Immobilized Zn(OAc)2on bipyridine-based periodic mesoporous organosilica for N -formylation of amines with CO2and hydrosilanes

Article abstract of DOI:10.1039/d1nj01204e

Zinc acetate (Zn(OAc)2) was successfully immobilized on a bipyridine-based periodic mesoporous organosilica (BPy-PMO-TMS), as confirmed by solid-state NMR and energy-dispersive X-ray spectroscopies, X-ray diffractometry, and nitrogen adsorption/desorption isotherm analyses. The immobilized Zn complex, Zn(OAc)2(BPy-PMO-TMS), exhibited good catalytic activity during the N-formylations of amines and amides with CO2 and PhSiH3 to produce the corresponding formamides. Zn(OAc)2(BPy-PMO-TMS) with a lower Zn loading was found to exhibit higher catalytic activity.

Full text of DOI:10.1039/d1nj01204e

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Immobilized Zn(OAc) on bipyridine-based
2
periodic mesoporous organosilica for
Cite this: New J. Chem., 2021,
45, 9501
N-formylation of amines with CO and
2
hydrosilanes†
ab
a
c
Xiao-Tao Lin, Kazuhiro Matsumoto,
Yoshifumi Maegawa,
a
a
a
Katsuhiko Takeuchi,
Norihisa Fukaya,
Kazuhiko Sato,
ac
ab
Shinji Inagaki
*
and Jun-Chul Choi
*
2
Zinc acetate (Zn(OAc) ) was successfully immobilized on a bipyridine-based periodic mesoporous
Received 12th March 2021,
Accepted 3rd May 2021
organosilica (BPy-PMO-TMS), as confirmed by solid-state NMR and energy-dispersive X-ray spectroscopies,
X-ray diffractometry, and nitrogen adsorption/desorption isotherm analyses. The immobilized Zn complex,
Zn(OAc)
with CO
loading was found to exhibit higher catalytic activity.
2
(BPy-PMO-TMS), exhibited good catalytic activity during the N-formylations of amines and amides
DOI: 10.1039/d1nj01204e
2
and PhSiH to produce the corresponding formamides. Zn(OAc) (BPy-PMO-TMS) with a lower Zn
3
2
rsc.li/njc
have recently been developed as effective catalysts for olefin
epoxidation and alkyne hydrosilylation, respectively.
Introduction
0
Periodic mesoporous organosilica (PMO) containing a 2,2 -
On the other hand, formamides, as a class of chemical, are
bipyridine (BPy) unit in its framework has emerged as an widely employed in industry as solvents and raw materials
efficient solid support for metal complexes due to its unique for the syntheses of pharmaceuticals and agrochemicals. The
1
,2
0
pore wall structure. Since the 2,2 -bipyridine unit is regularly N-formylation of an amine with CO
2
(as the C-1 feedstock) and
arranged on the pore surface, 1 : 1 metal complexes of BPy are a hydrosilane (as the reductant) is an attractive route to
1
1
selectively formed on the wall. Moreover, the large (3.8 nm formamides. Since the discovery of organic base-catalyzed
2
ꢀ1
12
diameter) pores and large (4600 m g ) surface area of the N-formylation chemistry by Cantat and co-workers, various
material enable substrate molecules to smoothly diffuse in its organocatalysts and metal-based catalysts have been developed
1
3
mesopores. These solid support characteristics are advantageous for this reaction. We also recently reported that Zn(OAc)2
for the immobilization of metal complexes and their application effectively catalyzes the N-formylation and N-methylation of
as heterogeneous catalysts for organic transformations. nitrogen nucleophiles, such as amines and amides, in the
1
4
Consequently, a wide variety of transition-metal complexes, presence of N-donor ligands, typically 1,10-phenanthroline.
including Mn, Ru, Rh, Re, Ir, Pt, and Au complexes, have It is noteworthy that Zn(OAc)
3
4
5
6
7
8
9
2
itself cannot promote the
been immobilized on BPy-PMO, and these immobilized reaction. To further develop this chemistry, herein we report
complexes have been used as heterogeneous catalysts for several the immobilization of Zn(OAc) on BPy-PMO-TMS and its use as
2
transformations. While BPy-PMO has a free silanol group on its a heterogeneous catalyst for the N-formylations of nitrogen
organosilicate framework, BPy-PMO-TMS, in which the silanols nucleophiles using CO and hydrosilanes.
2
are capped with trimethylsilyl (TMS) groups, is also available,
10
8b
2 2
and Mo(O) Cl(OH)(BPy-PMO-TMS) and PtMe (BPy-PMO-TMS)
Results and discussion
a
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
E-mail: junchul.choi@aist.go.jp
Immobilizing Zn(OAc)
2
on BPy-PMO-TMS and characterizing
Zn(OAc) (BPy-PMO-TMS)
2
b
c
Graduate School of Pure and Applied Sciences, University of Tsukuba,
Zn(OAc) was immobilized on BPy-PMO-TMS by simply mixing
2
1
-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
Zn(OAc) and BPy-PMO-TMS (with an initial Zn/BPy molar ratio
2
Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan.
E-mail: inagaki@mosk.tytlabs.co.jp
of 30/100) in MeOH or THF at 60 1C for 24 h (Scheme 1).
Zn(OAc)
2
(BPy-PMO-TMS) 1 (immobilized in MeOH) and 2
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj01204e
(immobilized in THF) were subsequently collected by filtration.
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corresponding to acetoxy groups at 22 ppm (methyl) and
1
83 ppm (carbonyl) that are almost identical to those of the
0
Zn(OAc) (2,2 -bipyridine) complex. However, the spectrum of
2
Zn(OAc) (BPy-PMO-TMS) 1 immobilized in MeOH reveals a
2
significant decline in the intensity of the peak at around
0
ppm, indicative of partial cleavage of the TMS caps in
BPy-PMO-TMS under the immobilization conditions, and
1
TMSOMe was detected by GC/MS and H NMR analysis of the
filtrate. On the other hand, no notable decrease in peak intensity
was observed in the spectrum of Zn(OAc) (BPy-PMO-TMS) 2
2
immobilized in THF. These results indicate that a protonolysis
of the O–TMS bonds occurred in protic MeOH.
In addition to 2, Zn(OAc) (BPy-PMO-TMS) 3 and 4 were
2
prepared in THF with initial Zn/BPy molar ratios of 10/100 (3)
and 5/100 (4). The physicochemical properties of 1–4 are listed
in Table 1. The combined filtrate and wash recovered from the
immobilization process were subjected to EDX, which revealed
that almost all of the Zn(OAc)
2
was immobilized on BPy-PMO-
TMS in THF, with zinc-atom loadings calculated to be 0.79 (2),
ꢀ
1
0
.29 (3) and 0.15 mmol g (4), respectively, whereas incom-
2
Scheme 1 Immobilization of Zn(OAc) on BPy-PMO-TMS.
plete immobilization was observed in MeOH (Zn amount of
: 0.69 mmol g ). Zn(OAc) (BPy-PMO-TMS) 1–4 showed
2
similar type-IV isotherms in nitrogen adsorption/desorption
experiments (Fig. S1–S5, ESI†), which indicates that the
uniform mesoporosity of the parent BPy-PMO-TMS was maintained
ꢀ
1
1
1
3
Comparing the
C CP/MAS NMR spectra of the parent
BPy-PMO-TMS and the immobilized zinc complexes 1 and 2,
0
15
with that of the Zn(OAc)
2
(2,2 -bipyridine) complex clearly
0
2 2
in Zn(OAc) (BPy-PMO-TMS) 1–4. Zn(OAc) (BPy-PMO-TMS) 3
reveals that the 2,2 -bipyridine unit of BPy-PMO-TMS is coordi-
0
was subjected to XRD, which confirmed that the mesoporous
structure of BPy-PMO-TMS was maintained during the
immobilization process (Fig. S6, ESI†).
nated to Zn(OAc)
2
to form the desired Zn(OAc)
2
(2,2 -bipyridine)-
like complexes on the solid support (Fig. 1). The NMR spectra
of Zn(OAc) (BPy-PMO-TMS) 1 and 2 show two new signals
2
Catalytic performance of Zn(OAc)
We next investigated the heterogeneous catalytic N-formylation
of N-methylaniline (5a) with CO (0.5 MPa) and PhSiH
1 mmol) at 60 1C over Zn(OAc) (BPy-PMO-TMS) 1–4 (Table 2).
Zn(OAc) (BPy-PMO-TMS) 2 immobilized in THF showed slightly
2
(BPy-PMO-TMS)
2
3
(
2
2
higher yield than Zn(OAc) (BPy-PMO-TMS) 1 immobilized in
2
MeOH (entries 1 and 2). It is noteworthy that catalytic performance
is strongly affected by the Zn loading on the solid support. Even in
the presence of the same Zn-based catalyst loading (2 mol% Zn),
ꢀ
1
2
Zn(OAc) (BPy-PMO-TMS) 3 (0.29 mmol Zn g ) showed higher
ꢀ1
catalytic activity than 2 (0.79 mmol Zn g ) (entries 2 and 3).
A similar trend was observed during N-formylations using 1 mol%
Table
1
Physicochemical
properties
of
BPy-PMO-TMS
and
Zn(OAc)
2
(BPy-PMO-TMS) 2–4
a,b
c
c
Zn amount
S
BET
d
DFT
ꢀ
1
2
ꢀ1
(mmol g
)
(m g
)
(nm)
BPy-PMO-TMS
—
877
630
804
856
866
4.4
4.6
4.1
4.3
4.4
1
2
3
4
0.69 (30/100)
0.79 (30/100)
0.29 (10/100)
0.15 (5/100)
a
b
Calculated from unimmobilized Zn amount. The number in parentheses
1
3
Fig. 1
C CP/MAS NMR spectra of (a) BPy-PMO-TMS, (b) Zn(OAc)
2
(BPy-
c
is an initial Zn/BPy molar ratio. Determined from nitrogen adsorption–
(BPy-PMO-TMS) 2 immo- desorption isotherms at liquid-nitrogen temperature and calculated using the
PMO-TMS) 1 immobilized in MeOH, (c) Zn(OAc)
bilized in THF, and (d) Zn(OAc)
2
0
2
(2,2 -bipyridine) complex.
BET method (SBET) or density functional theory (dDFT).
9
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Table
2
Zn(OAc)
2
(BPy-PMO-TMS)-catalyzed
N-formylations
of
N-methylaniline (5a) using CO
2
and phenylsilane
x
Yield of
6a (%)
Yield of
7a (%)
a
a
Entry
Cat.
(mol%)
1
2
3
4
5
6
7
8
9
1 (28 mg)
2 (25 mg)
3 (69 mg)
3 (35 mg)
4 (67 mg)
4 (34 mg)
2
2
2
1
1
0.5
—
1
2
39
43
94
86 (80)
93
78
6
Trace
97
1
1
2
2
2
2
1
b
BPy-PMO-TMS (69 mg)
Zn(OAc)
Zn(OAc)
2
Trace
2
0
2
(2,2 -bipyridine)
a
b
Determined by HPLC analysis. Isolated yield.
Scheme 2 Substrate scope of Zn(OAc)
2
(BPy-PMO-TMS)-catalyzed
ꢀ
1
ꢀ1
Zn: 3 (0.29 mmol Zn g ) and 4 (0.15 mmol Zn g ) (entries 4 and 5).
a
N-formylation of nitrogen nucleophiles 5. After the reaction of PhSiH
at 60 1C for 17 h, substrate was added and the mixture was stirred
3
Since it is well-known that zinc carboxylates generate oligomeric and CO
2
15a,16
b
at room temperature for 30 min. After the reaction of PhSiH
and CO at 60 1C for 17 h, substrate was added and the mixture was stirred
at 100 1C for 24 h.
3
(4 mmol)
and polymeric species through carboxylate bridges,
Zn(OAc) (BPy-PMO-TMS) with a higher Zn loading is expected to
contain a somewhat higher amount of inactive zinc species that are
not coordinated to BPy groups but form acetate-bridged oligomers.
2
2
The parent BPy-PMO-TMS also provided formamide 6a under the the standard conditions (12% yield). We assume that 5f coor-
above mentioned conditions, albeit in only 6% yield (entry 7). dinates to the zinc center to inhibit the production of silyl
In addition, a trace amount of 6a was detected when Zn(OAc)
2
was formate. Therefore, after the silyl formate from PhSiH
(2,2 -bipyridine) showed was generated at 60 1C for 17 h, 5f was added to the reaction
high catalytic activity comparable to that of Zn(OAc) (BPy-PMO- mixture; an 86% yield of 6f was obtained under these stepwise
3 2
and CO
0
used as the catalyst (entry 8), and Zn(OAc)
2
2
TMS) 3 and 4 (entry 9). These results support the notion that the conditions. Although benzamide (5g) was also able to be
0
Zn(OAc) (2,2 -bipyridine)-like complex on the solid support N-formylated, the yield of 6g was only 36% even at 100 1C due
2
observed in Fig. 1 is an active N-formylation catalyst or precatalyst. to its low nucleophilicity. Progress during the N-formylations of
2
While Beller and co-workers reported that Zn(OAc) is a good amides reveals that primary amines should be able to be double
17
catalyst for the reductions of amides with hydrosilanes, the N-formylated. Indeed, the double formylated product 8e was
formation of N,N-dimethylaniline (7a), which is formed by the obtained in 74% yield when aniline was treated with excess
reduction of N-methyl formamide 6a, was sufficiently suppressed PhSiH
3
(4 mmol) at 100 1C for 24 h. Benzylamine was two-fold
18
under the reaction conditions.
N-formylated to give 8f using the stepwise method with excess
While Zn(OAc) (BPy-PMO-TMS) 4 showed better catalytic PhSiH (4 mmol), albeit in a moderate yield of 33%.
2
3
performance per zinc than 3, the performance of 4 per unit
Although the detailed mechanism is still unclear, a plausible
weight was higher for 3. Therefore, Zn(OAc)
was applied for N-formylation of various nitrogen nucleophiles of the N-formylation using homogeneous Zn(OAc)
2
(BPy-PMO-TMS) 3 pathway is illustrated in Scheme 3, based on the previous study
/1,10-
In the presence of Zn(OAc)
reacts with phenylsilane to give the
(n = 1–3).
2
1
4
5
(Scheme 2). N-Methylaniline derivative 5b, which bears an phenanthroline catalyst.
2
electron-donating (MeO–) group on its benzene ring, reacted (BPy-PMO-TMS), CO
smoothly to give the corresponding formamide 6b in 86% yield. corresponding silyl formates, PhSiH3ꢀn(OC (Q O)H)
2
n
A lower (60%) yield of the corresponding formamide 6c Amine nucleophiles can attack the silyl formates even in the
was obtained when N-methylaniline derivative 5c, with an absence of the Zn catalyst to afford the desired N-formylated
electron-withdrawing (Cl–) group, was used; however, extend- products with the coproduction of phenylsilanols, PhSiH3
ꢀn
m
ing the reaction time to 24 h led to an improved yield of 80%. (OC (Q O)H)n–m(OH) (n = 1–3, m = 1–3, n Z m). In the reaction
While morpholine (5d) and aniline (5e) were also good N-for- of amide nucleophiles, which are less nucleophilic than
mylation substrates to give the corresponding formamides 6d amines, the Zn catalyst facilitates the N-formylation process
(88%) and 6e (70%), benzylamine (5f) reacted sluggishly under at higher temperature of 100 1C.
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Conclusions
We immobilized a zinc complex on a periodic mesoporous
organosilica to form Zn(OAc) (BPy-PMO-TMS), and used it as a
heterogeneous catalyst for the N-formylations of nitrogen
2
nucleophiles with CO
the reductant. Zn(OAc)
2
as the C-1 source and phenylsilane as
(BPy-PMO-TMS) with a lower Zn loading
2
exhibited higher catalytic performance for the N-formylation of
N-methylaniline. However, the co-production of phenylsilanol
derivatives during N-formylation led to occlusion of the solid-
support mesopores, which prevented catalyst reuse.
Conflicts of interest
Scheme 3 A plausible pathway to N-formylated products: (a) formation
2
of silyl formates from CO and phenylsilane and (b) N-formylation of
There are no conflicts to declare.
nitrogen nucleophiles with silyl formates.
Acknowledgements
Analyzing the recovered catalyst
The authors thank Yuta Hashiguchi for XRD analysis and Dr Yu
Shinke and Takahito Sakuraba for nitrogen adsorption/
desorption measurements.
We investigated the recovery and reuse of Zn(OAc) (BPy-PMO-
TMS) 3. After N-methylaniline had been N-formylated, the
immobilized catalyst was recovered by filtration, washed with
2
3
CH CN, dried in vacuo, and used in a second reaction; however,
a significantly lower product yield (79%) was observed. The
C CP/MAS NMR spectrum of the recovered catalyst shows a
Notes and references
1
3
remarkable decline in intensity of the peak at around 0 ppm
and new signals at around 140–120 ppm, indicative of
the partial cleavage of the TMS caps of BPy-PMO-TMS and
contamination by benzene derivatives (Fig. S7, ESI†). The
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9
appearance of a shoulder at around ꢀ70 ppm in the Si CP/
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(BPy-PMO-
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