Efficient 1,4-addition of enones and boronic acids catalyzed by a Ni-Zn hydroxyl double salt-intercalated anionic rhodium(III) complex

Article abstract of DOI:10.1021/cs501267h

Intercalation of an in situ prepared [Rh(OH)₆]^3- complex into an anion exchangeable Ni-Zn layered hydroxy double salt (Rh/NiZn) was demonstrated. The resulting Rh/NiZn effectively catalyzed the 1,4-addition of diverse enones and phenylboronic acids to their corresponding β-substituted carbonyl compounds. In the case of 2-cyclohexen-1-one and phenylboronic acid, a turnover frequency (TOF) of 920 h^-1 based on Rh was achieved. The [Rh(OH)₆]^3- complex maintained its original monomeric trivalent state within the NiZn interlayer following catalysis, attributable to a strong electrostatic interaction between the NiZn host and anionic Rh(III) complex.

Full text of DOI:10.1021/cs501267h

Research Article
pubs.acs.org/acscatalysis
Efficient 1,4-Addition of Enones and Boronic Acids Catalyzed by a
Ni−Zn Hydroxyl Double Salt-Intercalated Anionic Rhodium(III)
Complex
†
†
†
‡
‡
Takayoshi Hara, Nozomi Fujita, Nobuyuki Ichikuni, Karen Wilson, Adam F. Lee,
,†
and Shogo Shimazu*
†Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33, Yayoi, Inage, Chiba
63-8522, Chiba, Japan
2
‡
European Bioenergy Research Institute, School of Engineering and Applied Sciences, Aston University, Aston Triangle, Birmingham
B4 7ET, U.K.
*
S Supporting Information
ABSTRACT: Intercalation of an in situ prepared [Rh-
OH)6]3 complex into an anion exchangeable Ni−Zn layered
−
(
hydroxy double salt (Rh/NiZn) was demonstrated. The
resulting Rh/NiZn effectively catalyzed the 1,4-addition of
diverse enones and phenylboronic acids to their corresponding
β-substituted carbonyl compounds. In the case of 2-cyclo-
hexen-1-one and phenylboronic acid, a turnover frequency
−
1
(
[
TOF) of 920 h
based on Rh was achieved. The
Rh(OH)6]3 complex maintained its original monomeric
−
trivalent state within the NiZn interlayer following catalysis,
attributable to a strong electrostatic interaction between the
NiZn host and anionic Rh(III) complex.
KEYWORDS: Ni−Zn hydroxyl double salt, anion exchange, intercalation, rhodium, 1,4-addition
integrated [(D-valine)Pd(OH) ] /PO43−/NiZn intercalation
−
1
. INTRODUCTION
2
catalyst effectively drives the aerobic oxidation of alcohols
Although anion-exchangeable clays are relatively rare compared
with cationic clays in nature, their high surface area, sorptive,
and ion-exchange properties have been utilized in catalytic
applications for decades. Among these materials, layered
hydroxy double salts (HDSs), which consist of positively
charged layers and exchangeable interlayer anions, have
received considerable interest as anion-exchangeable layered
compounds because of their potential applications as catalyst
supports. The general formula of HDS families can be
3a
into their corresponding carbonyl compounds. In this case, a
Brønsted-basic PO43 anion, intercalated into the NiZn
interlayer along with the anionic Pd(II)-amino acid complex,
was able to act as “a substrate activator”. Our recent success
with the intercalation of well-defined anionic Pd(II)-hydroxide
complexes prompted us to examine whether this general
synthetic methodology could be extended to rhodium.
β-Substituted carbonyl compounds are versatile intermedi-
ates in the synthesis of high-value organic chemicals. Much
attention has been devoted to the rhodium-catalyzed 1,4-
addition reaction between arylboronic acids to form α,β-
unsaturated carbonyl compounds since the first report by
Miyaura et al. in 1997. Thereafter, the development of
homogeneous chiral rhodium catalysts with various well-
defined ligands was reported. However, to facilitate recycling
of the expensive rhodium, numerous families of heterogeneous
rhodium catalysts, including polymer-bound catalysts, surface-
immobilized catalysts, and polymer-intercalated bimetallic
nanoparticle catalysts have also been developed. Miyaura
and Hayashi et al. reported that monovalent Rh hydroxide
−
1
2
8
2
+
1
2+
n‑
represented as M
M′ 2x(OH)2A
·mH O (0.15 < x <
−x
2x/n 2
2+
2+
0
.25), where M and M′ are divalent metal cations such as
2+
2+
2+
2+
n‑
Zn , Cu , Co , or Ni and A represents various anions. We
have continued to develop “intercalation catalysts” using a Ni−
Zn mixed basic salt (NiZn), which is considered an HDS. By
exploiting the anion-exchange ability of NiZn, researchers have
investigated the intercalation of various guest anions, including
simple inorganic anions such as AcO , NO , SO , CO3
9
3
10
4
−
−
2−
2−
,
3
4
11
3
−
−
−
5
PO4 , Cl , or Br ; anionic metal complexes; organic
12
6
7
carboxylates; and sulfates or sulfonates into the NiZn
interlayer. Our choice of NiZn as a catalyst support is
motivated by the following advantages: (i) simple preparation,
13
14
15
(
ii) high crystallinity, (iii) isolation of anion-exchangeable sites
2
+
and neighboring Zn cations, and (iv) strong electrostatic
interactions between the guest interlayer anions and the Zn2
cations. We have recently demonstrated that a function-
Received: August 26, 2014
Revised: October 2, 2014
Published: October 6, 2014
+
©
2014 American Chemical Society
4040
dx.doi.org/10.1021/cs501267h | ACS Catal. 2014, 4, 4040−4046

ACS Catalysis
Research Article
species exhibited high catalytic activity toward the 1,4-addition
reaction because the transmetalation of an aryl group from
boron to rhodium occurs immediately. Here, we present a new
strategy for the design of a novel immobilized Rh hydroxide
complex catalyst based on the anion-exchange ability of NiZn
and demonstrate the aforementioned 1,4-addition reaction by
use of the resulting synthesized heterogeneous catalyst. In our
After NaOHaq was added to the aqueous Na RhCl solution,
the solution changed from dark red to clear yellow (Figure S1).
3
6
In the UV−vis spectrum of the aqueous Na RhCl solution,
3
6
peaks at 509 and 403 nm corresponding to the d−d transition
1
9
shifted to 422 and 335 nm, respectively, after NaOH
aq
20
addition (Figure S2). In the Rh K-edge XANES spectrum,
the edge jump for the aqueous Na RhCl solution appeared at
3
6
catalyst system, monomeric trivalent Rh hydroxide [Rh-
higher energy than those of Rh foil and Rh O reference
2 3
−
(
OH)6]3 acts as the catalytically active site, fixed within
standards (Figures 1a-c and Table S2), consistent with the
NiZn interlayers. Furthermore, the hydroxyl anion, which may
activate phenylboronic acid, can also be integrated into the
NiZn matrix together with the anionic Rh species. Because of
the strong electrostatic interaction between the interlayer guest
anion and the NiZn host, conventionally unstable monomeric
Rh(OH)6]3 species can be stabilized within the NiZn
−
[
interlayer.
2
. EXPERIMENTAL SECTION
−
2
.1. Preparation of CH COO /NiZn. Acetate anion-
3
−
intercalated Ni−Zn hydroxy double salt (CH COO /NiZn)
was prepared according to the literature procedures. Ni-
3
16
(
(
OCOCH ) ·4H O (134 mmol) and Zn(OCOCH ) ·2H O
3 2 2 3 2 2
66 mmol) were dissolved in deionized water (200 mL). The
solution was hydrolyzed by heating in a Teflon-linked pressure
bottle at 200 °C for 24 h. The resulting precipitates were
filtered, washed with deionized water, and dried under vacuum,
yielding ca. 5 g of Ni0.63Zn0.37 (OCOCH ) (OH) ·1.93H O
3 0.37
2
2
−
(
CH COO /NiZn) as a light green powder.
3
2
.2. Preparation of the Rh/NiZn Catalyst. The NiZn-
3−
intercalated [Rh(OH)6] complex (Rh/NiZn catalyst) was
prepared by a simple intercalation technique. Na RhCl ·nH O
3
6
2
(
Rh: 0.05 mmol, concentration of rhodium species was
determined by AAS) was placed in a round-bottom flask, and
the total volume was adjusted to 40 mL with deionized water.
The red solution was heated with stirring at 50 °C for 30 min
after addition of 10 M NaOH aq. (10 mL), turning the solution
yellow. CH COO /NiZn (1 g) was added to the resulting
−
3
solution and stirred at 50 °C for 8 h. The obtained slurry was
filtered, washed with deionized water, and dried under vacuum,
yielding Rh/NiZn as a light green powder.
2.3. Typical Procedure for 1,4-Addition between 2-
Cyclohexen-1-one and Phenylboronic Acid. Into a
Schlenk tube with a reflux condenser, Rh/NiZn catalyst
Figure 1. Rh K-edge XANES spectra for (a) Rh foil, (b) Rh O , (c)
2
3
(
0.034 g, Rh: 0.1 mol %), 2-cyclohexen-1-one (1 mmol),
Na
RhCl aq, (d) Na RhCl aq, with NaOH, (e) fresh Rh/NiZn, and
3 6 3 6
(f) the recovered Rh/NiZn catalyst.
phenylboronic acid (1 mmol), 1,5-cyclooctadiene (6 μmol),
toluene (2.5 mL), and H O (2.5 mL) were placed. The
2
resulting mixture was heated at 100 °C for 1 h. After the
reaction, biphenyl was added to the resulting mixture as an
internal standard. The conversion and product yield were
determined by GC analysis using an internal standard
technique. After the removal of the Rh/NiZn catalyst by
simple filtration, evaporation of the solvent followed by silica
gel chromatography (n-hexane/ethyl acetate = 9/1) gave
analytically pure 3-phenylcyclohexanone.
presence of a high oxidation state (trivalent) Rh species.
Subsequent NaOHaq addition induced a significant change in
the XANES line shape (Figure 1d) but had negligible impact
upon the edge jump, indicating a change in the coordination
environment but common oxidation state. Phase-shift un-
3
corrected Fourier Transform (FT) of the k -weighted Rh K-
edge EXAFS spectra reveals the loss of a peak at ∼0.19 nm,
associated with a Rh−Cl bond, upon addition of NaOH to
aq
Na RhCl, coincident with the appearance of a new scattering
3
3. RESULTS AND DISCUSSION
peak at ∼0.15 nm associated with Rh−O bond formation;
3
−
Preparation of the rhodium hydroxy anion, [Rh(OH) ] , via
features characteristic of the Rh foil and Rh O standards at
6
2
3
the hydrolysis of Rh(III) salts in alkaline water has been
0.22, 0.25, and 0.32 nm (corresponding to Rh−Rh and Rh−O−
Rh bonds) were absent (Figures 2a-d). Following phaseshift
correction, the first peak in Figure 2d fitted well to a Rh−O
scattering shell with six oxygen atoms at 0.206 nm for the
Na RhCl complex with NaOH (Table 1). Hence UV−vis
17
previously reported. In this work, the prepared Na RhCl ·
3
6
1
8
1
2H O was treated with an aqueous solution of 10 M NaOH.
2
The coordinated structure of the Rh complex in the solution
state was confirmed by UV−vis and Rh K-edge XAFS analysis.
3
6
aq
4
041
dx.doi.org/10.1021/cs501267h | ACS Catal. 2014, 4, 4040−4046

ACS Catalysis
Research Article
−
anion-exchange reaction in water. Treatment of CH COO /
3
NiZn with the aforementioned [Rh(OH)6]3 yielded the NiZn-
intercalated Rh(III) hydroxyl complex, Rh/NiZn, as a green
powder (Rh content: 0.034 mmol/g, as determined by AAS).
−
XRD profiles revealed that the d001 peak shifted to a larger angle
−
relative to the d001 peak of the parent CH COO /NiZn, with
3
the associated C.S. (clearance space = basal spacing (d001) −
thickness of the brucite layer (0.46 nm)) falling to 0.36 nm for
Rh/NiZn, from 0.84 nm for the parent NiZn (Figures 3a and
3
Figure 2. FT of k -weighted Rh K-edge EXAFS spectra for (a) Rh foil,
b) Rh O , (c) Na RhCl aq, (d) Na RhCl aq with NaOH, (e) fresh
(
2
3
3
6
3
6
Rh/NiZn, and (f) the recovered Rh/NiZn catalyst. Phase shifts were
not corrected.
−
Figure 3. XRD profiles for (a) CH COO /NiZn, (b) Rh/NiZn, (c)
recovered Rh/NiZn, and (d) OH /NiZn.
3
−
and Rh K-edge XAS evidenced the formation of an anionic
[
−
Rh(OH)6]3 complex in aqueous, basic solution.
−
3b). The C.S. of OH /NiZn prepared by the same procedure
but without the Rh species was also 0.36 nm (Figure 3d),
suggesting that OH intercalation occurs alongside anionic Rh
species under our strongly basic conditions. Additional
−
Table 1. Curve-Fitting Results of Rh K-Edge EXAFS
a
Spectra
3
−
experiments performed with a higher [Rh(OH)6] loading
(0.5 mmol/g) evidenced a shift of the d001 peaks in the
diffractogram to lower angle, indicative of an expanded
interlayer clearance space (C.S.) as shown in Figure S6,
confirming that [Rh(OH)6]3 must be incorporated within the
NiZn interlayers.
X-ray photoelectron spectroscopy (XPS) was undertaken to
determine the oxidation state of the Rh species within the NiZn
interlayer. Figure 4 shows the Rh 3d XPS spectrum of Rh/
NiZn, which fitted well to a unique chemical environment with
b
c
d
sample
e
shell
Rh−Rh
Rh−O
Rh−(O)−Rh
Rh−Cl
Rh−O
CN
r (nm)
σ (nm)
Rh foil
(12)
(6)
(4)
(6)
(0.269)
(0.205)
(0.292)
(0.238)
0.206
e
Rh O3
2
−
e
Na RhCl aq
3
6
Na RhCl aq + NaOH
6.0
0.0058
0.0057
0.0058
3
6
fresh Rh/NiZn
Rh−O
Rh−O
6.2
5.9
0.205
recovered Rh/NiZn
0.205
a
b
e
Inverse FT were performed for the regions of 0.112−0.199 nm.
c
d
a set of doublets (spin−orbit splitting = 4.74 eV) and 3d
binding energy of 309.6 eV consistent with that of bulk Rh O ,
2 3
21
Coordination number. Bond distance. σ is Debye−Waller factors.
5/2
Data from X-ray crystallography.
i.e. a Rh(III) oxidation state, confirming a trivalent Rh species
within the NiZn interlayer. The Ni 2p and Zn 2p XP spectra
and Ni:Zn surface atomic ratios exhibited minimal changes
following Rh exchange into the interlayer of the parent NiZn
acetate (Figure S5), indicating the electronic structure and
stoichiometry of the host cationic layers was preserved. Rh K-
edge XAS (Figure 1e and 2e) of the Rh/NiZn solid were almost
indistinguishable to those of the parent [Rh(OH)6] solution
(Figure 1d and 2d); fitted EXAFS of the Rh/NiZn was likewise
consistent with six oxygen atoms at 0.205 nm coordinated to a
−
Acetate anion-intercalated NiZn (CH COO /NiZn) was
3
16
synthesized according to a previously reported procedure.
The chemical formula and anion-exchange capacity of the
parent NiZn were Ni0.63Zn0.37(OH) (CH COO) ·1.93H O
Ni/Zn = 2.20) and 2.44 mmol/g, respectively, on the basis of
X-ray fluorescence (XRF) and thermogravimetric-differential
thermal analysis (Figure S3 and Table S2). The rhodium
species was intercalated into the NiZn interlayer via a simple
2
3
0.37
2
(
3
−
4
042
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ACS Catalysis
Research Article
Table 2. Screening of Reaction Conditions for 1,4-Addition
a
Reaction between 1 and 2 Catalyzed by Rh/NiZn
toluene/H O
1/2
(mmol/mmol)
1.5-COD (equiv
relative to Rh)
yield
b
2
entry
(mL/mL)
(%)
1
2
3
4
5
6
7
8
9
4/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/2
1/3
1/1
1/1
1/1
0.5
1
34
47
54
64
73
78
69
39
81
92
64
34
nd
nd
nd
4/1
4/1
2
4/1
3
4/1
5
4/1
6
4/1
10
6
4.5/0.5
3.3/1.7
2.5/2.5
2.5/2.5
2.5/2.5
2.5/2.5
2.5/2.5
2.5/2.5
6
1
1
1
1
1
1
0
1
2
3
4
5
6
Figure 4. Fitted Rh 3d XP spectrum of the fresh Rh/NiZn catalyst.
6
6
monomeric Rh(III) center (Table 1). In concert, these
observations demonstrate that the Rh species in the NiZn
interlayer is a monomeric hexahydroxy rhodate anion, [Rh-
c
6
d
e
6
6
3−
−
(
OH) ] , accompanied by intercalated OH , as shown in
6
a
Rh/NiZn catalyst (Rh: 1 μmοl), 1 (1 mmol), 2, 1,5-COD, solvent (5
Scheme 1.
b
mL), 100 °C, air 1 h. Determined by GC using an internal standard
c
d
−
e
−
technique. Without catalyst. CH COO /NiZn (0.05 g). OH NiZn
3
Scheme 1. Proposed Local Structure of Rh Species in a NiZn
Interlayer
(
0.05 g).
1
2c,d
−1
12f
porous silica-grafted Rh:
12 h ; Rh-TPPTS/HT: 5.3
11d
−
1
−1
h ; PS−PEG-supported Rh:
1.4 h ; and chiral PS-
11c
−1
supported Rh: 3.3 h ). The order in phenylboronic acid 2
was also investigated (entries 10−12), revealing an inverse
dependence on 3-phenylcyclohexanone yield; maximal yield of
3
was obtained for one equivalent of 2, indicating that excess
phenylboronic acid may inhibit the catalytic cycle. This latter
phenomenon was not observed in studies employing a
polymer-Janaphos Rh catalyst and 1.3 equiv of 2.
The efficiency of the resulting Rh/NiZn material toward 1,4-
addition reactions was first explored employing a catalytic
amount of 1,5-cyclooctadiene (1,5-COD). Previous homoge-
neous Rh complexes have shown the utility of 1,5-COD ligands
11a,b
Control
reactions showed negligible activity in the absence of catalyst or
−
−
10
using the parent CH COO /NiZn or OH /NiZn without
3
for such 1,4-additions, in which their weak electron-donating
effect upon Rh centers may accelerate transmetalation or
addition of phenyl groups. 2-Cyclohexen-1-one (1) and
phenylboronic acid (2) were chosen as model substrates for
the initial optimization (Table 2), employing 0.034 g of the Rh/
NiZn catalyst (1 μmol Rh), 1 mmol of substrate, and 0.5 equiv
of 1,5-COD relative to Rh in a solvent mixture comprising 4
mL of toluene and 1 mL of water; stirred reactions were
conducted at 100 °C under air. A 34% yield of 3-phenyl-
cyclohexanone (3) was obtained after 1 h (entry 1). This was
the only product observed in all reactions employing 2-
cyclohexen-1-one. The amount of 1,5-COD was subsequently
varied from 1 to 10 equiv relative to Rh (entries 1−7), with the
best yield of 78% of 3 achieved using 6 equiv of 1,5-COD
incorporated Rh species (entries 13−15). Catalytic activity was
strongly dependent on the choice of olefin (Table 3), with 1,5-
COD unique as the only diene to significantly promote the
23
addition reaction (entry 1); 1,3-cyclooctadiene, 1,4-cyclo-
Table 3. Effect of Olefins on the Rh/NiZn-Catalyzed 1,4-
a
Addition Reaction
b
entry
olefin
yield of 3 (%)
1
2
3
4
5
6
7
8
9
1,5-cyclooctadiene
1,3-cyclooctadiene
1,4-cyclohexadiene
2,5-norbornadiene
1,5-hexadiene
92
5
trace
4
15
(
entry 6). The impact of the toluene:H O solvent ratio was also
2
cyclooctene
trace
trace
trace
4
explored (entries 7−10), with an equivolume mixture
comprising 2.5 mL of toluene and 2.5 mL of H O affording a
9
cyclohexene
2
2-norbornene
22
−1
2% yield (entry 10) after 1 h. TOFs approached 920 h per
1,5,9-cyclododecatriene
none
Rh under these reaction conditions, significantly higher than
1
0
nd
reported for alternative, heterogeneous Rh catalyst systems
1
3
a
(
1
HT:
polymer-intercalated chiral Rh/Ag bimetallic nanoparticles:
Rh/NiZn catalyst (Rh: 1 μmοl), 1 (1 mmol), 2 (1 mmol), olefin (6
−1
11a,b
−1
9 h ; recyclable polymer-Janaphos Rh:
3.3 h ; Rh/
17 h ; hydrophobized meso-
equiv relative to Rh), toluene (2.5 mL), H O (2.5 mL), 100 °C, air, 1
2
1
2a,b
−1
12e
−1
b
580 h ; RhFAP:
h. Determined by GC using an internal Standard technique.
4
043
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ACS Catalysis
Research Article
hexadiene, 2,5-norbornadiene, 1,5-hexadiene, and 1,5,9-cyclo-
dodecatriene proved ineffective (entries 2−5 and 9). No
reaction occurred in the presence of monoenes such as
cyclooctene, cyclohexene, and 2-norbornene (entries 6−8) or
in the absence of olefins (entry 9). This suggests that the 1,5-
COD ligand may coordinate to Rh during reaction to form a
stable, activated complex.
The 1,4-addition between 1 and 2 catalyzed by Rh/NiZn was
also successfully undertaken on a preparative scale. For
example, 1 (0.96 g; 10 mmol) and 3 (1.21 g; 10 mmol)
successfully gave 3 (1.62 g; 93% of isolated yield) in the
presence of the Rh/NiZn catalyst (0.1 g, Rh: 0.029 mol %) for
4
h, wherein the TON was up to 3200.
The superior activity and selectivity of Rh/NiZn for 1,4-
addition reactions was apparent from benchmarking against a
0
range of alternative rhodium catalysts such as Rh /carbon,
Rh O , and Rh(OH) under the same reaction conditions
2
3
3
(
Table 4). RhCl ·nH O and Na RhCl ·12H O also afforded 3
Figure 5. Time profile of 1,4-addition reaction of 1 and 2 in the
presence of the Rh/NiZn catalyst. Reaction conditions: Rh/NiZn
catalyst (Rh: 0.12 mol %) 1 (1 mmol), 2 (1 mmol), 1,5-COD (1 equiv
3
2
3
6
2
Table 4. 1,4-Addition Reaction between 1 and 2 with
Various Rh Catalysts
a
relative to Rh), toluene/H O (5 mL, v/v: 5/1), 100 °C.
2
b
entry
1
Rh catalyst
yield of 3 (%)
probable that catalysis is confined within the NiZn interlayers;
Rh leaching was not observed by AAS or any changes in
environment by XAS (Figures 1f and 2f), and the parent
Rh/NiZn
92
trace
trace
trace
80
c
0
2
Rh /carbon (5 wt %)
3
4
5
6
Rh O
2
3
−
−
CH COO /NiZn or OH /NiZn were unable to catalyze
3
Rh(OH)3
reaction in its absence (Table 2, entries 14 and 15), it is
inconceivable that migration out from the interlayers would not
result in solution phase Rh detectable by elemental analysis or
significant changes in the Rh chemical environment.
The scope of 1,4-addition reactions was examined for a
variety of electron-deficient olefin substrates (Table 5) under
RhCl ·nH O
3
2
Na RhCl ·12H O
61
3
6
2
a
Rh catalyst (Rh: 1 μmοl), 1 (1 mmol), 2 (1 mmol), 1,5-COD (6
equiv relative to Rh), toluene (2.5 mL), H O (2.5 mL), 100 °C, air, 1
2
b
h. Determined by GC using an internal standard technique.
c
Purchased from Aldrich.
Table 5. Substrate Screening of 1,4-Addition Reaction with
the Rh/NiZn Catalyst
a
in moderate yields of 80% and 61%, respectively (entries 5 and
3
−
6). Anionic [Rh(OH)6] intercalated into the NiZn interlayer
thus appears a highly efficient catalyst for this coupling reaction,
attributed to a strong electrostatic interaction between NiZn
sheets and interlayer [Rh(OH)6]3 species which hinders their
deactivation via Rh aggregation into Rh O or metal nano-
−
2
3
12b,24
particles, as common with other unstabilized complexes.
In order to assess whether the Rh/NiZn catalyst underwent
on-stream deactivation by e.g. restructuring of the active metal
species or HDS layers, or leaching of Rh into the reaction
mixture, an experiment was conducted in which additional
quantities of reactants 1 and 2 were added to the reaction
mixture following an initial catalytic run. Figure 5 shows that
1
,4-addition of the second batch of reactants proceeded
identically over the used catalyst, with no change in kinetics
of 3 production, while elemental analysis (AAS) of the filtrate
evidenced negligible leached Rh (below 4 × 10− g).
6
25
Spectroscopic analysis confirmed the stability of our Rh/
NiZn catalyst, with Rh K-edge XANES and fitted EXAFS of
fresh and spent materials indistinguishable (Figures 1d, 2d and
Table 1). These measurements confirm that Rh species within
the NiZn matrix remain in a trivalent, monomeric state
throughout the 1,4-addition reaction; a strong electrostatic
3
−
interaction between anionic [Rh(OH)6] species and the
layered NiZn host is hypothesized to inhibit rhodium
aggregation. The C.S. of the recovered Rh/NiZn catalyst
expanded slightly from 0.36 to 0.41 nm (Figure 3b and 3c),
which may reflect interlayer water incorporation alongside
a
Rh/NiZn catalyst (0.034 g, Rh: 0.1 mol %), enone (1 mmol), boronic
acid (1 mmol), 1,5-COD (6 equiv relative to Rh), toluene/H O (5
mL, 1/1 v/v), 100 °C. Determined by GC using an internal standard
technique. Enone (2 mmol). Yield was based on 2. Rh/NiZn (0.068
2
b
c
d
Rh(OH)6]3 and OH anions during reaction. It seems highly
−
−
g, Rh: 0.2 mol %)
[
4
044
dx.doi.org/10.1021/cs501267h | ACS Catal. 2014, 4, 4040−4046

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Research Article
optimized conditions using a 1,5-COD and 1:1 toluene:H O
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mixed solvent. 3-Buten-2-one and 2-cyclopentan-1-one were
good acceptors for the reaction with 2 (entries 1 and 2). 3-
Nonen-2-one and chalcone also reacted with 2, generating 4-
phenyl-nonan-2-one and 1,3,3-triphenylpropan-1-one, respec-
tively, in excellent yields (entries 7 and 8). Rh/NiZn was also
efficacious toward various p-substituted phenylboronic acids
1
868−1881. (e) Kaneda, K.; Ebitani, K.; Mizugaki, T.; Mori, K. Bull.
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(
entries 3−6), resulting in high yields of coupled products. We
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propose the following reaction pathway for 1,4-additions
catalyzed by Rh/NiZn, which mirrors that previously reported
for homogeneous Rh catalysts: (i) transmetalation of the
interlayer [Rh(OH)6] species and arylboronic acid to
generate a Rh−Ar species; (ii) olefin insertion into the Rh−
Ar bond, yielding an oxa-π-allyl rhodium enolate, which is
readily protonated by water; (iii) elimination of the desired
addition product, regenerating the initial [Rh(OH)6]3 species.
Phenylboronic acids might also be quarternized with interlayer
3
−
2
685. (e) Morioka, H.; Tagaya, H.; Karasu, M.; Kadokawa, J.; Chiba,
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−
−
3− 26
OH anions to facilitate transmetalation to [Rh(OH) ] .
6
1
(
1, 2034−2040.
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4
. CONCLUSIONS
3
−
Synthesis of a well-defined [Rh(OH) ] /NiZn catalyst system
6
(
which efficiently catalyzes the 1,4-addition reaction between
various enones and phenylboronic acids into their correspond-
ing β-substituted carbonyl compounds has been demonstrated.
The intercalated, monomeric, trivalent Rh hydroxide species
was stabilized by the NiZn matrix during reaction, enabling
recycling with negligible deactivation. We are currently
investigating the application of our catalyst toward asymmetric
2
3
350−2356. (c) Kandare, E.; Hossenlopp, J. M. Inorg. Chem. 2006, 45,
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alkynes with phenylboronic acid into substituted alkenes for
which we have obtained promising preliminary results.
(
5) (a) Rajamathi, J. T.; Raviraj, N. H.; Ahmed, M. F.; Rajamathi, M.
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ASSOCIATED CONTENT
Supporting Information
The photoimages of preparation solution, UV−vis spectra, TG-
DTA data, bulk Ni/Zn ratio and surface elemental composition,
■
Barriga, C.; Rives, V. Microporous Mesoporous Mater. 2008, 112, 262−
*
S
́
72. (c) Rojas, R.; Barriga, C.; Ulibarri, M. A.; Rives, V. J. Solid State
2
Chem. 2004, 177, 3392−3401.
(6) (a) Kandare, E.; Hossenlopp, J. M. J. Phys. Chem. B 2005, 109,
the E values in Rh K-edge XANES, curve-fitting results of FT
8469−8475. (b) Tronto, J.; Leroux, F.; Dubois, M.; Taviot-Gueho, C.;
Valim, J. J. Phys. Chem. Solids 2006, 67, 978−982. (c) Arulraj, J.;
Rajamathi, J. T.; Prabhu, K. R.; Rajamathi, M. Solid State Sci. 2007, 9,
0
of EXAFS, XPS, and NMR data. This material is available free
of charge via the Internet at http://pubs.acs.org.
8
12−816. (d) Richardson-Chong, S. S. D.; Patel, R.; Williams, G. R.
Ind. Eng. Chem. Res. 2012, 51, 2913−2921.
7) (a) Nityashree, N.; Rajamathi, M. J. Phys. Chem. Solids 2013, 74,
AUTHOR INFORMATION
Corresponding Author
E-mail: shimazu@faculty.chiba-u.jp.
■
*
(
1
164−1168. (b) Rajamathi, J. T.; Ravishankar, N.; Rajamathi, M. Solid
State Sci. 2005, 7, 195−199.
(8) For reviews, see: (a) Tian, P.; Dong, H.-Q.; Lin, G.-Q. ACS Catal.
Notes
2
012, 2, 95−119. (b) Edwards, H. J.; Hargrave, J. D.; Penrose, S. D.;
Frost, C. G. Chem. Soc. Rev. 2010, 39, 2093−2105. (c) Defieber, C.;
Grutzmacher, H.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47,
482−4502. (d) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535−
553. (e) Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103, 2829−2844.
9) Sakai, M.; Hayashi, H.; Miyaura, N. Organometallics 1997, 16,
The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
■
4
1
This study was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan (25420824). The Rh K-edge
XAFS experiments were conducted at a facility in the Photon
Factory (KEK-PF, Proposal No. 2012G596). We are also
grateful to Mr. Muneharu Nozawa and Ms. Tomoko Fukusaki
for their invaluable contributions to the work described here.
A.F.L. thanks the EPSRC for the award of a Leadership
Fellowship (EP/G007594/4), and K.W. thanks the Royal
Society for the award of an Industry Fellowship.
(
4229−4231.
(10) Recently reported examples, see: (a) Korenaga, T.; Ko, A.;
Shimada, K. J. Org. Chem. 2013, 78, 9975−9980. (b) Khiar, N.;
́
́
Salvador, A.; Valdivia, V.; Chelouan, A.; Alcudia, A.; Alvarez, E.;
́
Fernandez, I. J. Org. Chem. 2013, 78, 6510−6521. (c) Takaya, H.;
Iwaya, T.; Ogata, K.; Isozaki, K.; Yokoi, T.; Yoshida, R.; Yasuda, N.;
Seike, H.; Takenaka, T.; Nakamura, M. Synlett 2013, 24, 1910−1914.
(d) Chen, Q.; Chen, C.; Guo, F.; Xia, W. Chem. Commun. 2013, 49,
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433−6435. (e) Fairhurst, N. W. G.; Munday, R. H.; Carbery, D. R.
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(
(
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4
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