Intramolecular H-Ar ligand exchange between silicon and boron: Functionality transfer of Si-H to B-H

Article abstract of DOI:10.1002/asia.201100678

o-C₆H₄(SiR_3-nH_n)(BMes ₂) (1; R=Me, Ph; n=1, 2) undergo Mes-H (Mes=mesityl) ligand exchange between the silicon atom and the boron atom to form o-C₆H ₄(SiMesR_3-nH_n

Full text of DOI:10.1002/asia.201100678

FULL PAPERS
DOI: 10.1002/asia.201100678
À
Intramolecular H Ar Ligand Exchange between Silicon and Boron:
À
À
Functionality Transfer of Si H to B H
Atsushi Kawachi,* Hiroshi Morisaki, Norimasa Nishioka, and Yohsuke Yamamoto[a]
Abstract: o-C6H
(SiR3ÀnHn)
E
afford their corresponding benzyloxy-
boranes (5). A DFT study of model
compounds reveals the transition states
of the ligand exchange. A hydride ab-
straction from the silicon atom by the
boron center is key to reaching the
transition states, which include the tri-
coordinate silyl-cation moiety and the
tetracoordinate hydridoborate moiety.
R=Me, Ph; n=1, 2) undergo Mes H
(Mes=mesityl) ligand exchange be-
tween the silicon atom and the boron
atom to form o-C6H4(SiMesR3ÀnHnÀ1)-
ACHTUNGTRENNUNG(BMesH) (6) upon heating. The result-
ing hydroborane intermediates (6) im-
Keywords: boron · exchange inter-
actions · hydroboration · silicon ·
transition states
mediately react with benzaldehyde to
Introduction
(Scheme 1); compound 1a reacted with methanol, ethanol,
propan-2-ol, and tert-butanol in tetrahydrofuran at room
temperature to give the corresponding alkoxysilanes (2)
with H2 evolution in good yields.[6a]
À
Si H bond activation is currently attracting a lot of atten-
tion because it has paved the way for a variety of useful
transformations of organic and organometallic compounds.[1]
À
There are mainly three modes of Si H bond activation: The
first one is the transition-metal-catalyzed reaction.[2] Transi-
À
tion-metal catalysts activate the Si H bond via oxidative ad-
dition or through the formation of a s complex. The second
mode is the hypercoordination of hydrosilanes.[3] Inter- or
intramolecular coordination of a ligand to the silicon atom
À
forms a pentacoordinate silicon species in which the Si H
bond is highly polarized and the silicon center becomes
more Lewis acidic. The third is electrophilic activation of
(C6F5)3.[4,5]
À
the Si H bond by a strong Lewis acid, such as B
The highly electrophilic boron center coordinates to the hy-
À
drogen atom on the silicon center, which renders the Si H
bond polarized (Sid+
dÀ) and makes the silicon center sus-
À
H
ceptible to nucleophilic attack.
Scheme 1. Reactions of compound 1 with alcohols and fluoride ions.
To extend the electrophilic-activation strategy for intra-
molecular reactions, we prepared
a series of ortho-
(hydrosilyl)(boryl)benzenes (1), in which a hydrosilyl group
N
ortho-(Hydrosilyl)ACTHNGUTERN(UNG boryl)benzenes 1a and 1b also reacted
(SiR3ÀnHn (n=1, 2)) and a dimesitylboryl group (BMes2) are
with KF in the presence of a cryptand to form hydridobo-
rates 3, which involved the unprecedented intramolecular
linked through an ortho-phenylene skeleton.[6] This skeleton
[7]
may allow the boryl group to activate the Si H bond.
F Si···H B interaction (Scheme 1).[6d] The nucleophilic
attack of a fluoride ion on the silicon atom triggers the hy-
dride transfer from the silicon center onto the boron atom.
In this context, we anticipated that the reactions of com-
pound 1 with various carbonyl compounds would yield hy-
À
À
À
ortho-(Hydrosilyl)
(boryl)benzene 1a underwent a dehy-
drogenative condensation reaction with various alcohols
[a] Prof. Dr. A. Kawachi, H. Morisaki, N. Nishioka,
Prof. Dr. Y. Yamamoto
À
drosilylation products 4, in which the Si H bond is activated
by the Lewis acidic boryl group whereas the silicon atom is
exposed to nucleophilic attack by the carbonyl oxygen atom
(Scheme 2). Herein, we report our unexpected finding that
the reactions of compound 1 with carbonyl compounds in
refluxing toluene affords hydroboration products 5, during
Department of Chemistry
Graduate School of Science
Hiroshima University
Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526 (Japan)
Fax : (+81)82-4240723
E-mail: kawachi@sci.hiroshima-u.ac.jp
À
which compound 1 undergoes H Mes ligand exchange be-
tween the silicon atom and the boron atom to form hydro-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100678.
546
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 546 – 553

hand, compound 1c decom-
posed to give a complex mix-
ture under similar reaction con-
ditions.[8]
The reaction of compound
1d with benzaldehyde was
monitored by 1H NMR spec-
troscopy, as shown in Figure 2.
As the reaction proceeded, the
broad signals of the SiH2
moiety in compound 1d dimin-
À
Scheme 2. H Ar ligand-exchange between hydrosilane 1 and hydroborane 6.
borane intermediates 6. The ligand-exchange mechanism is
analyzed by quantum-mechanical computations.
Results and Discussion
The reaction of compound 1a with benzaldehyde did not
proceed at room temperature; however, heating compound
1a and benzaldehyde in toluene at 1208C afforded benzy-
loxyborane 5a in 74% yield (Scheme 3), although the ex-
Figure 1. Crystal structures of compounds 5b (left) and 5d (right).
Table 1. Crystallographic data for compounds 5b and 5d.
Scheme 3. Reactions of compound 1 with benzaldehyde.
5b
5d
Formula
Mw
T [K]
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
C43H43BOSi
614.67
293
triclinic
P-1
12.090(5)
12.408(5)
13.679(5)
81.551(5)8
81.004(5)8
59.800(5)8
1746.1(12)
2
C37H39BOSi
538.58
173
triclinic
P-1
10.0084(6)
10.5696(6)
16.0316(10)
94.7430(14)
99.0128(14)
112.7339(14)
1525.62(16)
2
pected benzyloxysilane 4a was not obtained at all. Hydrodi-
phenylsilyl derivative 1b reacted slowly with benzaldehyde
to give compound 5b in 60% yield. Dihydrosilyl derivatives
1c and 1d reacted faster with benzaldehyde than the mono-
hydrosilyl derivatives to form compounds 5c (42%) and 5d
(67%), respectively.
Benzyloxyboranes 5 were characterized by NMR spec-
troscopy and X-ray crystallographic analysis. Analysis of
compounds 5 by 11B NMR spectroscopy revealed signals at
around d=48 ppm. These signals were shifted upfield com-
pared to those of their corresponding ortho-(alkoxysilyl)bor-
ylbenzenes (2; d=73–74 ppm),[6a] which are structurally
analogous to compound 4, perhaps due to the p donation
from the adjacent oxygen atom. The 29Si spectra of 5
showed signals at d=À8.50–À28.30 ppm, which are also
shifted upfield from those of 2 (d=9.7–À1.2 ppm). The mo-
lecular structures of compounds 5b and 5d were finally de-
termined by X-ray crystallographic analysis (Figure 1 and
Table 1). On the other hand, crystals obtained for com-
pounds 5a and 5c were not suitable for X-ray crystallo-
graphic analysis.
g [8]
V [ꢁ3]
Z
1calcd [MgmÀ3
]
1.169
0.100
656
0.50ꢂ0.40ꢂ0.20
3.02 to 27.488
1.172
0.105
576
0.40ꢂ0.30ꢂ0.20
3.08 to 27.488.
absorption coefficient [mmÀ1
]
F
N
crystal size [mm3]
2q max [8]
index ranges
À15< =h< =15 À12< =h< =12,
À15< =k< =16 À13< =k< =13,
17< =l< =17
18361
À20< =l< =20
16027
6957
0.0322
6957/0/371
1.040
total reflns
unique reflns
Rint
data/restraints/parameters
GOF
7992
0.0334
7992/0/421
1.048
0.0446
R (I>2 sigma(I))
Rw (all data)
0.0504
0.1350
0.1302
Heating compound 1a in the absence of benzaldehyde in
toluene resulted in its complete recovery; on the other
Largest diff. peak and hole [eꢁÀ3
]
0.287 and À0.280 0.330 and À0.260
Chem. Asian J. 2012, 7, 546 – 553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
547

A. Kawachi et al.
FULL PAPERS
ed reaction of dimesitylphenylborane and diphenylsilane in
refluxing toluene for 45 hours did not proceed at all
(Scheme 5).
À
Scheme 5. Attempted intermolecular H Mes exchange.
However, there is another possibility: the reaction path-
way involves the hydrosilylation of compound 1 to benzalde-
À
hyde to give compound 4 and then subsequent H alkoxy ex-
Figure 2. Monitoring the reaction of compound 1d with benzaldehyde by
change would yield compound 5. Benzyloxysilane 4a, which
was prepared by the dehydrogenation of compound 1a with
benzyl alcohol,[6a] was exposed to the same reaction condi-
tions as used for the conversion of compound 1a into 5a,
but showed no reaction (Scheme 6).
1H NMR spectroscopy.
ished and the intensities of a sharp signal assignable to the
SiH moiety and two sets of doublet assignable to the diaste-
reotopic benzyl protons of compound 5d increased; howev-
er, signals corresponding to intermediate 6d could not be
observed throughout the reaction.
Next we investigated the transformation of a carbonyl
compound into an alcohol by using compound 1. The reac-
tions of compounds 1a, 1c, and 1d with acetophenone in re-
fluxing toluene afforded benzyloxyboranes 7a, 7c, and 7d,
respectively (Scheme 4). Without being isolated, compounds
7a, 7c, and 7d were treated directly with tetra-n-butylam-
monium fluoride (TBAF) at room temperature and subse-
quently with water to produce 1-phenylethanol (8) in mod-
erate to good yields.[5d,9]
À
Scheme 6. Preparation of alkoxysilane 4a and attempted alkoxy Mes ex-
change.
It is plausible that coordination of the carbonyl oxygen
atom to the silicon atom is involved in the transition state
and promotes the hydride migration. However, this coordi-
nation seems less likely because the Si O bond would be
formed prior to the mesityl migration owing to the high oxo-
We postulated that the reaction pathway proceeds as fol-
À
lows: Coordination of the ortho-boryl group to the Si H
À
bond renders the silicon atom more positively charged and
À
renders the B C bonds on the boron atom more labile. In
À1 [10]
À
the absence of external nucleophiles, the neighboring elec-
tron-rich mesityl group interacts with the positive silicon
atom to undergo migration. This exchange process may be
reversible, as the sum of the bond energies are roughly com-
philicity of silicon (Si O: 452 kJmol ),
as observed in
(C6F5)3-catalyzed hydrosilylation reac-
the intermolecular B
A
tions of carbonyl compounds.[5]
Our postulated mechanism was examined by quantum-
mechanical computations.[12] The geometries of compounds
1 and 6 were optimized at the B3LYP/6-31G(d) level[13] and
their single-point energies were calculated at the MP2/6-
31G(d) level. Hydroboranes 6 were less stable than their
corresponding hydrosilanes (1) by a few kcalmolÀ1: relative
energies=+6.85 (6a), +4.32 (6b), +5.33 (6c), +4.33 kcal
molÀ1 (6d).
À1
À1
À
À
parable: Si H (326 kJmol ) and B C (372 kJmol ) versus
À
À1
À1 [10]
À
Si C (301 kJmol ) and B H (381 kJmol ).
However,
once compounds 6 are formed, they react immediately with
benzaldehyde to yield compounds 5 under the refluxing con-
ditions.[11]
À
The possibility of an intermolecular H Mes exchange
process was excluded by a control experiment. The attempt-
To gain further insight into
the ligand-exchange process, we
looked for the transition states
using dihydrosilyl model com-
pounds 9 and 10 (R=reactant;
TS=transition state; P=prod-
uct), in which the mesityl
groups in compounds 1c and
1d were replaced by phenyl
groups (Scheme 7).
Scheme 4. Transformation of acetophenone into benzyl alcohol using compound 1.
548
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 546 – 553

À
Intramolecular H Ar Ligand Exchange between Silicon and Boron
Two transition states (9-TSa and 9-TSb) were found for
the ligand-exchange reaction of 9-R (Figure 3). Intrinsic re-
action coordinate (IRC) calculations revealed two corre-
states 9-TSa and 9-TSb is so small that there may be no se-
lectivity between the two reaction pathways.
In transition state 9-TSa, the H1 atom migrates noticeably
1
1
À
À
towards the boron atom (B H =1.271 ꢁ vs. Si H =
1.928 ꢁ) whereas the migrating phenyl group remains on
À
À
the boron atom (B Cipso =1.677 ꢁ vs. Si Cipso =2.346 ꢁ),
being located on the same side of the ortho-phenylene plane
as the methyl group on the silicon atom. This geometry may
involve steric repulsion between the migrating phenyl group
and the methyl group. Thus, transition state 9-TSa includes
the tetracoordinate hydridoborate moiety and the tricoordi-
nate silyl cation moiety. The natural charge[14] at the silicon
atom becomes more positive in transition state 9-TSa
(+1.82) than in 9-Ra (+1.40) (Table 2). The valency calcu-
lated using the Wiberg bond index (WBI)[14] is 3.01 at the
silicon atom and 3.61 at the boron atom. The geometry at
the silicon atom is still pyramidal (SaSi=340.98), which in-
dicates bonding interactions of the silicon atom with the H1
and the Cipso atoms. The bonding interaction of the silicon
atom with the Cipso atom may be reflected in the angles de-
fined by the vector between the boron atom and the Cipso
À
Scheme 7. Model reaction of H Ar ligand exchange.
sponding reaction pathways (Scheme 8), either starting from
global minimum 9-Ra (B···H1-Si=3.051 ꢁ) or its conforma-
tional isomer 9-Rb (B···H1-Si=3.055 ꢁ). Compound 9-Ra
leads to transition state 9-TSa and this, in turn, yields the
product 9-Pa. On the other hand, compound 9-Rb leads to
transition state 9-TSb, which, in turn, affords product 9-Pb.
Products 9-P are less stable than reactants 9-R by a few kcal
molÀ1. The difference in relative-energy between transition
À
atom, and by the Cipso Cpara vector in transition state 9-TSa.
The angle is slightly bent to 1678 rather than linear (this is
also the case in 9-TSb=1718,
10-TSa=1708, and 10-TSb=
1718).
In transition state 9-TSb, the
migrating phenyl group is locat-
ed on the same side of the
ortho-phenylene plane as the
H2 atom. The geometrical pa-
rameters in transition state 9-
TSb are almost similar to those
in transition state 9-TSa except
that the Si···Cipso distance is
slightly shorter in 9-TSb.
The ligand-exchange process
of compound 10 is quite similar
to that of compound 9, as
shown in Figure 4 and
Figure 3. Optimized structures of compounds 9-TSa and 9-TSb at the B3LYP/6-31G(d) level.
Scheme 9. The reaction path-
way begins either from global
1
À
minimum 10-Ra (B···H Si=
3.102 ꢁ) or its conformational
1
À
isomer
10-Rb
(B···H Si=
3.081 ꢁ). Again, the energy dif-
ference between two transition
states (10-TSa and 10-TSb) is
so small that there may be no
selectivity between the two re-
action pathways. The migrating
phenyl group in transition state
10-TSa is located on the same
side of the silicon atom as the
phenyl group, whereas in transi-
tion state 10-TSb, the phenyl
group is located on the same
Scheme 8. MP2/6-31G(d) relative energies (kcalmolÀ1) for compounds 9-R, 9-TS, and 9-P.
Chem. Asian J. 2012, 7, 546 – 553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
549

A. Kawachi et al.
FULL PAPERS
Table 2. Natural charges and valencies for selected atoms of compounds
9 and 10.
and the valencies of compound 10 exhibited similar trends
to those of compound 9 (Table 2).
Natural charge[a]
Valency[b]
Si B
The nature of the interactions between cationic silicon
species with phenyl groups has attracted much recent atten-
tion.[15] It has been proposed that s complexes have wide
angles (e; ca. 1168–1428) whereas p complexes have acute
angles (ca. 908), in which the angle e is defined by the
vector between the silicon atom and the Cipso atom and by
Si
B
Cipso
H1
9-Ra
9-Rb
1.395
1.395
1.818
1.821
1.404
1.403
1.824
1.830
0.965
0.960
0.243
0.256
0.959
0.958
0.252
0.275
À0.442
À0.447
À0.475
À0.476
À0.450
À0.447
À0.458
À0.484
À0.239
À0.239
À0.042
À0.058
À0.229
À0.232
À0.050
À0.070
3.42
3.41
3.01
3.01
3.41
3.41
3.02
3.02
2.91
2.92
3.61
3.60
2.92
2.92
3.60
3.59
9-TSa
9-TSb
10-Ra
10-Rb
10-TSa
10-TSb
À
the Cipso Cpara vector. According to this proposal, the inter-
action between the silicon atom and the migrating phenyl
group in transition states 9-TS and 10-TS can be termed as a
s complex: e=1228 (9-TSa), 1178 (9-TSb), 1188 (10-TSa),
1178 (10-TSb).
[a] calculated by natural population analysis. [b] calculated with WBI.
Based on the transition states
of the model compounds, we
tried to find the transition
states for the real mesityl sys-
tems. Transition states 11-TSa
and 11-TSb for the ligand ex-
change of compound 1c, and
12-TSa and 12-TSb for that of
compound 1d, were successfully
found (Figure 5 and 6, respec-
tively). The corresponding natu-
ral charges and valencies of se-
lected atoms are shown in
Table 3. When the geometrical
parameters in transition states
11-TS and 12-TS are compared
with the corresponding parame-
ters in transition states 9-TS
Figure 4. Optimized structures of compounds 10-TSa and 10-TSb at the B3LYP/6-31G(d) level.
and 10-TS, we can notice some
1
À
differences. The Si H bond
lengths are shorter, and re-
versely, the B H bond lengths
1
À
are elongated. The Si···Cipso dis-
tances are also longer. The nat-
ural charges on the H1 atoms in
transition states 11-TS and 12-
TS are more negative than
those in transition states 9-TS
and 10-TS. These parameters
indicate that transition states
11-TS and 12-TS are closer to
the reactants than transition
states 9-TS and 10-TS.
Scheme 9. MP2/6-31G(d) relative energies (kcalmolÀ1) for compounds 10-R, 10-TS, and 10-P.
We also found that the rela-
tive energies of transition states
side as the H2 atom. The steric repulsion between the mi-
grating phenyl group and the phenyl group on the silicon
atom in transition state 10-TSa may explain why it is the
longest Si···Cipso distance of the four transition states. The
shortest Si···Cipso distance, observed in transition state 10-
TSb (2.308 ꢁ), may be due to Coulomb interactions be-
tween the largest positive charge on the silicon atom
(+1.830) and the largest negative charge on the Cipso atom
(À0.484) for the four transition states. The natural charge
11-TS and 12-TS are slightly smaller in magnitude than
those of 9-TS and 10-TS. This observation may not be be-
cause the mesityl substituents stabilize the transition states
but because they destabilize the reactants. The steric repul-
sion induced by bulky mesityl groups is more predominant
in compounds 1c and 1d than in transition states 11-TS and
12-TS because the B C bond lengths are elongated by 2–
6% in transition states 11-TS and 12-TS relative to com-
pounds 1c and 1d.
À
550
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 546 – 553

À
Intramolecular H Ar Ligand Exchange between Silicon and Boron
cation moiety and the tetra-
coordinate hydridoborate
moiety, in which the silicon and
boron centers are bridged by
the hydride and aryl groups. We
have demonstrated that
a
Mes2B group, which is less
Lewis acidic than (C6F5)3B, can
abstract a hydride from a neigh-
boring hydrosilyl group owing
to the ortho-phenylene skele-
ton.
Figure 5. Optimized structures of compounds 11-TSa and 11-TSb at the B3LYP/6-31G(d) level and the relative
energies (kcalmolÀ1) at the MP2/6-31G(d) level.
Experimental Section
General Procedures
1H (400 MHz), 11B (128.3 MHz), 13C
(100 MHz), and 29Si NMR spectra
(79.4 MHz) were recorded on a JEOL
EX-400 or AL-400 spectrometer. 1H
and 13C NMR chemical shifts were ref-
erenced to internal CDCl3 (1H d=
7.26; 13C d=77.00) and [D6]benzene
(1H d=7.20; 13C d=128.0). 11B NMR
chemical shifts were referenced to ex-
ternal BF3·OEt2 (d=0). 29Si NMR
chemical shifts were referenced to ex-
ternal tetramethylsilane (d=0). Melt-
ing points were measured with
a
Yanaco Micro Melting Point Appara-
tus and were uncorrected. Elemental
analyses were performed with
a
Perkin–Elmer 2400CHN elemental an-
alyzer in our laboratory. Column chro-
matography on silica gel was per-
formed using Kieselgel 60 (230–400
mesh) (Merck). Thin-layer chromatog-
raphy was performed on plates of
Figure 6. Optimized structures of compounds 12-TSa and 12-TSb at the B3LYP/6-31G(d) level and the relative
energies (kcalmolÀ1) at the MP2/6-31G(d) level.
Table 3. Natural charges and valencies for selected atoms of compounds
11 and 12.
Silica gel 60/Kieselguhr F254 (Merck). Recycling preparative HPLC was
performed with LC918 instrument equipped with a JAIGEL SIL SH-043-
15 column and recycling preparative reversed-phase HPLC was per-
formed with LC928 instrument equipped with a JAIGEL ODS-AP.
Natural charge[a]
Valency[b]
Si B
Si
B
Cipso
H1
1c
1d
11-TSa
11-TSb
12-TSa
12-TSb
1.403
1.413
1.810
1.812
1.816
1.821
1.012
1.011
0.298
0.318
0.314
0.341
À0.459
À0.465
À0.488
À0.484
À0.461
À0.478
À0.247
À0.237
À0.078
À0.097
À0.090
À0.105
3.42
3.41
3.03
3.02
3.04
3.03
2.84
2.84
3.58
3.57
3.58
3.56
Toluene was distilled over sodium diphenylketyl under a nitrogen atmos-
phere. n-Hexane was either distilled over sodium under a nitrogen atmos-
phere or the dehydrated solvent (<10 ppm; Kanto Chemical Co., Inc.)
was dried by a solvent dispensing system (Glass Contour) under a nitro-
gen atmosphere. THF was either distilled over sodium diphenylketyl
under
a nitrogen atmosphere or the dehydrated solvent (<10 ppm;
Kanto Chemical Co., Inc.) was dried with the solvent dispensing system
under a nitrogen atmosphere. All reactions were carried out under an
inert gas atmosphere.
[a] calculated by natural population analysis. [b] calculated with WBI.
Conclusions
Reaction of Compound 1a with Benzaldehyde
A solution of compound 1a (112 mg, 0.29 mmol) and benzaldehyde
(95 mg, 0.90 mmol) in toluene (1.5 mL) was heated at 1208C for 48 h.
After the solvent was evaporated, the remaining benzaldehyde was re-
moved at 1008C under vacuum. The resulting pale-yellow oil (133 mg)
was subjected to recycling HPLC (n-hexane/EtOAc, 20:1) to give com-
pound 5a (105 mg, 74% yield) as a white solid.
5a: M.p. 508C (dec. in a sealed tube); 1H NMR (C6D6): d=0.81 (s, 6H),
2.02 (s, 6H), 2.22 (s, 3H), 2.25 (s, 3H), 2.31 (s, 6H), 4.62 (s, 2H), 6.76–
6.88 (m, 6H), 7.03–7.10 (m, 3H), 7.14 (t, J=8 Hz, 1H), 7.31 (td, J=8 Hz
and 1 Hz, 1H), 7.71 ppm (d, J=8 Hz, 1H); 13C NMR (CDCl3): d=3.89
(CH3), 20.94 (CH3), 21.22 (CH3), 21.92 (CH3), 25.19 (CH3), 69.47 (CH2),
À
In conclusion, Ar H ligand exchange between the silicon
atom and the boron atom in compound 1 occurred upon
À
heating to convert the Si H function into the more-labile
B H function, which, in turn, underwent hydroboration in
the presence of carbonyl compounds. The Ar H ligand-ex-
change mechanism was clarified by quantum-mechanical
computations. The hydride transfer from the silicon atom to
the boron atom is the key step in reaching the transition
states. The transition states include the tricoordinate silyl-
À
À
Chem. Asian J. 2012, 7, 546 – 553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
551

A. Kawachi et al.
FULL PAPERS
127.04 (CH), 127.18 (CH), 127.63 (CH), 127.84 (CH), 127.86 (CH),
128.98 (CH), 130.56 (CH), 134.21 (CH), 134.75 (C), 137.42 (C), 137.54
(C), 138.80 (C), 139.66 (CH), 142.37 (br, C), 143.39 (C), 149.62 ppm (C)
(two tertiary carbon atoms could not be observed); 29Si NMR (CDCl3):
d=À8.5 ppm (s); 11B NMR (CDCl3): d=48 ppm (br); elemental analysis
calcd (%) for C33H39BOSi: C 80.80, H 8.01; found: C 80.41, H 7.63%.
137.59 (C), 138.37 (C), 138.63 (C), 138.79 (CH), 142.40 (C), 144.0 (br, C),
144.47 ppm (C); 11B NMR (CDCl3): d=48 ppm (br); 29Si NMR (CDCl3):
d=À27.66 ppm (d, 1JSiÀH =211 Hz); elemental analysis calcd (%) for
C37H39BOSi: C 82.51, H 7.30; found: C 82.34, H 7.08%.
Heating Compound 1a in Refluxing Toluene
Heating a solution of compound 1a (12 mg, 0.025 mmol) to reflux in
[D8]toluene (0.5 mL) for 1 day did not alter the H NMR spectra.
Reaction of Compound 1b with Benzaldehyde
1
A solution of compound 1b (150 mg, 0.29 mmol) and benzaldehyde
(95 mg, 0.90 mmol) in toluene (1.5 mL) was heated at 1208C for 96 h.
After the solvent was evaporated, the remaining benzaldehyde was re-
moved at 1008C under vacuum. The resulting white solid (193 mg) was
recrystallized from n-hexane at À188C to form colorless crystals of com-
pound 5b (83 mg, 46% yield). The mother liquor was concentrated in
vacuo. The residue (129 mg) was subjected to column chromatography
on silica gel (n-hexane/EtOAc, 30:1) and subsequently to recycling
HPLC (n-hexane/EtOAc, 40:1) to give compound 5b (26 mg, 14% yield)
as a white solid.
Heating Compound 1c in Refluxing Toluene
A solution of compound 1c (37 mg, 0.10 mmol) in [D8]toluene (0.5 mL)
was heated to reflux for 3 days to afford a complex mixture that was con-
1
firmed by H NMR spectroscopy.
Typical Procedure for the Reduction of Acetophenone with Compound 1
to form Benzyl Alcohol
A solution of compound 1c (114 mg, 0.31 mmol) and acetophenone
(42 mg, 0.35 mmol) in toluene (1.5 mL) was heated at 1208C for 24 h.
After the solvent was evaporated, the residue was dissolved in THF
(1.5 mL) and the reaction mixture was treated with a solution of TBAF
in THF (1m, 0.60 mL, 0.60 mmol) at RT for 1 day. To the reaction mix-
ture was added sat. NH4Cl aq. (3 mL). The aqueous layer was extracted
with Et2O (3ꢂ5 mL) and the combined organic layer was dried with
Mg2SO4. After evaporation, the colorless residue (262 mg) was subjected
to column chromatography on silica gel (n-hexane/EtOAc, 5:1) and sub-
sequently to recycling HPLC (n-hexane/EtOAc, 5:1) as an eluent to give
compound 8 (31 mg, 83% yield) as a colorless oil. 8: Rf =0.20; 1H NMR
(CDCl3): d=1.51 (d, J=7 Hz, 3H), 1.80 (1H), 4.91 (q, J=6 Hz, 4H),
7.28–7.30 (m, 1H), 7.34–7.40 ppm (m, 4H).
5b: Rf =0.21–0.28; m.p. 204.5–205.58C (in
a
sealed tube); 1H NMR
(CDCl3): d=1.67 (s, 6H), 1.97 (s, 6H), 2.29 (s, 3H), 2.33 (s, 3H), 4.19 (s,
2H), 6.41 (d, J=8 Hz, 2H), 6.74 (s, 2H), 6.79 (s, 2H), 6.98 (t, J=8 Hz,
2H), 7.11 (t, J=8 Hz, 1H), 7.20–7.26 (m, 5H), 7.27–7.33 (m, 3H), 7.46
(dd, J=8 Hz and 1 Hz, 1H), 7.53–7.59 ppm (m, 5H); 13C NMR (CDCl3):
d=21.14 (CH3), 21.22 (CH3), 21.65 (CH3), 26.21 (CH3), 69.62 (CH2),
126.91(CH), 127.05 (CH), 127.24 (CH), 127.61 (CH), 128.16 (CH), 128.35
(CH), 128.86 (CH), 129.41 (CH), 130.19 (CH), 130.43 (C), 136.56 (CH),
137.22 (C), 137.30 (C), 138.05 (CH), 138.39 (C), 138.83 (C), 139.10 (C),
140.81 (CH), 145.52 ppm (C) (three tertiary carbon atoms could not be
observed); 11B NMR (CDCl3): d=48 ppm (br); 29Si NMR (CDCl3): d=
À15.98 ppm (s); elemental analysis calcd (%) for C43H43BOSi: C 84.02,
H 7.05; found: C 83.71, H 6.81%.
À
Attempted Intermolecular H Mes Exchange Reaction
A solution of diphenylsilane (195 mg, 1.1 mmol) and dimesitylphenylbor-
ane (320 mg, 0.98 mmol) in toluene (1.0 mL) was heated at 1208C for
45 h. 1H NMR analysis of the reaction mixture showed no change in the
substrates.
Reaction of Compound 1c with Benzaldehyde
A solution of compound 1c (185 mg, 0.50 mmol) and benzaldehyde
(64 mg, 0.60 mmol) in toluene (2.0 mL) was heated at 1208C for 28 h.
After the solvent was evaporated, the remaining benzaldehyde was re-
moved at 1008C under vacuum. The resulting white solid (247 mg) was
subjected to recycling reverse-phase HPLC (CH3CN) to give compound
5c (99 mg, 42% yield) as a white solid.
5c: M.p. 508C (dec. in a sealed tube); 1H NMR (CDCl3): d=0.61 (d, J=
4 Hz, 3H), 1.77–1.93 (br, 3H), 1.96–2.10 (br, 3H), 2.23 (s, 6H), 2.26 (s,
3H), 2.31 (s, 3H), 4.68 (d, J=13 Hz, 1H), 4.81 (d, J=13 Hz, 1H), 5.52
(q, J=4 Hz, 1H), 6.72–6.83 (m, 4H), 6.99–7.07 (m, 2H), 7.16–7.23 (m,
3H), 7.27–7.29 (br, 1H), 7.34–7.43 (m, 2H), 7.69 ppm (d, J=7 Hz, 1H);
13C NMR (CDCl3): d=À2.03 (Si-CH3), 21.08 (CH3), 21.25 (CH3), 21.5–
22.1 (br, CH3), 24.22 (CH3), 69.38 (CH2), 127.05 (CH), 127.12 (CH),
127.15 (CH), 127.99 (CH), 128.45 (CH), 128.47 (CH), 130.41 (CH),
132.46 (CH), 135.74 (C), 137.57 (C), 138.37 (C), 138.55 (C), 138.75 (CH),
138.83 (C), 143.71 (C), 144.01 ppm (C) (two tertiary carbons could not be
observed); 11B NMR (CDCl3): d=48 ppm (br); 29Si NMR (CDCl3): d=
À28.3 ppm (d, 1JSiÀH =202 Hz); elemental analysis calcd (%) for
C32H37BOSi: C 80.65, H 7.83; found: C 80.29, H 7.47%.
Preparation of Compound 4a
To a solution of compound 1a (195 mg, 0.51 mmol) in THF (1.0 mL) was
added benzyl alcohol (0.061 mL, 0.56 mmol) via a microsyringe at RT. A
gas was evolved immediately. The reaction mixture was stirred at RT for
1 day and then concentrated in vacuo to afford a viscous oil. The residual
benzyl alcohol was removed azeotropically with n-hexane (2 mL) to give
compound 4a as a white solid. The white solid was washed with CH3CN
(4 mL) to give pure compound 4 (147 mg, 59% yield) as a white solid.
1
M.p. 123.0–124.08C (in a sealed tube); H NMR (C6D6): d=0.06–0.21 (br,
6H), 1.6–2.6 (br, 18H), 4.56 (s, 2H), 6.6–6.9 (br, 4H), 7.10–7.17 (m, 1H),
7.19–7.26 (m, 4H), 7.28–7.31 (m, 2H), 7.54 (d, 3J=8 Hz, 1H), 7.81 ppm
(d, 3J=8 Hz, 1H); 13C {1H} NMR (CDCl3): d=À0.16 (br), 21.22, 22.7–
23.8 (br), 64.41, 126.37, 126.86, 127.52, 127.76, 128.05, 128.6–128.8 (br),
129.06, 133.76, 34.79, 140.93, 142.4–142.8 (br), 156.14 ppm (two tertiary
carbon atoms could not be observed); 11B NMR (CDCl3): d=74 ppm
(br); 29Si NMR (C6D6): d=9.5 ppm. HRMS (ESI): m/z calcd for
[C33H39SiBO+Na]+: m/z 513.2755; found: 513.2757.
Reaction of Compound 1d with Benzaldehyde
Heating Compound 4a in Refluxing Toluene
A solution of compound 1d (130 mg, 0.30 mmol) and benzaldehyde
(98 mg, 0.92 mmol) in toluene (1.5 mL) was heated at 1208C for 24 h.
After the solvent was evaporated, the remaining benzaldehyde was re-
moved at 1008C under vacuum. The resulting white solid (158 mg) was
recrystallized from n-hexane at À308C to form colorless crystals of com-
pound 5d (109 mg, 67% yield).
5d: M.p. 133.0–134.08C (in a sealed tube); 1H NMR (C6D6): d=2.00 (s,
3H), 2.14 (s, 3H), 2.21 (s, 3H), 2.26 (s, 3H), 2.34 (s, 6H), 4.61 (d, J=
12 Hz, 1H), 4.70 (d, J=12 Hz, 1H), 6.43 (s, 1H), 6.79 (s, 2H), 6.81 (s,
2H), 6.86–6.98 (m, 2H), 7.05–7.11 (m, 5H), 7.20–7.24 (m, 5H), 7.72–
7.79 ppm (m, 4H); 13C NMR (CDCl3): d=21.17 (CH3), 21.26 (CH3),
21.7–22.0 (br, CH3), 24.52 (CH3), 69.10 (CH2), 127.02 (CH), 127.33 (CH),
127.76 (CH), 127.84 (CH), 128.52 (CH), 128.65 (CH), 128.70 (CH), 128.9
(br, C), 130.036 (C), 130.55 (CH), 135.84 (CH), 137.36 (CH), 137.42 (C),
Heating a solution of compound 4a (40 mg, 0.08 mmol) in [D8]toluene
(0.45 mL) at 1208C for 48 h did not alter the H NMR spectra.
1
X-ray Crystallographic Analysis of Compounds 5b and 5d
Single crystals of compounds 5b and 5d suitable for X-ray crystallo-
graphic analysis were obtained by recrystallization from n-hexane. A
crystal was mounted using silicone oil onto a cryoloop (Hampton Re-
search). Data were collected at 173 K on a Rigaku SCX-Mini CCD with
graphite-monochromated MoKa radiation (l=0.71075 ꢁ). The structures
were solved by direct methods using the SIR97 program.[16] Refinement
on F2 was carried out with full-matrix least-squares procedures using the
SHELXL-97 program.[17] CCDC 838195 (5b) and CCDC 838196 (5d)
contain the supplementary crystallographic data for this paper. These
552
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 546 – 553

À
Intramolecular H Ar Ligand Exchange between Silicon and Boron
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data request/cif.
[6] a) A. Kawachi, M. Zaima, A. Tani, Y. Yamamoto, Chem. Lett. 2007,
36, 362; b) A. Kawachi, M. Zaima, Y. Yamamoto, Organometallics
2008, 27, 4691; c) A. Kawachi, H. Morisaki, M. Zaima, T. Teranishi,
Y. Yamamoto, J. Organomet. Chem. 2010, 695, 2167; d) A. Kawachi,
H. Morisaki, A. Tani, M. Zaima, Y. Yamamoto, Heteroat. Chem.
2011, 22, 471; e) L. Kçnczçl, A. Kawachi, D. Szieberth, Organome-
tallics, in press.
Computations
Computations were performed with the Gaussian 09 program package.[12]
To find the initial geometries, conformational searches of compounds 1,
6, 9-R, 10-R, 9-P, and 10-P were carried out using the MMFF molecular
mechanics model with Spartan’04 Windows prior to DFT calculations.[18]
The geometries were optimized at the B3LYP/6-31G(d) level. The opti-
mized geometries were subjected to single-point energy calculations at
the MP2/6-31G(d) level. The transition states were optimized at the
B3LYP/6-31G(d) level and single-point energy calculations were per-
formed at the MP2/6-31G(d) level. The transition states were confirmed
to have one imaginary frequency by frequency calculations at the
B3LYP/6-31G(d) level and they were found to connect the corresponding
reactants and products by IRC calculations at the same level. Natural
bond orbital (NBO)[14] and WBI were calculated at the MP2/6-31G(d)
level for the optimized geometries at the B3LYP/6-31G(d) level.
À
[7] For representative examples of Si H···B intramolecular interactions,
see: a) B. Wrackmeyer, O. L. Tok, Y. N. Bubnov, Angew. Chem.
1999, 111, 214; Angew. Chem. Int. Ed. 1999, 38, 124; b) B. Wrack-
meyer, W. Milius, O. L. Tok, Chem. Eur. J. 2003, 9, 4732; c) B.
Wrackmeyer, O. L. Tok, Y. N. Bubnov, Appl. Organomet. Chem.
2004, 18, 43; d) B. Wrackmeyer, O. L. Tok, W. Milius, A. Khan, A.
Badshah, Appl. Organomet. Chem. 2006, 20, 99.
[8] Heating compounds 1b and 1d in the absence of benzaldehyde in
toluene afforded dibenzosilaborins, which were formed by the intra-
À
À
molecular dehydrogenation of the B H unit and the ortho-C H
unit of a phenyl group on the silicon atom in hydroboranes 6b and
6d. This finding will be reported in the near future.
[9] a) M. Fujita, T. Hiyama, J. Am. Chem. Soc. 1984, 106, 4629; b) M.
Fujita, T. Hiyama, J. Am. Chem. Soc. 1985, 107, 8249; c) M. Fujita,
T. Hiyama, J. Org. Chem. 1988, 53, 5405.
Acknowledgements
[10] J. Emsley, The Elements, 2nd ed., Clarendon, Oxford, 1991.
[11] Forselected reviews of hydroboration reactions, see: a) E. J. Corey,
C. J. Helal, Angew. Chem. 1998, 110, 2092; Angew. Chem. Int. Ed.
1998, 37, 1986; b) N. Arai, T. Ohkuma in Modern Reduction Meth-
ods (Eds: P. G. Andersson, I. J. Munslow), Wiley-VCH, Weinheim,
2008, pp. 159–181; c) M. Zaidlewicz, M. M. Pakulski in Science of
Synthesis Stereoselective Synthesis (Eds: J. G. D. Vries, G. A. Mo-
lander, P. A. Evans), 2011, 2, pp. 59–131.
[12] Gaussian 09, Revision B.01, M. J. Frisch et al., Gaussian Inc., Wall-
ingford, CT, 2010, see the Supporting Information.
[13] B3LYP: a) A. D. Becke, Phys. Rev. A 1988, 38, 3098; b) C. Lee, W.
Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785; c) A. D. Becke, J.
Chem. Phys. 1993, 98, 5648; d) B. G. Johnson, P. M. W. Gill, J. A.
Pople, J. Chem. Phys. 1993, 98, 5612.
This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas “Molecular Activation Directed toward Straightfor-
ward Synthesis” from the Ministry of Education, Culture, Sports, Science
and Technology, Japan, and the Tokuyama Science Foundation, Japan.
[1] For selected reviews, see: a) I. Ojima, Z. Li, J. Zhu in The Chemistry
of Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Ape-
loig), Wiley, Chichester, 1998, Chap. 29, pp. 1687–1792; b) S. Ren-
dler, M. Oestreich in Modern Reduction Methods (Eds: P. G. Ander-
sson, I. J. Munslow), Wiley-VCH, Weinheim, 2008, pp. 183–207.
[2] For selected examples, see: a) S. Rendler, G. Auer, M. Oestreich,
Angew. Chem. 2005, 117, 7793; Angew. Chem. Int. Ed. 2005, 44,
7620; b) M. Murata, H. Yamasaki, K. Uogoshi, S. Watanabe, Y.
Masuda, Synthesis 2007, 2944; c) M. Tanabe, K. Osakada, Organo-
metallics 2010, 29, 4702; d) A. Lesbani, H. Kondo, Y. Yabusaki, M.
Nakai, Y. Yamanoi, H. Nishihara, Chem. Eur. J. 2010, 16, 13519.
[3] For selected reviews, see: a) C. Chuit, R. J. P. Corriu, C. Reye, J. C.
Young, Chem. Rev. 1993, 93, 1371; b) D. Kost, I. Kalikhman in The
Chemistry of Organic Silicon Compounds (Ed: Z. Rappoport, Y.
Apeloig), Wiley, Chichester, 1998, Vol. 2, Chapter 23, pp. 1339–
1446.
[14] NBO Version 3.1, E. D. Glendening, A. E. Reed, J. E. Carpenter, F.
Weinhold.
[15] For examples of interactions between silylium ions and aryl groups,
see: a) C. A. Reed, Z. Xie, R. Bau, A. Benesi, Science 1993, 262,
402; b) C. A. Reed, Acc. Chem. Res. 1998, 31, 325; c) N. Choi, P. D.
Lickiss, M. McPartlin, P. C. Masangane, G. L. Veneziani, Chem.
Commun. 2005, 6023; d) R. Panisch, M. Bolte, T. Mꢄller, Organome-
tallics 2007, 26, 3524; e) N. Lꢄhmann, H. Hirao, S. Shaik, T. Mꢄller,
Organometallics 2011, 30, 4087; f) P. Romanato, S. Duttwyler, A.
Linden, K. K. Baldridge, J. Y. Siegel, J. Am. Chem. Soc. 2011, 133,
11844.
[16] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giaco-
vazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R. Spagna, J.
Appl. Crystallogr. 1999, 32, 115.
[17] G. M. Sheldrick, SHELX-97, University of Gçttingen, Gçttingen,
Germany, 1997.
[4] For BACHTUNGTRENNUNG(C6F5)3-catalyzed reaction of hydrosilanes with alcohols, see:
a) J. M. Blackwell, K. L. Foster, V. H. Beck, W. E. Piers, J. Org.
Chem. 1999, 64, 4887; b) V. Gevorgyan, M. Rubin, S. Benson, J.-X.
Liu, Y. Yamamoto, J. Org. Chem. 2000, 65, 6179; c) J. Chojnowski,
S. Rubinsztajn, J. A. Cella, W. Fortuniak, M. Cypryk, J. Kurjata, K.
Kꢃzmierski, Organometallics 2005, 24, 6077.
[5] For BACHTUNGTRENNUNG(C6F5)3-catalyzed reaction of hydrosilanes with carbonyl com-
[18] Spartan ꢅ04 Windows, Wavefunction, Inc., Irvine, CA, 2004.
pounds, see: a) D. J. Parks, W. E. Piers, J. Am. Chem. Soc. 1996, 118,
9440; b) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000,
65, 3090; c) J. M. Blackwell, D. J. Morrison, W. E. Piers, Tetrahedron
2002, 58, 8247; d) S. Rendler, M. Oestreich, Angew. Chem. 2008,
120, 6086; Angew. Chem. Int. Ed. 2008, 47, 5997.
Received: August 10, 2011
Published online: December 23, 2011
Chem. Asian J. 2012, 7, 546 – 553
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
553