Heteroatom Chemistry

Heteroatom Chemistry / 2019 / Article

Research Article | Open Access

Volume 2019 |Article ID 1459681 | https://doi.org/10.1155/2019/1459681

Rodrigo Abonia, Luisa F. Gutierrez, Angela T. Zwarycz, Sebastián Correa Smits, Kenneth K. Laali, "An Efficient Selectfluor-Mediated Oxidative Thio- and Selenocyanation of Diversely Substituted Indoles and Carbazoles", Heteroatom Chemistry, vol. 2019, Article ID 1459681, 10 pages, 2019. https://doi.org/10.1155/2019/1459681

An Efficient Selectfluor-Mediated Oxidative Thio- and Selenocyanation of Diversely Substituted Indoles and Carbazoles

Academic Editor: Gianluigi Broggini
Received18 Oct 2018
Accepted14 Nov 2018
Published02 Jan 2019

Abstract

A facile Selectfluor-mediated oxidative method for direct introduction of SCN and SeCN groups into diversely substituted indoles and carbazoles is described, by employing readily available thiocyanate and selenocyanate salts, and the scope of the method is underscored by providing 24 examples. The feasibility to sequentially introduce SCN followed by SeCN on a carbazole framework and to synthesize the corresponding S-tetrazole and Se-tetrazole derivatives is also demonstrated.

1. Introduction

Selectfluor,1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-octane bis(tetrafluoroborate), also known as F-TEDA-BF4) is a safe, bench stable, nontoxic, and highly reactive onium dication salt that delivers an equivalent of electrophilic or radical fluorine [1, 2]. For that reason, it is widely used in the synthesis of organofluorine compounds [38], but it also serves as a versatile catalyst and mediator in transformations involving oxidizable functional groups [913].

Aryl thio- and selenocyanates are versatile intermediates for various sulfur- and selenium-containing compounds that are of synthetic and biological interest [1426]. Aryl thiocyanates are widely employed as building blocks in the synthesis of diverse sulfides [27, 28], thiocarbamates [29], thionitriles [30], sulfonic acids [31], sulfonyl chlorides [32], thioesters [33], and sulfonyl cyanides [31] and mainly in the synthesis and functionalization of heterocyclic compounds [25, 26, 3440]. Particularly, selenium-containing organic compounds have recently attracted the interest of the scientific community due to their promising chemopreventive properties in connection with cancer therapy [32, 4144], and also as antioxidant agents [45, 46].

Owing to the aforementioned chemical versatility and biological activity, functional organic compounds bearing the thiocyanato and selenocyanate derivatives are highly sought after, and over the years a number of strategies and reagents have been utilized to enable the direct introduction of SCN and SeCN moieties into organic motifs. In particular, direct oxidative thiocyanation of C-H bonds have been achieved by using thiocyanate salts in the presence of oxidizing agents such as Mn(OAc)3 [15], NCS [47], CAN [48], hypervalent iodine reagents [49], DDQ [50], oxone [51], oxygen [52], DEAD [53], and TBHP [22]. In comparison, direct selenocyanation approaches have not been reported under similar oxidative conditions [2026]. Therefore, developing new and broad-based protocols for direct introduction of SCN and SeCN moieties into organic structures, in particular those motifs that are important for medicinal chemistry as building blocks of pharmaceuticals, continues to be relevant.

In a limited study in 2008, Yadav et al. [54] provided examples of arene thiocyanation by using Selectfluor, but a broader investigation to determine tolerance to a wider range of substituents and to gauge efficacy in selenocyanation remained unexplored.

In connection with previous studies from our laboratory on the use of Selectfluor as a versatile mediator for generation of incipient electrophiles through oxidation [55, 56], we report here a mild and efficient Selectfluor-mediated direct thio- and selenocyanation of C-H bonds (Scheme 1), focusing mainly on diversely substituted indoles 3 and carbazoles 6. Sequential introduction of SCN and SeCN and synthesis of thio- and selenotetrazoles on a carbazole motif are also demonstrated, in preliminary studies using this chemistry.

2. Results and Discussion

Following an initial feasibility study with 2-methylfuran 1 and 2-methylindole 3a (entries 1 and 2, Table 1), we focused on the indole derivatives bearing electron-withdrawing substituents, in particular HCO, F, Br, NO2, and CN groups.


EntrySubstrateProductYield ()

151
268
370
472
563
679
792
896
989
1067
1187
1257
1366
1472
1590
1672
1785
1860

In relation to other studies in our laboratory on the synthesis of heterocyclic curcuminoids and their antitumor properties [57], we were especially interested in the indole backbone bearing isomeric formyl substituents. Fortunately, both thio- and selenocyanation of the isomeric indole-carboxaldehydes 3b-c proved successful with the formyl group remaining intact (entries 3-5, Table 1). All other aforementioned substituents were tolerated and the corresponding SCN and SeCN derivatives were isolated in yields ranging from 96% to 57% (Table 1).

In the next phase of the study, we focused on the carbazole motif 6 (Table 2). Thus, parent carbazole 6a and its N-isopropyl derivative 6b reacted to furnish the corresponding 3-SCN and 3-SeCN derivatives (7-8)a and (7-8)b, respectively, in yields ranging from 87% to 75%. In other studies, the isolated carbazole-SCN derivatives 7a and 7b were successfully selenocyanated to produce the SCN/SeCN disubstituted derivatives 9 and 10, respectively (entries 23-24, Table 2). However, when this transformation was performed in the reverse order by subjecting the carbazole-SeCN derivatives 8a and 8b to thiocyanation conditions the corresponding seleno/thio-bis-adducts 9 and 10 could not be obtained. This is likely due to ease of oxidation of carbazole-SeCN derivative which could prevent further SCN introduction. 77Se NMR of the indole-SeCN derivatives fall in a relatively close range irrespective of the nature of the substituents (δ 149-162 ppm). Remarkably though 77Se chemical shifts for the carbazole-SeCN compounds 8a and 8b and the disubstituted derivatives 9 and 10 are significantly different (δ 313-317 ppm) (see experimental section and SI file available here).


EntrySubstrateProductYield ()

1987
2075
2181
2286
2343
2416

Finally, by employing the chemistry we reported previously for the synthesis of tetrazoles using Cu-Zn alloy nanopowder [58], in an exploratory study we subjected the carbazole-SCN 7b and carbazole-SeCN 8b derivatives to the reaction conditions shown in Scheme 2 and successfully obtained the first examples of thio- and selenotetrazoles (11b and 12b, respectively), on a carbazole motif 6. Observation of line broadening at room temperature in the proton NMR spectra of the corresponding tetrazoles 11-12 (SI file) is indicative of a rapid 2H- to 1H-tetrazole tautomerization.

3. Conclusions

In summary, a practical and easy to perform method for oxidative thio- and selenocyanation of diversely substituted indoles 3 as well as carbazoles 6 employing Selectfluor is presented. The feasibility to sequentially introduce the SCN followed by SeCN on a carbazole motif 6 is also demonstrated, and first examples of carbazole-S-tetrazole and carbazole-Se-tetrazole derivatives (11b and 12b, respectively) are provided.

4. Experimental

4.1. General Experimental Information

The reagents employed were high-purity commercial samples and were used without further purification. Column chromatography was performed on silica gel (32–63 particle size). Melting points were measured with a MEL-TEMP apparatus and are uncorrected. Multinuclear NMR spectra (1H, 13C, 19F, and 77Se) were recorded on a Varian INOVA 500 instrument. 19F and 77Se NMR spectra were externally referenced (relative to CFCl3 and Me2Se) as set by the instrument and verified by comparison with known reference samples. Some 1H and 13C NMR spectra were recorded on a Bruker Avance 400 instrument. HRMS analyses were performed on a Finnigan Quantum ultra-AM in electrospray mode using methanol as solvent. Other HRMS spectra were recorded on a Waters Micromass AutoSpec-Ultima spectrometer (equipped with a direct inlet probe) operating at 70 eV. Additional mass spectra were run on a Shimadzu-GCMS 2010-DI-2010 spectrometer (equipped with a direct inlet probe) operating at 70 eV.

4.2. General Procedure for the Thiocyanate and Selenocyanate Products 2, 4, 5, and 7-10

For a solution of ammonium thiocyanate (107 mg, 1.5 equiv.) or potassium selenocyanate (203 mg, 1.5 equiv.) and Selectfluor® (500 mg, 1.5 equiv.) in MeCN (10 mL), the appropriated substrate 1, 3, 6, 7a, or 7b (1 equiv.) was added and the mixture was stirred at room temperature under inert atmosphere for 2h. After completion (monitored by TLC), the reaction mixture was concentrated under reduced pressure, and water was added. The aqueous solution was extracted with ethyl acetate, and the combined organic extracts were dried with anhydrous Na2SO4. After removal of the solvent, the crude products were purified by column chromatography using hexane: ethyl acetate (7:3) as eluent.

4.2.1. 2-Methyl-5-thiocyanatofuran (2)

Yellow oil, Rf 0.71 (15% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 6.79 (d, 1H, J = 3.3 Hz), 6.12 - 6.13 (m, 1H), 2.38 (s, 3H, CH3) ppm. 13C NMR (CDCl3, 125 MHz): δ 159.4, 127.7, 123.4, 109.2, 108.8 (SCN), 14.1 (CH3) ppm.

4.2.2. 2-Methyl-3-thiocyanato-1H-indole (4a)

Beige solid, mp 96-99°C, Rf 0.62 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.49 (bs, 1H, NH), 7.71 - 7.69 (m, 1H), 7.34 - 7.32 (m, 1H), 7.29 - 7.23 (m, 2H), 2.56 (s, 3H, CH3) ppm. 13C NMR (CDCl3, 125 MHz): δ 141.9, 135.1, 128.7, 123.0, 121.6, 118.2, 111.9, 111.2, 89.2 (SCN), 12.1 (CH3) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C10H9N2S: 189.04864; found: 189.04207.

4.2.3. 3-Thiocyanato-1H-indole-4-carbaldehyde (4b)

Beige solid, mp 169-171°C, Rf 0.31 (30% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.50 (bs, 1H, NH), 10.85 (s, 1H, CHO), 8.06 (d, 1H, J = 3.5 Hz), 7.93 (dd, 1H, J = 8.0, 1.0 Hz), 7.87 (dd, 1H, J = 7.5, 1.0 Hz), 7.47 (t, 1H, J = 8.0 Hz) ppm. 13C NMR (acetone-d6, 125 MHz): δ 190.6, 138.5, 134.1, 129.3, 124.9, 122.8, 119.1, 119.0, 112.0, 92.3 (SCN) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C10H7N2OS: 203.02791; found: 203.02448.

4.2.4. 3-Thiocyanato-1H-indole-5-carbaldehyde (4c)

White solid, mp 174-175°C, Rf 0.22 (30% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.52 (bs, 1H, NH), 10.19 (s, 1H, CHO), 8.38 (d, 1H, J = 0.7 Hz), 8.11 (d, 1H, J = 2.8 Hz), 7.88 (dd, 1H, J = 8.5, 1.5 Hz), 7.76 (d, 1H, J = 8.5 Hz) ppm. 13C NMR (acetone-d6, 125 MHz): δ 191.6, 140.1, 134.7, 131.3, 127.8, 122.9, 122.6, 113.5, 110.9, 93.3 (SCN) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C10H7N2OS: 203.02791; found: 203.02432.

4.2.5. 3-Selenocyanato-1H-indole-5-carbaldehyde (5c)

Orange-brown solid, mp > 150°C decomposes, Rf 0.22 (30% EtOAc in hexane). 1H NMR (DMSO-d6, 500 MHz): δ 12.30 (bs, 1H, NH), 10.10 (s, 1H, CHO), 8.21 (d, 1H, J = 1.0 Hz), 8.04 (d, 1H, J = 3.0 Hz), 7.77 (dd, 1H, J = 8.0, 2.0 Hz), 7.66 (d, 1H, J = 8.5 Hz) ppm. 13C NMR (DMSO-d6, 125 MHz): δ 193.2, 140.2, 135.9, 130.6, 129.1, 124.1, 122.8, 113.7, 105.0, 92.4 (SeCN) ppm. 77Se NMR (DMSO-d6, 95 MHz): δ 162.1 (s) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C10H7N2OSe: 250.97236; found: 250.95178.

4.2.6. 1-Butyl-3-thiocyanato-1H-indole (4d)

Yellow oil, Rf 0.74 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 7.85 - 7.83 (m, 2H), 7.44 - 7.42 (m, 2H), 7.39 - 7.32 (m, 2H), 4.11 (t, 2H, J = 7.2 Hz, CH2), 1.86 - 1.80 (m, 2H, CH2), 1.40 - 1.32 (m, 2H, CH2), 0.98 (t, 3H, J = 7.4 Hz, CH3) ppm. 13C NMR (CDCl3, 125 MHz): δ 136.6, 134.2, 128.6, 123.3, 121.5, 119.0, 112.0, 110.5, 89.7 (SCN), 46.8 (CH2), 32.0 (CH2), 20.1 (CH2), 13.7 (CH3) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C13H15N2S: 231.09559; found: 231.10324.

4.2.7. 1-Butyl-3-selenocyanato-1H-indole (5d)

Yellow oil, Rf 0.74 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 7.78 - 7.75 (m, 1H), 7.45 (s, 1H), 7.41 - 7.43 (m, 2H), 7.36 - 7.30 (m, 2H), 4.16 (t, 2H, J = 7.2 Hz, CH2), 1.89 - 1.83 (m, 2H, CH2), 1.40 - 1.35 (m, 2H, CH2), 0.97 (t, 3H, J = 7.4 Hz, CH3) ppm. 13C NMR (CDCl3, 125 MHz): δ 136.5, 135.0, 129.6, 123.1, 121.4, 120.0, 110.2, 101.8, 87.2 (SeCN), 46.8 (CH2), 32.1 (CH2), 20.1 (CH2), 13.6 (CH3) ppm. 77Se NMR (CDCl3, 95 MHz): δ 149.6 (s) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C13H15N2Se: 279.04004; found: 279.04199.

4.2.8. 5-Bromo-3-thiocyanato-1H-indole (4e)

White solid, mp 133-136°C, Rf 0.35 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.8 (bs, 1H, NH), 7.92 (d, 1H, J = 1.2 Hz), 7.52 (d, 1H, J = 1.4 Hz), 7.39 (dd, 1H, J = 8.6, 1.9 Hz), 7.30 (d, 1H, J = 9.4 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 134.6, 132.1, 129.3, 127.0, 121.4, 115.4, 113.6, 111.6, 92.0 (SCN) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C9H6BrN2S: 252.94351/254.94146; found: 252.93964/254.93404.

4.2.9. 5-Bromo-3-selenocyanato-1H-indole (5e)

Beige solid, mp 155-157°C, Rf 0.35 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.7 (bs, 1H, NH), 7.90 (d, 1H, J = 1.7 Hz), 7.56 (d, 1H, J = 2.7 Hz), 7.42 (dd, 1H, J = 8.6, 1.8 Hz), 7.33 (d, 1H, J = 9.0 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 134.7, 132.8, 130.5, 127.0, 122.4, 115.4, 113.3, 101.3, 89.4 (SeCN) ppm. 77Se NMR (CDCl3, 95 MHz): δ 151.9 (s) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C9H6BrN2Se: 300.88796/302.88591; found 300.88208/302.87494.

4.2.10. 5-Fluoro-3-thiocyanato-1H-indole (4f)

Off-white solid, mp 108-110°C, Rf 0.51 (40% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.2 (br. s, 1H, NH), 7.99 (d, 1H, J = 2.5 Hz), 7.61 (dd, 1H, J = 8.5, 4.5 Hz), 7.45 (dd, 1H, J = 9.0, 2.5 Hz), 7.12 (td, 1H, J = 9.0, 3.0 Hz) ppm. 13C NMR (acetone-d6, 125 MHz): δ 158.8 (d, 1JCF = 236.6 Hz), 134.4 (d, JCF = 20 Hz), 133.3, 128.6 (d, JCF = 10.5 Hz), 114.1 (d, JCF = 10.5 Hz), 111.6 (d, JCF = 25.7 Hz), 111.1, 102.9 (d, JCF = 25 Hz), 91.0 (d, 4JCF = 4.7 Hz) ppm. 19F NMR (acetone-d6, 470 MHz): δ -123.2 (m) ppm. HRMS (ESI): m/z [M+H]+ calcd for C9H6N2FS: 193.02357; found: 193.01202.

4.2.11. 5-Fluoro-3-selenocyanato-1H-indole (5f)

Light-brown solid, mp 109-111°C, Rf 0.33 (40% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.13 (s, 1H, br, NH), 7.93 (d, 1H, J = 2.5 Hz), 7.59 (dd, 1H, J = 8.7, 4.5 Hz), 7.38 (dd, 1H, J = 9.5, 2.5 Hz), 7.10 (dt, 1H, J = 9.0, 3.0 Hz). 13C NMR (acetone-d6, 125 MHz): δ 157.7 (d, 1JCF = 236.6 Hz), 134.8 (d, JCF = 20.0 Hz), 133.2, 129.8 (d, JCF = 10.5 Hz), 113.8 (d, JCF = 5.6 Hz), 111.3 (d, JCF = 26.6 Hz), 103.8 (d, JCF = 24.7 Hz), 101.5, 89.0 ppm. 19F NMR (acetone-d6, 470 MHz): δ -123.68 (m) ppm. 77Se NMR (acetone-d6, 95 MHz): δ 149.7 (s). HRMS (ESI): m/z [M+H]+ calcd for C9H6N2FSe: 240.96802; found: 240.95600.

4.2.12. 1-Butyl-5-fluoro-3-thiocyanato-1H-indole (4g)

Yellow oil, Rf 0.26 (10% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 7.48 (s, 1H), 7.44 (dd, 1H, J = 8.2, 2.5 Hz), 7.33 (dd, 1H, J = 9.0, 4.0 Hz), 7.08 (dt, 1H, J = 8.7, 2.5 Hz), 4.14 (t, 2H, J = 7.0 Hz), 1.84 (m, 2H), 1.36 (m, 2H), 0.97 (t, 3H, J = 7.5 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 158.9 (d, 1JCF = 238.3 Hz), 135.4, 133.0, 129.4 (d, 3JCF = 10.5 Hz), 112.0 (d, 2JCF = 25.0 Hz), 111.5 (d, JCF = 12.5 Hz), 111.4, 104.3 (d, 2JCF = 24.8 Hz), 89.9 (d, 4JCF = 4.7 Hz), 32.0, 20.1, 13.6 ppm. 19F NMR (CDCl3, 470 MHz): δ -121.36 (m) ppm. HRMS (ESI): m/z [M+H]+ calcd for C13H14N2FS: 249.08617; found: 249.07428.

4.2.13. 1-Butyl-5-fluoro-3-selenocyanato-1H-indole (5g)

Brown oil, Rf 0.32 (10% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 7.48 (s, 1H), 7.39 (dd, 1H, J = 9.0, 2.5 Hz), 7.33 (dd, 1H, J = 9.2, 4.5 Hz), 7.07 (dt, 1H, J = 8.7, 3.0 Hz), 4.14 (t, 2H, J = 7.5 Hz), 1.85 (m, 2H), 1.36 (m, 2H), 0.97 (t, 3H, J = 7.0 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 158.9 (d, 1JCF = 237.4 Hz), 136.3, 133.0, 130.4 (d, JCF = 11.3 Hz), 111.8 (d, JCF = 21.6 Hz), 111.3 (d, JCF = 9.5 Hz), 105.0 (d, JCF = 24.7 Hz), 101.58, 87.0 (d, JCF = 4.8 Hz), 47.1, 32.0, 20.1, 13.6 ppm. 19F NMR (CDCl3, 470 MHz): δ -121.6 (m) ppm. 77Se NMR (CDCl3, 95 MHz): δ 152.0 (s) ppm. HRMS (ESI): m/z [M+H]+ calcd for C13H14N2FSe: 297.03062; found: 297.02386.

4.2.14. 6-Nitro-3-thiocyanato-1H-indole (4h)

Pale-yellow solid, mp 182-183°C, Rf 0.51 (40% EtOAc in hexane). 1H NMR (DMSO-d6, 500 MHz): δ 12.64 (s, 1H, NH), 8.43 (d, 1H, J = 2.0 Hz), 8.37 (d, 1H, J = 3.5 Hz), 8.10 (dd, 1H, J = 9.0, 2.5 Hz), 7.85 (d, 1H, J = 9.0 Hz) ppm. 13C NMR (DMSO-d6, 125 MHz): δ 143.8, 139.4, 135.3, 132.6, 118.9, 116.6, 112.4, 110.0, 92.2 ppm. HRMS (ESI negative ion mode): m/z [M-H]- calcd for C9H4O2N3S: 218.002442; found: 218.08749.

4.2.15. 6-Nitro-3-selenocyanato-1H-indole (5h)

Pale-yellow solid, mp 183-184°C, Rf 0.51 (40% EtOAc in hexane). 1H NMR (DMSO-d6, 500 MHz): δ 12.50 (bs, 1H, NH), 8.43 (dd, 1H, J = 2.1, 0.6 Hz), 8.25 (d, 1H, J = 2.8 Hz), 8.08 (dd, 1H, J = 8.8, 2.1 Hz), 7.76 (dt, 1H, J = 8.8, 0.6 Hz) ppm. 13C NMR (DMSO-d6, 125 MHz): δ 143.4, 139.5, 135.1, 133.9, 119.8, 116.2, 109.6, 105.0, 91.9 (SeCN) ppm. 77Se NMR (DMSO-d6, 95 MHz): δ 160.2 (s) ppm. HRMS (ESI negative ion mode): m/z [M-H]- calcd for C9H5N3O2Se: 266.95470; found: 266.01712.

4.2.16. 1-Benzyl-6-nitro-3-thiocyanato-1H-indole (4i)

Yellow solid, mp 144-146°C, Rf 0.69 (30% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 8.60 (d, 1H, J = 2.5 Hz), 8.40 (s, 1H), 8.20 (dd, 1H, J = 8.7, 2.0 Hz), 7.96 (d, 1H, J = 8.5 Hz), 7.41-7.31 (m, 5H), 5.77 (s, 2H) ppm. 13C NMR (acetone-d6, 125 MHz): δ 144.4, 140.9, 136.3, 135.6, 133.1, 129.0, 128.2, 127.5, 119.1, 116.6, 110.7, 108.3, 92.3, 50.7 ppm. HRMS (ESI): m/z [M+H]+ calcd for C16H12O2N3S: 310.0650; found: 310.0584.

4.2.17. 1-Benzyl-6-nitro-3-selenocyanato-1H-indole (5i)

Yellow solid, mp 153-154°C, Rf 0.59 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.38 (d, 1H, J = 2.0 Hz), 8.21 (dd, 1H, J = 9.0, 2.0 Hz), 7.85 (d, 1H, J = 9.0 Hz), 7.71 (s, 1H) 7.40 - 7.37 (m, 3H), 7.21-7.19 (m, 2H), 5.44 (s, 2H, CH2) ppm. 13C NMR (CDCl3, 125 MHz): δ 144.5, 139.9, 135.5, 134.5, 134.2, 129.4, 128.9, 127.3, 120.4, 117.1, 107.5, 100.7, 89.6, 51.3 ppm. 77Se NMR (CDCl3, 95 MHz): δ 153.7 (s) ppm. HRMS (ESI): m/z [M+H]+ calcd for C16H12O2N3Se: 358.0095; found: 358.0082.

4.2.18. 3-Thiocyanato-1H-indole-5-carbonitrile (4j)

Beige solid, mp 200°C decomposes, Rf 0.13 (30% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.60 (bs, 1H, NH), 8.23 (dt, 1H, J = 1.5, 0.7 Hz), 8.16 (d, 1H, J = 2.9 Hz), 7.80 (dd, 1H, J = 8.5, 0.8 Hz), 7.64 (dd, 1H, J = 8.5, 1.5 Hz) ppm. 13C NMR (acetone-d6, 125 MHz): δ 138.5, 135.3, 127.7, 126.0, 123.5, 119.4, 114.1, 110.8, 104.7, 92.6 (SCN) ppm. HRMS (ESI) m/z [M+H]+ calcd. For C10H6N3S: 200.02824; found: 200.01814.

4.2.19. 3-Thiocyanato-9H-carbazole (7a)

White solid, mp 103-104°C, Rf 0.47 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.35 (bs, 1H, NH), 8.25 (s, 1H), 8.03 (d, 1H, J = 8.0 Hz), 7.55 (dd, 1H, J = 8.5, 2.0 Hz), 7.51 - 7.45 (m, 2H), 7.40 (d, 1H, J = 8.5 Hz), 7.30 (pseudo-dt, 1H, J = 7.2, 1.5 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 140.2, 139.9, 129.6, 127.1, 125.3, 124.8, 122.1, 120.6, 120.4, 112.7, 112.3, 112.1, 111.0 (SCN) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C13H9N2S: 225.04864; found: 225.03801.

4.2.20. 3-Selenocyanato-9H-carbazole (8a)

Beige solid, mp 135-137°C, Rf 0.47 (30% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.35 (d, 1H, J = 1.7 Hz), 8.33 (bs, 1H, NH), 8.04 (d, 1H, J = 7.8 Hz), 7.65 (dd, 1H, J = 8.4, 1.7 Hz), 7.50 - 7.45 (m, 2H), 7.38 (d, 1H, J = 8.4 Hz), 7.29 (pseudo-dt, 1H, J = 7.2, 1.5 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ 140.1, 139.7, 131.6, 127.4, 127.0, 125.0, 122.2, 120.6, 120.4, 112.4, 111.0, 109.6, 103.0 (SCN) ppm. 77Se NMR (CDCl3, 95 MHz): δ = 317.2 (s) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C13H9N2Se: 272.99309; found: 272.96500.

4.2.21. 9-Isopropyl-3-thiocyanato-9H-carbazole (7b)

White solid, mp 124-126°C, Rf 0.58 (15% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 8.53 (d, 1H, J = 2.0 Hz), 8.29 (dd, 1H, J = 7.8, 1.3 Hz), 7.85 (d, 1H, J = 8.8 Hz), 7.76 (d, 1H, J = 8.5 Hz), 7.72 (dd, 1H, J = 8.7, 1.9 Hz), 7.53 (pseudo-dt, 1H, J = 8.4, 1.0 Hz), 7.29 (pseudo-dt, 1H, J = 7.9, 0.9 Hz), 5.22 (sept, 1H, J = 7.0 Hz, CH), 1.73 (s, 3H, CH3), 1.72 (s, 3H, CH3) ppm. 13C NMR (acetone-d6, 125 MHz): δ 140.2, 140.1, 129.3, 126.7, 125.3, 124.7, 122.2, 120.7, 119.7, 112.2, 111.8, 111.4, 110.9 (SCN), 47.0 (CH), 19.9 (CH3) ppm. HRMS (ESI) m/z [M+H]+ calcd. for C16H15N2S: 267.09559; found: 267.09363.

4.2.22. 9-Isopropyl-3-selenocyanato-9H-carbazole (8b)

Beige solid, mp 161-162°C, Rf 0.58 (15% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 8.60 (t, 1H, J = 1.2 Hz), 8.27 (dd, 1H, J = 7.8, 1.3 Hz), 7.80 (d, 2H, J = 1.2 Hz), 7.75 (d, 1H, J = 8.4), 7.52 (dt, 1H, J = 8.0, 1.3 Hz), 7.28 (dt, 1H, J = 7.9, 1.0 Hz), 5.21 (sept, 1H, J = 7.0 Hz, CH), 1.73 (s, 3H, CH3), 1.72 (s, 3H, CH3) ppm. 13C NMR (acetone-d6, 125 MHz): δ 140.0, 139.9, 131.3, 127.2, 126.5, 124.8, 122.3, 120.6, 119.5, 112.08, 110.8, 109.8, 102.7 (SeCN), 46.9 (CH), 19.9 (CH3) ppm. 77Se NMR (acetone-d6, 95 MHz): δ 312.9 (s). HRMS (ESI) m/z [M+H]+ calcd. for C16H15N2Se: 315.04004; found: 315.03387.

4.2.23. 3-Selenocyanato-6-thiocyanato-9H-carbazole (9)

Beige solid, mp 150°C decomposes, Rf 0.25 (30% EtOAc in hexane). 1H NMR (acetone-d6, 500 MHz): δ 11.09 (bs, 1H, NH), 8.72 (s, 1H), 8.65 (s, 1H), 7.86 (dd, 1H, J = 8.5, 1.8 Hz), 7.74 - 7.78 (m, 2H), 7.71 (d, 1H, J = 8.5 Hz) ppm. 13C NMR (acetone-d6, 125 MHz): δ 141.2, 141.1, 132.6, 130.3, 130.3, 127.8, 125.7, 123.6, 113.4, 113.3, 113.2, 113.1, 111.6, 111.5, 102.7 ppm. 77Se NMR (acetone-d6, 95 MHz): δ 317.3 (s) ppm. EIMS: m/z (%) = 328 (68) [M]+, 284 (30), 271 (24), 249 (57), 57 (65), 43 (100).

4.2.24. 9-Isopropyl-3-selenocyanato-6-thiocyanato-9H-carbazole (10)

Orange yellow solid, mp 126-128°C, Rf 0.44 (40% EtOAc in hexane). 1H NMR (CDCl3, 500 MHz): δ 8.43 (d, 1H, J = 2.0 Hz), 8.32 (d, 1H, J = 2.0 Hz), 7.79 (dd, 1H, J = 8.7, 2.0 Hz), 7.68 (dd, 1H, J = 9.0, 2.0 Hz), 7.62 (d, 1H, J = 8.5 Hz), 7.57 (d, 1H, J = 8.9 Hz), 5.00 (sept, 1H, J = 7.0 Hz), 1.72 (d, 6H, J = 6.5 Hz) ppm. 13C NMR (CDCl3, 125 MHz): δ140.6, 140.5, 132.3, 130.1, 127.6, 125.3, 123.9, 123.8, 113.0, 112.3, 112.2, 112.0, 110.4, 102.3, 47.6, 20.8 ppm. 77Se NMR (CDCl3, 95 MHz): δ 317.5 (s). GC-MS: m/z 371 (M+).

4.3. General Procedure for the Synthesis of Tetrazoles (11-12)b

Cu-Zn alloy nanopowder (38 mg) was added to a mixture of the thiocyanate or selenocyanate compound 7b or 8b, respectively (2 mmol), and sodium azide (2.8 mmol) in DMF (6 mL), and the mixture was stirred under ambient conditions for 8h at 90°C. After completion (monitoring by TLC), the resulting mixture was allowed to cool to room temperature and the catalyst was separated by centrifugation and washed with ethyl acetate (three times). After removal of DMF and ethyl acetate under reduced pressure from the resulting centrifugate, it was treated with acetic acid and ethyl acetate with stirring. The organic layer was separated and the aqueous solution left behind was extracted further with ethyl acetate. The combined organic extracts were washed with water and concentrated. Compounds (11-12)b were purified by column chromatography by using chloroform-methanol (20:1).

4.3.1. 3-((2H-tetrazol-5-yl)thio)-9-isopropyl-9H-carbazole (11b)

White solid, mp 182 -186°C, Rf 0.31 (5% MeOH in Chloroform). 1H NMR (DMSO-d6, 400 MHz): δ 8.50 (br. s, 1H), 8.23 (d, 1H, J = 7.7 Hz), 7.79 (s, 1H), 7.75 (d, 1H, J = 8.4 Hz), 7.63 (br. s, 1H), 7.48 (t, 1H, J = 7.6 Hz), 7.24 (t, 1H, J = 7.3 HZ), 5.19 – 5.12 (m, 1H, CH), 1.65 (br. s, 3H, CH3), 1.63 (br. s, 3H, CH3) ppm. 13C NMR (DMSO-d6, 100 MHz): δ 139.9, 139.9, 139.8, 131.4, 126.8, 126.7, 124.1, 122.4, 121.2, 119.7, 117.0, 112.2, 111.2, 46.8 (CH), 20.9 (CH3) ppm. EIMS: m/z (%) 309 (46) [M]+, 253 (12), 240 (20), 198 (100), 69 (20), 43 (89), 41 (66).

4.3.2. 3-((2H-tetrazol-5-yl)selanyl)-9-isopropyl-9H-carbazole (12b)

White solid, mp 156 - 160°C, Rf 0.53 (5% MeOH in Chloroform). 1H NMR (CDCl3, 400 MHz): δ 8.65 (br. s, 1H), 8.14 (d, 1H, J = 7.6 Hz), 7.91 (br. d, 1H, J = 8.4 Hz), 7.65 (d, 1H, J = 7.8 Hz), 7.59 (d, 1H, J = 8.3 Hz), 7.52 (t, 1H, J = 7.6 Hz), 7.28 (t, 1H, J = 7.4 Hz), 5.03 (sep, 1H, J = 7.1 Hz, CH), 1.73 (d, 6H, J = 7.0 Hz, CH3) ppm. 13C NMR (CDCl3, 100 MHz): δ 141.7, 140.0, 140.0, 126.5, 123.9, 123.0, 121.0, 120.9, 119.8, 119.2, 119.2, 110.8, 110.5, 47.2 (CH), 20.8 (CH3) ppm. EIMS: m/z (%) 496 (37) [M]+, 416 (100), 358 (15), 331 (24), 288 (33), 246 (31), 193 (35), 166 (35), 43(48), 41 (30).

Data Availability

The data used to support the findings of this study are provided in experimental section and in supplementary material. Additional inquiries may be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

Kenneth K. Laali thanks University of North Florida for the outstanding faculty scholarship and presidential professorship awards, faculty scholarship, and UNF Foundation Board grants. Rodrigo Abonia and Luisa F. Gutierrez thank COLCIENCIAS (no. 110665842661) and Universidad del Valle (CI. 71111) for financial support.

Supplementary Materials

Multinuclear NMR spectra of the reported products are gathered in the supplementary file. (Supplementary Materials)

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