A Highly Efficient, Metal-Free, Heterogeneous Catalyst for the Click

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Dec 19, 2015 - temperatures, and with superior conversion rates, stability toward traces ... explore OSU-6 as a catalyst for the click synthesis of tetrazoles from ...
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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; [email protected] (B.N.); [email protected] (N.P.M.); [email protected] (R.P.) * Correspondence: [email protected]; 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 [6], new materials [7–9], and ligands in coordination compounds [10,11]. 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 [16,17]. Additionally, other heterogeneous catalysts, including CoY zeolites [18,19], SiO2 –H2 SO4 [20], Amberlyst-15 [21], and cuttlebone [22], as well as soluble additives NH4 OAc and NH4 Cl [23,24] have also been used to facilitate this reaction. We therefore 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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2. Results and Discussion OSU-6, an MCM-41 type hexagonal mesoporous silica [25], has recently proven useful as a

mildly acidic, several transformations in our laboratory, productsIn inorder explore OSU-6 as areusable catalystcatalyst for thefor click synthesis of tetrazoles from azide andaffording nitriles [29,30]. excellent with of minimal purification [26–28].cyanide Encouraged these results, to gauge the yields feasibility this process, the requirements reaction of benzyl (1a, by 1 equiv.) with we sodium to explore as a catalyst for the click synthesis tetrazoles from and nitriles [29,30]. azidesought (1.2 equiv.) to OSU-6 generate 5-benzyl-1H-tetrazole (2a)ofwas chosen as azide a model reaction. Various In order to gauge the feasibility of this process, the reaction of benzyl cyanide (1a, 1 equiv.) with sodium solvents were evaluated using 15 wt % of the catalyst (relative to 1a) under varying temperature azide (1.2 equiv.) to generate 5-benzyl-1H-tetrazole (2a) was chosen as a model reaction. Various conditions (Table 1). Our optimization study determined that the reaction performed in DMF solvent solvents were evaluated using 15 wt % of the catalyst (relative to 1a) under varying temperature at 90 ˝ C using 15 wt % of OSU-6, afforded the highest yield (94%) of the tetrazole product. More conditions (Table 1). Our optimization study determined that the reaction performed in DMF solvent or less catalyst did not%improve yields, lower led toproduct. inefficient conversions. at 90 °C using 15 wt of OSU-6,the afforded theand highest yieldtemperatures (94%) of the tetrazole More or less In general, media gaveand superior results sinceled they solubilized both reacting partners catalystpolar did notaprotic improve the yields, lower temperatures to inefficient conversions. In general, at temperatures ě90 ˝ C. Polar protic andsince nonpolar media, on thereacting other hand, led unsatisfactory polar aprotic media gave superior results they solubilized both partners at to temperatures ≥90 °C.Solventless Polar protic and nonpolar afforded media, on reasonable the other hand, led to unsatisfactory outcomes. Solventless outcomes. conditions conversions for the model reaction, but were conditions afforded reasonable conversions for the model reaction, but were not practical for solid not practical for solid nitriles, and the requisite higher temperatures led to greater impurity profiles. ˝ C for nitriles, and the requisite higher temperatures led to greater impurityof profiles. Earlier syntheses [23,31] and Earlier syntheses [23,31] generally utilized reaction temperatures 120–150 this reaction, generally utilized reaction temperatures of 120–150 °C for this reaction, and thus, the conditions thus, the conditions employed in this work are somewhat milder. employed in this work are somewhat milder.

Table 1. Reaction optimization. Table 1. Reaction optimization.

Entry Solvent Entry Solvent 1 EtOH 2 CH3EtOH CN CH3CN 3 2 dioxane dioxane 4 3 THF 5 4 solventless THF 6 5 DMSO solventless 7 6 DMF DMSO 8 7 DMF DMF 9 DMF 8 DMF 10 a DMF 9 DMF 11 DMF a 10 DMF 12 DMF 11 DMF 13 DMF 12 DMF 13 DMF

OSU-6 OSU-6 (wt %) Temperature (o C) Time (h) Isolated Yield (%) Temperature (oC) Time (h) Isolated Yield (%) (wt15%) 90 12 trace 15 9090 12 24 trace18 15 15 9095 24 18 18trace 15 15 9570 18 18 trace10 15 70120 18 6 10 76 15 140 15 120 6 6 76 84 50 120 15 140 6 6 84 88 25 120 6 50 120 6 88 87 15 120 6 90 25 120 6 87 15 90 4 94 15 120 6 90 15 75 6 10 15 9090 4 6 94 73 10 15 75 6 10trace 0 120 12 10 a 90 6 73 Optimized conditions. 0 120 12 trace a

Optimized conditions.

Based on our preliminary findings, we now report our investigation of this catalyst for the synthesisBased of 5-benzyland 5-aryl-1H-tetrazoles using click approach. The optimized conditions on our preliminary findings, we now report ourainvestigation of this catalyst for the synthesis (15 wt OSU-6, and DMF, 90 ˝ C, 4–12 h) proved for promoting the conversion of benzyl of% 5-benzyl5-aryl-1H-tetrazoles usinggeneral a click approach. The optimized conditions (15 wtand % aryl OSU-6, DMF, 90 °C, 4–12 h) proved promoting the conversion of benzyl andmethyl, aryl nitriles nitriles to tetrazoles. In addition to thegeneral parentfor systems, substrate derivatives bearing methoxy, to tetrazoles. In addition to the parent systems, substrate derivatives methoxy, fluoro, fluoro, chloro, nitro, and 3-butenyl moieties were evaluated, and thebearing resultsmethyl, are summarized in Table 2. chloro, nitro, and 3-butenyl moieties were evaluated, and the results are summarized in Table 2. All All of these groups survived the reaction conditions and gave high yields of products, regardless of of these groups survived the reaction conditions and gave high yields of products, regardless of their their electronic character or position on the ring, thus demonstrating the general applicability of OSU-6 electronic character or position on the ring, thus demonstrating the general applicability of OSU-6 in in this transformation. Furthermore, in the conversion of 4-(3-butenyl)benzonitrile (1p) to tetrazole this transformation. Furthermore, in the conversion of 4-(3-butenyl)benzonitrile (1p) to tetrazole 2p, 2p, the excellentchemoselectivity chemoselectivity nitrile over terminal alkene. Finally, thereaction reaction showed showed excellent forfor thethe nitrile over the the terminal alkene. Finally, attempts to react aliphatic nitriles substitutiongave gave incomplete conversion attempts to react aliphatic nitrileslacking lackingaromatic aromatic substitution incomplete conversion to theto the targettarget tetrazoles andand unacceptable impurity ourconditions. conditions. tetrazoles unacceptable impuritylevels levels under under our 2

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Table 2. Synthesis of tetrazoles. Table 2. Synthesis of tetrazoles. Table 2. Synthesis of tetrazoles.

Substrate R Product Time (h) Isolated Yield (%) 2a 1a 4 94 C6H5CH2 2b 1b 3C6H4CH2 5 87 4-CH Substrate R R Product Time (h)(h) IsolatedYield Yield Substrate Product Time Isolated (%)(%) 1c 2c 4-CH 3OC6H4CH2 4 92 2a 1a 1a 5 CH 2 4 94 C6H C6 H5 CH2 2a 4 94 1q 1b 2d2b 4-ClC 6H4CH2 90 4-CH C64H 8787 2b 1b 3C36H CH 2 2 55 5 4-CH 4 CH 1e 2e 4-FC 6 H 4 CH 2 5 86 4-CH 9292 3 OC 4 CH 1c 1c 2c2c 4-CH 3OC 6H64H CH 2 2 4 4 1q 4-ClC H CH 2d 5 90 2f 1f 6H 5 8 89 C 4 2 2 1q 2d 4-ClC6H64CH 5 90 1e 4-FC H CH 2e 5 86 6 4 2 1g 2g 4-CH 3C6H4 90 1e 2e 4-FC 6H4CH2 58 8 1f C6 H5 2f 8986 1h 2h 2-CH 3C6H4 6 94 2f2g 1f 1g 8 8 C6H53 C6 H4 4-CH 9089 1i 1h 2i2h 4-CH 3OCC 6H4 92 2-CH 9490 1g 2g 4-CH 3C36H64H4 86 6 1j 2j 4-NO 2C 6H46 H4 12 84 1i 4-CH OC 2i 6 92 3 1h 2h 2-CH3C6H4 6 94 1j 4-NO 2j 12 8480 64H4 1k 2k 3-NO 2C26C H 12 1i 1k 2i2k 4-CH 3OC6H4 6 12 92 3-NO2 C6 H4 80 1l 2l 4-ClC 6H4 8 91 1j 1l 2j2l 4-NO 2C6HH 4 12 8 4-ClC 9184 6 4 1m 2m 4-FC 6H4 8 87 4-FC 8780 1k 1m 2k2m 3-NO 2C66 HH44 12 8 1n 2n (C 6 H 5 ) 2 CH 12 94 (C6 H6H 9491 5 )24CH 1l 1n 2l2n 4-ClC 8 12 4-CH )66C 4 8787 1o 1o 2o2o 4-CH 3(CH 2)62C H64H4 3 (CH 1m 2m 4-FC 6H4 84 87 4-CH ) C64H4 2p 4 9595 1p 2 =CH(CH 1p 2p 4-CH 2=CH(CH 2)22C26H 4 1n 2n (C6H5)2CH 12 94 1o 2o 4-CH3(CH2)6C6H4 4 87 The1p 1,3-dipolar4-CH addition of azide to nitriles2phas the potential to proceed via 2=CH(CH2)2C6H4 4 95 a concerted or stepwise mechanism. In either event, OSU-6 would serve as a mild proton source to convert azide to protonation (Scheme hydrazoic acid or to activate byhas protonation (Scheme 1). Hydrazoic acid has two The 1,3-dipolar additionthe of nitrile azide function to nitriles the potential to proceed via a concerted or resonance these, the reaction best, major resonance contributors, A and B. Of these, contributor B illustrates concerted stepwise mechanism. In either event, OSU-6 would serve as a mild proton source to convert azide to undergoing a smooth smooth six-electron cyclization with 2a2a to to form tetrazole It isItalso possible that six-electron cyclization with nitrile form tetrazole 3a. is also possible hydrazoic acid or to activate the nitrile function bynitrile protonation (Scheme 1). 3a. Hydrazoic acid has two OSU-6 protonates the nitrile toAsome which would this group toward byattack azide that OSU-6 protonates the nitrile to degree, some degree, which activate would activate this groupattack toward major resonance contributors, and B. Of these, contributor B illustrates the concerted reaction best, in a stepwise process. Many earlier papers, both with [14,32,33] and without [12,19,23] metal catalysts, by azide in a stepwise process. Many earlier papers, both with [14,32,33] and without [12,19,23] undergoing a smooth six-electron cyclization with nitrile 2a to form tetrazole 3a. It is also possible that have presented stepwise mechanisms formechanisms this transformation. Inthis our experiments, however, no metal catalysts, presented stepwise foractivate this transformation. In our experiments, OSU-6 protonateshave the nitrile to some degree, which would group toward attack by azide intermediates were observed byobserved thin layer during the[12,19,23] coursethe of the catalysts, reaction. however, no process. intermediates were bychromatography thin chromatography during course of the in a stepwise Many earlier papers, both withlayer [14,32,33] and without metal Additionally, the minimal substitution in the reactants provided no stereochemical evidence to reaction. Additionally, the minimal substitution the reactants provided stereochemical evidence have presented stepwise mechanisms for thisintransformation. In our no experiments, however, no illuminate thethe concerted or stepwise nature of the to illuminate concerted or stepwise nature ofprocess. the process. 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. 1. Plausible Plausible mechanisms mechanisms for the reaction reaction of of azide azide with with nitrile nitrile 2a 2a in in the the presence presence of of OSU-6 OSU-6 to to Scheme for the form tetrazole tetrazole 3a. 3a. form 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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One One of of the the most mostattractive attractivefeatures featuresof ofOSU-6 OSU-6in inour ourprevious previousstudies studies[26–28] [26–28]was wasits itsrecyclability, recyclability, and and thus, thus, the the catalyst catalyst was was evaluated evaluated for for this this possibility. possibility. The The acidity acidity of of this this silicic silicic material material apparently apparently derives from aging in derives from2015, aging in 22 M M HCl HCl for for two two weeks weeks during during its its preparation preparation [34]. [34]. In In the the current current application, application, Molecules 20, page–page OSU-6 OSU-6 was was reused reused up up to to five five times times without without significant significant loss loss of of activity activity (Figure (Figure 1) 1) and and required required only only One of the mostafter attractive of OSU-6 in our previous studiesfiltering [26–28] was recyclability, minimal each cycle. reconditioning involved the catalyst, washing minimal reconditioning reconditioning after eachfeatures cycle. This This reconditioning involved filtering theits catalyst, washing ˝ Cacidity andEtOH:H thus, the2O, catalyst was evaluated for this possibility. The of Employing this silicic material apparently with under vacuum at °C for this OSU-6 with 1:1 and drying drying under high high vacuum at 80 80 for 22 h. h. Employing this protocol, protocol, OSU-6 2 O, and derives from aging in 2 M HCl for two weeks during its preparation [34]. In the current application, retained retained its its proton donor properties without the need for acid treatment between reactions. 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. Figure 1. Recyclability of OSU-6. Figure 1. activity Recyclability of OSU-6.we monitored changes to the catalyst In order to understand the high catalytic of OSU-6, In order to understand the high catalytic activity of OSU-6, monitored changes to the of catalyst surface over five cycles of the reaction using scanning electron we microscopy (SEM). A series SEM In order to understand the high catalytic activity of electron OSU-6, we monitored changes to the catalyst surface over five cycles of the reaction using scanning microscopy (SEM). A series of SEM images at ~26,000× magnification (Figure 2) show 100 µm2 areas on the catalyst surface before and 2 areas surface over fivemagnification cycles of the reaction using scanning electron microscopy (SEM). A series of SEM and images at ~26,000ˆ (Figure 2) show 100 µm on the catalyst surface before after use. Comparison of the surface topography of fresh OSU-6 (A) with material recovered after the 2 areas on the catalyst surface before and images at ~26,000×of magnification (Figure 2) show 100 µm after (B) use. Comparison the surface topography ofsome fresh OSU-6 (A) occurred with material recovered after the third and fifth (C) reactions revealed that while roughening due to the loss of water, after use. Comparison of the surface topography of fresh OSU-6 (A) with material recovered after the third (B) and fifth (C) reactions revealed that while some roughening occurred due to the loss of water, the exposed catalyst remained largely unchanged. the to catalyst third (B)features and fifthof (C)the reactions revealed that while some roughening Overall, occurred due the lossmorphology of water, the exposed features of the catalyst remained largely unchanged. Overall, the catalyst morphology showed minor observable degradation after five iterations. theonly exposed features of the catalyst remained largely unchanged. Overall, the catalyst morphology showed only minor observable degradation after five iterations. showed only minor observable degradation after five iterations.

(A) (A)

(B) (B)

(C)(C)

Figure 2. SEM images of OSU-6: (A) Fresh; (B)After After three cycles; and (C) After fivefive cycles. Figure Figure 2. 2. SEM SEMimages images of of OSU-6: OSU-6: (A) (A)Fresh; Fresh;(B) (B) After three three cycles; cycles; and and (C) (C) After After five cycles. cycles.

The change in catalyst structure during multiple reactions was further investigated for one

The change in catalyst structure during multiple reactions was further investigated for one charge of catalyst usingstructure a Brunauer-Emmett-Teller (BET) surface area determination. results The change in catalyst during multiple reactions was further investigatedThe for one charge charge of catalyst using a Brunauer-Emmett-Teller (BET) surface area determination. The results 2/g. Measurements after each reaction cycle revealed thatafresh OSU-6 had a total surface areasurface of 880 m of catalyst using Brunauer-Emmett-Teller (BET) area determination. The results revealed that revealed that afresh OSU-6 had aintotal surface area oftotal 880 loss m2/g. Measurements after reaction cycle 2/geach 2 showed gradual decrease surface area, with a of only 21.6% to 690 m after five cycles fresh OSU-6 had a total surface area of 880 m /g. Measurements after each reaction cycle showed a 2/g after five cycles showed a gradual decrease in surface area, with a total loss ofand only 21.6% to retain 690 mits (Figure 3). Thus, the catalyst only minimal damage to pore size and gradual decrease in surface area,suffered with a total loss of only 21.6% towas 690 able m2 /g after five cycles (Figure 3). (Figure 3). Thus, the catalyst suffered only minimal damage and was ablethe to structural retain its stability pore size volume throughout the five-reaction sequence. These observations verified of and Thus, the catalyst suffered only minimal damage and was able to retain its pore size and volume OSU-6 toward conditions requiringsequence. extended exposure to a polar aprotic solvent 90 °C. volume throughout the five-reaction These observations verified the at structural stability of throughout the five-reaction sequence. These observations verified the structural stability of OSU-6 OSU-6 toward conditions requiring extended exposure to a polar aprotic solvent at 90 °C. toward conditions requiring extended exposure to a polar aprotic solvent at 90 ˝ C. 4

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Figure 3. 3. BET BET measurements of the the surface surface area area of of OSU-6. OSU-6. Figure Figure 3. BET measurements measurements of of the surface

In a further further study study of of the the behavior behavior of of OSU-6 OSU-6 over over an an extended extended reaction reaction series, series, thermogravimetric thermogravimetric In In aa 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. products. analysis analysis (TGA) (TGA) was was used used to to assess assess the the extent extent of ofclogging cloggingin inthe thecatalyst catalystpores poresby byreagents reagentsand and products. The results revealed a weight loss of 2%–4% below 150 °C, corresponding to the loss of adsorbed The results revealed revealeda aweight weightloss loss 2%–4% below corresponding theofloss of adsorbed ˝ C, °C, The results of of 2%–4% below 150150 corresponding to thetoloss adsorbed water. water. Upon Upon further further heating, heating, relatively relatively little little additional additional weight weight loss loss (≤6%) (≤6%) was was noted noted from from the the reused water. Upon further heating, relatively little additional weight loss (ď6%) was noted from the reused reused OSU-6 OSU-6 samples between between 150–500 °C, indicating indicating that that the the catalyst catalyst pores pores experienced experienced no no significant OSU-6 °C, ˝ C, indicating samplessamples between 150–500150–500 that the catalyst pores experienced no significantsignificant clogging clogging during during the the reaction (Figure (Figure 4). 4). Lastly, Lastly, thoughnot not shown in in the the Figure, Figure, OSU-6 OSU-6 did did not not breakdown breakdown clogging during the reaction reaction (Figure 4). Lastly, thoughthough not shown shown in the Figure, OSU-6 did not breakdown until until the the temperature temperature reached reached ca. ca.˝ 800 800 °C, °C, confirming confirming the the robust robust character character of of the the material. material. until the temperature reached ca. 800 C, confirming the robust character of the material.

Weight Weight (%) (%)

100 100 95 95 90 90

Cycle 11 Cycle Cycle 22 Cycle Cycle 33 Cycle Cycle 44 Cycle Cycle 55 Cycle

85 85 80 80

50 50

100 100

150 150

200 200

250 250

300 300

350 350

400 400

450 450

500 500

Temperature ((οοC) C) Temperature Figure 4. Thermogravimetric Thermogravimetric analysis (TGA) (TGA) of recovered recovered OSU-6. Figure Figure 4. 4. Thermogravimetric analysis analysis (TGA) of of recovered OSU-6. OSU-6.

Finally, to to more more fully fully elucidate elucidate the the role role of of OSU-6 OSU-6 in in the the reaction, reaction, two two separate separate experiments experiments were were Finally, Finally, to more fully elucidate the role of OSU-6 in the reaction, two separate experiments were performed on on the the model model conversion conversion of of benzyl benzyl cyanide cyanide (1a, (1a, 1.0 1.0 mmol) mmol) and and sodium sodium azide azide (1.2 (1.2 mmol) mmol) performed performed model conversion benzyl cyanide (1a, 1.0 mmol) azide to to 2a 2a under underon thethe optimized conditionsof (15 wt % % of of OSU-6, OSU-6, DMF, 90 °C, °C, 44and h). sodium In the the first first run,(1.2 themmol) reaction to the optimized conditions (15 wt DMF, 90 h). In run, the reaction ˝ C, 4 h). In the first run, the reaction 2a under the optimized conditions (15 wt % of OSU-6, DMF, 90 was heated heated in in the the presence presence of of OSU-6 OSU-6 for for aa period period of of 22 h, h, after after which which the the catalyst catalyst was was removed removed by by was was heated in the presence of OSU-6 for a period of 2 h, after which the catalyst was removed by filtration and and the the filtrate filtrate was was heated heated for for aa second second 2-h 2-h period. period. This This procedure procedure gave gave roughly roughly 68% 68% of of 2a 2a filtration filtration and the2 filtrate was for a second period. This procedure gave 2-h roughly 68%The of during the the first h, but but only only heated an additional additional 5% of of2-h product during the subsequent subsequent period. during first 2 h, an 5% product during the 2-h period. The 2a during was the first h, but only an additional 5% of during the subsequent 2-h period. The sequence then2reversed, reversed, heating the reaction reaction inproduct the absence absence of OSU-6 OSU-6 for the the initial initial 2-h period, period, sequence was then heating the in the of for 2-h sequence was then reversed, heating the reaction in the absence of OSU-6 for the initial 2-h period, followed by by introduction introduction of of catalyst catalyst and and heating heating for for another another 22 h. h. This This procedure procedure yielded yielded only only ~10% ~10% followed followed bythe introduction of catalyst and heating for another 2 h. This procedure yielded onlyclearly ~10% of 2a after first 2 h, but an additional 76% during the second 2-h period. These results of 2a after the first 2 h, but an additional 76% during the second 2-h period. These results clearly of 2a after the 2 h,was butessential an additional 76% during the second period. Thesetetrazoles. results clearly established thatfirst OSU-6 in promoting promoting the click click addition2-h process to form form established that OSU-6 was essential in the addition process to tetrazoles. established that OSU-6 was essential in promoting the click addition process to form tetrazoles.

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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: Varian Scimitar FTS 800 spectrophotometer (Randolph, MA, USA); 1 H- (400 MHz) and 13 C-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 mmol, 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 ˆ 15 mL). The combined extracts were washed with H2 O (3 ˆ 15 mL) and saturated aq. NaCl (1 ˆ 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 ; 1 H-NMR (DMSO-d6 ): δ 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); 13 C-NMR (DMSO-d6 ) δ 155.7, 136.4, 129.1, 128.8, 127.5, 29.4; HRMS (ESI): m/z Calcd for C8 H8 N4 : 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 ; 1 H-NMR (DMSO-d6 ): δ 16.1 (br s, 1H), 7.15 (apparent s, 4H), 4.23 (s, 2H), 2.27 (s, 3H); 13 C-NMR (DMSO-d6 ) δ 154.6, 135.5, 132.2, 128.6, 127.9, 27.9, 20.0; HRMS (ESI): m/z Calcd for C9 H10 N4 : 175.0984 [M + H]; Found: 175.0982. 5-(4-Methoxybenzyl)-1H-tetrazole (2c): Yield: 119 mg (92%) as an off-white solid, mp 160–162 ˝ C (lit. [35] mp 162–164 ˝ C); IR: 2834, 1613, 1514, 1246, 836 cm´1 ; 1 H-NMR (DMSO-d6 ): δ 16.1 (br s, 1H), 7.19 (d, J = 8.2 Hz, 2H), 6.90 (d, J = 8.2 Hz, 2H), 4.21 (s, 2H), 3.72 (s, 3H); 13 C-NMR (DMSO-d6 ) δ 157.7, 155.0, 129.2, 127.1, 113.7, 54.5, 27.4; HRMS (ESI): m/z Calcd for C9 H10 N4 O: 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 ; 1 H-NMR (DMSO-d6 ): δ 16.1 (br s, 1H), 7.41 (d, J = 8.1 Hz, 2H), 7.31 (d, J = 8.1 Hz, 2H), 4.30 (s, 2H); 13 C-NMR (DMSO-d6 ) δ 154.5, 134.3, 131.1, 130.0, 128.0, 27.6; HRMS (ESI): m/z Calcd for C8 H7 N4 Cl: 195.0438, 197.0408 (ca. 3:1) [M + H]; Found: 195.0442, 197.0411 (ca. 3:1). Anal. Calcd for C8 H7 N4 Cl: 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 ; 1 H-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); 13 C-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 C8 H7 N4 F: 179.0733 [M + H];

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Found: 179.0737. Anal. Calcd for C8 H7 N4 F: 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 ; 1 H-NMR (DMSO-d6 ): δ 8.09–8.03 (m, 2H), 7.66–7.57 (m, 3H), tetrazole NH off-scale; 13 C-NMR (DMSO-d6 ) δ 130.7, 128.8, 126.4, 123.5; HRMS (ESI): m/z Calcd for C7 H6 N4 : 147.0671 [M + H]; Found: 147.0668. 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 ; 1 H-NMR (DMSO-d6 ): δ 7.94 (d, J = 7.8 Hz, 2H), 7.42 (d, J = 7.8 Hz, 2H), 2.40 (s, 3H), tetrazole NH off-scale; 13 C-NMR (DMSO-d6 ) δ 154.5, 140.6, 129.7, 126.3, 120.9, 20.5; HRMS (ESI): m/z Calcd for C8 H8 N4 : 161.0827 [M + H]; Found: 161.0829. 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 ; 1 H-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; 13 C-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 C8 H8 N4 : 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 ; 1 H-NMR (DMSO-d6 ): δ 8.01 (d, J = 8.4 Hz, 2H), 7.18 (d, J = 8.4 Hz, 2H), 3.86 (s, 3H), tetrazole NH off-scale; 13 C-NMR (DMSO-d6 ) δ 160.9, 154.1, 128.0, 115.7, 114.3, 54.9; HRMS (ESI): m/z Calcd for C8 H8 N4 O: 177.0776 [M + H]; Found: 177.0773. 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 ; 1 H-NMR (DMSO-d6 ): δ 8.46 (d, J= 8.4 Hz, 2H), 8.32 (d, J = 8.4 Hz, 2H), tetrazole H off-scale; 13 C-NMR (DMSO-d6 ) δ 154.9, 148.2, 130.0, 127.6, 124.0; HRMS (ESI): m/z Calcd for C7 H5 N5 O2 : 192.0522 [M + H]; Found: 192.0525. 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 ; 1 H-NMR (DMSO-d6 ): δ 8.84 (s, 1H), 8.49 (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; 13 C-NMR (DMSO-d6 ) δ 155.5, 148.7, 133.5, 131.7, 126.6, 126.1, 122.0; HRMS (ESI): m/z Calcd for C7 H5 N5 O2 : 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 262 ˝ C); IR: 1635, 1535, 1492, 1419, 886, 755 cm´1 ; 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; 13 C-NMR (DMSO-d6 ) δ 155.4, 136.4, 130.0, 129.2, 123.7; HRMS (ESI): m/z Calcd for C7 H5 N4 Cl: 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, 1499, 1444, 1241, 839, 749 cm´1 ; 1 H-NMR (DMSO-d6 ): δ 8.14–8.04 (m, 2H), 7.48 (t, J = 8.6 Hz, 2H), tetrazole NH off-scale; 13 C-NMR (DMSO-d6 ) δ 163.1 (d, J = 247. Hz), 154.2 (br), 128.9 (d, J = 9.1 Hz), 120.3, 116.0 (d, J = 21.5 Hz); HRMS (ESI): m/z Calcd for C7 H5 N4 F: 165.0577 [M + H]; Found: 165.0581. Anal. Calcd for C7 H5 N4 F: 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 ; 1 H-NMR (DMSO-d6 ): δ 7.40–7.25 (m, 10H), 5.97 (s, 1H), tetrazole NH off-scale; 13 C-NMR (DMSO-d6 ) δ 139.5, 128.1, 127.8, 126.6, 45.1; HRMS (ESI): m/z Calcd for C20 H16 N4 : 313.1453 [M + H]; Found: 313.1461.

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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 ; 1 H-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; 13 C-NMR (DMSO-d6 ) δ 145.4, 145.3, 128.7, 126.3, 120.9, 34.4, 30.6, 30.1, 28.0, 27.9, 21.5, 13.3; HRMS (ESI): m/z Calcd for C14 H20 N4 : 245.1766 [M + H]; Found: 245.1771. Anal. Calcd for C14 H20 N4 : 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 ; 1 H-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; 13 C-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 C11 H12 N4 : 201.1140 [M + H]; Found: 201.1137. Anal. Calcd for C11 H12 N4 : 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 1 H and 13 C-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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Sample Availability: Samples of the compounds are not available from authors (or from MDPI). © 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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