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. 2022 Mar 29;2(4):306–311. doi: 10.1021/acsorginorgau.2c00008

Visible-Light-Mediated (sp3)Cα–H Functionalization of Ethers Enabled by Electron Donor–Acceptor Complex

Sourav Roy 1, Indranil Chatterjee 1,*
PMCID: PMC9955270  PMID: 36855592

Abstract

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A synthetically beneficial visible-light-mediated protocol has been disclosed to achieve C–H amination of readily available feedstocks cyclic and acyclic ethers. A rarely identified N-bromosuccinamide–tetrahydrofuran electron donor–acceptor complex served as an initiator to functionalize both α-diazoketones and dialkyl azodicarboxylates. This developed methodology gives an alternative and milder way to construct the C–N bond and can be explored for the formation of C–C bond to perform arylation and allylation reactions.

Keywords: (sp3)Cα−H amination, ethers, N-bromosuccinamide, EDA complex, hydrazonation, radical chain process

Introduction

Visible light has served as a robust tool in sustainable synthesis, operative via radical pathways under metal or metal-free conditions, enriching the chemistry of radical-based transformations.1,2 Photoinduced excitation of organic molecules can drive remarkable transformation, generally inaccessible via conventional thermal routes. Among light-driven protocols, one of the most valuable techniques is the electron donor–acceptor (EDA) complex strategy due to its economic and synthetic utilities in modern-day chemistry.3−5 The photoexcited EDA complex can break into a radical anion and radical cation, leading to various C–H and C–X bond functionalization.

Likewise, selective (sp3)Cα–H functionalization of ethers has also gained tremendous attention in recent years. Traditionally, functionalization of (sp3)Cα–H bonds requires external catalysts, additives, elevated temperature, extended reaction time, or excess synthetic oxidant, possessing low functional group tolerability and selectivity, and affords the product moderately.6−13 Readily available feedstocks like cyclic and acyclic ethers are widely used to form α-oxyalkyl C-radicals that can be trapped by various acceptors. This strategy can be applied over the construction of eminent C–N bonds, offering an elegant synthetic approach to forge N-functionalized molecules.14−16

Over the years, diazo compounds remained as the prime building block in synthetic chemistry. Several methodologies have been developed by using these compounds as carbene precursors, C-nucleophiles, and 1,3-dipoles in the presence of metal or under photochemical conditions.17−22 However, we are interested to use the diazo compound as the N-terminal electrophile to forge the C–N bond. The typical example for this type of reactivity is the Japp–Klingemann reaction, where β-ketoesters or acids were allowed to react with aryldiazonium salts, forging the hydrazone moiety.23 In 2006, the Takamura and Zhao groups reported the use of Grignard reagents as a nucleophile with diazo esters.24−26 Following this, some valuable reports came into the picture, giving novel access to various hydrazonation reactions.27−31

In contrast, Nikolaev et al. demonstrate photosensitizer guided and UV-light mediated insertion of α-C–H bond of tetrahydrofuran (THF) into the terminal N-atom of the diazo group, suffering from a mixture of products with limited substrate scope (Scheme 1a).32,33 In recent years, seminal work was done by the Yu and Xu groups individually with photoredox catalysis. Concurrently, two reports from the Lu group demonstrate the exploitation of aryl diazoacetates to synthesize hydrazone moieties under cobalt catalysis.34−36

Scheme 1. Metal-Free Radical-Based C–H Amination Reactions Using Diazo Compounds and Azodicarboxylates.

Scheme 1

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Similar to diazo compounds, dialkyl azodicarboxylates were also employed for radical-based amination reactions.37−41 In 2020, the Chudasama and Kokotos groups individually reported HFIP mediated and CFL lamp-promoted C–H amination, respectively (Scheme 1a).42,43 Very recently, Laulhé et al. were able to functionalize the (sp3)Cα–H bond of aliphatic cyclic ethers in the presence of visible light and traces of oxygen (Scheme 1a).44

Despite the previous methods mentioned above, there is still considerable room to develop new radical-based C–H amination techniques in an eco-friendly and atom-economical fashion, excluded from any strong oxidants, bases, or metal catalysts, and in a cheaper way with sound potential importance. Our literature survey did not offer any report that dealt with EDA-guided, metal-free, photosensitizer, or photocatalyst-free conditions to functionalize (sp3)Cα–H bonds of ethers by utilizing diazo compounds and dialkyl azodicarboxylates as radical acceptor under visible light irradiation.

On our ongoing interest in seeking different reactivities of diazo compounds under photochemical conditions, herein, we devised a catalytic amount of N-bromosuccinamide (NBS) as an inexpensive and efficient promoter in (sp3)Cα–H bond functionalization of ethers. This resulted in synthesizing a wide range of hemiaminal ethers where α-diazoketones and dialkyl azodicarboxylates served as an amination reagent (Scheme 1b) guided by catalytic EDA system.45,46

Results and Discussion

Based on the previous literature,32,33 we hypothesized that an α-diazo-β-keto compound might act as a good N-terminal acceptor. Therefore, we geared our investigation by choosing α-diazo-β-keto ester 1a as a model substrate in THF (0.5 M) 2a at room temperature to commence the visible-light promoted C–H amination protocol. Here, THF can serve as both the solvent and α-oxyalkyl C-radical precursor under aerobic conditions (Table 1, entry 1). However, the formation of no product indicates that a radical initiator is required to trigger this process. Next, we screened several radical initiators (see the Supporting Information (SI) for more information, section 4.1) and NBS (1.0 equiv) gives the best result to furnish aminated product 3a in 72% yield (Table 1, entry 2). Variation in the solvent concentration was also checked, suggesting 0.1 (M) THF to be the best-fitted concentration for this transformation (Table 1, entries 2–5). We were pleased to find that using a catalytic amount of NBS (20 mol %) instead of a stoichiometric one does not alter the reaction yield significantly (Table 1, entry 6). Reaction times and other light source screenings indicate that blue-LEDs (450 nm) irradiation for 16 h remained more potent over others (Table 1, entries 7–11). In comparison, a decrease or increase in the reaction temperatures led to a decrease in the yield of 3a (Table 1, entries 12 and 13). No conversion for the reaction in the dark at room temperature and at high temperature (60 °C) suggests that the light source is essential for this reaction (Table 1, entries 14 and 15).

Table 1. Optimization Study and Reaction Setupa.

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entry 2a (M) NBS (mol %) light source time yield %b
1c 0.5 0 450 nm 16 h 0
2 0.5 100 450 nm 16 h 72
3 0.25 100 450 nm 16 h 75
4 0.05 100 450 nm 16 h 62
5 0.1 100 450 nm 16 h 85
6 0.1 20 450 nm 16 h 82 (78)g
7 0.1 20 450 nm 24 h 77
8 0.1 20 450 nm 10 h 55
9 0.1 20 420 nm 16 h 60
10 0.1 20 405 nm 16 h 51
11 0.1 20 365 nm 16 h 32
12d 0.1 20 450 nm 16 h 48
13e 0.1 20 450 nm 16 h 75
14 0.1 20 in dark 16 h 0
15f 0.1 20 in dark 16 h 0
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a

Unless specified, reactions were performed on a 0.1 mmol scale under argon atmosphere by taking freshly distilled and deoxygenated THF.

b

Measured by 1H NMR analysis of the crude reaction mixture using dibromomethane as an internal standard. Isolated yield in parentheses.

c

Reaction was performed under aerobic conditions.

d

Reaction was carried out at 10 °C.

e

Reaction was performed at 45 °C.

f

Reaction was performed at 60 °C.

g

Reaction performed on a 2.0 mmol scale gives 62% (360 mg) isolated yield of 3a.

After establishing the optimal reaction conditions (Table 1, entry 6), we investigated the generality of this C–H amination protocol between α-diazo-β-keto compound 1 and THF 2a (Scheme 2). Under visible light, the electron-donating substituent and electron-withdrawing substitutions on the aryl ring, such as methyl, halogen, and cyano groups with meta- and para-substituted ones, smoothly furnished the product (Scheme 2, entries 3b–3f) with moderate to good yield and moderate Z/E isomeric ratio. Moreover, an aryl ring containing two methoxy substitutions at the meta- and para-positions delivered the desired product 3g in a 48% yield. Next, amide and heterocycle substituted diazo compounds were investigated, which form the corresponding hydrazones 3h–3j, with moderate yields. Symmetrical acyclic and cyclic α-diazo-β-keto compounds also lead to a facile transformation to achieve compounds 3j–3l in good yield. Although acceptor/acceptor–diazo compounds react well under these reaction conditions, donor/acceptor–diazo compounds such as α-aryldiazoacetates and α-aryldiazoketones remained incompatible. We reasoned this might be due to, under blue LEDs, α-aryldiazoacetates being prone to undergo dinitrogen extrusion to generate the carbene intermediate, which cannot take part in the reaction.19−21 Next, several cyclic and acyclic ethers (2b–2f; see SI section 3.3) were tested as α-oxyalkyl C-radical precursors, but unfortunately, they also remained unreactive under this reaction condition.

Scheme 2. Substrate Scope for Amination from Diazo Compounds.

Scheme 2

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Unless specified, reactions were performed on a 0.1 mmol scale under argon atmosphere by taking freshly distilled and deoxygenated THF. Yields refer to isolated ones. Z/E ratio was measured by 1H NMR analysis of the crude reaction mixture using dibromomethane as an internal standard.

Reactions carried out for 10 h.

To check the versatility of the developed protocol, we envisioned whether dialkyl azodicarboxylates could act as a radical acceptor to forge the C–N bond.37−44 To our delight, THF radical was successfully trapped by diisopropylazodicarboxylate (4a, see SI) under blue LEDs irradiation to deliver amine product with moderate yield. A series of light sources and reaction times were then screened to check the best-fitted one (See SI for the optimization table, section 4.4). We found purple LEDs are the most efficient to promote this reaction, which gives 91% yield of 5a. Here, azodicarboxylates substituted with different alkyl partners furnished the corresponding amines with good yield (Scheme 3, entries 5a-5d). Subsequently, a range of cyclic and acyclic ethers participated in this transformation (Scheme 3, entries 5e-5g). In addition, S-heterocycle and N-heterocycle were also successfully employed with high efficiency (Scheme 3, entries 5h and 5i).

Scheme 3. Substrate Scope for Amination from Azodicarboxylates.

Scheme 3

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Unless specified reactions were performed on a 0.1 mmol scale under argon atmosphere by taking 4 (1.0 equiv) and 2 (1.0 mL). Yields refer to isolated ones.

Reaction performed on a 2.0 mmol scale gives 80% (440 mg) isolated yield of 5a.

10.0 equiv of 2 was used in 0.1 mL of benzene.

We executed several control experiments to shed light on the plausible reaction pathways of this visible-light-driven transformation. The reaction between 1a and dry-deoxygenated THF 2a without NBS, under blue LEDs irradiation and oxygen atmosphere, delivered no product 3a (Scheme 4a). Reaction under aerobic conditions and use of AR grade THF 2a (Scheme 4b) did not provide the desired product. The yield of 3a drops significantly when performing the reaction between 1a and AR grade THF 2a in the presence of NBS, under argon atmosphere, or aerobic conditions in the presence of blue LEDs (Scheme 4c, d). Moreover, reactions under dark conditions did not offer any product formation (Scheme 4e). These results suggest that NBS, blue LED irradiation, and entirely inert conditions are necessary to facilitate this transformation. Next, when diisopropyl azodicarboxylate 4a was allowed to react with dry-deoxygenated THF 2a under purple LED irradiation and oxygen atmosphere in the absence of NBS, the reaction gives traces amount of 5a. However, on increasing the reaction time (5 min to 24 h), a considerable amount of product 5a (45%) was obtained. Reactions performed by taking AR grade THF 2a without NBS under aerobic condition and argon atmosphere led to the formation of 22% and 25% of 5a, respectively (reaction time 24 h). Similarly, the experiment in the dark suggested the importance of the light source (see the SI for the control experiments, section 6.1).

Scheme 4. Control Experiments.

Scheme 4

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The colored solution of the reaction mixtures encouraged us to check ultraviolet–visible (UV/vis) spectra. A mixture of diazo compound 1a and THF 2a gives an absorbance band near the UV region. Pleasingly, a combination of NBS and THF (dry and deoxygenated) developed a yellow-colored solution. This solution shows a bathochromic shift in the visible spectral region in its optical absorption spectrum, diagnostic of an EDA complex (samples need to prepare in the glovebox to avoid oxygen). In contrast, NBS in acetonitrile does not show any colored solution, and its absorption falls near the UV region. Therefore, a catalytic amount of NBS can act as an acceptor in this system and THF acts as a donor to form the NBS-THF EDA complex (see SI section 6.2, for the UV–visible data).

Next, the existence of a radical intermediate was tested by adding (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) as a radical scavenger to the reaction mixture where THF radical VII was successfully trapped (detected in HRMS, Scheme 4g) and the reaction was completely stopped.

To validate whether the reaction proceeds via a radical chain process, we performed a ‘“light-on/light-off”’ experiment. Irradiating the reaction mixture under blue LEDs for 2 h and then stirring the reaction in the dark for 14 h resulted in 54% of 3a formation (Scheme 4f). A tool to better corroborate the chain process is to measure the reaction’s quantum yield (ϕ) (λ = 405 and 450 nm, using potassium ferrioxalate as the actinometer).47 We performed this measurement for both the amination reactions and found very high quantum yields (ϕ = 142.97 and 902.41 for the synthesis of 3a and 5a, respectively; see SI section 6.4, for the quantum yield calculations).

The above results offer compelling evidence to rationalize the mechanism. Based on the mechanistic experiments and literature precedence,48 we envisioned that the reaction initiated by forming an activated colored NBS-THF EDA complex I. Irradiation of this EDA complex I by visible light (blue or purple LEDs) facilitates the generation of radical cation II and radical anion III. Elimination of bromine radical IV leads to the formation of anionic species V. Intermediate V then abstracts one proton from intermediate II to deliver the THF radical VII. Consequently, the generation of succinimide VI also takes place via path a. This crucial radical intermediate VII then adds to substrate 1 or 4 to forge the C–N bond and fabricate their respective intermediates VIII and IX, respectively. Now, intermediate VIII and IX abstracts a hydrogen atom (HAT) from 2a to facilitate the radical chain process and furnishes the aminated products (3 or 5) as well (Scheme 5). However, as this a radical chain process paths a′, b, and b′, as described in Scheme 5, cannot be discarded as an alternative pathway for the generation of THF radical VII. Here, path b represents the formation of bromide ion IV′and radical intermediate V′ from intermediate III. Intermediate IV′ undergoes a deprotonation step with intermediate II to generate THF radical VII. Similarly, a HAT process between intermediate V′ or IV with 2a leads to the THF radical VII formation via path a′ and path b′, respectively.

Scheme 5. Mechanistic Proposal for C–H Amination.

Scheme 5

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The general applicability of any developed protocol showcases its usefulness. Gratifyingly, the catalytic NBS-THF EDA system also remained potent over some arylation and allylation reactions (Scheme 6). These reactions proceed via a similar mechanism described in Scheme 5 to generate the key THF radical VII. This carbon-centered radical VII then adds to various aryl and allyl sulfones to give the desired THF incorporated product via desulfonylation (Scheme 6, entries 8a, 9a–9d). Additionally, the aryl ring substituted with a halogen atom (e.g., N-Boc 2-chlorobenzimidazole 6b, 2-chlorobenzoxazole 6c; see SI section 3.5) as a leaving group instead of sulfones efficiently provides their corresponding products (Scheme 6, entries 8b and 8c). Here sodium acetate is used as an additive to neutralize in situ generated hydrochloric acid, increasing the reaction yield.49 This desulfonylation process follows an activated nucleophilic substitution reaction via an addition–elimination pathway. The calculated quantum yield (ϕ) for these reactions is 97.58 (for the synthesis of 8a) and 99.85 (for the synthesis of 9a), respectively, which supports the radical chain pathway (see SI section 6.4 for the quantum yield calculations).

Scheme 6. Substrate Scope for Arylation and Allylation Reactions.

Scheme 6

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Unless specified, reactions were performed on a 0.1 mmol scale under argon atmosphere by taking 4 (1.0 equiv), freshly distilled and deoxygenated THF. Yields refer to isolated ones.

Reaction performed on a 2.0 mmol scale gives 59% (240 mg) isolated yield of 8a.

1.0 equiv NaOAc was used as an additive.

Purple LEDs were used instead of blue LEDs.

Reaction performed on a 2.0 mmol scale gives 54% (200 mg) isolated yield of 9a.

Conclusions

In summary, the developed protocol provides a modular method to render a series of amine, C(sp2)–H, and C(sp3)–H alkylating products with various ethers. Visible lights were utilized to carry out this metal-free, oxidant-free C–N and C–C bond-forming event. The generation and identification of the catalytic NBS-THF electron donor–acceptor (EDA) complex were crucial, permitting direct and selective (sp3)Cα–H bond functionalization. A radical chain mechanism is proposed based on the literature and our experimental evidence. It is noteworthy that the developed methodology overcomes significant disadvantages like long reaction time and harsh reaction conditions associated with previous (sp3)Cα–H bond functionalization techniques.

Experimental Details

General Procedure for Amination

In an argon filled glovebox, to an oven-dried vial equipped with a Teflon-coated stir bar, 1 or 4 (0.1 mmol, 1.0 equiv) and N-bromosuccinamide (NBS) (0.02 mmol, 0.2 equiv) were added. Then ether 2 (0.1 M, 1.0 mL) was added to the reaction mixture and sealed. The vial was taken out from the glovebox and then irradiated under blue/purple LEDs for 5 min to 16 h at room temperature. The crude reaction mixture was used to measure the Z/E ratio by 1H NMR if required after completion of the reaction. The solvent was evaporated under reduced pressure. Analytically pure products were obtained by column chromatography on silica gel using petroleum ether and ethyl acetate as eluents.

Acknowledgments

I.C. wishes to acknowledge financial assistance from SERB, DST, India (ECR/2018/000098) and IIT Ropar. S.R. thanks IIT Ropar for his research fellowship.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.2c00008.

  • Experimental procedures, mechanistic evidence, and spectral data for the newly synthesized products (PDF)

The authors declare no competing financial interest.

Supplementary Material

gg2c00008_si_001.pdf (7.8MB, pdf)

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