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DOI: 10.1039/C9QO01055F (Research Article) Org. Chem. Front., 2020, 7, 113-118

Transition-metal-free direct C-3 cyanation of quinoxalin-2(1H)-ones with ammonium thiocyanate as the “CN” source†

Jiayang Wang a, Bin Sun a, Liang Zhang b, Tengwei Xu b, Yuanyuan Xie b and Can Jin *b
aCollaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P. R. China
bCollege of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310014, P. R. China. E-mail: jincan@zjut.edu.cn

Received 28th August 2019 , Accepted 19th November 2019

First published on 19th November 2019

Abstract

A novel avenue of C–CN bond construction was facilely achieved via the TBHP-mediated oxidative coupling reaction between quinoxalin-2(1H)-one derivatives and ammonium thiocyanate without any metal catalyst. Applying this protocol, a variety of 3-cyanoquinoxalin-2(1H)-one derivatives were prepared in moderate to good yields with good functional-group tolerance under mild conditions. Additionally, this methodology featured a broad substrate scope, excellent regioselectivity and operational simplicity.

As a common functional group, cyanide plays an important role in natural products, chemistry, biology and medicine.1 Meanwhile, the cyano functional group is highly versatile in organic synthesis, acting as a privileged precursor of carboxyl, carbonyl, amines, amides and other heterocyclic groups.2 Thus, developing convenient and efficient methods for the installation of cyano groups into biological molecules is of great interest. In the past decade, transition metal-catalyzed C–H cyanation with various cyanide sources has been applied to construct C–CN bonds. However, most of the procedures required expensive and complex metals such as ruthenium,3 rhodium4 or palladium.5 Moreover, some cyanide sources involved in the process, like KCN, NaCN, TMSCN, CuCN and Zn(CN)2,6 are highly toxic. Although several mild cyanating agents have been developed for C–H cyanation such as diaminomaleonitrile,7 aryl cyanates,8 tosyl cyanide,9 cyanobenziodoxole,10 aryl(cyano)iodonium triflates,11N-cyano-N-phenyl-p-toluenesulfonamide,12tert-butyl isonitrile,13 cyanocarbonyls14 and Zhdankin's reagent,15 these cyanide sources should also be prepared from highly toxic precursors, and most of them were demonstrated to be easily decomposed. Therefore, the development of novel, environmentally friendly and metal-free C–H cyanation using low toxic and easily available cyano sources on a broad substrate scope is still highly desirable.

The quinoxalin-2(1H)-one skeleton is a crucial structural motif since it is endowed with significant biological activities and outstanding pharmaceutical properties such as anti-angiogenesis, antitumor, antidiabetic, anti-inflammatory, antimicrobial and aldose reductase inhibitor activities.16 The direct C–H functionalization at the C-3 position of available quinoxalin-2(1H)-ones has served as an ideal and powerful method to afford C-3 substituted quinoxalin-2(1H)-ones. Consequently, much effort has been made in the preparation of quinoxalin-2(1H)-one derivatives,17 including C-3 arylation,18 alkylation,19 acylation,20 amidation,21 amination,22 benzylation23 and phosphonation24 (Scheme 1, a). Despite the significance of these investigations, to the best of our knowledge, the synthesis of quinoxalin-2(1H)-ones bearing a cyano substituent at the C3 position via direct C–H bond functionalization has not been studied. With our ongoing studies on developing green methods for C–H functionaliztions,25 herein, we report a TBHP-mediated oxidative C–H cyanation of quinoxalin-2(1H)-ones utilizing inexpensive and low toxicity ammonium thiocyanate as the cyanide source under metal-/base-/ligand-free conditions.


image file: c9qo01055f-s1.tif
Scheme 1 Direct C–H bond functionalizations.

In the preliminary experiment, we chose the cyanation of 1-methylquinoxalin-2(1H)-one 1aa as a model reaction to optimize the reaction conditions. The transformation was initially carried out in the presence of TBHP (70% solution in water) in CH3CN at room temperature under a N2 atmosphere by employing ammonium thiocyanate 2 as the cyanide source, and the corresponding product 3aa was obtained, even though in a relatively low yield. Subsequently, a variety of other commonly used solvents such as toluene, DMF, EtOAc, acetone and H2O were then studied, while this transformation could not proceed well in these screened solvents (Table 1, entries 2–6). To our delight, it was observed that adding an appropriate amount of water was beneficial for this reaction, and the yield of 3aa could be sharply increased to 68% when MeCN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) was employed as the solvent (Table 1, entries 7–10). Replacing aqueous tBuOOH with anhydrous tBuOOH resulted in a lower reaction efficiency (Table 1, entry 11). Next, a series of oxidants including O2, oxone, K2S2O8, PhI(OAc)2, PhI(OTf)2 and DTBP were examined, while all of these oxidants completely inhibited this reaction (Table 1, entries 12–17). The effect of the TBHP loading was also screened. As a result, 4.0 eq. was found to be the best choice and the yield could reach 72% (Table 1, entries 18–20). In addition, other cyanide sources such as NaSCN or KSCN were also demonstrated to be suitable “CN” sources for this reaction system (Table 1, entries 21–22). In light of the higher reactivity, NH4SCN was believed to be more suitable as a “CN” source. Subsequently, the effect of temperature was also tested. The experimental results indicated that raising the temperature from room temperature to 50 °C would exhibit a negative effect on this reaction, and the yield of 3aa was decreased from 72% to 58% (Table 1, entry 23). Furthermore, the experiment results revealed that shortening or prolonging the reaction time also failed to give better results (Table 1, entry 24). Thus, it could be concluded that the optimized reaction should be performed in the presence of TBHP (70% solution in water) in a mixed solvent (CH3CN/H2O = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) at room temperature under a N2 atmosphere for 30 h by employing ammonium thiocyanate 2 as the cyanide source.

Table 1 Optimization of the reaction conditionsa

image file: c9qo01055f-u1.tif
Entry Oxidant (equiv.) Solvent Yieldb (%)
a Reaction conditions: 1aa (0.4 mmol), 2 (1.2 mmol), oxidant (1.2 mmol), solvent (3.0 mL), rt, N2 atmosphere, 30 h. b Isolated yield. c Anhydrous tBuOOH (5.5 M solution in decane). d KSCN was added instead of NH4SCN. e NaSCN was added instead of NH4SCN. f 50 °C. g 24 h. h 36 h. NR = no reaction.
1 tBuOOH (3) CH3CN 32
2 tBuOOH (3) Toluene 15
3 tBuOOH (3) DMF Trace
4 tBuOOH (3) EtOAc NR
5 tBuOOH (3) Acetone NR
6 tBuOOH (3) H2O 23
7 tBuOOH (3) CH3CN/H2O (4[thin space (1/6-em)]:[thin space (1/6-em)]1) 44
8 tBuOOH (3) CH3CN/H2O (2[thin space (1/6-em)]:[thin space (1/6-em)]1) 62
9 tBuOOH (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 68
10 tBuOOH (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]2) 42
11c tBuOOH (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 63
12 O2 (balloon) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
13 Oxone (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
14 K2S2O8 (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
15 PhI(OAc)2 (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
16 PhI(OTf)2 (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
17 DTBP (3) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) NR
18 tBuOOH (2) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 54
19 tBuOOH (4) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 72
20 tBuOOH (5) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 67
21 tBuOOH (4) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 52d
22 tBuOOH (4) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 43e
23 tBuOOH (4) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 58f
24 tBuOOH (4) CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) 54g, 66h

With the optimized conditions in hand, a range of quinoxalin-2(1H)-ones were applied to yield the corresponding 3-cyanoquinoxalin-2(1H)-one derivatives. The effects of the N-substituents of quinoxalin-2(1H)-one on this transformation were examined (Table 2). To our delight, the N-substituted quinoxalin-2(1H)-one analogues 1aa–1ao showed good reactivity with ammonium thiocyanate 2 under the optimal conditions to give the anticipated products 3aa–3ao in 57–88% yields. For N-alkyl groups, it was found that the short chain alkyl (methyl, ethyl) and long chain alkyl (octyl) groups were all compatible for this reaction and gave the desired products 3aa–3ac in 68% to 72% yields. In addition, the substrates with N-aryl groups such as phenyl 1ad were also tolerated and gave the cyanated product 3ad in moderate yield.

Table 2 Scope of N-substituted quinoxalin-2(1H)-onesa
a All reactions were performed with 1 (0.3 mmol), 2 (0.9 mmol) and tBuOOH (70% solution in water, 1.2 mmol) in 2 ml of CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) at room temperature under a N2 atmosphere for 30 h.
image file: c9qo01055f-u2.tif

Quinoxalin-2(1H)-ones 1ae–1al containing a phenyl or an ester group on N-alkyl could also undergo this coupling process smoothly to give the corresponding products 3ae–3al in moderate to excellent yields. It was worth noting that the N-protecting group containing an alkenyl or an alkynyl group proved to be well tolerated for this transformation, providing the coupling products 3am and 3an in 66% and 57% yields, respectively, whereas the alkenyl or alkynyl group was not affected during the process. Notably, the unprotected quinoxalin-2(1H)-one 1ao also displayed a high reactivity in the reaction process, furnishing the desired product 3ao in satisfactory yield.

Subsequently, the effects of different substituents on the phenyl ring of quinoxalin-2(1H)-ones were studied, and the results are summarized in Table 3. The experimental results indicated that substrates bearing either electron-donating or electron-withdrawing groups were all compatible with satisfactory efficiency, affording the corresponding products 3ba–3bp in 32% to 81% yields. For example, the electron-donating groups at the 6-positon of the benzene ring, such as methoxy 3ba and t-butyl 3bb, could provide the desired products in 67% and 72% yields. Additionally, quinoxalin-2(1H)-ones containing electron-withdrawing groups such as bromo, chloro, fluoro, ester, acyl and nitro groups at the 6-positon of the benzene ring proceeded smoothly to deliver the corresponding products 3bc–3bh in moderate to good yields (43–78%). Subsequently, quinoxalin-2(1H)-ones with substituents at the 7-position of the benzene ring were also investigated, and the experimental results revealed that the substituted position did not affect the efficiency of the reaction, and 7-substituted quinoxalin-2(1H)-ones also engaged in this reaction efficiently and gave the desired products 3bi–3bl in moderate yields. Moreover, it was noted that the disubstituted quinoxalin-2(1H)-ones containing two electron-withdrawing or electron-donating groups at 6- and 7-positions could also undergo this coupling process well and the expected products 3bm–3bo were obtained in good yields. Finally, 1-methylbenzo[g]quinoxalin-2(1H)-one 1bp was also tested under the standard conditions, and we found that the target product 3bp was obtained in an acceptable yield.

Table 3 Scope of different substituents on the phenyl ring of quinoxalin-2(1H)-onesa
a All reactions were performed with 1 (0.3 mmol), 2 (0.9 mmol) and tBuOOH (70% solution in water, 1.2 mmol) in 2 ml of CH3CN/H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1) at room temperature under a N2 atmosphere for 30 h.
image file: c9qo01055f-u3.tif

Further transformation of product 3aa was investigated to illustrate the applicability of this reaction (Scheme 2). When product 3aa was treated with 5 mol% Pd(OAc)2 and 10 mol% La(OTf)3 in AcOH/H2O (2[thin space (1/6-em)]:[thin space (1/6-em)]1) (3 mL) at 60 °C for 12 h,26 1-methyl-1,2-dihydroquinoxalin-2-one-3-carboxamide 4 was obtained in 72% isolated yield. The experimental results indicated that 3-cyanoquinoxalin-2(1H)-one could be conveniently converted into quinoxalinone-3-carboxamides, which exhibited a wide range of medicinal and biological properties.27


image file: c9qo01055f-s2.tif
Scheme 2 Synthetic applications of 3aa.

In order to study the mechanism of the target product formation, some control experiments were carried out. As a result, it was found that the yield of product 3aa decreased greatly from 72% to 28% when 3.0 equiv. of BHT (2,6-ditert-butyl-4-methylphenol) was applied to a model reaction system (Scheme 3a). We continued to examine the mechanistic studies and found that the addition of 1,1-diphenylethylene (3.0 equiv.) resulted in trace product formation (Scheme 3b).


image file: c9qo01055f-s3.tif
Scheme 3 Control experiment.

These results indicate that a radical mechanism may be involved in this oxidative coupling process.

On the basis of the above-mentioned results and literature reports,28 a plausible catalytic cycle is proposed and depicted in Scheme 4. Initially, 1-methylquinoxalin-2(1H)-one 1aa was oxidized by tBuOOH to generate radical cation A, accompanied by the formation of a hydroxide anion and a tert-butyloxy radical. Subsequently, the cyanide ion that was generated by the oxidation of thiocyanate ions as suggested in eqn (1) was regioselectively added to the C-3 position of intermediate A, affording nitrogen radical intermediate B, which then underwent the single-electron transformation (SET) process to produce nitrogen cation C. Finally, the expected coupling product 3aa was obtained after deprotonation.


image file: c9qo01055f-s4.tif
Scheme 4 Proposed reaction mechanism.

Conclusions

In summary, we have developed a novel and environmentally friendly direct C–H cyanidation of quinoxalin-2(1H)-ones by employing ammonium thiocyanate as the cyanide source under mild conditions. This transformation exhibited an excellent substrate scope and a variety of 3-cyanoquinoxalin-2(1H)-ones with different functional groups were obtained in moderate to excellent yields under the optimized conditions. In addition, the strategy featured room temperature, low toxicity, excellent regioselectivity and operational simplicity, thus affording an attractive approach to 3-cyanoquinoxalin-2(1H)-ones.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21606202). We are also grateful to the College of Pharmaceutical Sciences, the Zhejiang University of Technology and the Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals for the financial help.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qo01055f
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