtered off
under an N2 atmosphere. All volatiles in the filtrate were removed
under vacuum to yield Cp*(Mes)BCl as a pale yellow oil, which was
used without further purification. The resulting Cp*(Mes)BCl was
then mixed with Li[B(C6F5)4] (4.52 g, 4.61 mmol) in 20 mL of
DCM. After removal of LiCl by filtration, the filtrate was dried under
vacuum to yield [1][B(C6F5)4] as a purple powder, which was washed
with diethyl ether and recrystallized from DCM/hexane to givea
colorless microcrystalline powder (2.29 g, 2.30 mmol). Yield: 50%
(calculated on the basis of Cp*BCl2). 1H NMR (400.2 MHz, CD2Cl2,
298 K): δ 6.84 (s, 2 H, Mes), 2.31 (s, 15 H, Cp*), 2.22 (s, 3 H, para-
Me of Mes), 2.17 ppm (s, 6 H, ortho-Me of Mes). 13C{1H} NMR
Author Contributions
∥H.-C.T. and C.-T.S. contributed equally and share the first
authorship.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology of Taiwan (Grant MOST 104-2113-M-002-018-
MY3 and 105-2119-M-002-031-MY2) and National Taiwan
University (NTU 107L880104). We are also grateful for JSPS
KAKENHI grants JP15H02163 and JP18H05261.
1
(100.6 MHz, CD2Cl2, 298 K): δ 148.5 (d, JC−F = 241.0 Hz, C6F5),
1
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