Access to SCN-containing thiazolines via electrochemical regioselective thiocyanothiocyclization of N-allylthioamides

Yan-An Zhang , Zhong Ding , Peng Liu , Wei-Si Guo *, Li-Rong Wen * and Ming Li
State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China. E-mail: nick8110@163.com; wenlirong@qust.edu.cn

Received 6th March 2020 , Accepted 8th April 2020

First published on 9th April 2020


An electrochemical thiocyanothiocyclization of N-allylthioamides has been developed for the synthesis of SCN-containing 2-thiazolines. This method provides a green and efficient approach to generate 5-exo-cyclization 2-thiazolines with a broad substrate scope and good yields. In addition, 6-endo-cyclization isothiocyanato thiazines are formed regioselectively when cyclic thioamides are used as reactants. The reaction is easy to proceed under catalyst-, additive- and oxidant-free conditions.


Introduction

Electrochemical synthesis, which is atom economical and sustainable, has attracted much attention in the past decade.1 It provides a green and efficient approach for the generation of radical intermediates using electrons as traceless reagents, and could avoid the use of toxic oxidants and expensive catalysts. Recently, electrochemical difunctionalization of alkenes has been utilized to synthesize a variety of functionalized heterocycles.2 Anodic oxidation can be used to form C–C and C–heteroatom bonds under environmentally friendly conditions. However, an electrochemical approach for the synthesis of 2-thiazolines has not yet been explored.

The 2-thiazoline scaffold is a privileged heterocycle found in natural products and bioactive molecules.3 In addition, thiazolines are representative chiral ligands in asymmetric synthesis.4 Therefore, various approaches have been developed in the past decade for the synthesis of thiazoline derivatives.5 The cyclization based on the toxic β-aminothiols remains the most frequently used approach.6 Furthermore, these reactions usually accompanied by harsh conditions, limited substrate scope and stoichiometric waste formation. Compared to the intramolecular cyclization of N-allylcarboxamides to form oxazolines,7 a similar strategy for the synthesis of thiazolines using N-allylthioamides has rarely been reported, possibly because thioamides are easily oxidized and prone to desulfurization to form amides under traditional oxidative conditions.8 Recently, the Hong group developed a hypervalent iodine-mediated aminothiolation reaction for the synthesis of 5-amino-thiazolines using excess N-allylthioamides as starting materials (Scheme 1a).9 The Nicewicz group developed a photocatalyzed intramolecular hydrothiolation reaction for the synthesis of thiazolines (6 examples) using thiophenol as a hydrogen atom donor (Scheme 1b).10 Despite this progress, the synthesis of functionalized thiazolines from N-allylthioamides under mild reaction conditions is still challenging.


image file: d0qo00300j-s1.tif
Scheme 1 Synthesis of 2-thiazolines from N-allylthioamides.

Organothiocyanates have versatile functionality in natural products and biologically active compounds.11 They are also useful precursors to transform into thiocarbamates, thiotetrazoles, and other sulfur-containing derivatives.12 Thus, a variety of synthetic protocols have been established for the synthesis of SCN-containing compounds.13 Recently, various SCN-containing heterocycles have been constructed by the intramolecular cyclization of alkenes with thiocyanate salts, such as pyrazolines,14 dihydrofurans,15 and oxazines.16 However, the synthetic method for thiocyanato-substituted thiazolines remains undeveloped.17 We speculated that the SCN anion could be transformed into (SCN)2 under mild electrochemical conditions to trigger intramolecular cyclization, as shown in Scheme 1c. Although the proposed strategy seems reasonable, its realization remains challenging. First, the desulfurization of N-allylthioamides should be avoided under electrochemical oxidative conditions.18 Second, thioamides might also be oxidized under electrochemical conditions, resulting in a less regioselective cyclization.10 With our continued interest in thioamide chemistry and electrocatalysis,19 we developed the first example of an electrochemical synthesis of 5-thiocyanatomethyl-2-thiazolines using readily available N-allylthioamides under catalyst- and oxidant-free conditions. When cyclic thioamides were used as starting materials, 6-endo-cyclization isothiocyanato thiazines were formed regioselectively. Moreover, the cheap thiocyanate salt play a dual role, and an additional supporting electrolyte is not required for the reaction.

Results and discussion

Initially, N-allylthioamide 1a and NH4SCN 2 were selected as starting materials to investigate the optimized reaction conditions. The best yield (85%) of the desired thiazoline 3a was obtained by using a graphite rod as both an anode and a cathode at room temperature in CH3CN with a constant voltage of 2.0 V in an undivided cell for 5 h (Table 1, entry 1). A similar yield (81%) was obtained with a platinum plate as a cathode (Table 1, entry 2). However, graphite felt gave a low yield (Table 1, entry 3). The addition of the electrolyte nBu4NPF6 and LiClO4 did not promote the yield of 3a (Table 1, entries 4 and 5). Both an increase and a decrease in the constant voltage than 2.0 V were less efficient, while the use of 5 mA constant current also failed to improve the reaction yield (Table 1, entries 6–8). Moreover, different solvents were screened, CH3OH gave a 59% yield and THF led to a trace amount of 3a (Table 1, entries 9 and 10). Instead of NH4SCN, KSCN resulted in a relatively close yield (Table 1, entry 11). Not surprisingly, no product 3a was detected without electricity (Table 1, entry 12).
Table 1 Optimization of the reaction conditionsa

image file: d0qo00300j-u1.tif

Entry Deviation from standard conditions Yieldb (%)
a Standard conditions: C (Φ 5 mm) anode, C (Φ 5 mm) cathode, constant voltage = 2.0 V, 1a (0.2 mmol), 2 (0.4 mmol), CH3CN (4.0 mL), RT, air, 5 h. b Isolated yields; n.d. = not detected.
1 None 85
2 C (+)|Pt (−) instead of C (+)|C (−) 81
3 C felt instead of C (+)|C (−) 36
4 Add nBu4NPF6 (1 equiv.) 83
5 Add LiClO4 (1 equiv.) 44
6 3.0 V instead of 2.0 V 77
7 1.5 V instead of 2.0 V 73
8 5.0 mA instead of 2.0 V 75
9 THF instead of CH3CN Trace
10 MeOH instead of CH3CN 59
11 KSCN instead of NH4SCN 80
12 Without electricity n.d.


Having established the optimal reaction conditions, the substrate scope of N-allylthioamides 1 was explored (Table 2). The reactions were compatible with different substituted groups (e.g., Me, MeO, Ph, Cl, Br, CF3, and COOMe) on the phenyl ring (R1) regardless of the electronic nature and substitution position, products 3a–j were obtained in 70–88% yields. Thioamides containing 3,5-dimethyl groups on the phenyl ring were also compatible, affording 3k in moderate yield. In addition, a substrate with a naphthyl group furnished thiazoline 3l in 63% yield. Gratifyingly, substrates bearing furan, thiophene, and pyridine moieties were also tolerated to generate products 3m–o in good yields. N-Allylcinnamide was also a good substrate, and the desired product 3p was obtained in 44% yield. Subsequently, 1,1-disubstituted alkenes were performed under the optimized reaction conditions, and the desired thiazolines 3q–aa were also obtained regioselectively, regardless of the electronic properties of the functional group on the two aryl rings (4-OMe, 4-Cl, 4-Br, 2-pyridyl, and 4-CF3). Steric hindrance may influence the transformation, and the corresponding products were obtained in moderate yields. The reaction of 1,2-disubstituted alkenes also proceeded successfully to generate the desired thiazolines 3ab and 3ac in 66% and 63% yields, respectively. Alkyl- and alkene-substituted substrates were also cyclized smoothly, albeit they gave 3ad and 3ae in low yields. It is noteworthy that trisubstituted alkenes were well tolerated, and the desired product 3af was obtained in 58% yield. The structures of compounds 3 were undoubtedly confirmed by the X-ray crystallographic structure of 3ab (see ESI, Fig. S1).

Table 2 Substrate scope of thioamide 1 with 2a,b
a Standard conditions: C (Φ 5 mm) anode, C (Φ 5 mm) cathode, constant voltage = 2.0 V, 1 (0.2 mmol), 2 (0.4 mmol), CH3CN (4.0 mL), RT, air, 5 h. b Isolated yields. c The reaction time was 8 h. d The diastereoselectivity ratio is >19[thin space (1/6-em)]:[thin space (1/6-em)]1.
image file: d0qo00300j-u2.tif


1,2-Dihydronaphthalene substrates were synthesized to test the tolerance of the reaction under the optimized reaction conditions (Table 3). The 6-endo-cyclization products 4a–c were isolated regioselectively in moderate yields, and no 5-exo-cyclization products were detected. Although the explanation for the regioselectivity is unclear, we speculate that the reactions of the substrates bearing cyclic alkenes are prone to occur through 6-endo-cyclization with less steric hindrance. Interestingly, only the C–N bond forming isothiocyanates were generated selectively.20 The structure of compound 4b was confirmed by X-ray crystallography analysis (see ESI, Fig. S2). In addition, the reactions with dihydrochromene- and dihydrothiochromene-derived substrates proceeded smoothly, the corresponding tricyclic isothiocyanates 4d and 4e were obtained in 58% and 60% yields, respectively. Indene thioamide was also tolerated to produce the desired product 4f, albeit in a low yield.

Table 3 Substrate scope of thioamide 1 with 2a,b
a Standard conditions: C (Φ 5 mm) anode, C (Φ 5 mm) cathode, constant voltage = 2.0 V, 1 (0.2 mmol), 2 (0.4 mmol), CH3CN (4.0 mL), RT, air, 8 h. b Isolated yields; all the products were obtained with a >19[thin space (1/6-em)]:[thin space (1/6-em)]1 diastereoselectivity ratio.
image file: d0qo00300j-u3.tif


To demonstrate the practicality of this method, a gram-scale experiment was performed using substrate 1a. The desired product 3a was formed in 71% yield (1.66 g). Furthermore, the synthetic applications of 3 were also explored. As shown in Scheme 2, hydrolyzed thiocarbamate 5 was obtained in 87% yield using sulfuric acid. The thiocyanate group could react with NaN3 through cycloaddition to give tetrazole 6 in 91% yield. Phosphonothioate 7 was obtained in a good yield when diphenylphosphine oxide was used as a nucleophile. The trifluoromethylthio group is widely used in pharmaceuticals and agrochemicals. Similarly, the reaction of 3a with TMSCF3 at room temperature afforded trifluoromethylthio derivative 8 in 78% yield. In addition, 3a can be oxidized to thiazole 9 smoothly in the presence of DDQ.


image file: d0qo00300j-s2.tif
Scheme 2 Gram-scale and follow-up reactions.

Several control experiments were carried out to elucidate the reaction mechanism. A radical scavenger 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT) was added into the reaction mixture under standard conditions. The reaction was not inhibited and 3a was obtained in 68% or 57% yield, respectively (Scheme 3, eqn (1)). Furthermore, a radical clock experiment was also carried out using 1ag as the starting material. Product 3ag was obtained in 72% yield with the cyclopropyl group retained, which excluded the formation of a thiyl radical intermediate (Scheme 3, eqn (2)).10 Meanwhile, the cyclic voltammetry experiments of 1a and 2 were performed, and the oxidation peaks were observed at 1.67 V and 1.15 V, respectively (see the ESI, Fig. S3). This result indicated that NH4SCN was easier to oxidize under electrochemical conditions.


image file: d0qo00300j-s3.tif
Scheme 3 Control experiments.

On the basis of the above experimental results and literature reports, a possible mechanism for the tandem reaction is shown in Scheme 4. Initially, the anode oxidation of the SCN anion generates (SCN)2,21 which reacts with N-allylthioamide 1 to form sulfonium intermediate A.22 Subsequently, intramolecular regioselective nucleophilic ring-opening by thioamides, followed by deprotonation, affords thiazoline 3 or thiazine 4. Meanwhile, protons were reduced to hydrogen at the cathode.


image file: d0qo00300j-s4.tif
Scheme 4 Proposed reaction mechanism.

Conclusions

In conclusion, we have developed an efficient tandem reaction for the synthesis of thiocyanato-substituted thiazoline derivatives. This environmentally benign electrochemical strategy is performed under catalyst-, additive-, and oxidant-free conditions with a constant voltage at room temperature. The reaction also features a broad substrate scope, high regioselectivity, and easily scaled-up and simple operation. Furthermore, isothiocyanato-fused thiazines were obtained when cyclic thioamides were used as reactants. Further application of the electrochemical tandem reactions with thioamides is currently under investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21572110) and the Natural Science Foundation of Shandong Province (ZR2019MB010).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. CCDC 1960652 and 1960653. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qo00300j

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