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DOI: 10.1039/D4GC03057E (Paper) Green Chem., 2024, Advance Article

Radical cascade synthesis of γ-amino acids or γ-lactams via carboxyl-mediated intramolecular C–H amination†

Tao Huang, Can Liu, Pan-Feng Yuan, Tao Wang, Biao Yang, Yao Ma and Qiang Liu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China. E-mail: liuqiang@ lzu.edu.cn

Received 24th June 2024 , Accepted 13th August 2024

First published on 15th August 2024

Abstract

The γ C–H amination of carboxylic acid presents a promising and sustainable strategy for synthesizing high-value pharmaceutical chemicals. Radical reaction pathways initiated by aroyloxy radical-involved hydrogen atom transfer (HAT) provide diverse but challenging opportunities for remote C–H functionalization. In this report, the first example of intramolecular γ C–H amination of carboxylic acids using a commercially available oxime auxiliary has been achieved. This innovative approach employs a radical relay chaperone, facilitating selective C–H functionalization via 1,5-HAT/radical cross-coupling and enabling the net incorporation of ammonia at the γ carbon of carboxylic acids. In addition, this protocol enables the recycling of the by-product benzophenone, and both product isolation and by-product recycling are silica gel-free. The reactions offer high chemo- and regio-selectivities, operate under mild reaction conditions, boast a broad substrate scope, exhibit good functional group compatibility, and are easily scalable.

Introduction

In recent years, it has been discovered that amino acids serve not only as cell signaling molecules but also as regulators of gene expression and the protein phosphorylation cascade.1 Additionally, the amino acid market size was estimated at USD 34.39 billion in 2023 and USD 36.59 billion in 2024, and is expected to grow at a CAGR of 6.63% to reach USD 53.93 billion by 2030 (CAGR, compound annual growth rate).2 In contrast to the ubiquitous α-amino acids, γ-amino acid derivatives are emerging as promising drug carriers due to their biocompatibility, degradability, and multifunctionality.3 And γ-aminoalkyl benzoic acid has been discovered to exhibit therapeutic properties in the treatment of conditions including weakness attacks, hypokinesia, clinical disorders, neurogenic disorders, depression, anxiety, panic, neuropathic pain, neuropathological disorders, and sleep disorders4 (Fig. 1a). The lactam motif is also widely employed as one of the most commonly utilized structural units in drugs, natural products, and functionalized materials.5 The γ-lactam motifs have been found to possess therapeutic properties in the treatment of various conditions, including inhibitors of Mnk1 and Mnk2,6 modulators of chemokine receptors,7 inhibitors of DYRK1A,8 and agonists of dopamine receptors9 (Fig. 1a).
image file: d4gc03057e-f1.tif
Fig. 1 (a) Medicinal prevalence of γ-amino acids and γ-lactams. (b) Previous strategies for intramolecular C–H activation mediated by carboxyl groups. (c) This work: visible-light-induced radical cascade synthesis of γ-amino acids or γ-lactams via intramolecular C–H amination mediated by carboxyl groups.

From the perspective of atom economy and sustainable chemistry, direct γ C–H amination is one of the worthiest research approaches for γ-amino acid preparation, yet it remains unresolved and requires further investigation. While considerable advancements have been achieved in synthesizing amino acids (primarily analogues or precursors), these traditional synthetic routes to γ-amino acids are constrained by tedious synthesis steps and a limited substrate scope, significantly reducing the synthesis process's efficiency.10 The reported Ritter-type aminations require the production of benzyl carbocations under oxidative conditions (using oxidants or anodic oxidation), which can then be nucleophilically attacked by nitrile compounds acting as amine sources. While benzyl carbocations with simple aliphatic carbon chains favor the Ritter reaction, more stable benzyl carbocations containing arenes resist the attack of nitrile compounds.11 Moreover, the yields of these reactions are typically modest, and the processes necessitate excessive amounts of oxidants, such as Selectfluor. Furthermore, it is noteworthy that Chang's group developed dioxazolones through transition-metal-catalyzed C–H amidation reactions.12 These metal-nitrene species (Ir, Ru, Fe, Ni, Cu, Co, Rh, etc.) exhibit high chemo- and enantioselectivity, operate under mild conditions, accommodate a wide range of substrates, and demonstrate excellent compatibility with various functional groups. Without a doubt, the pursuit of a green and practical strategy to obtain a wide range of γ-amino acids from abundant feedstock, ideally under mild conditions, represents a highly desirable yet challenging goal in organic synthesis.

Carboxylic acids are omnipresent in organic chemistry, playing crucial roles as essential bulk chemicals, fundamental synthetic building blocks, and components of natural products.13 Although conjugate addition to α,β-unsaturated carbonyl compounds continue to be a strategic disconnection for introducing β- or γ-substituents, the past two decades have seen rapid advancements in the functionalization of C(sp3)–H bonds mediated by carboxylic acids.14 These advancements have facilitated the formation of C–C and C–X (where X represents a heteroatom) bonds, surpassing the capabilities of conventional methods. The major challenge involves the development of reactions that demonstrate consistent selectivity in complexly functionalized environments, particularly within frequently encountered structural motifs.15 Although most metal-catalyzed reactions effectively modulate the reaction site through a cyclic transition state at the β C–H position of the carboxyl group (usually involving the formation of a cyclic transition state between a transition metal and a carboxylic acid),16 the remote C–H functionalization of the carboxylic acid (typically, γ C–H bonds) has remained unresolved and requires further research14d,17 (Fig. 1b). Iodine reagent18 and electrochemically19 and visible-light-induced photoredoxcatalyst20 initiated carboxyl oxygen radical population offers an attractive approach for γ C–H functionalization via intramolecular H-atom transfer, despite the strategies being limited to the synthesis of lactones (Fig. 1b). Therefore, there is a need to develop new radical reactions to achieve remote C–H functionalization of carboxylic acids.

Oxime esters are emerging as the first-line building blocks in modern heterocyclic chemistry because they are accessible and can be easily prepared from a simple reaction of easily available oximes with a carboxylic acid and their derivatives.21 Nevertheless, carboxyoxime ester chemistry primarily targets bifunctionalization reactions22 and decarboxylation radical coupling reactions.23 In this report, we present a novel radical relay chaperone strategy,24 whereby a carboxylic acid undergoes transient conversion to an oxime ester, facilitating intramolecular HAT by an aroyloxy radical to synthesise γ-amino acids and γ-lactams (Fig. 1c). This regioselective 1,5-HAT now serves as the first example for γ C–H amination of carboxylic acid – by harnessing energy transfer catalysis [Ir(dFppy)2(phpzpy)]PF6 (Ir-1)25 – to drive selective radical generation. The main challenges of this reaction (the rate of intramolecular 1,5-HAT of aroyloxy radicals – k = 2.7 × 107 s−1)26 stem from two aspects: the aroyloxy radicals may add to other aryl rings to produce ester by-products (k = 106–108 M−1 s−1),27 or undergo decarboxylative coupling reactions (k = 1.4 × 106 s−1),28 which have been extensively studied. In this work, both the process of obtaining the desired γ-amino acids and the recovery of the by-product benzophenone are silica gel-free and highly advantageous for scale-up, aligning well with the principles of green chemistry.

Results and discussion

The development of this γ C–H amination is shown in Table 1. Crucially, the carboxyoxime ester is easily installed onto benzoic acids in a single step by combination with commercially available diphenylmethanone oxime (1 equiv.) with an excellent yield.29 After the visible-light-driven radical relay chaperone is completed, obtaining the corresponding C–H amination products via acidification with an HCl solution is easy.30 We initiated our project with diphenylmethanone O-(2-benzylbenzoyl) oxime S1 as a standard substrate to generate 2-(amino(phenyl)methyl)benzoic acid hydrochloride 1 as the desired product (Table 1). In the presence of [Ir(dFppy)2(phpzpy)]PF6 (Ir-1) as a photocatalyst in ethyl acetate (EA) with irradiation (λmax = 395 nm), the desired γ C–H amination product 1 was obtained in 82% yield with high selectivity and almost no other decarboxylation and addition by-products (entry 1). A variety of reaction conditions with other photocatalysts, solvents and light sources were also tested to give lower conversions and yields (entries 2–10). Control experiments revealed that photocatalysts and visible light played essential roles in the reaction (entries 11–13). Having established the optimized reaction conditions, we investigated the substrate scope (Fig. 2). A broad range of electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) exhibited tolerance at the meta- or para-positions of the arene moiety, leading to the synthesis of the desired products 1–16 with moderate-to-excellent yields. Substrates incorporating diverse functional groups, including the trifluoromethoxyl (4), fluoro (6, 8 and 14), and bromo (7) groups, underwent smooth conversion to their corresponding products. This successful transformation facilitated subsequent downstream manipulations. Encouragingly, the efficiency of the protocol remained unhindered by the presence of an aromatic heterocyclic ring (17 and 18), resulting in the corresponding γ C–H amination products in good-to-excellent yields. Substrates with different substituents on the aliphatic chain were also suitable for such a C–H amination, furnishing products 19–39 in 67–99% yields. Notably, a simple methyl group located at the O-position of the arene moiety was also compatible with the reaction, affording the products (19 and 23) in good yields. A particularly noteworthy outcome was the successful 10 mmol scale reaction of S23, delivering the desired pain agent 23 in a 70% yield.4 We were delighted to find that substrate S27 containing the tetrahydronaphthalene moiety could also occur under such conditions, giving the γ C–H amination product 27 in 99% yield. Moreover, aliphatic chains incorporating alkenyl groups (28), heteroatoms (30), and aryl groups (24–26, 32–34 and 39) smoothly participated in the reaction, yielding the desired γ C–H amination products in good-to-excellent yields. The γ-lactam moiety is a common structural motif present in a variety of biologically active natural products and pharmaceuticals,5 and seeking an efficient and simple approach for the construction of indolines is of continuous interest. It is worth mentioning that Lei's group reported a novel approach for synthesizing N-acetyl γ-lactams using an electrochemical oxidative strategy, with o-alkylbenzoic acid and alkyl nitrile serving as the nitrogen source.11b Encouraged by the above results, we further turned our attention to the selective synthesis of NH γ-lactams via carboxyl-mediated intramolecular C–H amination, as depicted in Fig. 3. We observed that mono-substituents on the aromatic ring had minimal impact on these reactions, leading to the successful formation of NH γ-lactams 40–47 with satisfactory yields. Encouragingly, the thiophene ring also underwent the reaction smoothly, providing NH γ-lactam 48. Furthermore, substrates with aliphatic chains were suitable for this transformation, yielding products 49–51 in the 56–77% yield range. Impressively, this system also facilitated the conversion of tertiary C–H substrates (52–63) to produce spiro-NH lactams containing quaternary carbon centers in yields of 63–87%.
image file: d4gc03057e-f2.tif
Fig. 2 Synthesis of γ-amino acids via carboxyl-mediated intramolecular HAT. The reactions were carried out with S1 (0.2 mmol) and Ir-1 (1 mol%) in EA (4 mL) under Ar and irradiation (λmax = 395 nm) at room temperature for 3 h, and then stirred with HCl (2 N) and Et2O at rt for 2 h. a The reaction was performed on a 5 mmol scale. b The reaction was performed on a 10 mmol scale.

image file: d4gc03057e-f3.tif
Fig. 3 Synthesis of γ-lactams via carboxyl-mediated intramolecular HAT. After irradiation, the reaction was stirred in THF (2 mL)/H2O (50 equiv. 180 μL) and TFA (1.0 equiv.) for 12 h at room temperature.
Table 1 Optimization of the reaction conditionsa

image file: d4gc03057e-u1.tif
Entry Variations as shown Yield of 1b
N.D. = not detected. r.t. = room temperature.a Unless otherwise noted, the reactions were carried out with S1 (0.2 mmol) and Ir-1 (1 mol%) in EA (4 mL) under Ar and irradiation (λmax = 395 nm) at room temperature for 3 h, and then stirred with HCl (2 N) and Et2O at rt for 2 h.b Isolated yields.c The reaction was irradiated at 450 nm.d 5 mol% PC was used.
1 None 82%
2c Ru-1 instead of Ir-1 N.D.
3c Ir-3 instead of Ir-1 N.D.
4d Thioxanthone instead of Ir-1 73%
5 Ir-2 instead of Ir-1 57%
6 The reaction was carried out at 80 °C 61%
7 DCM instead of EA 60%
8 THF instead of EA 68%
9 CH3CN instead of EA 44%
10 MeOH instead of EA 49%
11 w/o Ir-1 N.D.
12 w/o light N.D.
13 w/o light, and carried out at 80 °C N.D.

To gain deeper insights into the underlying mechanisms of this reaction, a series of control experiments were meticulously carried out, as illustrated in Fig. 4. First, when the reaction was performed in the presence of the radical scavenger 2,2,6,6-tetramethyl-1-piperidiny-1-oxy (TEMPO), the formation of product 1 was completely inhibited; TEMPO-adducts 65 and 66 and radical–radical cross-coupled byproduct 67 were detected by high-resolution mass spectroscopy. These results clearly hinted at the radical nature of the reaction and also demonstrated the generation of iminyl radicals (Fig. 4a). Second, radical probe experiments using S64 formed the corresponding aroyloxy radical-mediated ring-closed product 68 in a 73% yield, showing the involvement of aroyloxy radicals in this annulation reaction (see ESI 3.1.2†). Moreover, radical clock experiments under the standard reaction conditions using S65 formed the corresponding ring-opened product 69 in a 25% yield, suggesting the participation of benzyl radicals formed from the γ-position of aroyloxy radicals via 1,5-intramolecular HAT (Fig. 4b). Since both iminyl radicals and aroyloxy radicals are critical intermediates in the reaction, the homolysis of O–N bonds of oxime esters is possible. Subsequently, experiments involving light on/off and quantum yield measurement (Φ = 0.24, see ESI 3.1.4†) indicated the unlikelihood of an extended radical chain mechanism (Fig. 4c). To thoroughly understand the nature of the interaction, a comparison of various photocatalysts with differing properties was conducted (Fig. 4d). As a result, the yields of product 1 correlate to the triplet state energy of photocatalysts while they are unrelated to the redox properties.31 These results implied that an EnT process is likely to have been operational in the reaction. Furthermore, cyclic voltammetry measurement was conducted. As shown in Fig. 4e, no obvious reduction peak of S1 was observed before −1.4 V versus SCE, which means that the compound could not be reduced by *Ir-1 image file: d4gc03057e-t1.tif. Therefore, the thermodynamic feasibility of a single-electron transfer (SET) reduction of S1 by the excited Ir-1 was also excluded by means of cyclic voltammetry. The observed inhibition effect with typical triplet-state quenchers such as stilbene or oxygen provides further support for an energy transfer mechanism32 (see ESI Table 3,† entries 3 and 4).


image file: d4gc03057e-f4.tif
Fig. 4 Mechanistic investigations. (a) TEMPO trapping experiment. The reaction of S1 was completely inhibited by using TEMPO as a radical scavenger. (b) Radical clock experiment. S65 was converted into the ring-opening compound 69 in 25% yield. (c) Light on/off experiment. (d) Comparison of different photocatalysts for 1. An EnT process is likely to have been operational in the reaction. (e) Cyclic voltammetry measurements. No obvious reduction peaks of S1 were observed before −1.4 V versus SCE, which means that S1 could not be reduced by *Ir-1 image file: d4gc03057e-t2.tif. N.D. = not detected.

A feasible mechanism is proposed upon consolidating the aforementioned findings, as depicted in Fig. 5. The reaction starts with an interaction between S1 and the excited Ir-1 through a photo-induced EnT process to generate the excited S1*. The triplet energy of S1 (ET = ∼46 kcal mol−1)21b is sufficient to undergo triplet–triplet EnT with an excited Ir-1 (ET = 62.8 kcal mol−1). Then, S1* undergoes N–O bond cleavage to form a transient aroyloxy radical and a persistent iminyl radical (III).


image file: d4gc03057e-f5.tif
Fig. 5 Proposed mechanisms.

Based on the persistent radical effect,33 the transient carboxyl oxygen radical predominantly abstracts a hydrogen atom from the δ-position, resulting in the generation of a C-centered radical (IV), followed by a radical–radical coupling process with the iminyl radical to yield the desired products of γ C–H amination (V). Through further transformations, diverse products can be selectively obtained. Treatment with HCl (2 N) yields the corresponding γ-amino acid hydrochloride salt (VI), while reacting with TFA yields the corresponding γ-lactam (VII).

As depicted in Fig. 6, subsequent experiments on benzophenone recovery further demonstrated the industrial applicability of our reaction, achieving a recovery yield of 91% based on the generated benzophenone oxime (see ESI 2.8†).


image file: d4gc03057e-f6.tif
Fig. 6 Recycling of the benzophenone.

Conclusions

In summary, we have illustrated a method for conducting carboxyl-mediated intramolecular C–H amination to synthesize high-value pharmaceutical γ-amino acids or NH γ-lactams. This is achieved through the use of a commercially available oxime auxiliary. Our approach involves the reaction with [Ir(dFppy)2(phpzpy)]PF6 to facilitate selective radical generation, leading to C–H functionalization via an EnT process. The efficient hydrogen atom transfer (HAT) is crucial in overcoming decarboxylation processes. This protocol enables the recycling of the by-product benzophenone and is driven by visible light under redox-neutral conditions. Furthermore, product isolation and benzophenone separation do not require column chromatography, aligning with sustainable chemistry principles. We believe this method represents an operationally simple, effective, and practical route to γ C–H amination of carboxylic acids in academic research and industry.

Author contributions

Q.L. directed the project. T.H. conceived and performed the experiments. C.L., P.-F.Y., T.W. and B.Y. helped with data collection. Q.L. and T.H. wrote the paper. All authors contributed to the scientific discussion.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and its ESI.† Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Center under the deposition number CCDC 2336872 (60).† Extra data are available from the authors upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (No. 22171120) and the Natural Science Foundation of Gansu Province of China (22JR5RA472).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2336872 (60). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4gc03057e
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