Transition-metal-free dibenzoxazepinone synthesis by hypervalent iodine-mediated chemoselective arylocyclizations of N-functionalized salicylamides

Abstract

We have developed transition-metal-free synthetic methodologies for dibenzoxazepinones utilizing salicylamides as starting materials and employing two distinct types of successive hypervalent iodine-mediated arylocyclizations. This synthetic protocol encompasses selective phenol O-arylation of salicylamides with diaryliodonium salts, followed by electrophilic aromatic amination utilizing chemically or electronically generated hypervalent iodine reagents in the second stage of the process.

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Dibenzoxazepinone, a seven-membered heterocyclic compound, possesses a unique structural motif commonly found in bioactive compounds, pharmaceuticals, and their intermediates (Fig. 1).¹ Various synthetic approaches to dibenzoxazepinones involving diaryl ether and amide moiety formation have been documented (Fig. 2).²

Fig. 1 Bioactive compounds of dibenzoxazepinone derivatives.

Fig. 2 C–O bond formation approach for dibenzoxazepinone synthesis.

The synthesis of dibenzoxazepinones typically involves the formation of diaryl ether bonds as the key step. The condensation of ortho-functionalized benzoic acids and aminophenols, along with diaryl ether bond formation via a transition-metal-catalyzed coupling or aromatic nucleophilic substitution (S_NAr), represents a straightforward approach (Fig. 2a).³ Starting with salicylic esters and ortho-fluoronitroarenes also provides an efficient combination for the construction of dibenzoxazepinone structures, involving diaryl ether bond formation followed by reductive lactamization with iron (Fig. 2b).⁴ Notably, salicylamides undergo a double S_NAr reaction to form both aryl C–O and C–N bonds in a single procedure. However, the starting material is limited to electron-deficient aromatic compounds (Fig. 2c).⁵ In contrast, transition-metal-catalyzed coupling reactions address this limitation; the reaction of benzamides and ortho-bromophenol derivatives in the presence of a copper catalyst yields dibenzoxazepinones in a single procedure (Fig. 2d).⁶ Moreover, transition-metal catalysts enable CO insertion, leading to a three-component coupling for dibenzoxazepinone synthesis (Fig. 2e).⁷ However, the use of transition-metal catalysts presents drawbacks, including high cost and toxicity, and efficient transition-metal-free methods are highly desirable, particularly for drug synthesis.

As a transition-metal-free approach to the formation of aryl–heteroatom bonds, the use of diaryliodonium salts as arylating agents represents an attractive strategy,⁸ with the initial phenol O-arylation using diaryliodonium salts reported in 1953.⁹ Over approximately ten years, advanced phenol O-arylation methods, incorporating considerations of the base, counter anion, and aryl ligand, have been developed by many researchers and our group.^10,11 The fluoride and acetate counter anions of diaryliodonium salts directly activate phenol nucleophilicities, enabling these reactions to proceed under milder primary conditions. Furthermore, trimethoxyphenyl (TMP)-iodonium salts are readily accessible,¹² exhibit unified selectivity for aryl transfer,^13,14 and have been employed in phenol O-arylations.

In this context, we focused on the selective O-arylation of salicylamides under transition-metal-free conditions (Fig. 3, step a). Thus, the obtained O-arylated salicylamides undergo electrophilic aromatic amination¹⁵ to give dibenzoxazepinones (Fig. 3, step b). This study reports the synthesis of transition-metal-free dibenzoxazepinones via chemoselective arylocyclization, leveraging the unique reactivity of diaryliodonium salts. Furthermore, it demonstrates a one-pot strategy for synthesizing dibenzoxazepinone through chemical- or electro-oxidation, utilizing persistent iodoarene as catalyst. The challenge lies in the selective phenol O-arylation of salicylamides because of the competitive arylation that can occur at both the nitrogen and oxygen atoms of the amide.^14b,c,16 In fact, the copper-catalyzed coupling of salicylamide and halobenzene has been investigated; this reaction proceeded with selective amide N-arylation.¹⁷

Fig. 3 New approach for dibenzoxazepinone synthesis.

The investigation began with the O-arylation of N-functionalized salicylamides using TMP-iodonium salts (Table 1 and Table S1, ESI†). The reaction between N-methoxy-functionalized salicylamide (1a) and phenyl(TMP)-iodonium acetate (2a) was conducted in the presence of Na₂CO₃ (1.5 equiv.) in a (CH₂Cl)₂–H₂O (1 : 1) mixture at 70 °C for 24 h. The corresponding O-arylated salicylamide 3aa was obtained in 42% yield (entry 1). When N-tert-butoxy-functionalized salicylamide was used as the starting material, the corresponding O-arylated salicylamide 3aa was not obtained (entry 2). The starting materials were not recovered in these reactions, suggesting that the previously reported side reactions involving amide N- and/or O-arylation had occurred.^14f The introduction of phthalimide group (NPhth) to amide moiety could reduce the reactivity for amide arylation.^14c In fact, the reaction employing N-NPhth-functionalized salicylamide resulted in chemoselective O-arylation of hydroxy group generating 3aa in 41% yield with recovery of the starting material (entry 3). Screening of various bases (NaHCO₃, K₂CO₃) and solvents ((CH₂Cl)₂, H₂O, CHCl₃) revealed that the use of K₂CO₃ in CHCl₃ resulted in 74% yield (entries 4–8). The influence of the counter anion of the phenyl(TMP)iodonium salt was investigated, with trifluoroacetate emerging as the optimal counter anion due to its lower nucleophilicity compared to acetate (entry 9). The tosylate counter anion exhibited similar reactivity and produced salicylamide diaryl ether 3aa in 88% yield (entry 10).

Table 1 Optimization of reaction conditions for O-arylation^a

Entry	R	Base	Solvent	L	Yield^b (%)
a Reaction conditions: 1a (0.20 mmol), 2a (1.2 equiv.), and base (1.5 equiv.) in solvent (2.0 mL) at 70 °C for 24 h. b Yields were determined by ^1H NMR. c Isolated yield. TMP = 2,4,6-trimethoxyphenyl. NPhth = phthalimide.
1	OMe	Na2CO3	(CH2Cl)2/H2O (1 : 1)	OAc	42
2	O^tBu	Na2CO3	(CH2Cl)2/H2O (1 : 1)	OAc	0
3	NPhth	Na2CO3	(CH2Cl)2/H2O (1 : 1)	OAc	41
4	NPhth	NaHCO3	(CH2Cl)2/H2O (1 : 1)	OAc	40
5	NPhth	K2CO3	(CH2Cl)2/H2O (1 : 1)	OAc	52
6	NPhth	K2CO3	(CH2Cl)2	OAc	70
7	NPhth	K2CO3	H2O	OAc	Trace
8	NPhth	K2CO3	CHCl3	OAc	74
9	NPhth	K2CO3	CHCl3	OCOCF3	95 (90)^c
10	NPhth	K2CO3	CHCl3	OTs	88

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Based on the optimized reaction conditions, we investigated the scope of the chemoselective arylation of N-NPhth salicylamide 1 with aryl(TMP)iodonium trifluoroacetate 2 (Fig. 4). Salicylamides bearing 4-(1d), 5-(1b), 6-methyl (1c), and 4-methoxy (1e) substituents were used, and the desired O-arylated salicylamides 3ba–3ea were obtained in good yields. The O-arylation of halogen-functionalized salicylamides 1f–1i also proceeded efficiently without dehalogenation, yielding O-arylated salicylamides 3fa–3ia in 63–89% yield. Aryl(TMP)iodonium trifluoroacetate containing electron-donating (methyl (2b)), halogen (2c–2e), and electron-withdrawing (phenyl (2f), methyl ester (2g), ketone (2h), and cyano (2i)) groups were effectively converted into the desired products 3ab–3ai in moderate to good yields. Cyano-functionalized aryl(TMP) iodonium trifluoroacetate 2i decomposed more rapidly than it reacted with salicylamide at 70 °C; thus, the reaction yield was improved at 40 °C. meta- and ortho-functionalized aryl(TMP)iodonium trifluoroacetates were also well-tolerated, yielding corresponding salicylamide diaryl ethers 3aj–3al in 62–88% yields.

Fig. 4 Substrate scope of O-arylation with aryl(TMP)iodonium salts. ^a Reaction conditions: 1 (0.50 mmol), 2 (1.2 equiv.), and K₂CO₃ (1.5 equiv.) in CHCl₃ (5.0 mL) at 70 °C for 24 h. ^b For 36 h. ^c At 90 °C. ^d At 40 °C.

The O-arylated salicylamides obtained serve as valuable precursors for the intramolecular electrophilic aromatic amination, yielding dibenzoxazepinones (Fig. 5). Hypervalent iodine-mediated electrophilic aromatic amination is an attractive methodology for transition-metal-free C–N bond formation,^18–20 and we applied it to the synthesis of dibenzoxazepinone. Following the evaluation of various iodine catalysts and reaction conditions, the reaction was conducted using 2,2′-diiodo-1,1′-biphenyl (5 mol%), m-chloroperbenzoic acid (mCPBA, 1.1 equiv.), and trifluoroacetic acid (2.0 equiv.) in CH₂Cl₂–HFIP (1 : 1) at room temperature (Table S2, ESI†). This reaction proceeded efficiently, yielding dibenzoxazepinones with electron-donating (4ba–4ea and 4ab), electron-withdrawing (4ag–4ai), and halogen atoms (4fa–4ia, 4ac–4ae) in moderate to good yields. When O-(para-fluorinated) salicylamide 3ac was employed, defluoro-spirocyclization occurred,^19b,20e yielding dibenzoxazepinone 4ac in low yield. Salycylamides bearing ortho-chlorophenyl (3aj) and ortho-tolyl (3ak) groups were also applicable, producing the desired dibenzoxazepinones 4aj and 4ak. The C–N coupling reaction of salicylamide having meta-tolyl group 3al yielded the dibenzoxazepinones 4al and 4al′ in 54% yield as a mixture of two regioisomers.

Fig. 5 Substrate scope of N-arylation with μ-oxo hypervalent iodine reagents. ^a Reaction conditions: 3 (0.20 mmol), iodine catalyst (5 mol%), mCPBA (1.1 equiv.), and TFA (2.0 equiv.) in CH₂Cl₂/HFIP (1 : 1, 2.0 mL) at room temperature for 24 h. ^b Iodine catalyst (10 mol%) and mCPBA (1.5 equiv.) were used.

We subsequently investigated the one-pot synthesis of dibenzoxazepinones 4 from N-NPhth salicylamides 1 by combining two successive arylations; O-arylation with diaryliodonium salts and intramolecular N-cyclization with hypervalent iodine reagents generated under chemical or electrolytic conditions (Fig. 6). Arylation with diaryliodonium salts is typically accompanied by the coproduction of a stoichiometric amount of iodoarenes. To minimize chemical waste, iodoarenes have been utilized as aryl sources in successive transition-metal-catalyzed coupling reactions.²¹ However, no examples exist of their use as oxidizing reagents in successive bond-forming steps. In this one-pot dibenzoxazepinone synthesis strategy, the first step involved a chemoselective O-arylation of N-NPhth salicylamide with diaryliodonium salt, followed by hypervalent iodine-mediated electrophilic aromatic amination, which occurred via the chemical- or electro-oxidation of the persistent iodoarene generated in the first step (Fig. 6a). For this purpose, mCPBA was employed as a chemical reoxidant for the second amination step, and dibenzoxazepinones 4 were obtained in 30–70% yields (Fig. 6b). Subsequently, the C–N coupling conditions were optimized using salicylamide diaryl ether 3da under electrolytic conditions (Table S3, ESI†). In this reaction, potassium trifluoroacetate, generated by adding trifluoroacetic acid and potassium bicarbonate, serves as a suitable electrolyte. N-NPhth salicylamides 1a, 1b, 1d, and 1e and diaryliodonium salts containing para-chloro (2d′) and bromo (2e′) were tolerated, and the corresponding dibenzoxazepinones 4 were obtained in acceptable yields, without addition of external iodoarene. Control experiments were conducted under electrolytic conditions using diaryl ether 3da (Table S4 and Fig. S1, ESI†). The presence of iodobenzene was crucial for this reaction; in its absence, complete decomposition of 3da was observed. Compared to chemical oxidation, the reaction time was shorter under the electrolytic conditions, likely due to the specific formation of highly reactive μ-oxo PIFA²² under electrolytic conditions. Indeed, μ-oxo PIFA and PIFA were specifically observed as major species by ¹H and ¹⁹F NMR measurements under electrolytic conditions in the absence of diaryl ether 3da.

Fig. 6 One-pot dibenzoxazepinone synthesis through chemical and electrolytic conditions. ^a Reaction conditions (O-arylation): 1 (0.20 mmol), 2′ (1.2 equiv.), and K₂CO₃ (1.5 equiv.) in (CH₂Cl)₂ (2.0–2.5 mL) at 70 °C for 24 h. ^b Reaction conditions (chemical oxidation): mCPBA (1.5 equiv.) in (CH₂Cl)₂/HFIP (1 : 1, 4.0 mL) at room temperature for 24 h. ^c Reaction conditions (electro-oxidation): TFA (10 equiv.) in (CH₂Cl)₂/HFIP (1 : 1, 5.0 mL) at room temperature. Glassy carbon anode, platinum foil cathode, constant current electrolysis (CCE) at 10 mA for 4.5 F mol⁻¹.

In conclusion, we developed efficient synthetic methods for dibenzoxazepinones by combining two distinct types of successive hypervalent-mediated arylocyclization reactions. When aryl(TMP)iodonium salts were employed as arylation agents, O-arylated salicylamides were isolated, followed by electrophilic aromatic amination using μ-oxo hypervalent iodine catalyst. Furthermore, successive electrophilic aromatic aminations could be conducted without adding an external iodoarene under chemical and electrolytic conditions, achieving a one-pot metal-free synthesis of dibenzoxazepinones from salicylamides.

T. D. and K. K. acknowledge support from JSPS KAKENHI grant number 19K05466 (T. D.) and 23K04827 (K. K.), JST CREST grant number JPMJCR20R1, and the Ritsumeikan Global Innovation Research Organization (R-GIRO) project. N. M. thanks JST the establishment of university fellowships towards the creation of science technology innovation grant number JPMJSP2101.

Data availability

The data underlying this study are available in the published article and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, characterization date and NMR spectra for all new compounds. See DOI: https://doi.org/10.1039/d4cc05908e

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