Photoredox-catalyzed intramolecular oxy- and aminoacylation of alkenes with acyl oxime esters: facile synthesis of acylated saturated heterocycles

Abstract

In the present paper, an efficient photoredox-catalyzed intramolecular oxy- and aminoacylation of internal alkenes equipped with pendant oxygen- or nitrogen-centered nucleophiles with acyl oxime esters is reported. Under visible-light irradiation, a variety of structurally diverse acylated saturated heterocycles, such as tetrahydrofurans, tetrahydropyrans, δ-valerolactones, tetrahydropyrroles, piperidines, tetrahydro-1,3-oxazepines, etc., were efficiently synthesized from easily accessible unsaturated alcohols, carboxylic acids, tosyl-protected amines, and O-homoallyl benzimidates via an acyl radical-mediated cascade acylation/cyclization process. This light-driven transformation features a broad substrate scope, good functional group compatibility, high regio- and diastereoselectivity, and mild reaction conditions, representing a general and practical procedure toward the construction of acylated saturated heterocycles as well as their derivatives.

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Introduction

Functionalized saturated heterocycles are valuable core scaffolds that are ubiquitous in biologically active molecules, pharmaceuticals, agrochemicals, and fine chemicals.¹ Consequently, the development of a practical and general synthetic approach toward the construction of such structural scaffolds is of increasing interest to the synthetic chemistry community. In particular, in recent decades, in the promotion of electrochemically enabled² and photoredox-mediated³ annulation, impressive green and efficient synthetic methodologies toward the preparation of versatile heterocyclic compounds have been developed. Among them, radical-initiated cascade cyclizations of alkenes equipped with pendant heteroatom-containing nucleophiles, which not only allow the synthesis of saturated heterocyclic frameworks but also enable the simultaneous introduction of valuable functional groups via a one-pot process, could provide streamlined and facile synthetic protocols to access structurally diverse heterocycles.⁴ In this regard, in particular, the acyl radical-triggered transformation has emerged as a hot research topic due to the potential derivatization applications of the acyl substituent in heterocyclic compounds.⁵ To date, several common carbonyl compounds including aromatic aldehydes,⁶ carboxylic acids,⁷ anhydrides,⁸ acyl chlorides,⁹ α-ketocarboxylic acids,¹⁰ and other acyl sources¹¹ have been widely utilized in a series of acyl-radical-involved transformations. Notably, Xu,¹² Li,¹³ and other groups¹⁴ successfully developed the acyl radical-driven cyclization reaction of alkenes towards acyl-containing heterocycles. However, despite the impressive advances in this field, it should be noted that these existing methods still face several challenges, for instance, the use of stoichiometric chemical oxidants, poor compatibility of easily oxidizable functional groups under oxidative conditions, and a narrow alkene substrate scope. Undoubtedly, continuous efforts toward the exploration of the acyl radical-involved cyclization of alkenes under mild and environmentally benign conditions to construct structurally varied heterocycles carrying an acyl group are still highly desired.

In 2019, acyl oxime esters were first employed as acyl radical precursors by Wu and coauthors to react with Michael acceptors (i.e., acrylamide) under visible light irradiation, delivering various heterocyclic 2-oxindole derivatives.¹⁵ In this reaction, a visible-light-induced single-electron transfer (SET) from a photocatalyst to oxime esters occurs followed by a fast β-fragmentation, affording acyl radicals for the subsequent transformation. Using this elegant acyl formation approach, such precursors would provide fresh opportunities to expand the potential of acyl radical chemistry with regard to both reactivity and selectivity. Following this seminal work, acyl oxime esters have been widely utilized as acylating reagents for visible-light-mediated difunctionalizations of alkenes to generate diverse linear and cyclic carbonyl compounds (Scheme 1a).¹⁶ It is worth mentioning that most of the reported examples focused on terminal alkenes while the use of internal alkene derivatives still remains underexplored and challenging, probably due to their relatively low reactivity and the difficulty in controlling their regioselectivity and diastereoselectivity. More recently, we successfully developed an efficient electrochemical intramolecular amino- and oxysulfonylation of internal alkenes equipped with pendant nitrogen or oxygen-based nucleophiles, producing a variety of sulfonylated N-heterocycles and O-heterocycles with high efficiency and excellent diastereoselectivity (Scheme 1b).¹⁷ Inspired by our previous work and other pioneering studies, we wondered whether the acyl oxime esters would be amenable for tandem cyclization of internal alkenes bearing inner nucleophiles toward the synthesis of acyl-substituted heterocyclic building blocks. To the best of our knowledge, there is no report on such a tandem reaction of internal alkenes involving the formation of various heterocycles and simultaneous incorporation of valuable acyl groups using a single procedure. Herein, we present a photoredox-catalyzed intramolecular oxy- and aminoacylation of easily accessible internal alkenes, efficiently furnishing structurally diverse acylated saturated heterocycles, such as tetrahydrofurans, tetrahydropyrans, δ-valerolactones, tetrahydropyrroles, piperidines, and tetrahydro-1,3-oxazepines (Scheme 1c).

Scheme 1 Research background and summary of this work.

Results and discussion

We commenced the reaction optimization using the internal alkene substrate S1 and the easily prepared oxime ester (E)-2-(acetoxyimino)-1-phenylpropan-1-one as the model substrates (Table 1). After efforts toward systematically investigating the reaction conditions, mainly including the photocatalyst, solvent, reaction time, gas atmosphere, etc. (see Table S1† for details of screening studies), we determined the reaction using Ir(ppy)₃ (1 mol%) as the catalyst, oxime esters (3.0 equiv.) as the acyl source in tetrahydrofuran under the irradiation of 14 W blue LEDs at room temperature as the standard conditions, which delivered the acylated tetrahydrofuran P1 in 91% NMR yield (Table 1, entry 1). Similar results were obtained when the reaction was conducted in MeCN, acetone, or EtOAc, while the use of MeOH gave a low yield (Table 1, entries 2–5). Other tested photocatalysts, such as Ru(bpy)₃Cl₂, Ru(bpy)₃(PF₆)₂, Eosin Y, thioxanthone, rhodamine B, and 4CzlPN, were all less effective in promoting the expected reaction compared to the original choice of Ir(ppy)₃ (Table 1, entries 6–11). Evaluation of light sources revealed that white light, purple light, red light, and green light could irradiate the reaction to give the expected compound in lower yields (Table 1, entries 12–15). Either O₂ or an air atmosphere could dramatically inhibit the expected transformation and caused the decomposition of the starting material S1 to aromatic aldehyde (Table 1, entries 16 and 17). Furthermore, some control experiments showed that both light and photocatalyst were required for this desired transformation, confirming the nature of visible light catalysis (Table 1, entries 18 and 19).

Table 1 Optimization of the reaction conditions^a

Entry	Variation from the standard conditions	Yield^b (%)
a Reaction conditions: unless mentioned otherwise, all reactions were performed with alkene S1 (0.2 mmol, 1.0 equiv.), (E)-2-(acetoxyimino)-1-phenylpropan-1-one (0.6 mmol, 3.0 equiv.), and Ir(ppy)3 (0.002 mmol, 1 mol%) in 2.0 mL of solvent by using LED irradiation under an argon atmosphere at room temperature. b ^1H NMR analysis of the reaction mixture determined yields using CH2Br2 as the internal standard. c n.d. = not detected.
1	None	91
2	MeCN instead of THF	81
3	Acetone instead of THF	79
4	EtOAc instead of THF	83
5	MeOH instead of THF	45
6	Ru(bpy)3Cl2 instead of Ir(ppy)3	17
7	Ru(bpy)3(PF6)2 instead of Ir(ppy)3	10
8	Eosin Y instead of Ir(ppy)3	12
9	Thioxanthone instead of Ir(ppy)3	14
10	Rhodamine B instead of Ir(ppy)3	8
11	4CzlPN instead of Ir(ppy)3	21
12	White light instead of blue light	74
13	Purple light instead of blue light	65
14	Red light instead of blue light	17
15	Green light instead of blue light	73
16	O2 instead of Ar	41
17	Air instead of Ar	64
18	In the dark	n.d.^c
19	Without Ir(ppy)3	n.d.^c

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With the optimized conditions in hand, we subsequently investigated the substrate scope of this photochemical transformation. The initial focus was on reacting structurally different unsaturated alcohols with (E)-2-(acetoxyimino)-1-phenylpropan-1-one (Table 2). As exemplified by P1–P17, the reaction worked well over a range of 4-aryl-but-3-en-1-ol to give 3-acyl-substituted tetrahydrofurans in 40%–91% yields with excellent diastereoselectivity. Among these internal alkenes investigated, the obvious electronic effect of the substituents on the aryl ring was observed. In general, internal alkenes with electron-donating groups on the phenyl ring could give much higher yields than those bearing electron-withdrawing groups under the standard conditions. The cyano group and halogen substituents on the phenyl ring of unsaturated alcohols were untouched and endowed the heterocycle with possibilities for further derivatization. On replacing the phenyl group of the 1-aryl-substituted alkene substrate with a naphthyl or a furan substituent, the corresponding cyclized products P16 and P17 were obtained in 83% and 86% yields, respectively. Additionally, when 5-phenylpent-4-en-1-ol and 6-phenylhex-5-en-1-ol were subjected to the photoredox-catalyzed reaction with (E)-2-(acetoxyimino)-1-phenylpropan-1-one, they afforded the acylated tetrahydropyrans and oxepanes, albeit in slightly low yields (P18–P20). Moreover, the reaction was also applicable to 1-diaryl-substituted internal alkene substrates and terminal alkene substrates, affording the corresponding compounds with a quaternary carbon center in moderate to good yields (P21–P27). To further illustrate the synthetic utility of this method, a gram-scale experiment was carried out with (E)-5-(4-butoxyphenyl)pent-4-en-1-ol S1 as an example, affording the desired cyclized product P1 in 70% isolated yield.

Table 2 Substrate scope with different unsaturated alcohols^a

a Unless indicated otherwise, all yields are based on the isolated product on a 0.2 mmol scale. b With 1.2 g of the alkene substrate S1.

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To further test the generality of this light-driven cascade acylation/cyclization reaction, diverse acyl oxime esters and internal alkenes bearing different nucleophilic pendants were also examined (Table 3). As exemplified by P28–P36, various aryl and aliphatic acyl precursors could react well with (E)-5-(4-butoxyphenyl)pent-4-en-1-ol, affording the corresponding 3-acyl-substituted tetrahydrofurans in moderate to high yields with excellent diastereoselectivity. As for the aryl acyl subunit of oxime esters, the obvious electronic property and steric effect on aryl substitutions were not observed. Acyl oxime esters bearing electron-donating (p-OMe, m-Me, and o-Me) and electron-withdrawing (p-Br, m-Cl, and o-F) substituents at different positions on the phenyl ring displayed good reactivity to reassemble into the internal alkene. On replacing the phenyl group of the oxime ester substrate with a furan substituent, the corresponding product P35 was obtained in 87% yield. Meanwhile, the structure and relative configuration of compound P35 were unambiguously confirmed by single-crystal X-ray diffraction analysis (CCDC 2286313†). Besides unsaturated alcohols, unsaturated carboxylic acids, tosyl-protected amines, and O-homoallyl benzimidate were also suitable for the photo-catalyzed transformation. These substrates containing 1-aryl-substituted alkene, 1,1-diaryl-substituted alkene, and terminal alkene could react well with (E)-2-(acetoxyimino)-1-phenylpropan-1-one to furnish the corresponding 3-acyl δ-valerolactones (P37–P41), oxepan-2-one (P42), tetrahydropyrroles (P43, P44 and P46), piperidines (P45), and tetrahydro-1,3-oxazepines (P47–P50) in 42%–88% yields.

Table 3 Photoredox-catalyzed synthesis of 3-acyl tetrahydrofurans, tetrahydropyrans, δ-valerolactones, tetrahydropyrroles, piperidines, and tetrahydro-1,3-oxazepines^a

a All yields are based on the isolated yield on a 0.2 mmol scale.

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The synthetic utility of the products was demonstrated by the derivatization of the newly-formed acyl group and a Pd-catalyzed Suzuki coupling reaction (Scheme 2). As shown in Scheme 2a, using some well-developed procedures of ketone transformation, the ketocarbonyl of compound P1 could be easily converted to the oxime (P51), amine (P52), alcohol (P53 and P54), and methylene (P55) group, respectively, in acceptable yields. In addition, the halogenated compound P24 could react smoothly with arylboronic acids via a Pd-catalyzed Suzuki coupling reaction to further extend the carbon chain of heterocycles (Scheme 2b). Importantly, the parent heterocycle is untouched in the above-mentioned transformations, which could endow the photoredox-catalyzed acylation/cyclization with more synthetic potential for the construction of structurally complex and diverse heterocycles.

Scheme 2 Synthetic transformations of acyl heterocycles.

To better understand the reaction process, several mechanism experiments were carried out, as depicted in Scheme 3. First, we observed that the E-alkene S1 could undergo isomerization to give the thermodynamically less stable Z-alkene in an almost quantitative isolated yield under light irradiation (Scheme 3A). When the reaction was performed in 1 h, 2 h, and 24 h under the standard conditions, both the Z-alkene substrate and the E-alkene substrate could deliver the desired product P1 with similar yields and diastereoselectivities. Meanwhile, the use of E-alkene substrate could generate a trace amount of the Z-alkene at the initial stage of the reaction, which was gradually consumed as the reaction progressed (Scheme 3B). These results showed that the isomerization reaction of partial E-alkene followed by subsequent transformation could be involved in the visible light-mediated reaction. Second, when a radical scavenger (TEMPO, BHT, and 1,1-diphenyethylene) was added into the reaction system, all these reactions were dramatically inhibited under the otherwise standard conditions, resulting in a trace amount of compound P1 and the simultaneously delivery of adducts of the acyl radical (Scheme 3C). These control experiments revealed that the acyl radical could be involved in the light-mediated transformation. Next, a light on–off experiment was conducted. It was found that the reaction could not proceed in the absence of light, revealing that the reaction may proceed through a catalytic radical reaction instead of a radical chain pathway (Scheme 3D).

Scheme 3 Mechanistic study.

On the basis of the mechanistic experiments presented above and related studies in this field,^15–17 a possible mechanism for this photoredox-catalyzed intramolecular oxy- and aminoacylation of internal alkenes with oxime esters is proposed, as depicted in Scheme 4. First, under visible-light irradiation, Ir^III(ppy)₃ is excited to the photoexcited catalyst. The latter could not only drive the isomerization of the thermodynamic E-alkene to the less stable Z-isomer via an energy transfer pathway but also reduce the acyl oxime esters (E_1/2 Ir^IV/Ir^III* = −1.73 vs. SCE for Ir(ppy)₃, E^red_1/2 = −1.67 V vs. SCE for aliphatic acyl oxime ester (E)-3-(acetoxyimino)butan-2-one, E^red_1/2 = −1.42 V vs. SCE for aryl acyl oxime ester (E)-2-(acetoxyimino)-1-phenylpropan-1-one)^16avia single-electron transfer to deliver the radical anion I and the Ir^IV intermediate. Second, the fragmentation of the N–O bond in the radical anion I formed the nitrogen-based radical intermediate II along with an acetoxyl anion species, which could serve as a base in the following transformation. The radical intermediate II could undergo fast C–C bond homolysis to generate the acyl radical III and the acetonitrile molecule. Third, the highly active acyl radical III preferentially attacks the C2 position of the E-alkene or the Z-alkene and generates the stabilized benzylic radical IV, which could be further oxidized viaIr^IV(Path A) or an acyl oxime ester (Path B) to the benzylic carbocation V. During the production of intermediate V, the newly-formed photocatalyst Ir^III(Path A) or anion I(Path B) could be used for subsequent transformation. Considering that the reaction could not proceed further in the absence of light in the above light on–off experiment, path B may be ruled out. Next, the benzylic carbocation intermediate V was intramolecularly attacked by the nitrogen or oxygen-centered nucleophiles to give the acyl-substituted saturated heterocycles.

Scheme 4 Proposed reaction mechanism.

Conclusions

In summary, we have developed a photoredox-catalyzed intramolecular oxy- and aminoacylation of internal alkenes equipped with pendant nucleophiles with acyl oxime esters. Using acyl oxime esters as the precursor of acyl radicals and bases, this light-induced cascade acylation/cyclization process proceeds smoothly under mild conditions without stoichiometric amounts of chemical redox reagents and bases. Through the use of this methodology, a series of acylated saturated O-heterocycles and N-heterocycles, such as tetrahydrofurans, tetrahydropyrans, δ-valerolactones, tetrahydropyrroles, piperidines, tetrahydro-1,3-oxazepines, etc., were prepared in moderate to good yields with excellent regio- and diastereoselectivity from easily accessible unsaturated alcohols, carboxylic acids, tosyl-protected amines and O-homoallyl benzimidates. In addition, the gram-scale preparation and derivatization experiments of products demonstrated the practicality of this light-driven method. As a beneficial complement to the construction of functionalized heterocycles, this method features broad substrate specificity, good functional group compatibility, excellent regio- and diastereoselectivity, and mild reaction conditions, and will provide a general and practical procedure for the synthesis of acylated five-to-seven saturated heterocycles as well as their derivatives.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from the Natural Science Foundation of China (22161056), the Guizhou Science and Technology Department (QKHJC-ZK [2021]-037, QKHPTRC-GCC [2022]032-1, QKHPTRC-CXTD [2022]012), the Zunyi Science and Technology Department (ZSKRPT-2020-5, ZSKRPT-2021-5), and the Zunyi Medical University (202210661194, S202210661093, and ZYDC202301036).

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Footnotes

† Electronic supplementary information (ESI) available: Details of the reaction condition screening, experimental procedures, spectra data of products, and single crystal data of product 35. CCDC 2286313. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qo01659e

‡ These authors contributed equally.

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