Silica-Supported Triptycene-Type Phosphine. Synthesis, Characterization, and Application to Pd-Catalyzed Suzuki-Miyaura Cross-Coupling of Chloroarenes

Article abstract of DOI:10.1021/acscatal.5b00904

A silica-supported triptycene-type phosphine, Silica-TRIP, comprising a 9-phospha-10-silatriptycene (TRIP) and silica gel as a P-coordination center and a solid support, respectively, was synthesized and structurally characterized by nitrogen absorption measurements and solid-state CP/MAS NMR spectroscopy. Silica-TRIP exhibited a mono-P-ligating feature toward a Pd(II) complex, resulting in selective formation of a 1/1 Pd-P species even with an excess amount of the ligand. As a result, Silica-TRIP enabled Pd-catalyzed Suzuki-Miyaura cross-coupling reactions of chloroarenes under mild conditions, regardless of the moderate electron-donating nature of the triarylphosphine-based ligand.

Full text of DOI:10.1021/acscatal.5b00904

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Article
Silica-supported Triptycene-type Phosphine. Synthesis, Characterization, and
Application to Pd-Catalyzed Suzuki–Miyaura Cross-coupling of Chloroarenes
Tomohiro Iwai, Shota Konishi, Tatsuya Miyazaki, Soichiro Kawamorita,
Natsumi Yokokawa, Hirohisa Ohmiya, and Masaya Sawamura
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00904 • Publication Date (Web): 29 Oct 2015
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ACS Catalysis
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Silica-supported Triptycene-type Phosphine. Synthesis, Characteri-
zation, and Application to Pd-Catalyzed Suzuki–Miyaura Cross-
coupling of Chloroarenes
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Tomohiro Iwai, Shota Konishi, Tatsuya Miyazaki, Soichiro Kawamorita, Natsumi Yokokawa, Hirohisa
Ohmiya and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
KEYWORDS: silica-supported catalyst, phosphine, triptycene, palladium, chloroarene
ABSTRACT: A silica-supported triptycene-type phosphine, Silica-TRIP, comprising a 9-phospha-10-silatriptycene (TRIP) and
silica gel as a P-coordination center and a solid support, respectively, was synthesized and structurally characterized by nitrogen
absorption measurements and solid-state CP/MAS NMR spectroscopy. Silica-TRIP exhibited a mono-P-ligating feature toward a
Pd(II) complex, resulting in selective formation of a 1:1 Pd-P species even with an excess amount of the ligand. As a result, Silica-
TRIP enabled Pd-catalyzed Suzuki–Miyaura cross-coupling reactions of chloroarenes under mild conditions, regardless of the mod-
erate electron-donating nature of the triarylphosphine-based ligand.
INTRODUCTION
Silica-supported ligands,1 which are generally synthesized
(a)
(b)
P
by silane coupling between silicon-functionalized organic
molecules and a silanol-containing surface, have been widely
studied to prepare heterogenized transition metal catalysts
with practical merits such as easy separation and reusability.2,3
However, steric hindrance of the solid surface toward the ac-
tive site often causes a decrease in catalytic performance com-
pared to the corresponding homogeneous molecular catalysts.
Moreover, commonly used flexible linkers (e.g., alkyl chains)
between ligands and silica gel impart significant mobility to
the coordination centers, resulting in difficulty in ligand design
based on the features of the solid surface. We envisaged to
solve these problems by limiting the mobility of the ligand on
the silica surface by introducing a rigid linker system between
the metal-coordinating atom and the surface, for allowing the
use of the solid surface as a tool for producing highly active
catalyst species rather than merely for having the benefit of
easy separation, reuse, and recycling of the catalyst.
To this end, we have recently designed and synthesized a
silica-supported monophosphine, Silica-SMAP (Figure 1a),
providing a novel example of increased catalytic activity by
surface immobilization.4,5 The rigidity of the caged trial-
kylphosphine (SMAP)6 and the cage-to-surface immobiliza-
tion made the P-coordination center stand upward on the sur-
face and hence exist in isolation (Figure 1b).7 Thus, Silica-
SMAP could form 1:1 metal-phosphine complexes exclusively
with a range of transition metal species regardless of its com-
pactness. The transition metal complexes prepared in this
manner performed as useful heterogeneous catalysts for vari-
ous reactions.4
P
P
Si
O
SiMe3
O
Si
Si
Si
Si
O
O
O
O
O
O
O
Si
O
O
SiO2
Silica-SMAP
O
SiO2
Figure 1. (a) Structure of Silica-SMAP and (b) restricted mo-
bility of the SMAP moiety on silica surface.
On the basis of these considerations, we focused on 9-
phospha-10-silatriptycenes (TRIP), which were previously
synthesized by Tsuji and Tamao et al.,8 as a new motif for sili-
ca-supported ligands. We expected that silica-supported trip-
tycene-type phosphine Silica-TRIP (Figure 2) would function
as a supported ligand complementary to Silica-SMAP-based
systems. SMAP and TRIP moieties are trialkyl- and tri-
arylphosphines, respectively, thus having contrasting elec-
tronic natures; the former should be much more electron-
donating. The two ligands also differ in steric demand; the
latter is significantly bulkier. In fact, a Silica-TRIP-Rh catalyst
system realized heteroatom-directed borylation of C(sp3)–H
bonds of amides, ureas, and 2-aminopyridines at the position α
to the N atom, for which Silica-SMAP was not effective, af-
fording the corresponding primary and secondary α-
aminoalkylboronates (Scheme 1, top),9 while both Silica-
SMAP and Silica-TRIP were effective for the Ir-catalyzed N-
directed C(sp3)–H borylation of 2-alkylpyridines (Scheme 1,
middle).4j This article describes details of our studies on the
synthesis of Silica-TRIP, its coordination properties toward
Pd(II) complexes, and its catalytic application to Pd-catalyzed
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Suzuki–Miyaura cross-coupling of chloroarenes (Scheme 1,
Page 2 of 12
the silanol group was supported by the observation of a broad
bottom).10,11 The use of bulky and electron-rich monophos-
phine12 or NHC13 ligands is a common strategy for enabling
the Pd-catalyzed cross-coupling of chloroarenes under mild
conditions, and there are only a limited number of reports on
effective catalyst systems with moderately electron-donating
triarylphosphine-based ligands; chloroarenes are more desira-
ble but less reactive than bromoarenes and iodoarenes.14–17
absorption band at 3347 cm–1 by IR spectroscopy, which was
assignable to an (Si)O–H stretching vibration.
1
2
3
4
5
6
7
8
1) tBuLi (6 equiv)
1) iPrMgBr (3.5 equiv)
THF, –20 ºC, 3 h
Et2O/THF (5:1)
–78 ºC, 4 h
P
I
P
3
Si
Br 2) PCl3 (1 equiv)
Br
2) SiCl4 (1 equiv)
Et2O/THF (5:1)
–78 °C, 4 h
OH
TMEDA (3.5 equiv)
THF, –20 °C
1
2 57%
(3.5 equiv)
HO-TRIP (3)
9
P
then 0 °C, 14 h
70%
3) workup with SiO2
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SiMe3
O
Si
O
Si
Scheme 2. Preparation of silanol HO-TRIP (3).
Si
O
O
O
O
O
O
SiO2
Silica-TRIP
Figure 2. Structure of Silica-TRIP.
N-Adjacent C–H transformation (Ref. 9)
cat.
[Silica-TRIP-Rh]
X
H
X
N
Bpin
R2
pinB–Bpin
+
Y
N
R1
R2
Y
N
R1
cat.
C–H transformation (Ref. 4j)
[Silica-SMAP-Ir]
or
[Silica-TRIP-Ir]
N
H
Bpin
R3
Figure 3. Molecular structure of 3 at 50% probability level; a
solvent molecule (CHCl3) and one of the disordered hydrogen
atoms of the silanol moiety (SiO–H) are omitted for clarity.
pinB–Bpin
+
R3
C–Cl transformation (This work)
cat.
[Silica-TRIP-Pd]
+
Cl
(HO)2B
base
R4
R5
R4
R5
Scheme 1. Transformations of C(sp3)–H and C(sp2)–Cl bonds
with Silica-TRIP-metal catalyst systems.
RESULTS AND DISCUSSION
Preparation of Silica-Supported Triptycene-Type Phos-
phine (Silica-TRIP). For the synthesis of the precursor for
Silica-TRIP, a soluble triptycene-type phosphine 3 having a
silanol group at the bridgehead, we followed Tsuji and
Tamao’s procedure for the synthesis of 9-phospha-10-
silatriptycenes with a slight modification.8 Thus, commercially
available 1-bromo-2-iodobenzene (1) was converted to tris(o-
bromophenyl)phosphine (2) by treatment with iPrMgBr for
Mg-I exchange at –20 °C,18 followed by the reaction with PCl3
in the presence of N,N,N',N'-tetramethylethylenediamine
(TMEDA) as an additive. This procedure avoids the extremely
low temperature conditions (–110 °C) employed in Tsuji and
Tamao’s procedure.8 The trilithiated species generated from 2
with 6 equivalents of tBuLi in Et2O/THF was reacted with
SiCl4. Purification by silica gel chromatography gave the si-
lanol HO-TRIP (3) in 70% yield as an air- and moisture-stable
solid. Single-crystal X-ray diffraction analysis of 3 confirmed
its three-dimensional molecular structure having a triptycene
cage, the bridgehead of which was substituted with P and Si
atoms (Figure 3). Interestingly, six molecules of 3 in the crys-
tal structure adopted a chair form hydrogen-bonding network
consisting of six silanol groups as shown in Figure 4 (average
distance of Ar3SiO···OSiAr3; 2.651 Å).19,20 The existence of
Figure 4. Chair form hydrogen-bonding network (blue-dotted
lines) of 3 in the crystal structure.
Interestingly, the triarylsilanol 3 did not react with silica gel
during the chromatography, while the purified 3 underwent
slow self-condensation to form the corresponding disiloxane
(TRIP)2O (4). The self-condensation was more rapid in the
presence of a base. In fact, heating of 3 with imidazole in ben-
zene at 80 °C caused its complete consumption, forming white
precipitates (Scheme 3).21 Filtration of the precipitates fol-
lowed by washing with benzene gave the disiloxane 4 in 52%
isolated yield. Treatment of the silanol
trimethylsilylimidazole provided the 1,1,1-trimethyl-
substituted disiloxane TMSO-TRIP (5) in 87% yield (Scheme
4).
3 with N-
2
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Figure 5 shows nitrogen adsorption-desorption isotherms
P
1
2
3
4
5
6
7
8
and Barrett-Joyner-Halenda (BJH) plots for Silica-TRIP. The
structural parameters (specific surface area, pore diameter and
pore volume) are summarized in Table 1. Silica-TRIP exhib-
ited a hysteresis loop, indicating the existence of mesopores
with a broad pore-diameter distribution at an average of 17.2
nm. The surface modification of CARiACT Q10 silica gel
(75–150 µm) for the preparation of Silica-TRIP as well as
Silica-SMAP reasonably reduced their structural parameters
such as surface area and pore volume.4c A surface P density of
Silica-TRIP on the basis of the surface area (244 m2/g) and the
estimated TRIP loading (0.078 mmol/g) was calculated to be
0.19 nm–2. This value was comparable to that of Silica-SMAP
(0.19 nm–2).
imidazole
(0.4 equiv)
Si
O
3
(2 equiv)
benzene
80 °C, 10.5 h
Si
P
(TRIP)2O (4) 52%
Scheme 3. Preparation of disiloxane (TRIP)2O (4).
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P
Me3Si-imidazole
(5 equiv)
Si
3
O
CH2Cl2, rt, 1 h
Me3Si
TMSO-TRIP (5) 87%
(a)
Scheme 4. Preparation of disiloxane TMSO-TRIP (5).
The preparation of silica-supported triptycene-type phos-
phine (Silica-TRIP) with a direct disiloxane linkage is shown
in Scheme 5. We made slight modifications to the procedure
described in our original report4h for more expeditious opera-
tion in large-scale synthesis. Specifically, the silanol phos-
phine 3 was grafted to a silica gel surface (CARiACT Q10,
75–150 µm) in the presence of imidazole and toluene under
gentle stirring at 100 °C over 16 h. The resulting colorless
solids were collected by filtration, washed successively with
degassed toluene, toluene-MeOH (1:1), and MeOH, and then
dried under vacuum to afford phosphine-functionalized silica-
gel Silica-TRIP(SiOH). Unreacted surface silanols were
Me3Si-endcapped through treatment with excess N-
trimethylsilylimidazole in THF at 60 °C for 24 h, to furnish
Silica-TRIP. The P loading to silica gel was calculated to be
0.078 mmol/g based on the ability of the gel to bind a Pd(II)
complex in a 1:1 Pd/P coordination mode (vide infra). The
phosphine moiety of Silica-TRIP underwent oxidation by m-
chloroperoxybenzoic acid in CH2Cl2 at room temperature, to
give the immobilized phosphine Silica-TRIP oxide.22
(b)
P
P
silica gel
imidazole
Me3Si-
imidazole
Si
O
Si
SiMe3
O
Si
O
Si
3
OH
Si
toluene
100 °C
16 h
THF
60 °C
24 h
Si
O O
O
O
O
O
O
O
O
O
O
O
Figure 5. (a) Nitrogen adsorption–desorption isotherms and
(b) BJH pore-diameter distribution plots of Silica-TRIP.
SiO2
SiO2
Silica-TRIP(SiOH)
Silica-TRIP
δ –52 (31P CP/MAS)
O
P
Table 1. Structural parameters.
surface area pore diameter pore volume
materials
mCPBA
(BET, m2/g)
244
(nm)
17.2
17.3
17.8
(mL/g)
1.05
1.08
SiMe3
O
Si
O
Si
CH2Cl2
rt, 1 h
Silica-TRIP
Si
O
O
Silica-SMAPa
CARiACT Q10a
220
284
O
O
O
O
SiO2
Silica-TRIP oxide
δ –8 (31P CP/MAS)
1.32
a Data were taken from ref 4c.
Scheme 5. Preparation of Silica-TRIP and Silica-TRIP oxide.
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Solid-State NMR Spectra of Silica-TRIP. The silica- TRIP with an excess amount of PdCl2(py)2 (Pd/P 2:1) in
1
2
3
4
5
6
7
8
supported phosphine Silica-TRIP was characterized by solid-
state 31P, 13C, and 29Si NMR spectroscopies with comparison to
the solution NMR spectra of the soluble disiloxane phosphine
5 in CDCl3.3 The 31P CP/MAS NMR spectrum of Silica-TRIP
showed a singlet signal at δ –52 (ppm) assignable to the P
atom of the TRIP moiety (Figure 6a) (31P NMR for 5; δ –54.9).
The 13C CP/MAS spectrum showed a sharp signal at δ 2 ppm
for the Me carbons of the trimethylsilyl endcaps and multiple
broad signals around δ 130 for the aromatic carbons of the
TRIP moiety (Figure 6b). In the 29Si CP/MAS NMR spectrum
(Figure 6c), a weak signal at δ –36 was assigned to the bridge-
head Si atom of the TRIP moiety (29Si NMR for 5; δ –38.9).
CH2Cl2 at room temperature for 0.5 h produced pale yellow
silica gel, indicating that Pd atoms were bound to the gel. Un-
reacted PdCl2(py)2 was recovered from the filtrate, and its
weight was measured. This procedure allowed us to estimate
the amount of P loading on the silica gel, which was calculated
to be 0.078 mmol/g under the assumption of selective for-
mation of PdCl2(py)(Silica-TRIP) with a Pd/P stoichiometry of
1:1 (vide infra for the structure assignment). On the other
hand, the inductively coupled plasma-atomic emission spec-
troscopic (ICP-AES) analysis of PdCl2(py)(Silica-TRIP) gave
P and Pd loading values of 0.063 and 0.072 mmol/g, respec-
tively. For convenience, the value of 0.07 mmol/g was used
for P loading in metal complexations and catalytic applications
(vide infra). The 31P CP/MAS NMR spectrum of the silica-
supported Pd complex obtained in this manner showed a sin-
glet peak at –5 ppm (Figure 7a). The comparison of this chem-
ical shift value with those of homogeneous Pd complexes pre-
pared from TMSO-TRIP (5) (Pd/P 0:1, 1:1, 1:2 in Figure 8) is
consisted with the formation of mono-P-ligated Pd complex
PdCl2(py)(Silica-TRIP) without forming a bis-P-ligated Pd
complex PdCl2(Silica-TRIP)2 [31P NMR in CDCl3 for
PdCl2(py)(5) δ –6.5; for PdCl2(5)2 δ –19.5]. Notably, the
mono-P-ligated Pd complex was formed selectively even
when excess P was present (Pd/P 1:2, in CH2Cl2 at room tem-
perature for 0.5 h) (Figure 7b).
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(a)
(b)
(c)
(a)
(b)
Figure 7. 31P CP/MAS NMR spectra obtained from (a)
PdCl2(py)2/Silica-TRIP (Pd/P 2:1) and (b) PdCl2(py)2/Silica-
TRIP (Pd/P 1:2).
Figure 6. CP/MAS NMR spectra of Silica-TRIP for (a) 31P (b)
13C and (c) 29Si nuclei.
Coordination toward a Pd(II) Complex. The coordination
property of Silica-TRIP in the reaction with PdCl2(py)2 (py =
pyridine) was investigated. Specifically, the reaction of Silica-
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Figure 8. 31P NMR spectra obtained from PdCl2(py)2 and
TMSO-TRIP (5) [in CDCl3, Pd/P (a) 0:1; (b) 1:1; (c) 1:2].
Figure 10. 31P NMR spectra obtained from PdCl2(py)2 and
PPh2[4-Me2(iPrO)Si-C6H4] (6) [in CDCl3, Pd/P (a) 0:1; (b) 1:1;
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(c) 1:2].
In contrast, the reaction of PdCl2(py)2 with silica-supported
non-cage-type triarylphosphine Silica-1p-TPP4h,17b (Pd/P 1:2,
in CH2Cl2 at room temperature for 1 h) gave well-separated
two 31P signals at δ 29 and δ 23 (Figure 9), which were assign-
able to a 1:1 Pd/P complex PdCl2(py)(Silica-1p-TPP) and a 1:2
Pd/P complex PdCl2(Silica-1p-TPP)2, respectively, on the ba-
sis of the 31P NMR studies using the corresponding soluble
phosphine PPh2[4-Me2(iPrO)Si-C6H4] (6)17b (Pd/P 0:1, 1:1 or
1:2; in CDCl3, Figure 10), along with a signal for the free
phosphine at δ –4.23 Comparison of the Pd coordination prop-
erties of Silica-TRIP and Silica-1p-TPP indicated that the
cage-to-surface direct immobilization was an effective means
for site isolation of the P centers to allow selective mono-P-
ligation.
Pd-Catalyzed Suzuki–Miyaura Cross-Coupling of Chlo-
roarenes. To demonstrate ligand characteristics of Silica-TRIP
for catalytic applications, we examined Pd-catalyzed Suzuki–
Miyaura coupling of chloroarenes, in which mono-ligation of
two-electron donor ligands is important for high catalytic ac-
tivity.12–14 Specifically, the reaction of 4-chlorotoluene (7a, 0.5
mmol) and phenylboronic acid (8a, 0.6 mmol) was conducted
at 60 °C for 12 h in the presence of K3PO4 as a base and a
palladium source (0.5 mol%). The results are summarized in
Table 2.
The pre-formed immobilized mono-P-ligated Pd complex
PdCl2(py)(Silica-TRIP) initiated the coupling reaction to give
the desired product 9a in 56% yield, regardless of the moder-
ate electron-donor power of Silica-TRIP as a triarylphosphine
(Table 2, entry 1). However, the heterogeneous catalyst pre-
pared in-situ from PdCl2(py)2 and Silica-TRIP were less effi-
cient (5%, entry 2). In both cases, their solution phases
changed from colorless to dark brown during the reactions
probably due to leaching of Pd. More active catalyst was pro-
duced by a combination of Silica-TRIP and commercially
available Pd(OAc)2 (93%, entry 3).24 The supernatant of the
mixture after the coupling reaction was colorless; however,
3% Pd leaching into solution was observed by the ICP-AES
analysis. Using unmodified silica gel CARiACT Q10 or TMS-
endcapped CARiACT Q10 in place of Silica-TRIP under the
conditions of entry 3 caused no reaction, indicating that
Pd(OAc)2 directly bound to silica gel was not effective (entries
4 and 5).
Figure 9. 31P CP/MAS NMR spectrum obtained from
PdCl2(py)2/Silica-1p-TPP (Pd/P 1:2).
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Table 2. Pd-catalyzed Suzuki–Miyaura cross-coupling be-
tween 7a and 8a.a
(7b–7c) and electron-withdrawing (7d–7g) substituents at the
1
2
3
4
5
6
7
8
para position of the aromatic ring smoothly reacted with 8a,
yielding the corresponding biaryls in good to high yields (en-
tries 1–6). The reaction between 3-chloropyridine (7h) and 4-t-
butylphenylboronic acid (8b) afforded the 3-arylpyridine 9h in
88% yield (entry 7). Sterically hindered chloroarenes such as
2-chlorotoluene (7i) and 2,6-dimethylchlorobenzene (7j) were
also suitable substrates (entries 8 and 9). The use of preformed
catalyst PdCl2(py)(Silica-TRIP) was more efficient than that of
in situ prepared complex Pd(OAc)2/Silica-TRIP in the reaction
of 2,4,6-trimethoxychlorobenzene (7k) (entry 10). ortho-
Substituted arylboronic acids such as 1-naphthylboronic acid
(8c) and 2-tolylboronic acid (8d) were applicable to the cou-
pling reactions, providing the corresponding biaryls in high
yields (entries 11 and 12). Sterically congested 2,2’,6-
trimethylbiphenyl was obtained through the coupling reaction
between 7j and 8d (entry 13).
[Pd] (0.5 mol%)
Additive
+
Me
Cl
(HO)2B Ph
Me
Ph
K3PO4 (1.5 mmol)
THF, 60 °C, 12 h
7a (0.5 mmol) 8a (0.6 mmol)
9a
entry
Pd source [Pd]
additive
yield (%)b
1
PdCl2(py)(Silica-TRIP)
PdCl2(py)2
Pd(OAc)2
none
Silica-TRIP
Silica-TRIP
56
5
93 (88)
2c
3c
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CARiACT Q10
(unmodified)
CARiACT Q10
(TMS-endcapped)
4d
Pd(OAc)2
0
5d
Pd(OAc)2
0
a
Conditions: 7a (0.5 mmol), 8a (0.6 mmol), [Pd] (0.0025 mmol,
b
0.5 mol%), K3PO4 (1.5 mmol), THF (1.5 mL), 60 °C, 12 h.
Yields of 9a were determined by 1H NMR. Isolated yield is given
c
d
in parentheses. Silica-TRIP (0.030 mmol, 0.6 mol%). CAR-
iACT Q10 (42.9 mg).
Table 3. Ligand effects in Pd-catalyzed Suzuki–Miyaura
cross-coupling between 7a and 8a.a
Pd(OAc)2 (0.5 mol%)
Ligand (0.6 mol%)
Effects of phosphine ligands shown in Figure 11 were in-
vestigated in the presence of a catalytic amount of Pd(OAc)2
(0.5 mol%, Table 3). Cage-type trialkylphosphine Silica-
SMAP4 and tripodally immobilized triarylphosphine Silica-3p-
TPP,17b which possess mono-P-ligating features toward transi-
tion metals, promoted the coupling reaction, but their reaction
efficacies were slightly less than that of Silica-TRIP (Table 2,
entry 3 vs. Table 3, entries 1 and 2). The conventional mon-
opodally silica-immobilized ligand Silica-1p-TPP, in which
site-isolation of the P center was not enough as shown in Fig-
ure 9, was much less effective (9% yield, entry 3). In contrast
to the silica-supported catalyst systems, their homogeneous
counterparts TMSO-TRIP (5), Ph-TRIP,4h Ph-SMAP,6 or PPh3
induced no or trace of coupling reactions (entries 4–7). These
results indicated that immobilization of ligands for obtaining a
mono-P-ligating feature (e.g., cage-to-surface or tripod),
which was supported by NMR studies of Pd-P coordination
(Figure 7), is crucial for the high catalytic activities obtained
with Silica-TRIP. Under the present reaction conditions, steri-
cally demanding homogeneous trialkylphosphines such as
PCy3 and PtBu3 were not as effective as Silica-TRIP (entries 8
and 9), while a (dicyclohexylphosphino)biphenyl-type ligand
XPhos,12a which is one of the most efficient ligands reported to
date in the cross-coupling reactions, gave 9a quantitatively
(entry 10)
The heterogeneous Silica-TRIP-Pd catalyst has the advanta-
ge of catalyst-product separation over homogeneous molecular
catalyst systems. Thus, after the reaction between 7a and 8a in
the presence of the Silica-TRIP/Pd(OAc)2 catalyst system (Ta-
ble 2, entry 3), the reaction mixture was filtered through
Celite®. The ICP-AES analysis indicated that Pd residues in
the filtrate were 3% of the loaded Pd. This value was much
less than the corresponding value (58% of the loaded Pd) in
the experiment for the homogeneous system with the XPhos
ligand (Table 3, entry 10). The hot filtration test with the Sili-
ca-TRIP/Pd(OAc)2 catalyst showed no catalytic activity in the
solution phase (Figure S6).25
+
Me
Cl
(HO)2B Ph
Me
Ph
K3PO4 (1.5 mmol)
THF, 60 °C, 12 h
7a (0.5 mmol) 8a (0.6 mmol)
9a
entry
ligand
yield (%)b
1c
2
Silica-SMAP
Silica-3p-TPP
Silica-1p-TPP
TMSO-TRIP (5)
Ph-TRIP
82d
83
9
3
4
5
1
0
6
7
8
9
Ph-SMAP
PPh3
PCy3
PtBu3
XPhos
0
<1
51
14
>99
10
a
Conditions: 7a (0.5 mmol), 8a (0.6 mmol), Pd(OAc)2 (0.0025
mmol, 0.5 mol%), ligand (0.003 mmol, 0.6 mol%), K3PO4 (1.5
b
mmol), THF (1.5 mL), 60 °C, 12 h. Yields of 9a were deter-
1
c
d
mined by H NMR. 0.5 mol% of ligand, for 10 h. Data were
taken from ref. 4g.
Me
P
Me
Me
Ph2P
Si
Me
SiMe3
Me
Si
O
Me
O
SiMe3
O
Si
O
Si
O
O
Me
Me
Si
Si
O
Si
O
O
Si
Si
O
O
O
O
O
Si
O
O
O
O
O
O
O
O
O
O
SiO2
SiO2
Silica-3p-TPP
P
Silica-1p-TPP
P
Si
Si
PCy2
iPr
iPr
Ph-TRIP
XPhos
Ph-SMAP
iPr
The utility of Silica-TRIP for the Pd-catalyzed Suzuki–
Miyaura coupling of various chloroarenes 7 and arylboronic
acids 8 is summarized in Table 4. In the presence of 0.5–1
mol% of the Pd catalyst, chloroarenes with electron-donating
Figure 11. Heterogeneous and homogeneous phosphine lig-
ands employed in Table 3.
6
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Page 7 of 12
ACS Catalysis
Table 4. Silica-TRIP-Pd catalyzed Suzuki–Miyaura cross-coupling of 7 and 8.a
1
2
3
4
Pd(OAc)2 (0.5–1 mol%)
Silica-TRIP (0.6–2.5 mol%)
+
Cl
(HO)2B
8 (0.6 mmol)
R2
R2
R1
R1
K3PO4 (1.5 mmol)
THF (1.5 mL), 60 °C, 12 h
7 (0.5 mmol)
9
5
6
7
chloroarenes
arylboronic acids
[Pd] loading
(mol%)
[P] loading
(mol%)
product
yield
entry
7
8
9
(%)b
8
9
MeO
Cl
Cl
Cl
Cl
MeO
8a
8a
8a
8a
8a
8a
1
1
1.2
2.5
1
94
83
92
78
74
88
88
7b
7c
9b
9c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H2N
H2N
2
3
4
5
6
7
1
F3C
F3C
0.5
0.5
0.5
0.5
1
7d
9d
O
O
1
Me
Me
7e
9e
O
O
Cl
0.6
1
MeO
MeO
7f
9f
NC
Cl
NC
7g
9g
Cl
tBu
(HO)2B
tBu
1.2
N
N
8b
7h
9h
Me
Me
8a
8a
8
1
2.5
94
Cl
7i
9i
Me
Me
Me
Cl
9
1
2.5
91
Me
7j
9j
OMe
Cl
OMe
OMe
10c,d
11c,d
12d
1
1
80
86
80
MeO
MeO
8a
OMe
7k
9k
9l
(HO)2B
F3C
7d
7i
0.7
0.5
0.7
0.7
8c
Me
(HO)2B
Me
8d
Me
9m
Me
13d
0.5
0.8
88e
7j
8d
Me Me
9n
a
Conditions: 7 (0.5 mmol), 8 (0.6 mmol), Pd(OAc)2 (0.0025–0.005 mmol, 0.5–1 mol%), Silica-TRIP (0.003–0.0125 mmol, 0.6–2.5
mol%), K3PO4 (1.5 mmol), THF (1.5 mL), 60 °C, 12 h. b Yields of isolated products. c PdCl2(py)(Silica-TRIP) was used instead of Silica-
TRIP/Pd(OAc)2. d 8 (1 mmol) was used. e Isolated product was contaminated with 9m (4%).
Unfortunately, reusability of the Silica-TRIP/Pd(OAc)2
catalyst system was unsatisfactory under the present condi-
tions. Specifically, the reaction of 7a (0.5 mmol) and 8a (0.75
mmol) with K3PO4 (1.5 mmol) in THF (1 mL) at 60 °C for 1 h
in the presence of a heterogeneous catalyst prepared from
Pd(OAc)2 (1 mol%) and Silica-TRIP (1.5 mol%) gave 9a in
97% yield (1H NMR). Insoluble solids were recovered by fil-
tration through a cotton plug followed by washing successive-
ly with H2O, THF, and Et2O. The catalysts recovered in this
way gave decreased product yields with the increasing of the
reuse (2nd run, 28%; 3rd run, 3%). Black color of the solid
phase was supportive of the formation of inactive Pd species
in the solid phase. The gradual Pd leaching was also observed
during the catalyst reuse experiment; during the second run,
color of the solution phase turned to dark brown. This Pd
leaching may be due to the moderate coordination ability of
the triarylphosphine center of Silica-TRIP.26
7
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ACS Catalysis
Page 8 of 12
CONCLUSION
were performed at Hokkaido University Sousei Hall. A mi-
1
2
3
4
5
6
7
8
crowave digestion system (Milestone, ETHOS One) was used
to prepare samples for ICP-AES analysis. IR spectra were
measured with a Perkin-Elmer Spectrum One. GLC analyses
were conducted on a Shimadzu GC-14B equipped with a
flame ionization detector. Melting points were determined on a
micro melting point apparatus (Yanaco MP-500D).
Silica-supported triptycene-type phosphine Silica-TRIP,
comprising a 9-phospha-10-silatriptycene (TRIP) and silica
gel as a coordinating moiety and a solid support, respectively,
was synthesized in an efficient way. This material was charac-
terized by nitrogen adsorption measurements and 13C, 29Si and
31P CP/MAS NMR spectroscopies. Because of the cage-to-
surface direct immobilization, Silica-TRIP exhibited a mono-
P-ligating feature toward a Pd(II) complex, resulting in selec-
tive formation of a 1:1 Pd-P species even with the P-ligand in
excess. This coordination behavior was confirmed by 31P
CP/MAS NMR spectroscopy. Usefulness of Silica-TRIP was
previously demonstrated in the application to the Rh-catalyzed
heteroatom-directed borylation of C(sp3)–H bonds of amides,
ureas, and 2-aminopyridines at the position α to the N atom. In
the present work, Silica-TRIP was used as a ligand for the Pd-
catalyzed Suzuki–Miyaura cross-coupling reaction with chlo-
roarenes. This catalytic reaction occurred under mild condi-
tions, regardless of the moderate electron-donating nature of
the triarylphosphine-based ligand. The facile catalyst-product
separation by filtration was demonstrated as the merit of the
heterogeneous Pd catalyst system over homogeneous systems.
However, catalyst deactivation in the Suzuki–Miyaura cou-
pling hampered the efficient reuse of the Pd catalyst.
Preparation of Tris(2-bromophenyl)phosphine (2).8a
A
solution of iPrMgBr in THF (0.92 M, 38 mL, 35 mmol), which
was freshly prepared from iPrBr and Mg, was added over 25
min to a solution of 1-bromo-2-iodobenzene (1, 9.90 g, 35
mmol) in THF (35 mL) at –20 °C. The reaction mixture turned
into a gray slurry after stirring at –20 °C for an additional 3 h.
After complete consumption of 1, which was confirmed by
GC analysis of a small aliquot of the reaction mixture,
TMEDA (5.25 mL, 35 mmol) and PCl3 (872 µL, 10 mmol)
were added in a dropwise manner in that order at –20 °C. The
resulting mixture was allowed to warm to 0 °C and stirred at
this temperature for an additional 14 h to give a clear pale-
yellow solution. After quenching with NH4Cl aq. at 0 °C, the
reaction mixture was extracted with EtOAc. The organic layer
was washed with brine, dried over MgSO4, filtered, and con-
centrated. The residue was passed through a short silica gel
column with toluene as an eluting solvent, and the eluent was
evaporated under vacuum. The residual solids were recrystal-
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
lized
from
benzene/MeOH
to
give
tris(2-
bromophenyl)phosphine as a while solid (2.86 g, 57% yield).
1H NMR (CDCl3): δ 6.74–6.77 (m, 3H), 7.22–7.28 (m, 6H),
7.63–7.66 (m, 3H). 13C NMR (CDCl3): δ 127.78, 130.37 (d,
JC–P = 34.4 Hz), 130.73, 133.19 (d, JC–P = 1.9 Hz), 134.69,
136.72 (d, JC–P = 11.5 Hz). 31P NMR (CDCl3): δ –2.8.
EXPERIMENTAL SECTION
General. All reactions were carried out under nitrogen or
argon atmosphere. Materials were obtained from commercial
suppliers or prepared according to standard procedures unless
otherwise noted. Although Silica-SMAP4a,c and Silica-TRIP4h
are commercially available,5 the silica-supported ligands for
this work were prepared according to the reported procedure
or the modified procedure described below, respectively.
CARiACT Q-10 silica gel (Catalyst grade, 75–150 µm, Fuji
Silysia Chemical, Ltd.) was dehydrated by heating at 120 °C
under vacuum for 10 h and stored in a glove box before use.
Silica-3p-TPP,17b Silica-1p-TPP,17b Ph-SMAP,6 and Ph-TRIP4h
were prepared according to the reported procedure. Pd(OAc)2
was purchased from Aldrich Co., Ltd., and PdCl2(py)2 was
prepared according to the literature.27 Phenylboronic acid (8a)
was purchased from TCI Co., Ltd., and was recrystallized
from hot water before use. K3PO4 was purchased from Junsei
Chemicals Co., Ltd., and dried at 150 °C for 10 h under vacu-
um. All solvents for catalytic reactions were degassed via three
freeze–pump–thaw cycles.
Preparation of Silanol 3. For the preparation of 3, Tsuji
and Tamao’s procedure for the synthesis of 10-chloro-9-
phospha-10-silatriptycene8a was modified as follows. A solu-
tion of tBuLi in pentane (1.77 M, 5.1 mL, 9.0 mmol) was add-
ed over 15 min to a solution of 2 (748 mg, 1.5 mmol) in THF
(4.5 mL) and Et2O (23 mL) at –78 °C. After stirring for 4 h,
SiCl4 (172 µL, 1.5 mmol) was added in a dropwise manner at
–78 °C, and the resulting mixture was stirred at this tempera-
ture for an additional 4 h. After quenching the excess organo-
lithium species with Me3SiCl (1.1 mL, 9 mmol) at –78 °C, the
mixture was allowed to warm to room temperature and stirred
for an additional 13 h. After evaporation of the volatiles, the
residue was dissolved in toluene and filtered through a Celite®
pad. The filtrate was concentrated, and the crude product was
purified by silica gel column chromatography (hexane/EtOAc
100:0–80:20) followed by reprecipitation from CH2Cl2/hexane
to give silanol 3 as a white solid (321 mg, 70% yield). Single
crystals of 3 suitable for X-ray diffraction studies were ob-
tained by recrystallization from a CHCl3/hexane solution
(CCDC: 1060983).
Solution NMR spectra were recorded on a JEOL ECX-II
1
(400 MHz for H NMR, 100.5 MHz for 13C NMR, 79.4 MHz
for 29Si NMR and 161.8 MHz for 31P NMR). Chemical shift
values are referenced to Me4Si (1H and 29Si), the residual sol-
vent (13C), and H3PO4 (31P). Magic angle spinning (MAS)
NMR spectra were recorded on a Bruker MSL-300 spectrome-
ter, operating at 75.5 MHz for 13C NMR, 59.6 MHz for 29Si
NMR, and 121.5 MHz for 31P NMR. Combustion elemental
analyses (J-SCIENCE Micro Corder JM10 or Yanako MT-6)
and high-resolution mass spectra (Thermo Scientific Exactive
or JEOL JMS-T100LC for ESI-MS, and JEOL JMS-T100GCv
for EI-MS) were recorded at the Instrumental Analysis Divi-
sion, Equipment Management Center, Creative Research Insti-
tution, Hokkaido University. N2 adsorption (Quantachrome
Autosorb-6) and ICP-AES analysis (Shimadzu ICPE-9000)
1
M.p.: 120 °C (decomp.). H NMR (CDCl3): δ 3.32 (s, 1H),
7.21–7.28 (m, 6H), 7.81 (d, J = 6.4 Hz, 3H), 7.87 (dd, J =
10.8, 7.2 Hz, 3H). 13C NMR (CDCl3): δ 127.86, 128.10 (d, JC–P
= 15.2 Hz), 131.56, 134.69 (d, JC–P = 45.8 Hz), 141.13, 146.12
(d, JC–P = 8.6 Hz). 29Si NMR (CDCl3): δ –30.6 (d, JSi–P = 8.5
Hz). 31P NMR (CDCl3): δ –54.0. IR (ATR): 3347 (br), 3052,
2964, 1688, 1572, 1430, 1260, 896, 755, 666 cm–1. HRMS–
ESI (m/z): [M–H]– calcd for C18H12OPSi, 303.04005; found,
303.04086.
Preparation of Disiloxane 4. Silanol 3 (30.0 mg, 0.1
mmol) and imidazole (1.4 mg, 0.02 mmol) were dissolved in
benzene (0.5 mL), and the mixture was stirred at 80 °C for
8
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Page 9 of 12
ACS Catalysis
10.5 h. After cooling to room temperature, the insoluble solids (18.8 mg, 0.056 mmol) were placed in a vial containing a
1
2
3
4
5
6
7
8
were filtered and washed with benzene to give the disiloxane 4
as a white solid (15.0 mg, 52% yield). Due to the low solubili-
ty of 4 into organic solvents, clear NMR data were not ob-
tained.
magnetic stirring bar. Anhydrous, degassed CH2Cl2 (2 mL)
was added, and the tube was sealed with a screw cap. The
mixture was stirred at room temperature for 0.5 h. The suspen-
sion was filtered and washed with CH2Cl2. The filtrate was
evaporated to recover unreacted PdCl2(py)2 (8.3 mg, 0.025
mmol). The pale yellow silica gel was dried under vacuum to
give PdCl2(py)(Silica-TRIP) (375.1 mg). Thus, P loading on
Silica-TRIP was calculated to be 0.078 mmol/g based on a Pd
to P stoichiometry of 1:1 [{(18.8–8.3)/335.53}/0.4015 =
0.078]. For convenience, the value of 0.07 mmol/g was used
for P loading in metal complexations and catalytic applica-
tions.
1
White solid. M.p. 430 °C (decomp.). H NMR (CDCl3): δ
7.22–7.26 (m, 6H), 7.31 (t, J = 7.2 Hz, 6H), 7.91 (d, J = 6.4
Hz, 6H), 7.98 (dd, J = 11.2, 7.2 Hz, 6H). 13C NMR (CDCl3): δ
128.27, 128.47 (d, JC–P = 15.3 Hz), 131.77, 134.98 (d, JC–P
=
45.8 Hz), 140.77, 146.31 (d, JC–P = 9.6 Hz). 31P NMR (CDCl3):
δ –55.1. HRMS-EI (m/z) Calcd for [M]+ C36H24OP2Si2,
590.08409; found, 590.08389.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Preparation of TMSO-TRIP 5. A solution of silanol 3
(45.7 mg, 0.15 mmol) in CH2Cl2 (2 mL) was added dropwise
to a solution of N-trimethylsilylimidazole (105 mg, 0.75
mmol) in CH2Cl2 (2 mL). After stirring at room temperature
for 1 h, MeOH (2 mL) was added to quench the excess N-
trimethylsilylimidazole, and then the mixture was stirred for
additional 10 min. The volatiles were removed under reduced
pressure. The crude product was purified by silica gel column
chromatography (eluting with CH2Cl2) to give 10-
13C CP/MAS NMR: δ 3, 120–158 (br m). 31P CP/MAS
NMR: δ –5.
Reaction of PdCl2(py)2 and Silica-TRIP (Pd/P 1:2). Sili-
ca-TRIP (399.2 mg, 0.028 mmol, 0.07 mmol/g) and PdCl2(py)2
(4.8 mg, 0.014 mmol) were placed in a vial containing a mag-
netic stirring bar. Anhydrous, degassed CH2Cl2 (2 mL) was
added, and the tube was sealed with a screw cap. The mixture
was stirred at room temperature for 0.5 h. The suspension was
filtered and washed with CH2Cl2. The pale yellow silica was
dried under vacuum to give a mixture of PdCl2(py)(Silica-
TRIP) and unreacted Silica-TRIP (363.5 mg).
{(trimethylsilyl)oxy}-9-phospha-10-silatriptycene (5) as
a
white solid (49.0 mg, 87% yield).
1
M.p.: 225–228 ºC. H NMR (CDCl3): δ 0.52 (s, 9H), 7.19–
7.28 (m, 6H), 7.76 (dd, J = 6.8, 0.8 Hz, 3H), 7.85 (dd, J =
11.6, 6.8 Hz, 3H). 13C NMR (CDCl3): δ 2.48, 127.79–127.94
(m), 131.58, 134.61 (d, JC–P = 45.8 Hz), 142.09, 146.12 (d, JC–
31P CP/MAS NMR: δ –52, –5.
Reaction of PdCl2(py)2 and TMSO-TRIP (Pd/P 1:1, 1:2).
TMSO-TRIP (5) (0.02 mmol) and PdCl2(py)2 (0.02 mmol for
Pd/P 1:1; 0.01 mmol for Pd/P 1:2) were placed in a 5-mL glass
tube containing a magnetic stirring bar. CDCl3 (0.7 mL) was
added and stirred at room temperature for 5 min. The mixture
was analyzed with 31P NMR spectroscopy.
= 7.6 Hz). 31P NMR (CDCl3): δ –54.9. 29Si NMR (CDCl3): δ
P
–38.9 (d, JSi–P = 9.1 Hz), 14.4. HRMS-EI (m/z) Calcd for [M]+
C21H21OPSi2, 376.08685; found, 376.08541.
Preparation of Silica-TRIP. Silanol 3 (365 mg, 1.2 mmol),
CARiACT Q-10 silica gel (10.6 g), imidazole (408 mg, 6.0
mmol), and anhydrous, degassed toluene (42 mL) were placed
in a 200-mL three-necked flask equipped with a mechanical
stirrer under argon atmosphere. The suspension was gently
stirred at 100 °C for 16 h. After cooling to room temperature,
the mixture was filtered, washed successively with degassed
toluene, toluene-MeOH (1:1), and MeOH, and dried under
vacuum at 120 °C overnight. Next, a solution of N-
trimethylsilylimidazole (5.3 mL) in THF (30 mL) was added
to the flask with the functionalized silica gel Silica-
TRIP(SiOH). The suspension was stirred at 60 °C for 24 h
under argon atmosphere. After cooling to room temperature,
solids were collected by filtration through a glass filter,
washed with MeOH, and dried under vacuum at 120 °C over-
night to give 9.4 g of Silica-TRIP.
Preparation of PdCl2(py)(5). A solution of 5 (18.8 mg,
0.05 mmol) in CHCl3 (1.5 mL) was added dropwise to a solu-
tion of PdCl2(py)2 (17.8 mg, 0.05 mmol) in CHCl3 (1.5 mL).
The mixture was stirred at room temperature for 0.5 h. The
volatiles were evaporated. The residue was extracted with
benzene. After filtration, the solution was evaporated. The
residue was recrystallized from CHCl3/hexane to give
PdCl2(py)(5) as a yellow solid (22.8 mg, contaminated with
3% of PdCl2(py)2, 71% yield)
1
M.p.: 190 °C (decomp.). H NMR (CDCl3): δ 0.53 (s, 9H),
7.30–7.39 (m, 6H), 7.50 (t, J = 7.2 Hz, 2H), 7.71 (d, J = 6.8
Hz, 3H), 7.88 (t, J = 7.2 Hz, 1H), 9.12 (dd, J = 13.6, 7.3 Hz,
3H), 9.16–9.19 (m, 2H). 13C NMR (CDCl3): δ 2.37, 124.89 (d,
JC–P = 3.8 Hz), 127.96 (d, JC–P = 13.4 Hz), 128.73, 130.49 (d,
JC–P = 7.6 Hz), 136.74 (d, JC–P = 19.2 Hz), 138.57, 140.04 (d,
JC–P = 49.8 Hz), 140.99, 151.47. 31P NMR (CDCl3): δ –6.5.
Attempts to obtain MS spectra (ESI or FAB) of PdCl2(py)(5)
were unsuccessful.
13C CP/MAS NMR: δ 2, 123–150 (br m). 29Si CP/MAS
NMR: δ –110, −102, –36, 14. 31P CP/MAS NMR: δ –52. Ele-
mental Anal. found: C 5.33; H 1.19.
Preparation of Silica-TRIP oxide. Silica-TRIP (200 mg,
0.014 mmol, 0.07 mmol/g) and m-chloroperbenzoic acid
(≤77%, 9.4 mg, 0.042 mmol) were placed in a vial containing
a magnetic stirring bar. CH2Cl2 (1 mL) was added, and the
tube was sealed with a screw cap. The mixture was stirred at
room temperature for 1 h. The suspension was filtered, washed
with CH2Cl2, and dried under vacuum to give Silica-TRIP
oxide (183.4 mg). The sample for analysis was prepared after
drying under vacuum at 120 °C overnight.
Preparation of PdCl2(5)2. The mixture of 5 (37.6 mg, 0.10
mmol), PdCl2(py)2 (17.8 mg, 0.05 mmol) and CHCl3 (2 mL)
was stirred at room temperature for 1 h. The volatiles were
evaporated. The residue was washed with CHCl3 to give
PdCl2(5)2 as a yellow solid (39.6 mg, 43% yield).
1
M.p.: 310 ºC (decomp.). H NMR (CDCl3): δ 0.55 (s, 18H),
7.30–7.43 (m, 12 H), 7.78 (d, J = 7.6 Hz, 6H), 9.18–9.24 (m,
6H). 31P NMR (CDCl3): δ –19.5. HRMS-ESI (m/z) Calcd for
[M+Na]+
C42H42Cl2O2P2PdSi4Na,
953.00508;
found
13C CP/MAS NMR: δ 2, 130 (br), 143 (br). 29Si CP/MAS
NMR: δ –110, –101, –38, 14. 31P CP/MAS NMR: δ –8. Ele-
mental Anal. found: C 5.42; H 0.99.
953.00236. 13C NMR data are not available due to the low
solubility of PdCl2(5)2 in organic solvents.
Reaction of PdCl2(py)2 and Silica-1p-TPP (Pd/P 1:2). Sil-
ica-1p-TPP (200 mg, 0.018 mmol, 0.09 mmol/g) and
PdCl2(py)2 (3.0 mg, 0.0089 mmol) were placed in a vial con-
Reaction of PdCl2(py)2 and Silica-TRIP (Pd/P 2:1). Sili-
ca-TRIP (401.5 mg, 0.028 mmol, 0.07 mmol/g) and PdCl2(py)2
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taining a magnetic stirring bar. Anhydrous, degassed CH2Cl2 to determine the yield of 4-methylbiphenyl (9a, 93% yield).
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7
8
(2 mL) was added, and the tube was sealed with a screw cap.
The mixture was stirred at room temperature for 1 h. The sus-
pension was filtered and washed with CH2Cl2. The pale yellow
silica was dried under vacuum to give a mixture of
PdCl2(py)(Silica-1p-TPP), PdCl2(Silica-1p-TPP)2 and unreact-
ed Silica-TRIP (200 mg).
The crude product was purified by silica gel chromatography
(eluting with hexane) to give 9a (74.3 mg, 0.44 mmol, 88%
yield).
ICP-AES Measurements for Pd Residue. After the reac-
tion of 7a (0.5 mmol) and 8a (0.6 mmol) in the presence of 0.5
mol% the Silica-TRIP-Pd catalyst (Table 2, entry 3), the reac-
tion mixture was diluted with Et2O, and filtered through a
Celite® pad (eluting with Et2O). The volatiles were evaporated.
A small aliquot of the residue (ca. 5 mg) was taken for a mi-
crowave-assisted acid digestion with aqua regia (1000 W,
220 °C) followed by a mixture of nitric acid and perchloric
acid (1000 W, 220 °C). The resulting mixture was diluted in
H2O (20 mL). This solution was subjected to ICP-AES analy-
sis.
31P CP/MAS NMR: δ –4, 23, 29.
Reaction of PdCl2(py)2 and PPh2[4-Me2(iPrO)Si-C6H4]
(Pd/P 1:1, 1:2). PPh2[4-Me2(iPrO)Si-C6H4] (6) (0.02 mmol)
and PdCl2(py)2 (0.02 mmol for Pd/P 1:1; 0.01 mmol for Pd/P
1:2) were placed in a 5-mL glass tube containing a magnetic
stirring bar. CDCl3 (0.7 mL) was added and stirred at room
temperature for 5 min. The mixture was analyzed with 31P
NMR spectroscopy.
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Preparation of PdCl2(py)(6). A solution of 6 (18.9 mg,
0.05 mmol) in CHCl3 (1.5 mL) was added dropwise to a solu-
tion of PdCl2(py)2 (17.8 mg, 0.05 mmol) in CHCl3 (1.5 mL).
The mixture was stirred at room temperature for 0.5 h. The
volatiles were evaporated. The residue was extracted with
benzene. After filtration, the solution was evaporated. The
residue was recrystallized from benzene/hexane to give
PdCl2(py)(6) as a yellow solid (16.8 mg, contaminated with
6% of PdCl2(py)2, 51% yield)
Hot Filtration Test. In a nitrogen-filled glove box, Silica-
TRIP (0.07 mmol/g, 42.9 mg, 0.003 mmol) and a solution of
Pd(OAc)2 (0.56 mg, 0.0025 mmol) in anhydrous, degassed
THF (0.5 mL), and THF (1 mL) were placed in a 10-mL glass
tube containing a magnetic stirring bar. After stirring at room
temperature for 5 min, phenylboronic acid (8a, 73.2 mg, 0.6
mmol), K3PO4 (318 mg, 1.5 mmol), p-chlorotoluene (7a, 63.3
mg, 0.5 mmol), and 1,2-diphenylethane (91.1 mg, 0.5 mmol,
internal standard) were added successively. The tube was
sealed with a screw cap, and the mixture was stirred at 60 °C.
A small aliquot was taken out from the reaction mixture at
suitable interval, and the samples were diluted with Et2O and
analyzed by GC. After 15 min, the hot mixture was filtered
through a Celite® pad. The filtrate was placed in a new 10-mL
glass tube containing 8a (0.5 mmol), K3PO4 (1.5 mmol), and a
magnetic stirring bar. Again, the mixture was stirred at 60 °C.
The yields of the product 9a were determined by GC analysis.
Reuse of Silica-TRIP-Pd System. In a nitrogen-filled
glove box, Silica-TRIP (0.07 mmol/g, 107 mg, 0.0075 mmol)
and a solution of Pd(OAc)2 (1.1 mg, 0.0050 mmol) in anhy-
drous, degassed THF (1 mL) were placed in a 10-mL glass
tube containing a magnetic stirring bar. After stirring at room
temperature for 5 min, K3PO4 (318 mg, 1.5 mmol), phenyl-
boronic acid (8a, 91.4 mg, 0.75 mmol) and p-chlorotoluene
(7a, 63.3 mg, 0.5 mmol) were added successively. The tube
was sealed with a screw cap and removed from the glove box.
The mixture was stirred at 60 °C for 1 h. After cooling to room
temperature, the mixture was diluted with Et2O and filtered
through a glass pipette equipped with a cotton plug (eluting
with Et2O). The inorganic salts were removed by washing
H2O, and then solvent displacement was performed with THF
followed by Et2O. After drying under vacuum, the recovered
gel was placed in the vial tube that was used for second run.
Anhydrous, degassed THF (1 mL), K3PO4 (318 mg, 1.5
mmol), 7a (63.3 mg, 0.5 mmol) and 8a (91.4 mg, 0.75 mmol)
were added, and the mixture was stirred at 60 °C for 1 h.
1
M.p.: 210 °C (decomp.). H NMR (CDCl3): δ 0.37 (s, 6H),
1.15 (d, J = 6.4 Hz, 6H), 4.02 (sept, J = 6.4 Hz, 1H), 7.35–7.51
(m, 8H) 7.64 (dd, J = 8.4, 2.8 Hz, 2H), 7.76–7.84 (m, 7H),
8.99–9.01 (m, 2H). 13C NMR (CDCl3): δ –1.16, 25.68, 65.51,
124.59 (d, JC–P = 2.8 Hz), 128.09 (d, JC–P = 11.5 Hz), 129.21
(d, JC–P = 58.4 Hz), 130.16 (d, JC–P = 56.5 Hz), 131.02 (d, JC–P
= 2.8 Hz), 133.02 (d, JC–P = 10.6 Hz), 133.90 (d, JC–P = 9.5
Hz), 134.82 (d, JC–P = 9.5 Hz), 138.20, 142.33 (d, JC–P = 1.9
Hz), 151.60. 31P NMR (CDCl3): δ 29.3. Attempts to obtain MS
spectra (ESI or FAB) of PdCl2(py)(6) were unsuccessful.
Preparation of PdCl2(6)2. The mixture of 6 (22.7 mg, 0.06
mmol), PdCl2(py)2 (10.1 mg, 0.03 mmol) and CHCl3 (1 mL)
was stirred at room temperature for 1 h. The volatiles were
evaporated. The resulting residue was recrystallized from
CH2Cl2/hexane to give PdCl2(6)2 as a yellow solid (26.5 mg,
contaminated with traces of impurities, 47% yield).
1
M.p.: 192–195 °C. H NMR (CDCl3): δ 0.36 (s, 12H), 1.14
(d, J = 6.4 Hz, 12H), 4.01 (sept, J = 6.4 Hz, 2H), 7.35–7.45
(m, 12H), 7.59 (d, J = 7.6 Hz, 4H), 7.68–7.73 (m, 12H). 13C
NMR (CDCl3): δ –1.15, 25.68, 65.44, 128.03 (vt, JC–P = 4.8
Hz), 129.51 (vt, JC–P = 24.9 Hz), 130.48, 130.59 (vt, JC–P
=
23.9 Hz), 132.97 (vt, JC–P = 4.8 Hz), 134.15 (vt, JC–P = 5.7 Hz),
135.00 (vt, JC–P = 6.7 Hz), 141.48. 31P NMR (CDCl3): δ 23.7.
HRMS-ESI (m/z) Calcd for [M+Na]+ C46H54Cl2O2P2PdSi2Na,
957.14513; found 957.14797.
Typical Procedure for the Suzuki–Miyaura Coupling
Reaction. In a nitrogen-filled glove box, Silica-TRIP (0.07
mmol/g, 42.9 mg, 0.003 mmol) and a solution of Pd(OAc)2
(0.56 mg, 0.0025 mmol) in anhydrous, degassed THF (0.5
mL) were placed in a 10-mL glass tube containing a magnetic
stirring bar. After stirring at room temperature for 5 min, phe-
nylboronic acid (8a, 73.1 mg, 0.6 mmol), K3PO4 (318 mg, 1.5
mmol), p-chlorotoluene (7a, 63.3 mg, 0.5 mmol), and THF (1
mL) were added successively. The tube was sealed with a
screw cap and removed from the glove box. The mixture was
stirred at 60 °C for 12 h. After cooling to room temperature,
the mixture was diluted with Et2O and filtered through a
Celite® pad (eluting with Et2O). The volatiles were evaporated,
and an internal standard (1,1,2,2-tetrachloroethane) was added
1
Yields of the products were determined by H NMR analysis
with 1,1,2,2-tetrachloroethane as an internal standard. This
recycling procedure was repeated two times (1st run, 97%; 2nd
run, 28%; 3rd run, 3%).
X-ray Crystallographic Analysis of 3. Crystal data for
3{3·1/3(CHCl3)} (CCDC 1060983; recrystallization from
CHCl3/hexane). C55H40Cl3O3P3Si3, M = 1032.46, trigonal,
space group P–3–c1 (#165), a = 15.5297(5) Å, c = 24.1564(11)
Å, V = 5045.3(4) Å3, Z = 4, density (calc.) = 1.359, total re-
flections collected = 39823, unique reflections = 3860 (Rint
0.0542), GOF = 1.139, R1 (I>2σ(I)) = 0.0461, wR2 = 0.1077.
=
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ACS Catalysis
Data were collected on a Rigaku Mercury 70 CCD diffrac- T.; Sawamura, M. Adv. Synth. Catal. 2012, 354, 3440–3444.
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tometer with graphite monochromated Mo-Kα radiation (λ =
0.71075 Å) at 150 K, and processed using the CrystalClear
software.28 Structures were solved by a direct method using
SIR-2004,29 and refined by full-matrix least-square method
using SHELXL-97.30 Non-hydrogen atoms were refined aniso-
tropically. All hydrogen atoms except for the silanol protons
were located on the calculated positions and refined using a
riding model. The silanol protons (H1 and H2; disordered, 1:1)
were located in the difference Fourier map and refined isotrop-
ically. All calculations were performed using the CrystalStruc-
ture software package.31
(j) Kawamorita, S.; Murakami, R.; Iwai, T.; Sawamura, M. J.
Am. Chem. Soc. 2013, 135, 2947–2950. (k) Konishi, S.; Ka-
wamorita, S.; Iwai, T.; Steel, P. G.; Marder, T. B.; Sawamura,
M. Chem. Asian J. 2014, 9, 434–438. (l) Murakami, R.; Tsu-
noda, K.; Iwai, T.; Sawamura, M. Chem. Eur. J. 2014, 20,
13127–13131.
(5) Silica-SMAP and Silica-TRIP can be purchased from
Wako Pure Chemical Industries.
(6) (a) Ochida, A.; Hara, K.; Ito, H.; Sawamura, M. Org. Lett.
2003, 5, 2671–2674. (b) Ochida, A.; Ito, S.; Miyahara, T.; Ito,
H.; Sawamura, M. Chem. Lett. 2006, 35, 294–295. (c) Ochida,
A.; Hamasaka, G.; Yamauchi, Y.; Kawamorita, S.; Oshima, N.;
Hara, K.; Ohmiya, H.; Sawamura, M. Organometallics 2008,
27, 5494–5503.
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ASSOCIATED CONTENT
Supporting Information. Experimental procedures and charac-
terization of the products. This material is available free of
charge via the Internet at http://pubs.acs.org.
(7) For selected examples of immobilized phosphines with
rigid linkers, see: (a) Yang, Y.; Beele, B.; Blümel, J. J. Am.
Chem. Soc. 2008, 130, 3771–3773. (b) Beele, B.; Guenther, J.;
Perera, M.; Stach, M.; Oeser, T.; Blümel, J. New J. Chem.
2010, 34, 2729–2731.
AUTHOR INFORMATION
Corresponding Author
sawamura@sci.hokudai.ac.jp
(8) (a) Tsuji, H.; Inoue, T.; Kaneta, Y.; Sase, S.; Kawachi, A.;
Tamao, K. Organometallics 2006, 25, 6142–6148. (b) Tsuji,
H.; Inoue, T.; Sase, S.; Tamao, K. Acta Cryst. E 2006, 62,
m535–m537.
(9) Kawamorita, S.; Miyazaki, T.; Iwai, T.; Ohmiya, H.;
Sawamura, M. J. Am. Chem. Soc. 2012, 134, 12924–12927.
(10) Portions of this work on the synthesis of Silica-TRIP (ref.
4h) and its application to Pd-catalyzed Suzuki–Miyaura cross-
coupling of chloroarenes (ref. 17b) have been reported.
(11) For selected reviews, see: (a) Littke, A. F.; Fu, G. C. An-
gew. Chem., Int. Ed. 2002, 41, 4176–4211. (b) Suzuki, A. An-
gew. Chem., Int. Ed. 2011, 50, 6722–6737.
ACKNOWLEDGMENT
This work was supported in part by Grants-in-Aid for Young
Scientists (B) (No. 25810056) JSPS, to T.I. and by CREST and
ACT-C, JST, to M.S. S.K. thanks the JSPS for scholarship
support.
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Miyaura coupling of chloroarenes, see: (a) Hu, Q.-S.; Lu, Y.; (25) (a) Phan, N. T. S.; Sluys, M. V. D.; Jones, C. W. Adv.
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(26) The 31P CP/MAS NMR spectrum of the silica gel Pd-
Silica-TRIP recovered after the coupling reaction in a larger
scale (7a, 2.5 mmol) showed a distinct major signal at δ 4
together with minor signals at δ –52 (Silica-TRIP) and –8
(overlapped with Silica-TRIP oxide) (Figure S7a). On the oth-
er hand, a sample obtained from the solution phase of the reac-
tion mixture showed no signal in 31P NMR spectroscopy (in
CDCl3, Figure S8). These results indicate that at least most of
the phosphine moiety remained bound to the solid surface.
However, partial degradation of the Silica-TRIP structure dur-
ing the catalytic reaction cannot be ruled out due to poor sig-
nals of the aromatic carbons and the bridgehead Si atom of the
TRIP moiety in the 13C and 29Si CP/MAS NMR spectra, re-
spectively (Figures S7b and S7c).
(18) Kowada, T.; Yamaguchi, S.; Ohe, K. Org. Lett. 2010, 12,
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(21) Without imidazole as an additive, a significant amount of
silanol 3 remained in the crude product (ca. 20% conversion of
3 after 12 h).
(27) Krogul, A.; Cedrowski, J.; Wiktorska, K.; Ozimiński, W.
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(22) The oxidation of TMSO-TRIP (5) with mCPBA afforded
the phosphine oxide TMSO-TRIP oxide (S1). See the Support-
ing Information for details.
(23) The coordination behavior of Silica-1p-TPP toward a
Pd(II) center using PdCl2(PhCN)2 was studied previously. See,
ref 17b.
(24) Reactions of Pd(OAc)2 with Silica-TRIP (Pd/P 2:1 or
1:1), or TMSO-TRIP (Pd/P 2:1 or 1:1) were conducted to in-
vestigate their coordination properties toward the Pd center
(Figures S3–S5).
(29) SIR-2004: Burla, M. C.; Caliandro, R.; Camalli, M.; Car-
rozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.;
Polidori, G.; Spagna, R. J. Appl. Cryst. 2005, 38, 381–388.
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112–122.
(31) CrystalStructure 4.0: Crystal Structure Analysis Package;
Rigaku Corporation: Tokyo, Japan, 2000–2010.
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