Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane System frameworks bearing multiple tuning factors: Geometric and substituent effects on their lewis acid properties

Article abstract of DOI:10.1002/chem.201002789

Boron complexes that contain new tridentate ligands, tris(o-oxyaryl) methanes and -silanes, were prepared. These complexes had a cage-shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open-shaped boron compounds. The cage-shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L·B(OC₆H₄)₃CH, L·B(OC ₆H₄)₃SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage-shaped borates had a large dihedral angle (C_ipso-O-B-O) compared with open-shaped borates. The geometric effect due to the dihedral angle means that compared with open-shaped, the cage-shaped borates have a greater Lewis acidity. The introduction of electron-withdrawing groups on the aryl moieties in the cage-shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C _ipso-O-B-O. The ligand-exchange rates of the para-fluoro-substituted compound B(OC₆H₃F)₃CH and the ortho-phenyl-substituted compound B(OC₆H₃Ph)₃CH were less than that of the unsubstituted borate B(OC₆H ₄)₃CH. The ligand-exchange rate of B(OC₆H ₄)₃SiMe was much faster than that of B(OC ₆H₄)₃CH. A hetero Diels-Alder reaction and Mukaiyama-type aldol reactions were more effectively catalyzed by cage-shaped borates than by the open-shaped borate B(OPh)₃ or by the strong Lewis acid BF₃·OEt₂. The cage-shaped borates with the bulky substituents at the ortho-positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.

Full text of DOI:10.1002/chem.201002789

DOI: 10.1002/chem.201002789
Cage-Shaped Borate Esters with Tris(2-oxyphenyl)methane or -silane System
Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent
Effects on Their Lewis Acid Properties
Makoto Yasuda,*[a, b] Hideto Nakajima,[a] Ryosuke Takeda,[a] Sachiko Yoshioka,[a]
Satoshi Yamasaki,[a] Kouji Chiba,[c] and Akio Baba*[a]
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Chem. Eur. J. 2011, 17, 3856 – 3867

FULL PAPER
Abstract: Boron complexes that con-
tain new tridentate ligands, tris(o-oxy-
aryl)methanes and -silanes, were pre-
pared. These complexes had a cage-
shaped structure around a boron center
and showed higher Lewis acidity and
catalytic activity than open-shaped
boron compounds. The cage-shaped lig-
rates. The geometric effect due to the
dihedral angle means that compared
with open-shaped, the cage-shaped bo-
rates have a greater Lewis acidity. The
introduction of electron-withdrawing
groups on the aryl moieties in the
cage-shaped framework increased the
Lewis acidity. Substitution of a bridge-
head Si for a bridgehead C decreased
the Lewis acidity of the boron com-
plexes because the large silicon atom
reduces the dihedral angle of Cipso-O-
B-O. The ligand-exchange rates of the
para-fluoro-substituted compound B-
ACHTUNGTRENNUNG
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
B
G
CHTUNGTRENNUNG
BACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
action and Mukaiyama-type aldol reac-
tions were more effectively catalyzed
by cage-shaped borates than by the
ACHTUNGTRENNUNGands determined the properties of the
borates by altering the geometry and
were consistently bound to the metal
center by chelation. The synthesized
open-shaped borate BACHTUNRGTNEUNG(OPh)3 or by the
strong Lewis acid BF3·OEt2. The cage-
shaped borates with the bulky substitu-
ents at the ortho-positions selectively
catalyzed the reaction with less sterical-
ly hindered substrates, while the unsub-
stituted borate showed no selectivity.
compounds were L·B
ACHTNUGRTNE(NUGN OC6H4)3CH,
L·B(OC6H4)3SiMe, and its derivatives
ACHTUNGTRENNUNG
(L=THF or pyridine as an external
ligand). Theoretical calculations sug-
gested that the cage-shaped borates
had a large dihedral angle (Cipso-O-B-
O) compared with open-shaped bo-
Keywords: aldol reaction · borates ·
Diels–Alder reaction · homogene-
ous catalysis · Lewis acids
Introduction
Group 13 elements are very important to Lewis acid chemis-
try.[1] Among them, boron-containing compounds have been
widely studied as typical Lewis acids, and have been applied
to various types of organic transformations.[2] One of the
most commonly available compounds is the boron trihalide
BX3 A, as shown in Scheme 1a. In this compound, a Lewis
basic substrate (Sub) interacts with a strong Lewis acid A to
form the adduct B. The activated substrate may react with
an external reagent to give the product complex C that in-
ꢀ
cludes a B Pro bond (Pro=product). Usually, the metal
ꢀ
center releases the original ligand X due to a stronger B
Pro bond.[3] In this pathway, an equimolar amount of the
Lewis acid A is consumed to complete the reaction. Conse-
quently, high Lewis acidity and high catalytic turnovers are
[a] Dr. M. Yasuda, H. Nakajima, R. Takeda, S. Yoshioka, S. Yamasaki,
Prof. Dr. A. Baba
Department of Applied Chemistry
Graduate School of Engineering
Osaka University, 2–1 Yamadaoka, Suita
Osaka 565-0871 (Japan)
Scheme 1. Working hypothesis for conceptually new Lewis acid.
Fax : (+81)6-6879-7387
E-mail: yasuda@chem.eng.osaka-u.ac.jp
baba@chem.eng.osaka-u.ac.jp
incompatible. In contrast, the borate ester B(OR)3 A’ has
low Lewis acidity due to the overlap between lone pairs on
oxygen atoms with
a
vacant
p
orbital on boron
[b] Dr. M. Yasuda
(Scheme 1b).[4,5] Unfortunately, the Lewis acidity of the
borate ester B(OR)3 A’ is too low to react with the sub-
strate. To overcome the problems associated with A and A’,
we have designed a new type of Lewis acid, A’’, with a cage-
shaped ligand system, as shown in Scheme 1c. We used the
“cage” to generate the appropriate Lewis acidity, as confor-
mational changes in the frame of the cage could be used to
activate the substrate and to keep the original oxy ligands
Center for Atomic and Molecular Technologies
Graduate School of Engineering, Osaka University
2–1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
[c] K. Chiba
Science and Technology Division
Ryoka Systems Inc., 1-28-38 Shinkawa
Chuo-ku, Tokyo 104-0033 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002789.
Chem. Eur. J. 2011, 17, 3856 – 3867
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3857

M. Yasuda, A. Baba et al.
on boron due to the chelating effect of the cage frame.
Therefore, if designed appropriately, both high Lewis acidity
and high catalytic turnovers would be compatible in the new
type of Lewis acid A’’, if it has labile boron–ligand bonds.
Various types of metal complexes can be controlled by
changing the geometry around the metal using special lig-
ands,[6] but this approach often causes relatively large altera-
tions in the metal complex properties, which interfere with
its catalytic activity. The proposed cage-shaped borate A’’
could be a suitable Lewis acid catalyst and finely tuned by
many factors, such as the size of the cage or steric and elec-
tronic effects, based on the ligand design.
In this paper, we report on borate compounds with cage-
shaped frameworks and their application to the catalysis of
organic transformations.[7,8] We synthesized new borate com-
pounds 1B with a triaryloxy ligand 1 that combined with
bridgehead atom X, as shown in Scheme 2. The properties
of these compounds were effectively controlled by varying
R and X; that is, R provided substituent control due to elec-
tronic and steric factors,[9] while the bridgehead atom X al-
lowed control of the geometric control.
Results and Discussion
Synthesis of various ligands of tris(o-oxyphenyl)methanes or
-silanes 1H3: Based on the novel concept presented in
Scheme 1c, we prepared borate 1B as a structurally strained
Lewis acid. Reportedly, organic components that include
preorganized phenoxy moieties are effective ligand systems
for metal complexes.[10,11] Scheme 3 shows the synthetic
routes to ligands 1a–hH3 that were used for the formation
of the various compounds 1B.[12] Tris(2-hydroxyphenyl)
AHCTUNGTERGaNNUN ne (1aH3) was synthesized as follows. ortho-Lithiation of
ACHTUNGTRENNUNGmeth-
anisole (2a) followed by treatment with ethyl chloroformate
gave the triarylmethanol 3a. The treatment of 3a with p-tol-
uenesulfonic acid in THF/MeCN directly gave the reduced
compound 4a. The in situ-generated carbenium cation,
which was stabilized by electron-donating groups,[13] was re-
duced by THF probably by means of either an ionic or a
single-electron transfer (SET) mechanism.[14] The desired
compound 1aH3 was obtained after deprotection of 4a by
BBr3. Bromination of 1aH3 in AcOH/CCl4 gave the o- and
p-brominated compound 1bH3. Bromination of 4a afforded
p-brominated 4c, which was deprotected by BBr3 to give
1cH3.[15] The fluorinated compound 1dH3 was prepared
from 2-bromo-4-fluoroanisole (2d) in a manner similar to
that used to prepare 1aH3. The phenyl- and naphthyl-substi-
tuted compounds 1eH3 and 1 fH3 were prepared from the
substituted anisole derivatives 2e and 2 f, respectively.
For synthesis of the silane derivatives 1gH3 and 1hH3, a
different protecting group was required. Although we ob-
tained (o-MeOC6H4)3SiMe by the reaction of o-lithioanisole
with MeSiCl3, deprotection with BBr3 failed and gave an un-
Scheme 2. Cage-shaped borate esters 1B with triaryloxy ligand 1.
ꢀ
desired product due to weak Si aryl bonds. Among the pro-
tecting groups examined,
a
dimethylcarbamoyl group
Scheme 3. Synthesis of triphenolic methanes and silanes 1H3.
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Chem. Eur. J. 2011, 17, 3856 – 3867

Borate Esters
FULL PAPER
worked very well for 1gH3, as shown in Scheme 3.[16] Protec-
tion of 2-bromophenol with dimethylcarbamoyl chloride and
its subsequent lithiation followed by treatment with MeSiCl3
gave the triarylmethylsilane 4g. Deprotection of 4g by
LiAlH4 effectively afforded 1gH3. In a similar manner, the
isobutyl derivative 1hH3 was obtained.
Table 1. NMR chemical shifts of the cage-shaped borates.
Ar3CH
Ar3SiC
d(1H)
d(1H)
dG
(13C)
d
N
d
E
dACHTUNGTRENNUNG
(11B)
1aH3
1bH3
1cH3
1dH3
1eH3
1 fH3
6.07
6.31
6.03
6.06
6.46
6.51
38.4
39.7
37.9
38.1
38.3
38.8
Generation of cage-shaped borates: The treatment of 1H3
with BH3·THF readily generated the cage-shaped borates
1B·THF (Scheme 4). These compounds were thermally
1gH3
1hH3
ꢀ18.2
ꢀ19.8
0.95
1.50
ꢀ3.0
22.2
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
1hB·THF
1aB·Py
1bB·Py
1cB·Py
1dB·Py
1eB·Py
1 fB·Py
1gB·Py
1hB·Py
5.13
4.94
4.88
4.88
5.37
5.51
57.4
56.5
55.8
56.6
58.2
58.2
5.52
4.32
5.13
5.27
5.04
4.40
7.11
5.87
4.45
4.12
4.06
4.19
4.46
3.54
4.32
3.99
16.52
ꢀ21.3
ꢀ24.4
1.06
1.73
ꢀ6.7
18.8
5.19
5.01
4.95
4.95
5.44
5.58
57.9
57.0
56.3
57.1
58.8
58.8
ꢀ21.1
ꢀ24.0
1.08
1.77
ꢀ6.4
20.8
BACTHNUTRGEN(UNG OPh)3
the methine hydrogen was confirmed relative to that of
1aH3 (6.07!5.13 ppm). This shift has been observed for
ACHTUNGTRENNUNG
similar cage-shaped compounds.[17] The chemical shift d(13C)
of the methine carbon of 1aB·THF was observed at
57.4 ppm (38.4 ppm for 1aH3).[17] The ligated THF showed
broadening and downfield-shifted signals at d(1H)=4.46 and
2.13 ppm (free THF: d(1H)=3.73 and 1.84 ppm). The boron
NMR signals appeared at d
ACHTUNGTRENNUNG
while the open-shaped borate
BACHTUNGTRENNUNG
16.52 ppm.[18] Similar NMR chemical shifts were observed
for the cage-shaped pyridine complex 1aB·Py. The chemical
shifts for the methine moiety were d(1H)=5.19 ppm, d-
Scheme 4. Generation of cage-shaped borates 1B·L with various substitu-
ents.
ACHUTNGRENNUG CAHTUNGTRENNUGN
(13C)=57.9 ppm, and d(11B)=4.45 ppm. The ligated pyridine
stable, but decomposed in air (O2 and/or water), and, thus,
purification and recrystallization were performed in a nitro-
gen-filled glove box and NMR spectroscopy measurements
were performed under nitrogen. The THF-free 1B was not
observed under these conditions, which suggests that cage-
shaped borate had a higher Lewis acidity than that of planar
showed downfield-shifted signals around d(1H)=9.24, 8.20,
and 7.78 ppm (free pyridine: d(1H)=8.61, 7.61, and
7.28 ppm). Other cage-shaped complexes 1b–fB·L showed
analogous chemical shifts for their 1H, 13C, and 11B NMR
spectra, as shown in Table 1. The order of the downfield
shifts Dd(1H) of the ligated THF on 1a–dB as compared to
free THF was as follows: 1bB·THF (4.85 and 2.35 ppm)>
1cB·THF (4.53 and 2.28 ppm)>1dB·THF (4.52 and
2.26 ppm)>1aB·THF (4.46 and 2.13 ppm). The sharper sig-
nals of the ligated THF were observed in the same order.[19]
These results were probably due to the Lewis acidity of the
cage-shaped borates. The number and type of halogen
atoms precisely controlled the Lewis acidity; the magnitude
of the effect on the enhancement of Lewis acidity was dibro-
mo>monobromo>monofluoro>unsubstituted compounds
on one phenyl group in the cage-shaped borates.[20] For the
ortho-phenyl substituted compound 1eB·THF, the ligand
borates, such as BACHTUNGTRENNUNG(OPh)3. The generated cage-shaped bo-
rates 1a–hB·L with various substituents and bridgehead
atoms are shown in Scheme 4. The pyridine complexes
1B·Py were formed by addition of pyridine to the generated
1B·THF, and were thoroughly analyzed by X-ray crystallog-
raphy (described later).
NMR data for cage-shaped borates 1B·L: Selected NMR
signals for the cage-shaped borates 1B·L and their ligands
1H3 are shown in Table 1. The NMR data of the generated
THF-ligated cage-shaped borates (1B·THF) showed charac-
teristic signals. For 1aB·THF, the significant upfield shift of
THF showed
a broadening and upfield-shifted signals
Chem. Eur. J. 2011, 17, 3856 – 3867
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3859

M. Yasuda, A. Baba et al.
around d(1H)=3.18 and 1.24 ppm. These results indicate
that the ligated THF was surrounded by ortho-substituted
phenyl rings and was affected by an anisotropic effect. Simi-
lar changes in the NMR chemical shifts were observed for
the cage-shaped pyridine complex 1eB·Py. The ligated pyri-
dine showed upfield-shifted signals at d(1H)=7.77, 7.67 and
6.84 ppm. The ortho-naphthyl-substituted, cage-shaped
borate 1 fB·L exists as a mixture of conformational isomers
owing to bulky substituents, and showed large upfield shifts
for ligated THF (1.56 and 0.03 ppm) and pyridine (6.53 and
5.84 ppm; 6.20 and 5.49 ppm for 2- and 3-H, respectively).[21]
For the silicon-bridging compound 1gB·THF, the NMR data
showed a characteristic shift for the Me group on Si. A
downfield shift of d(1H) of the Me group (0.95!1.06 ppm)
and an upfield shift of dACHTUGNTRENNNUG
firmed relative to those of the ligand 1gH3. The 29Si NMR
spectrum showed an upfield chemical shift (ꢀ18.2!
ꢀ21.3 ppm) relative to 1gH3. The ligated THF signals ap-
peared in lower fields with a broadening (d(1H)=3.91 and
2.14 ppm) relative to those of free THF. The pyridine com-
plex 1gB·Py showed similar spectral changes with down-
field-shifted pyridine signals (d(1H)=9.26, 8.16, and
7.75 ppm). Similar spectral data were obtained for the isobu-
tyl derivatives 1hB·THF and 1hB·Py.
X-ray crystallographic analysis of cage-shaped borates: The
pyridine complexes of cage-shaped borates 1a–hB·Py pro-
duced crystals of sufficient quality to be analyzed as single-
crystal structures.[22] Selected crystal data and structural re-
finement parameters are shown in Table 2. The ORTEP
drawings are shown in Figure 1. The selected bond lengths,
angles, and tetrahedral character (THC)[23] of the boron
atom are shown in Table 3. For 1aB·Py, boron has a distort-
ed tetrahedral coordination sphere with average bond
angles of O-B-O, 114.28 and N-B-O, 104.28. This compound
is the first example of a triphenolic methane-based mononu-
clear complex that acts as a Lewis acid.[24,25] The top view of
1aB·Py (Figure 2) clearly shows a nearly C3-symmetric pro-
peller shape. The aromatic rings deviate from a plane per-
pendicular to that of the three oxygen atoms (B-Cbridge-C-
C=19.28), and, thus, the complex has a chirality that is
Figure 1. ORTEP drawings of cage-shaped borates 1B·Py (some hydro-
gen atoms are omitted for clarity).
Table 2. X-ray data for all crystallographically characterized complexes.
1aB·Py
1bB·Py
1cB·Py
1dB·Py
1eB·Py
1 fB·Py
1gB·Py
1hB·Py
formula
Mr
space group
C24H18BNO3 C24H12BBr6NO3 C24H15BBr3NO3 C24H15BF3NO3 C42H30BNO3 C54H36BNO3 C24H20BNO3Si C27H26BNO3Si
379.22
Pbca
0.087
14.8638(6)
15.4624(6)
16.3768(6)
–
852.60
Pbca
615.91
P21/n
5.436
10.4648(10)
10.6734(12)
20.055(2)
–
433.19
P1
0.116
607.51
P1
0.082
757.69
P212121
0.619
13.2269(2)
13.3959(3)
21.9181(12)
–
409.32
P21/n
0.142
9.19210
15.3839(2)
14.4803(4)
–
451.40
Pbca
¯
¯
m
N
]
9.290
9.2617(3)
22.1780(6)
25.3996(7)
–
0.126
9.1369(3)
20.4629(5)
25.7290(6)
–
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
g [8]
V [ꢁ3]
Z
8.5360(4)
10.5824(4)
12.5929(5)
64.1478(12)
71.7351(13)
83.6755(13)
971.65(7)
2
10.0002(10)
10.2302(9)
16.9575(16)
100.935(3)
106.528(3)
107.048(3)
1517.8(2)
2
–
–
–
–
92.540(3)
–
–
–
98.4086(15)
–
–
–
3763.9(2)
8
5217.2(3)
8
2237.8(4)
4
3883.59(12)
4
2025.65(6)
4
4810.5(2)
8
R1
wR2
0.0677
0.1602
0.0828
0.0859
0.0414
0.0988
0.0402
0.0636
0.0335
0.0689
0.0732
0.1177
0.0803
0.0906
0.0462
0.0553
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Chem. Eur. J. 2011, 17, 3856 – 3867

Borate Esters
FULL PAPER
Table 3. Selected bond lengths and angles [8], and THCs for the cage-shaped borates.
1aB·Py
1bB·Py
1cB·Py
1dB·Py
1eB·Py
1 fB·Py
1gB·Py
1hB·Py
ꢀ
bond lengths [ꢁ]
B O
ꢀ
B O
1.432(4)
1.440(5)
1.457(5)
1.443
1.628(5)
2.979(5)
–
1.392(12)
1.450(11)
1.451(11)
1.431
1.619(13)
2.992(13)
–
0.950(8)
–
1.477(12)
1.503(11)
1.514(11)
1.498
1.439(7)
1.443(8)
1.452(8)
1.445
1.611(9)
3.025(9)
–
1.436(2)
1.452(2)
1.453(2)
1.447
1.626(2)
3.010(2)
–
1.417(5)
1.441(5)
1.462(6)
1.440
1.647(6)
2.984(7)
–
1.438(7)
1.440(8)
1.456(8)
1.445
1.631(8)
2.990(8)
–
1.426(5)
1.437(5)
1.445(6)
1.436
1.655(5)
–
3.158(5)
–
1.857(6)
–
1.432(5)
1.437(4)
1.447(4)
1.439
1.645(5)
–
3.159(4)
–
1.879(3)
–
ꢀ
B O
average
ꢀ
B N
ꢀ
B Cbridge
ꢀ
B Sibridge
ꢀ
Ar3C H
0.95(3)
–
0.951(5)
–
0.898(1)
–
0.950(4)
–
0.950(5)
–
ꢀ
Ar3Si C
ꢀ
Ar C
1.515(4)
1.517(5)
1.525(4)
1.519
–
–
–
–
1.521(8)
1.526(8)
1.529(8)
1.525
–
–
–
–
1.530(2)
1.530(3)
1.531(2)
1.530
–
–
–
–
1.528(4)
1.536(6)
1.546(5)
1.537
–
–
–
–
1.508(8)
1.535(7)
1.538(7)
1.527
–
–
–
–
ꢀ
Ar C
–
–
–
–
–
–
ꢀ
Ar C
average
ꢀ
Ar Si
–
–
–
–
1.864(3)
1.872(4)
1.874(3)
1.870
1.873(3)
1.882(3)
1.882(3)
1.879
ꢀ
Ar Si
ꢀ
Ar Si
average
bond angles [8]
torsion angles [8]
THC [%]
O-B-O
O-B-O
O-B-O
total
N-B-O
N-B-O
N-B-O
total
113.4(3)
113.7(3)
115.6(3)
342.7
102.3(3)
104.4(3)
105.9(3)
312.6
113.2(7)
114.2(7)
114.3(7)
341.7
103.4(7)
104.5(7)
105.7(7)
313.6
112.2(5)
113.8(5)
114.0(5)
340.0
104.4(5)
105.3(5)
106.1(5)
315.8
112.3(1)
113.1(1)
115.1(1)
340.5
104.2(1)
105.1(1)
106.0(1)
315.3
112.1(4)
115.0(4)
115.7(4)
342.8
103.0(4)
103.9(4)
105.3(4)
312.2
113.0(5)
113.2(5)
115.0(5)
341.2
104.2(4)
104.7(4)
105.4(4)
314.3
114.2(3)
114.4(3)
114.5(3)
343.1
102.7(3)
104.1(3)
105.0(3)
311.8
113.6(3)
114.4(3)
114.5(3)
342.5
102.7(3)
104.6(3)
105.3(3)
312.6
B-Cbridge-C-C
B-Cbridge-C-C
B-Cbridge-C-C
average
B-Sibridge-C-C
B-Sibridge-C-C
B-Sibridge-C-C
average
16.7(3)
19.5(3)
21.6(3)
19.3
–
–
–
–
17.3(7)
17.7(8)
20.4(7)
18.5
–
–
–
–
19.2(5)
19.8(6)
22.6(5)
20.5
–
–
–
–
19.3(1)
19.8(1)
22.2(1)
20.4
–
–
–
–
14.8(4)
16.9(4)
17.3(4)
16.3
–
–
–
–
17.5(5)
17.5(5)
20.0(5)
18.3
–
–
–
–
–
–
–
–
–
–
–
–
16.4(3)
17.3(3)
17.8(3)
17.2
19.9(2)
21.5(2)
22.1(2)
21.2
boron
67
69
73
72
66
70
65
67
stituted compounds 1b,cB·Py and F-substituted compound
1dB·Py have structures that are similar to 1aB·Py and their
THCs also are very similar, as shown in Table 3. The
ꢀ
phenyl-substituted borate 1eB·Py has a longer B N bond
length (1.647(6) ꢁ) with a larger S(O-B-O) angle (342.88)
G
and a smaller S
N
bly because of the steric hindrance of the ortho-phenyl
groups. In the bulkier naphthyl-substituted borate 1 fB·Py,
ꢀ
the B N bond length (1.631(8) ꢁ) is less than that of
1eB·Py, presumably due to crystal-packing effects and/or a
p–p interaction between the pyridine and naphthyl rings. A
silicon-based compound with a pyridine-ligand 1gB·Py was
also analyzed by X-ray crystallography. The larger size of
Figure 2. ORTEP drawing of 1aB·Py. Top view (pyridine is omitted for
clarity).
ꢀ
the bridging Si atom resulted in longer Si aryl bonds (aver-
ꢀ
age 1.870 ꢁ) in 1gB·Py than the C aryl bonds (average
1.519 ꢁ) in 1aB·Py, and, therefore, it directly affected the
caused by the cage shape.[26] A similar borate structure, with
its phenolic rings connected to nitrogen, was reported, but
its coordination to boron resulted in nearly perpendicular
geometry of the cage. The boron has a distorted tetrahedral
coordination sphere and the SACTHNUGTRNENUG(O-B-O) and SACHUTNGTREN(NUNG N-B-O) bond
angles are 343.1 and 311.88, respectively, while the angles of
1aB·Py are 342.7 and 312.68, respectively. The geometries
around boron in 1gB·Py and 1aB·Py were nearly identical,
aromatic rings.[27] The bond length of B N in 1aB·Py is
ꢀ
1.628(5) ꢁ, and the sum of the angles for O-B-O and N-B-O
ꢀ
around boron are 342.7 and 312.68, respectively. The Br-sub-
but that of 1gB·Py was more planar. The B N bond length
Chem. Eur. J. 2011, 17, 3856 – 3867
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3861

M. Yasuda, A. Baba et al.
in 1gB·Py is 1.655(5) ꢁ, which is longer than that in 1aB·Py.
This geometry suggests that 1gB·Py has a lower Lewis acidi-
ty than 1aB·Py. It is worth noting that the silicon atom in
1gB·Py has an almost tetrahedral structure (sum of Ar-Si-
Ar; 330.38), while the carbon atom at the bottom of 1aB·Py
has a distorted structure with bond angles that equaled
343.78. The iBuSi-bridging borate 1hB·Py was also analyzed
by X-ray crystallography, and has a framework similar to
that of 1gB·Py. The sum of the bond angles for 1gB·Py are
among the cage-shaped borates 1B was observed for the o-
and p-Br-substituted compound 1bB (entry 2). This shift
corresponds to the chemical shifts observed for the ligated
THF discussed above. Interestingly, replacement of the
bridgehead C with a bridgehead Si decreased the Lewis
acidity. The silicon-bridging compounds 1gB and 1hB
showed smaller downfield shifts of the pyrone, +5.844 and
+4.519 ppm (entries 7 and 8), respectively, than that of the
carbon-bridging compound 1aB (+6.782 ppm). The strength
as follows: S
(O-B-O), 342.58; S
E
N
of the Lewis acidity based on the measurement of Dd
was: BF3 >1bB>1cB>1dB>1aB>1gB>1hB>B(OPh)3.
The Lewis acidities of 1eB and 1 fB could not be estimated
by using this Dd
(13C) measurement method, because the
ACHTUNGTRENNUNG
(13C)
ꢀ
Ar), 328.08. The average Si aryl bond length for 1gB·Py is
1.879 ꢁ.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Lewis acidity of the cage-shaped borates: To investigate the
ability of the cage-shaped borates 1B to activate carbonyl
compounds, we synthesized their complexes with the 2,6-di-
ortho-aryl groups had a significant anisotropic effect on the
chemical shifts of ligated 5. Table 4 describes the fine-tuning
of Lewis acidity using the new cage-shaped template over a
range of moderate Lewis acidity that would be useful for
catalysts.
methyl-g-pyrone 5. A DdACHTNUTGRNEUNG
(13C) shift of C3 in 5 clearly shows
the degree of Lewis acidity. These data provide an estimate
of the Lewis acidity that is more precise than the chemical
shift of ligated THF coordinated to boron, as discussed
above in the section on NMR data for cage-shaped borates.
Theoretical calculations: The characteristic properties of
cage-shaped borates 1B were investigated by theoretical cal-
culations. Optimized structures and unoccupied molecular
orbitals contributing to the Lewis acidity of the cage-shaped
The Dd
ous borates. For comparison with other Lewis acids, the
planar borate B(OPh)3 and the strong Lewis acid BF3·OEt2
were also employed. In fact, the complexation of 5 with
BF3·OEt2 showed the largest downfield shift (Dd
(13C)=
+8.708 ppm) in C3 (entry 10), and B(OPh)3 showed only a
small chemical shift (Dd
(13C)=+0.774 ppm) (entry 9). It is
worth noting that all cage-shaped borates 1B showed a
Lewis acidity that lay between that of (OPh)3 and
ACHTUNGTRENNUNG
(13C) shifts of C3 in 5 are shown in Table 4 for vari-
R
borates 1B and the open-shaped borate BACTHNGUETRNNU(G OPh)3 are shown
in Figure 3. Some of the calculated data are shown in
Table 5. The optimized structures of the cage-shaped borates
1B show that the geometries around the boron centers are
nearly planar, and the sums of the three O-B-O angles are
nearly 3608 in each case (Figure 3 and Table 5). Figure 3
also shows the molecular orbital (MO) diagram of unoccu-
pied MOs[28] of the cage-shaped borates 1a–dB, 1gB, and
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
BACHTUNGTRENNUNG
BF3·OEt2. The unsubstituted borate 1aB showed a down-
field shift of +6.782 ppm (entry 1). The introduction of an
electron-withdrawing group onto the aryl rings in cage-
shaped borates allowed for precise control of the Lewis
acidity. Introduction of F onto the aryl rings of the cage-
shaped borate 1dB resulted in a higher Lewis acidity than
that observed for the unsubstituted borate 1aB (entry 4).
The p-Br-substituted compound 1cB had a higher Lewis
acidity (entry 3) than 1dB, and the highest Lewis acidity
1hB, and the open-shaped borate B
shaped borates include a large and accessible lobe on boron,
while the corresponding lobe in B(OPh)3 (LUMO in this
ACHTUNGRTEN(NUNG OPh)3. The cage-
AHCTUNGTRENNUNG
case) is small and buried. From the optimized borate struc-
tures, we observe that the bridgehead atom significantly af-
fects the cage structure—for example, the dihedral angle
(Cipso-O-B-O) q (Table 5). The bridgehead atoms, either
carbon or silicon, control the dihedral angles (Cipso-O-B-O)
q (1aB 48.48; 1gB 45.48; 1hB 43.68; entries 1, 5 and 6). As
the angles become smaller, the cage-shaped borates showed
the lower energy levels of the unoccupied MOs[29] of Lewis
acids. Their eigenvalues are on the order of 1aB<1gB<
Table 4. Complexation of boron compounds with the carbonyl compound
5. [B]=complexed boron compound
1hB. In fact, the open-shaped borate BACTHNUTRGENUG(N OPh)3 has a nearly
planar structure (small dihedral angle, q=2.08) and a high
eigenvalue (entry 7). To examine the correlation between
the dihedral angle and the eigenvalue of the corresponding
MO, a series of theoretical calculations were carried out.
The change of the eigenvalue could be traced by changing
the dihedral angle (H-O-B-O) q under the constraints on
the geometry around the boron center (planar structure in
sp2 hybridization) (Scheme 5). The angle of q=08 gave the
highest MO energy level because of the effective conjuga-
tion between the p orbitals on O and B. Gradual changes in
the MO level can be realized by varying q, even with three
Entry
Boron compound
DdACTHNGUTERNNUG
(13C) of C3 [ppm]
1
2
3
4
5
6
7
8
9
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
1hB·THF
+6.782
+7.630
+7.605
+7.243
+5.564
+3.877
+5.844
+4.519
+0.774
+8.708
BACHTUNGTRENNUNG
ꢀ
B O bonds, by keeping the structure in-plane. These results
10
show that the dihedral angle q controls the overlap between
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Borate Esters
FULL PAPER
Scheme 5. Relationship between the dihedral angle q and the energy
level of the lowest unoccupied MO relative to a Lewis acid.
4), the energy levels of the unoccupied MOs[29] that contrib-
ute to the Lewis acids are controlled by the electronic fac-
tors of the substituents on the aryl rings. The eigenvalues
are on the order of 1bB<1cB<1dB<1aB. It is under-
standable that electron-withdrawing groups lead to favora-
ble interactions between the borates and the Lewis basic
substrates. In total, the order of the eigenvalues is 1bB<
1cB<1dB<1aB<1gB<1hB<BACHTNUGTRENUNG(OPh)3. The pyridine
complexation energies, DE, also showed the same order.
Thus, we were able to fine-tune the Lewis acidity of the bo-
rates by structural and electronic controls. The Lewis acidity
data were highly consistent with the experimental NMR
data for pyrone 5.
Ligand exchange rate of cage-shaped borates: The ligand
exchange rate of the cage-shaped borates 1B was investigat-
ed to obtain information on the kinetics of the ligand associ-
ation–dissociation process that controls a Lewis acid catalyst
reaction. Dimethylaminopyridine (DMAP) complexes of 1B
were dissolved in [D5]pyridine, and the ligand dissociation
rate was measured during ligand exchange from DMAP to
pyridine. The results are shown in Table 6. The unsubstitut-
ed cage-shaped borate 1aB has a dissociation rate constant
of k=2.32ꢂ10ꢀ9 sꢀ1, whereas that for the fluoro-substituted
compound 1dB is smaller (k=8.37ꢂ10ꢀ12; entries 1 and 2).
The activation enthalpy DH° of 1dB is much larger than
that of 1aB, because the electron-withdrawing effect of fluo-
rine increased the Lewis acidity of the boron center by sta-
bilization of the negative charge generated during complex-
Figure 3. Optimized structures and unoccupied MO diagrams of cage-
shaped and open-shaped borates.
Table 5. Theoretical calculations for borates.
Table 6. Kinetic parameters for ligand dissociation of the cage-shaped
borates 1B.
Entry Borate
SACHTUNGTRENNUNG(O-B-O) Dihedral angle Eigenvalue DE
[b]
[8]
G
[kcalmolꢀ1
]
1
2
3
4
5
6
7
1aB
1bB
1cB
1dB
1gB
1hB
359.7
359.4
359.6
359.6
360.0
359.9
48.4
46.6
48.1
47.5
45.4
43.6
2.0
ꢀ0.79
ꢀ1.67
ꢀ1.31
ꢀ1.12
ꢀ0.73
ꢀ0.71
ꢀ0.54
ꢀ19.2
ꢀ31.4
ꢀ22.5
ꢀ20.8
ꢀ13.2
ꢀ13.1
ꢀ5.0
BACHTUNGTRENNUNG(OPh)3 360.0
Entry Borate DH°
[kcalmolꢀ1
DS°
DG°
k
[a] Eigenvalues of MOs depicted in Figure 3. [b] For pyridine complexa-
tion.
[a]
[a]
]
[calKꢀ1 molꢀ1
]
[kcalmolꢀ1
]
ACHTUNGTRENNUNG ]
[sꢀ1
1
2
3
4
1aB·L 35.1
22.3
40.0
7.5
28.6
31.9
29.0
22.3
2.32ꢂ10ꢀ9
8.37ꢂ10ꢀ12
1.16ꢂ10ꢀ9
1.28ꢂ10ꢀ4
1dB·L 43.6
1eB·L 31.2
1gB·L 26.4
the p orbitals on O and B, which allows fine-tuning of the
MO energy level. While the differences among the carbon-
bridging borates 1a–dB are minimal (qꢁ47–488; entries 1–
14.2
[a] DG° and k are calculated at T=208C.
Chem. Eur. J. 2011, 17, 3856 – 3867
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www.chemeurj.org
3863

M. Yasuda, A. Baba et al.
ation. For 1eB, which has sterically demanding ortho-phenyl
substituents, a decrease in the rate constant (k=1.16ꢂ
10ꢀ9 sꢀ1) is also observed (entry 3). The activation entropy
DS° of 1eB is much lower than that of 1aB, although the ac-
tivation enthalpy is nearly identical. The steric repulsion
caused by the bulky ortho-substituents during ligand dissoci-
ation controls the entropic effect. The electronic factor con-
trols the ligand exchange rate by changing DH°, and the
steric factor influences the rate with different DS°. It is
worth noting that the silicon-bridging compound 1gB has a
low DG°, mainly due to a lower DH° =26.4 kcalmolꢀ1
(entry 4). This result shows that the geometry of the cage
shape is an important determinant of the character of a
Lewis acid catalyst.
The Mukaiyama aldol reaction[32,33] was also examined
using the ketene silyl acetal 9 substituted with methyls at
the terminal olefinic moiety, as shown in Table 8. In contrast
Table 8. Mukaiyama aldol reaction using aldehyde catalyzed by various
borates.
Entry
Nucleophile
Time [h]
Borate
Product
Yield [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
9
9
9
9
9
9
9
9
9
10
10
10
10
10
10
10
10
10
6
6
6
6
6
6
6
6
6
4
4
4
4
4
4
4
4
4
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
11
11
11
11
11
11
11
11
11
12
12
12
12
12
12
12
12
12
<5
98
17
<5
69
90
20
<5
9
30
91
50
43
67
54
31
26
8
Catalytic activity of cage-shaped borates of organic transfor-
mation: The cage-shaped borates were applied as catalysts
in the hetero Diels–Alder reaction of the Danishefsky diene
6
with benzaldehyde (7a); the results are shown in
BACTHNGUTRENU(NG OPh)3
Table 7.[30] For a Lewis acid catalyst, the balance between
BF3·OEt2
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
Table 7. Hetero Diels–Alder reaction catalyzed by various borates.
BACHTUNGTRENNUNG
Entry
Borate
Yield [%]
1
2
3
4
5
6
7
8
9
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
77
14
25
29
77
65
85
7
to the results of a hetero Diels–Alder reaction, the appropri-
ate Lewis acid was the dibromo-substituted cage-shaped
borate 1bB, while the unsubstituted borate 1aB was ineffec-
tive (entries 1 and 2). In this case, the activation step, rather
than the catalyst-regeneration process (releasing step), was
more important in the catalytic cycle. Interestingly, a rela-
tively high yield of the product 11 was obtained by using the
phenyl-substituted borate 1eB (entry 5). The open-shaped
borate catalyst showed no catalytic activity (entry 8), and
the strong Lewis acid afforded very low yields (entry 9).
When the unsubstituted silyl nucleophile 10 was used, 1bB
also afforded the product 12 in high yield (entry 11), and the
cage-shaped borates also worked well. It should be noted
that different reactions had different suitable catalysts, even
among the cage-shaped borate catalysts.
In the case of the Mukaiyama aldol reaction that used the
acetal 13 as an electrophile,[34] the dibromo- and monobro-
mo-substituted borates 1bB and 1cB gave high yields
among the series of cage-shaped borate catalysts (Table 9,
entries 2 and 3). The phenyl- and naphthyl-substituted bo-
rates 1eB and 1 fB did not give the product, probably be-
cause steric hindrance prevented the approach of the bulky
electrophile 13 (entries 5 and 6). Based on these results, we
thought that the ortho-aryl substituted cage-shaped borates
1eB and 1 fB recognized the bulkiness of the substrates.
We next investigated the generality of the aldehydes in
the hetero Diels–Alder reaction using the cage-shaped bo-
BACHTUNGTRENNUNG
<5
the Lewis acidity and the ability to exchange ligands is im-
portant.[31] In fact, both the weak Lewis acid B
(OPh)3 and
AHCTUNGTRENNUNG
the strong Lewis acid BF3·OEt2 gave low yields (entries 8
and 9). The cage-shaped borates 1B afforded higher yields
(entries 1–7), because of their moderate Lewis acidity. The
unsubstituted cage-shaped borate 1aB·THF gave the cyclo-
addition product 8a in 77% yield. The halogen-substituted
cage-shaped borates yielded the product on the order of
1dB>1cB>1bB. This result suggests that a Lewis acidity
that is too high leads to a strong affinity between the boron
center and the product, which reduces the catalytic turnover.
The silicon-bridging borate 1gB·THF was the best catalyst
for the hetero Diels–Alder reaction, giving the product in
85% yield (entry 7). In this case, the Lewis acidity of the
borate 1gB was slightly lowered by bridgehead-control,
which promotes release of the product, contributing to an
increase in catalytic activity. The phenyl-substituted borate
1eB gave the product in satisfactory yield (entry 5).
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Borate Esters
FULL PAPER
Table 9. Mukaiyama aldol reaction using acetal catalyzed by various bo-
rates.
A careful comparison between 1aB and 1eB was per-
formed by competitive reaction using a mixture of the ben-
zaldehyde (7a) and o-phenylbenzaldehyde (7c) (Table 11).
Table 11. Competitive reaction of the Danishefsky diene with two types
of aldehyde 7a and 7c catalyzed by the borates 1aB·THF, 1eB·THF, or
1 fB·THF.[a]
Entry
Borate
Yield [%]
1
2
3
4
5
6
7
8
9
1aB·THF
1bB·THF
1cB·THF
1dB·THF
1eB·THF
1 fB·THF
1gB·THF
8
62
61
17
<5
<5
<5
<5
31
BACHTUNGTRENNUNG
Entry
Borate
Solvent
Yield [%]
Ratio 8a/8c
1
2
3
4
5
1aB·THF
1eB·THF
1aB·THF
1eB·THF
1 fB·THF
CH2Cl2
CH2Cl2
acetonitrile
acetonitrile
acetonitrile
86
87
67
73
69
1.1
3.6
0.97
6.3
12.8
rates 1aB and 1eB at room temperature for 4 h (Table 10).
The reaction of 6 with benzaldehyde 7a gave the product 8a
in 72% yield in the presence of a catalytic amount of 1aB
[a] The reactions were carried out using 6 (1.0 mmol), 7a (1.0 mmol), 7c
(1.0 mmol), and borate catalyst (0.1 mmol).
Table 10. Hetero Diels–Alder reaction catalyzed by two types of borates:
1aB·THF and 1eB·THF.
When the unsubstituted borate 1aB in dichloromethane was
used as the catalyst for the competitive reaction, the product
ratio of 8a/8c was 1.1 (=52:48; entry 1). In contrast, when
the phenyl-substituted borate 1eB was used, the product
ratio was 3.6 (=78:22), and, therefore, the selectivity was in-
creased 3.27-fold (entry 2). This result can be explained by
the steric effects of the ortho-phenyl substituent that pre-
vented access of the bulky substrate to the metal center.[35,36]
The use of 1eB with acetonitrile as the solvent showed a
6.49-fold increase compared to 1aB (entries 3 and 4). The
coordinative solvent somewhat retarded complexation with
the substrate aldehyde, and, hence, the selectivity was en-
hanced. Notably, the naphthyl-substituted borate 1 fB gave a
much higher value of 12.8 (13.2-fold greater than 1aB) due
to effective steric hindrance for substrate-selectivity (en-
tries 3 and 5). The cage-shaped borates provide steric hin-
drance due to their inflexible structure; therefore, they ef-
fectively controlled the substrate-selective reaction system,
as shown in Table 11.
Entry
Aldehyde
Borate
Product
Yield of 8 [%]
1
2
1aB·THF
1eB·THF
8a
8a
72
74
7a
7b
7c
3
4
1aB·THF
1eB·THF
8b
8b
64
64
5
6
1aB·THF
1eB·THF
8c
8c
53
67
7
8
1aB·THF
1eB·THF
8d
8d
54
61
7d
7e
9
10
1aB·THF
1eB·THF
8e
8e
56
63
11
12
1aB·THF
1eB·THF
8 f
8 f
76
65
7 f
7g
13
14
1aB·THF
1eB·THF
8g
8g
38
42
Conclusion
We have synthesized cage-shaped borates with a tris(o-oxy-
phenyl)methane or -silane moiety, with various substituents.
The cage shape resulted in a novel boron center, which is a
unique Lewis acid. Both the Lewis acidity and the catalytic
activity of organic transformation were successfully en-
hanced. A moderate Lewis acidity was attained by tuning
factors such as the substituents (electronic and steric con-
trol) and the bridgehead atoms (geometric control). Theo-
retical calculations suggested that the energy levels of the
unoccupied molecular orbitals, which greatly contributed to
activation of the substrate, are finely tuned by the substitu-
(entry 1). The substituted aldehydes 7b and 7c also gave the
products, although a small decrease in yields was observed
because of the steric effects of the ortho-substituents (en-
tries 3 and 5). The aliphatic aldehydes 7d–g were also ap-
plied to this catalytic reaction system to give the corre-
sponding products 8d–g (entries 7, 9, 11, and 13). Unexpect-
edly, the use of phenyl-substituted borate 1eB showed
almost the same results in the hetero Diels–Alder reaction
as use of 1aB, in spite of the bulky ortho-phenyl groups in
1eB.
Chem. Eur. J. 2011, 17, 3856 – 3867
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3865

M. Yasuda, A. Baba et al.
[9] For elegant examples of stereo-controlled aryloxy aluminum Lewis
acids, see: a) H. Yamamoto, S. Saito, Pure Appl. Chem. 1999, 71,
239–245; b) K. Maruoka, Y. Araki, H. Yamamoto, J. Am. Chem.
Soc. 1988, 110, 2650–2652; c) H. Yamamoto, Tetrahedron 2007, 63,
8377–8412.
[10] T. Matsuo, H. Kawaguchi, Chem. Lett. 2004, 33, 640–645.
[11] a) M. Fujita, O. C. Lightbody, M. J. Ferguson, R. McDonald, J. M.
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on the cage-shaped borate controlled the selectivity of the
competitive reaction between sterically different aldehydes.
The cage-shaped template can be modified in many ways by
altering either the geometry[37] or the substituents, and it is a
promising template for other metal complexes to be used in
catalysts, new metal complexes, or materials.
Experimental Section
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 20036036, “Synergistic Effects for Creation of Func-
tional Molecules”), Scientific Research on Innovative Areas (No.
22106527, “Organic Synthesis Based on Reaction Integration. Develop-
ment of New Methods and Creation of New Substances”), and for Scien-
tific Research (No. 21350074) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. This work was financially support-
ed by the General Sekiyu Research & Development Encouragement &
Assistance Foundation. We thank Dr. Nobuko Kanehisa for the valuable
advice regarding X-ray crystallography. Thanks are due to Mr. H. Mori-
guchi, Faculty of Engineering, Osaka University, for assistance in obtain-
ing the MS spectra. H.N. thanks the Yoshida Scholarship Foundation.
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[20] The difference of the chemical shifts between 1cB·THF and
1dB·THF was quite small and difficult to compare their Lewis acidi-
ties accurately owing to broadening signals. Therefore we investigat-
ed Lewis acidity by using other ligand and describe later (Lewis
acidity of the cage-shaped borates).
[21] Two sets of pyridine signals were observed by 1H NMR spectrosco-
py, but their 4-H signals were obscured by other aryl signals.
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[29] The described MOs are the lowest unoccupied orbitals with suitable
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Chem. Eur. J. 2011, 17, 3856 – 3867

Borate Esters
FULL PAPER
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[38] CCDC-794455 (1bB·Py), 794456 (1cB·Py), 794457 (1dB·Py), 794458
(1eB·Py), and 794459 (1 fB·Py) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: September 29, 2010
Revised: December 22, 2010
Published online: March 7, 2011
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