Heterogeneous double-activation catalysis: Rh complex and tertiary amine on the same solid surface for the 1,4-addition reaction of aryl- and alkylboronic acids

Article abstract of DOI:10.1039/c5cy00133a

Double-activation catalysis by a rhodium complex/tertiary amine catalyst for the 1,4-addition of organoboronic acids was investigated. A rhodium complex and a tertiary amine were co-immobilized on the same silica surface by silane-coupling reactions follo

Full text of DOI:10.1039/c5cy00133a

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Heterogeneous double-activation catalysis: Rh
complex and tertiary amine on the same solid
surface for the 1,4-addition reaction of aryl- and
alkylboronic acids†
Cite this: Catal. Sci. Technol., 2015,
5, 2714
Hiroto Noda,a Ken Motokura,a Wang-Jae Chun,b Akimitsu Miyaji,a Sho Yamaguchia
a
*
and Toshihide Baba
Double-activation catalysis by
a rhodium complex/tertiary amine catalyst for the 1,4-addition of
organoboronic acids was investigated. A rhodium complex and a tertiary amine were co-immobilized on
the same silica surface by silane-coupling reactions followed by complexation of the Rh species. Structures
of the Rh complex and tertiary amine on the silica surface were determined by solid-state 13C and 29Si
MAS NMR, XAFS, and XPS measurements. The immobilized tertiary amine accelerated the Rh-catalyzed
1,4-addition of phenylboronic acid to cyclohexenone. The role of the anchored tertiary amine in the
1,4-addition reaction was clarified by solid-state 11B MAS NMR: it activates the arylboronic acid forming a
four-coordinate boron species, which then accelerates the transmetallation step in the Rh complex-
catalyzed 1,4-addition. The silica-supported Rh complex/tertiary amine catalyst system could be applicable
to the reaction of various aryl- and even alkyl-boronic acids.
Received 28th January 2015,
Accepted 16th February 2015
DOI: 10.1039/c5cy00133a
www.rsc.org/catalysis
synergistic and double-activation catalysis. For example,
heterogeneous multifunctional catalysts having protonic
Introduction
Intense research activity has been devoted to the development
of concerted catalysis for the acceleration of a single transfor-
mation.1 Such catalytic systems can be classified as follows:
(i) synergistic catalysis (Scheme 1a)2 and (ii) double-activation
catalysis (Scheme 1b).3 When two substrates (A and B) are
activated by two different catalysts simultaneously to acceler-
ate a single transformation, the catalysis system is classified
as synergistic (Scheme 1a). In contrast, when two separate cat-
alysts both activate only one of the reactants (B) concertedly,
the process is classified as double-activation catalysis
(Scheme 1b).
To realize reaction systems involving two catalytic sites,
the co-immobilization of those sites on the same solid sur-
face is a key strategy. A solid surface enables incompatible
catalytic species to co-exist by suppressing their interactions
and allows the close accumulation of catalytic functionalities;
therefore, heterogeneous catalysts could have potential utility
in the acceleration of a variety of organic transformations by
acids, metal nanoparticles, metal cations, and metal com-
plexes with organic bases on the same surface have been
demonstrated.4–7
Scheme 2(a) displays heterogeneous synergistic catalysis
by a metal complex and an organic base on the same solid
surface. Our group previously reported heterogeneous syner-
gistic catalysis by a Pd complex and a tertiary amine on a sil-
ica surface for the Tsuji–Trost allylation.8 In the reaction, the
Pd complex activates an allylating reagent to form an η3-allyl-
palladium species, while the tertiary amine simultaneously
activates a nucleophile by abstraction of an α-proton. In this
catalysis system, the addition of a homogeneous amine
decreased the catalytic activity,8 which indicated that self-
quenching was avoided by anchoring both the metal complex
and the amine. When catalytic sites are immobilized on the
same surface, they can co-exist and activate substrates
simultaneously.
On the basis of the previous results, we hypothesized that
a surface co-immobilization strategy could extend to double-
activation catalysis (Scheme 2b). One reaction potentially
amenable to heterogeneous double-activation catalysis is the
1,4-addition of an arylboronic acid to an α,β-unsaturated car-
bonyl compound catalyzed by a Rh complex.9 The 1,4-addi-
tion of arylboron reagents to enones is a widely used reaction
a Department of Environmental Chemistry and Engineering, Tokyo Institute of
Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan
b Graduate School of Arts and Sciences, International Christian University,
Mitaka, Tokyo, 181-8585, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00133a
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Scheme 1 Classification of catalytic systems involving two catalysts accelerating a single transformation.
Scheme 2 Metal complex and organic base on the same surface for (a) heterogeneous synergistic and (b) double-activation catalysis.
for C–C bond formation, giving β-substituted carbonyl com-
Here, we predicted that, if a Rh complex and an organic
amine existed closely on the same solid surface, the amine
would trap the arylboronic acid as the 4-coordinate boron
species,12 ensuring proximity of the activated boron species
to the Rh complex, and thus accelerate the transmetalation
step. In addition, we speculated that a strong Lewis base,
which would be incompatible with the active Rh complex in
a homogeneous system, could co-exist with that complex in a
heterogeneous system and should powerfully activate poor
reactive substrate such as alkylboronic acid.
Herein, we demonstrate heterogeneous double-activation
catalysis by a Rh complex and a tertiary amine on a silica sur-
face. The silica-supported tertiary amine accelerates the Rh
complex-catalyzed 1,4-addition of arylboronic acids to α,β-
unsaturated carbonyl compounds. We also clarify the mecha-
nism of the anchored tertiary amine-accelerated reaction cat-
alyzed by a Rh complex on the same surface. The catalyst sys-
tem could be applicable for the 1,4-addition reaction using
various aryl- and alkyl-boronic acids.
pounds.9 It is known that the reaction is accelerated by
bases.9c,j Scheme
3 shows the reaction mechanism for
1,4-addition in the presence of a base. The catalytic cycle
using a Rh–OH complex with a base involves (i) activation of
the arylboronic acid by the base to form a 4-coordinate boron
species; (ii) transmetalation of the aryl group from the acti-
vated boron to Rh; (iii) insertion of the α,β-unsaturated car-
bonyl compound into the aryl–Rh bond; and (iv) hydrolysis,
giving the 1,4-addition product and the Rh–OH species.
Transmetalation step (ii), which is usually rate-limiting,10
proceeds smoothly when an arylboronic acid is activated by a
base to form the 4-coordinate boron species.11 This activation
mechanism is one of double-activation catalysis because
PhBIJOH)2 can be activated by both the base and the Rh spe-
cies. Actually, Hara and co-workers very recently reported the
highly efficient 1,4-addition reaction of arylboronic acids cat-
alyzed by a Rh complex with a OH− site in a layered
material.9m
Scheme 3 Mechanism of 1,4-addition of phenylboronic acid to cyclohexenone catalyzed by a Rh–OH complex and a base.
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Results and discussion
Catalyst preparation and characterization
Table 1 summarizes the elemental analysis results of the
prepared samples. The presence of carbon and nitrogen in
all samples indicates the incorporation of the organic
amines. For example, SiO2/diamine/Rh-a contained 6.38
mmol g−1 of carbon, 1.07 mmol g−1 of nitrogen, and 0.528
mmol g−1 of rhodium atom. The C/N/Rh ratio was 12/2/1,
which is consistent with the theoretical ratio (Scheme 4B).
When the charged amount of the silane-coupling reagent was
decreased, a catalyst having reduced organic and Rh contents
was obtained (SiO2/diamine/Rh-b). This indicates that the
amount of immobilized complex is easily controlled by chang-
ing the raw material loading. The SiO2/diamine/Rh/NEt2-a cat-
alyst had 8.21 mmol g−1 of carbon, 1.31 mmol g−1 of nitrogen,
and 0.412 mmol g−1 of rhodium atom. The Rh/N ratio was
1/3, which suggests the presence of almost equimolar
Scheme 4A shows a typical procedure for the synthesis of the
silica-supported rhodium complex/tertiary amine catalyst
IJSiO2/diamine/Rh/NEt2).
3-IJ2-Aminoethylamino)-propyl-
trimethoxysilane and 3-diethylaminopropyltrimethoxysilane
were immobilized on SiO2 using a silane-coupling reaction to
afford the silica-supported amines IJSiO2/diamine/NEt2). This
material was treated with ijRhIJcod)OH]2 (equimolar with the
immobilized diamine) in dioxane solvent to afford
SiO2/diamine/Rh/NEt2. A catalyst having only the rhodium
complex immobilized on silica (SiO2/diamine/Rh, Scheme 4B)
was also prepared using the silane-coupling reaction followed
by complexation.
Scheme 4 Syntheses of (A) SiO2/diamine/Rh/NEt2 and (B) SiO2/diamine/Rh catalysts.
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Table 1 Elemental analyses of silica-supported Rh catalysts
Molar ratio of raw materials IJRhIJ=diamine)/tertiary Cb
amine)a (mmol g−1
Nb
Rhb
Amount of tertiary aminec
Catalyst
)
(mmol g−1
)
(mmol g−1
)
(mmol g−1
)
SiO2/diamine/Rh-a
SiO2/diamine/Rh-b
—
—
6.38
2.14
8.21
2.63
4.51
7.33
7.16
4.99
3.64
5.93
6.46
1.07
0.15
1.31
0.21
0.59
1.04
1.15
0.80
0.42
0.83
0.83
0.528
0.074
0.412
0.054
0.064
0.065
0.319
0.227
0.109
0.365
—
—
—
SiO2/diamine/Rh/NEt2-a 1/1
SiO2/diamine/Rh/NEt2-b 1/1
SiO2/diamine/Rh/NEt2-c 1/7
SiO2/diamine/Rh/NEt2-d 1/15
SiO2/diamine/Rh/NEt2-e 1/1
SiO2/diamine/Rh/NEt2-f 1/1
SiO2/diamine/Rh/NEt2-g 1/1
0.44
0.07
0.46
0.91
0.38
0.27
0.14
—
SiO2/diamine/Rh/Me
SiO2/NEt2
—
—
0.83
a
Charged molar ratio of the diamine/tertiary amine. Details of the amounts of raw materials are summarized in Table S1 in the ESI.
The amounts of C and N were determined by elemental analysis. The amount of Rh was determined by inductively coupled plasma-atomic
b
emission spectroscopy (ICP-AES). c The tertiary amine content was calculated as follows: (amount of tertiary amine) = (total amount of nitrogen)
− (amount of Rh) × 2.
amounts of the diaminorhodium complex and tertiary amine.
As will be described in the spectroscopic analysis discussion,
all the diamines on the SiO2 surface coordinated to Rh cen-
ters after the complexation reaction. Therefore, the amount
of tertiary amine on the SiO2/diamine/Rh/NEt2-a catalyst was
calculated to be 0.44 mmol g−1 based on the amount of Rh
and total N content. The amounts of N and Rh in the pre-
pared samples were found to correspond closely to the
amounts of the introduced raw materials, silane-coupling
reagents, and Rh precursor. This enables the preparation of
catalysts that differ in the total amounts and ratios of
organic functionalities (Rh complex and tertiary amine
group) by selecting the corresponding raw material charges.
Catalysts which contained (i) approximately 0.06 mmol g−1
Rh complex and various quantities of tertiary amine
IJSiO2/diamine/Rh/NEt2-b, c, and d) and (ii) various amounts
of organic functionalities with similar ratios of tertiary
amine/Rh (ca. 1/1) IJSiO2/diamine/Rh/NEt2-a, e–g) were also
prepared. For these catalysts, the elemental analysis results
were in agreement with the theoretical values.
comparison with the elemental analysis results for the
SiO2/diamine/Rh/NEt2-a (Table 1). These 29Si MAS NMR data
indicate that the organic reagents are immobilized on the
silica surface by SiIJsurface)–O–Si covalent bonding.14
To clarify the structures of the Rh complexes and tertiary
amines anchored on the silica surface, 13C cross polarization
(CP)/MAS NMR measurements were carried out (Fig. 1). No
significant shifts were observed in the spectra before and
after immobilization of the diamine and tertiary amine func-
tionalities (cf. Fig. 1A and B with Fig. 1C), except for the car-
bons adjacent to the silicon atoms. These results indicate
that the organic amines are anchored on the SiO2 surface
while maintaining their carbon skeletons. Fig. 1D displays
the 13C NMR spectrum of SiO2/diamine/Rh-a. After treatment
of the SiO2/diamine with ijRhIJcod)OH]2, the signal derived
from the carbon next to the nitrogen atom was shifted to
lower field [42 → 43 ppm (▲)] and that of the central carbon
in the propyl chain was shifted to higher field [24 → 22 ppm
(●)]. The signal derived from the olefinic carbons of the
cyclooctadiene ligand (cod) was also shifted from 73 to 80–90
ppm.15 These changes in the 13C NMR chemical shifts indi-
cate complexation of the Rh species with the ethyl-
enediamine function on the silica surface. Fig. 1E shows the
13C NMR spectrum of SiO2/diamine/Rh/NEt2-a. Comparing
the spectra before and after complexation (Fig. 1E versus C),
similar shifts for the SiO2/diamine/Rh-a component (Fig. 1D)
were observed, which indicate that the complexation of ethyl-
enediamine also occurs in the case of SiO2/diamine/NEt2-a.
Moreover, signals derived from the tertiary amine did not
shift after complexation (cf. the peak at 56 ppm (◇)),
suggesting that tertiary amine moiety does not have any
interaction with the Rh complex on the silica surface.
The immobilization of organic functionalities on the silica
surface was confirmed by solid-state 29Si magic angle spin-
ning (MAS) NMR spectroscopy (Fig. S1†). The 29Si NMR spec-
trum of SiO2/diamine/Rh/NEt2-a presented two groups of typ-
ical signals (Fig. S1B;† Q and T sites) distributed from −90
to −130 ppm and from −50 to −70 ppm, respectively. In the
spectrum, the upfield
whereas the downfield
Q
T
signals originate from silica,
sites are derived from the
immobilized silane-coupling reagents. The T site [Tm
=
RSiIJOH)3−m(OSi)m] was observed to appear after immobiliza-
tion (cf. Fig. S1A and B†). In the parent SiO2, the Q sites
showed three signals at −111, −101, and −91 ppm, corre-
sponding to the Q4, Q3, and Q2 species of the silica frame-
work, respectively [Qn = SiIJOSi)nIJOH)4−n]. After the silane-
coupling reaction, the Q3/IJQ2 + Q3 + Q4) ratio decreased
from 0.23 to 0.17 and the Q2/IJQ2 + Q3 + Q4) ratio decreased
from 0.02 to <0.01. The decrease in silanol content was cal-
To clarify the acceleration effect of the immobilized ter-
tiary amine on the Rh-catalyzed reaction, it was necessary to
confirm that the prepared catalysts possessed the same local
structure for the Rh complex in both the presence and
absence of the surface-bound tertiary amine. To determine
the local structure and electronic state of the immobilized Rh
culated to be 1.5 mmol g−1 13
This value is reasonable in
.
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Fig. 1 Liquid-state 13C NMR spectra of (A) 3-diethylaminopropyltrimethoxysilane and (B) 3-IJ2-aminoethylamino)propyltrimethoxysilane. Solid-state
13C CP/MAS NMR spectra of (C) SiO2/diamine/NEt2, (D) SiO2/diamine/Rh-a, and (E) SiO2/diamine/Rh/NEt2-a. (F) Liquid-state 13C NMR spectrum of
ijRhIJcod)OH]2.
complex, X-ray absorption fine structure (XAFS) measure-
ments were conducted. Fig. 2 shows the Rh K-edge X-ray
absorption near-edge structure (XANES) spectra of the pre-
pared silica-supported Rh catalysts together with those of Rh0
foil, RhIII2O3, and ijRhIIJcod)OH]2 as reference samples. The
absorption edge varies with respect to the oxidation state of
the rhodium. The XANES spectrum of the Rh foil has two dis-
tinct peaks around 23 230 and 23 256 eV (Fig. 2A). In the case
of Rh2O3, the spectrum has two peaks around 23 232 and
23 244 eV (Fig. 2B). On the other hand, the spectrum of the
ijRhIJcod)OH]2 has one broad peak around 23 236 eV (Fig. 2C).
The spectral features of the silica-supported catalyst
IJSiO2/diamine/Rh/NEt2-a, Fig. 2D) resemble that of the mono-
valent complex, which indicates that the Rh species in the
catalyst is in the +1 oxidation state. X-Ray photoelectron
spectroscopy (XPS) analysis of Rh 3d5/2 also indicates that the
surface Rh species is RhIJI) (binding energy = 308.5–308.9 eV)
(Fig. S2†).16 These XANES and XPS results strongly indicate
the absence of metallic Rh particles on the SiO2 surface. In
the case of SiO2/diamine/Rh-a without the tertiary amine
groups (Fig. 2E), the XANES spectrum was almost identical to
that of SiO2/diamine/Rh/NEt2-a, which also indicates the
monovalent state of the Rh species. In comparing the catalysts
SiO2/diamine/Rh/NEt2-b and -f with SiO2/diamine/Rh/NEt2-a,
similar spectra are observed (Fig. S3†), which indicate that the
loading of the Rh complex on the SiO2 surface (0.054–0.412
mmol g−1) does not affect its oxidation state. After the catalytic
1,4-addition reaction, the recovered SiO2/diamine/Rh/NEt2-a
maintained the +1 oxidation state (Fig. 2F).
The k3-weighted extended X-ray absorption fine structure
(EXAFS) spectrum of SiO2/diamine/Rh/NEt2-a is almost identi-
cal to those of SiO2/diamine/Rh-a and SiO2/diamine/Rh/NEt2-b
(Fig. 3). Thus, the local structure of the Rh complex is not
affected by the presence of the tertiary amine and the loading
amounts of the organic functionalities. Moreover, the recovered
SiO2/diamine/Rh/NEt2-a was also consistent with the fresh
catalyst (Fig. S4†), which indicates that the local structure of
the Rh complex used for the reaction is maintained.
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bonds (Fig. 4A). The peaks around 1.6 and 2.5 Å in the Rh2O3
spectrum can be identified as Rh–O and Rh–O–Rh bonds,
respectively (Fig. 4B). ijRhIJcod)OH]2 has two peaks around 1.7
and 2.6 Å, derived from Rh–C/O and Rh–O–Rh, respectively
(Fig. 4C). In the spectrum of the SiO2/diamine/Rh/NEt2-a, two
peaks can be observed around 1.6 and 2.5 Å (Fig. 4D). The
first shell was well fitted with the Rh–C/O parameter (Fig.
S5†). Table 2 summarizes the results of the curve-fitting anal-
ysis of the EXAFS data. In the case of SiO2/diamine/Rh/NEt2-
a, the coordination number (N) in the first shell was evalu-
ated as 6.8. Since EXAFS is not able to distinguish between
light-scattering atoms (in this study O, C, N),17 this N value is
consistent with the theoretical value containing four Rh–C
bonds, two Rh–N bonds, and one Rh–O bond.18 In addition,
the peak around 2.5 Å was not fitted with Rh–O–Rh at all,
suggesting that the Rh complex in the SiO2/diamine/Rh/NEt2-
a catalyst did not form a dimeric complex. Overall, the pro-
posed surface structure of SiO2/diamine/Rh/NEt2 is shown in
Fig. 1E. The results of the curve-fitting analyses of the other
prepared catalysts are similar to those of SiO2/diamine/Rh/NEt2-a
and also agree with the theoretical values (Table 2). These
Fig. 2 Rh K-edge XANES spectra of (A) Rh foil, (B) Rh2O3, (C) ijRhIJcod)-
OH]2, (D) SiO2/diamine/Rh/NEt2-a, (E) SiO2/diamine/Rh-a, and (F)
recovered SiO2/diamine/Rh/NEt2-a after the 1,4-addition between 1
and 2.
Fig. 3 EXAFS spectra of SiO2/diamine/Rh/NEt2-a (black solid line),
SiO2/diamine/Rh-a (black dashed line), and SiO2/diamine/Rh/NEt2-b
(red solid line).
Fig. 4 shows the Fourier transforms (FT) of the k3-
weighted EXAFS spectra of the prepared catalysts and the ref-
erence samples (Rh0 foil, RhIII2O3, and ijRhIIJcod)OH]2). Rh
foil has a strong peak around 2.4 Å, derived from Rh–Rh
Fig. 4 Fourier transforms of the k3-weighted Rh K-edge EXAFS spectra
for (A) Rh foil, (B) Rh2O3, (C) ijRhIJcod)OH]2, (D) SiO2/diamine/Rh/NEt2-a,
(E) SiO2/diamine/Rh-a, and (F) recovered SiO2/diamine/Rh/NEt2-a after
the 1,4-addition between 1 and 2. The k range for FT was k = 3–13 (Å−1).
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Table 2 Curve-fitting analysis for the prepared silica-supported catalystsa
Sample
Nb
rc (Å)
σ2d (Å2 × 10−3
)
ΔE0 (eV)
Rf (%)
SiO2/diamine/Rh/NEt2-a
SiO2/diamine/Rh/NEt2-ae
SiO2/diamine/Rh/NEt2-b
SiO2/diamine/Rh/NEt2-f
SiO2/diamine/Rh-a
6.8 1.5
6.6 1.4
6.6 1.0
6.9 1.4
6.8 1.5
6.8 1.3
2.10 0.02
2.10 0.02
2.10 0.01
2.07 0.01
2.09 0.02
2.10 0.01
5.44 1.75
5.65 1.71
5.57 1.27
5.43 1.71
5.35 1.68
5.41 1.62
−8.46 3.81
−9.09 3.74
−7.01 2.58
−7.21 3.37
−10.2 4.06
−7.80 3.30
0.022
0.022
0.018
0.010
0.022
0.017
SiO2/diamine/Rh/Me
a
b
c
The fitting range of the R-space was 1.2–2.0 (Å). The k range for Fourier transformation was k = 3–13 (Å−1). Coordination number. Bond
distance. d Debye–Waller factor. e Recovered SiO2/diamine/Rh/NEt2-a.
curve-fitting results indicate that the local structure of the Rh
complex is not affected by its loading and the presence of ter-
tiary amine groups on the same SiO2 surface.
proceed at all (Table 3, entries 3 and 4), indicating that the
Rh complex was necessary to promote the reaction. These
results also show that the tertiary amine accelerates the Rh-
catalyzed 1,4-addition reaction. When the SiO2/diamine/Rh-a
catalyst was used for the reaction with a homogeneous ter-
tiary amine, the product yield was lower than that in the case
of SiO2/diamine/Rh-a alone (Table 3, entry 5 versus 2). In
addition, an increase in the amount of additional free tertiary
amine caused a decrease in the catalytic activity (Fig. S6†).
Furthermore, in the case of the SiO2/diamine/Rh/NEt2-a cata-
lyst, the presence of free tertiary amine also deactivated the
catalyst (Table 3, entry 6 versus 1). The results for entries 5
and 6 indicate that a tertiary amine which is not anchored on
the surface inhibits the Rh-catalyzed reaction. The product
yield was not enhanced by the addition of SiO2/NEt2, i.e., sil-
ica with only a tertiary amine, to the reaction mixture with
SiO2/diamine/Rh-a (Table 3, entry 7). To remove the possibil-
ity that the enhancement of catalytic activity was derived
from the decreased amount of surface silanol groups, a cata-
lyst in which the surface silanols had been capped with
methyltrimethoxysilane (SiO2/diamine/Rh/Me) was tested.
The catalytic activity was not enhanced compared with SiO2/
diamine/Rh-a (Table 3, entry 8 versus 2).
To summarize, the NMR, XAFS, and XPS analyses show
that the (i) Rh species on the prepared catalyst is monova-
lent; (ii) the local structure of the Rh complex is not affected
by the presence of the tertiary amine and the amount of
immobilized organic functionalities; (iii) the ligands for the
Rh complex on the prepared catalyst include the diamine,
cyclooctadiene, and hydroxyl ligands; and (iv) the structure of
the Rh complex is maintained after the catalytic 1,4-addition
reaction.
1,4-Addition reaction using the silica-supported Rh catalyst
The 1,4-addition reaction between cyclohexenone (1) and
phenylboronic acid (2) in the presence of the Rh catalysts
was examined; the results are summarized in Table 3. Prod-
uct
3
was obtained in 28% yield when using
which contains both the
SiO2/diamine/Rh/NEt2-a,
immobilized Rh complex and the tertiary amine (Table 3,
entry 1). The yield was 10% for SiO2/diamine/Rh-a (Table 3,
entry 2). Without the rhodium species, the reaction did not
Table 3 1,4-Addition between cyclohexenone and phenylboronic acid using Rh catalystsa
Entry
Catalyst
Conversion of 1b (%)
Yield of 3b (%)
1
2
3
4
5
6
7
8
SiO2/diamine/Rh/NEt2-a
SiO2/diamine/Rh-a
SiO2/diamine/NEt2
28
10
0
0
8
11
10
7
28
10
0
0
8
11
10
7
c
None
SiO2/diamine/Rh-a + free tertiary amined
SiO2/diamine/Rh/NEt2-a + free tertiary amined
SiO2/diamine/Rh-a + SiO2/NEt2
SiO2/diamine/Rh/Me
a
Reaction conditions: cyclohexenone (1) (1.0 mmol), phenyl boronic acid (2) (1.5 mmol), silica-supported Rh catalyst (Rh: 6.0 μmol), 1,4-dioxa-
ne/H2O IJ10/1, 2.0 mL), 60 °C, 1 h. Determined by 1H NMR using an internal standard technique. 1,3,5-Triisopropylbenzene was used as an
b
internal standard. c 1.3 × 10−2 g of SiO2/diamine/NEt2 was used. d Diethylbutylamine (6.0 μmol) was added.
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Next, to confirm that the reaction proceeds on the cata-
lyst surface, a filtration test was conducted in which the
SiO2/diamine/Rh/NEt2 catalyst was removed from the reaction
mixture by filtration at approximately 35% conversion. After
the solid catalyst was removed, the reaction did not proceed
further in the filtrate (Fig. S7†), which indicates that the Rh-
catalyzed reaction proceeds at the solid surface. The
immobilized tertiary amine enhances the Rh complex-
catalyzed reaction on the silica surface.
To investigate the effect of the amount of immobilized ter-
tiary amine on the catalytic activity, catalysts which had simi-
lar loadings of the Rh complex and varying amounts of ter-
tiary amine (0.07–0.91 mmol g−1) were used for the reaction.
The results are summarized in Table 4. Upon increasing the
amount of immobilized tertiary amine on the surface, the
1,4-addition product yield increased. This result strongly sup-
ports the acceleration of the Rh-catalyzed reaction by the
immobilized tertiary amine.
Next, catalysts which had various total amounts of organic
functionalities (tertiary amine + Rh complex) with similar Rh
complex/tertiary amine ratios (ca. 1/1) were used for the reac-
tion. As shown in Table 5, increasing the total amount of
immobilized organic functionalities increased the product
yield, suggesting that the catalytic activity depends on the
surface density of the organic functionalities.
The SiO2/diamine/Rh/NEt2-a and SiO2/diamine/Rh/NEt2-d
catalysts have nearly equivalent amounts of total organic
functionalities, but the loadings of the Rh complex are quite
different. However, both catalysts afforded similar product
yields (Table 6). In addition, in the cases of SiO2/diamine/Rh/NEt2-c
and SiO2/diamine/Rh/NEt2-f, which both had ca. 0.5 mmol g−1
total organic functionalities, the same phenomena occurred
(Table 6).
the catalytic activity should depend on the distance between the
tertiary amine and the Rh complex. Thus, it is strongly suggested
that heterogeneous double-activation catalysis (Scheme 2b) is
effected by the tertiary amine/Rh complex on the SiO2 surface
for the 1,4-addition reaction.
Reaction mechanism
One possible explanation for the acceleration of the 1,4-addi-
tion reaction by the tertiary amine is the enhancement of the
rate-determining step in the reaction using SiO2/diamine/Rh
catalyst. Kinetic studies show a zero-order dependence on the
concentration of cyclohexenone (Fig. S8†), which indicates
that the insertion step (Scheme 3(ii)) proceeds smoothly and
is not rate-determining. On the other hand, the first-order
dependence on the concentration of phenylboronic acid (Fig. S9†)
shows that the transmetalation step (Scheme 3(i)) is rate-
determining. It is conceivable that the tertiary amine
anchored on the silica surface accelerates transmetalation by
the formation of 4-coordinate boron species in the catalytic
cycle of the reaction. To confirm the formation of such spe-
cies by reaction of the phenylboronic acid with the
immobilized tertiary amine, solid-state 11B MAS NMR mea-
surements were conducted. After the adsorption of PhBIJOH)2
on a silica-supported tertiary amine IJSiO2/NEt2), a new peak
can be observed at around 1.1 ppm (Fig. 5). This peak is
identified as the four-coordinate boron species,19 whereas
three-coordinate sites derived from phenylboronic acid itself
cover a larger chemical shift range. The peak at around 1.1
ppm did not appear when SiO2 was treated with
phenylboronic acid. This result indicates that the tertiary
amine on the SiO2 surface coordinates to phenylboronic acid
to afford the 4-coordinate boron species. The formation of
this species was also confirmed by 11B multiple-quantum
(MQ) MAS NMR measurements (Fig. 6). The resonances cor-
responding to phenylboronic acid were visible in the region
from δF2 = −6–28 ppm and δF1 = 14 ppm IJBIJIII)a; Fig. 6A). After
the treatment of SiO2/NEt2 with PhBIJOH)2, the resonances
To summarize these catalytic reaction results, we found
that (i) the surface tertiary amine accelerates the Rh-catalyzed
reaction and (ii) a higher density of the total organic func-
tionalities leads to high catalytic activity. If the organic func-
tionalities are homogeneously dispersed on the SiO2 surface,
Table 4 Effect of the amount of immobilized tertiary amine on the product yielda
Catalyst
Rh (mmol g−1
)
Tertiary amine (mmol g−1
)
Yield of 3b (%)
SiO2/diamine/Rh-b
0.074
0.054
0.064
0.065
—
4
6
12
30
SiO2/diamine/Rh/NEt2-b
SiO2/diamine/Rh/NEt2-c
SiO2/diamine/Rh/NEt2-d
0.07
0.46
0.91
a
Reaction conditions: cyclohexenone (1) (1.0 mmol), phenylboronic acid (2) (1.5 mmol), silica-supported Rh catalyst (Rh: 6.0 μmol), 1,4-dioxane/
H2O IJ10/1, 2.0 mL), 60 °C, 1 h. Determined by 1H NMR using an internal standard technique. 1,3,5-Triisopropylbenzene was used as an
b
internal standard.
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Table 5 Effect of the total amount of immobilized Rh complex and tertiary amine on the product yielda
Catalyst
Rh (mmol g−1
)
Tertiary amine (mmol g−1
)
Total amount of organic functionalities (mmol g−1
)
Yield of 3b (%)
SiO2/diamine/Rh/NEt2-a 0.412
0.44
0.38
0.27
0.14
0.07
0.85
0.70
0.50
0.25
0.12
28
16
13
7
SiO2/diamine/Rh/NEt2-e
SiO2/diamine/Rh/NEt2-f
0.319
0.227
SiO2/diamine/Rh/NEt2-g 0.109
SiO2/diamine/Rh/NEt2-b 0.054
6
a
Reaction conditions: cyclohexenone (1) (1.0 mmol), phenylboronic acid (2) (1.5 mmol), SiO2/diamine/Rh/NEt2 (Rh: 6.0 μmol), 1,4-dioxane/H2O
IJ10/1, 2.0 mL), 60 °C, 1 h. Determined by 1H NMR using an internal standard technique. 1,3,5-Triisopropylbenzene was used as an internal
b
standard.
Table 6 Effect of the total amount of immobilized organic functionalities on the product yielda
Catalyst
Rh (mmol g−1
)
Tertiary amine (mmol g−1
)
Total amount of organic functionalities (mmol g−1
)
Yield of 3b (%)
SiO2/diamine/Rh/NEt2-a
SiO2/diamine/Rh/NEt2-d 0.065
SiO2/diamine/Rh/NEt2-c
SiO2/diamine/Rh/NEt2-f
0.412
0.44
0.91
0.46
0.27
0.85
0.98
0.52
0.49
28
30
12
13
0.064
0.227
a
Reaction conditions: cyclohexenone (1) (1.0 mmol), phenylboronic acid (2) (1.5 mmol), SiO2/diamine/Rh/NEt2 (Rh: 6.0 μmol), 1,4-dioxane/H2O
IJ10/1, 2.0 mL), 60 °C, 1 h. Determined by 1H NMR using an internal standard technique. 1,3,5-Triisopropylbenzene was used as an internal
b
standard.
shifted to around δF2 = 0 and δF1 = 29 ppm, which were
assigned to the four-coordinate boron species IJBIJIV)a; Fig. 6B).
When the SiO2/diamine/Rh/NEt2-a catalyst was treated with
phenylboronic acid (Scheme 5), two signals appeared: BIJIV)a
IJδF2 : δF1 = 0 : 29 ppm) and BIJIV)b IJδF2 : δF1 = 0 : 25 ppm) (Fig. 6C).
The former signal IJBIJIV)a) was identified as the four-coordinate
phenylboronic acid, and the latter signal IJBIJIV)b) was derived
from the four-coordinate boronic acid.20 This result indicates
that the tertiary amine on the SiO2/diamine/Rh/NEt2-a reacts
with phenylboronic acid to form a four-coordinate boron spe-
cies on the catalyst surface (4), as shown in Scheme 5. Next,
compound 4 was treated with cyclohexenone (1) (Scheme 6).
The 1,4-addition reaction proceeded smoothly and product 3
was obtained (Scheme 6). Then, the resonance due to the
four-coordinate phenylboronic acid IJBIJIV)a) disappeared (Fig. 6D).
Taken together, these results indicate that the 1,4-addi-
Fig. 5 Solid-state 11B MAS NMR spectra of (A) phenylboronic acid and
(B) phenylboronic acid with SiO2/NEt2.
tion reaction pathway mediated by SiO2/diamine/Rh/NEt2
includes the transmetalation of the tertiary amine-activated
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Fig. 6 Solid-state 11B MQ MAS NMR spectra of (A) phenylboronic acid, (B) phenylboronic acid with SiO2/NEt2, (C) phenylboronic acid with
SiO2/diamine/Rh/NEt2 (4), and (D) after treatment of 4 with cyclohexenone.
Scheme 5 Treatment of SiO2/diamine/Rh/NEt2-a with phenylboronic acid.
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Scheme 6 Treatment of 4 with cyclohexenone.
four-coordinate boron species on the SiO2 surface. The trans-
metalation, which is the rate-determining step of the reac-
tion, is accelerated by the surface tertiary amine, leading to
the enhancement of the catalytic activity.
Overall, the proposed mechanism for the transformation
in the presence of the SiO2/diamine/Rh/NEt2 catalyst is
shown in Scheme 7. The catalytic cycle involves (i) formation
of the four-coordinate boron species from reaction of the
phenylboronic acid with the tertiary amine, (ii) trans-
metalation of the aryl group from the activated boron species
to the Rh complex, (iii) insertion of an α,β-unsaturated car-
bonyl compound into the aryl–Rh bond, and (iv) hydrolysis,
giving the 1,4-addition product and the Rh–OH species.
Substrate scope and application
To explore the scope of the SiO2/diamine/Rh/NEt2-a catalyzed
1,4-addition reaction, we examined the reaction of various
phenylboronic acids with cyclohexenone (1). The results are
summarized in Table 7. The product yield reached >99%
within 1 h in the presence of 12.0 μmol of the rhodium spe-
cies (Table 7, entry 1). Electron-donating substituents such as
methoxy and methyl groups at the para position of the phe-
nyl ring resulted in excellent yields (Table 7, entries 2 and 3).
In the case of electron-withdrawing groups such as acetyl and
chloro moieties, the reaction also proceeded in 80–84% yields
(Table 7, entries 4 and 5). The catalyst was also applicable for
Scheme 7 Proposed double-activation mechanism of the 1,4-addition reaction in the presence of the SiO2/diamine/Rh/NEt2 catalyst.
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Table 7 1,4-Addition using various phenylboronic acids with cyclo-
(Table 7, entry 7). As shown in Scheme 8, the 1,4-addition
reaction using an alkylboronic acid as a reactant proceeded
in the presence of the SiO2/diamine/Rh/NEt2-a catalyst. To
the best of our knowledge, this is the first example of
1,4-addition using an alkylboronic acid.
hexenone catalyzed by SiO2/diamine/Rh/NEt2-aa
Entry
1c
Phenylboronic acid
Product
Yieldb (%)
>99
Conclusions
2
3
4
5
85
97
80d
84
A silica-supported Rh complex/tertiary amine catalyst was
synthesized and characterized. Double-activation catalysis
was observed when the surface contained both the
immobilized Rh complex and the tertiary amine. The pres-
ence of the tertiary amine immobilized on the same surface
with the Rh complex accelerated the rhodium-catalyzed
1,4-addition of phenylboronic acids to cyclohexenone. The
solid-state 11B MQ MAS NMR measurements revealed that
the 1,4-addition reaction proceeded through the formation of
a four-coordinate boron species from the reaction of the
phenylboronic acid with the surface-bound tertiary amine.
The catalyst was applied in the reaction of various
organoboronic acids, including both aryl and alkyl types.
Experimental section
Preparation of SiO2/diamine/NEt2 and SiO2/diamine/Rh/NEt2
6
7
51 (42)e
86 (70)e
SiO2 (Aerosil® 300, SiO2 content: >99.9%, NIPPON
AEROSIL Co.) was pretreated at 120 °C for 3 h under
vacuum. Dried SiO2 (0.64 g) was placed in a round-bottom
flask and treated with 15.0 mL of toluene solution containing
3-IJ2-aminoethylamino)propyltrimethoxysilane (0.40 mmol,
9.0 × 10−2 g) and 3-diethylaminopropyltrimethoxysilane (0.40
mmol, 9.5 × 10−2 g) at 40 °C for 1 h. Toluene was removed by
vacuum evaporation, affording SiO2/diamine/NEt2. Then, 0.21
g of SiO2/diamine/NEt2 was treated with 3.0 mL of dioxane
containing ijRhIJcod)OH]2 (5.0 × 10−2 mmol; Rh: 0.10 mmol) at
90 °C for 4 h. The resulting mixture was evaporated and dried
under vacuum, affording SiO2/diamine/Rh/NEt2-a.
a
Reaction conditions: cyclohexenone (1) (0.5 mmol), phenylboronic
acid (0.75 mmol), SiO2/diamine/Rh/NEt2-a (Rh: 6.0 μmol), 1,4-
dioxane/H2O IJ10/1, 0.5 mL), 80 °C, 1 h. Determined by 1H NMR
b
using an internal standard technique. 1,3,5-Triisopropylbenzene was
used as an internal standard. c Reaction conditions: cyclohexenone (1)
(0.5 mmol), phenylboronic acid (0.75 mmol), SiO2/diamine/Rh/NEt2
d
(Rh: 12.0 μmol), 1,4-dioxane/H2O IJ10/1, 0.5 mL), 60 °C, 1 h. 24 h.
e
SiO2/diamine/Rh-a was used as a catalyst.
1,4-Addition reaction using silica-supported Rh catalyst
A silica-supported Rh catalyst (6.0 μmol), dioxane/H2O IJ10/1,
2.0 mL), cyclohexenone (1) (1.0 mmol), and phenylboronic
acid (2) (1.5 mmol) were placed in a Pyrex glass reactor. The
resulting mixture was stirred vigorously for 1 h at 60 °C
under Ar. Product formation was confirmed by GC-MS and
NMR. Yields and conversion were determined by 1H NMR
using a CDCl3 solution of the reaction mixture.
the reaction using trans-2-phenylvinylboronic acid (Table 7,
entry 6). The product yield was higher than that in the case
of SiO2/diamine/Rh-a. When a boron ester was used instead
of the phenylboronic acid, the SiO2/diamine/Rh/NEt2-a cata-
lyst also afforded a higher yield than SiO2/diamine/Rh-a
Scheme 8 1,4-Addition reaction using alkylboronic acid as a reactant.
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Treatment of SiO2/NEt2 with phenylboronic acid for 11B NMR
analysis of the reaction intermediate
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SiO2/NEt2 (0.10 g, N: 0.083 mmol), dioxane/H2O IJ10/1, 1.0
mL), and phenylboronic acid (2) (0.083 mmol) were placed in
a Pyrex glass reactor. The resulting mixture was vigorously
stirred for 5 min at 40 °C under Ar. Then, the solvent was
evaporated under vacuum, and the resulting solid was trans-
ferred to the NMR sample rotor.
Treatment of SiO2/diamine/Rh/NEt2 with phenylboronic acid
for 11B NMR analysis of the reaction intermediate
6 For Lewis acids or metal particles, and organic bases,
see: (a) M. Waki, S. Muratsugu and M. Tada, Chem.
Commun., 2013, 49, 7283; (b) K. Motokura, Y. Ito, H. Noda,
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SiO2/diamine/Rh/NEt2 (0.19 g, tertiary amine: 0.085 mmol),
dioxane/H2O IJ10/1, 1.0 mL), and phenylboronic acid (2)
(0.085 mmol) were placed in a Pyrex glass reactor. The
resulting mixture was vigorously stirred for 5 min at 40 °C
under Ar. Then, the solvent was evaporated under vacuum,
and the resulting solid was transferred to the NMR sample
rotor.
Acknowledgements
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We thank the Kyusyu Synchrotron Light Research Center
(SAGA-LS) for their technical support. XAFS measurements
were carried out under the approval of the SAGA-LS Advisory
Committee (Proposal No. 1404015F). This study was
supported by JSPS KAKENHI (grant no. 24686092 and
25630362) and JSPS Grant-in-Aid for Scientific Research on
Innovative Areas “3D Active-Site Science” (grant no.
26105003). H.N. thanks Grant-in-Aid for JSPS Fellows (grant
no. 2612038).
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