Electrochemical synthesis of dendritic diarylcarbenium ion pools

Article abstract of DOI:10.1016/j.tet.2011.04.065

Dendritic diarylcarbenium ion pools were synthesized by the low temperature electrochemical oxidation of the corresponding dendritic (diarylmethyl) trimethylsilanes, which were prepared by use of the iterative method consisting of electrochemical activati

Full text of DOI:10.1016/j.tet.2011.04.065

Tetrahedron 67 (2011) 4664e4671
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Electrochemical synthesis of dendritic diarylcarbenium ion pools
*
Toshiki Nokami, Takashi Watanabe, Naoki Musya, Takafumi Suehiro, Tatsuya Morofuji, Jun-ichi Yoshida
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 March 2011
Received in revised form 8 April 2011
Accepted 14 April 2011
Available online 27 April 2011
Dendritic diarylcarbenium ion pools were synthesized by the low temperature electrochemical oxidation
of the corresponding dendritic (diarylmethyl)trimethylsilanes, which were prepared by use of the iter-
ative method consisting of electrochemical activation and FriedeleCrafts type coupling. Time-course
NMR studies revealed that thermal stability of dendritic diarylcarbenium ions depends both on the
generation of the dendritic structure and on the para-substituents of the terminal phenyl groups.
Dendritic diarylcarbenium ions up to the third generation exhibited high reactivity as a carbon
electrophile.
Keywords:
Dendrimer
Carbocation
Ó 2011 Elsevier Ltd. All rights reserved.
Electrochemical oxidation
FriedeleCrafts alkylation
1. Introduction
anodic
X
C
Nu
C
Nu-
oxidation
+
C
Organic cations (carbocations and onium ions) are important
reactive intermediates in organic synthesis.1 It is noteworthy that
the manner in which we carry out reactions of organic cations is
different from that for carbanions. Usually, organic cations are
produced in the presence of nucleophiles to achieve a desired
transformation. In contrast, carbanions are produced and accu-
mulated in a solution in the absence of electrophiles, and an elec-
trophile is added to the solution of the pre-formed carbanion. This
is probably because organic cations that are often used in organic
synthesis are unstable and transient in conventional reaction media
and should be trapped in situ by nucleophiles immediately after
their preparation. In this context, development of a new method
that enables production and accumulation of organic cations in the
absence of nucleophiles is strongly needed for expanding the scope
of cation chemistry in organic synthesis. In 1999 we have de-
veloped the ‘cation pool’ method,2 whereby organic cations are
produced in the absence of nucleophiles and are used for the
subsequent reactions (Fig. 1). Various organic cation including
N-acyliminium ions and alkoxycarbenium ions can be produced
and accumulated as ‘cation pools’.
Bu4NBF4/CH2Cl2
-78 oC
X = H or SiMe3
cation pools
F
F
+
+
O
O
+
+
N
N
+
n-Bu COOMe
+
COOMe
O
H
Fig. 1. The general scheme of the ‘cation pool’ method and some examples of organic
cations, which have been produced and accumulated as ‘cation pools’.
carried out at low temperatures, such as ꢀ78 ꢁC. Counter anions,
which are considered to be very weak nucleophiles, such as BFꢀ4 are
normally used to prevent the nucleophilic attack on the cationic
center. After electrolysis is complete, a nucleophile is then added to
obtain the desired product. The ‘cation pool’ method serves as
a flexible way of using organic cations in organic synthesis in
comparison with the conventional in situ preparation method. It is
also important to note that the ‘cation pool’ method is applicable
to spectroscopic characterization and studies on stability and
reactivity of unstable organic cations.
Recently, we developed an iterative process based on the ‘cation
pool’ method, which consists of a sequence of synthesis of a dia-
rylcarbenium ion pool4 followed by coupling with (diphe-
nylmethyl)trimethylsilane as a building block (Fig. 2).5 The process
enables convergent synthesis of dendritic molecules.6 During the
course of this study, we have produced and accumulated dendritic
The ‘cation pool’ method is based on the irreversible oxidative
production of organic cations. In the first step, a cation precursor is
electrochemically3 oxidized to produce and accumulate an organic
cation in the solution in the absence of a nucleophile. To avoid
thermal decomposition of the cation, the electrolysis should be
* Corresponding author. Tel.: þ81 75 3832726; e-mail address: yoshida@
sbchem.kyoto-u.ac.jp (J.-ichi Yoshida).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.04.065

T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
4665
cation production
and accumulation
coupling
X
X
anodic
oxidaton
H
H
SiMe3
[G-N]+
X
X
X
X
next
sequence
H
SiMe3
[G-N]
X
X
H
SiMe3
[G-(N+1)]
Fig. 2. An iterative process based on the sequential synthesis of diarylcarbenium ions
and coupling with (diphenylmethyl)trimethylsilane.
Fig. 3. X-ray structure of 3b. Thermal ellipsoids are drawn at the 50% probability level.
diarylcarbenium ions in the solution. To the best of our knowledge,
structures, stability, and reactivity of such large-size organic cations
have not been studied comprehensively.7 In this article, we report
the full details of our studies on synthesis, characterization, and
thermal stability of dendritic diarylcarbenium ion pools.
H
SiMe3
H H
2
E
ox = 1.81 V (vs SCE)
Eox = 0.98 V (vs SCE)
2. Results and discussion
Fig. 4. The oxidation potential of diphenylmethane and (diphenylmethyl)trimethylsi-
lane (2).
We initiated our study by synthesizing precursors of the first
generation dendritic diarylcarbenium ion pools. Although it was
difficult to produce diphenylcarbenium ion (1a) from diphenyl-
methane by the cation pool method,2o the low temperature elec-
trochemical oxidation of di(p-fluorophenyl)methane in Bu4NBF4/
CH2Cl2 using an H-type divided cell at ꢀ78 ꢁC (3.0 F/mol) gave
a pool of di(p-fluorophenyl)carbenium ion (1b), which was allowed
to react with (diphenylmethyl)trimethylsilane (2) to give the
organosilicon precursor 3b [G-1] in 79% yield (Scheme 1).8 The
structure of 3b was confirmed by the X-ray crystal analysis. An
ORTEP diagram of 3b is shown in Fig. 3.
To confirm the effect of the trimethylsilyl group at the benzylic
position on the reactivity, the reaction of 1b with dipheylmethane
was carried out under the same condition. The reaction did not give
the disubstituted compound. Instead 1:1 adduct 4 was obtained in
very low yield (11%).
F
anodic
oxidation
X
X
X
X
F
H H
4
Bu4NBF4, CH2Cl2
-78 oC
H
H H
1
1a (X = H)
1b
X
X
(X = F)
Diarylcarbenium ions 1 were also easily produced in situ by the
conventional chemical method using the corresponding diary-
lmethyl chlorides as precursors (Scheme 2). In this case simple
diphenylcarbenium ion 1a could be produced and used for the
reaction with 2. The reaction of diphenylmethyl chloride with
borontrifluoride diethyletherate (BF3$OEt2) in the presence of 2 at
H SiMe3
2
X
X
-78 oC
H
SiMe3
3
[G-1] 3a (X = H), -
X
X
3b
(X = F), 79%
.
X
X
BF3 OEt2
Scheme 1. Preparation of precursors of the first generation of dendritic diary-
lcarbenium ions 3 [G-1].
neat, 70 oC
H
Cl
H
(X = H or F)
Choice of a building block was important to construct dendritic
structure having the triarylmethane repeating unit. It was reported
that the introduction of a silyl group at the benzylic position in-
creases the HOMO level of the benzene ring to enhance its re-
X
X
H SiMe3
2
activity.9 The interaction of the CeSi
s-orbital with the p-orbital of
the benzene is responsible. In fact, the oxidation potential of 2 is
much lower than that of diphenylmethane (Fig. 4). It is also known
that if the outer-sphere electron transfer takes place, the CeSi bond
in the resulting radical cation is readily cleaved.10e12 However,
Mayr and co-workers reported that the outer-sphere electron
transfer is definitely excluded for the reaction of diarylcarbenium
ions with nucleophiles.4f
X
X
H SiMe3
3
3a
[G-1]
(X = H), 64%
(X = F), 81%
3b
Scheme 2. Preparation of precursors of the first generation of dendritic diary-
lcarbenium ions 3 [G-1] by the conventional chemical reaction.

4666
T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
70 ꢁC gave 3a in 64%. Compound 3b was also prepared by the same
method from di(p-fluorophenyl)methyl chloride in 81% yield.
Fortunately, the conventional Lewis-acid mediated reactions
afforded both organosilicon precursors 3a and 3b, which were
subsequently used as precursors for producing the first generation
dendritic diarylcarbenium ions 5 [G-1]þ using the ‘cation pool’
method (Scheme 3). The lowtemperatureelectrochemical oxidation
of 3a and 3b was carried out at ꢀ78 ꢁC in Bu4NBF4/CD2Cl2 to produce
5a and 5b, respectively (2.5 F/mol). The resulting anodic solution
was quickly transferred to an NMR tube with cold glass syringes.
1H NMR at ꢀ80 ꢁC (Fig. 5) indicated the selective formation 5a and
5b by the cleavage of the CeSi bond without affecting benzylic CeH
0
0
bonds (vide infra). There were two sets of peaks Hb/Hb and Hc/Hc ,
which were assigned as protons of phenyl groups adjacent to the
cation carbon. These peaks became time averaged signals above
ꢀ40 ꢁC due tothe freerotation of phenyl groups (See Supplementary
data for details). The accumulation of 5a and 5b was also confirmed
by their reduction with Et3SiH to give 6a and 6b, respectively.
X
X
X
X
H
[G-1]
SiMe3
3a
3
(X = H)
(X = F)
3b
Bu4NBF4
anodic
oxidation
CD2Cl2, -78 oC
X
X
1
Fig. 5. H NMR spectra of dendritic diarylcarbenium ions 5a (a) and 5b (b) at ꢀ80 ꢁC.
X
X
Ha
5 [G-1]+
9.87 5a (X = H)
5b
9.92
(X = F)
Et3SiH
X
X
Fig. 6. Decomposition profiles of the first generation of dendritic diarylcarbenium ions
5 [G-1]þ obtained by time-course NMR analyses (5a: at ꢀ20 ꢁC and 5b: at 0 ꢁC) (rel-
ative intensity of Ha proton at 15 min).
X
X
H H
6a (X = H)
6b
Presumably the coupling of the cationic carbon of 5a and the para-
position of terminal phenyl groups took place to give the oligomers.
In the case of 5b, however, the para-positions are substituted by
fluorine atoms, and therefore the phenyl groups are less reactive
toward the cationic species, making 5b thermally more stable.
Upon decomposition, two signals appeared at around 4 ppm, which
are assigned to ArCH2Ar, in 1H NMR spectrum (See Supplementary
data for details). The signal at 3.94 ppm corresponds to 6b, in-
dicating that simple reduction of 5b occurred, although the hydride
source is not clear at present. The signal at 4.27 ppm seems to in-
dicate the formation of triarylcarbenium ion, which is similar to 7
(Scheme 4), because the chemical shift is very similar. This means
that the migration of the cationic center took place, although the
mechanism of hydride shift is not clear at present.
(X = F)
Scheme 3. Preparation of the first generation of dendritic diarylcarbenium ions 5
[G-1]þ by the ‘cation pool’ method and the subsequent treatment with triethylsilane.
1H NMR spectra of the first generation dendritic diary-
lcarbenium ions did not change during measurements at ꢀ80 ꢁC.
Thus, the thermal stability of 5a and 5b was investigated by the
time-course NMR analysis at elevated temperature (Fig. 6). The
intensity of the signal of the methine proton (Ha) of 5a decreased
gradually and disappeared completely in 4 h at ꢀ20 ꢁC. De-
composition of 5b was slower even at higher temperatures, such as
0 ꢁC. Decomposition of 5a led to the formation of oligomeric
products, although it was difficult to fully characterize them.
In the preparation of the first generation dendritic diary-
lcarbenium ions 5a and 5b, the effect of the trimethylsilyl (Me3Si)

T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
4667
ions 5 were allowed to react with 2 to obtain the corresponding
precursors 8 in reasonable yields.14 The low-temperature electro-
chemical oxidation of 8a and 8b afforded the ‘cation pools’ of 9a
and 9b, respectively, which were also characterized by low-
temperature 1H and 13C NMR analyses.
6a
Bu4NBF4
anodic
oxidation
CD2Cl2, -78 o
C
(
13C-NMR)
207
The thermal stability of 9a and 9b was investigated by the time-
course NMR measurements. As shown in Fig. 7, 9b decomposed
slower than 9a, presumably because fluorine substituents at the
para-positions in 9b suppress the coupling of the cationic carbon
and terminal phenyl groups. The significant difference in thermal
stability between the first generation and the second generation is
interesting. The second generation dendritic diarylcarbenium ion
9a is less stable than the first generation 5a. In fact, 9a decomposed
completely in 1.5 h at ꢀ20 ꢁC, whereas it took 3.5 h for 5a to de-
compose at the same temperature. Similarly, it took 3.5 h for 9b to
decompose completely at 0 ꢁC, whereas it took 10 h for 5b to de-
compose at the same temperature. Upon decomposition, several
signals appeared at around 4 ppm, which are assigned to ArCH2Ar,
in 1H NMR spectrum (See Supplementary data for details). Pre-
sumably, both simple reduction and migration of the cationic
center took place, although the details are not clear at present.
H
H
H
H H
H
5a
194
(
13C-NMR)
7
67% conversion (5a:7 = 72:28)
Scheme 4. Preparation of the ‘cation pool’ from compound 6a as a precursor.
group as an electroauxiliary is remarkable. In fact, the oxidation
potential of 3a (1.26 V vs SCE), which has a Me3Si group at the
carbon bearing two aryl groups, is much lower than that of the
corresponding compound without the Me3Si group (6a, 1.73 V vs
SCE). It is also noteworthy that the carbon bearing two aryl groups
was oxidized selectively without affecting the carbons bearing
three aryl groups and that the CeSi bond was cleaved selectively
without affecting CeH bonds in the same molecule. In contrast, the
electrochemical oxidation of 6a gave a mixture of 5a and 7 (72:28);
the latter being produced by the oxidation of the carbon bearing
three aryl groups (Scheme 4). Therefore, the Me3Si group acted as
an electroauxiliary, which activates substrate molecules toward
electron transfer and controls the subsequent chemical process to
give the desired cation selectively.13
Next, preparation of the second generation dendritic diary-
lcarbenium ion pools 9 [G-2]þ was achieved using the same
method (Scheme 5). The first generation dendritic diarylcarbenium
5 [G-1]+
Fig. 7. Decomposition profiles (relative intensity of Ha proton at 15 min) of the first
and the second generation of dendritic diarylcarbenium ions 9a and 9b obtained by
time-course NMR analyses.
-78 oC
H
SiMe3
Ar
The third generation dendritic diarylcarbenium ion was pre-
pared in the similar manner (Scheme 6). Because of better thermal
stability, we focused on the fluorine-substituted dendritic diary-
lcarbenium ions. The precursor 10 [G-3] was prepared by the re-
action of 9b with 2 at 0 ꢁC in 62% yield,15 and was oxidized
electrochemically to synthesize the cation pool of 11 [G-3]þ, which
was characterized by the low-temperature NMR spectroscopy. 1H
NMR spectrum indicated the complete conversion of 10 and the
2
Ar
Ar
Ar
Ar
Ar
formation of 11 as a single species. A signal observed at
d 9.84
H SiMe3
Ar
Ar
(Ar = Ph) 80%
(singlet) was assigned to the proton adjacent to the cationic carbon.
Several attempts to observe the cationic carbon of 11 by the 13C
NMR were failed, although the reason is not clear at present.
The thermal stability of 11 [G-3]þ was also investigated by the
time-course NMR measurements and was compared with those of
lower generation dendritic diarylcarbenium ions 5b [G-1]þ and 9b
[G-2]þ. As shown in Fig. 8, 11 decomposed faster than 9b and 5b,
indicating that the thermal stability of the dendritic diary-
lcarbenium ions decreases with an increase in the generation of the
dendritic structure. Although the reason for the tendency has not
yet been fully clarified, it seems to be reasonable from 1H NMR
spectra of decomposition products (See Supplementary data for
details) to consider that migration of the cationic center is one of
the major pathways of decomposition of the dendritic diary-
lcarbenium ions. Thus, we assume that rapid migration of the
cationic center is responsible for faster decomposition of the higher
generation of dendritic diarylcarbenium ions because the number
of the triarylmethyl moiety is greater than that in the lower
generations.
8 [G-2]
8a
8b (Ar = p-FC6H4) 77%
anodic
Bu4NBF4
CD2Cl2, -78 oC
oxidation
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ha
Ar
9a
9.87
9.89
(Ar = Ph)
Ar = p-FC6H4
+
9
[G-2]
9b
Scheme 5. Preparation of the second generation of dendritic diarylcarbenium ion
pools 9 [G-2]þ by the ‘cation pool’ method.

4668
T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
+
[G-2]
11 [G-3]+
9b
-78 oC then 0 o
C
-78 oC then 0 oC
SiMe3
H
SiMe3
H
2
2
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar Ar
Ar Ar
Ar
Ar
Ar
Ar
SiMe3
H
Ar
Ar
Ar
Ar
SiMe3
10 [G-3]
62%
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
H
Ar
Ar
Ar
Ar
12
[G-4]
Bu4NBF4
46%
anodic
oxidation
CD2Cl2, -78 o
C
Ar Ar
Ar Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar = p-FC6H4
Scheme 7. Reaction of the third generation of dendritic diarylcarbenium ions 11
[G-3]þ with diphenylmethyltrimethylsilane.
+
Ha
Ar
Ar
Ar
Ar
9.84
generation in the solution based on the ‘cation pool’ method. The
dendritic diarylcarbenium ions that are reactive as an electrophile
have sufficient stability to characterize in detail by NMR analyses at
low temperature. We believe that these findings open a new pos-
sibility of carbocation chemistry, organic electrochemistry, and
dendrimer synthesis. Scope and limitations of functional groups
and further applications including the dendronized polymer syn-
thesis, taking advantage of high reactivity of dendritic diary-
lcarbenium ions, are currently in progress in our laboratory.
11 [G-3]+
Ar
Ar
Ar = p-FC6H4
Ar
Ar
Scheme 6. Preparation of the third generation of dendritic diarylcarbenium ions 11
[G-3]þ by the ‘cation pool’ method.
4. Experimental section
4.1. General
1H and 13C NMR spectra were recorded on JEOL ECA-600P (1H
600 MHz, 13C 150 MHz). Chemical shifts are reported using the
methine signal of CHCl3 at d d
7.26 (1H NMR) and 77.0 (13C NMR) as
internal standards. FAB mass spectra were recorded on JEOL JMS-
HX110A spectrometer. IR spectra were recorded on Shimadzu
IRAffinity-1 FT-IR spectrophotometer equipped with MIRacle ATR
sampling accessory. Melting points were measured on Electro-
thermal Mel-Temp manual melting point apparatus. Preparative gel
permeation chromatography was performed on Japan Analytical
Industry LC-918 (an eluent: CHCl3). X-ray single crystal structure
analysis was performed on RIGAKU R-AXIS RAPID. Dichloro-
methane was washed with water, distilled from P2O5, redistilled
from dried K2CO3 to remove a trace amount of acid, and stored over
molecular sieves 4A. Compounds and cation species 3b, 4, 5b, 8b,
9b, and 10 have already reported in the previous communication.5
Unless otherwise noted, all materials were obtained from com-
mercial suppliers and used without further purification.
Fig. 8. Decomposition profiles (relative intensity of Ha proton in 15 min) of the first
(5b), the second (9b), and third (11) generation of dendritic diarylcarbenium ions
obtained by time-course NMR analyses at 0 ꢁC.
Finally, the production and accumulation of 11 [G-3]þ was also
confirmed by the reaction with 2, which gave the corresponding 2:1
adduct 12 [G-4] in 46% yield, although the yield was not optimized
(FAB-MS: m/z calcd for C406H288F32AgSi [MþAgþ]: 5905.0840,
found: 5905.0845) (Scheme 7). This result clearly showed that se-
lective activation of carbonesilicon bond occurred with such a huge
organosilicon compound 10 and the third generation dendritic
diarylcarbenium ion 11 has reasonble reactivity as an electrophile.
4.2. Preparation of the first generation dendritic
3. Conclusion
organosilicon compounds
In conclusion, we have developed a new method for producing
and accumulating dendritic diarylcarbenium ions up to the third
4.2.1. The electrochemical method. The anodic oxidationwas carried
out in an H-type divided cell (4G glass filter) equipped with a carbon

T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
4669
felt anode (Nippon Carbon JF-20-P7, ca. 160 mg, dried at 250 ꢁC/
1 mmHg for 2.5 h before use) and a platinum plate cathode
(20 mmꢂ10 mm). In the anodic chamber were placed a solution of
4,40-difluorodiphenylmethane (75.1 mg, 0.368 mmol) and 0.3 M
Bu4NBF4/CH2Cl2 (8.0 mL). In the cathodic chamber were placed tri-
140.7, 140.3, 129.4, 129.2, 128.5, 128.2, 126.2, 56.4, 45.2, ꢀ1.7. HRMS
(FAB) m/z calcd for C42H40Si: 572.2899, found 572.908. IR (cmꢀ1):
3024,1597,1492,1447,1246. Bis(4-bis(4-fluorophenyl)methyl-phenyl)-
methyltrimethylsilane (3b). The reaction of chlorobis(4-fluorophenyl)
methane (10.0 g, 42 mmol) and trimethylsilyl-diphenyl methane
(3.99 g, 16.6 mmol) in BF3$OEt2 afforded a mixture of 3b (8.59 g,
13.3 mmol) in 80% yield (isolated yield by flash chromatography
and precipitation from methanol).
fluoromethanesulfonic acid (64 mL, 0.72 mmol) and 0.3 M Bu4NBF4/
CH2Cl2 (8.0 mL). The constant current electrolysis (12.0 mA) was
carried out at ꢀ78 ꢁC with magnetic stirring until 3.0 F/mol of
electricity was consumed. Then (diphenylmethyl)trimethylsilane
(36.2 mg, 0.151 mmol) was added to the anodic chamber at ꢀ78 ꢁC
and the mixture was stirred for 20 min. The resulting mixture was
treated with triethylamine (0.3 mL) at ꢀ78 ꢁC. After stirring at room
temperature for 10 min, the solvent was removed under reduced
pressureand the residuewas quickly filteredthrough a short column
(2ꢂ3 cm) of silica gel to remove Bu4NBF4 by using hexane/AcOEt¼
1/1 as an eluent. Removal of the solvent of the combined filtrate
under reduced pressure and the crude product thus-obtained was
purified with flash chromatography to obtain 3b as a colorless
crystal in 79% yield (NMR yield). Bis(4-bis(4-fluorophenyl)methyl-
phenyl)methyl-trimethylsilane (3b). Mp 125 ꢁC. 1H NMR (400 MHz,
4.3. The low-temperature and the time-course NMR analyses
of the first generation of dendritic diarylcarbenium ions
4.3.1. NMR analysis of the first generation of dendritic diarylcarbenium
ions. The anodic oxidation was carried out in an H-type divided cell
(4G glass filter) equipped with a carbon felt anode (Nippon Carbon
GF-20-P21E, ca. 160 mg, dried at 250 ꢁC/1 mmHg for 2.5 h before use)
and a platinum plate cathode (10 mmꢂ10 mm). In the anodic
chamber were placed 3a (115 mg, 0.20 mmol) and 0.3 M Bu4NBF4 in
CD2Cl2 (5.0 mL). In the cathodic chamber were placed tri-
fluoromethanesulfonic acid (35 ml, 0.40 mmol) and 0.3 M Bu4NBF4 in
CDCl3)
d
7.20e7.18 (m, 4H), 7.07e7.04 (m, 8H), 7.00e6.96 (m, 12H),
CD2Cl2 (5.0 mL). The constant current electrolysis (5.0 mA) was car-
ried out at ꢀ78 ꢁC with magnetic stirring. After 2.5 F/mol of electricity
was consumed, an aliquot of the anodic solution was transferred to
5.48 (s, 2H), 3.50 (s, 1H), 0.05 (s, 9H). 13C NMR (100 MHz, CDCl3)
d
161.2 (d, J¼236.5 Hz), 140.8, 140.0, 139.5 (d, J¼2.8 Hz), 130.6 (d,
J¼7.9 Hz), 128.9, 128.5, 115.0 (d, J¼21.1 Hz), 54.9, 45.4, ꢀ1.5. HRMS
(FAB) m/z calcd for C42H36F4Si: 644.2522, found 644.2523. IR (cmꢀ1):
2955, 1601, 1504, 1223, 1157. X-ray data for 3b: C42H36F4Si,
a 5 mm f NMR tube with a septum cap under Ar atmosphere at
ꢀ78 ꢁC. The NMR measurement was carried out at ꢀ80 ꢁC. Chemical
shifts were reported using signals of CH2Cl2 at 5.32 ppm (1H NMR)
and CD2Cl2 at 53.8 ppm (13C NMR) as standards. Selected data for the
first generation of dendritic diarylcarbenium ion 5a; 1H NMR
ꢀ
M¼644.80, triclinic, space group Pꢀ1 (No. 2), a¼10.9102(4) A,
3
ꢀ
ꢀ
ꢀ
b¼11.5590(4) A, c¼14.7982(5) A, ¼77.4670(10), V¼1744.86(11) A ,
b
Z¼2, Dc¼1.227 g/cm3,
m
¼1.17 cmꢀ1. Intensity dataweremeasured on
(600 MHz, CD2Cl2)
J¼8.3 Hz, 2H), 7.63 (d, J¼8.2 Hz, 2H), 7.56 (d, J¼8.2 Hz, 2H), 7.35e7.04
(m), 5.79 (s, 2H). 13C NMR (150 MHz, CD2Cl2)
194.0 (cationic carbon).
The anodic oxidation of 3b (208 mg, 0.32 mmol) afforded 5b.
Selected peaks for 5b, 1H NMR (600 MHz, CD2Cl2)
9.87 (s,1H), 8.37
d
9.87 (s, 1H), 8.38 (d, J¼8.2 Hz, 2H), 8.28 (d,
a Rigaku R-AXIS imaging plate area detector with graphite-
monochromated Mo K
a
q
radiation. The data were collected at
d
100ꢃ1 K to maximum 2
value of 27.5ꢁ. A total of 17,196 reflections
were collected. The structure was solved by SHELX-9716 and ex-
panded using Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were refined by using the
riding model. The final cycle of full-matrix least-squares refinement
d
(d, J¼7.6 Hz, 2H), 8.31 (d, J¼6.2 Hz, 2H), 7.63 (d, J¼8.2 Hz, 2H), 7.51
(d, J¼6.9 Hz, 2H), 7.00e6.88 (m), 5.72 (s, 2H), 5.50 (s, 4H). 13C NMR
(150 MHz, CD2Cl2)
d 194.4 (cationic carbon).
on F2 was based on 7912 observed reflections (I>2.00
s(I)) and 460
variable parameters and converged (largest parameter shift was
0.00 times its esd) with unweighted and weighted agreement fac-
tors of R¼0.041 (Rw¼0.149). GOF¼1.11, CCDC-802918. All calcula-
tions were performed using the Yadokari-XG crystallographic
software package.17
4.3.2. Preparation of the extended diarylmethane. The anodic oxi-
dation was carried out in an H-type divided cell (4G glass filter)
equipped with a carbon felt anode (Nippon Carbon JF-20-P7, ca.
160 mg, dried at 250 ꢁC/1 mmHg for 2.5 h before use) and a plati-
num plate cathode (20 mmꢂ10 mm). In the anodic chamber were
placed of bis(4-bis(phenyl)methyl-phenyl)methyltrimethylsilane
3a (229.0 mg, 0.400 mmol) and 0.3 M Bu4NBF4/CH2Cl2 (9.0 mL). In
the cathodic chamber were placed trifluoromethanesulfonic acid
(4-Bis(4-fluorophenyl)methylphenyl)phenylmethane (4). Synthesis
of the diarylcarbenium ion pool 1 from 4,40-difluorodiphenyl-
methane (78.1 mg, 0.38 mmol) and its reaction with diphenyl-
methane (28.9 mg, 0.17 mmol) afforded a mixture of 4 (7.0 mg,
0.021 mmol) as a colorless oil in 11% yield (isolated yield by pre-
(70 mL, 0.79 mmol) and 0.3 M Bu4NBF4/CH2Cl2 (9.0 mL). The con-
stant current electrolysis (9.0 mA) was carried out at ꢀ78 ꢁC with
magnetic stirring until 2.5 F/mol of electricity was consumed. Then
triethylsilane (121.0 mg, 1.04 mmol) was added to the anodic
chamber at ꢀ78 ꢁC and the mixture was stirred for 1 h. The
resulting mixture was treated with triethylamine (0.2 mL) at
ꢀ78 ꢁC. After stirring at room temperature for 10 min, the solvent
was removed under reduced pressure and the residue was quickly
filtered through a silica gel short column to remove Bu4NBF4 by
using hexane/AcOEt 1:1 as an eluent. Removal of the solvent of the
combined filtrate under reduced pressure and the crude product
thus-obtained was purified with flash chromatography and GPC to
obtain Bis(4-bis(phenyl)methylphenyl)methane (6a) as a pale yellow
solid in 54% yield (108 mg, 0.22 mmol). Mp 144 ꢁC. 1H NMR
(400 MHz, CDCl3) 7.30e7.24 (m, 10H), 7.23e7.18 (m, 4H), 7.12e7.08
(m, 10H), 7.03e7.00 (m, 4H), 5.50 (s, 2H), 3.92 (s, 2H). 13C NMR
(100 MHz, CDCl3) 144.0, 141.6, 139.0, 129.45, 129.40, 128.8, 128.3,
126.2, 56.5, 41.1. HRMS (FAB) m/z calcd for C39H32: 500.2504, found:
500.2504. IR (cmꢀ1): 3024, 1597, 1508, 1492, 1447.
parative GPC). 1H NMR (400 MHz, CDCl3)
d 7.31e7.27 (m, 2H),
7.22e7.19 (m, 3H), 7.13e7.11 (d, J¼8.0 Hz, 2H), 7.06e7.03 (m, 4H),
7.00e6.95 (m, 6H), 5.47 (s, 1H), 3.96 (s, 2H). 13C NMR (100 MHz,
CDCl3)
d
161.4 (d, J¼243.6 Hz),141.2,140.9,139.5 (d, J¼3.2 Hz),139.4,
130.7 (d, J¼7.9 Hz), 129.3, 129.0, 128.5, 126.1, 115.1 (d, J¼21.0 Hz),
54.9, 41.5. HRMS (FAB) m/z calcd for C26H20F2: 370.1533, found
370.1531. IR (cmꢀ1): 3028, 1601, 1504, 1223, 1157.
4.2.2. The conventional chemical method. Chlorodiphenylmethane
(10.0 g, 10.4 mmol), (diphenylmethyl)trimethylsilane (0.95 g,
3.94 mmol) were dissolved in BF3$OEt2 (2.84 g, 20.0 mmol). After
stirring at 70 ꢁC for 12 h, the reaction was treated with ice water
and extracted with AcOEt and dried over MgSO4. After evaporation
of the solvent, the crude product was purified with flash chroma-
tography and GPC to obtain 3a as a white solid in 64% yield (1.45 g,
2.53 mmol). Bis(4-bis(phenyl)methylphenyl)-methyltrimethylsilane
(3a). Mp 120 ꢁC. 1H NMR (400 MHz, CDCl3)
7.22e7.18 (m, 4H), 7.15e7.10 (m, 12H), 6.98 (d, 4H), 5.49 (s, 2H), 3.45
(s, 1H), 0.00 (s, 9H). 13C NMR (100 MHz, CDCl3)
144.14, 144.13,
d 7.29e7.25 (m, 8H),
Bis(4-bis(4-fluorophenyl)methylphenyl)methane (6b). Prepara-
tion of the first generation dendritic diarylcarbenium ion pool 5b
d

4670
T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
from 3b (1.93 g, 3.0 mmol) and its reaction with triethylsilane
(0.94 mL, 6.0 mmol) afforded a mixture of 6b (866 mg, 1.51 mmol)
as a white solid in 50% yield (isolated yield by preparative GPC). Mp
142 ꢁC. 1H NMR (400 MHz, CDCl3) 7.12 (d, J¼8.2 Hz, 4H), 7.06e7.03
(m, 8H), 7.00e6.96 (m, 12H), 5.47 (s, 2H), 3.94 (s, 2H). 13C NMR
(100 MHz, CDCl3) 161.4 (d, J¼243.5 Hz), 141.3, 139.5 (d, J¼3.6 Hz),
139.2, 130.7 (d, J¼7.9 Hz), 129.3, 129.0, 115.1 (d, J¼21.5 Hz), 54.9,
41.0. HRMS (EI) m/z calcd for C39H28F4: 572.2127, found: 572.2127.
IR (cmꢀ1): 2986, 1601, 1504, 1219, 1157.
obtain 8a (158 mg, 0.127 mmol) as a white solid in 80% yield. 1H
NMR (400 MHz, CDCl3) 7.32e7.28 (m, 16H), 7.24e7.21 (m, 8H),
7.16e7.14 (m, 20H), 7.03e6.99 (m, 20H), 5.53(s, 4H), 5.44 (s, 2H),
3.45 (s, 1H), 0.02 (s, 9H). 13C NMR (100 MHz, CDCl3)
144.0, 142.1,
141.6, 140.7, 140.4, 129.4, 129.3, 129.21, 129.17, 128.4, 128.2, 126.2,
56.5, 55.6, 45.2, ꢀ1.7. HRMS (FAB) m/z calcd for C94H80NaSi
[MþNa]þ: 1259.5922, found: 1259.5927. IR (cmꢀ1): 3024, 1597,
1493, 1450, 1250, 1018.
The second generation dendritic organosilicon compound 8b.
Synthesis of the ‘cation pool’ from 3b (1.04 g, 1.61 mmol) and its
reaction with benzhydryltrimethylsilane (154 mg, 0.638 mmol)
afforded a mixture of 8b (683 mg, 0.494 mmol) as a white solid in
77% yield (isolated yield by preparative GPC). 1H NMR (400 MHz,
d
d
4.3.3. NMR analysis of the ‘cation pool’ synthesized from the pre-
cursor without trimethylsilyl group. The anodic oxidation was car-
ried out in an H-type divided cell (4 G glass filter) equipped with
a carbon felt anode (Nippon Carbon JF-20-P7, ca. 100 mg, dried at
250 ꢁC/1 mmHg for 2.5 h before use) and a platinum plate cathode
(10 mmꢂ10 mm). In the anodic chamber were placed 6a (100 mg,
0.200 mmol) and 0.3 M Bu4NBF4 in CD2Cl2 (5.0 mL). In the cathodic
CDCl3)
d
7.14 (d, J¼8.3 Hz, 4H), 7.06e6.96 (m, 52H), 5.46 (s, 4H),
5.41 (s, 2H), 3.43 (s, 1H), ꢀ0.01 (s, 9H). 13C NMR (150 MHz, CDCl3)
d
161.6 (d, J¼242.7 Hz), 142.5, 141.4, 140.9, 140.4, 139.6, 130.8 (d,
J¼7.2 Hz), 129.5, 129.3, 129.1, 128.6, 115.3 (d, J¼21.5 Hz), 55.6, 55.0,
45.4, ꢀ1.6. HRMS (FAB) m/z calcd for C94H71F8Si [MꢀH]þ:
1379.5197, found 1379.5184. IR (cmꢀ1): 3036, 1601, 1504, 1223,
1157, 1018.
chamber were placed trifluoromethanesulfonic acid (35 mL,
0.40 mmol) and 0.3 M Bu4NBF4 in CD2Cl2 (5.0 mL). The constant
current electrolysis (5.0 mA) was carried out at ꢀ78 ꢁC with mag-
netic stirring. After 2.5 F/mol of electricity was consumed, the re-
The third generation dendritic organosilicon compound 10. Syn-
thesis of the ‘cation pool’ from 8b (346 mg, 0.25 mmol) and its
reaction with benzhydryltrimethylsilane (24.3 mg, 0.10 mmol)
afforded a mixture of 10 (178 mg, 0.062 mmol) as a pale yellow
solid in 62% yield (isolated yield by preparative GPC). 1H NMR
action mixture of the anodic chamber was transferred to a 5 mm
f
NMR tube with a septum cap under Ar atmosphere at ꢀ78 ꢁC. The
NMR measurement was carried out at ꢀ80 ꢁC. Chemical shifts are
reported using the signal of CH2Cl2 at d d 53.8
5.32 for 1H NMR and
for 13C NMR, respectively. Peaks of
and 193.9 are selected for determining the existence of diary-
lcarbenium ion 5a and peak of
4.22 (s, 2H, Hb (integral: 5.52)) and
d
9.87 (s, 1H, Ha (integral: 7.16))
(400 MHz, CDCl3) d 7.11e6.92 (m, 121H), 5.45 (s, 8H), 5.42 (s, 4H),
d
5.38 (s, 2H), 3.42 (s, 1H), ꢀ0.01 (s, 9H). 13C NMR (150 MHz, CDCl3)
d
d
161.4 (d, J¼244.2 Hz), 142.2, 141.5, 141.4, 140.7, 140.4, 139.5, 130.7
d
207.0 are selected for determining the existence of triar-
(d, J¼8.6 Hz), 129.4, 129.3, 129.22, 129.16, 129.06, 128.4, 115.2 (d,
J¼20.1 Hz), 55.8, 55.7, 54.9, 45.4, ꢀ1.7. HRMS (FAB) m/z calcd for
C198H144F16Si: 2853.0782, found 2853.0764. IR (cmꢀ1): 1601, 1504,
1223, 1157, 1018.
ylcarbenium ion 7. The conversion of the starting material 6a (Hc
(integral: 9.15)) and the ratio 5a/7 are determined from the integral
of protons Ha, Hb, and Hc.
The fourth generation dendritic organosilicon compound 12. Syn-
thesis of the ‘cation pool’ from 10 (529.4 mg, 0.185 mmol) and its
reaction with benzhydryltrimethylsilane (16.8 mg, 0.070 mmol)
afforded a mixture 12 (186 mg, 0.032 mmol) as a pale yellow solid
in 46% yield (isolated yield by preparative GPC). 1H NMR (400 MHz,
4.3.4. The time-course NMR analysis of the first generation of den-
dritic diarylcarbenium ions. A solution of the first generation of
dendritic diarylcarbenium ion 5 (ca. 0.5 mL, ca. 0.02 mmol) in the
anodic chamber was transferred to a 5 mm NMR tube with a sep-
tum cap under an argon atmosphere at ꢀ78 ꢁC. The NMR tube was
put into the cooling bath at the second temperature (0 ꢁC or
ꢀ20 ꢁC). The time-course NMR measurement was performed at the
second temperature until the peak of proton Ha disappeared. The
decompose profiles in Fig. 6 were obtained from these results (see
Supplementary data for details).
CDCl3)
d
7.10 (d, J¼8.0 Hz, 8H), 7.00e6.88 (m, 240H), 5.41(s, 16H),
5.38 (s, 8H), 5.35 (m, 6H), 3.41 (s, 1H), ꢀ0.03 (s, 9H). 13C NMR
(100 MHz, CDCl3)
d
161.4 (d, J¼244.2 Hz), 142.1, 141.9, 141.64, 141.56,
141.4, 140.7, 140.4, 139.4, 130.7 (d, J¼7.2 Hz), 129.4, 129.3, 129.2,
129.1, 128.4, 115.1 (d, J¼21.5 Hz), 55.8, 55.7, 54.9, 45.3, ꢀ1.7. HRMS
(FAB) m/z calcd for C406H288F32AgSiþ: 5905.0840, found:
5905.0845. IR (cmꢀ1): 1601, 1504, 1223, 1157, 1018.
4.4. Preparation of the second, the third, and the fourth
generation of dendritic organosilicon compounds
4.5. The low-temperature and the time-course NMR analyses
of the second and the third generation of dendritic
diarylcarbenium ions
The anodic oxidation was carried out in an H-type divided cell
(4G glass filter) equipped with a carbon felt anode (Nippon Carbon
JF-20-P7, ca. 160 mg, dried at 250 ꢁC/1 mmHg for 2.5 h before use)
and a platinum plate cathode (20 mmꢂ10 mm). In the anodic
chamber were placed a solution of 3a (234.8 mg, 0.410 mmol) and
0.3 M Bu4NBF4/CH2Cl2 (8.0 mL). In the cathodic chamber were
4.5.1. NMR analysis of dendritic diarylcarbenium ions. The anodic
oxidation was carried out in an H-type divided cell (4G glass filter)
equipped with a carbon felt anode (Nippon Carbon JF-20-P7, ca.
100 mg, dried at 250 ꢁC/1 mmHg for 2.5 h before use) and a plati-
num plate cathode (10 mmꢂ10 mm). In the anodic chamber were
placed 8a (166 mg, 0.13 mmol) and 0.3 M Bu4NBF4 in CD2Cl2
(5.0 mL). In the cathodic chamber were placed trifluorometh-
placed trifluoromethanesulfonic acid (70 mL, 0.79 mmol) and 0.3 M
Bu4NBF4/CH2Cl2 (8.0 mL). The constant current electrolysis
(9.0 mA) was carried out at ꢀ78 ꢁC with magnetic stirring until
2.7 F/mol of electricity was consumed. Then benzhydryl-
trimethylsilane (38.3 mg, 0.159 mmol) was added to the anodic
chamber at ꢀ78 ꢁC and the mixture was stirred for 1 h. The
resulting mixture was treated with triethylamine (0.2 mL) at
ꢀ78 ꢁC. After stirring at room temperature for 10 min, the solvent
was removed under reduced pressure and the residue was quickly
filtered through a short column (2ꢂ3 cm) of silica gel to remove
Bu4NBF4 by using hexane/AcOEt 1:1 as eluent. Removal of the
solvent of the combined filtrate under reduced pressure and the
crude product thus-obtained was purified with preparative GPC to
anesulfonic acid (35 mL, 0.40 mmol) and 0.3 M Bu4NBF4 in CD2Cl2
(5.0 mL). The constant current electrolysis (5.0 mA) was carried out
at ꢀ78 ꢁC with magnetic stirring. After 2.5 F/mol of electricity was
consumed, the reaction mixture of the anodic chamber was
transferred to a 5 mm
f NMR tube with a septum cap under Ar
atmosphere at ꢀ78 ꢁC. The NMR measurement was carried out at
ꢀ80 ꢁC. Chemical shifts are reported using the methylene signal of
CH2Cl2 at
d d
5.32 (1H NMR) and 53.8 (13C NMR) as internal stan-
dards, respectively. Selected peaks for 9a, 1H NMR (600 MHz,

T. Nokami et al. / Tetrahedron 67 (2011) 4664e4671
4671
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Am. Chem. Soc. 2007, 129, 1902e1903; (o) Nokami, T.; Shibuya, A.; Tsuyama, H.;
Suga, S.; Bowers, A. A.; Crich, D.; Yoshida, J. J. Am. Chem. Soc. 2007, 129,
10922e10928; (p) Okajima, M.; Soga, S.; Watanabe, T.; Terao, K.; Nokami, T.;
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CD2Cl2)
d
9.87 (s,1H), 8.36 (br s, 2H), 8.15 (br s, 2H), 5.70 (s, 2H), 5.52
193.7 (cationic carbon).
(s, 4H). 13C NMR (150 MHz, CD2Cl2)
d
The second generation dendritic diarylcarbenium ion 9b. The NMR
measurement was carried out at ꢀ80 ꢁC. Selected peaks for 9b, 1H
NMR (600 MHz, CD2Cl2)
d
9.87 (s,1H), 8.37 (d, J¼7.6 Hz, 2H), 8.31 (d,
J¼6.2 Hz, 2H), 7.63 (d, J¼8.2 Hz, 2H), 7.51 (d, J¼6.9 Hz, 2H),
7.00e6.88 (m), 5.72 (s, 2H), 5.50 (s, 4H). 13C NMR (150 MHz, CDCl3)
d
193.7, 167.0, 160.7 (d, J¼242.0 Hz), 147.4, 142.3, 139.8, 138.9, 138.3,
135.1, 133.3, 132.3, 130.4 (d, J¼6.0 Hz), 129.2, 129.1, 114.7 (d,
J¼20.2 Hz), 57.5.
3. Recent reviews on organic electrochemistry: (a) Moeller, K. D. Tetrahedron
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The third generation dendritic diarylcarbenium ion 11. The NMR
measurement was carried out at ꢀ60 ꢁC. Selected peaks for 11, 1H
NMR (600 MHz, CD2Cl2)
d 9.84 (br s, 1H), 8.31 (br s, 2H), 5.63 (br s,
2H), 5.42 (br s, 12H). Cationic carbon was not observed by 13C NMR.
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4.5.2. The time-course NMR analysis of dendritic diarylcarbenium
ions. A solution of the second and the third generation of dendritic
diarylcarbenium ions (ca. 0.5 mL, ca. 0.01 mmol for 9a, ca.
0.02 mmol for 9b, and ca. 0.01 mmol for 11) in the anodic chamber
was transferred to a 5 mm NMR tube with a septum cap under an
argon atmosphere at ꢀ78 ꢁC. The NMR tube was put into the
cooling bath at the second temperature (0 ꢁC or ꢀ20 ꢁC). The time-
course NMR measurement was performed at the second temper-
ature until the peak of proton Ha disappeared. The decompose
profiles in Figs. 7 and 8 were obtained from these results (see
Supplementary data for details).
ꢁ
Mayr, H. Eur. J. Org. Chem. 2010, 1435e1439; (q) Denegri, B.; Matic, M.; Kronja,
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Acknowledgements
This work was partially supported by the Grant-in-Aid for Sci-
entific Research on Innovative Areas (No. 2105) from the MEXT. T.N.
thanks the Asahi Glass Foundation and the Research for Promoting
Technological Seeds from JST for financial supports. The authors
thank Mr. Haruo Fujita, Mr. Tadashi Yamaoka, and Dr. Keiko Kuwata
of Kyoto University for assistance in NMR, MS, and X-ray crystal
analyses.
€
Newkome, G. R., Moorefield, C. N., Vogtle, F., Eds.; Wiley-VHC: Weinheim, 2001;
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Supplementary data
These data include 1H and 13C NMR spectra, the VT-NMR anal-
yses, and the time-course NMR analyses of dendritic diary-
lcarbenium ions. Supplementary data related to this article can be
found online at doi:10.1016/j.tet.2011.04.065.
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References and notes
1. (a) Superelectrophiles and Their Chemistry; Olah, G. A., Klumpp, D. A., Eds.; John
Wiley: Hoboken, New Jersey, 2008; (b) Recent Developments in Carbocation
and Onium Ion Chemistry; Laali K. K. Ed.; ACS Symposium Series 965; American
Chemical Society: Washington, DC, 2007; (c) Carbocation Chemistry; Olah, G. A.,
Prakash, G. K. S., Eds.; John Wiley: Hoboken, New Jersey, 2004; (d) Onium Ions;
Olah, G. A., Laali, K. K., Wang, Q., Prakash, G. K. S., Eds.; John Wiley: New York,
NY, 1998; (e) Stable Carbocation Chemistry; Prakash, G. K. S., Schleyer, P. V. R.,
Eds.; John Wiley: New York NY, 1997.
12. Chemical electron transfer: (a) Baciocchi, E.; Crescenzi, M.; Fasella, E.; Mattioli,
M. J. Org. Chem. 1992, 57, 4684e4689; (b) Hirao, T.; Morimoto, C.; Takada, T.;
Sakurai, H. Tetrahedron 2001, 57, 5073e5079.
13. (a) Yoshida, J.; Maekawa, T.; Murata, T.; Matsunaga, S.; Isoe, S. J. Am. Chem. Soc.
1990, 112, 1962e1970; (b) Yoshida, J.; Nishiwaki, K. J. Chem. Soc., Dalton Trans.
1998, 2589e2596.
14. Although we applied the same Lewis-acid mediated reaction, 8a was obtained
in 31% yield.
2. (a) Yoshida, J.; Suga, S.; Suzuki, S.; Kinomura, N.; Yamamoto, A.; Fujiwara, K. J.
Am. Chem. Soc. 1999, 121, 9546e9549; (b) Suga, S.; Suzuki, S.; Yamamoto, A.;
Yoshida, J. J. Am. Chem. Soc. 2000, 122, 10244e10245; (c) Suga, S.; Okajima, M.;
Fujiwara, K.; Yoshida, J. J. Am. Chem. Soc. 2001, 123, 7941e7942; (d) Suga, S.;
Suzuki, S.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 30e31; (e) Suga, S.; Watanabe,
M.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 14824e14825; (f) Yoshida, J.; Suga, S.
Chem.dEur. J. 2002, 8, 2651e2658; (g) Suga, S.; Nishida, T.; Yamada, D.; Nagaki,
A.; Yoshida, J. J. Am. Chem. Soc. 2004, 126, 14338e14339; (h) Nagaki, A.;
Kawamura, K.; Suga, S.; Ando, T.; Sawamoto, M.; Yoshida, J. J. Am. Chem. Soc.
15. The yield of 10 was decreased to 40% when the reaction was performed at
ꢀ78 ꢁC.
16. Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112e122.
17. Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. J. Cryst. Soc. Jpn. 2009, 51, 218e222.