Rh-catalyzed 1,4-addition reactions of arylboronic acids accelerated by co-immobilized tertiary amine in silica mesopores

Article abstract of DOI:10.1016/j.mcat.2019.04.016

Mesoporous silica-supported Rh complex catalysts were prepared by simple silane-coupling, followed by complexation, and characterized by FT-IR, SEM, Rh K-edge XAFS, and elemental analysis. Local structures of the Rh complexes in each sample were almost similar to those of a nonporous silica-supported diaminorhodium complex. Co-immobilization of a tertiary amine on the same silica surface induced slight changes to the Rh complex structure in the case of the support with smaller pores. The prepared catalysts showed high activity for the 1,4-addition reaction of phenylboronic acids. Co-immobilization of the tertiary amine increased the reaction rate by more than 7-fold, with turnover number of nearly 8500. The catalytic performance achieved with this novel system is with much higher than that reported previously with a nonporous silica-supported catalyst. The mesoporous silica-supported Rh complex-tertiary amine showed a wide substrate scope, including unsaturated ketones and nitriles. This co-immobilized tertiary amine may activate phenylboronic acid to enhance its reactivity in the transmetalation step with Rh-OH species.

Full text of DOI:10.1016/j.mcat.2019.04.016

MolecularCatalysis472(2019)1–9
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Rh-catalyzed 1,4-addition reactions of arylboronic acids accelerated by co-
immobilized tertiary amine in silica mesopores
⁎
Ken Motokuraa,b, , Kohei Hashiguchia, Kyogo Maedaa, Masayuki Namboa, Yuichi Manakaa,c
,
Wang-Jae Chund
a Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,
Yokohama, 226-8502, Japan
b PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan
c Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology, 2-2-9 Machiikedai, Koriyama, Fukushima 963-0298, Japan
d Graduate School of Arts and Sciences, International Christian University, Mitaka, Tokyo, 181-8585, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rh complex
1,4-Addition reaction
Phenylboronic acid
Mesoporous silica
Concerted catalysis
Mesoporous silica-supported Rh complex catalysts were prepared by simple silane-coupling, followed by com-
plexation, and characterized by FT-IR, SEM, Rh K-edge XAFS, and elemental analysis. Local structures of the Rh
complexes in each sample were almost similar to those of a nonporous silica-supported diaminorhodium com-
plex. Co-immobilization of a tertiary amine on the same silica surface induced slight changes to the Rh complex
structure in the case of the support with smaller pores. The prepared catalysts showed high activity for the 1,4-
addition reaction of phenylboronic acids. Co-immobilization of the tertiary amine increased the reaction rate by
more than 7-fold, with turnover number of nearly 8500. The catalytic performance achieved with this novel
system is with much higher than that reported previously with a nonporous silica-supported catalyst. The me-
soporous silica-supported Rh complex-tertiary amine showed a wide substrate scope, including unsaturated
ketones and nitriles. This co-immobilized tertiary amine may activate phenylboronic acid to enhance its re-
activity in the transmetalation step with Rh-OH species.
1. Introduction
amine cooperativity, Jones et al. investigated the effect of mesopore
size on aldol-type condensation reactions [4]. Although concerted cat-
Concerted catalysis by two isolated functional groups embedded on
a solid surface offers highly efficient approaches in organic synthesis
[1]. Catalytic nucleophilic additions, such as aldol and Henry reactions,
on silica surfaces bearing both acidic functionalities and basic amines
have received considerable attention due to the excellent catalytic
performance of the embedded moieties [2]. This cooperative strategy
has been recently extended to the synthesis of fine chemicals using
metal complexes as immobilized active centers [3]. For example, Cόr-
dova et al. reported efficient asymmetric cascade transformations via
catalysis on silica surface containing a Pd complex and a secondary
amine [3d]. Fernandes et al. demonstrated the superior catalytic oxi-
dations of alcohols to aldehydes using a click chemistry immobilized Cu
complex via cooperative action with the co-immobilized TEMPO [3f].
With continued exploration in this area of catalysis, understanding
the effects of the support structure on such cooperative catalysis by
more than two functionalities is the next research frontier. For acid-
alysis using metal complexes on solid supports is one of the most ac-
tively pursued topics in the field of heterogeneous catalysis, the precise
effect of the support on such cooperativity is still unclear. Our group
recently reported the allylation of nucleophiles by concerted catalysis
between a precisely designed Pd complex and a tertiary amine [5].
While this cooperative phenomenon was observed initially on a non-
porous silica surface, the catalytic performance increased significantly
in silica mesopores [6]. Detailed characterization of the Pd complex
indicated no significant change in its local coordination structure,
suggesting that the enhanced cooperativity between Pd complex and
amine in the mesopores results from the different environments of their
distance and/or direction from nonporous surface [6]. Encouraged by
these findings, we expanded our studies on cooperative catalysis in
mesopores to other metal complex-catalyzed reactions and the results
herein.
Rh-catalyzed 1,4-addition is one of the most powerful strategies for
⁎ Corresponding author at: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259
Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan.
E-mail address: motokura.k.ab@m.titech.ac.jp (K. Motokura).
https://doi.org/10.1016/j.mcat.2019.04.016
Received 22 March 2019; Received in revised form 22 April 2019; Accepted 23 April 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
the synthesis of β-arylcarbonyl compounds. Following the seminal re-
port by Miyaura et al. in 1997 [7], various novel homogeneous [8] and
heterogeneous Rh catalysts [9] have been developed for this transfor-
mation. Shimazu et al. reported a heterogeneous NiZn/Rh catalyst
containing a Rh aqua complex and hydroxide ions in the NiZn inter-
layer space [9k]. Our group also reported a nonporous silica-supported
Rh complex and tertiary amine system for the 1,4-addition reaction
[10]. “Double-activation catalysis” by Rh and basic functionalities is the
key step in such heterogeneous catalytic systems which activates the
phenylboronic acid by formation of the tetra-coordinated boron species,
resulting in an accelerated CeB bond cleavage and faster transmetala-
tion between Rh−OH and Ph-B [10]. Based on this mechanistic ratio-
nale, changes in the location of the Rh complex and the basic site, such
as immobilized amine, should have a marked effect on its catalytic
performance.
Scheme 1. MS/diamine/Rh/NEt2-catalyzed 1,4-addition reactions of ar-
ylboronic acid.
Table 1
Physicochemical properties of synthesized MS supports.
a
b
MS Support
Surface area [m2 g−1
]
Pore size [nm]b
Pore vol. [ml/g]
We report here our studies on the development of an efficient 1,4-
addition reaction of arylboronic acid catalyzed by mesoporous silica-
supported Rh complex and tertiary amine (Scheme 1). The use of me-
soporous silica as a support significantly enhances the cooperative
catalysis compared with previously reported nonporous silica support
system. The effects of pore size on the Rh complex structure and its
MS(C8)
MS(C12)
MS(C18)
1865
1175
876
1.6
2.3
3.1
1.46
1.01
1.01
a
Determined by N2 adsorption-desorption isotherm measurement.
Determined by BJH method.
b
Scheme 2. Preparation of (A) MS/diamine/Rh and (B) MS/diamine/Rh/NEt2.
2

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
Table 2
Elemental analysis results of MS-supported Rh catalysts.
a
b
Catalyst
Molar ratio of raw materials [Rh:N(diamine):N(tertiary amine)]
Element (mmol g−1
)
C
N
Rh
MS(C8)/diamine/Rh
MS(C12)/diamine/Rh
MS(C18)/diamine/Rh
MS(C8)/diamine/Rh/NEt2
MS(C12)/diamine/Rh/NEt2
MS(C18)/diamine/Rh/NEt2
MS(C18)/diamine/Rh/NEt2(12)
MS(C18)/diamine/Rh/NEt2(7.5)
1 : 2 : 0
1 : 2 : 0
1 : 2 : 0
1 : 2 : 15
1 : 2 : 15
1 : 2 : 15
1 : 2 : 12
1 : 2 : 7.5
5.8
6.9
5.3
21.7
18.5
16.1
15.9
12.6
0.6
0.5
0.4
3.1
2.6
2.2
2.1
1.6
0.312
0.229
0.196
0.192
0.199
0.143
0.161
0.157
a
Charged molar ratio of the diamine/tertiary amine.
b
The amounts of C and N were determined by elemental analysis. The amount of Rh was determined by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES).
Fig. 1. SEM image of (A) MS(C8)/diamine/Rh and (B) MS(C8)/diamine/Rh/NEt2 and their EDS line analysis results for O, Si, N, and Rh elements.
3

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
Fig. 2. Rh K-edge XANES (left) and Fourier Transform of k3-
weighted Rh K-edge EXAFS spectra (right) of (A) Rh foil, (B)
Rh2O3, (C) [Rh(cod)OH]2, (D) MS(C8)/diamine/Rh, (E) MS
(C12)/diamine/Rh, (F) MS(C18)/diamine/Rh, (G) MS(C8)/
diamine/Rh/NEt2, (H) MS(C8)/diamine/Rh/NEt2, (I) MS
(C18)/diamine/Rh/NEt2. The k range for FT was k = 3.0-
11 Å−1
.
Table 3
a
Curve-fitting analysis for the prepared SiO2-supported Rh catalysts
.
b
d
Sample
N
R
c(Å)
Δσ2 (Å × 10−3
)
ΔE0 (eV)
Rf (%)
MS(C8)/diamine/Rh
MS(C12)/diamine/Rh
MS(C18)/diamine/Rh
6.7( 1.1)
6.6( 1.2)
6.5( 1.1)
6.1( 1.0)
6.2( 1.0)
6.5( 1.1)
6.8( 1.5)
6.8( 1.5)
2.09 ( 0.01)
2.09 ( 0.01)
2.09 ( 0.01)
2.09 ( 0.01)
2.10 ( 0.01)
2.11 ( 0.01)
2.09 ( 0.02)
2.10 ( 0.02)
5.81 ( 0.29)
7.64 ( 0.32)
4.50 ( 0.29)
1.02 ( 0.36)
1.80 ( 0.32)
2.30 ( 0.32)
5.35 ( 1.68)
5.44 ( 1.75)
−4.1( 2.3)
−5.3( 2.6)
−3.6( 2.4)
−0.9( 2.6)
−0.8( 2.6)
−1.1( 2.4)
−10.2( 4.1)
−8.5( 2.3)
0.799
1.048
0.792
0.928
0.934
0.895
0.022
0.022
MS(C8)/diamine/Rh/NEt2
MS(C12)/diamine/Rh/NEt2
MS(C18)/diamine/Rh/NEt2
e
SiO2/diamine/Rh
e
SiO2/diamine/Rh/NEt2
a
The fitting range of the R-space was 1.0–2.0 (Å). The k range for Fourier Transformation was k = 3–13 (Å−1). Shell: Rh-C/O.
Coordination number.
Bond distance.
Debye-Waller factor.
b
c
d
e
Nonporous SiO2-supported Rh samples. Data from ref [10a].
catalytic activity are also evaluated.
diamine ligand (MS/diamine). A solution of [Rh(cod)OH]2 in dioxane
was added to MS/diamine to afford MS-supported diaminorhodium
complex (MS/diamine/Rh). The Rh and tertiary amine immobilized
catalyst MS/diamine/Rh/NEt2, was prepared by attaching 3-(2-ami-
2. Results and discussion
noethylamino)propyltrimethoxysilane
trimethoxysilane simultaneously onto the MS surface in 1:15 to 1:7.5 M
ratios, followed by complexation with [Rh(cod)OH]2.
and
3-diethylaminopropyl-
2.1. Preparation and characterization of catalysts
The mesoporous silica (MS) support used in this study was synthe-
sized according to a well-known sol-gel procedure using primary
amines as template molecules [11]. Physicochemical properties of the
synthesized MS supports are summarized in Table 1 and are denoted MS
(C8), MS(C12), and MS(C18) with 1.6, 2.3, 3.1 Å pore sizes respec-
tively, according to their carbon chain lengths of the template.
The immobilization process of Rh complex and tertiary amine on
MS support is shown in Scheme 2. The MS-supported Rh catalysts were
prepared according to a similar procedure we reported the preparation
of a nonporous silica support [10]. Silane-coupling of 3-(2-aminoethy-
lamino)propyltrimethoxysilane with the silica surface afforded MS with
The elemental analysis of prepared MS/diamine/Rh and MS/dia-
mine/Rh/NEt2 is summarized in Table 2. The carbon and nitrogen
amounts for MS/diamine/Rh with different pore diameters, are 5.3–6.9
and 0.4−0.6 mmol/g respectively. The Rh content of 0.2−0.3 mmol/g
for MS/diamine/Rh indicates a N/Rh ratio of around 2, suggesting the
formation of a diaminorhodium complex. In comparison, the elemental
analysis of MS/diamine/Rh/NEt2 reveals a larger amount of nitrogen
and carbon compared to Rh, which clearly indicates the presence of an
excess amount of tertiary amine with respect to Rh in MS/diamine/Rh/
NEt2. Samples of MS/diamine/Rh/NEt2 containing lower amounts of
4

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
Fig. 3. k3-weighted EXAFS spectra of (A) MS
(C18)/diamine/Rh/NEt2 (solid line) and MS
(C18)/diamine/Rh (dashed line), (B) MS(C18)/
diamine/Rh (black), MS(C12)/diamine/Rh
(blue), MS(C8)/diamine/Rh (red), and (C) MS
(C18)/diamine/Rh/NEt2 (black), MS(C12)/
diamine/Rh/NEt2 (blue), MS(C8)/diamine/
Rh/NEt2 (red) (For interpretation of the refer-
ences to colour in this figure legend, the reader
is referred to the web version of this article).
Table 4
a
Effect of co-immobilized tertiary amine on 1,4-addition reaction
.
b
Entry
Catalyst
Yield (%)
TON
1
2
3
4
5
6
MS(C18)/diamine/Rh/NEt2
MS(C18)/diamine/Rh
MS(C18)/diamine/Rh + triethylamine
MS(C18)/diamine/Rh + diisopropylethylamine
MS(C18)/diamine/Rh/NEt2(12)
MS(C18)/diamine/Rh/NEt2(7.5)
65
33
50
66
54
29
430
220
330
440
360
190
c
d
a
Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol), Rh catalyst (Rh: 1.5 μmol), dioxane/H2O (2.0 mL, 10/1), 60 °C, 60 min.
b
c
Determined by 1H NMR.
Triethylamine (23 μmol).
Diisopropylethylamine (23 μmol).
d
tertiary amine were also synthesized. Samples with 1:12 and 1:7.5 ra-
SEM-EDX analysis data clearly support the homogeneous dispersion of
Rh complex and amine functionalities inside the mesopores of MS.
Retention of the immobilized tertiary amine structure was con-
firmed by FT-IR analysis (Figure S1, Supporting Information). The co-
ordination environment of Rh with the diamine ligand in supported
catalysts was analyzed by Rh K-edge XAFS. Fig. 2 shows the XANES and
FT-EXAFS spectra for MS/diamine/Rh and MS/diamine/Rh/NEt2 with
three types of MS pore diameters along with reference samples. Shapes
of the XANES spectra for MS-supported Rh complexes [(D)-(I)] were
similar to that of [Rh(cod)OH]2 (C) but differ largely from Rh2O3 (B)
and Rh foil (A). In the FT-EXAFS spectra, a strong signal at 1.6 Å as-
signed as the Rh-O/C/N bond was observed in supported Rh complexes
[(D)-(I)]. This signal fits well with the Rh-C/O parameter. The result of
tios of Rh:tertiary amine were designated MS/diamine/Rh/NEt2(12)
and MS/diamine/Rh/NEt2(7.5), respectively. Elemental analysis of
these two samples confirmed the lower carbon and nitrogen contents
(Table 2).
Homogeneous dispersion of the Rh complex and amine functional-
ities inside the mesoporous silica was confirmed by SEM-EDX analysis.
MS particles with an average size of approx. 0.5 μm were observed for
both MS(C8)/diamine/Rh and MS(C8)/diamine/Rh/NEt2 in the re-
corded SEM images and the line analysis of elements, shown in Fig. 1.
The EDS line analysis results indicate similar location profiles for Rh
and N elements relative to Si and O, the main components of MS sup-
port, both in the presence and absence of the tertiary amine. These
5

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
Table 5
MS/diamine/Rh, especially in MS supports with smaller pores (MS(C8)
and MS(C12)). Fig. 3 represents k3-weighted EXAFS spectra of MS-
supported Rh complexes. As shown in Fig. 3(A) and (B), EXAFS spectra
of MS(C18)/diamine/Rh/NEt2, MS(C18)/diamine/Rh, MS(C12)/dia-
mine/Rh, and MS(C8)/diamine/Rh are almost identical. The results are
in good agreement with similar curve-fitting analysis results of these
samples. In the case of MS/diamine/Rh/NEt2 with three types of MS
pores, EXAFS spectra of the MS(C12) and MS(C8) show small differ-
ences with MS(C18) (Fig. 3(C)). These results imply that coordination
environment of a part of Rh complex in small pores, MS(C8) and MS
(C12), are slightly changed by co-immobilized tertiary amine, due to
the direct coordination of tertiary amine to Rh center and/or the steric
bulk of the immobilized functionalities with high density in the con-
fined space.
a
Effect of pore size of MS support on 1,4-addition reaction
.
b
c
Entry
Catalyst
Yield (%)
Initial TOF (min−1
)
1
2
3
4
MS(C8)/diamine/Rh
MS(C12)/diamine/Rh
MS(C18)/diamine/Rh
MS(C8)/diamine/Rh/
NEt2
81
11
33
> 99
7.3
1.2
3.8
49.3
5
6
MS(C12)/diamine/Rh/
NEt2
MS(C18)/diamine/Rh/
NEt2
20
65
57
2.6
8.0
6.4
2.2. Catalysis of 1,4-addition reaction of phenylboronic acid
d
7
a
SiO2/diamine/Rh/NEt2
The 1,4-addition catalysis with the prepared MS supported catalysts
was evaluated using the addition of phenylboronic acid to cyclohex-
enone. Reactions of 1.0 mmol of cyclohexenone (1) and 1.5 mmol of
phenylboronic acid (2) were examined using 1.5 μmol of Rh in the
synthesized MS(C18)-supported Rh catalysts (Table 4). The MS(C18)/
diamine/Rh (entry 2) afforded 33% yield which increased to 65% with
the use of MS(C18)/diamine/Rh/NEt2 (entry 1), indicating the bene-
ficial effect of the tertiary amine co-immobilization on the MS surface.
The co-immobilization effect of tertiary amine was investigated using a
control experiment: addition of triethylamine to MS(C18)/diamine/Rh
system slightly enhanced product yield (entry 2 vs 3), however, the
TON value was smaller than that of co-immobilized catalyst (entry 1).
Addition of diisopropylethylamine was effective for improvement of
product yield to 66% (entry 4). Bulkiness of diisopropylethylamine may
prevent the direct interaction between amine and Rh center, indicating
that the appropriate spacing between two active functionalities is ne-
cessary for effective promotion of 1,4-addtion reaction. Therefore, in
the case of MS/diamine/Rh/NEt2, the characteristics of the support
Reaction conditions:
1 (1.0 mmol), 2 (1.5 mmol), Rh catalyst (Rh:
1.5 μmol), dioxane/H2O (2.0 mL, 10/1), 60 °C, 60 min.
b
Determined by 1H NMR.
c
Initial TOF (min−1). Calculated from initial reaction rate within 10 min
and/or at less than 30% conversion.
d
Nonporous silica (Aerosil300, SiO2) support was used.
curve-fitting analysis of the EXAFS spectra are summarized in Table 3.
For MS/diamine/Rh, the coordination numbers (N) and bond distances
(R) were evaluated to be 6.5 ˜ 6.7 and 2.09 Å, respectively. These values
are well in agreement with those reported previously for the nonporous
SiO2-supported Rh complex [10], suggesting the formation of a dia-
minorhodium complex on MS surface, as shown in Scheme 1(A). The
difference of curve-fitting analysis results of MS/diamine/Rh from
simply adsorbed Rh complex (SiO2/Rh) [3h] also implies coordination
of diamine ligand to the Rh center. The bond distances (R) for MS/
diamine/Rh/NEt2 are almost similar to MS/diamine/Rh (2.09–2.11 Å).
However, coordination numbers (N) are slightly lower than those of
Table 6
a
Substrate scope for MS/diamine/Rh/NEt2-catalyzed 1,4-addition reaction of phenylboronic acids
.
Entry
1
Unsaturated compound
Arylboronic acid (Ar-)
Ph-
Time / h
2
Yield / %
93
(1)
2
3
4
5
6
4-Cl-Ph-
24
24
4
4
24
92
99
92
72
90
4-CH3C(O)-Ph-
4-CH3O-Ph-
4-CH3-Ph-
Ph-
b
7
8
Ph-
Ph-
5
94
24
83
9
Ph-
Ph-
24
6
35
94
10
c
11
a
Ph-
24
89
Reaction conditions: Unsaturated compound (1.0 mmol), ArB(OH)2 (1.5 mmol), MS(C18)/diamine/Rh/NEt2 (Rh: 4.5 μmol), dioxane/H2O (2.0 nl), 60 °C.
b
c
90 °C.
100 °C.
6

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
Table 7
a
High TON 1,4-addition reaction using MS/diamine/Rh/NEt2
.
b
Entry
1 / mmol
Rh /μmol
T / oC
Time / h
Yield (%)
TON
1
2
3
4
5
1.0
1.0
1.0
3.0
3.0
1.50
0.29
0.29
0.29
0.29
60
60
100
100
100
1
> 99
71
85
51
82
670
60
24
24
96
2500
3000
5300
8500
c
c
a
Reaction conditions: 2 (1.5 equiv. to 1), dioxane/H2O (2.0 mL, 10/1), MS(C18)/diamine/Rh/NEt2.
b
c
Determined by 1H NMR.
MS(C8)/diamine/Rh/NEt2.
Scheme 3. Proposed reaction mechanism of MS/diamine/Rh/NEt2-catalzyed 1,4-addition reaction of arylboronic acid.
structure, such as pore size, should influence the cooperative catalytic
activity. Decreasing the amount of the co-immobilized tertiary amine
induced a decrease in catalytic activity (entries 5 and 6), pointing out
the beneficial effect of higher loading density of the co-immobilized
tertiary amine in accelerating the 1,4-addition reaction.
We next examined in detail, the effect of MS pore size on the 1,4-
addition reaction (Table 5). Among the supports used, MS(C8) posses-
sing the smallest mesopores showed higher catalytic activity, both with
7

K. Motokura, et al.
MolecularCatalysis472(2019)1–9
and without co-immobilized tertiary amines (entries 1 and 4), in-
dicating enhancement of the 1,4-addition reaction in confined spaces
even in the absence of tertiary amine. After co-immobilization of ter-
tiary amine, the activity increased for all MS supports, with MS(C8)/
diamine/Rh/NEt2 delivering the highest catalytic performance. The
activity of nonporous silica-supported catalyst (SiO2/diamine/Rh/
NEt2) was lower than catalysts with MS(C8) and MS(C18) as a support
(entry 7). This could be a result of accumulation of the Rh complex,
tertiary amine, and surface silanol group in restricted space, which may
enhance the active/adsorption site density in mesopore, resulting the
effective accumulation/activation of polar substrate molecules [6]. On
the other hand, the catalyst with the larger mesopore, MS(C18)/dia-
mine/Rh/NEt2, showed higher performance than the catalyst with the
smaller mesopore, MS(C12)/diamine/Rh/NEt2 possibly due to lower
diffusion barrier in larger-pore materials. The detailed underlying rea-
sons for these conflicting results in the pore size effect are unclear.
However, one possible explanation is formation of inhomogeneous,
highly active catalytic site formation in MS(C8)/diamine/Rh/NEt2 by
accumulation of active functional groups into restricted spaces.
The scope and limitation of 1,4-addition reaction of phenylboronic
acid to unsaturated carbonyl compounds are summarized in Table 6. To
exclude the size effect of large substrate molecules, MS(C18)/diamine/
Rh/NEt2 was used as the heterogeneous Rh catalyst. The presence of
electron-withdrawing and donating groups at the para-position of
phenylboronic acid had no adverse impact on the reaction, with for-
mation of the corresponding cyclohexenone 1,4-addition product in
good to excellent yields (entries 1–5). Other ketones substrates such as
cyclopentenone and chalcone afforded high yields under MS(C18)/
diamine/Rh/NEt2-catalysis (entries 6–8). The unsaturated aldehyde
afforded the product in lower yield (entry 9). The reaction system tol-
erates the presence of unsaturated nitriles as acceptors (entries 10 and
11) without any loss in yield. This catalyst system was not applicable to
the reaction with olefins without electron-withdrawing group: no ad-
dition product was obtained with 1-octene.
highly unlikely, we propose both the direct and indirect (activation by
OH-) pathways (C). The Rh-Ph species on MS surface can easily react
with the unsaturated ketone, followed by hydration to form the 1,4-
addition product and the regeneration of MS/diamine/Rh/NEt2 cata-
lyst.
3. Conclusion
MS/diamine/Rh/NEt2 was found to be a highly active catalyst for
1,4-addition reaction of phenylboronic acids to unsaturated ketones.
Several studies were undertaken to probe the effect of the solid support
on catalytic performance. XAFS analysis revealed the local structure of
Rh on the MS surface. In all prepared catalysts, the Rh complexes
maintained their structure without aggregation. Detailed CF analysis
indicates that the pore size of the support slightly affects the local
structure of Rh in the case of MS/diamine/Rh/NEt2, suggesting differ-
ences in the interactions between Rh complex and immobilized tertiary
amine in each mesopore. The catalytic activity of the immobilized Rh
complex improved significantly by co-immobilization of a tertiary
amine on the same support surface, delivering a high TON of 8500. The
MS/diamine/Rh/NEt2-catalyzed reaction was found compatible with a
wide range of substrates. These results affirm the relevance of co-
operative catalysis between the two immobilized functionalities such as
the metal complex and organic group in confined spaces for highly
efficient synthesis of fine chemicals.
Acknowledgments
This study was supported by JSPS Grant-in-Aid for Scientific
Research on Innovative Areas (Grant no. 18H04242 and 26105003),
and Innovative Research Initiative project in Tokyo Tech.
Appendix A. Supplementary data
We next studied the optimization of the reaction conditions to
achieve high turnover number (TON) in the MS/diamine/Rh/NEt2-
catalyzed 1,4-addition reaction system (Table 7). 85% yield of the
product was obtained even with 0.29 μmol of Rh in MS(C18)/diamine/
Rh/NEt2 at 100 °C. The reaction of 3.0 mmol of 1 with 0.29 μmol of Rh
in MS(C8)/diamine/Rh/NEt2 afforded 51% yield of 1,4-addition pro-
duct with a TON of 5300 after 24 h. After a prolonged reaction time, we
were pleased to find an increase in the yield and TON to 82% and 8500,
respectively. These high TONs compare exceeding well to literature
reports. For example, the TONs [12] for reported heterogeneous Rh
catalytic systems are as follows: Rh/NiZn, 3200 (4 h);[9k] Rh/HT, 3500
(6 h);[9i] PS-PEG-diene-Rh, 1073 (63 h);[9p] and Rh/Ag nanoparticles,
2200 (16 h and 6 recycle runs).[9l] Our MS/diamine/Rh/NEt2 system
showed higher TON and comparable turnover frequency (TOF) per-
formances compared with reported heterogeneous Rh catalysts.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.04.016.
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