OSU-6: A highly efficient, metal-free, heterogeneous catalyst for the click synthesis of 5-benzyl and 5-aryl-1H-tetrazoles

Article abstract of DOI:10.3390/molecules201219881

OSU-6, an MCM-41 type hexagonal mesoporous silica with mild Br?nsted acid properties, has been used as an efficient, metal-free, heterogeneous catalyst for the click synthesis of 5-benzyl and 5-aryl-1H-tetrazoles from nitriles in DMF at 90 °C. This catalyst offers advantages including ease of operation, milder conditions, high yields, and reusability. Studies are presented that demonstrate the robust nature of the catalyst under the optimized reaction conditions. OSU-6 promotes the 1,3-dipolar addition of azides to nitriles without significant degradation or clogging of the nanoporous structure. The catalyst can be reused up to five times without a significant reduction in yield, and it does not require treatment with acid between reactions.

Full text of DOI:10.3390/molecules201219881

Article
OSU-6: A Highly Efficient, Metal-Free,
Heterogeneous Catalyst for the Click Synthesis of
5-Benzyl and 5-Aryl-1H-tetrazoles
Baskar Nammalwar, Nagendra Prasad Muddala, Rajasekar Pitchimani and Richard A. Bunce *
Received: 18 November 2015; Accepted: 11 December 2015; Published: 19 December 2015
Academic Editor: Derek J. McPhee
Department of Chemistry, Oklahoma State University, Stillwater, OK 74078-3071, USA;
baskar@okstate.edu (B.N.); npiict@gmail.com (N.P.M.); raja2765@yahoo.com (R.P.)
*
Correspondence: rab@okstate.edu; Tel.: +1-405-744-5952; Fax: +1-405-744-6007
Abstract: OSU-6, an MCM-41 type hexagonal mesoporous silica with mild Brönsted acid properties,
has been used as an efficient, metal-free, heterogeneous catalyst for the click synthesis of 5-benzyl and
5-aryl-1H-tetrazoles from nitriles in DMF at 90 ˝C. This catalyst offers advantages including ease of
operation, milder conditions, high yields, and reusability. Studies are presented that demonstrate the
robust nature of the catalyst under the optimized reaction conditions. OSU-6 promotes the 1,3-dipolar
addition of azides to nitriles without significant degradation or clogging of the nanoporous structure.
The catalyst can be reused up to five times without a significant reduction in yield, and it does not
require treatment with acid between reactions.
Keywords: 5-benzyl and 5-aryl-1H-tetrazoles; carboxylic acid bioisosteres; click 1,3-dipolar addition;
heterogeneous catalysis; recyclable catalyst
1. Introduction
Tetrazoles are versatile heterocyclic systems, which have attracted considerable interest in diverse
applications ranging from pharmaceuticals [
1
–3
] and agrochemicals [
4
] to photographic compounds [
5],
explosives [ ], new materials [ ], and ligands in coordination compounds [10,
6
7
–
9
11]. In biological
studies, tetrazoles play a critical role as pharmacophores and also as metabolic surrogates for carboxylic
acids in various therapeutic agents to treat cancer, AIDS, bacterial infections, hypertension, convulsions,
and allergies [12,13]. Currently, the commercial antihypertensives Losartan and Valsartan [14] as well
as an experimental 2-arylcarbapenem antibiotic [15] all incorporate a tetrazole ring within their
structures. Tetrazoles have been synthesized primarily by the reaction of azides with nitriles in polar
aprotic media. This process has been promoted by various metal-based agents, including aluminum
chloride, aluminum bisulfate, cadmium chloride, copper-, zinc- and iron-based salts, copper delafossite
nanoparticles, palladium complexes, silver benzoate, metal-based triflates, tungstates, zinc-copper
alloys, and zinc sulfate nanospheres [16
CoY zeolites [18 19], SiO2–H2SO4 [20], Amberlyst-15 [21], and cuttlebone [22], as well as soluble
additives NH4OAc and NH4Cl [23 24] have also been used to facilitate this reaction. We therefore
,17]. Additionally, other heterogeneous catalysts, including
,
,
wish to report a new catalyst which performs this conversion under metal-free conditions, at moderate
temperatures, and with superior conversion rates, stability toward traces of water, and recyclability.
2. Results and Discussion
OSU-6, an MCM-41 type hexagonal mesoporous silica [25], has recently proven useful as a mildly
acidic, reusable catalyst for several transformations in our laboratory, affording products in excellent
yields with minimal purification requirements [26
–
28]. Encouraged by these results, we sought to
Molecules 2015, 20, 22757–22766; doi:10.3390/molecules201219881
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Molecules 2015, 20, 22757–22766
explore OSU-6 as a catalyst for the click synthesis of tetrazoles from azide and nitriles [29,30]. In order
to gauge the feasibility of this process, the reaction of benzyl cyanide (1a, 1 equiv.) with sodium
azide (1.2 equiv.) to generate 5-benzyl-1H-tetrazole (2a) was chosen as a model reaction. Various
solvents were evaluated using 15 wt % of the catalyst (relative to 1a) under varying temperature
conditions (Table 1). Our optimization study determined that the reaction performed in DMF solvent
˝
at 90 C using 15 wt % of OSU-6, afforded the highest yield (94%) of the tetrazole product. More
or less catalyst did not improve the yields, and lower temperatures led to inefficient conversions.
In general, polar aprotic media gave superior results since they solubilized both reacting partners
˝
at temperatures ě90 C. Polar protic and nonpolar media, on the other hand, led to unsatisfactory
outcomes. Solventless conditions afforded reasonable conversions for the model reaction, but were
not practical for solid nitriles, and the requisite higher temperatures led to greater impurity profiles.
Earlier syntheses [23,
31] generally utilized reaction temperatures of 120–150 ˝C for this reaction, and
thus, the conditions employed in this work are somewhat milder.
Table 1. Reaction optimization.
Entry
Solvent
OSU-6 (wt %)
Temperature (oC)
Time (h)
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
CH3CN
dioxane
THF
solventless
DMSO
DMF
15
15
15
15
15
15
50
25
15
15
15
10
0
90
90
95
12
24
18
18
6
6
6
6
6
4
6
6
12
trace
18
trace
10
76
84
88
87
90
94
70
120
140
120
120
120
90
75
90
120
DMF
DMF
DMF
DMF
DMF
DMF
10 a
11
12
13
10
73
trace
a
Optimized conditions.
Based on our preliminary findings, we now report our investigation of this catalyst for the
synthesis of 5-benzyl- an˝d 5-aryl-1H-tetrazoles using a click approach. The optimized conditions
(15 wt % OSU-6, DMF, 90 C, 4–12 h) proved general for promoting the conversion of benzyl and aryl
nitriles to tetrazoles. In addition to the parent systems, substrate derivatives bearing methyl, methoxy,
fluoro, chloro, nitro, and 3-butenyl moieties were evaluated, and the results are summarized in Table 2.
All of these groups survived the reaction conditions and gave high yields of products, regardless of
their electronic character or position on the ring, thus demonstrating the general applicability of OSU-6
in this transformation. Furthermore, in the conversion of 4-(3-butenyl)benzonitrile (1p) to tetrazole
2p, the reaction showed excellent chemoselectivity for the nitrile over the terminal alkene. Finally,
attempts to react aliphatic nitriles lacking aromatic substitution gave incomplete conversion to the
target tetrazoles and unacceptable impurity levels under our conditions.
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Table 2. Synthesis of tetrazoles.
Substrate
R
Product
Time (h)
Isolated Yield (%)
1a
1b
1c
1q
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
C6H5CH2
4-CH3C6H4CH2
4-CH3OC6H4CH2
4-ClC6H4CH2
4-FC6H4CH2
C6H5
4-CH3C6H4
2-CH3C6H4
4-CH3OC6H4
4-NO2C6H4
3-NO2C6H4
4-ClC6H4
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
4
5
4
5
5
8
8
6
6
12
12
8
8
12
4
94
87
92
90
86
89
90
94
92
84
80
91
87
94
87
95
4-FC6H4
(C6H5)2CH
4-CH3(CH2)6C6H4
4-CH2=CH(CH2)2C6H4
1p
4
The 1,3-dipolar addition of azide to nitriles has the potential to proceed via a concerted or
stepwise mechanism. In either event, OSU-6 would serve as a mild proton source to convert azide to
hydrazoic acid or to activate the nitrile function by protonation (Scheme 1). Hydrazoic acid has two
major resonance contributors, A and B. Of these, contributor B illustrates the concerted reaction best,
undergoing a smooth six-electron cyclization with nitrile 2a to form tetrazole 3a. It is also possible
that OSU-6 protonates the nitrile to some degree, which would activate this group toward attack
by azide in a stepwise process. Many earlier papers, both with [14,32,33] and without [12,19,23]
metal catalysts, have presented stepwise mechanisms for this transformation. In our experiments,
however, no intermediates were observed by thin layer chromatography during the course of the
reaction. Additionally, the minimal substitution in the reactants provided no stereochemical evidence
to illuminate the concerted or stepwise nature of the process.
Scheme 1. Plausible mechanisms for the reaction of azide with nitrile 2a in the presence of OSU-6 to
form tetrazole 3a.
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Molecules 2015, 20, 22757–22766
One of the most attractive features of OSU-6 in our previous studies [26–28] was its recyclability,
and thus, the catalyst was evaluated for this possibility. The acidity of this silicic material apparently
derives from aging in 2 M HCl for two weeks during its preparation [34]. In the current application,
OSU-6 was reused up to five times without significant loss of activity (Figure 1) and required only
minimal reconditioning after each cycle. This reconditioning involved filtering the catalyst, washing
˝
with 1:1 EtOH:H2O, and drying under high vacuum at 80 C for 2 h. Employing this protocol, OSU-6
retained its proton donor properties without the need for acid treatment between reactions.
Figure 1. Recyclability of OSU-6.
In order to understand the high catalytic activity of OSU-6, we monitored changes to the catalyst
surface over five cycles of the reaction using scanning electron microscopy (SEM). A series of SEM
images at ~26,000
after use. Comparison of the surface topography of fresh OSU-6 (
third ( ) and fifth ( ) reactions revealed that while some roughening occurred due to the loss of water,
ˆ
magnification (Figure 2) show 100
µ
m2 areas on the catalyst surface before and
) with material recovered after the
A
B
C
the exposed features of the catalyst remained largely unchanged. Overall, the catalyst morphology
showed only minor observable degradation after five iterations.
(A)
(B)
(C)
Figure 2. SEM images of OSU-6: (A) Fresh; (B) After three cycles; and (C) After five cycles.
The change in catalyst structure during multiple reactions was further investigated for one charge
of catalyst using a Brunauer-Emmett-Teller (BET) surface area determination. The results revealed that
fresh OSU-6 had a total surface area of 880 m2/g. Measurements after each reaction cycle showed a
gradual decrease in surface area, with a total loss of only 21.6% to 690 m2/g after five cycles (Figure 3).
Thus, the catalyst suffered only minimal damage and was able to retain its pore size and volume
throughout the five-reaction sequence. These observations verified the structu˝ral stability of OSU-6
toward conditions requiring extended exposure to a polar aprotic solvent at 90 C.
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Molecules 2015, 20, 22757–22766
Figure 3. BET measurements of the surface area of OSU-6.
In a further study of the behavior of OSU-6 over an extended reaction series, thermogravimetric
analysis (TGA) was used to assess the extent of clogging in the catalyst pores by reagents and products.
˝
The results revealed a weight loss of 2%–4% below 150 C, corresponding to the loss of adsorbed water.
Upon further heating, relatively little additional weight loss ( 6%) was noted from the reused OSU-6
ď
samples between 150–500 ˝C, indicating that the catalyst pores experienced no significant clogging
during the reaction (Figure 4). Lastly, though not shown in the Figure, OSU-6 did not breakdown until
˝
the temperature reached ca. 800 C, confirming the robust character of the material.
100
95
90
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
85
80
50
100
150
200
250
300
350
400
450
500
Temperature (οC)
Figure 4. Thermogravimetric analysis (TGA) of recovered OSU-6.
Finally, to more fully elucidate the role of OSU-6 in the reaction, two separate experiments were
performed on the model conversion of benzyl cyanide (1a, 1.0 mmol) and sodium azide (1.2 mmol) to
2a under the optimized conditions (15 wt % of OSU-6, DMF, 90 ˝C, 4 h). In the first run, the reaction
was heated in the presence of OSU-6 for a period of 2 h, after which the catalyst was removed by
filtration and the filtrate was heated for a second 2-h period. This procedure gave roughly 68% of
2a during the first 2 h, but only an additional 5% of product during the subsequent 2-h period. The
sequence was then reversed, heating the reaction in the absence of OSU-6 for the initial 2-h period,
followed by introduction of catalyst and heating for another 2 h. This procedure yielded only ~10%
of 2a after the first 2 h, but an additional 76% during the second 2-h period. These results clearly
established that OSU-6 was essential in promoting the click addition process to form tetrazoles.
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Molecules 2015, 20, 22757–22766
3. Experimental Section
3.1. General Information
The OSU-6 catalyst can be purchased from XploSafe, LLC (Product No. 9001, Stillwater, OK,
USA; www.xplosafe.com). All reactions were run under dry N2. Reactions were monitored by thin
layer chromatography (TLC) on silica gel GF plates (Analtech No. 21521, Newark, DE, USA). Column
chromatography, when necessary, was performed using silica gel (Davisilr grade 62, 60–200 mesh
)
mixed with UV-active phosphor (Sorbent Technologies, No. UV-05, Norcross, GA, USA); band
elution was monitored using a hand held UV lamp. The following instrumentation was used: Mp
determinations: Laboratory Devices Mel-temp apparatus (Cambridge, MA, USA); FT-IR spectra:
1
Varian Scimitar FTS 800 spectrophotometer (Randolph, MA, USA); H- (400 MHz) and 13C-NMR
(100 MHz) spectra: Bruker Avance 400 system (Billerica, MA, USA); HRMS: Thermo LTQ-Orbitrap
XL system (Waltham, MA, USA); BET surface areas: Quantachrome Autosorb 1 instrument (Boynton
Beach, FL, USA); SEM images: Hitachi S4800 system (Shaumburg, IL, USA); TGA measurements:
Mettler-Toledo TGA/DSC 1 instrument (Columbus, OH, USA).
3.2. Representative Procedure for the Preparation of 5-Benzyl and 5-Aryl-1H-tetrazoles
5-Benzyl-1H-tetrazole (2a). To a DMF solution of benzyl cyanide (1a, 100 mg, 0.85 mmol) and NaN3
(67 mg, 1.02 mm˝ol, 1.2 eq) was added OSU-6 (15 mg, 15 wt% relative to 1a). The reaction mixture
˝
was heated at 90 C (oil bath temperature 95–100 C) for 4 h at which time TLC indicated the reaction
was complete. The crude reaction mixture was filtered to remove the catalyst, and the filtrate was
added to water and extracted with EtOAc (3
(3 ˆ 15 mL) and saturated aq. NaCl (1 15 mL), dried (MgSO4), filtered, and concentrated under
vacuum to give 2a (129 mg, 94%) as a white solid, mp 121–122 ˝C (lit. [31] mp 123–124 ˝C). IR: 1603,
1549, 1532, 1494, 1457, 1073, 733, 695 cm´1; 1H-NMR (DMSO-d6):
16.1 (br s, 1H), 7.34 (t, J = 7.4 Hz,
2H), 7.27 (d, J = 7.4 Hz, 3H), 4.29 (s, 2H); 13C-NMR (DMSO-d6)
155.7, 136.4, 129.1, 128.8, 127.5, 29.4;
ˆ
15 mL). The combined extracts were washed with H2O
ˆ
δ
δ
HRMS (ESI): m/z Calcd for C8H8N4: 161.0827 [M + H]; Found: 161.0831. Other tetrazoles (below)
were prepared in the same fashion by heating for the times indicated in Table 2.
5-(4-Methylbenzyl)-1H-tetrazole (2b): Yield: 115 mg (87%) as a white solid, mp 151–152 ˝C (lit. [31] mp
153–154 ˝C); IR: 1635, 1540, 1496, 1448, 1382, 869 cm´1; 1H-NMR (DMSO-d6):
δ
16.1 (br s, 1H), 7.15
154.6, 135.5, 132.2, 128.6, 127.9, 27.9,
20.0; HRMS (ESI): m/z Calcd for C9H10N4: 175.0984 [M + H]; Found: 175.0982.
(apparent s, 4H), 4.23 (s, 2H), 2.27 (s, 3H); 13C-NMR (DMSO-d6)
δ
˝
5-(4-Methoxybenzyl)-1H-tetrazole (2c): Yield: 119 mg (92%) as an off-white solid, mp 160–162 C (lit. [35
]
1
mp 162–164 ˝C); IR: 2834, 1613, 1514, 1246, 836 cm´1; H-NMR (DMSO-d6):
δ
16.1 (br s, 1H), 7.19
157.7,
(d, J = 8.2 Hz, 2H), 6.90 (d, J = 8.2 Hz, 2H), 4.21 (s, 2H), 3.72 (s, 3H); 13C-NMR (DMSO-d6)
δ
155.0, 129.2, 127.1, 113.7, 54.5, 27.4; HRMS (ESI): m/z Calcd for C9H10N4O: 191.0933 [M + H]; Found:
191.0938.
˝
5-(4-Chlorobenzyl)-1H-tetrazole (2d): Yield: 115 mg (90%) as a white solid, mp 155–156 C; IR: 1641, 1586,
1536, 1493, 1409, 827, 764 cm´1; 1H-NMR (DMSO-d6):
(d, J = 8.1 Hz, 2H), 4.30 (s, 2H); 13C-NMR (DMSO-d6)
δ
16.1 (br s, 1H), 7.41 (d, J = 8.1 Hz, 2H), 7.31
154.5, 134.3, 131.1, 130.0, 128.0, 27.6; HRMS
δ
(ESI): m/z Calcd for C8H7N4Cl: 195.0438, 197.0408 (ca. 3:1) [M + H]; Found: 195.0442, 197.0411 (ca. 3:1).
Anal. Calcd for C8H7N4Cl: C, 49.37; H, 3.63; N, 28.79. Found: C, 49.46; H, 3.55; N, 28.89.
5-(4-Fluorobenzyl)-1H-tetrazole (2e): Yield: 113 mg (86%) as a light yellow solid, mp 154–155 ˝C; IR:
1601, 1582, 1508, 1413, 1223, 828, 767 cm´1; 1H-NMR (DMSO-d6):
δ
16.1 (br s, 1H). 7.33 (m, 2H), 7.18
(t, J = 8.7 Hz, 2H), 4.30 (s, 2H); 13C-NMR (DMSO-d6)
δ 160.7 (d, J = 241 Hz), 154.7, 131.5 (d, J = 4 Hz),
130.1 (d, J = 8.1 Hz), 114.9 (d, J = 22.2 Hz), 27.5; HRMS (ESI): m/z Calcd for C8H7N4F: 179.0733 [M + H];
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Molecules 2015, 20, 22757–22766
Found: 179.0737. Anal. Calcd for C8H7N4F: C, 53.93; H, 3.96; N, 31.45. Found: C, 54.02; H, 3.92;
N, 31.37.
˝
˝
5-Phenyl-1H-tetrazole (2f): Yield: 126 mg (89%) as an off-white solid, mp 215–216 C [lit. [36] mp 216 C
(dec)]; IR: 1608, 1563, 1485, 1465, 1409, 1162, 746, 703 cm´1; 1H-NMR (DMSO-d6):
δ
8.09–8.03 (m, 2H),
7.66–7.57 (m, 3H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
m/z Calcd for C7H6N4: 147.0671 [M + H]; Found: 147.0668.
δ 130.7, 128.8, 126.4, 123.5; HRMS (ESI):
5-(4-Methylphenyl)-1H-tetrazole (2g): Yield: 122 mg (90%) as a tan solid, mp 249–250 ˝C (lit. [31] mp
247.5–247.7 ˝C); IR: 1612, 1569, 1505, 1369, 822, 742 cm´1; 1H-NMR (DMSO-d6):
7.94 (d, J = 7.8 Hz,
2H), 7.42 (d, J = 7.8 Hz, 2H), 2.40 (s, 3H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
154.5, 140.6,
129.7, 126.3, 120.9, 20.5; HRMS (ESI): m/z Calcd for C8H8N4: 161.0827 [M + H]; Found: 161.0829.
δ
δ
˝
5-(2-Methylphenyl)-1H-tetrazole (2h): Yield: 128 mg (94%) as an off-white solid, mp 152–153 C (lit. [31
]
˝
mp 153.2–153.8 C); IR: 1608, 1564, 1465, 1387, 782, 744 cm´1; 1H-NMR (DMSO-d6):
δ
7.70 (d, J = 7.6 Hz,
1H), 7.54–7.36 (m, 3H), 2.51 (d, J = 2.0 Hz, 3H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
δ 154.7,
136.5, 130.7, 130.1, 128.8, 125.7, 123.3, 19.9; HRMS (ESI): m/z Calcd for C8H8N4: 161.0827 [M + H];
Found: 161.0831.
˝
5-(4-Methoxyphenyl)-1H-tetrazole (2i): Yield: 122 mg (92%) as a white solid, mp 230–232 C (lit. [35] mp
232–233 ˝C); IR: 2859, 1608, 1249, 828, 749 cm´1; 1H-NMR (DMSO-d6):
8.01 (d, J = 8.4 Hz, 2H), 7.18
(d, J = 8.4 Hz, 2H), 3.86 (s, 3H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
160.9, 154.1, 128.0, 115.7,
114.3, 54.9; HRMS (ESI): m/z Calcd for C8H8N4O: 177.0776 [M + H]; Found: 177.0773.
δ
δ
˝
5-(4-Nitrophenyl)-1H-tetrazole (2j): Yield: 109 mg (84%) as a yellow solid, mp 219–221 C (lit. [35
mp 218–220 ˝C); IR: 1603, 1551, 1512, 1337, 1318, 1293, 861, 728 cm´1; 1H-NMR (DMSO-d6):
8.46
(d, J= 8.4 Hz, 2H), 8.32 (d, J = 8.4 Hz, 2H), tetrazole H off-scale; 13C-NMR (DMSO-d6)
154.9, 148.2,
130.0, 127.6, 124.0; HRMS (ESI): m/z Calcd for C7H5N5O2: 192.0522 [M + H]; Found: 192.0525.
]
δ
δ
˝
5-(3-Nitrophenyl)-1H-tetrazole (2k): Yield: 103 mg (80%) as a tan solid, mp 153–155 C (lit. [31] mp
144.7–145.6 C); IR: 1625, 1525, 1348, 1250, 872, 823, 712 cm´1; 1H-NMR (DMSO-d6):
(d, J = 7.7 Hz, 1H), 8.44 (d, J = 8.3 Hz, 1H), 7.92 (t, J = 8.0 Hz, 1H), tetrazole NH off-scale; 13C-NMR
(DMSO-d6) 155.5, 148.7, 133.5, 131.7, 126.6, 126.1, 122.0; HRMS (ESI): m/z Calcd for C7H5N5O2:
δ 8.84 (s, 1H), 8.49
˝
δ
192.0522 [M + H]; Found: 192.0528.
5-(4-Chlorophenyl)-1H-tetrazole (2l): Yield: 119 mg (91%) as a yellow solid, mp 259–260 ˝C (lit. [33] mp
1
262 ˝C); IR: 1635, 1535, 1492, 1419, 886, 755 cm´1; H-NMR (DMSO-d6):
δ
8.07 (d, J = 8.2 Hz, 2H),
7.71 (d, J = 8.2 Hz, 2H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
δ 155.4, 136.4, 130.0, 129.2, 123.7;
HRMS (ESI): m/z Calcd for C7H5N4Cl: 181.0281, 183.0252 (ca. 3:1) [M + H]; Found: 181.0283, 183.0255
(ca. 3:1).
˝
5-(4-Fluorophenyl)-1H-tetrazole (2m): Yield: 117 mg (87%) as an off-white solid, mp 203–205 C; IR: 1606,
1
1499, 1444, 1241, 839, 749 cm´1; H-NMR (DMSO-d6):
δ
8.14–8.04 (m, 2H), 7.48 (t, J = 8.6 Hz, 2H),
δ 163.1 (d, J = 247. Hz), 154.2 (br), 128.9 (d, J = 9.1 Hz),
tetrazole NH off-scale; 13C-NMR (DMSO-d6)
120.3, 116.0 (d, J = 21.5 Hz); HRMS (ESI): m/z Calcd for C7H5N4F: 165.0577 [M + H]; Found: 165.0581.
Anal. Calcd for C7H5N4F: C, 51.22; H, 3.07; N, 34.13. Found: C, 51.36; H, 3.11; N, 34.29.
˝
5-(Diphenylmethyl)-1H-tetrazole (2n): Yield, 109 mg, (94%) as a white solid, mp 161–163 C (lit. [31] mp
˝
164.2–165.2 C); IR: 1564, 1494, 1452, 767, 718 cm´1; 1H-NMR (DMSO-d6):
δ 7.40–7.25 (m, 10H), 5.97 (s,
139.5, 128.1, 127.8, 126.6, 45.1; HRMS (ESI): m/z
1H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
δ
Calcd for C20H16N4: 313.1453 [M + H]; Found: 313.1461.
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Molecules 2015, 20, 22757–22766
5-(4-Heptylphenyl)-1H-tetrazole (2o): Yield: 105 mg (87%) as a white solid, mp 184–186 ˝C; IR: 1615,
1504, 1438, 1352, 844, 751, 722 cm´1; 1H-NMR (DMSO-d6):
δ
7.94 (d, J = 7.8 Hz, 2H), 7.43 (d, J = 7.9 Hz,
2H), 2.66 (t, J = 7.7 Hz, 2H), 1.60 (quintet, J = 7.0 Hz, 2H), 1.30 (m, 4H), 1.25 (m, 3H), 0.85 (t, J = 6.6 Hz,
3H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
145.4, 145.3, 128.7, 126.3, 120.9, 34.4, 30.6, 30.1, 28.0,
δ
27.9, 21.5, 13.3; HRMS (ESI): m/z Calcd for C14H20N4: 245.1766 [M + H]; Found: 245.1771. Anal. Calcd
for C14H20N4: C, 68.82; H, 8.25; N, 22.93. Found: C, 68.87; H, 8.19; N, 22.85.
5-[4-(3-Butenyl)phenyl]-1H-tetrazole (2p): Yield: 121 mg (95%) as a yellow solid, mp 189–191 ˝C; IR:
1641, 1618, 1509, 1478, 998, 918, 844, 743 cm´1; 1H-NMR (DMSO-d6):
δ
7.95 (d, J = 7.8 Hz, 2H), 7.45
(d, J = 7.9 Hz, 2H), 5.84 (ddt, J = 17.1, 10.2, 6.5 Hz, 1H), 5.05 (d, J = 17.1 Hz, 2H), 4.98 (d, J = 10.2 Hz,
1H), 2.77 (t, J = 7.6 Hz, 2H), 2.38 (q, J = 7.0 Hz, 2H), tetrazole NH off-scale; 13C-NMR (DMSO-d6)
δ
145.1, 137.7, 130.0, 129.4, 126.9, 121.7, 115.5, 34.5, 34.3; HRMS (ESI): m/z Calcd for C11H12N4: 201.1140
[M + H]; Found: 201.1137. Anal. Calcd for C11H12N4: C, 65.98; H, 6.04; N, 27.98. Found: C, 65.90; H,
6.11; N, 28.09.
4. Conclusions
In summary, we have successfully used OSU-6 as an efficient, metal-free, heterogeneous catalyst
for the high-yield click synthesis of 5-benzyl- and 5-aryl-1H-tetrazoles from nitriles in DMF at
90 ˝C. This MCM-41 type hexagonal mesoporous silica permits the synthesis of these targets using a
simple procedure, under mild conditions, and can be readily recycled. Studies are presented which
demonstrate the robust properties of the catalyst under the optimized reaction conditions. The catalyst
promotes the 1,3-dipolar addition without significant surface erosion or clogging of the nanoporous
structure. The catalyst can be reused up to five times without a significant reduction in yield, and it
does not require treatment with acid between reactions.
Supplementary Materials: Electronic Supplementary Information (ESI) available: Copies of the 1H and 13C-NMR
spectra for each tetrazole prepared, see http://www.mdpi.com/1420-3049/20/12/19881/s1.
Acknowledgments: The authors wish to thank XploSafe, LLC (Stillwater, OK, USA) for a generous gift of OSU-6.
The authors are also grateful to the Oklahoma State University College of Arts and Sciences for funds to purchase
a new 400 MHz NMR for the Oklahoma State-wide NMR facility. This facility was established with support from
NSF (BIR-9512269), the Oklahoma State Regents for Higher Education, the W. M. Keck Foundation, and Conoco,
Inc. (Houston, TX, USA).
Author Contributions: B.N. and N.P.M. performed the compound synthesis work, R.P. assisted with the
acquisition and interpretation of the SEM, BET and TGA data, and R.A.B. wrote the paper. All authors read and
approved the final version of the manuscript before submission.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons by Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
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