Palladium-Catalyzed β-Elimination of Aminoboranes from (Aminomethylsilyl)boranes Leading to the Formation of Silene Dimers

Article abstract of DOI:10.1021/acs.organomet.7b00695

Silylboronic esters bearing a N,N-dialkylaminomethyl group on the silicon atoms were synthesized and used in palladium-catalyzed reactions. Five silylboronic esters were synthesized through a reaction of the [(N,N-dialkylaminomethyl)diorganosilyl]lithiums

Full text of DOI:10.1021/acs.organomet.7b00695

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX-XXX
Palladium-Catalyzed β‑Elimination of Aminoboranes from
(Aminomethylsilyl)boranes Leading to the Formation of Silene
Dimers
Toshimichi Ohmura,* Hiroki Nishiura, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Silylboronic esters bearing a N,N-dialkylamino-
methyl group on the silicon atoms were synthesized and used in
palladium-catalyzed reactions. Five silylboronic esters were
synthesized through
a reaction of the [(N,N-
dialkylaminomethyl)diorganosilyl]lithiums with i-PrOB(pin).
An addition reaction of the silylboronic ester to ethynylbenzene
(silaboration) took place efficiently by a phosphine- and
isocyanide-free palladium catalyst to give (E)-1-silyl-2-boryl-1-
phenylethene in a regio- and stereoselective manner. On the
other hand, the silylboronic esters underwent β-elimination
reactions in the presence of a palladium catalyst and styrene to afford 1,3-disilacyclobutanes and aminoboranes in good yields.
This may be due to the generation of silene intermediates during the reaction. It should be noted that use of styrene as a ligand
was essential for the generation of the silene species.
Scheme 1. Mechanistic Classification of Transition-Metal-
Catalyzed Generation of Silene Intermediate
INTRODUCTION
■
Since a report on the first experimental evidence in 1967,1 there
has been much interest in the isolation, structure, reactivity, and
synthetic application of silene derivatives (R2SiCR′2), which
contain a highly reactive silicon−carbon double bond.2 With a
view to the synthetic utilization of silene species, various
thermal and photochemical reactions have been utilized for the
generation of silene species under rather harsh reaction
conditions, including the use of strong bases or UV irradiation,
which often make their selective reaction difficult. Use of
transition-metal catalysis for the generation of silene species or
their equivalents has also attracted much interest, because these
catalysts may promote not only silene generation but also
subsequent reactions through the formation of transition-
metal−silene complexes. Several transition-metal-catalyzed
reactions involving the generation of silene have been reported
to date.3−6 They can be classified into four categories on the
basis of the reaction mechanism (Scheme 1): (a) oxidative
addition of the Si−H bond of alkylsilane followed by β-H
elimination to afford η2-silene complex A,3 (b) oxidative
addition of the Si−Si bond of alkynyldisilane and subsequent
1,3-rearrangement to 3-sila-1,2-propadienyl complex B,4 (c)
oxidative addition of the Si−Si bond of 3,4-benzo-1,2-
disilacyclobutene to give o-quinodisilane C,5 and (d) formation
of a silyl-substituted carbene complex from an α-diazocarbonyl
compound followed by 1,2-rearrangement of one of the
substituents on the silicon atom to afford D.6 Reaction types
a−c, which rely on Si−E (E = H, Si) bond activation, require
high reaction temperatures (generally 150−220 °C). Although
reaction type d can be carried out under rather mild conditions,
the accessible silene is limited to that stabilized with carbonyl.
For wider utilization of silene or its equivalents in the synthesis
of organosilicon compounds, new catalytic reactions for the
generation of silene under mild conditions are highly desirable.
Received: September 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00695
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In this regard, we have focused our attention on the use of Si−
B bond activation for mild generation of silene species.
Scheme 3. Preparation of Silylboronic Esters 3 Bearing an
N,N-Dialkylaminomethyl Group on Silicon
We and other groups have developed various catalytic
reactions of silylboronic esters through activation of Si−B
bonds by transition-metal catalysts.7−9 In addition to simple
addition reactions including asymmetric variants, silaborative
C−C bond forming reactions have also been established.9 The
Si−B bond can be activated by palladium catalysts under mild
reaction conditions.7a−i,l Most notably, we have reported that
silylboronic esters bearing a dialkylamino group on the silicon
atom reacts as a synthetic equivalent of silylene (R2Si:) under
palladium-catalyzed reaction conditions.10 In this conversion,
elimination of aminoborane from the boryl(aminosilyl)
palladium intermediate E, which is formed by oxidative
addition of the Si−B bond of silylborane to the Pd(0) species,
is involved as the key step (Scheme 2i). Upon looking at the
Scheme 2. Concept of Catalytic Generation of Silylene and
Silene: α- and β-Elimination Reactions of Amino-Substituted
Silylboronic Esters
dium. The reaction of 3Aa with ethynylbenzene was examined
in the presence of a palladium catalyst. We envisioned that if
“MePhSiCH2” is formed through β-elimination, it is
spontaneously trapped by ethynylbenzene. Indeed, 3Aa reacted
with ethynylbenzene (1.2 equiv) efficiently in toluene at room
temperature in the presence of Pd(OAc)2 (1 mol %) (eq 1).
generation of silylene species via the formal α-elimination of
aminoborane from (aminosilyl)borane, we envisioned that
silene species could be similarly generated via formal β-
elimination of aminoborane from (aminomethylsilyl)boronic
ester through activation of the Si−B bond to form F (Scheme
2ii).11 Herein, we describe the synthesis of (aminomethylsilyl)-
boronic esters and their reactivities in the presence of palladium
catalysts.
However, the reaction gave (Z)-1-boryl-2-silylethene 4Aa in
79% yield, indicating that silaboration of alkyne took place
instead of β-elimination.7 11B NMR chemical shifts of 4Aa (δ
11.9 ppm) indicated coordination of the nitrogen atom to the
boron center. It should be noted that the silaboration
proceeded efficiently without the use of tertiary phosphines
or isocyanides as ligands, which have been essential in
palladium-catalyzed silaboration of alkynes with Me2PhSi−
B(pin).7a−i,l Indeed, silaboration of ethynylbenzene with
Me2PhSi−B(pin) did not proceed under identical conditions
even in the presence of N-methylpiperidine (1.0 equiv) (eq 2).
These results indicate that coordination of the nitrogen atom of
3Aa to palladium promotes activation of the intramolecular Si−
B bond.
Reaction of 3 in the Presence of Styrene: Palladium-
Catalyzed β-Elimination Reaction. Styrene was then
examined as a reaction partner of 3Aa. The reaction was
carried out in toluene at room temperature in the presence of
Pd(OAc)2 (1 mol %) and styrene (0.5−2.2 equiv) (entries 1−
3, Table 1). In sharp contrast to the reaction with
ethynylbenzene, this reaction gave aminoboronate 6a in 46−
93% yields within 5 min, indicating that β-elimination took
place efficiently under the mild conditions. The reaction also
afforded 1,3-dimethyl-1,3-diphenyl-1,3-disilacyclobutane (5A),
RESULTS AND DISCUSSION
■
Preparation of Silylboronic Esters Bearing an N,N-
Dialkylaminomethyl Group on Silicon. According to the
procedure reported by Strohmann and co-workers,12 silyl-
lithium 2 bearing a N,N-dialkylaminomethyl group on the
silicon atom was prepared from diaryldiorganosilanes 1 by
reductive cleavage of the Si−Ar bond using Li (Scheme 3i).
Reaction of 1 with Li (5 equiv) in THF at 0 °C gave a mixture
of 2 and ArLi after 6 h. The solution was then treated with i-
PrOB(pin) (4 equiv) in THF/hexane at 0 °C.13 Silylboronic
esters 3 and ArB(pin) were formed and then separated by
distillation. Piperidino-, pyrrolidino-, and diethylamino-sub-
stituted 3Aa−c were prepared in 60−85% yields (Scheme 3ii).
(Piperidinomethyl)silane derivatives 3Ba and 3Ca, bearing o-
tolyl and ethyl groups, respectively, were also synthesized. 11B
NMR chemical shifts of 3Aa−c, 3Ba, and 3Ca (δ 35−36 ppm)
indicated no coordination of the nitrogen atom to the boron
center. Compounds 3Aa−c, 3Ba, and 3Ca were thermally
stable and storable at room temperature under an atmosphere
of nitrogen for several months.
Reaction of 3Aa with Ethynylbenzene: Silaboration
Catalyzed by Phosphine- and Isocyanide-Free Palla-
B
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a
yield (96%, entry 4). The conversion of ethyl-substituted 3Ca
to give 5C was sluggish at room temperature; it required
forcing reaction conditions (50 °C, 24 h) with use of an
increasing amount of styrene (4.4 equiv) (entry 5). In contrast,
aminosilane 7 and hydrosilane 8 did not undergo β-elimination,
even when the reaction was carried out at 50 °C using 5 mol %
of Pd(OAc)2 (Scheme 4). This indicated that both cleavage of
the Si−B bond by Pd(0) and formation of the thermodynami-
cally stable N−B bond are essential to achieve successful β-
elimination.
Table 1. Reaction of 3a in the Presence of Styrene
b
yield (%)
styrene
entry
1
catalyst
(equiv)
temp (°C)
time
5A
6a
c
Pd(OAc)2
0.5
room
temp
5 min
28
68
88
85
46
c
c
c
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(dba)2
1.0
2.2
2.2
room
temp
5 min
5 min
5 min
85
93
88
Scheme 4. Structure Requirements for β-Elimination
room
temp
room
temp
d
5
Pd(OAc)2
−
−
Ni(cod)2
Pt2(dba)3
[RhCl(cod)]2
0
110
110
110
110
110
110
24 h
24 h
24 h
24 h
24 h
24 h
0
0
0
0
0
0
14
0
6
7
2.2
0
0
The palladium-catalyzed reaction of 3Aa was carried out
using electronically and sterically different styrene derivatives as
additives (Table 3). To slow down the reaction rates for tracing
d
8
d
2.2
2.2
2.2
19
14
6
9
d
10
a
Table 3. Reaction of 3Aa in the Presence of Electronically
and Sterically Different Styrenes
Conditions unless specified otherwise: a catalyst (0.0010 mmol), an
additive (0−0.22 mmol), and 3Aa (0.10 mmol) were reacted in
a
b
toluene (0.1 mL) at the indicated temperature. 1H NMR yield.
c
d
cis:trans = 1:1. 5 mol % of catalyst was used.
a cyclic dimer of “MePhSiCH2”, in 28−88% yields, while
styrene remained unreacted. Pd(dba)2 was equally effective as a
catalyst precursor in the reaction (entry 4), indicating that the
catalytic cycle involves Pd(0). In the absence of styrene, the
reaction did not give 5A, even after increasing the loading of
Pd(OAc)2 (5 mol %) with heating to 110 °C and a longer
reaction time, although a small amount of 6a was formed (entry
5). No reaction took place in the absence of palladium catalyst
(entries 6 and 7), indicating that both palladium and styrene
are essential for the formation of 5A. No formation of 5A was
observed when using Ni(cod)2, Pt2(dba)3, and [RhCl(cod)]2 as
catalysts (entries 8−10).
Silylboronic esters 3 were subjected to the β-elimination
reaction (Table 2). The reactions of 3Aa−c, bearing different
dialkylamino groups, were complete within 5 min at room
temperature to give 5A in 73−74% yields (entries 1−3). o-
Tolyl-substituted 3Ba reacted efficiently to afford 5B in high
c
conversn yield (%)
b
entry
Ar
n
t1/2 (min)
5A
6a
1
2
3
4
5
6
7
8
Ph
2.2
2.2
2.2
2.2
2.2
2.2
4.4
6.5
54
>300
190
11
43
23
71
4-MeOC6H4
4-MeC6H4
4-CF3C6H4
4-ClC6H4
2-MeC6H4
Ph
58
d
d
30
61
e
e
46
74
<5
57
17
68
81
46
>300
<5
84
f
f
Ph
<5
71
87
a
Conditions: Pd(OAc)2 (0.00021 mmol), styrene (0.154−0.455
mmol), 3Aa (0.070 mmol), and dibenzyl ether (0.10 mmol, internal
standard) were reacted in C6D6 (0.7 mL) at room temperature. The
time required for 50% conversion of 3Aa. Conversion yield after 60
b
c
d
e
min. Conversion yield after 55 min. Conversion yield after 45 min.
f
Conversion yield after 30 min.
1
by H NMR, the reaction was carried out in dilute C6D6
Table 2. Palladium-Catalyzed β-Elimination Reaction to
Give 1,3-Disilacyclobutanes 5
a
solution (ca. 0.1 M) using a smaller amount of Pd(OAc)2 (0.3
mol %). When the reaction was carried out with styrene (2.2
equiv), the time required for 50% conversion of 3Aa (t1/2) was
54 min (entry 1). No induction period was observed (see the
Supporting Information). Consumption of 3Aa and formation
of 5A and 6a proceeded in a parallel manner, and no
intermediates were observed (see the Supporting Information).
The conversion yields of 5A and 6a after 60 min (43% and
71%, respectively) indicated that the formation of 5A from the
silene intermediate was less efficient than the reaction under
the optimized conditions (88% and 93%, respectively; entry 3
in Table 1). Styrenes having electron-rich aryls led to slower
reactions (entries 2 and 3), whereas rate acceleration was
observed in the reactions with electron-deficient styrenes
(entries 4 and 5). Sterically demanding o-methylstyrene
resulted in a slow reaction (entry 6). It is noted that slow
conversion of 3Aa resulted in a decrease in conversion yield of
b
entry
Si−B
product
yield (%)
1
2
3
4
3Aa
3Ab
3Ac
3Ba
3Ca
5A (R1 = Me, R2 = Ph)
5A (R1 = Me, R2 = Ph)
5A (R1 = Me, R2 = Ph)
5B (R1 = Me, R2 = o-Tol)
5C (R1 = Et, R2 = Ph)
73
73
74
96
65
c
5
a
Conditions unless specified otherwise: Pd(OAc)2 (0.0020 mmol),
styrene (0.44 mmol), and 3 (0.20 mmol) were reacted in toluene (0.2
b
c
mL) at room temperature for 5 min. Isolated yield. At 50 °C for 24 h
with styrene (4.4 equiv).
C
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(399.89 MHz) spectrometer. 13C NMR spectra were recorded on a
Varian 400-MR (100.55 MHz) spectrometer. 11B NMR spectra were
recorded on a Varian 400-MR (128.30 MHz) spectrometer. H NMR
data were reported as follows: chemical shifts in ppm downfield from
tetramethylsilane, multiplicity (s = singlet, d = doublet, t = triplet, q =
quartet, and m = multiplet), coupling constant (J), and integration. 13C
and 11B NMR data were reported in ppm downfield from
tetramethylsilane (13C) and BF3·OEt2 (11B), respectively. High-
resolution mass spectra were recorded on JEOL JMS-MS700 (EI) or
Thermo Scientific Exactive (MALDI, APCI) spectrometers.
5A after 60 min (entries 2, 3, and 6), indicating that a high
concentration of silene species leads to the more efficient
formation of 5A. Indeed, when the reaction was carried out
with an increased amount of styrene (4.4 and 6.5 equiv), t1/2
was reduced (<5 min) and the conversion yield of 5A was
improved (entries 7 and 8). On the basis of these results, we
assume that styrene coordinates to the palladium center as a
spectator ligand, which stabilizes a (η2-silene)Pd intermediate.14
A possible mechanism for the palladium-catalyzed reaction of
3 is shown in Scheme 5. The first step is oxidative addition of
1
Materials. Pd(OAc)2 (Aldrich) and Ni(cod)2 (Strem) were used as
received from commercial sources. Pd(dba)2,18 Pt2(dba)3,19 and
20
[RhCl(cod)]2 were synthesized by the method reported previously.
Scheme 5. Possible Mechanism
Toluene (Kanto) and tetrahydrofuran (THF, Wako) were degassed by
purging with argon (15 min + 10 min) and then dried by The
Ultimate Solvent System (GlassContour). 2-Isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (i-PrOB(pin)) was synthesized by
the reported method.21 Styrene (Wako), 4-methoxystyrene (Wako), 4-
methylstyrene (TCI), 4-chlorostyrene (Wako), and 2-methylstyrene
(TCI) were distilled prior to use. Compounds 1 were synthesized
through a reaction of secondary amines with alkyldiaryl-
(chloromethyl)silanes.22 7 was prepared via iridium-catalyzed C−H
borylation of chlorotrimethylsilane.22 8 was synthesized through a
reaction of piperidine with (chloromethyl)methylphenylsilane.22
Preparation of 3 (Scheme 3). Compounds 3 were synthesized by
the reaction of i-PrOB(pin) with silyllithiums 2 according to the
reported procedure.13 Silyl lithiums 2 were generated from 1 according
to the method reported by Strohmann.12 A typical procedure is given
for the preparation of 3Aa.
2-[Methylphenyl(piperidinomethyl)silyl]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (3Aa). A 100 mL two-necked, round-bottomed
flask, equipped with a magnetic stirring bar, was charged with cut
lithium (nacalai, 0.69 g, 100 mmol). The flask was evacuated and back-
filled with argon. THF (15 mL) and methyldiphenyl-
(piperidinomethyl)silane (1Aa; 5.91 g, 20 mmol) were placed in the
flask. After a color change of the solution (colorless to pale yellow),
the mixture was cooled to 0 °C and stirred for 6 h. The solution color
changed during the reaction (red, green, and then black). An another
100 mL two-necked, round-bottomed flask, equipped with a magnetic
stirring bar, was charged with i-PrOB(pin) (1.49 g, 80 mmol) and
hexane (17 mL). The flask was cooled to 0 °C by ice-water bath, and
the black solution including silyl lithium was added slowly to the flask.
The mixture as stirred for 12 h at room temperature. Volatiles were
removed in vacuo, and the residue was filtered through Celite. 3Aa
(5.85 g, 85%) was obtained as a colorless liquid after bulb-to-bulb
distillation (120−130 °C/0.17 mmHg). Data for 3Aa are as follows.
1H NMR (400 MHz, C6D6): δ 7.86−7.91 (m, 2H), 7.23−7.29 (m,
the Si−B bond of 3 to Pd(0) to give H, in which the bond
activation is facilitated by preceding coordination of the
nitrogen atom of 3 to the palladium center (G). In the
reaction with ethynylbenzene, insertion of the C−C triple bond
into the Pd−B bond takes place in a regio- and stereoselective
manner to afford I, which undergoes reductive elimination to
give 4 and Pd(0) (path a).15 In contrast, the reaction in the
presence of styrene leads to β-amino elimination, which is
followed by reductive elimination of aminoborane to give (η2-
silene)Pd complex J (path b). The following reaction of J with
either 3 or J results in the formation of silene dimer 5.
Dissociation of free silene from J followed by dimerization to
form 5 is unlikely because no products derived from styrene are
formed, even though styrene is one of the typical trapping
reagents of free silene.16,17
2H), 7.17−7.22 (m, 1H), 2.37−2.52 (m, 4H), 2.42 [d (AB pattern), J
= 14.0 Hz, 1H], 2.33 [d (AB pattern), J = 14.0 Hz, 1H], 1.50−1.58 (m,
4H), 1.24−1.33 (m, 2H), 1.07 (s, 12H), 0.61 (s, 3H). 13C NMR (101
MHz, C6D6): δ 138.3, 135.1, 129.0, 128.1, 83.5, 58.7, 50.4, 26.8, 25.06
[CH3 of B(pin), 2C], 25.01 [CH3 of B(pin), 2C], 24.3, −4.9. 11B
NMR (128 MHz, C6D6): δ 35.6. HRMS (MALDI): m/z calcd for
C19H33BNO2Si+ [M + H]+, 346.2368; found, 346.2377.
CONCLUSION
■
We synthesized silylboronic esters bearing a N,N-dialkylami-
nomethyl group on the silicon atoms and successfully
established the palladium-catalyzed silaboration of ethynylben-
zene and the β-elimination reaction in the presence of styrene.
Remarkably, the latter reaction proceeds efficiently under mild
conditions (within 5 min at room temperature) to give silene
dimers. We found that the use of styrene as a ligand is essential
for the generation of silene species. Our findings open up new
possibilities for the development of new catalytic reactions
involving silene intermediates. Application of the catalytic
system described here to the synthesis of new organosilicon
compounds is underway in our laboratory.
2-[Methylphenyl(pyrrolidinomethyl)silyl]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (3Ab). According to the procedure given for
3Aa, methyldiphenyl(pyrroridinomethyl)silane (1Ab; 2.81 g, 10
mmol), lithium (0.35 g, 50 mmol), and i-PrOB(pin) (0.75 g, 40
mmol) were reacted. 3Ab (1.99 g, 60%) was obtained as a colorless
liquid after bulb-to-bulb distillation (120−130 °C/0.17 mmHg). Data
1
for 3Ab are as follows. H NMR (400 MHz, C6D6): δ 7.86−7.92 (m,
2H), 7.23−7.29 (m, 2H), 7.16−7.22 (m, 1H), 2.47−2.58 (m, 4H),
2.52 [d (AB pattern), J = 14.0 Hz, 1H], 2.47 [d (AB pattern), J = 14.0
Hz, 1H], 1.58−1.67 (m, 4H), 1.06 (s, 12H), 0.63 (s, 3H). 13C NMR
(101 MHz, C6D6): δ 138.1, 135.1, 129.0, 128.1, 83.4, 58.1, 46.0, 25.03
[CH3 of B(pin), 2C], 24.97 [CH3 of B(pin), 2C], 24.5, −5.0. 11B
NMR (128 MHz, C6D6): δ 35.7. HRMS (MALDI): m/z calcd for
C18H31BNO2Si+ [M + H]+, 332.2212; found, 332.2221.
EXPERIMENTAL SECTION
■
General Considerations. Column chromatography was per-
formed with Ultra Pure Silica Gel (SILICYCLE, pH 7.0, 40−63 μm,
60 Å). Gas chromatography (GC) was performed with a Shimadzu
GC-2014 instrument with Agilent J&W GC Column DB-1 (i.d. 0.32
2-[(N,N-diethylaminomethyl)methylphenylsilyl]-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (3Ac). According to the procedure given
1
mm × 15 m). H NMR spectra were recorded on a Varian 400-MR
D
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for 3Aa, (N,N-diethylaminomethyl)methyldiphenylsilane (1Ac; 2.83 g,
10 mmol), lithium (0.35 g, 50 mmol), and i-PrOB(pin) (0.75 g, 40
mmol) were reacted. 3Ac (2.67 g, 80%) was obtained as a colorless
liquid after by bulb-to-bulb distillation (120−130 °C/0.17 mmHg).
Data for 3Ac are as follows. 1H NMR (400 MHz, C6D6): δ 7.86−7.91
(m, 2H), 7.23−7.30 (m, 2H), 7.17−7.23 (m, 1H), 2.45−2.61 (m, 4H),
2.46 [d (AB pattern), J = 14.4 Hz, 1H], 2.38 [d (AB pattern), J = 14.4
Hz, 1H], 1.06 (s, 12H), 0.99 (t, J = 7.2 Hz, 6H), 0.62 (s, 3H). 13C
NMR (101 MHz, C6D6): δ 138.4, 135.1, 129.0, 128.1, 83.4, 50.4, 44.1,
stopcock (J. Young), equipped with a magnetic stirring bar, was
charged with a catalyst precursor (0.0010 mmol for metal), toluene
(0.1 mL), additive (0 or 0.22 mmol), dibenzyl ether (20 mg, 0.10
mmol, internal standard), and 3Aa (0.10 mmol). The tube was sealed
with a stopcock and was taken out from the glovebox. The mixture was
stirred at 110 °C for 24 h by a heating magnetic stirrer with an
aluminum heating block (hole size 21 mm diameter × 33 mm depth).
The resulting mixture was analyzed by 1H NMR to determine the yield
of 5A and 6a. For 1,3-dimethyl-1,3-diphenyl-1,3-disilacyclobutane
1
25.08 [CH3 of B(pin), 2C], 25.00 [CH3 of B(pin), 2C], 12.1, −5.1. 11
B
(5A), the structure was confirmed by comparison of the H and 13C
NMR (128 MHz, C6D6): δ 35.7. HRMS (MALDI): m/z calcd for
NMR data with an authentic sample, which was prepared by treatment
of (chloromethyl)methylphenylsilyl chloride with Mg.23 In all
reactions that gave 5A (entries 1−4), 5A was obtained as a cis/trans
mixture (1/1). The isomers could be separated by HPLC (nacalai
COSMOSIL 5SL-II, 20 × 250 mm, 10 mL/min, UV 254 nm, eluent
hexane). Data for cis-5A are as follows. 1H NMR (400 MHz, CDCl3):
δ 7.54−7.60 (m, 4H), 7.32−7.38 (m, 6H), 0.66−0.74 (m, 2H), 0.59 (s,
6H), 0.42−0.49 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 139.7,
C18H33BNO2Si+ [M + H]+, 334.2368; found, 334.2375.
2-[Methyl(piperidinomethyl)-o-tolylsilyl]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (3Ba). According to the procedure given for
3Aa, methyl(piperidinomethyl)di-o-tolylsilane (1Ba; 3.24 g, 10 mmol),
lithium (0.35 g, 50 mmol), and i-PrOB(pin) (0.75 g, 40 mmol) were
reacted. After bulb-to-bulb distillation (120−130 °C/0.17 mmHg),
3Ba was obtained with a small amount of unreacted 1Ba (2.16 g,
3Ba:1Ba = 85:15). The yield of 3Ba was estimated as 51%. Data for
3Ba are as follows. 1H NMR (400 MHz, C6D6): δ 7.88−7.96 (m, 1H),
7.07−7.21 (m, 3H), 2.61 (s, 3H), 2.50 [d (AB pattern), J = 14.0 Hz,
1H], 2.38−2.50 (m, 4H), 2.39 [d (AB pattern), J = 14.0 Hz, 1H],
1.47−1.57 (m, 4H), 1.24−1.36 (m, 2H), 1.06 (s, 12H), 0.64 (s, 3H).
13C NMR (101 MHz, C6D6): δ 144.1, 136.7, 135.7, 129.8, 129.4,
125.4, 83.4, 58.7, 49.9, 26.8 [CH3 of B(pin), 2C], 26.7 [CH3 of B(pin),
2C], 25.0, 24.4, 23.4, −4.1. 11B NMR (128 MHz, C6D6): δ 35.7.
HRMS (MALDI): m/z calcd for C20H35BNO2Si+ [M + H]+, 360.2525;
found, 360.2531.
2-[Ethylphenyl(piperidinomethyl)silyl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (3Ca). According to the procedure given for 3Aa,
ethyldiphenyl(piperidinomethyl)silane (1Ca; 1.54 g, 5 mmol), lithium
(0.17 g, 25 mmol), and i-PrOB(pin) (3.7 g, 20 mmol) were reacted.
3Ca (0.70 g, 39%) was obtained as a colorless liquid after bulb-to-bulb
distillation (120−130 °C/0.17 mmHg). Data for 3Ca are as follows.
1H NMR (400 MHz, C6D6): δ 7.89−7.95 (m, 2H), 7.24−7.31 (m,
+
133.3, 129.1, 127.8, 2.3, 1.0. HRMS (EI): m/z calcd for C16H20Si2
[M]+ 268.1098; found, 268.1091. Data for trans-5A are as follows. H
1
NMR (400 MHz, CDCl3): δ 7.62−7.69 (m, 4H), 7.38−7.46 (m, 6H),
0.57 (s, 4H), 0.46 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 139.9,
+
133.3, 129.1, 127.9, 2.0, 1.3. HRMS (EI): m/z calcd for C16H20Si2
[M]+, 268.1098; found, 268.1098. Data for 2-piperidino-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (6a) are as follows.24 1H NMR (400
MHz, C6D6): δ 3.14−3.26 (m, 4H), 1.30−1.42 (m, 6H), 1.14 (s,
12H). 11B NMR (128 MHz, C6D6): δ 24.0.
Palladium-Catalyzed Reaction of 3 To Give 1,3-Disilacyclo-
butanes 5 (Table 2). General Procedure. In a glovebox, a 4 mL
screw cap vial, equipped with a magnetic stirring bar, was charged with
Pd(OAc)2 (0.01 M solution in toluene, 0.20 mL), styrene (0.44
mmol), and 3 (0.20 mmol). The mixture was stirred at room
1
temperature for 5 min. The resulting mixture was analyzed by H
NMR to determine the cis/trans ratio. 5 was purified by column
chromatography on silica gel.
Reaction of 3Aa To Give 5A (Entry 1, Table 2). According to the
general procedure, 3Aa (70 mg, 0.20 mmol) was reacted in toluene
(0.2 mL) at room temperature in the presence of Pd(OAc)2 (0.0020
mmol) and styrene (46 mg, 0.44 mmol). 5A (20 mg, 73%) was
obtained as a cis/trans mixture (1/1) after column chromatography on
silica gel (eluent hexane/Et2O = 10/1). For characterization data of
cis- and trans-5A, see above.
2H), 7.16−7.24 (m, 1H), 2.36−2.53 (m, 4H), 2.44 [d (AB pattern), J
= 14.0 Hz, 1H], 2.40 [d (AB pattern), J = 14.0 Hz, 1H], 1.49−1.51 (m,
4H), 1.23−1.33 (m, 5H), 1.10−1.15 (m, 2H), 1.09 (s, 12H). 13C
NMR (101 MHz, C6D6): δ 137.4, 135.5, 129.0, 128.0, 83.4, 58.8, 48.8,
26.8, 25.04 [CH3 of B(pin), 2C], 25.03 [CH3 of B(pin), 2C], 24.3, 8.8,
5.1. 11B NMR (128 MHz, C6D6): δ 35.7. HRMS (MALDI): m/z calcd
for C20H35BNO2Si+ [M + H]+, 360.2525; found, 360.2532.
Reaction of 3Ab To Give 5A (Entry 2, Table 2). According to the
general procedure, 3Ab (68 mg, 0.20 mmol) was reacted in toluene
(0.2 mL) at room temperature in the presence of Pd(OAc)2 (0.0020
mmol) and styrene (46 mg, 0.44 mmol). 5A (20 mg, 73%) was
obtained as a cis/trans mixture (1/1) after column chromatography on
silica gel (eluent hexane/Et2O 10/1). For characterization data of cis-
and trans-5A, see above.
Reaction of 3Ac To Give 5A (Entry 3, Table 2). According to the
general procedure, 3Ac (65 mg, 0.20 mmol) was reacted in toluene
(0.2 mL) at room temperature in the presence of Pd(OAc)2 (0.0020
mmol) and styrene (46 mg, 0.44 mmol). 5A (20 mg, 74%) was
obtained as a cis/trans mixture (1/1) after column chromatography on
silica gel (eluent hexane/Et2O 10/1). For characterization data of cis-
and trans-5A, see above.
Reaction of 3Aa with Ethynylbenzene (Eq 1). In a glovebox, a
4 mL screw cap vial, equipped with a magnetic stirring bar, was
charged with Pd(OAc)2 (1.1 mg, 0.0047 mmol) and 3Aa (173 mg,
0.50 mmol). Ethynylbenzene (63 mg, 0.62 mmol) and toluene (0.5
mL) were added, and the resulting mixture was stirred at room
temperature for 1 h. After removal of the volatiles in vacuo, the residue
was purified by bulb-to-bulb distillation (110−130 °C/0.17 mmHg).
(Z)-1-[Methylphenyl(piperidinomethyl)silyl]-2-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-1-phenylethene (4Aa; 176 mg, 79%) was
1
obtained as a viscous liquid. Data for 4Aa are as follows. H NMR
(400 MHz, CDCl3): δ 7.65−7.73 (m, 2H), 7.29−7.41 (m, 3H), 7.16−
7.26 (m, 4H), 7.08−7.16 (m, 2H), 2.84−3.04 (m, 4H), 2.72 [d (AB
pattern), J = 14.4 Hz, 1H], 2.68 [d (AB pattern), J = 14.4 Hz, 1H],
1.42−1.62 (m, 6H), 1.23 (s, 12H), 0.49 (s, 3H). 13C NMR (101 MHz,
CDCl3): δ 157.5 (broad, C−B), 145.4, 137.5, 134.6, 129.1, 128.0,
127.8, 126.2, 125.8, 80.4, 54.1, 41.7, 27.4, 27.1, 23.2, 20.3, −3.2. 11B
NMR (128 MHz, CDCl3): δ 11.9. HRMS (ESI): m/z calcd for
C27H39BNO2Si [M + H]+, 448.2838; found, 448.2828.
Reaction of 3Ba To Give 1,3-Dimethyl-1,3-di-o-tolyl-1,3-disilacy-
clobutane (5B) (Entry 4, Table 2). According to the general
procedure, 3Ba (3Ba:1Ba = 85:15, 70 mg, 0.17 mmol of 3Ba) was
reacted in toluene (0.2 mL) at room temperature in the presence of
Pd(OAc)2 (0.0020 mmol) and styrene (46 mg, 0.44 mmol). 5B (24
mg, 96%) was obtained as a cis/trans mixture (1/1) by column
chromatography on silica gel (eluent hexane/CH2Cl2 10/1). The
isomers could be separated by HPLC (nacalai COSMOSIL 5SL-II, 20
× 250 mm, 10 mL/min, UV 254 nm, eluent: hexane). Data for cis-5B
Reaction of 3a in the Presence of Styrene (Table 1).
Procedure for the Reaction at Room Temperature (Entries 1−4,
Table 1). In a glovebox, a 4 mL screw cap vial, equipped with a
magnetic stirring bar, was charged with Pd(OAc)2 (0.0010 mmol),
toluene (0.1 mL), additive (0.05−0.22 mmol), dibenzyl ether (20 mg,
0.10 mmol, internal standard), and 3Aa (0.10 mmol). The mixture was
stirred at room temperature for 5 min. The resulting mixture was
1
are as follows. H NMR (400 MHz, CDCl3): δ 7.50−7.56 (m, 2H),
7.25 (dt, J = 7.6, 1.6 Hz, 2H), 7.10−7.18 (m, 4H), 2.46 (s, 6H), 0.75−
0.84 (m, 2H), 0.55−0.64 (m, 2H), 0.55 (s, 6H). 13C NMR (101 MHz,
CDCl3): δ 143.1, 138.1, 134.1, 129.4 (two nonequivalent carbons
overlapped), 124.9, 22.3, 3.7, 1.6. HRMS (EI): m/z calcd for
1
analyzed by H NMR to determine the yield of 5A and 6a.
Procedure for the Reaction at 110 °C (Entries 5−10, Table 1). In a
glovebox, a glass tube (outside diameter 20 mm) having PTFE
E
DOI: 10.1021/acs.organomet.7b00695
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Organometallics
Article
C17H21Si2+ [M − CH3]+, 281.1176; found, 281.1180. Data for trans-5B
JP26288048 for Scientific Research (B) (TO) and JP15H05811
for Scientific Research on Innovative Areas in Precisely
Designed Catalysts with Customized Scaffolding (MS). T.O.
also expresses his appreciation for support by The Kyoto
University Research Fund for Senior Scientists (Core) FY2017.
1
are as follows. H NMR (400 MHz, CDCl3): δ 7.64 (d, J = 7.6 Hz,
2H), 7.32 (dt, J = 7.6, 1.2 Hz, 2H), 7.17−7.27 (m, 4H), 2.51 (s, 6H),
0.69 (s, 4H), 0.37 (s, 6H). 13C NMR (101 MHz, CDCl3): δ 143.4,
137.9, 134.3, 129.5 (two nonequivalent carbons overlapped), 124.9,
+
22.5, 3.2, 2.2. HRMS (EI): m/z calcd for C17H21Si2 [M − CH3]+,
281.1176; found, 281.1182.
Reaction of 3Ca To Give 1,3-Diethyl-1,3-diphenyl-1,3-disilacy-
clobutane (5C) (Entry 5, Table 2). According to the general
procedure, 3Ca (71 mg, 0.20 mmol) was reacted in toluene (0.2
mL) at 50 °C for 24 h in the presence of Pd(OAc)2 (0.0020 mmol)
and styrene (92 mg, 0.88 mmol). 5C (19 mg, 65%) was obtained as a
cis/trans mixture (1/1) by column chromatography on silica gel
(eluent hexane/CH2Cl2 10/1). The isomers could be separated by
HPLC (nacalai COSMOSIL 5SL-II, 20 × 250 mm, 10 mL/min, UV
254 nm, eluent: hexane). Data for cis-5C are as follows. 1H NMR (400
MHz, CDCl3): δ 7.48−7.55 (m, 4H), 7.27−7.37 (m, 6H), 1.02−1.12
(m, 6H), 0.93−1.02 (m, 4H), 0.44−0.58 (m, 4H). 13C NMR (101
MHz, CDCl3): δ 138.6, 133.6, 129.0, 127.7, 9.1, 7.2, −2.3. HRMS
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1
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge on the
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met.7b00695.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.O.: ohmura@sbchem.kyoto-u.ac.jp.
*E-mail for M.S.: suginome@sbchem.kyoto-u.ac.jp.
■
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Stratakis, M. Org. Lett. 2014, 16, 1430−1433. (l) Khan, A.; Asiri, A. M.;
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ORCID
Toshimichi Ohmura: 0000-0002-2534-3769
Michinori Suginome: 0000-0003-3023-2219
Notes
The authors declare no competing financial interest.
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M. J. Am. Chem. Soc. 2006, 128, 13366−13367.
ACKNOWLEDGMENTS
We thank Mr. Ikuo Sasaki for his assistance of experiments.
This work was supported by JSPS KAKENHI Grant Numbers
■
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Organometallics
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