NaIO₄-driven oxidative dimerization and Cu(I)-catalyzed oxidative decarbonylation: modular synthesis of 1,2-naphthoquinones and aryl naphtho[2,b]furans

Abstract

Sodium metaperiodate-mediated oxidative C–C homocoupling of diverse β-naphthols to 1,2-naphthoquinones in an aqueous medium, using 18-crown-6 as an additive, is reported. Subsequent Cu(I) chloride-assisted decarbonylative oxidation of these resulting 1,2-diketones under an oxygen atmosphere affords the corresponding naphthofurans in good yields. These findings demonstrate the effectiveness of orthogonal oxidative protocols in facilitating the efficient synthesis of functionalized biaryl scaffolds from readily available β-naphthols.

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Introduction

1,2-Naphthoquinones (NQs) and their functionalized derivatives are important structural motifs commonly found in a wide range of bioactive natural products and functional materials.¹ These NQ-containing natural products exhibit diverse biological activities, including antimicrobial, antidiabetic, anticancer, and anti-inflammatory properties.² Another intriguing class of chemical compounds, naphtho[2,b]furans, constitute essential structural components in numerous natural products and synthetic compounds.³ Structurally related derivatives from both 1,2-naphthoquinones and naphthofurans are also widely present in polyketide natural products and play a significant role in imparting different activity profiles to these bioactive natural products (Fig. 1).⁴ Binaphthyls, an important class of naphthol derivatives, are traditionally synthesized through transition metal-catalyzed C–C coupling reactions of aryl precursors, typically pre-activated as halogenated or metallated derivatives.⁵ In recent years, several transition metal-free approaches employing various types of inorganic or organic oxidants and oxidant-free electrochemical methods have been reported for the chemo- and regioselective synthesis of biaryls, including binaphthyl derivatives.^6–8

Fig. 1 Representative examples of bioactive natural products harbouring 1,2-naphthoquinone and naphthofuran moieties.

As far as the typical oxidative dimerization of 2-naphthols is concerned, different methods have surfaced from time to time (Fig. 1). These include, dimerization triggered by titanium or molybdenum based peroxo complexes⁹ (Fig. 2a, i); regio- and enantioselective photoinduced oxidative coupling enabled by a homochiral Ru/Pd heterometallic coordination cage^7e (Fig. 2a, ii); coupling of naphthols in the presence of K₂S₂O₈/CF₃COOH^7d (Fig. 2a, iii); and hypoiodite/Lewis-acid assisted Brønsted acid (LBA)-co-catalysed tandem oxidation/cross-coupling of 1,2-dihydroxynaphthalene and 2-naphthol^7f (Fig. 2a, iv). Similarly, for the synthesis of naphthofurans, different methods have emerged, including: base mediated intramolecular condensation/decarboxylation of benzophenone acids¹⁰ (Fig. 2b, i); acid-promoted [3 + 2] cascade annulation of 2-naphthols with Z-enoate propargylic alcohols¹¹ (Fig. 2b, ii); cascade annulation of propargyl amines with β-naphthols¹² (Fig. 2b, iii); and cascade intramolecular Friedel–Crafts reaction, rearrangement, and aromatization sequence catalyzed by Brønsted acid¹³ (Fig. 2b, iv).

Fig. 2 Recent approaches towards oxidative dimerization of 2-naphthols, naphthofuran synthesis and our work.

While the oxidation protocols for 2-naphthols discussed above have proven valuable within their respective domains, their broader applicability across structurally diverse 2-naphthols remains underexplored. This gap presents an opportunity for the development of novel and more versatile methodologies.

In line with our ongoing efforts toward advancing oxidative transformations and functionalization strategies for 2-naphthols, we herein report a NaIO₄-mediated, peroxide free, oxidative dimerization approach for substituted 2-naphthols, performed in water with 18-crown-6 as an additive (Fig. 2a, v). Furthermore, we explored a copper(I)-mediated decarbonylative oxidation of the cyclic 1,2-diketones using inexpensive Cu(I)Cl as the catalyst under an oxygen atmosphere, with pyridine as the solvent (Fig. 2b, v). Both these oxidation strategies are operationally simple, compatible with a variety of 2-naphthol substrates, and consistently deliver the desired products in good to excellent yields.

Results and discussion

Given the considerations outlined above, there is a clear need and significant opportunity to develop more efficient and sustainable methods for the oxidative dimerization of 2-naphthols and, more critically, look for the decarbonylative oxidation of cyclic 1,2-diketones – a transformation for which reported strategies remain scarce. In this context, we disclose two efficient oxidative protocols: NaIO₄-mediated oxidative dimerization and Cu(I)Cl-mediated decarbonylative oxidation which can be implemented sequentially to afford cyclic 1,2-diketones and naphthofurans, respectively. Our study began with the screening of commonly available oxidants such as KMnO₄, Pb(OAc)₄, HIO₄, NMO, PCC, PDC etc., using 2-naphthol (9a) as the starting material. Initial results showed that some oxidants, notably HIO₄ and m-CPBA, led to the decomposition of the substrate, while others were either ineffective or afforded the desired product in suboptimal yields. Specifically, PCC and PDC in a THF : H₂O (1 : 1 v/v) solvent system yielded the target compound in low amounts via intermediate 9ai. Increasing the equivalents of these oxidants led to significant decomposition of both the starting material and the intermediate.

In the presence of NaIO₄ under the same reaction conditions, the desired product was obtained in a low yield (27%), but a substantial portion of the starting material was converted into the intermediate. This observation prompted us to focus further on NaIO₄ and explore different solvent systems. Reactions in pure aprotic solvents such as THF and CH₂Cl₂ showed no conversion, directing our attention to polar protic solvents. These protic solvents, either alone or in combination with water, gave the desired product, albeit in insufficient yields. Using pure water as the solvent resulted in particularly low yields, likely due to solubility limitations.

A breakthrough came when 18-crown-6 was employed as an additive in combination with NaIO₄ (3 equiv.) in water at room temperature. Under these conditions, the reaction proceeded efficiently without solubility issues, and the desired compound was obtained in a significantly improved yield of 63%. Further attempts to enhance the reaction yields using other additives such as TBAB or micellar solutions to improve solubility and local substrate confinement were unsuccessful. Consequently, we settled with the conditions mentioned in entry 21 (Table 1) as the optimized protocol.

Table 1 General standardisation of oxidative dimerization reaction

Entry	Oxidant	Solvent/additive	Time (h)	Yield^a (%)
a Yields reported are isolated yields. b Solvent systems represent micellar solutions above CMG; ND = not detected; IM = intractable mixture.
1	KMnO4 (1 eq.)	THF : H2O	24	ND
2	Pb(OAc)4 (1 eq.)	—	—	ND
3	HIO4 (1 eq.)	—	12	IM
4	NMO (1 eq.)	—	24	ND
5	TEMPO (1 eq.)	—	—	ND
6	PCC (1 eq.)	—	6	20%
7	PDC (1 eq.)	—	9	12%
8	SeO2 (1 eq.)	—	24	ND
9	m-CPBA (1 eq.)	—	12	IM
10	NalO4 (1 eq.)	—	—	27%
11	−(1 eq.)	CH2Cl2	24	ND
12	−(1 eq.)	THF	24	ND
13	−(1 eq.)	EtOH : H2O	10	35%
14	−(1 eq.)	EtOH	20	24%
15	−(1 eq.)	MeOH	15	34%
16	−(1 eq.)	MeOH : H2O	12	38%
17	−(1 eq.)	CH2Cl2 : H2O	2.5	28%
18	−(1 eq.)	H2O	24	15%
19	−(1 eq.)	H2O, 18-crown-6	12	42%
20	−(2 eq.)	H2O, 18-crown-6	8	55%
21	−(3 eq.)	H 2 O, 18-crown-6	6	63%
22	−(3 eq.)	H2O, SDS^b	9	34%
23	−(3 eq.)	H2O, TBAB	24	ND
24	−(3 eq.)	H2O, CTAB^b	9	41%
25	−(3 eq.)	H2O, TWEEN-20^b	20	33%

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With the standardized reaction conditions in hand, we explored the substrate scope of transformation. A variety of substituted 2-naphthols (9b–9q), either commercially available or synthesized via Suzuki coupling, were subjected to the reaction. Remarkably, all substrates successfully underwent oxidative dimerization to afford the corresponding biaryl products (10b–10q) in moderate to good yields (Table 2). In order to demonstrate the practical applicability of this methodology, two reactions (9a and 9m) were executed at the gram scale (>2 g) and both these experiments proceeded smoothly and furnished the corresponding products (10a and 10m) with negligible changes in yield and reaction profile (Table 2).

Table 2 General standardisation of the oxidative dimerization reaction

a Yields reported are isolated yields. b 10a and 10m were synthesized at a >1.0 g scale.

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Regarding the plausible mechanism of this reaction, the starting material 9a reacts with NaIO₄ to generate intermediate I. This intermediate then undergoes an intramolecular oxygen transfer to the most nucleophilic ortho position, leading to the formation of intermediate II. Subsequent rearrangement of II yields the cyclic intermediate III. Cleavage of III, followed by reaction with another molecule of 2-naphthol furnishes intermediate IV. Aromatization of IV to V followed by final oxidation generates enone, ultimately leading to the formation of the final compound 10a, as depicted in Scheme 1.

Scheme 1 Plausible mechanism of oxidative dimerization.

Following our successful results with 2-naphthols, we next investigated the reactivity of 1-naphthol under the optimized reaction conditions. Interestingly, it initially formed the same intermediate (9ai) as observed in the 2-naphthol series, which over time converted exclusively to the corresponding naphthoquinone (see the SI).

With access to a diverse set of cyclic diketones, our next objective was to explore the synthetic utility of these interesting scaffolds, focusing primarily on Lewis acid- or metal-catalyzed oxygenation/oxidation strategies. Toward this end, we carried out a detailed optimization of the reaction conditions under molecular oxygen atmosphere with pyridine as the solvent, using the dimerized compound 10a as the model substrate.

A broad screening of various Lewis acids and metal catalysts was then conducted at room temperature (Table 3). Among the tested catalysts, only Cu(I)Cl proved effective, enabling an oxidative decarbonylation of diketone 10a to furnish the naphthofuran 11a in a highly regioselective manner and in good yield of 77%. In contrast, all other catalysts screened failed to produce the desired product. Furthermore, no reaction occurred when solvents other than pyridine were employed, underscoring the critical role of both the metal catalyst and solvent in facilitating this transformation.

Table 3 General standardisation of the decarbonylative oxidation reaction

Entry	Catalyst	Solvent	Time (h)	Yield^a (%)
a Yields reported are isolated yields; ND = not detected; IM = intractable mixture.
1	FeCl3	Pyridine	30	ND
2	NiCl2	—	—	ND
3	CuBr2	—	—	ND
4	Cu(OTf)2	—	—	ND
5	Cu(I)Br	—	18	25%
6	Cu(OAc)2	—	—	ND
7	CuCl2	—	24	ND
8	ZnCl2	—	—	ND
9	Pd(OAc)2	—	—	ND
10	PdCl2	—	—	ND
11	CoCl2	—	—	ND
12	FeCl2	—	—	ND
13	SnCl2	—	—	ND
14	CeCl3	—	—	ND
15	Sc(OTf)2	—	—	ND
16	Yb(OTf)2	—	—	ND
17	Cu(I)CI	—	6	77%
18	—	Xylene	24	ND
19	—	DME	—	ND
20	—	DMF	—	ND
21	—	Toluene	—	ND
22	—	DCM	—	ND
23	—	Chlorobenzene	—	ND

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To explore the substrate scope of the reaction, a series of dimerized diketones bearing various substituents (10a–10c, 10f–10g, 10i, 10k–10l, and 10o–10q) were subjected to the optimized reaction conditions. Gratifyingly, all substrates underwent the transformation smoothly, affording the corresponding naphthofurans (11a–11c, 11f–11g, 11i, 11k–11l, and 11o–11q) in good yields. The structures of the resulting compounds were unambiguously confirmed, with the structure of compound 11a being validated through single-crystal X-ray diffraction analysis (Table 4). Pertinent to mention, acyclic diketones like benzils and phenanthridine dione, under similar reaction conditions, resulted in the corresponding carboxylic acids, see the SI.¹⁴

Table 4 Substrate scope for decarbonylative oxidation

a Yields reported are isolated yields. b 11a was synthesized at a >0.5 g scale.

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Mechanistically, the transformation is proposed to proceed via a pyridine-accelerated oxidative addition, leading to the formation of metallacycle I. This intermediate then undergoes sequential decarbonylation to generate species II, followed by a disproportionative oxygen insertion to afford intermediate III. Finally, intramolecular cyclization triggered by free hydroxyl followed by reductive elimination in III furnishes the target compound 11a (Scheme 2).¹⁵

Scheme 2 Plausible reaction pathway for decarbonylative oxidation.

To further illustrate the synthetic versatility of the biaryl acid derivatives, compound 11a was subjected to a series of functional group transformations. Reduction of the carboxylic acid moiety in 11a using LiAlH₄ proceeded smoothly to furnish the corresponding primary alcohol 12 in excellent yield¹⁶ (91%). Subsequent esterification of 11a with boronic acid 13, under well-established reaction conditions, afforded ester 14 in 85% yield.¹⁷ Additionally, amide coupling of 11a with aniline 15, employing slightly modified reaction conditions, delivered the corresponding amide 16 in 88% yield¹⁸ (Scheme 3).

Scheme 3 Synthetic utility of naphthofuran acids.

Conclusions

In conclusion, we have developed an efficient two-step oxidative strategy for converting readily available β-naphthols into valuable aryl naphthofuran scaffolds via cyclic 1,2-diketones. The first step involves a sodium metaperiodate-mediated oxidative homocoupling of β-naphthols in aqueous medium, promoted by 18-crown-6, to generate 1,2-naphthoquinones. This is followed by a Cu(I) chloride-catalyzed oxidative decarbonylation under an oxygen atmosphere, affording the corresponding aryl naphthofurans in good yields. These orthogonal oxidative protocols offer a practical and modular approach for synthesizing functionalized naphthofurans from simple 2-naphthol precursors.

Experimental section

General procedure for the synthesis

(a) Substituted 1,2-naphthoquinones (10a–10q). A round-bottomed flask equipped with a magnetic stir bar was charged with 2-napthol, 9a (800 mg, 5.549 mmol, 1.0 equiv.) and 18-crown-6 (293.00 mg, 1.11 mmol, 0.2 equiv.) in water (40 mL). The reaction mixture was stirred at room temperature for 10 minutes, followed by the addition of sodium periodate (3.5 g, 16.65 mmol, 3 equiv.) and was allowed to stir at room temperature for 6 hours. The progress of the reaction was monitored by TLC. After completion, the reaction mixture was quenched with water and extracted with ethyl acetate (30 mL × 3). The combined organic layer was dried over anhydrous Na₂SO₄, concentrated under reduced pressure and subjected to column chromatography using hexanes : ethyl acetate (80 : 20, v/v) as an eluent to afford product 10a (525 mg, 63% yield). A similar procedure was followed for the synthesis of remaining compounds 10b–10q (61–69% yield).

(b) Substituted naphtho[2,b]furans. To a stirred solution of 10a (500 mg, 1.665 mmol, 1.0 equiv.) in pyridine (15 mL), copper(I) chloride (165 mg, 1.665 mmol, 1.0 equiv.) was added. The reaction mixture was stirred at room temperature under oxygen for 6 hours. The completion of reaction was monitored by TLC. After completion, the reaction mixture was quenched with a saturated aqueous solution of copper(II)sulfate and extracted with ethyl acetate (15 mL × 3). The combined organic layer was dried over anhydrous sodium sulphate and concentrated under reduced pressure to obtain a crude mixture, which was further purified by column chromatography using hexanes : ethyl acetate (70 : 30, v/v) as an eluent to afford the desired product 11a (370 mg, 77%).

A similar procedure was followed for the synthesis of compounds 11b, 11c, 11f–11g, 11i, 11k–11l, and 11o–11q (72–82% yield).

Author contributions

SR conceptualized the work and wrote the manuscript. MA, GAR, ART and WIL did the experimental work and compiled the SI.

Conflicts of interest

There are no conflicts to declare.

Data availability

All the analytical data (NMR and XRD) related to this publication is available in the SI.

Supplementary information contains the experimental details and characterization data of all the new compounds, including copies of ¹H and ¹³C NMR spectra. See DOI: https://doi.org/10.1039/d5ob01064k.

CCDC 2465277 contains the supplementary crystallographic data for this paper.¹⁹

Acknowledgements

SR acknowledges the grant support from DST-SERB, India [SRG/2023/000916 (GAP-3152)] and CSIR-India (CSPS24/RDSF/IIIM/IHP24/10). MA thanks DST-SERB for fellowship. ART and WIL thank AcSIR for Ph.D. registration (IIIM communication number: CSIR-IIIM/IPR/00953).

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