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Communication

Tetra(phenylethynyl)tin Is a New Reagent for Solvent-Free Alkynylation of Imines

by
Andrey S. Levashov
,
Elena V. Dvirnaya
,
Dzhamilay N. Konshina
and
Valery V. Konshin
*
Department of Chemistry and High Technology, Kuban State University, Stavropolskaya st 149, 350040 Krasnodar, Russia
*
Author to whom correspondence should be addressed.
Molbank 2023, 2023(1), M1534; https://doi.org/10.3390/M1534
Submission received: 26 November 2022 / Revised: 21 December 2022 / Accepted: 23 December 2022 / Published: 27 December 2022
(This article belongs to the Topic Catalysis: Homogeneous and Heterogeneous)
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Abstract

The first ZnCl2-catalyzed alkynylation of aldimines with tetra(phenylethynyl)tin was achieved under solvent-free conditions. The present methodology provides propargylamines in 38–62% yields.

1. Introduction

Along with the use of classical reagents (Li, Na, Mg, Zn acetylides, and alkynylsilanes) in organic synthesis for the introduction of an alkyne fragment into various substrates [1,2], considerable attention is paid to the development and study of the reactivity of new non-classical alkynylating reagents and new protocols for the use of well-known reagents. Variants of applying calcium carbide in organic synthesis for ethynylation of different substrates [3,4,5], including boron [6,7,8,9] and aluminum [10] alkynylides, 5-(alkynyl)dibenzothiophenium salts [11,12], alkynylbenziodoxolones [13,14], and sulfonyl acetylenes [15] are shown (Figure 1).
Tin alkynylides, which within more than half a century proved to be reliable reagents [16,17,18,19], continue to be used in organic synthesis; however, they have two significant disadvantages: first, they are highly toxic and, second, these compounds do not meet atom-sparing requirements and considerably reduce the reaction mass efficiency [20] due to a large ballast moiety (Bu3Sn and Me3Sn). We are developing a methodology for the use of easily available tin tetraalkynylides in organic synthesis [21], which have been found to be convenient reagents for the alkynylation of haloaromatic substrates under conditions of the Stille reaction [22], carbonyl compounds [23], and acyl chlorides under the Lewis catalysis [24] (Scheme 1).
Continuing this research trend, it seemed interesting to study whether tetra(phenylethynyl)tin could be used for the alkynylation of imines. Alkynylation of imines is an important reaction affording propargylamines, which are valuable building blocks for the synthesis of different bioactive compounds [25]. Current synthesis protocols for propargylamines mostly reduce to a ternary reaction, A3-coupling [26,27], and numerous versions of a catalytic alkynylation reaction of imines [28,29,30].

2. Results and Discussion

Screening for reaction conditions was carried out on a model reaction of tetra(phenylethynyl)tin 1 with (2,2-dimethylpropylidene)aniline 2a using an equimolar amount of alkynylating reagent, varying Lewis acid, solvent and its amount, reaction time, and temperature. The course of the reaction was controlled by chromatography–mass spectrometry (Scheme 2, Table 1). ZnCl2 (Entry 2) showed the best catalytic activity in the reaction using a solvent with the highest yield of 92% being observed at the minimum solvent amount (100 µL per 0.19 mmol 1), which favored more efficient stirring of the reaction mixture. An increase in the solvent amount led to a dramatic decrease in the yield of 3a to 7% (400 µL per 0.19 mmol 1) (Entry 2**). The catalytic activity of aluminum and indium(III) chlorides (Entry 3, 4) and boron trifluoride etherates (Entry 5) was far less efficient. Among solvents, toluene was found to be the optimum medium. The yield of 3a dramatically decreased in 1,2-dichloroethane (Entry 6), and only traces of the product formed using dichloromethane. 1,4-Dioxane strongly coordinating with a Lewis acid is not suitable (Entry 8). The reaction features the possibility of being carried out in a solvent-free manner: the best conversion (98%) to the product was achieved by using ZnCl2 (Entry 9); in this case, the reaction proceeded in a melt of reagents. Other Lewis acids were also less efficient when used without a solvent (Entries 10–12).
The preparative-scale reaction was carried out with imines 2a–c (Scheme 3). The effect of donor substituents in the benzene ring of the imine leads to a decrease in the yield of propargylamines, and 3b and 3c were synthesized in preparative yields of 44% and 38%, respectively. The target product, 3a–c, was purified by flash chromatography. The structure of product 3a–c was confirmed by 1H, 13C NMR, IR spectroscopy, and mass spectrometry.
When tetra(phenylethynyl)tin was replaced tetrakis(phenylethynyl)silane, the yield of 3a decreased to 7%. Tetrakis(phenylethynyl)germanium (IV) did not react with imine under the above-mentioned conditions.
In conclusion, a new approach is proposed to obtain propargylamines by the solvent-free ZnCl2-catalyzed alkynylation of imines with tetra(phenylethynyl)tin.

3. Materials and Methods

The reactions were monitored by GC/MS recorded on a GCMS−QP2010Plus (Shimadzu, Kyoto, Japan) and in EI ionization mode (70 eV and ionization chamber temperature 25 °C). The 1H-NMR, 13C-NMR spectra were acquired on ECA400 (JEOL, Tokyo, Japan) (400 and 100 MHz, respectively), spectrometers in CDCl3 were at room temperature. The chemical shifts δ were measured in ppm with reference to the residual solvent resonances (1H: CDCl3, δ = 7.25 ppm and 13C: CDCl3, δ = 77.2 ppm). The splitting patterns are referred to as s, singlet; d, doublet; t, triplet; and m, multiplet. Coupling constants (J) are given in hertz. IR spectra were recorded on an IR Prestige (Shimadzu, Kyoto, Japan), using tablets of samples with KBr. High-resolution and accurate mass measurements were carried out using a MaXis Impact (Bruker, Bremen,·Germany) (electrospray ionization/time of flight). The melting points were determined on a Stuart SMP30 apparatus and left uncorrected. Column chromatography was performed using silica 60 (40–63 µm, Mecherey-Nagel, Düren, Germany).
The commercial reagents employed in the synthesis were pivalaldehyde (96%, Aldrich, St. Louis, MO, USA), aniline, p-toluidine, o-anisidine (99%, ABCR GmbH & Co. KG). Imines 2a–c were synthesized from the anilines and pivalaldehyde in benzene medium. Their constants and parameters of the NMR spectra accorded to published data [31,32,33]. Tetra(phenylethynyl)tin, tetrakis(phenylethynyl)silane, and tetrakis(phenylethynyl)germanium (IV) were synthesized by previously described methods [21,34,35].

General Procedure for the Synthesis of Propargylamine

Lewis acid (0.077 mmol), solvent (0.1–0.4 mL), imine 2a 0.126 g (0.77 mmol) (0.135 g 2b and 0.147 g 3c), and 0.1 g (0.19 mmol) tetraphenylethynyltin were placed in a reaction vial. The reaction mixture was vigorously stirred at 30–100 °C for 3–12 h. Reactions were monitored by GC/MS. After the complete reaction, 1 mL of chloroform and 3 mL of water were added, the organic layer was separated, and the aqueous layer was extracted with chloroform (3 × 1 mL). The propargylamines 3a–c were purified by flash chromatography on silica gel using hexane—ethyl acetate (1:1) as the eluent.
N-(4,4-dimethyl-1-phenylpent-1-yn-3-yl)aniline 3a. Yield 0,126 g (62%); light yellowish crystals; and mp 82 °C. IR (KBr): ν = 3396 (NH), 3084, 3055, 3018 (Csp2-H), 2976, 2960, 2931, 2897, 2866 (Csp3-H), 1604, 1504 (Csp2-Csp2), 1489, 1431, 1363, 1313, 1253, 1105, 1070, 1028, 974, 920, and 871 cm−1 (SI, Figure S1). 1H NMR (CDCl3, 399.78 MHz): δ = 1.14 (s, 9H, CH3), 3.74 (br. s, 1H, NH), 4.02 (s, 1H, CH), 6.73–6.76 (m, 3H, CH), 7.17–7.22 (m, 2H, CH), 7.23–7.26 (m, 3H, CH), and 7.32–7.36 (m, 2H, CH) (SI, Figure S2). 13C NMR (CDCl3, 100.5 MHz): δ = 26.5 (CH3), 35.6 (C), 56.3 (CH), 83.6 (Csp), 89.2 (Csp), 114.1 (CH), 118.2 (CH), 123.2 (C), 127.9 (CH), 128.1 (CH), 129.1 (CH), 131.6 (CH), and 147.5 (C) (SI, Figure S3). MS (EI, 70 eV), m/z (Irel, %): 263 [M+] (3), 248 (1), 206 (100), 178 (2), 128 (4), 115 (3), and 104 (8) (SI, Figure S4). HRMS ESI TOF: m/z = 264,1746 [M + H]+ (264,1747 calcd for C19H21N) (SI, Figure S3). The compound is described earlier [36].
N-(4,4-dimethyl-1-phenylpent-1-yn-3-yl)-4-methylaniline 3b. Yield 0.094 g (44%); light yellowish oil; and IR (KBr): ν = 3379, 3363 (NH), 3099, 3076, 3059, 3016 (Csp2-H), 2958, 2924, 2864 (Csp3-H), 2164 (Csp-Csp), 1612, 1516 (Csp2-Csp2), 1489, 1475, 1440, 1367, 1319, 1292, 1244, 1126, 1085, 1031, 808, and 756 (SI, Figure S6). 1H NMR (CDCl3, 399.78 MHz): δ = 1.15 (s, 9H, CH3), 2.26 (s, 3H, CH3), 3.61 (br. s, 1H, NH), 3.98 (s, 1H, CH), 6.68–6.72 (m, 2H, CH), 7.01–7.05 (m, 2H, CH), 7.24–7.28 (m, 3H, CH), and 7.33–7.37 (m, 2H, CH) (SI, Figure S7). 13C NMR (CDCl3, 100.5 MHz): δ = 20.4 (CH3) 26.5 (CH3), 35.6 (C), 57.1 (CH), 83.6 (Csp), 89.4 (Csp), 114.4 (CH), 123.4 (C), 127.4 (C), 127.8 (CH), 128.1 (CH), 129.6 (CH), 131.6 (CH), and 145.3 (C) (SI, Figure S8). MS (EI, 70 eV), m/z (Irel, %): 277 [M+] (3), 220 (100), 204 (3), 118 (18), 102 (3), and 91 (33) (SI, Figure S9). HRMS ESI TOF: m/z = 278,1907 [M + H]+ (278,1903 calcd for C20H23N) (SI, Figure S10).
N-(4,4-dimethyl-1-phenylpent-1-yn-3-yl)-2-methoxyaniline 3c. Yield 0.086 g (38%); light yellowish oil; and IR (KBr): ν = 3431 (NH), 3059 (Csp2-H), 2997, 2958, 2904, 2866, 2833 (Csp3-H), 1600, 1510 (Csp2-Csp2), 1489, 1458, 1427, 1392, 1363, 1313, 1246, 1220, 1176, 1126, 1051, and 1028. (SI, Figure S11). 1H NMR (CDCl3, 399.78 MHz): δ = 1.18 (s, 9H, CH3), 3.86 (s, 3H, CH3O), 4.03 (s, 1H, CH), 4.47 (br. s, 1H, NH), 6.68–6.72 (m, 1H, CH), 6.78–6.85 (m, 2H, CH), 6.88–6.92 (m, 1H, CH), 7.23–7.27 (m, 3H, CH), and 7.34–7.37 (m, 2H, CH) (SI, Figure S12). 13C NMR (CDCl3, 100.5 MHz): δ = 26.5 (CH3), 35.7 (C), 55.6 (CH3O), 56.0 (CH), 83.3 (Csp), 89.4 (Csp), 109.7 (CH), 111.2 (CH), 116.9 (CH), 121.2 (CH), 123.4 (C), 127.8 (CH), 128.1 (CH), 131.7 (CH), 137.5 (C), and 147.2 (C) (SI, Figure S13). MS (EI, 70 eV), m/z (Irel, %): 293 [M+] (4), 236 (100), 220 (4), 193 (3), 134 (21), and 115 (6) (SI, Figure S14). HRMS ESI TOF: m/z = 294,1853 [M + H]+ (294,1852 calcd for C20H23NO) (SI, Figure S15). The compound is described earlier [37].

Supplementary Materials

Figure S1: IR-spectrum of 3a; Figure S2: 1H NMR of 3a; Figure S3: 13C NMR of 3a; Figure S4: MS of 3a; Figure S5: HRMS of 3a; Figure S6: IR-spectrum of 3b; Figure S7: 1H NMR of 3b; Figure S8: 13C NMR of 3b; Figure S9: MS of 3b; Figure S10: HRMS of 3b; Figure S11: IR-spectrum of 3c; Figure S12: 1H NMR of 3c; Figure S13: 13C NMR of 3c; Figure S14: MS of 3c; and Figure S15: HRMS of 3c.

Author Contributions

Conceptualization, V.V.K. and A.S.L.; methodology, V.V.K. and A.S.L.; software, D.N.K.; validation, V.V.K. and D.N.K.; formal analysis, D.N.K.; investigation, E.V.D.; resources, E.V.D.; data curation, V.V.K.; writing—original draft preparation, V.V.K. and D.N.K.; writing—review and editing, V.V.K. and D.N.K.; supervision, V.V.K.; project administration, V.V.K.; funding acquisition, V.V.K. and D.N.K. All authors have read and agreed to the published version of the manuscript.

Funding

This publication was financially supported by the Russian Ministry of Education and Science (project no. FZEN-2020-0022). The reported study was funded by the RFBR and Krasnodar region according to the research project no. 19-43-230009.

Data Availability Statement

Data available from the corresponding authors upon reasonable request.

Acknowledgments

The GC/MS, HRMS study was accomplished with the use of scientific equipment of the Collective Employment Centre “Ecoanalytical Centre”, Kuban State University (A. Z. Temerdashev).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Molbank 2023 m1534 g001
Figure 1. Examples of non-classical alkynylating reagents.
Figure 1. Examples of non-classical alkynylating reagents.
Molbank 2023 m1534 g001
Molbank 2023 m1534 sch001
Scheme 1. Examples of various alkynylation substrates with tin tetraalkynylides.
Scheme 1. Examples of various alkynylation substrates with tin tetraalkynylides.
Molbank 2023 m1534 sch001
Molbank 2023 m1534 sch002
Scheme 2. The model reaction of tetra(phenylethynyl)tin 1 with imine 2a.
Scheme 2. The model reaction of tetra(phenylethynyl)tin 1 with imine 2a.
Molbank 2023 m1534 sch002
Molbank 2023 m1534 sch003
Scheme 3. Reaction of tetra(phenylethynyl)tin 1 with imines.
Scheme 3. Reaction of tetra(phenylethynyl)tin 1 with imines.
Molbank 2023 m1534 sch003
Table 1. The effect of Lewis acid, solvent, and time on the yields 3a *.
Table 1. The effect of Lewis acid, solvent, and time on the yields 3a *.
EntryLewis Acid
(10 mol %)		SolventTemp, °CTime, hYield 3a,%
(GS/MS)
1ZnCl2PhMe100336
2ZnCl2PhMe100992 (7 **)
3InCl3PhMe100958
4AlCl3PhMe100917
5BF3·OEt2PhMe100952
6ZnCl2DCE80930
7ZnCl2DCM3093
8ZnCl21,4-dioxane1009-
9ZnCl2-1001298
10InBr3-100925
11Sc(OTf)3-10099
12Cu(OTf)2-100918
* 0.19 mmol 1, 0.77 mmol 2a, 0.0077 mmol Lewis acid and 0.1 mL solvent; ** 0.4 mL solvent.
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Levashov, A.S.; Dvirnaya, E.V.; Konshina, D.N.; Konshin, V.V. Tetra(phenylethynyl)tin Is a New Reagent for Solvent-Free Alkynylation of Imines. Molbank 2023, 2023, M1534. https://doi.org/10.3390/M1534

AMA Style

Levashov AS, Dvirnaya EV, Konshina DN, Konshin VV. Tetra(phenylethynyl)tin Is a New Reagent for Solvent-Free Alkynylation of Imines. Molbank. 2023; 2023(1):M1534. https://doi.org/10.3390/M1534

Chicago/Turabian Style

Levashov, Andrey S., Elena V. Dvirnaya, Dzhamilay N. Konshina, and Valery V. Konshin. 2023. "Tetra(phenylethynyl)tin Is a New Reagent for Solvent-Free Alkynylation of Imines" Molbank 2023, no. 1: M1534. https://doi.org/10.3390/M1534

APA Style

Levashov, A. S., Dvirnaya, E. V., Konshina, D. N., & Konshin, V. V. (2023). Tetra(phenylethynyl)tin Is a New Reagent for Solvent-Free Alkynylation of Imines. Molbank, 2023(1), M1534. https://doi.org/10.3390/M1534

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