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Heterocycle-guided synthesis of m-hetarylanilines via three-component benzannulation

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1Department of Chemistry, Perm State University, ul. Bukireva 15, Perm, 614990, Russian Federation
2Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab., 7/9, Saint Petersburg, 199034, Russian Federation
3Research Institute of Chemistry, Рeoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya Street, 6, Moscow, 117198, Russian Federation
  1. Corresponding author email
Associate Editor: N. Yoshikai
Beilstein J. Org. Chem. 2024, 20, 2208–2216. https://doi.org/10.3762/bjoc.20.188
Received 18 May 2024, Accepted 12 Aug 2024, Published 02 Sep 2024
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Abstract

A one-pot three-component synthesis of substituted meta-hetarylanilines from heterocycle-substituted 1,3-diketones has been developed. The electron-withdrawing power of the heterocyclic substituent (which can be estimated on the basis of calculated Hammett constants) in the 1,3-diketone plays a pivotal role in the studied reaction. The series of meta-hetarylanilines prepared (21–85% isolated yield) demonstrates the synthetic utility of the developed method.

Keywords: aniline; benzannulation; condensation; 1,3-diketone; Hammett constants; terphenyl

Introduction

The aniline moiety is omnipresent in the synthetic chemistry with applications ranging from building blocks to catalysis [1-4]. Among the possible substitution patterns, meta-substituted anilines hold a special place. These compounds are difficult to access due to the inherent ortho-/para-directional reactivity of the amino group, at the same time they are widely used in medicinal chemistry, resulting in several marketed drugs (Figure 1). On the other hand, 3,5-diarylanilines can be regarded as meta-terphenyls which are of great interest for material and coordination chemistry [5-16]. Moreover, compounds with diverse bioactivities and natural products contain the meta-terphenyl moiety as a key fragment [17-26].

[1860-5397-20-188-1]

Figure 1: The meta-hetarylaniline motif in bioactive molecules.

The aforementioned features have led to an extensive development of novel methods to access meta-substituted anilines which can be divided into two main strategies (Scheme 1A) [27]. The first strategy focuses on the decoration of the aromatic ring mainly via metal-catalyzed C–N or C–C bond formation. Despite recent advances in the area of remote C–H functionalization, this strategy still requires some pre-functionalization of the starting material or the use of directing groups [28-32]. An alternative strategy is based on aromatic ring formation via benzannulative inter- or intramolecular condensation of acyclic precursors [27,32-37].

[1860-5397-20-188-i1]

Scheme 1: Strategies to access meta-substituted anilines.

Within the latter strategy, [3 + 3] condensations are gaining much attention due to the availability of starting materials and the straightforward installation of the aryl(hetaryl) substituent in the meta-position [38-43]. For example, several methods based on the Michael condensation–oxidation sequence starting from α,β-unsaturated ketones have been described (Scheme 1B) [44-50]. Recently, several methods have been developed based on the interrupted Kröhnke reaction (Scheme 1B) [51,52]. The main step of this process is an intermolecular cyclization of the formed 1,5-diketone followed by aromatization.

Previously we have shown that 1,3-diketones bearing an electron-withdrawing group (EWG) adjacent to one of the carbonyls readily react with in situ-generated acetone imines in a (3 + 3) manner to afford meta-substituted anilines (Scheme 1C) [53,54]. Various EWGs (ester, carbamoyl, ketone, trifluoromethyl) have been successfully employed which motivated us to evaluate other possible EWGs.

Results and Discussion

Based on the fact, that many heterocycles are isoelectronic to an ester or a carbamoyl group, we were interested in testing various heterocycles as electron-withdrawing groups for activation of the carbonyl group in 1,3-diketones. In order to compare the electron-withdrawing ability of heterocycles and previously studied EWGs, we tried to utilize Hammett constants. Since only a few numbers of experimentally measured Hammett constants for heterocycles are known [55,56], this approach seems unsuitable at first. However, recently, a web-based tool for the calculation of the substituent descriptors compatible with the Hammett sigma constants was released [57] allowing direct comparison of different substituents.

On the basis of the calculated Hammett constants, we have selected a model series of 1,3-diketones, bearing heterocyclic substituents with a range of electron-withdrawing ability (Figure 2). For the amine model series, we used amines with different nucleophilicity, namely benzylamine (primary alkylamine), morpholine (secondary alkylamine) and aniline (aromatic amine).

[1860-5397-20-188-2]

Figure 2: The model series of synthesized 1,3-diketones and corresponding calculated Hammett constants of heterocyclic substituents. Previously studied EWGs [53] are shown in the dashed block. The numbers in parentheses are σm and σp calculated [57] constants.

We first examined the reaction of 1,2,4-oxadiazole-1,3-diketone 1a (σm/σp 0.463/0.575, which is quite close to the constants of the CO2Me group, so a successful reaction was expected, Figure 2) with morpholine under previously found conditions (Scheme 2). To our delight, the meta-1,2,4-oxadiazole aniline 3ab, formed by the sequence of aldol-type reactions, was isolated in 80% yield and its structure was confirmed by NMR and single crystal X-ray analysis (CCDC 2356151). No significant improvement in the yield was observed by varying the reaction conditions. Surprisingly, the reaction of 1,3-diketone 1a, morpholine and acetone without the use of molecular sieves and acid catalysis (conditions A) resulted in 81% yield of meta-substituted aniline 3ab.

[1860-5397-20-188-i2]

Scheme 2: Synthesis of meta-substituted anilines from 1,2,4-oxadiazol-5-yl substituted 1,3-diketone 1a. Conditions A: 1a (0.5 mmol), 2a,b (0.5 mmol) and acetone (2 mL), 60 °C. Conditions B: 1a (0.5 mmol), 2c (0.75 mmol), AcOH (30 mol %), molecular sieves 3 Å (300 mg) and acetone (2 mL), 60 °C.

Applying the latter conditions to the reaction with benzylamine, the target substituted aniline 3aa was formed in a good 73% yield. However, the reaction with less nucleophilic aniline was sluggish, requiring almost 15 days to achieve full conversion of the 1,3-diketone 1a (TLC control). In this case, performing the reaction in the presence of molecular sieves and 1.5-fold excess of aniline (conditions B) dramatically reduced the reaction time to 1 d, allowing isolation of diarylamine 3ac in 75% yield (Scheme 2).

Next, we started to screen various heterocyclic 1,3-diketones with the model amine series. The reaction of diketone 1b, bearing the less electron-withdrawing 1,3,4-oxadiazole moiety (σm/σp 0.335/0.443), with alkylamines proceeded well (73–74% yields), but required the addition of CHCl3 as co-solvent due to the low solubility of the starting 1,3-diketone 1b. On the other hand, the reaction of 1,3-diketone 1b with aniline resulted in low conversion of 1b even at prolonged reaction times (up to 10 days). The addition of molecular sieves, excess aniline, or acid catalysts did not significantly affect the conversion (Scheme 3).

[1860-5397-20-188-i3]

Scheme 3: Synthesis of meta-substituted anilines from 1,3,4-oxadiazol-substituted 1,3-diketone 1b. Conditions A: 1b (0.5 mmol), 2a,b (0.5 mmol) and acetone/CHCl3 (3 mL, 2:1), 60 °C. Conditions B: 1b (0.5 mmol), 2c (0.75 mmol), AcOH (30 mol %), molecular sieves 3 Å (300 mg) and acetone (2 mL), 60 °C.

1,3-Diketones with benzothiazole (1c, σm/σp 0.338/0.390) and oxazole (1d, σm/σp 0.267/0.305) substituents reacted with primary and secondary alkylamines, requiring prolonged heating and a 1.5 excess of amine (Scheme 4), to give meta-substituted arylamines in reasonable synthetic yields. In the case of 1,3-diketone 1d, the addition of molecular sieves is necessary in order to reduce the formation of the enamine side-product. Similar to 1,3-diketone 1b, an extremely low conversion of 1c and 1d was observed in the reaction with aniline.

[1860-5397-20-188-i4]

Scheme 4: Synthesis of meta-substituted anilines from benzothiazol-2-yl and oxazol-2-yl-substituted 1,3-diketones. Conditions A: 1c,d (0.5 mmol), 2a,b (0.75 mmol) and acetone/CHCl3 (2 mL, 1:1), 60 °C. Conditions B: 1c,d (0.5 mmol), 2a,c (0.75–2.5 mmol), AcOH (30 mol %), molecular sieves 3 Å (300 mg) and acetone/CHCl3 (2 mL, 1:1), 60 °C.

The isoxazol-3-yl or furan-2-yl substituents have calculated Hammett constants below 0.200 (Figure 2), thus based on the above observations, a slow reaction of 1,3-diketones 1e and 1f with aliphatic amines was expected. Indeed, the reaction of 1,3-diketones 1e or 1f with benzylamine was sluggish, and a number of undefined side-products were formed (LC control). m-Isoxazole arylamine 3ea was isolated in low yield (21%) after heating for 6 days (Scheme 5). In fact, arylamine 3fa, produced from furan-substituted 1,3-diketone 1f and a 5-fold excess of benzylamine, could be prepared in 18% crude yield (after 7 days, see Supporting Information File 1, page S9).

[1860-5397-20-188-i5]

Scheme 5: Synthesis of meta-substituted aniline from isoxazol-3-yl-substituted 1,3-diketone 1e. Conditions B: 1e (0.3 mmol), 2a (0.45 mmol), AcOH (30 mol %), molecular sieves 3 Å (300 mg) and acetone (1 mL), 60 °C.

Finally, all attempts to perform the reaction with 1,2,3-triazole 1,3-diketone 1g (σm/σp 0.043/0.011) with model amine series failed (7 days reaction time in each case). This result is in good agreement with low Hammett constants of the triazole-substituent of 1,3-diketone 1g, which are close to the calculated Hammett constants of the phenyl group (σm/σp 0.055/0.012).

Quantum-chemical/chemoinformatics calculations were also performed to correlate the observed reactivity with the charges on the carbonyl group of 1,3-diketones. Unfortunately, no reliable correlation was found between the charges and the reactivity of the 1,3-diketones with the present set of heterocyclic substituents or with the EWGs that were previously studied (see Supporting Information File 1).

The presence of electron-donating (3ha) or electron-withdrawing groups (3ia) in the aryl substituent of the 1,3-diketone does not affect the reaction outcome (Figure 3). We have further demonstrated the utility of three-component condensation by introducing additional functional groups into the amine moiety (Figure 3).

[1860-5397-20-188-3]

Figure 3: Scope of functionalized amines in three-component condensation. Conditions A: 1a,b,h,i (0.2–0.5 mmol), 2e,i–k (0.2–0.75 mmol) and acetone (1–2 mL) or acetone/CHCl3 (3 mL, 2:1), 60 °C. Conditions B: 1a (0.3–0.5 mmol), 2d,f–h (0.45–0.75 mmol), AcOH (30 mol %), molecular sieves 3 Å (300 mg) and acetone (1–2 mL), 60 °C.

Substituted arylamines bearing alcohol (3ae), phenol (3ad), alkene (3bi), dimethyl acetal (3bj) functionality can be accessed in good yields. Reaction of 1,3-diketone 1a with a non-amidine type heterocyclic amine, 3-aminopyridine, provided N-hetarylaniline (3af) in moderate yield. Furthermore, it is possible to synthesize sterically hindered anilines such as arylamine 3ah, which was prepared from 2,6-di(isopropyl)aniline in 53% yield. However, in the case of an electron-deficient amine (4-trifluoromethylaniline), the desired meta-heterocycle aniline 3ag was prepared in low yield. Finally, the developed method is suitable for the late-stage arylation of drug-like molecules such as deacetyllinezolid (3ak).

The meta-substituted anilines 3 are formed in a sequence of reactions shown in Scheme 6 [53]. Firstly, the reaction of acetone and amines 2 leads to the formation of acetone imine/enamine (reaction 1, Scheme 6). Nucleophilic addition of an enamine to the most electron-deficient carbonyl group (C1=O, adjacent to the EWG) of the 1,3-diketones 1 gives the acyclic carbinol I (reaction 2, Scheme 6), followed by the intramolecular addition of enamine I to the C3=O to form intermediate II, which dehydrates to cyclic carbinol III. Finally, dehydration of intermediate III yields anilines 3.

[1860-5397-20-188-i6]

Scheme 6: Proposed mechanism for the formation of meta-substituted anilines 3 via three-component benzannulation.

Conclusion

In summary, a method for the synthesis of substituted meta-hetarylanilines under mild conditions starting from 1,3-diketones with heterocyclic substituents, acetone and various amines has been developed. The success of this three-component reaction is governed by the electron-withdrawing ability of the heterocyclic substituent in the 1,3-diketone, which can be evaluated with computational Hammett constants. As a rule-of-thumb, 1,3-diketones bearing substituents with σm or σp > 0.300 afford meta-anilines from alkylamines in good synthetic yields, and higher σm or σp are required for three-component condensation with less nucleophilic arylamines. The developed one-pot three-component reaction is efficient (yields up to 85%), compatible with many functional groups, and allows to synthesize a series of difficult-to-access meta-substituted anilines of interest for medicinal and material chemistry.

Supporting Information

CCDC 2356152 (1g), 2356151 (3ab), 2356154 (3bb) and 2356153 (3ca) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures.

Supporting Information File 1: Full experimental details, characterization data of new compounds and NMR spectra.
Format: PDF Size: 5.3 MB Download
Supporting Information File 2: xyz files with Cartesian atomic coordinates for all model structures and CIF-files for compounds 1g, 3ab, 3bb and 3ca.
Format: ZIP Size: 434.6 KB Download

Funding

This study was performed under financial support of the Ministry of Science and Higher Education of the Russian Federation (FSNF-2023-0004). The DFT calculations were supported by the RUDN University Scientific Projects Grant System, project № 021342-2-000.

Data Availability Statement

The data that supports the findings of this study is available from the corresponding author upon reasonable request.

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