A reliable method to create adjacent acid-base pair sites on silica through hydrolysis of pre-anchored amide

Article abstract of DOI:10.1246/cl.190773

A method to create adjacent acid-base pair sites, which are carboxyl and amino groups, respectively, on silica through hydrolysis of pre-anchored amide is proposed. This method can produce an adjacent acid-base pair site. The catalyst showed excellent catalytic performance for aldol condensation of 4-nitrobenzaldehyde with acetone, overwhelming the catalyst having only amino group and an acid-base catalyst prepared in a conventional manner.

Full text of DOI:10.1246/cl.190773

Advance Publication Cover Page
A reliable method to create adjacent acid-base pair sites on silica
through hydrolysis of pre-anchored amide
Wontae Kim, Loida O. Casalme, Taiki Umezawa, Fuyuhiko Matsuda,
Ryoichi Otomo, and Yuichi Kamiya*
Advance Publication on the web November 16, 2019
doi:10.1246/cl.190773
© 2019 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
manuscripts and a portion of the graphical abstract can be released on the web as soon as the submission is accepted. Note that the
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Publication manuscripts.

1
A reliable method to create adjacent acid-base pair sites on silica
through hydrolysis of pre-anchored amide
Wontae Kim,1 Loida O. Casalme,1 Taiki Umezawa,2 Fuyuhiko Matsuda,2 Ryoichi Otomo,2 Yuichi Kamiya*2
1Graduate School of Environmental Science, Hokkaido University, Kita 10, Nishi 5, Kita-ku, Sapporo 060-0810, Japan
2Faculty of Environmental Earth Science, Hokkaido University, Kita 10, Nishi 5, Kita-ku, Sapporo 060-0810, Japan
E-mail: kamiya@ees.hokudai.ac.jp
50 reactions.12–15 While silanol group on SiO2 is weak Brønsted
51 acid site, the acid-base pair sites comprising strong one is
52 created by using silica alumina16 and acidic
53 montmorillonite17 instead of SiO2. In addition to the
1
2
3
4
5
6
7
8
A method to create adjacent acid-base pair sites, which
are carboxyl and amino groups, respectively, on silica
through hydrolysis of pre-anchored amide is proposed. This
method can produce an adjacent acid-base pair sites. The
catalyst showed excellent catalytic performance for aldol
condensation of 4-nitrobenzaldehyde with acetone,
overwhelming the catalyst having only amino group and an
acid-base catalyst prepared in a conventional manner.
54 immobilization
approach,
co-condensation
of
55 aminoalkylsilane with tetraethyl orthosilicate has been
56 extensively studied to synthesize the siliceous solids having
57 the acid-base pair sites on the surface.18
9
Keywords: Acid-base pair sites, Cooperative catalysis,
58
As alternative approaches, acid-base pair sites on the
10 Aldol condensation
59 surface can be created by the immobilizations of two
60 different organosilanes on metal oxides or the co-
61 condensation of the two.19–26 This has a great advantage for
62 installation of a variety of acid and base sites by simply
63 changing acid and base attached to organosilanes.21,23,24,26–28
64 However, the acidic and basic groups in silanes should be
65 protected or silanes should have functional groups that are
66 transformable to acidic and basic groups but not interactive
67 each other to avoid neutralization during the
68 synthesis,19,21,22,27,28 requiring burdensome experimental
69 processes to obtain the acidic and basic sites for the
70 synthesis. In addition, this approach gives the randomly
71 located acid and base sites, since no strong interaction
72 works between the two reagents. Therefore, in this approach,
73 it is hard to create a single acid-base pair sites with
74 prescribed distance that exert the synergetic effect on
75 catalysis.
11
Because acid and base in a solution unavoidably
12 neutralize, they lose the acid and base functions and
13 generally do not work as an efficient acid or base catalyst in
14 the solution. On the other hand, some molecules having both
15 acidic and basic groups enhance the reaction rates in the
16 solution.1–4 Proline is the representative molecule and
17 efficiently promotes various carbon-carbon bond formation
18 reactions.5–7 It is pointed out that acidic and basic groups in
19 such molecules cooperatively work for the reactions. The
20 distance between acidic and basic groups in the molecule is
21 important to determine the cooperative action as a catalyst.
22 In fact, synergetic effect arising from the acidic and basic
23 groups does not appear if they are placed away from each
24 other in the molecule.4 In other words, the molecule with the
25 adjacent acidic and basic groups, namely acid-base pair,
26 often show higher catalytic performance than distantly
27 positioned analogs or those having either of the two.
76
77
Step 1. Amide synthesis
28
In heterogeneous catalysts, some metal oxides
78
79
WSCD ▪ HCl in DMF
+
29 including Al2O3, ZnO, Al2O3-ZnO, and ZrO2-ZnO have both
30 acid and base sites on the surface and they cooperatively
31 promote the reactions.8–11 However, the distance between
32 the acid and base sites is widely distributed even for highly
33 crystalline metal oxides, since the surface of the metal
34 oxides is inhomogeneous at atomic level. In addition, the
35 structure of the surface of the metal oxides is different
36 depending on the crystal faces and correspondingly the non-
37 uniform distance between the acid and base sites is
38 unavoidable, while the distance is of critical importance to
39 exert the synergetic effect.
DIEA, HOBt
80
81
Step 2. Hydrosilylation
82
(DCPD)PtCl2
24 h, 85 o
HSi(OC2H5)3
83
84
85
86
87
88
89
90
91
92
93
94
95
96
C
Step 3. Immobilization of amide
1
SiO2 in toluene
reflux, 24 h
Amide / SiO2
40
Immobilization of organosilane with a basic group on
6 M HCl
41 metal oxides was proposed to control the distance between
42 acid and base sites as well as the density of them.12–15
43 Immobilization of (3-aminopropyl)triethoxysilane on SiO2
44 is a typical example to create the acid-base pair sites by this
45 approach, because silanol group on SiO2 acts as an acid
46 site.12,13 The acid and base sites formed by this approach
47 make the acid-base pair sites, working cooperatively, and
48 thus effectively promotes the carbon-carbon bond formation
49 reactions including aldol and Knoevenagel condensation
110 oC, 24h
Step 4. Hydrolysis
Acid-Base / SiO2
97 Figure 1. Synthetic procedure for adjacent acid-base pair
98 sites on SiO2 by use of hydrolysis of pre-anchored amide.
99

2
1
2
3
4
5
6
7
8
9
In the present study, we wish to propose a reliable
method to definitely create adjacent acid-base pair sites on
the surface through hydrolysis of secondary amide
immobilized on SiO2. In the method, the amide with
trialkoxysilyl groups at both ends is immobilized on SiO2
and subsequently is hydrolyzed to create the adjacent
carboxyl and amino groups, namely the acid-base pair sites
(Figure 1). The method has a great advantage to create only
adjacent acid-base pair sites in principle. We furthermore
60
Finally, amide bond in Amide/SiO2 was hydrolyzed in
61 hydrochloric acid (6 mol L–1) at reflux temperature for 12 h
62 to form carboxyl and amino groups (Step 4 in Figure 1). The
63 obtained material is denoted as Acid-Base/SiO2. The amount
64 of Cl remaining in Acid-Base/SiO2 was below detection
65 limit for elemental analysis. In the IR spectrum of Acid-
66 Base/SiO2, the absorption bands due to amide group were
67 absent and instead, two new ones assignable to N-H
68 deformation vibration of amino group and C=O stretching
69 vibration of carboxyl one arose at 1592 and 1730 cm–1,
70 respectively (Figure 2(c) and 2(f)), which were identical to
71 those observed for the samples in which each of 5-
10 demonstrate that the prepared catalyst exhibits excellent
11 catalytic performance for aldol condensation.
12
5-(triethoxysilyl)pentanoic
acid
3-
13 (triethoxysilyl)propylamide (1) was synthesized from 3-
14 amino-1-propene and 4-pentenoic acid by condensation,
15 followed by hydrosilylation with triethoxysilane (Steps 1
16 and 2 in Figure 1). Structure and purity of 1 were confirmed
17 by 1H and 13C solution NMR (Figure S1). Immobilization of
18 1 on SiO2 (Nippon Aerosil Co., Ltd., AEROSIL® 300, 300
19 m2 g–1) was performed in toluene at reflux temperature for
20 24 h (Step 3 in Figure 1). To prevent multi-layer deposition
21 of 1 on SiO2, the density of 1 on SiO2 was adjusted to 0.4
22 nm–2 with consideration for the molecular size of 1. The
23 obtained material is denoted as Amide/SiO2. The IR
24 spectrum of Amide/SiO2 showed absorption bands
25 assignable to amide at 1524 and 1652 cm–1 and methylene
26 groups at 2800 – 2900 cm–1 (Figure 2(b) and 2(d)). The
27 molar ratio of carbon to nitrogen (=C/N) determined by
28 CHN elemental analysis for Amide/SiO2 was 8.14 (Table
29 S1), indicating that almost all ethoxy groups in 1 were
30 eliminated by the reaction with silanol groups on SiO2 to
31 form Si-O-Si bonds. In fact, the decrease of silanol group on
32 SiO2 was confirmed as a sharp and intense negative band at
33 3740 cm–1 in the difference IR spectrum before and after the
34 immobilization (Figure 2(d)). 13C{1H} CP/MAS NMR
35 spectrum of Amide/SiO2 was almost identical to 13C
36 solution NMR spectrum of 1 except for weakened peaks due
37 to ethoxy group (Figure S2). These results demonstrated that
38 1 was successfully immobilized on SiO2 as illustrated in
39 Figure 1.
72 (triethoxysilyl)pentanoic
acid
and
(3-
73 aminopropyl)triethoxysilane was fixed on SiO2 (Figure S3).
74 The cleavage of amide bond was also confirmed by the peak
75 shift of carbonyl group from176.6 to 182.3 ppm in 13C{1H}
76 CP/MAS NMR spectra (Figure S2). There was no
77 substantial shift in the other peaks in 13C{1H} CP/MAS
78 NMR spectra with the hydrolysis. In addition, the contents
79 of carbon and nitrogen as well as the C/N ratio for Acid-
80 Base/SiO2 were almost the same as those for Amide/SiO2
81 (Table S1). Therefore, it was concluded that Acid-Base/SiO2
82 was successfully synthesized as Figure 1 shows.
83
Next, we evaluated the catalytic performance of Acid-
84 Base/SiO2 for aldol condensation of 4-nitrobenzaldehyde
85 with acetone and compared to the catalysts with only
86 carboxyl (Acid/SiO2) or amino group (Base/SiO2).
87 Acid/SiO2 and Base/SiO2 were prepared by the
88 modifications of SiO2 with 5-(triethoxysilyl)pentanoic acid
89 and (3-aminopropyl)triethoxysilane, respectively, in
90 similar manner to that for Amide/SiO2 (Details are shown in
91 Supporting Information).
a
92
Base/SiO2 showed catalytic activity for the aldol
93 condensation reaction and the conversion at 24 h was 34%
94 (Entry 3 in Table 1), while pristine SiO2 (Entry 1) and
95 Acid/SiO2 (Entry 2) were completely inactive, indicating
96 that the basic sites catalyzed the reaction as is reported.29 In
97 the experiment, we used Base/SiO2 with the amino group
98 density of 0.8 nm–2, because it showed the highest catalytic
99 activity among them with the various densities ranging from
100 0.1–2 nm–2. To confirm the synergetic effect by silanol
101 group on SiO2 as an acid site, we carried out the reaction
40
41
1.0
42
43
44
45
46
47
48
49
50
51
52
53
54
55
1730
1592
1592
f)
1730
102 over
Base/SiO2
treated
with
1,1,1,3,3,3-
1524
1592
103 hexamethyldisilazane to cap remaining silanol group with
104 trimethylsilyl group (Base/SiO2(TMS)) and the conversion
105 was only 9% (Entry 4), indicating that synergetic effect
106 between the amino group and silanol on SiO2 was present
107 for Base/SiO2. Amide/SiO2 did not show any activity (Entry
108 5). It should be noted that Acid-Base/SiO2 efficiently
109 promoted the reaction and the conversion for Acid-
110 Base/SiO2 was much higher than that for Base/SiO2 despite
111 no difference in the number of the basic sites (Entry 6). We
112 further confirmed that Acid-Base/SiO2 with the density of
113 0.4 nm–2, which was used here, was the most active catalyst
114 among them with various acid-base pair site densities.
c)
1652
1730
1652
1524
e)
d)
2800-2900
b)
a)
1652
1524
3740
3740
4000 3500 3000 2500 2000 1500
Wavenumber / cm-1
4000 3500 3000 2500 2000 1500
Wavenumber / cm-1
56 Figure 2. IR spectra for pristine (a) SiO2, (b) Amide/SiO2, 115 Equimolar physical mixture of Acid/SiO2 and Base/SiO2
57 (c) Acid-Base/SiO2, and difference spectra obtained by (d) 116 gave only low conversion (Entry 7). Therefore, we conclude
58 b–a, (e) c–a, and (f) c–b.
59
117 that the adjacency of carboxyl group to amino one created

3
1
2
3
4
5
by hydrolysis of the amide resulted in the excellent catalytic
activity of Acid-Base/SiO2 for the reaction.
50 acid on SiO2, followed by deprotection of Boc to give the
51 catalyst that had both acid and base sites (Details are shown
52 in Supporting Information). Acid-Base(CV)/SiO2 was less
53 active than Acid-Base/SiO2 (Entry 8), while the number of
54 the base sites for Acid-Base(CV)/SiO2 determined by
55 titration (0.16 mmol g–1) was almost the same as that for
56 Acid-Base/SiO2 (0.14 mmol g–1). We presumed that the low
57 activity of Acid-Base(CV)/SiO2 was due to the presence of
58 the less active amino group that was not adjacent to
59 carboxylic acid. In other word, our proposed method can
60 create efficiently the adjacent acid-base pair sites showing
61 the high catalytic activity. Jones and co-workers reported
62 that the modification of Base/SiO2 with methacrylic acid,
63 which was formed by deprotection of tert-butylmethacrylate
64 fixed in advance, to make acid-base pair sites lower the
65 catalytic activity for the aldol condensation22 unlike Acid-
66 Base(CV)/SiO2. The carboxyl group in Acid-Base(CV)/SiO2
67 is more flexible than that in their catalyst because of the
68 longer (C4) methylene chain, which probably makes the
69 distance of acid-base pair sites appropriate for the activation
70 of the reactants. It is interesting that the adjacent acid-base
71 pair sites on Acid-Base/SiO2 was more active than the
72 corresponding propylamine (Entry 9) that acted as a
73 homogenous catalyst and a mixture of propylamine and
74 pentanoic acid (Entry 10).
Table 1. Comparison of catalytic activities for aldol
condensation of 4-nitrobenzaldehyde with acetone.
Entry
Catalyst
Conv.a /%
1
2
3
SiO2
Acid/SiO2
Base/SiO2
0
0
34
4
Base/SiO2(TMS)
9
5
6
Amide/SiO2
Acid-Base/SiO2
0
66 (61, 63)b
7
8
9
10
Acid/SiO2 + Base/SiO2
Acid-Base(CV)/SiO2
Propylaminec
Propylamine + Pentanoic acidd
8
42
22
12
6
7
8
9
Reaction conditions: 4-nitrobenzaldehyde, 0.5 mmol; acetone,
1.5 mmol; catalyst, 0.025 mmol; toluene (solvent), 10 mL;
temperature, 50°C; time, 24 h. The amount of the catalyst was
defined according to the number of base sites estimated by
10 titration. In entries 1 and 2, 0.05 g of the catalysts were used. In
a
11 entry 5, 0.10 g of the catalyst was used. Conversion of 4-
12 nitrobenzaldehyde. bThe data in parenthesis are the conversions
13 for 1st and 2nd reuses. cHomogeneous catalyst. dMixture.
14
15
At 10 h for the reaction over Acid-Base/SiO2, the
16 catalyst was removed by filtration and the obtained solution
17 was further heated again at 50°C. No significant increase in
18 the conversion was observed (Figure 3), indicating that the
19 reaction proceeded over Acid-Base/SiO2. In addition, Acid-
20 Base/SiO2 was reusable for the reaction. After the reaction
21 for 24 h, the catalyst was collected by a centrifugation and
22 washed with toluene and ethanol, followed by drying at
23 60°C overnight. Then, the catalyst was afforded to the
24 second reaction under the same reaction conditions. It was
25 found that Acid-Base/SiO2 was reusable without any
26 significant loss of activity for at least 2 times reuse (Entry 6
27 in Table 1).
75
In the aldol condensation catalyzed by primary amine,
76 it is proposed based on the spectroscopic analyses with IR,
77 Raman and 13C CP/MAS NMR that the reaction proceeds
78 through the formation of enamine intermediate between
79 ketone and amine,30,31 while imine formation inhibits the
80 reaction.31 In this process, it is assumed that enough
81 nucleophilicity of amine is required for the addition to
82 carbonyl group. Thus, higher the nucleophilicity of amine
83 on the catalyst is, more the reaction is enhanced. To get
84 information on the nucleophilicity, we measured N 1s XPS
85 spectra for Base/SiO2 and Acid-Base/SiO2 (Figure S4). The
86 both catalysts gave the peaks with almost the same binding
87 energy, indicating similar nucleophilicity between Acid-
88 Base/SiO2 and Base/SiO2 having no carboxyl group for
28
29
80
30
89 neutralization of amino group.
60
31
90
91
92
93
94
95
96
97
98
99
100
101
102
103
32
40
33
34
Removal of the catalyst
20
35
36
37
38
0
0
10
20
Reaction time/h
39 Figure 3. Hot-filtration experiment for the aldol
40 condensation over Acid-Base/SiO2. (●) without removal of
41 the catalyst and (○) the catalyst was removed at 10 h.
42
43
To highlight the superiority of our proposed method,
44 we further compared Acid-Base/SiO2 with another acid-base
45 SiO2 catalyst (Acid-Base(CV)/SiO2) that was prepared by a
46 conventional manner25,26 and a homogeneous base catalyst.
47 Acid-Base(CV)/SiO2 was prepared by immobilizing tert-
104
105
106
48 butoxycarbonyl
(Boc)-protected
3-
107 Figure 4. Plausible mechanism for the acceleration of aldol
108 condensation over the adjacent acid-base pair sites on SiO2.
49 (triethoxysilyl)propylamine and 5-(triethoxysilyl)pentanoic

4
58 13
59
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1
2
3
4
5
6
7
8
9
The other possibility is that the acid and base sites on
Acid-Base/SiO2 cooperatively activate the reactants (Figure
4).30,31 In acid-base cooperative mechanism for the aldol
condensation, an enamine derived from acetone and amino
group in Acid-Base/SiO2 is first generated by the assistance
of carboxyl group. Subsequently, nucleophilic addition of
the enamine to the aldehyde smoothly occurs through
activation of the aldehyde by the carboxyl group to form the
60 14
61
62 15
63 16
64
65 17
66
67 18
68
10 carbon-carbon bond. Under these processes, the higher
11 acidity of the carboxyl group than the silanol and the
12 appropriate adjacency between the carboxyl and amino
13 groups are thought to be important for the potent catalytic
14 activity of Acid-Base/SiO2. We are now investigating
15 kinetic and spectroscopic studies to reveal how the acid-
16 base pair sites on Acid-Base/SiO2 catalyze the reaction and
17 thus will report them in the near future.
19
69 20
70
71 21
72
73 22
74
23
18
In conclusion, we have shown the excellent catalytic
24
25
26
19 performance of the present catalyst having adjacent acid-
20 base pair sites for the aldol condensation of 4-
21 nitrobenzaldehyde with acetone. The acid-base pair was
22 closely arranged by hydrolysis of amide immobilized on
23 SiO2, exhibiting much higher catalytic activity than other
24 catalysts examined such as only acid or base sites on SiO2.
25 This method enabled close proximity between acid and base
26 and further applicable to create acid-base pair sites with
27 precisely controlled distance by hydrolysis of amide
28 including linker moiety. (Figure S5)
27
28
29
30
31
29
30
This work is supported by the JSPS KAKENHI Grant-
75
31 in-Aid for Challenging Exploratory Research (Grant
32 Number JP19K22074).
33
34
Supporting
Information
is
available
on
35 https://doi.org/????????.
36
37
38
39
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