Electrochemically promoted diamidation of alkenes to dihydroimidazole skeleton

Abstract

The dihydroimidazole framework is a valuable heterocyclic compound widely found in natural products, ligands, molecular catalysts, pesticides, etc. The construction of a dihydroimidazole framework from alkenes is an attractive synthesis approach. Herein, we report a simple and environmentally friendly electrocatalytic diamination reaction of alkenes. The reaction involves constructing a dihydroimidazole framework from readily available alkenes, CH₃CN, and H₂O. CH₃CN serves as a solvent and a nitrogen source, whereas H₂O provides an oxygen source without the need for catalysts and oxidants. The synthesized 3,4-dihydroimidazole compound can be further hydrolyzed into vicinal diamines. As far as we know, the synthesis of dihydroimidazole frameworks through electrochemical Ritter-type diamination of alkenes has not been reported yet.

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Introduction

The 1,2-diamine motif is widely present in natural products, pharmaceutical compounds, and molecular catalysts (Fig. 1).¹ Difunctionalization of alkenes² with amination reagents, which is one of the most straightforward and practical strategies³ for assembling scaffolds, has achieved remarkable progress⁴ in the past few decades. However, these reported methods often require the utilization of transition metal reagents and even stoichiometric amounts of osmium or cobalt.^4a,5 Moreover, the use of stoichiometric amounts of chemical oxidants (e.g., iodine(III) reagents or organic peroxides)^4d,6 or azide reagents^4b,c,7 raises costs and environmental and safety issues, whereas the use of strong oxidizers may produce environmentally hazardous by-products and pose an explosion hazard when used with azide sources, particularly for large-scale applications.⁸ In addition, they are often limited in substrate scope and sometimes require special amination reagents (e.g., diaziridinone and its analogs^4e,9 or azido-iodine compounds).^4c Therefore, developing highly efficient, mild, and green synthetic strategies is still desirable, and an additional substrate combination for the 1,2-diamine motif should be found to address the aforementioned drawbacks.

Fig. 1 Dihydroimidazole-based drugs and bioactive molecules.

Electrochemistry offers an environmentally friendly, gentle, and efficient alternative to traditional chemical methods of redox conversion.¹⁰ In the alternative method, common organic starting materials can lose or gain electrons at the electrode surface without the need for an external redox agent; thus, highly reactive intermediates are readily produced. Electrochemistry allows the precise external control of chemoselectivity and flux of reactive intermediates by regulating the applied current or potential. Compared with the energy efficiency in traditional chemical methods, that in the alternative method is maximized because of the latter's precise external control. Recently, electrochemical anodic oxidation has shown great potential in the construction of a 1,2-diamine motif.

For instance, Xu and his colleagues¹¹ reported an electrochemical reaction of synthesizing 1,2-diamine compounds with diastereoselectivity using aryl alkenes and sulfonamides mediated by tris (2,4-dibromophenyl) amine. Lin's group^7b has also made remarkable progress in the electrocatalytic diazidation of alkenes (Scheme 1). Despite the remarkable progress made above, issues such as limited types of nitrogen sources and 1,2-diamine structures may exist. Dihydroimidazole frameworks are a class of important nitrogen-containing heterocyclic compounds¹² with the structural characteristics of two nitrogen atoms and two sp³ C–H bonds, which can be easily synthesized by 1,2-diamination of alkenes in theory. However, electrochemical synthesis of dihydroimidazole has not been reported because of the lack of well-matched nitrogen sources, alkenes, and electrochemical conditions.

Scheme 1 1,2-Diamination of alkenes.

Herein, we report a simple and environmentally friendly electrocatalytic diamination reaction of alkenes. The reaction involves constructing a dihydroimidazole framework from readily available alkenes, CH₃CN, and H₂O. CH₃CN serves as a solvent and a nitrogen source,¹³ whereas H₂O provides an oxygen source without the need for catalysts and oxidants. The synthesized 3,4-dihydroimidazole compound can be further hydrolyzed into vicinal diamines. As far as we know, the synthesis of dihydroimidazole frameworks through electrochemical Ritter-type diamination of alkenes has not been reported yet.

Results and discussion

We used 2-phenyl-1-propene (1a) as the model substrate to establish the optimal conditions for this reaction by optimizing the reaction conditions. Under the constant current condition of 8 mA, the reaction proceeded smoothly in a three-necked flask with Et₄NBF₄ as the electrolyte, reticulated vitreous carbon (RVC) electrode as the anode, and Ni foam electrode as the cathode. The required product 2a (Table 1, entry 1) was obtained with an 82% yield. The yield decreased when the cathode was replaced with platinum or nickel sheets (Table 1, entries 2 and 3), whereas the yield of 2a was 59% when the cathode and anode were replaced with platinum sheets (Table 1, entry 4). However, the yield of 2a was only 35% when the cathode and anode were replaced with carbon rods (Table 1, entry 5). Increasing or decreasing the current would decrease the reaction activity (Table 1, entries 6 and 7). Only trace amounts of product 2a were detected when Et₄NCl, Et₄NBr, and Et₄NI were used as electrolytes (Table 1, entry 8). The reaction could not proceed without current (Table 1, entry 9). In summary, the optimal conditions established are as follows: RVC as anode, foam nickel electrode as cathode, Et₄NBF₄ (4 equiv.) as electrolyte, extra dry CH₃CN and H₂O as solvent (extra dry CH₃CN = 6 mL; H₂O = 5 μL), and reaction at room temperature with a constant current of 8 mA for 3–6 h.

Table 1 Optimization of the reaction conditions^a

Entry	Variation from standard conditions	Yield^b^,^c (%)
a Standard conditions: RVC (100 PPI, 1 cm × 1 cm × 1.2 cm) anode, Ni foam (1 cm × 2 cm × 0.5 cm) cathode, extra dry CH3CN and H2O as solvent (extra dry CH3CN = 6 mL, H2O = 5 μL), undivided cell, constant current = 8 mA, 1a (0.2 mmol, 1 equiv.), Et4NBF4 (0.8 mmol, 4 equiv.), and room temperature. b Isolated yields. c Unless otherwise specified, extra dry CH3CN with a water content of ≤10 ppm was used.
1	None	82
2	RVC (+)|Pt (−) instead of RVC (+)|Ni foam (−)	68
3	RVC (+)|Ni (−) instead of RVC (+)|Ni foam (−)	61
4	Pt (+)|Pt (−) instead of RVC (+)|Ni foam (−)	59
5	C (+)|C (−) instead of RVC (+)|Ni foam (−)	35
6	10 mA instead of 8 mA	64
7	6 mA instead of 8 mA	62
8	Et4NCl, Et4NBr, and Et4NI instead of Et4NBF4	Trace
9	No electricity	0

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Under optimal conditions, the substrate range was explored. The results are shown in Schemes 2 and 3. First, the model substrate 2-phenyl-1-propene (1a) presented the target product in 82% yield, and it could obtain the target product with 76% (2b), 71% (2c), 73% (2d), 72% (2e), and 61% (2f) yields when the benzene ring was para-carried with groups such as fluorine, chlorine, bromine, trifluoromethyl, and nitro groups. When halogen groups such as fluorine and bromine groups exist in the meta position of the aromatic ring, the target product can be obtained with yields of 83% (2g) and 66% (2h), respectively. When a methyl group exists in the meta position of the aromatic ring, the target product can be obtained with a yield of 55% (2j). However, when the CH₃CN was replaced with butyronitrile, the target product (2k) could be obtained in 61% yield. When halogen groups such as chlorine are found in the adjacent position of the aromatic ring, the target product can be obtained with a yield of 65% (2i). When the R¹ group is replaced with ethyl, propyl, or phenyl, the yield of the target product gradually decreases as the steric hindrance increases. The target product can be obtained with yields of 74% (2l), 66% (2m), and 61% (2n). In addition, electrochemical methods can be used to modify natural products, such as L-menthol, 1-adamantane methanol, and 1-cyclododecanol, affording the corresponding derivatives (2o–2q) in 40%–55% yields.

Scheme 2 The range of alkenes with substituents at the α-position. Reaction conditions: RVC (100 PPI, 1 cm × 1 cm × 1.2 cm) anode, Ni foam (1 cm × 2 cm × 0.5 cm) cathode, undivided cell, constant current = 8 mA, 1a (0.2 mmol, 1 equiv.), Et₄NBF₄ (0.8 mmol, 4 equiv.), extra dry CH₃CN and H₂O as solvent (extra dry CH₃CN = 6 mL, H₂O = 5 μL); electrolysis was carried out at room temperature under air atmosphere.

Scheme 3 The range of alkenes without substituents at the α-position. Reaction conditions: RVC (100 PPI, 1 cm × 1 cm × 1.2 cm) anode, Ni foam (1 cm × 2 cm × 0.5 cm) cathode, undivided cell, constant current = 8 mA, 3a (0.2 mmol, 1 equiv.), Et₄NBF₄ (0.8 mmol, 4 equiv.), extra dry CH₃CN and H₂O as solvent (extra dry CH₃CN = 6 mL, H₂O = 5 μL); electrolysis was carried out at room temperature under air atmosphere.

When we replace 2-phenyl-1-propene (1a) with styrene (3a), the target product can still be obtained with a 75% (4a) yield. On the benzene ring of styrene, a series of different substitutions can be successfully carried out for electrochemical 1,2-diamination reactions, including fluorine, chlorine, bromine, trifluoromethyl, and methyl. Alkenes with 2,6-disubstituted benzene rings are also suitable substrates (4i). We also confirmed the structure of (4j) through X-ray diffraction analysis. In addition, the electrochemical reaction of 1,2-diamination is compatible with chloromethyl groups (4e). After switching to styrene, the yield is generally slightly lower than that of alkenes with substituents at the α-position. The possible reason is that when substituents exist at the α-position, stabilizing carbocations is easy, resulting in a high electrochemical reaction activity for 1,2-diamination. This finding is consistent with our speculated mechanism.

We conducted a gram-scale conversion of the electrocatalytic diamination reaction of alkenes under constant current conditions of 80 mA to demonstrate further the synthetic application of this electrochemical method. The reaction of alkene 1a with CH₃CN and H₂O was completed at a scale of grams, and the target product 2a (0.71 g) was obtained with a 41% yield (Scheme 4a). In addition, 1,2-diamine compounds are important synthetic units, such as drug intermediates, catalysts, and ligands. Therefore, synthesizing 1,2-diamine compounds from readily available alkenes is important. We dissolved 0.2 mmol 2a in potassium hydroxide, methanol, and ethylene glycol, and it was heated under reflux. After the reaction was completed, the diamine compound 2aaa was obtained with a 67% yield. In addition, 4e can be used to modify BOC Glycine, providing the corresponding derivative with a yield of 45% (4ee).

Scheme 4 Electrochemical applications.

We conducted several control experiments to gain a deep understanding of the reaction mechanism (Scheme 5). When 5 equiv. BHT and TEMPO are added to the reaction, only trace amounts of product 2a are detected, indicating that the reaction may involve the free radical pathway (Scheme 5a). We conducted an H₂¹⁸O labeling experiment to verify the source of oxygen in product 2a. When H₂¹⁸O (5 μL) is added to the reaction, the results show that the ¹⁸O labeled product 2aa can be detected by HRMS analysis (Scheme 5b and Fig. S2†). The results indicate that H₂O may provide oxygen atoms for the formation of the target product 2a. Finally, we conducted radical clock experiments using compounds 5a and 6a. Products 5aa and 6aa can be detected by HRMS analysis, indicating that the reaction is undergoing a free radical pathway (see ESI† for additional details).

Scheme 5 Control experiments.

We also conducted cyclic voltammetry analysis on compound 1a. The test results show that an irreversible oxidation peak is observed at 2.45 V (relative to Ag/AgCl) in the cyclic voltammogram of 1a, indicating that 1a may be oxidized at the anode (RVC).

On the basis of the above experimental results and previous literature reports,¹⁴ we proposed a possible mechanism (Scheme 6). 2-Phenyl-1-propene 1a is oxidized at the anode (RVC) to form alkene radical cation I, which is captured by the CH₃CN to form intermediate II. Then, intermediate II loses an electron to intermediate III. Intermediate III is captured by a molecular CH₃CN to form intermediate IV, which then forms a carbocation intermediate. After hydrolysis, it forms amide product VIII, which is then cyclized to obtain the target product 2a. This process is essentially a Ritter reaction,¹⁵ with the difference being the methods and synthetic precursors for generating carbocations.

Scheme 6 Possible reaction mechanism.

Conclusions

We have developed a simple and environmentally friendly electrochemically driven 1,2-diamination reaction of alkenes, which is an effective method for constructing 1,2-diamine framework compounds from readily available alkenes, CH₃CN, and H₂O. The reaction does not require additional catalysts and oxidants, and the raw materials are readily available. We hope that this protocol can provide a complementary way for dihydroimidazole compounds and be widely applied in modern synthetic chemistry and pharmaceutical research.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Guangxi Natural Science Foundation of China (2021GXNSFFA220005), National Natural Science Foundation of China (22061003, 22301048), the Natural Science Foundation of Guangxi Normal University (No. 2021JC004) for financial support.

References

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2368464. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01445f

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