Electrocatalytic conversion of ω-azido carboxylic acids to 1-pyrrolines via a combined process of oxidative decarboxylation and intramolecular Schmidt rearrangement

Abstract

1-Pyrrolines constitute a significant group of organic intermediates, and we present here an electrocatalytic method for their synthesis. Electrochemical oxidation of ω-azido carboxylic acids followed by an intramolecular Schmidt reaction was explored, and 26 1-pyrrolines were produced in up to 93% yield under an air atmosphere and in an undivided cell, where no external oxidants or catalysts were employed.

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Introduction

N-Heterocycles are the fundamental units in a vast majority of natural products, medicinal molecules and probe materials,^1–6 and they confer their unique functionalities. Generally, compounds containing the 1-pyrroline skeleton have been reported to have significant antiviral, anti-infective and antimicrobial effects in pharmaceutical research (Fig. 1).^7–10 Therefore, many strategies for the synthesis of 1-pyrrolines have been developed by synthetic chemists, and the common methods include the intramolecular cyclization of bifunctional compounds and intermolecular multicomponent cycloaddition^11–14 and [3 + 2] cycloaddition of 1,3-dipoles and alkenes or cyclopropanes (the Huisgen cycloaddition reaction).^15–19 The intramolecular Schmidt rearrangement of alkyl azides provides an alternative route to 1-pyrrolines,²⁰ although only moderate yields were observed in early studies.

Fig. 1 Examples of the biological activity of 1-pyrrolines.

The Schmidt rearrangement involves the reaction of an azide with an electrophile,^21–24 proceeding a 1,2-migration of an alkyl or an aryl group, or H to the electron-deficient nitrogen atom with the release of nitrogen gas. Generally, amines, nitriles, amides, or imines are produced from the reaction, and the corresponding products depend on the type of electrophile. The carbocation derived from an alkene or an alcohol has been explored as the electrophile for the rearrangement by Pearson's group,^25–27 and later the reaction was employed for the preparation of 2-aryl-1-pyrroline by Molina's group (Scheme 1A),²⁰ where excess TfOH was used as the promoter at 0 °C and 1-pyrrolines were isolated in only 30%–72% yields. To avoid the toxic reagents and harsh conditions, organic electrosynthesis would be a green and sustainable choice.^28–31 The intermolecular Schmidt reaction of an azide with a carbocation from electrocatalytic C–H oxidation under an argon atmosphere with a constant current of 20 mA has been reported by Jiao and coworkers (Scheme 1B),³² and the aromatic amines derived from the hydrolysis of the rearrangement product were isolated in 13%–91% yields, where the arenes containing electron-withdrawing functional groups resulted in the amines with only up to 35% yield. The decarboxylation reaction is widely used and is also commonly used in the preparation of amine compounds.³³ It has been reported that the Hofer–Moest decarboxylation reaction of carboxylic acids can efficiently produce carbocations.^34–37 Following our ongoing research on the Schmidt reaction of ω-azido carboxylic acids,^38–46 we envisaged that an ω-azido carboxylic acid could be oxidized to a carbocation under electrocatalytic conditions and the subsequent capture of the carbocation by an intramolecular azide would be reasonable. Herein, we would like to report the positive results of the research on the combined process of the Hofer–Moest reaction and intramolecular Schmidt reaction for the synthesis of 1-pyrrolines.

Scheme 1 Schmidt reactions.

Results and discussion

5-Azido-2-phenylpentanoic acid 1a was used as a standard substrate to explore the suitable conditions for the combined process (Table 1). After many screening experiments (see the ESI† for more details), 91% yield of 2-phenyl-1-pyrroline 2a was obtained as the best result, where the electrolysis of substrate 1a was conducted with a constant current of 4.5 mA in a solution of nBu₄NPF₆ and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) at room temperature under an air atmosphere (entry 1). For the reaction, dimerization of the radical intermediate from the initial decarboxylation under the electrocatalytic conditions might be the main competitive reaction,⁴⁷ so the anode material should be the critical factor affecting the product distribution. Different anode materials were screened (entries 2 and 3), and as expected the graphite electrode demonstrated the best performance. Previous reports revealed that the graphite anode exhibited strong adsorption of radical intermediates and thereby promoted their further efficient oxidation to carbocations.^48,49 Several organic bases were examined, and similar results were obtained with DBU, 2,4,6-collidine and pyridine (entries 4 and 5). HFIP was identified as the preferred solvent, and other solvents all failed to give a better conversion. 54% yield of 2-phenyl-1-pyrroline was isolated from the reaction performed in DCM, while no desired product could be obtained when methanol was employed, and only (4-azido-1-methoxybutyl)benzene from the interception of the carbocation with methanol was produced in 96% yield. Here, HFIP also very likely acted as the acid promoter for the subsequent rearrangement reaction of azides with carbocations, and it has been used for promoting the Schmidt reaction of alkyl azides with ketones by Aubé's group.⁵⁰ Furthermore, the screening experiments on electric currents showed that slightly lower yields of 2a would result from either reducing or increasing the operating current (entries 8 and 9). As an unexceptional result, no reaction took place without electricity (entry 10). Finally, the use of Me₄NPF₆ and LiClO₄ as the electrolyte could not improve the conversion (entries 11 and 12).

Table 1 Optimization of the reaction conditions^a

Entry	Variation from standard conditions	Yield^b (%)
a Reaction conditions: IKA ElectraSyn 2.0 Pro, undivided cell, graphite anode and cathode, 1a (0.1 mmol), HFIP (3 mL), nBu4NPF6 (0.1 M), DBU (0.15 mmol), constant current = 4.5 mA, rt, under air, 110 min, faradaic efficiency = 57%. b ^1H NMR yield using 1,3,5-trimethoxybenzene as an internal standard. c Isolated yield. d The product is (4-azido-1-methoxybutyl)benzene, isolated yield = 96%.
1	None	91 (87)^c
2	Pt(+)/C(–) instead of C(+)/C(–)	10
3	GC(+)/C (–) instead of C(+)/C(–)	40
4	2,4,6-Collidine instead of DBU	84
5	Pyridine instead of DBU	87
6	MeOH instead of HFIP	0^d
7	DCM instead of HFIP	54
8	4 mA instead of 4.5mA	79
9	5 mA instead of 4.5mA	52
10	Without electricity	0
11	Me4NPF6 instead of nBu4NPF6	61
12	LiClO4 instead of nBu4NPF6	5

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With the optimized reaction conditions available (Table 1, entry 1), we explored the scope and generality of 5-azido-2-arylpentanoic acids (Table 2). Many functional groups were tolerated due to the mild conditions used here, and the yields of the 2-aryl-1-pyrrolines ranged from 35% to 93%. It seemed that the electrical properties of the substituents at the para position of the aromatic ring did not affect the conversion significantly except for 2-(p-Ph-Ph)-1-pyrroline 2g with 47% yield, whereas 2-(p-Me-Ph)-1-pyrroline 2b, 2-(p-^tBu-Ph)-1-pyrroline 2e, 2-(p-MeO-Ph)-1-pyrroline 2h, 2-(p-F-Ph)-1-pyrroline 2m, 2-(p-Cl-Ph)-1-pyrroline 2n, 2-(p-Br-Ph)-1-pyrroline 2q and 2-(p-CF₃-Ph)-1-pyrroline 2t were all generated with high efficiency and in yields ranging from 79 to 93%. 2-(p-MeS-Ph)-1-pyrroline 2f was produced in only 54% yield, and this could be due to the electrocatalytic oxidation of the thioether.^51–54 The steric hindrance might have a strong influence on the reaction, and the pyrrolines with substituents on the meta or/and ortho positions all showed lower conversion yields. If the meta position of the aromatic ring was substituted, the pyrrolines were generally afforded in good yields (67% for 2c, 61% for 2i, 54% for 2o, and 64% for 2r), and only moderate yields were obtained with the pyrrolines where the ortho position was occupied (59% for 2d, 37% for 2p, and 35% for 2s). More substituents on the aryl ring means more site resistance, and the yield of the product will also be reduced (49% of 2j, 46% of 2l, 75% of 2u, 61% of 2v, and 50% of 2w); the pyrroline 2k with three methoxyl groups on the aryl ring was delivered with an isolated yield of 38%. Furthermore, the naphthyl pyrroline 2x could be obtained in 32% yield. The reaction of the 2-thiophyl substrate was examined, and pyrroline 2y was produced in 30% yield, and the low conversion efficiency may be due to the fact that thiophene is easily oxidized under electrolysis conditions.⁵⁵ A secondary azide was tested, and the desired pyrroline 2z was isolated in 45% yield, suggesting that the Schmidt reaction of an azide with a carbocation is sensitive to the steric hindrance. A scale-up reaction of 1a (2.0 mmol) was also carried out, affording 2a in 70% yield after 220 minutes of electrolysis.

Table 2 Substrate scope of the electrocatalytic Schmidt rearrangement reaction^a,b

a Standard reaction conditions, see the ESI† for detailed routes. b Yield of a 2 mmol scale reaction.

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Furthermore, the conversion of the product was investigated (Scheme 2). According to the literature, we converted 2a into pyrrole 3 by adding Pd/C to the reaction using xylene as the solvent (Scheme 2a).⁵⁶ Treatment of 2e with m-chloroperbenzoic acid resulted in C=N epoxidation, and the bicyclic oxaziridine 4 was afforded in 67% yield (Scheme 2b).⁵⁷ Enamide 5 was prepared in a yield of 94% by reacting the cyclic imine 2e with trifluoroacetic anhydride (Scheme 2c).⁵⁸ Finally, the reaction of 2e with benzaldehyde dimethyl acetal was studied with the assistance of TiCl₄ and triethyamine,⁵⁹ and it gave 6 in a satisfactory yield of 92% (Scheme 2d).

Scheme 2 Late-stage transformations.

Based on the experimental results and relevant literature, a proposed reaction mechanism is illustrated in Scheme 3. Initially, the deprotonated azido carboxylate I, which was obtained after the reaction of 1a with DBU, underwent a first oxidation at the anode followed by decarboxylation to give the benzyl radical II. The benzyl radical then experienced the second oxidation process at the anode, converting it into the benzyl carbocation III. Subsequently, the intramolecular addition of the azide onto the carbocation resulted in the formation of the aminodiazonium ion IV. Finally, the elimination of protons, along with the loss of nitrogen, gave pyrroline 2a as the ultimate product.

Scheme 3 Proposed reaction mechanism.

Conclusions

In summary, we have designed and realized the electrocatalytic Schmidt Rearrangement reaction of ω-azido carboxylic acids. This conversion combines the processes of electrocatalytic oxidative decarboxylation and intramolecular Schmidt rearrangement to produce 1-pyrroline compounds in moderate to good yields. Notably, HFIP has a facilitating effect on the Schmidt reaction. Other side reactions of electrode carboxylation were avoided and the reaction was carried out without external oxidants and catalysts, providing a mild and economical alternative to previous strategies.

Author contributions

This manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. K. Yang designed and performed the experiments. R. Li participated in designing the project and data discussion. P. Gu directed the project and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No 22061035) and the Natural Science Foundation of Ningxia Province (No 2023AAC02005 and 2022AAC03112).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo00273c

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