Isoselenazole Synthesis by Rh-Catalyzed Direct Annulation of Benzimidates with Sodium Selenite

Abstract

Organoselenium compounds have attracted significant research interest because of their potent therapeutic activities and indispensable applications in the organic chemistry field. The selenation reactions conventionally rely on the use of sensitive Se reagents; thus, new synthetic methods with improved efficiency and operational simplicity have recently been of particular interest. In this manuscript, we report a Rh-catalyzed direct selenium annulation using tractable sodium selenite (Na₂SeO₃) as the limiting reagent. The selenite species was converted to highly electrophilic SeO(OBz)₂ in situ upon treatment with Bz₂O, thereby undergoing C–H/N–H double nucleophilic selenation. A series of benzimidates successfully underwent selenation under mild reaction conditions to afford isoselenazole derivatives.

1. Introduction

Recent decades have witnessed considerable progress in selenium chemistry for the diverse functions of catalysis, electronic materials, biologically active compounds, selenoproteins, etc. [1,2,3,4,5,6,7,8,9,10,11]. It is notable that Ebselen derivatives have recently been extensively studied for their fascinating effect on COVID-19 protease inhibition [12,13]. The unique and flexible redox characteristic of Se species over other chalcogens would be a key factor in such a wide spectrum of applications. Unlike the relevant sulfur chemistry, selenation has not been established sufficiently to meet the growing demand in synthetic chemistry field, which may be attributed to the relatively limited availability of selenium reagents.

Selenation reactions can be classified into two categories based on the amount of bond formation onto the Se atom: single selenation and double selenation (Scheme 1a). For single selenation, selenium compounds with one leaving group (R–Se–X type reagents) are generally adopted. Diselenides are the first choice in this transformation because of their availability, stability, and robustness [14,15,16,17,18]. Other highly activated reagents, such as selenohalides, selenocyanides, and selenophthalimides, and selenol derivatives are also synthetically valuable, but have been less commonly utilized [19,20,21,22,23,24,25]. Meanwhile, double selenation requires nucleophilic M–Se–M (M = metal), electrophilic X–Se–X, or ambiphilic (specifically KSeCN) reagents to facilitate twofold bond formation in an annulation or three-component reaction [26,27,28,29,30,31,32]. In particular, electrophilic double selenation has relied entirely on the use of sensitive (oxy)halides or odorous and toxic SeO₂ [33,34]. One rare example utilized H₂SeO₃ for diarylation with anisole solvent [35].

To address this issue, herein, we use an inorganic selenite salt Na₂SeO₃ as a novel X–Se–X-type reagent in combination with acid anhydride. To the best of our knowledge, there have been no reports on the usage of stable, odorless, and readily available selenite salt in selenation. This strategy stems from our previous reports on Rh-catalyzed direct selenation, where a highly electrophilic SeO(OAc)₂ formed in situ upon treating Se powder with AgOAc (Scheme 1b) [36,37]. We would like to highlight this as a new activation mode in selenium chemistry; however, low Se atom economy due to the disproportionation process, as well as the use of excess Ag salt, are obvious disadvantages. In this work, we demonstrate a Rh-catalyzed annulative C–H/N–H double selenation of benzimidates adopting Na₂SeO₃ to afford isoselenazole derivatives (Scheme 1c) [38,39,40].

2. Materials and Methods

2.1. General Information

All manipulations were performed under N₂, using standard Schlenk techniques unless otherwise noted. PhCF₃ was purchased as a dehydrated solvent and stored with molecular sieves 4A. [Cp*RhCl₂]₂ and [Cp*Rh(MeCN)₃][SbF₆]₂ were prepared according to the procedure outlined in the literature [41]. Imidate derivatives 1 were prepared according to the procedure in the literature [42,43]. N-acyl imidate 3 was also prepared according to the procedure in the literature [44]. Rh-1b was prepared according to the procedure in the literature [45,46]. All other reagents were purchased from suppliers and were used without further purification. Nuclear magnetic resonance spectra (Bruker, Billerica, United States) were measured at 400 MHz (¹H NMR), at 100 MHz (¹³C NMR), and at 376 MHz (¹⁹F NMR) in 5 mm NMR tubes. ¹H NMR chemical shifts were reported in ppm relative to the resonance of TMS (δ 0.00) or the residual solvent signals at δ 7.26 for CDCl₃ and at δ 7.16 for C₆D₆. ¹³C NMR chemical shifts were reported in ppm relative to the residual solvent signals at δ 77.16 for CDCl₃ and at δ 128.10 for C₆D₆. Preparative gel permeation chromatography (GPC) was conducted with two in-line YMC-GPC T2000 preparative columns.

2.2. General Procedure for Rh-Catalyzed Annulation of Benzimidates with Sodium Selenite

To a screw-top glass tube, 1 (0.15 mmol), Na₂SeO₃ (17.3 mg, 0.1 mmol), benzoic anhydride (67.9 mg, 0.3 mmol), and [Cp*Rh(MeCN)₃][SbF₆]₂ (3.3 mg, 4.0 mol%) were added. The tube was flushed with N₂. PhCF₃ (0.5 mL) and AcOH (11 μL, 0.2 mmol) were added via syringe. The mixture was stirred at room temperature for 15 min, then heated at 100 °C for 14 h using an oil bath. After cooling to room temperature, the resulting mixture was filtered through a pad of Celite eluting with EtOAc. The filtrate was concentrated in vacuo, and the crude product was purified by silica gel column chromatography or preparative TLC, if indicated by GPC.

2.3. Rh-Catalyzed Annulation of 1a with Sodium Selenite (5.0 mmol Scale)

To a screw-cap glass tube, [Cp*Rh(MeCN)₃][SbF₆]₂ (166 mg, 4.0 mol%), sodium selenite (865 mg, 5.0 mmol), and benzoic anhydride (3.39 g, 15.0 mmol) were added. The tube was flushed with N₂. PhCF₃ (25 mL), 1a (1.22 g, 7.5 mmol), and AcOH (0.57 mL, 10.0 mmol) were added via syringe. The mixture was stirred at room temperature for 15 min, then heated at 100 °C for 14 h using an oil bath. After cooling to room temperature, the resulting mixture was filtered through a pad of Celite eluting with EtOAc. The filtrate was concentrated in vacuo, and the crude product was purified by silica gel column chromatography (hexane/EtOAc = 10/1) and GPC (CHCl₃) to produce 2a as white solid (1.00 g, 83% yield).

3. Results

The optimization of the reaction conditions began by seeking a suitable activator for Na₂SeO₃ to achieve selenium annulation with a benzimidate 1a in the presence of a cationic Cp*Rh(III) catalyst, [Cp*Rh(MeCN)₃][SbF₆]₂ (Cp* = cyclopentadienyl) (Table 1). No desired product was observed when acetyl chloride was used in combination with Na₂SeO₃ (entry 1). On the other hand, relatively mild activating reagents such as benzoyl fluoride (entry 2) and acid anhydrides (entries 3–5) provided the target isoselenazole 2a in moderate yields of 28~38%. The main side reactions were acylation of the imine nitrogen atom and imidate hydrolysis to the corresponding ester. These were considerably suppressed by the addition of acetic acid (entry 6). Further improvement of the yield was achieved with 1.5 equiv of 1a affording 2a in 82% isolated yield (entry 7). After screening the solvent, dioxane and MeCN were found to lead to similar results (entries 8–9, see Table S1 and S2 for more information). Other metal catalysts, such as [Cp*IrCl₂]₂, Cp*Co(CO)I₂, and [Ru(p-cymene)Cl₂]₂, exhibited poor productivity (entries 10–12). The conditions in entry 7 were chosen as the standard and were used for the following study.

The scope of the substrate was investigated with respect to benzimidates (Scheme 2). The reaction of 1a was successfully conducted at a 1.0 mmol (80% yield) and 5.0 mmol (83% yield) scale under the standard conditions. Substrates with a series of O-alkyl substituents were tolerated well (2b–2g, 69~83% yields). For the para-substituted benzimidates, functional groups involving alkyl (1a), alkoxy (1h), halogen (1i–1l), aryl (1m), and ester (1n) were all able to provide the corresponding products in high yields (2h–2n, 66~84% yields). It is notable that the meta-substituted substrates were converted with high regioselectivity, and 2o–2r were obtained as the sole regioisomers (63~79% yields). Unfortunately, the substituent at the ortho-position seemed to impair the C–H activation (2s, 26% yield), whereas modified conditions with increased Na₂SeO₃, Bz₂O, and AcOH slightly improved the yield to 39%. The thiophene (1t) and naphthalene (1u) analogs were also applicable.

In order to confirm the intermediacy of electrophilic Se(IV) species within the catalysis, a stoichiometric reaction was conducted (Schem 3a). The expected Se(IV) reagent SeO(OBz)₂ was prepared from SeOCl₂ upon treatment with AgOBz. After the removal of precipitated AgCl by filtration, the filtrate was subsequently reacted with a metallacycle complex Rh-1b in a heated PhCF₃ solution. As expected, the annulation product 2b was obtained at a 44% yield, whereas the reductant for Se(IV) to Se(II) was not identified here. Additionally, an N-acyl imidate 3, which may have been formed by the acylation of 1a with acid anhydride, did not participate in the selenation under standard conditions (Scheme 3b). These results support the role of Bz₂O activating Na₂SeO₃ to generate electrophilic Se(IV) species in order to ensure the annulation process.

Based on the control experiments and previous literature [36,37], we would like to propose a reaction mechanism, as shown in Scheme 3c. A catalytically active species, which is assumed to be Cp*Rh(OAc)(SbF₆), undergoes chelation-assisted C–H activation with imidate 1 to produce intermediate A (path a). The electrophilic SeO(OBz)₂ is generated in situ from Na₂SeO₃ and Bz₂O, thereby coordinating to the Rh species (A→B). Formal double nucleophilic substitution at the Se atom takes place (B→C), and the primary Se(IV) product is dissociated and reduced to give the desired isoselenazole 2. As an alternative reaction pathway, SeO(OBz)₂ might first react with imidate 1 to generate N-seleninyl imidate (path b). The subsequent C–H activation and intramolecular nucleophilic selenation would form the same intermediate (D→C). The coupling product would be liberated, and the active Rh complex would be recovered to close the catalytic cycle.

4. Conclusions

We have achieved the Rh-catalyzed direct annulative selenation of benzimidates by adopting Na₂SeO₃ as a versatile and user-friendly selenium source. This protocol requires neither an excess amount of Se reagent nor stoichiometric silver salt. The reaction conditions are considerably mild, and a series of functional groups were shown to be tolerated. The established activation mode would be valuable for designing new reaction systems in selenium chemistry, and some examples are currently in development in our research group.

Author Contributions

Conceptualization, Q.-F.X.-X. and Y.N.; investigation, Q.-F.X.-X.; writing—original draft preparation, Q.-F.X.-X.; writing—review and editing, Y.N. and M.M.; supervision, M.M.; funding acquisition, Y.N. and M.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available in the Supplementary Material.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Schemes and Table

Scheme 1. Synthetic approaches to organoselenium compounds.

Scheme 2. Scope of the substrates. Reaction conditions: 1 (0.15 mmol), Na2SeO3 (0.1 mmol), [Cp*Rh(MeCN)3][SbF6]2 (0.004 mmol), Bz2O (0.3 mmol), and AcOH (0.2 mmol) in PhCF3 (0.5 mL) at 100 °C for 14 h under N2. Isolated yields are shown. [a] With 1s (0.1 mmol), Na2SeO3 (0.2 mmol), Bz2O (0.4 mmol), and AcOH (0.5 mmol).

Scheme 3. Control experiments and proposed reaction mechanism.

Table 1

Optimization of the reaction conditions [a].

Scheme 3. Control experiments and proposed reaction mechanism.

Entry	Catalyst	Activator	Solvent	Yield [b]
1	[Cp*Rh(MeCN)3][SbF6]2	AcCl	PhCF3	n.d.
2	[Cp*Rh(MeCN)3][SbF6]2	PhCOF	PhCF3	38%
3	[Cp*Rh(MeCN)3][SbF6]2	Ac2O	PhCF3	36%
4	[Cp*Rh(MeCN)3][SbF6]2	Piv2O	PhCF3	28%
5	[Cp*Rh(MeCN)3][SbF6]2	Bz2O	PhCF3	58% (54%)
6 [c]	[Cp*Rh(MeCN)3][SbF6]2	Bz2O	PhCF3	62%
7 [c,d]	[Cp*Rh(MeCN)3][SbF6]2	Bz2O	PhCF3	84% (82%)
8 [c,d]	[Cp*Rh(MeCN)3][SbF6]2	Bz2O	dioxane	84% (80%)
9 [c,d]	[Cp*Rh(MeCN)3][SbF6]2	Bz2O	MeCN	80%
10 [c,d,e]	[Cp*IrCl2]2	Bz2O	PhCF3	27%
11 [c,d,e]	Cp*Co(CO)I2	Bz2O	PhCF3	n.d.
12 [c,d,e]	[Ru(p-cymene)Cl2]2	Bz2O	PhCF3	6%

[a] Reaction conditions: 1a (0.1 mmol), Na₂SeO₃ (0.1 mmol), catalyst (4.0 mol% metal), and activator (0.2 mmol) in solvent (0.5 mL) were stirred at rt for 15 min and then heated at 100 °C for 14 h under N₂. [b] NMR yield. Isolated yield in paratheses. [c] With AcOH (0.2 mmol). [d] 1a (0.15 mmol), Bz₂O (0.3 mmol). [e] With AgSbF₆ (15 mol%). n.d. = not detected.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry5040140/s1, Table S1: Optimization study 1; Table S2: Optimization study 2; Product Identification Data; Copy of NMR Spectra.

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