Access to tetrahydrocarbazoles via a photocatalyzed cascade decarboxylation/addition/cyclization reaction

Abstract

An efficient photocatalyzed decarboxylative coupling of indolepropionic acid NHPI esters with α,β-unsaturated carbonyl compounds has been developed. Various α,β-unsaturated carbonyl compounds including esters, ketones, aldehydes, and amides showed good compatibility, providing structurally diverse tetrahydrocarbazoles in moderate to good yields. This approach features a wide substrate scope and mild reaction conditions. The synthetic value of this method was further demonstrated by the post-transformation of the product into carbazole.

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Introduction

Tetrahydrocarbazole (THC) is one of the most privileged scaffolds that is widely distributed in alkaloids and a number of bioactive compounds, such as ondansetron, flutriciclamide F18, kopsihainanine and so on (Fig. 1).¹ Owing to its wide range of biological activities, scientists have paid a lot of attention to developing novel methods for its preparation.²

Fig. 1 Representative bioactive molecules and natural products possessing the THC framework.

A classic method for the construction of the tetrahydrocarbazole skeleton is Fisher indolisation, which involves the rearrangement of phenylhydrazone under acidic conditions.³ In addition, pre-functionalized indoles tethered to an alkenyl or alkynyl chain at the C2 or C3 position could undergo cyclization in the presence of various transition-metal catalysts to afford tetrahydrocarbazole (Scheme 1a).⁴ An alternative strategy to access tetrahydrocarbazole is via intramolecular Friedel–Crafts alkylation of an indolylaldehyde, indolylalcohol or indolylalkene.⁵ Moreover, several cycloaddition reactions have been developed for the preparation of tetrahydrocarbazole.⁶ Although these methods are able to successfully synthesize THC backbones, most of them suffer from the disadvantages of harsh reaction conditions, cumbersome reaction steps, and narrow substrate scopes.

Scheme 1 Methods for the synthesis of tetrahydrocarbazoles.

Visible-light photoredox catalysis is a novel technology with potential and application prospects that has been developed in recent years, and it has been widely used in the fields of organic synthesis, environmental treatment and energy conversion.⁷ In 2022, Han's group⁸ reported a visible-light-mediated method for the synthesis of sulfonylalkyl substituted THC derivatives from indole tethered alkenes and sulfonyl chlorides, which proceeded via a sequence of reduction, radical addition and intramolecular cyclization (Scheme 1b). Although it provided an efficient and green route for the synthesis of tetrahydrocarbazole derivatives, a cumbersome synthetic step was required to prepare the functionalized indole substrates, which increased the complexity of the operation.

Recently, great progress has been made in photocatalyzed decarboxylative coupling reactions involving N-hydroxyphthalimide (NHPI) esters, which can be easily prepared by the condensation of N-hydroxyphthalimide with alkyl carboxylic acids.⁹ NHPI esters have been considered as alkyl radical precursors that participate in various transformations to construct C–X bonds.¹⁰ However, the application of NHPI esters in tandem reactions to construct privileged scaffolds has rarely been reported. In continuation of our interest in photoredox reactions,¹¹ herein, we reported an efficient photocatalyzed decarboxylative coupling reaction between indolepropionic acid NHPI esters and α,β-unsaturated carbonyl compounds, involving decarboxylation, radical addition and cyclization (Scheme 1c). This method features mild reaction conditions and a wide substrate scope, affording a series of tetrahydrocarbazoles in moderate to good yields.

Results and discussion

This visible-light-promoted tandem decarboxylation/cyclization reaction study was carried out with N-benzylindolepropionic acid NHPI ester 1a and methyl methacrylate 2a as the model substrates. After a detailed screening of the reaction conditions (see the ESI† for details), we were pleased to find that the target product 3a was produced in 75% yield by the coupling of 1a and 2a with fac-Ir(ppy)₃ as the photocatalyst and DIPEA (N,N-diisopropylethylamine) as the base in DCM at room temperature for 24 h (Table 1, entry 1). The replacement of fac-Ir(ppy)₃ with Ir[dF(CF₃)(ppy)]₂(dtbpy)PF₆ or Ir[(ppy)₂(dtbpy)]PF₆ led to reduced yields (entries 2 and 3). When the reaction was performed in MeCN, product 3a was obtained in only 33% yield, accompanied by the formation of an uncyclized byproduct (entry 4). Other solvents such as THF and DCE displayed lower reaction efficiency (entries 5 and 6). Screening of various bases including TEA, K₂CO₃ and Cs₂CO₃ (entries 7–9) demonstrated that DIPEA was the optimal base. Control experiments suggested that a photocatalyst, a base, and blue light irradiation were indispensable for the reaction, as no product was formed in their absence (entries 10–12). Besides, the yield of product 3a dropped sharply when the reaction was carried out under an air atmosphere (entry 13).

Table 1 Optimization of the reaction parameters^a

Entry	Deviation from standard conditions	Yield^b (%)
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol, 2.0 equiv.), DIPEA (0.2 mmol, 2.0 equiv.), fac-Ir(ppy)3 (5 mol%), DCM (1.0 mL), r.t., 24 h, Ar. b ^1H NMR yield, using 4-methylbenzophenone as the internal standard. c Isolated yield.
1	None	75 (67)^c
2	Ir[dF(CF3)(ppy)]2(dtbpy)PF6 instead of fac-Ir(ppy)3	21
3	Ir[(ppy)2(dtbpy)]PF6 instead of fac-Ir(ppy)3	27
4	MeCN instead of DCM	33
5	THF instead of DCM	23
6	DCE instead of DCM	30
7	TEA instead of DIPEA	21
8	K2CO3 instead of DIPEA	24
9	Cs2CO3 instead of DIPEA	15
10	No fac-Ir(ppy)3	N.R.
11	No DIPEA	N.R.
12	No blue LEDs	N.R.
13	Air	25

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Having established the optimal reaction conditions, the scope of this photocatalyzed decarboxylative cyclization reaction was explored with respect to the indolepropionic acid NHPI esters (Scheme 2). Indole derivatives bearing electron-donating groups such –Me (3f) and –OMe (3b–3d and 3g) on the benzene ring were amenable to this reaction, providing the corresponding tetrahydrocarbazole products in 53%–66% yields. The position of the functional groups had no significant effect on the reaction (3b–3d). Indole derivatives substituted with electron-withdrawing functional groups (–CO₂Me and –Br) were also tolerated, although decreased yields were obtained (3e and 3h). Subsequently, indole derivatives with different substituents on the nitrogen atom were evaluated. Methyl (3j), ethyl (3k), allyl (3l), and even a free H (3e, 3i and 3o) showed good compatibility, delivering the corresponding products in moderate to good yields. Unfortunately, the N-Boc substituted indole derivative failed to generate the target product (3m) and the decarboxylative protonation byproduct was detected. Moreover, secondary indolepropionic NHPI esters underwent this decarboxylative cyclization reaction smoothly, affording tetrahydrocarbazoles in 61% (3n) and 79% (3o) yields, respectively.

Scheme 2 Substrate scope of indole derivatives. Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol, 2.0 equiv.), DIPEA (0.4 mmol, 2.0 equiv.), fac-Ir(ppy)₃ (5 mol%), DCM (1.5 mL), Ar atmosphere, 30 W blue LED irradiation for 24 h. Isolated yields.

Furthermore, various α,β-unsaturated carbonyl compounds were examined under the optimized reaction conditions, and the results are summarized in Scheme 3. The methyl group at the α-site of the carbonyl could be replaced with a hydrogen (4a) or a hydroxymethyl (4b) group. α,β-Unsaturated esters derived from aliphatic alcohols such as benzyl (4a), isopropyl (4c), and allyl (4d) alcohols were well tolerated. Aromatic unsaturated esters were also effective, providing the functionalized tetrahydrocarbazole products in moderate to good yields (4e–4j). The electronic effects of the substituents on the benzene ring did not significantly affect the reaction, as both electron-donating (–methylthio 4f, –methylenedioxy 4g, and –acetylamino 4i) and electron-withdrawing (–ester 4h and –trifluoromethyl 4j) functional groups were compatible. The replacement of the benzene ring with naphthalene was feasible and gave the target product 4k in 68% yield. Coumarin bearing α,β-unsaturated lactone (4l) was successfully coupled with the indolepropionic acid NHPI ester, although a relatively lower yield was obtained. Besides, α,β-unsaturated aldehyde, ketones, and amides could participate in this visible light-mediated decarboxylative cyclization reaction, delivering the corresponding tetrahydrocarbazole products in yields ranging from 38% to 87% (4m–4r). Unfortunately, styrene and vinylcyclohexane were not compatible.

Scheme 3 Substrate scope of α,β-unsaturated carbonyl compounds. Reaction conditions: 1j (0.2 mmol), α,β-unsaturated carbonyl compounds 2 (0.4 mmol, 2.0 equiv.), DIPEA (0.4 mmol, 2.0 equiv.), fac-Ir(ppy)₃ (5 mol%), DCM (1.5 mL), Ar atmosphere, 30 W blue LED irradiation for 24 h. Isolated yields.

To demonstrate the practicality of this photocatalyzed tandem decarboxylation/cyclization reaction, a scaled-up experiment was conducted. The reaction of 1j with 2o at the 2.0 mmol scale under the standard conditions afforded the tetrahydrocarbazole 4o in 73% yield (Scheme 4a). Besides, the obtained products could undergo different transformations (Scheme 4b). For instance, carbazole 5 was generated from compound 4mvia dehydrogenation with Pd/C.¹² Compound 3j was oxidized to tetrahydrocarbazole ketone 6 at the C4 position with DDQ (2,3-dichloro-5,6-dicyanobenzoquinone) in 61% yield.¹³

Scheme 4 Scaled-up experiment and transformation of the products.

To further understand the reaction mechanism, several mechanistical studies were conducted. Addition of TEMPO (2,2,6,6-tetramethylpiperidine oxide) to the reaction under the standard conditions completely inhibited product formation, and only 21% yield was obtained when BHT (2,6-di-tert-butylphenol) was added (Scheme 5a). These experiments demonstrated a radical mechanism for this photocatalyzed decarboxylation/cyclization. Next, a light on–off experiment was carried out, and it was found that product 3o was formed only under constant irradiation, suggesting that the reaction took place via a catalytic radical pathway rather than a radical chain propagation pathway (Scheme 5b). According to the fluorescence quenching experiments, the excited-state photocatalyst fac-Ir(ppy)₃* was quenched by NHPI ester 1a (Scheme 5c).

Scheme 5 Mechanistic studies.

Based on the above mechanistic studies and previous reports,¹⁴ a possible mechanism was proposed for this visible light-initiated decarboxylative radical reaction (Scheme 6). First, fac-[Ir(ppy)₃] A is irradiated with blue LED light to give the excited-state *fac-[Ir(ppy)₃] B. Then, single electron transfer (SET) reduction of indolepropionic acid NHPI ester 1j (E^red = −1.23 V vs. SCE in CH₃CN) by the excited-state photocatalyst B (E^IV/*^III = −1.73 V vs. SCE in CH₃CN)^14b takes place to generate the alkyl radical F, with the release of the phthalamide anion E and CO₂. Subsequently, the addition of the alkyl radical F to compound 2 forms the radical intermediate G, which undergoes intramolecular cyclization at the C-2 position of the indole moiety to give intermediate H. Subsequently, intermediate H is oxidized by Ir^IVC to produce the carbon cation I, which undergoes deprotonation to afford the corresponding tetrahydrocarbazole product 3 or 4.

Scheme 6 Proposed mechanism.

Conclusions

In summary, we have developed a novel photocatalyzed decarboxylative coupling reaction between indolepropionic acid NHPI esters and α,β-unsaturated carbonyl compounds. This sequential decarboxylation/alkylation/cyclization procedure provided a facile strategy to access structurally diverse tetrahydrocarbazoles in moderate to good yields. Notably, this method features a wide substrate scope, as α,β-unsaturated esters, ketones, aldehydes and amides were well tolerated. The applicability of this method was further demonstrated by the transformation of the product into carbazole.

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

This project was supported by the National Natural Science Foundation of China (No. 22101126) and the Natural Science Foundation of Hunan Province (No. 2024JJ5323).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01357c

‡ These authors contributed equally to this work.

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