Transfer hydrogenation of nitrogen heterocycles using a recyclable rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica

Article abstract of DOI:10.1039/c7cy02167d

Transfer hydrogenation of unsaturated nitrogen heterocycles using a rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica (BPy-PMO) is described. The immobilized catalyst was prepared by mixing [Cp?RhCl₂]₂ (Cp?

Full text of DOI:10.1039/c7cy02167d

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Transfer hydrogenation of nitrogen heterocycles
using a recyclable rhodium catalyst immobilized
on bipyridine-periodic mesoporous organosilica†
Cite this: DOI: 10.1039/c7cy02167d
Kazuma Matsui,a Yoshifumi Maegawa,b Minoru Waki,b
a
Shinji Inagaki b and Yoshihiko Yamamoto
*
*
Transfer hydrogenation of unsaturated nitrogen heterocycles using a rhodium catalyst immobilized on
bipyridine-periodic mesoporous organosilica (BPy-PMO) is described. The immobilized catalyst was pre-
pared by mixing [Cp*RhCl2]2 (Cp* = η5-C5Me5) with BPy-PMO powder in DMF at 60 °C and characterized
by nitrogen adsorption measurements, solid-state NMR spectroscopy, X-ray diffraction, energy-dispersive
X-ray spectroscopy, and transmission electron microscopy. In the presence of the catalyst, a wide variety
of unsaturated nitrogen heterocycles underwent transfer hydrogenation to afford the corresponding prod-
ucts in good yields. The immobilized catalyst could be readily recovered by centrifugation and reused sev-
eral times in the transfer hydrogenation process.
Received 21st October 2017,
Accepted 6th December 2017
DOI: 10.1039/c7cy02167d
rsc.li/catalysis
Inagaki and co-workers previously synthesized bipyridine-
periodic mesoporous organosilica (BPy-PMO), in which the
Introduction
Saturated nitrogen heterocycles are ubiquitous in natural
products and drug molecules.1 Therefore,
variety of
2,2′-bipyridine ligands were regularly arranged and exposed
on the pore surface.6 Various metal complexes can be directly
immobilized on the pore surfaces of BPy-PMO. Such direct
immobilization effectively suppresses the unfavorable interac-
tions between the metal catalysts and carrier surfaces, as well
as the aggregation of the immobilized metal catalysts. More-
over, BPy-PMO has a large pore size (diameter: 3.8 nm) that
enables smooth diffusion of the substrate and product mole-
cules. For these reasons, the catalysts immobilized on BPy-
PMO generally show higher catalytic performance as com-
pared to conventional heterogeneous catalysts.6,7 Therefore,
we prepared a recyclable rhodium catalyst immobilized on
BPy-PMO (Rh@BPy-PMO) and employed it in the transfer hy-
drogenation of diverse unsaturated nitrogen heterocycles.
a
methods have been developed for the synthesis of saturated
nitrogen heterocycles.1d,2 Among these, hydrogenation of un-
saturated heterocycles has been recognized as a straightfor-
ward route to saturated heterocycles.1d,3 Moreover, transfer
hydrogenation has a practical advantage over conventional hy-
drogenation in that safe and inexpensive hydrogen donors
can be utilized instead of hydrogen gas.4 Thus, transfer hy-
drogenation circumvents the use of pressure equipment. Al-
though transfer hydrogenation of unsaturated nitrogen
heterocycles has been realized using homogeneous
catalysts,1d,3 the application of such catalysts to the produc-
tion of fine chemicals is still limited because of the difficul-
ties in catalyst separation and recycling. Therefore, the devel-
opment of a high-performance recyclable catalyst for transfer
hydrogenation is of great importance. Nevertheless, the sub-
strate scope of transfer hydrogenation involving heteroge-
neous catalysts was restricted to aldehydes or ketones.4 Exam-
ples of nitrogen heterocycles in such cases were scarce, partly
because of catalyst inhibition by the strong coordination of
the resulting saturated nitrogen heterocycles.5
Results and discussion
Catalyst preparation
Recently, Xu and co-workers revealed that a combination of
[Cp*RhCl2]2 (Cp* = η5-C5Me5) and 2,2′-bipyridine as a ligand
showed high catalytic activity that was suitable for the trans-
fer hydrogenation of unsaturated nitrogen heterocycles.8
They utilized a formic acid/formate buffer that served as a hy-
drogen donor in H2O. Inspired by this study, we investigated
the transfer hydrogenation of nitrogen heterocycles using a
Rh catalyst immobilized on BPy-PMO (Rh@BPy-PMO).
Rh@BPy-PMO could be readily prepared by stirring [Cp*
RhCl2]2 with BPy-PMO powder in DMF at 60 °C (Scheme 1).
The amount of Rh immobilized on BPy-PMO was easily
a Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical
Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan.
E-mail: yamamoto-yoshi@ps.nagoya-u.ac.jp
b Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan.
E-mail: inagaki@mosk.tytlabs.co.jp
† Electronic supplementary information (ESI) available: Experimental details
and characterization data. See DOI: 10.1039/c7cy02167d
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sorption measurements were carried out. The TEM images
clearly showed the ordered structure of Rh@BPy-PMO, indi-
cating that the pore channel was maintained throughout the
PMO particle (Fig. S7, ESI†). However, the XRD profile
showed that the diffraction peak at 2θ = 1.88° that corre-
sponds to a periodic mesoporous structure was lower in
intensity compared to that of BPy-PMO. On the other hand,
the intensities of the diffraction peaks at 2θ = 7.54°, 15.2°,
and 22.9°, attributable to the molecular-scale periodicity of
the bipyridine ligands in the pore wall, were maintained (Fig.
S8, ESI†). The nitrogen absorption isotherm indicated a
poorer absorption behavior after the immobilization of the
Rh complex as compared to that of BPy-PMO (Fig. S9, ESI†).
The BET surface area, pore volume, and pore diameter of
Rh@BPy-PMO were 298 m2 g−1, 0.127 cc g−1, and 3.1 nm,
respectively.
Optimization of transfer hydrogenation conditions
To establish a viable protocol for the transfer hydrogenation
using Rh@BPy-PMO, the reaction conditions were optimized
in the case of 3-methylquinoxalinone 1a (Table 1). In the pres-
ence of Rh@BPy-PMO (0.5 mol% Rh), 1a was heated in a 1 : 1
HCO2H/HCO2Na buffer solution (pH 3.8, 5.0 mol L−1) at 80
°C for 2 h to afford the desired product 2a in 78% yield
(NMR), along with 9% of the unreacted 1a (entry 1). As a side
product, small amounts (9% NMR) of N-formylation product
2a′ were also detected (δ 1.33 (d, J = 7.3 Hz, 3H, CH3), 5.35 (q,
J = 7.3 Hz, 1H, CHMe), 6.93–7.21 (aromatic 4H), 8.63 (s, 1H,
CHO)). Next, the influence of buffer concentration and pH
was investigated (entries 2–4). The use of a 1 : 2 HCO2H/
HCO2Na buffer solution (pH 4.1, 0.6 mol L−1) gave the best
result (entry 4). Decreasing the catalyst loading to 0.2 mol%
Rh gave a comparable result, and 2a was isolated in 98%
Scheme 1 Immobilization of [Cp*RhCl2]2 on the pore surface of BPy-
PMO.
controlled by the initial loading of [Cp*RhCl2]2. We prepared
the catalyst sample with a Rh/BPy molar ratio of 0.06. The cor-
responding Rh loading was confirmed as 0.20 mmol g−1 by
energy-dispersive X-ray spectroscopy (EDX, Fig. S1 in the ESI†).
The obtained immobilized catalyst was characterized by
13C cross polarization (CP) magic-angle spinning (MAS) NMR
and 29Si MAS NMR spectroscopy. The 13C CP-MAS NMR spec-
trum of Rh@BPy-PMO clearly showed new signals derived
from the Cp* ligand in the PMO cavity at δ 7.9 and 97.4 ppm,
respectively (Fig. S2, ESI†), and these chemical shifts were
similar to those of the Cp* ligand in [Cp*RhCl2]2 (δ 9 and 97
ppm). The 29Si MAS NMR spectrum of Rh@BPy-PMO showed
T2 [SiC(OSi)2IJOH)] and T3 [SiC(OSi)3] signals, but almost no
Qn [SiIJOSi)nIJOH)4−n, n = 2–4] signals between −90 and −120
ppm were observed, suggesting almost complete preservation
of the Si–C bonds after the immobilization of the Rh complex
(Fig. S3, ESI†). Subsequently, the local structure of the
immobilized Rh complex was investigated by X-ray absorp-
tion fine structure (XAFS) measurements. The X-ray absorp-
tion near edge structure (XANES) and extended XAFS (EXAFS)
at the Rh K-edge of Rh@BPy-PMO were compared with those
of the homogeneous complex [Cp*RhCl2IJbpy)]. The XANES
spectrum and EXAFS Fourier transform of Rh@BPy-PMO
were similar to those of Cp*RhCl2IJbpy) (Fig. S4–S6, ESI†).
Curve fitting analysis suggested that the immobilized Rh
complex had almost the same electronic state as Cp*
RhCl2IJbpy) (Fig. S1, ESI†). These results suggested that the
Cp*RhCl2–bipyridine complex was formed successfully over
the pore surface of BPy-PMO.
Table 1 Optimization of transfer hydrogenation conditions
Entry
Rh, mol%
[HCO2−].a mol L−1
pH
Yield,b %
1
2
3
4
0.5
0.5
0.5
0.5
0.2
—
5.0
1.3
1.3
0.6
0.6
0.6
3.8
3.8
4.1
4.1
4.1
4.1
78c
89
95
98
5
(98)d
—
6e
a
The HCO2H/HCO2Na ratios are 2.5 M/2.5 M (entry 1), 0.40 M/0.90 M
In order to confirm the ordered mesoporous structure of
Rh@BPy-PMO, transmission electron microscopy (TEM),
X-ray diffraction (XRD) analysis, and nitrogen adsorption–de-
b
(entries 2 and 3), and 0.19 M/0.41 M (entries 4 and 5). Yields
c
determined by NMR. N-Formylation product 2a′ was obtained in 9%
yield. d Yields of isolated 2a. e BPy-PMO was used.
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yield by silica gel column chromatography (entry 5). BPy-
PMO alone did not catalyze the transfer hydrogenation (entry
6).
To gain insights into the catalytic process involving the
immobilized catalyst, a hot filtration experiment was carried
out.6 A mixture of 1a and the catalyst was heated in a
HCO2H/HCO2Na buffer solution at 80 °C for 30 min, and the
resultant suspension was filtered using a membrane filter
(0.1 μm) to remove Rh@BPy-PMO and the solid unreacted 1a.
After adding 1a, the resultant filtrate was heated at 80 °C for
another 23 h, but no noticeable increase in the product yield
was detected (Fig. 1). This result indicates that the reaction
virtually proceeded at the sites of the immobilized Rh com-
plex on the pore surface of Rh@BPy-PMO. However, ICP-AES
measurements of the filtrate revealed a low degree of
leaching of the Rh species (<1.2 ppm Rh) from Rh@BPy-
PMO. The leached Rh species may be attributed to the [Cp*
RhCl2]2 precursor adsorbed on the silanol groups (Si–OH)
present on the pore surface. It was previously reported that
[Cp*RhCl2]2 shows no catalytic activity for transfer
hydrogenation.8
Fig. 2 Catalyst recycling experiments using Rh@BPy-PMO. Isolated
yields of 2a: 1st 99%, 2nd 97%, 3rd 98%, 4th 90%, and 5th 71%.
PMO can be neglected. Moreover, the TEM images of the re-
covered catalyst showed interference stripes corresponding to
the ordered structure of PMO (Fig. S11, ESI†). Interestingly,
the 13C CP-MAS NMR spectra revealed that the intensities of
the signals corresponding to the Cp* ligand decreased after
the first reaction cycle (Fig. S12, ESI†). After the fourth reac-
tion cycle, the Cp* signals completely disappeared,
suggesting that the Cp* ligand dissociated during the trans-
fer hydrogenation. Miller and Winkler independently
reported that a hydrogen atom is transferred from the Rh
center to the Cp* ligand to produce an η4-cyclopentadiene
complex.9 Thus, it is assumed that a similar H-transfer oc-
curred in this study for the immobilized catalyst, and the re-
sultant η4-diene ligand dissociated from the Rh center
(Scheme 2). Because the catalytic hydrogenation was observed
after the Cp* ligand disappeared, bpy-bound Rh hydrides
without the Cp* ligand are also catalytically active species,
and this fact was not pointed out in the previous study.8 In
contrast, the 13C signals of bpy were observed after the 4th
cycle (Fig. S12, ESI†), indicating that the pyridine moiety
remained intact.
Catalyst recycling experiments
The reusability of Rh@BPy-PMO was investigated by
conducting recycling experiments (Fig. 2). After transfer hy-
drogenation of 3-methylquinoxalinone 1a under the opti-
mized conditions, the immobilized catalyst was recovered by
centrifugation, dried in vacuo, and used for the next reaction
cycle. On a 6.0 mmol scale, Rh@BPy-PMO maintained high
catalytic activity in at least three reaction cycles, affording 2a
in >95% yield. However, the yield of 2a gradually decreased
after the fourth reaction cycle, and the recovery of 1a
increased.
To clarify the nature of the immobilized catalyst after the
reaction cycles, the recovered catalyst was inspected by sev-
eral physicochemical analysis methods. In the 29Si MAS NMR
spectrum of the recovered catalyst, no signal was observed at
the Q site (Fig. S10, ESI†). Thus, the Si–C bond cleavage of
Fig.
1 Progress of transfer hydrogenation of 1a at 80 °C using
Rh@BPy-PMO (red and blue). Rh@BPy-PMO was removed after 30 min
(red).
Scheme 2 Plausible mechanism for Cp* dissociation.
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Table 2 Transfer hydrogenation of various nitrogen heterocyclesa
Table 2 (continued)
Run
12h
Substrate (time, h)
Product (yield, %)
Run
1b
Substrate (time, h)
Product (yield, %)
2b (95)
1b (5)
1m (3)
1n (3)
2m (80)e
2n (93)e
2b
13d
2c (86)
2d (99)
1c (19)
1d (2)
14b,d
3
1o (2)
1p (2)
3o (96)
2p (90)
4b,c
15b
1e (4)
2e (94)
2f (92)
5b
16b
1f (20)
2q (85)
2r (95)
2s (84)
1q (7)
6
17b
1r (24)
1g (6)
18b
2g (97)
2h (94)
2i (96)e
2j (85)
7b
1s (4)
1h (5)
1i (4)
a
8b,d
9b,d
10b,c,d
Standard reaction conditions: 1 (0.3 mmol), Rh@BPy-PMO (0.2
mL), 80 °C.
Isopropanol (1 mL) was added. HCO2H/HCO2Na (pH 4.1, 0.6 M, 4
mol% Rh), HCO2H/HCO2Na (pH 4.1, 0.6 M,
2
b
c
d
mL). HCO2H/HCO2Na (pH 4.1, 1.3 M, 2 mL (4 mL for entry 10)).
cis/trans ratios are 2 : 1 (entry 8), 15 : 1 (entry 12), and 7 : 1 (entry 13).
HCO2H/HCO2Na (pH 3.5, 1.3 M, 2 mL). 0.5 mol% Rh. HCO2H/
HCO2Na (pH 4.4, 2.0 M, 2 mL).
e
f
g
h
1j (10)
Substrate scope
We further investigated the scope of the nitrogen heterocy-
cles for transfer hydrogenation catalyzed by Rh@BPy-PMO.
The reaction of 3-isopropylquinoxalinone 1b was sluggish
even under the optimal conditions because of its poor solu-
bility. Thus, 2-propanol was added as a co-solvent. As a re-
sult, hydrogenation of 1b was complete within 5 h, affording
the desired product 2b in 95% yield (Table 2, run 1). In a
similar manner, 3-cyclopropyl derivative 1c was converted
into 2c in 86% yield, wherein the cyclopropyl ring was intact
(run 2). N-Substituted quinoxalinones 1d–h were also
subjected to transfer hydrogenation, and the corresponding
1k (2)
2k (75)
2l (70)
11b,f,g
1l (24)
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products 2d–h were obtained in >90% yields (runs 3–7). No-
tably, the N-allyl, N-cyanomethyl, N-propargyl, and N-benzyl
moieties remained intact under the reaction conditions.
The transfer hydrogenation of quinoxalines 1i–k
proceeded efficiently, although a 1.3 M HCO2H/HCO2Na
buffer solution was required (Table 2, runs 8–10). The reac-
tion of 2,3-dimethylquinoxaline 1i afforded 2i in 99% yield
with a cis/trans ratio of 2 : 1 (run 8), whereas only the
cis-isomer of 2j was obtained in 97% yield from 2,3-
diphenylquinoxaline 1j (run 9). The reaction of indanone-
fused quinoxaline 1k afforded 2k in 85% yield with the con-
comitant diastereoselective reduction of the ketone moiety
(run 10). The immobilized catalyst also enabled the transfer
hydrogenation of substituted quinolines under modified con-
ditions. The reaction of 2-phenylquinoline 1l was conducted
at pH 3.5 under an increased catalytic loading (0.5 mol%
Rh), affording 2l in 70% yield (run 11). The reaction of
quinoline-4-carboxylic acid 1m was performed at pH 4.4 with
a 2.0 M buffer solution, affording 2m in 80% yield and a cis/
trans ratio of 15 : 1 (run 12), while the reaction of quinoline-3-
carboxylate ester 1n at pH 4.1 with a 1.3 M buffer solution
produced 2n in 93% yield and a cis/trans ratio of 7 : 1 (run
13). The latter reaction deserves some comments. When the
reaction of 1n was conducted using a buffer of lower concen-
tration (0.6 M), the semireduction product 3n was selectively
generated in 96% yield (NMR) together with trace amounts of
2n (Scheme 3). This result suggests that primary hydrogena-
tion occurred and that the second hydrogenation of the resul-
tant 3n was slower because it required protonation of the less
electron-rich enamino ester moiety. The same reaction was
repeated using the homogeneous catalyst [Cp*RuCl2]2/bpy un-
der otherwise identical conditions, and a near 1 : 1 mixture of
2n and 3n was obtained. Therefore, the selective semihydro-
genation process is characteristic of the heterogeneous condi-
tions using Rh@BPy-PMO. Interestingly, the reaction of
cyclohexanone-fused quinoline 1o afforded 1,4-dihydroquin-
oline 2o as the sole product in 96% yield, even when a higher
buffer concentration of 1.3 M was used (run 14). This result
suggests that transfer hydrogenation proceeds faster for the
nitrogen heterocyclic moiety than for the ketone. This selec-
tivity can be ascribed to the preferable coordination of a
softer nitrogen heterocycle than a harder carbonyl oxygen.
The carbonyl group of 3o remained intact because it is incor-
porated into a vinylogous amide and has low electrophilicity.
In addition to the above six-membered heterocyclic com-
pounds, five and seven-membered heterocyclic compounds
were investigated as substrates. In a previous study using a
homogeneous catalyst, the reaction of indole produced the
corresponding hydrogenation product with concomitant
formylation of the nitrogen atom.8 Thus, extra manipulation
was required for the removal of the N-formyl group. There-
fore, indolenine 1p was subjected to heterogeneous transfer
hydrogenation, and protection-free indoline 2p was directly
obtained in 90% yield (Table 2, run 15). In contrast, when
indolenium 1q was used as a substrate, N-ethylindoline 2q
was also obtained in 85% yield (run 16). Moreover, benzo-
xazepine 1r and benzodiazepine 1s underwent transfer hydro-
genation to afford the corresponding products 2r and 2s in
95% and 84% yields, respectively (runs 17 and 18).
Conclusions
We have prepared a rhodium catalyst immobilized on BPy-
PMO (Rh@BPy-PMO) by mixing [Cp*RhCl2]2 with BPy-PMO in
DMF at 60 °C. Rh@BPy-PMO was characterized by several
physicochemical analyses, and the local structure of the
immobilized Rh complex was observed to be structurally sim-
ilar to the corresponding homogeneous complex Cp*
RhCl2IJbpy). Rh@BPy-PMO could be used as the catalyst for
the transfer hydrogenation of unsaturated nitrogen heterocy-
cles in a HCO2H/HCO2Na buffer solution at 80 °C. A wide va-
riety of nitrogen heterocycles underwent transfer hydrogena-
tion to afford the products in good yields under modified
reaction conditions. The transfer hydrogenation occurred at
the site of the immobilized Rh complex on the pore surface
of BPy-PMO, as was confirmed by hot-filtration experiments.
Rh@BPy-PMO was readily recovered from the reaction mix-
tures by centrifugation, and the recovered catalyst
maintained high catalytic activity up to four reaction cycles of
the model substrate.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by AMED (Platform for Drug Dis-
covery, Informatics, and Structural Life Science), ACT-C, from
Japan Science and Technology Agency, (Grant Number
JPMJCR12Y1), and in part, by a Grant-in-Aid for Scientific
Research on Innovative Areas ‘Artificial Photosynthesis’
(no. 2406) from JSPS. The XAFS measurements were
performed at SPring-8 (BL14B2: 2017A1822). The authors
thank Dr. Yasutomo Goto (Toyota Central R&D Laboratories,
Inc.) for the SEM-EDX analysis and TEM observations.
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