Mesoporous PbO nanoparticle-catalyzed synthesis of arylbenzodioxy xanthenedione scaffolds under solvent-free conditions in a ball mill.

A protocol for the efficient synthesis of arylbenzodioxy xanthenedione scaffolds was developed via a one-pot multi-component reaction of aromatic aldehydes, 2-hydroxy-1,4-naphthoquinone, and 3,4-methylenedioxy phenol using mesoporous PbO nanoparticles (NPs) as a catalyst under ball milling conditions. The synthesis protocol offers outstanding advantages, including short reaction time (60 min), excellent yields of the products (92-97%), solvent-free conditions, use of mild and reusable PbO NPs as a catalyst, simple purification of the products by recrystallization, and finally, the use of a green process of dry ball milling. This journal is © The Royal Society of Chemistry.

Recently, the ball milling technique has received great attention as an environmentally benign strategy in the context of green organic synthesis.[1] The process of “ball milling” has been developed by adding mechanical grinding to the mixer or shaker mills. The ball milling generates a mechanochemical energy, which promotes the rupture and formation of the chemical bonds in organic transformations.[1] Subsequently, detailed literature[1] and books on this novel matter have been published.[2] Several typical examples include carbon–carbon and carbon–heteroatom bond formation,[2] organocatalytic reactions,[2] oxidation by using solid oxidants,[2] dehydrogenative coupling, asymmetric, and peptide or polymeric material synthesis, which have been reported under ball milling conditions.[2] Hence, the organic reactions using ball milling activation carried out under neat reaction environments, exhibit major advantages,[2] including short reaction time, lower energy consumption, quantitatively high yields and superior safety with the prospective for more improvement than the additional solvent-free conditions and clear-cut work-up.[3-5]

On the other hand, the organic transformations using metal and metal oxide nanoparticles[6] are attracting enormous interest due to the unique and interesting properties of the NPs.[7,8,9] Particularly, PbO NPs[9] provide higher selectivity in some organic reactions[9] and find applications in various organic reactions, like Paal–Knorr reaction,[10] synthesis of diethyl carbonate,[11] phthalazinediones,[12] disproportionation of methyl phenyl carbonate to synthesize diphenyl carbonate,[13] the capping agent in organic synthesis, and selective conversion of methanol to propylene.[14] In addition, the PbO NPs are also used in many industrial materials.[15,16]

However, till date, PbO NPs have not been explored in MCRs leading to biologically important scaffolds. Among others, the xanthene scaffolds[17] are one of the important heterocyclic compounds[18] and are extensively used as dyes, fluorescent ingredients for visual imaging of the bio-molecules, and in optical device technology because of their valuable chemical properties.[19] The xanthene molecules have conjointly been expressed for their antibacterial activity,[20] photodynamic medical care, anti-inflammatory drug impact, and antiviral activity. Because of their various applications, the synthesis of these compounds has received a great deal of attention.[21] Similarly, vitamin K nucleus[22,23] shows a broad spectrum of biological properties, like anti-inflammatory, antiviral, antiproliferative, antifungal, antibiotic, and antipyretic.[24] As a consequence, a variety of strategies[24] have been demonstrated in the literature for the synthesis of xanthenes and their keto derivatives, like rhodomyrtosone-B,[25] rhodomyrtosone-I,[25] and BF-6 [25] as well as their connected bioactive moieties. Few biologically active xanthene scaffolds are shown in (Fig. 1).

Fig. 1

Some biologically important xanthenes and their keto derivatives.

Due to the significance of these compounds, the synthesis of xanthenes and their keto derivatives using green protocols is highly desirable. Reported studies reveal that these scaffolds are synthesized by three-component condensations using p-TSA[26] and scolecite[27] as catalysts. However, these methods suffer from the use of toxic acidic catalysts like p-TSA, long reaction times (3 h), harsh refluxing[26] or microwave reaction conditions,[27] and tedious work-up procedures. The previously reported methods for the synthesis of xanthenediones are shown in Scheme 1.

Scheme 1

Previous protocol for the synthesis of xanthenedione derivatives.

Herein, we report an economical and facile multicomponent protocol, using ball milling, for the synthesis of 7-aryl-6H-benzo[H][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-dione using PbO NPs as a heterogeneous catalyst (Scheme 2). The PbO NPs are non-corrosive, inexpensive, and easily accessible.

Scheme 2

General reaction scheme of PbO NP-catalyzed synthesis of the xanthenedione scaffolds under ball milling conditions.

In our protocol,[28] the PbO NPs were initially prepared by mixing sodium dodecyl sulphate (2.5 mmol) and sodium hydroxide (10 mL, 0.1 N) with an aqueous methanolic solution of lead nitrate (2 mmol) under magnetic stirring at 30 °C by continuing the reaction for 2 h. Then, the obtained white polycrystalline product was filtered, washed with H2O, and dried at 120 °C, followed by calcination at 650 °C for 2 h. During this step, the white PbO NPs turned pale yellow in colour. Eventually, the synthesized PbO was then characterized by spectroscopic and analytical techniques.

The powder X-ray diffraction (XRD) pattern revealed the crystalline nature of the PbO NPs as the diffraction peaks corresponding to (131), (311), (222), (022), (210), (200), (002), and (111) crystal planes were identified (Fig. 2). The XRD outline of the synthesized PbO NPs was further established for the formation of space group Pca21 [29] with a single orthorhombic structure (JCPDS card number 76-1796). The sharp diffraction peaks indicated good crystallinity, and the average particle size of the PbO NPs was estimated to be 69 nm, as calculated using the Debye–Scherer equation.

Fig. 2

The powder XRD pattern of PbO NPs.

The surface morphology of the PbO NPs was analyzed by scanning electron microscopy (SEM), and the SEM image revealed the discrete and spongy appearance of the PbO NPs (Fig. 3).

Fig. 3

The SEM image of PbO NPs.

Moreover, the elemental composition obtained from energy dispersive X-ray (EDX) analysis confirmed that the material contains Pb and O elements, and no other impurity was present (Fig. 4).

Fig. 4

The EDAX spectrum of crystalline PbO NPs.

The transmission electron microscopy (TEM) image shown in Fig. 5 indicated the formation of orthorhombic crystallites of PbO with several hexagon-shaped particles. The dark spot in the TEM micrograph further confirmed the synthesis of PbO NPs, as the selected area diffraction pattern associated with such spots reveals the occurrence of the PbO NPs in total agreement with the X-ray diffraction data (Fig. 6). The average size of the PbO nanocrystals by TEM was approximated to be around 20 nm.

Fig. 5

The TEM image of nanocrystalline PbO NPs.

Fig. 6

The SAED image of nanocrystalline PbO NPs.

The Fourier transform infrared (FT-IR) spectrum (ESI, S6†) of the PbO NPs displayed peaks at 575, 641, and 848 cm−1, which corresponds to the Pb–O vibrations. Furthermore, the absorption band at ∼3315 cm−1 was due to the presence of the hydroxyl group (–OH) in the NPs.

The N2 adsorption–desorption isotherms of the PbO nanoparticles shown in Fig. 7 was consistent with type IV adsorption–desorption isotherms with H1 hysteresis corresponding to the cylindrical mesoporous structure. Moreover, the surface area, pore-volume, and BJH pore diameter were found to be 32.0 m2 g−1, 0.023 cm3 g−1, and 30.9 Å, respectively.

Fig. 7

BET surface area and pore size of nanocrystalline PbO catalyst.

The catalytic activity of the synthesized PbO NPs was tested in a one-pot multicomponent synthesis of arylbenzodioxoloyl xanthenedione derivative under ball milling condition according to the reaction scheme 2a, with 3,4-dimethoxybenzaldehyde (166.2 mg, 1.0 mmol), 3,4-methylenedioxyphenol (138.0 mg, 1.0 mmol), and 2-hydroxy-1,4-naphthoquinone (174.0 mg, 1.0 mmol) as reactants. The reaction conditions, the ball milling parameters (speed, time, and ball to solids ratio), and the PbO nanocatalyst amount were first optimized to produce the highest yield using experimental design as shown in Table 1. The ball milling at 600 (6 balls) rpm for 60 min under solvent-free conditions was established as the optimized reaction conditions (entry 13, Table 1). Further, to understand the effect of ball milling, we also performed the model reaction in an RB flask under normal conditions without ball milling (entry 15, Table 1). However, the reaction did not initiate under these conditions, which demonstrated that ball milling provides sufficient energy to the reaction leading to the formation of the product.

The optimization of reaction conditions for the synthesis of xanthenedione derivative 2a

Entry	Conditions	Rotation (rpm)	Catalyst (mol%)	Time (min)	Yield (%)a
1	Ball milling	400	00	50	21
2	Ball milling	400	10	50	48
3	Ball milling	400	15	60	54
4	Ball milling	400	20	70	59
5	Ball milling	500	10	50	62
6	Ball milling	500	15	50	65
7	Ball milling	500	20	60	67
8	Ball milling	600	10	70	71
9	Ball milling	600	15	50	77
10	Ball milling	600	20	60	82
11	Ball milling	600	05	70	90
12	Ball milling	600	10	50	91
13	Ball milling b	600	15	60	97
14	Ball milling	600	20	70	97
15	No ball millingc	—	15	60	—

Isolated yield; model reaction: 3,4-dimethoxybenzaldehyde (166.2 mg, 1.0 mmol), 3,4-methylenedioxyphenol (138.1 mg, 1.0 mmol), 2-hydroxy-1,4-naphthoquinone (174.1 mg, 1.0 mmol) under ball milling.

Optimized reaction conditions.

The reaction was performed under stirring condition in a RB flask.

Next, by utilizing the general experimental procedure (ESI† for detail experimental procedure; S2) and the aforementioned optimized conditions (Table 1), we examined the range of conditions for the synthesis of arylbenzodioxoloyl xanthenedione scaffolds. Moreover, we demonstrated that using PbO NPs as a catalyst leads to outstanding yield (92–97%) of 7-aryl-6H-benzo[h][1,3] dioxolo [4,5-b]xanthene-5.6(7H)-dione (2a–l) by the condensation of a variety of substituted aromatic aldehydes, 2-hydroxy-1,4-naphthoquinone, and 3,4-methylenedioxyphenol under the optimized conditions.

With the optimal conditions and using general experimental procedure,[29] we also investigated the possible scopes of the reactants as revealed in Table 2. All these arylbenzodioxoloyl xanthenediones are well-known scaffolds and were simply recognized by the assessment of their spectroscopic information with earlier reports.[26] These data are available in S4 (see ESI† for the spectroscopic data). The aromatic aldehydes comprising both electron-withdrawing (e.g., nitro group) and electron-donating (e.g., –OMe, –OH, –Cl, –Me, and –Br) groups participated proficiently in the reaction without including any electronic effects. The aromatic aldehyde with electron-donating groups (e.g., –OMe, –OH, –Cl, –Me, and –Br) increased the product yield, while in the case of aryl aldehyde having an electron-withdrawing group (e.g., –NO2), both the product yield as well as the reaction rate decreased. These findings are depicted in Table 2. All the prepared xanthenedione derivatives were purified via recrystallization from hot ethanol and extracted as solids, and thus, any column chromatography step was not required in the present protocol. The compounds were then confirmed by measuring their melting points (MP), followed by spectroscopic analysis using 1H and 13C NMR spectra provided in S5, ESI.†

Scope of the PbO NP-catalyzed synthesis of arylbenzodioxoloyl xanthenedione derivatives

Following a previously reported mechanism,[26] a possible mechanism for the synthesis of arylbenzodioxoloyl xanthenedione derivative under ball milling at 600 rpm for 60 min is shown in Scheme 3. It is speculated that in the first step, the surface of the PbO NPs having free –O–H groups facilitated the carbon–carbon bond formation by activating aromatic aldehyde 1a to react with 2-hydroxy-1,4-naphthoquinone 1b leading to the intermediate B, which further undergoes dehydration, followed by the addition of 3,4-methylenedioxyphenol 1c, which upon cyclization leads to the formation of the product 2a with the recovery of the catalyst, PbO NPs.

Scheme 3

Plausible mechanism of PbO NP-catalyzed synthesis of arylbenzodioxoloyl xanthenedione (2a).

Further, to signify the advantages of the current methodology, a comparative study of known methods is provided in Table 3, which clarifies the effectiveness of the PbO NP catalyst over known p-TSA and scolecites in terms of product yields, reaction times, and catalyst recyclability. A comparative study of the present method to that of the reported methods for the synthesis of arylbenzodioxoloyl xanthenediones is shown in Table 3.

A comparative study of present and previous methods

Sr. no.	Catalyst	Reaction conditions	Yield (%)	Time (min)	Reusable?
1	p-TSA[26]	EtOH/90–120 °C	85–90	180	No
2	Scolecites[27]	EtOH/80 oC	90–92	4–15 MW	Yes up to 3 cycles
3	aPbO NPs	Ambient temperature	92–97	60	Yes up to 8 cycles

Present work.

Next, we investigated the reusability of the PbO nanocatalyst for the synthesis of 7-(3,4-dimethoxyphenyl)-6H-benzo[H][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-dione (2a) as a model reaction. After the reaction, PbO NPs were separated from the reaction mixture by centrifugation, washed consecutively with aqueous ethanol, dried, and reused for the next run. As shown in Fig. 8, the reaction yield was reduced by only 12% after eight consecutive runs. This slight decrease in the yield was observed due to the loss of PbO NPs (∼10 wt%) during the recycling process.

Fig. 8

Reusability of PbO NPs for the synthesis of 7-(3,4-dimethoxyphenyl)-6H-benzo[H][1,3]dioxolo[4,5-b]xanthenes-5,6 (7H)-dione as a model reaction.

The fate of the recycled PbO NPs was analyzed by performing SEM and TEM studies after the 8th run, and considerable agglomeration of NPs was observed. However, interestingly the particle size of the NPs reduced to ∼15 nm compared to fresh PbO NPs during the ball milling process (Fig. 9).

Fig. 9

(a) SEM and (b) TEM images of the recycled PbO NPs after 8th run.

In conclusion, we demonstrated a facile and efficient method for the synthesis of 7-aryl-6H-benzo[H][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-dione using PbO NPs as a catalyst. The entire synthesis process was very clean and provided very high yields (86–97%) of xanthenedione derivatives (2a–l) via mild ball milling. Moreover, the present protocol has demonstrated significant development in terms of higher isolated yields, faster rate of reaction (1 h), and most importantly, it is environment-friendly. Moreover, the use of solvent-free ball milling conditions allows simple isolation and purification of the products, with no column chromatography, as well as the mild PbO NPs as a reusable catalyst made the current synthetic method more suitable and environmentally benign in nature.

Conflicts of interest

There are no conflicts to declare.