Wood ash biocatalyst as a novel green catalyst and its application for the synthesis of benzochromene derivatives

Wood ash is a naturally alkaline derived substance containing organic and inorganic constituents. This study investigates the catalytic activity of wood ash as a heterogeneous catalyst for the synthesis of benzochromene derivatives. Several wood ash catalysts, comprising calcium- and potassium-rich carbonates, were prepared from different natural resources under various combustion temperatures. The prepared catalysts were characterized by Fourier transform infrared, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and X-ray diffraction techniques. Catalytic efficiency of the resultant catalysts was tested in the synthesis of benzochromene derivatives. The experimental studies clarified that the catalyst prepared at 850 °C could efficiently expedite the formation of three-component synthesis of benzochromene derivatives in water at 80 °C with high yields. Indeed, alkali, alkaline metal, and metal oxides such as Al2O3, SiO2, MgO, CaO, and Fe2O3, are widely utilized as both catalyst and catalyst support in the heterogeneous catalytic processes. The prepared wood ash catalysts (possessing metal oxides, e.g., CuO, Al2O3, SiO2, and CaO) could effectively prompt the electrophilic activity of the carbonyl groups during the nucleophilic attack intermediate, enhancing the efficiency of the reactions.

Chromenes are important moieties in medicinal and organic chemistry because of their broad spectrum of biological activities including antioxidant, antimicrobial, antimalarial, anticancer, and antibacterial [15][16][17][18][19] . Among various chromenes, benzochromenes are highly considerable compounds because of their applicability and biological properties in variable applications 20 . The preparation of benzochromenes has been investigated using different catalysts, e.g., Zn(l-proline) 2 , 1-butyl-3-methyl imidazolium hydroxide ([bmim]OH) lipase, triethylbenzylammonium chloride (TEBA), etc. [21][22][23][24][25] . Although there are many novel methods to prepare these compounds, several of them have critical disadvantages such as requirement of toxic solvent, high reaction times, non-reusable catalyst, etc. Consequently, developing efficient and inexpensive catalysts presenting high catalytic activity for the preparation of benzochromenes is highly desirable 14 . Indeed, the preparation of benzochromenes by multicomponent reactions (MCR) has garnered much attention because of good product yield and their applicability.
In continuation of our studies to explore new preparation procedure for main organic compounds [26][27][28][29][30][31][32] , we introduce a green method for the synthesis of benzochromene derivatives by an efficient three component reaction of 1-(6-hydroxy-2-isopropenyl-1-benzofuran-yl)-1-ethanone or euparin 1 33 , aldehydes 2, alkyl bromides 3 and triphenylphosphine 4 in the presence of water extract WA (WEWA) as a catalyst in water at 70 °C with good yields. In addition, the antioxidant activities of some of the synthesized derivatives were studied by ferric ion reducing power test and DPPH radical scavenging. To the best of our knowledge, the application of WA as a catalyst for MCR reactions has not previously been reported. The aim of this study is to acquire an active and inexpensive catalyst from waste WA for the synthesis of some benzochromene derivatives. For this purpose, several catalysts were prepared from different WA and characterized by the latest analytical techniques.

Results and discussion
Characterization of WA. Basicity. Our initial studies focused on soluble basicity measurement with the aim of finding optimum conditions. It was found that the source of the wood and combustion temperature both have a deep influence on basicity. Therefore, all the WA which prepared at four different burning temperatures are provided for pH measurement and the results of the WA samples are tabulated in Table 1.
It is observed that both source of wood and combustion temperature affect pH or basicity of the ash samples. The results indicated that the pH of Russian olive ash (ROA) is much more than pine ash (PiA) and poplar ash (PoA) and the pH of the ash samples increases with increasing of burning temperature for example for PiA from 9.45 to 11.65 from 450 to 850 °C and decreases to 10.94 with increase of combustion temperature (above 850 °C). The pH of the ROA 850 (Russian olive Ash at 850 °C) has the highest value 12.79.
Thermal decomposition of CaCO 3 (825 °C) to CaO is perhaps the possible reason for higher basicity with the increase of temperature, which because of higher solubility of CaO in water than CaCO 3 , an increase in basicity value is resulted 34 . But the reason that at temperatures above 1000 °C, the pH shows a great decrease is because of the formation of a highly stable silicate phase via the interaction of metal oxides (e.g., SiO 2 and CaO). The reason for the decrease in basicity is that the stable silicates are less soluble in the water (Fig. 1) 35 .  Figure 2a shows the scanning electron microscopy (SEM) analysis of WA from the combustion of the Russian olive wood at 850 °C (ROA 850 ). The SEM image of the ROA 850 illustrates the porous and spongy nature with rough surfaces and high surface areas 12 in WA particles. Most of the particles characterize sphere-shaped structure. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images show high crystallinity of the prepared wood ash at 850 °C (Fig. 2b). The energy dispersive X-ray (EDX) analysis used to provide the elemental composition and to evaluate a structural vision of the ROA 850 sample as shown in Fig. 2c. The elements identified were potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), oxygen (O), carbon (C), sulfur (S), aluminum (Al), silicon (Si), and sodium (Na). As expected, the main elements of C, and O were consistently dispersed on the surface of the RO WA. Additionally, according to the EDX analysis and elemental mapping, the main elemental compositions of ROA 850 were mainly C, K, Ca, Mg, and Si, which indicates the presence of calcium and potassium rich carbonates and oxides on the surface of RO WA (Fig. 2d).
X-ray diffraction. The X-ray diffraction (XRD) patterns of the Russian olive WA prepared at different combustion temperatures (ROA 450 , ROA 850 , and ROA 1050 ) are shown in Fig. 3 (Fig. 3). At higher temperature, the percentages of Na 2 O and MgO decrease due to their carbonates decomposition to oxides and the subsequent volatilization 36 . The decrease in potassium percentage is predominantly because of vaporization of KCl (Eq. 2). The probable cause for rising CaO content may be because of non-volatility of CaO from the WA 36 .
FT-IR analysis. The FT-IR spectra of burned WA samples at 450, 850, and 1050 °C provided in Fig. 4. The broad band at 3453 cm −1 belongs to hydroxyl stretching vibrations in several organic and inorganic constituents 37 , as can be seen in Fig. 4, the absorbance intensity decreases with increasing temperature due to burning of organic substances. The week absorbance bands at 2922 and 2853 cm −1 from C-H stretching vibrations are corresponded to aliphatic hydrocarbons in the WA and also the absorbance band at 1789 cm −1 and a shoulder peak at 1621 cm −1 belong to carbonyl and C=C groups, respectively 38  Catalytic activity of WEWA in synthesis of benzochromene derivatives. In this research work, we studied a green method for the preparation of some benzochromene derivatives by an efficient three component reaction of euparin 1, aldehydes 2, alkyl bromides 3 and triphenylphosphine 4 in the presence of a catalytic amounts of ROA 850 (%5) in water at 80 °C with high yields (Fig. 5).
In the starting step of this work, condensation reaction of euparin 1 33 , 4-methoxy benzaldehyde 2, ethyl bromopyruvate 3 and triphenylphosphine 4 at 80 °C in water was applied as a model reaction to obtain the optimum reaction conditions (Table 3).
These reactions did not progress without any catalyst even after 15 h (Table 3, entry 1). By increasing the reaction temperature to 70 and 90 °C, a trace amount of 5a generated after 15 h (Table 3, entries 2 and 3). To acquire better results, ROA 850 (%1) as a catalyst was added into the reaction mixture. Interestingly, 55% yield of 5a was produced after 2 h (Table 3, entry 4). Then, the reaction was performed in the presence of ROA 850 (%5). As it was expected, the yield of product 5a was accomplished in 99% after 3 h under these reaction conditions (Table 3, entry 5). Consequently, various amounts of WEWA catalyst were utilized to discover the optimal catalyst loading. The results displayed that 5% of WEWA (ROA 850 ) are enough for producing an excellent yield of 5a (Table 3, entry 5). To clearly evaluate the catalytic activity of WA as a base catalyst, different percentages of NaOH were used in this reaction. Consequently, these results confirmed the main function of WA as the effective catalyst in   (Table 3), ROA 850 (%5) as the catalyst in water at 70 °C was estimated to be the optimum amount of the catalyst for this reaction. The structures of compounds 5 were verified by FT-IR, 1 H NMR, 13 C NMR, and mass spectral data (for detain see supporting information). For instance, the 1 H NMR spectrum of 5a revealed two singlets at δ = 2.15 and 2.52 ppm for methyl protons, four singlets at 4.58, 5.37, 6.14 and 7.75 ppm for methine proton along with signals for aromatic moiety. In the 13 C NMR spectrum, the signals of the carbonyl group of 5a were observed at δ 160.2 and 197.6 ppm. Although there is no exact information to approve the mechanistic details, it can be proposed as shown in Fig. 6. Alkali, alkaline metal, and metal oxides (e.g., Al 2 O 3 , CaO, MgO, Fe 2 O 3 , and SiO 2 ) are widely used as both heterogeneous catalyst and catalyst support 40 . WA, a rich source of the aforementioned metal oxides, is an  www.nature.com/scientificreports/ appropriate candidate for the reactions requiring basic catalysts. In addition, WA has a good catalytic activity and can be used as a solid base catalyst, and WA due to presence of some metal oxides such as CuO, Al 2 O 3 , SiO 2 , and CaO which increases the electrophilic activity of the carbonyl groups can increase the nucleophilic attack in the reaction media. Therefore, we expect that with the preparation of some benzochromene derivatives, both the basic power and nucleophilic activity will increase by WA.
On the other hand, the main advantages of this procedure are green reaction conditions, economical procedure, utilization of small amounts of catalyst, high yield, short reaction times, and easy work-up, which are the required principles of green chemistry [41][42][43][44][45] . Under similar conditions, we also investigated the reaction between 2-hydroxyacetophenone 11, aldehyde 2, alkyl bromide 3 and triphenylphosphine 4 in the presence of ROA 850 (%5) catalyst in water at 80 °C for confirming diversity of these reactions (Fig. 7).

Experimental
Materials and reagents. Three wood samples used for WA preparation in this study, pine (Pinus alba), poplar (Populus nigra) and Russian olive (Elaeagnus angustifolia), were collected from Mazandaran province, Iran. All the wood samples were dried prior to catalyst preparation. The chemicals and solvents used in this work were obtained from Sigma Aldrich. Euparin was extracted from Petasites hybridus dried roots according to our previous research 29,33 . All aqueous solutions are freshly prepared using distilled water. FT-IR spectra were recorded using pressed KBr disks, using Perkin-Elmer 781 spectrophotometer. X-ray diffraction (XRD) analyses were carried out with a Philips powder diffractometer type PW 1373 goniometer. The X-ray wavelength (1.5405 Å) and the diffraction patterns were recorded in the 2 h range (10-80°) with scanning rate of 2 °C/min. Fresh WA analyzed for elemental composition using XRF using a Philips 1404 wavelength dispersive spectrom- Table 3. Effect of catalyst, its loading, and temperature on the condensation reaction of compound 5a. a Isolated yield  www.nature.com/scientificreports/ eter. All the samples were dried in an oven at 50 °C for 12 h to remove water content prior to the analyses. The morphology and particle dispersion were studied by SEM (Cam scan MV2300). The chemical composition of the prepared WA was confirmed by energy dispersive X-ray spectroscopy (EDS). The elemental analysis was employed to ascertain the resistance of C, H, and N using a Heraeus CHNO-Rapid analyzer. The mass spectra were collected using a FINNIGAN-MAT 8430 spectrometer operating at an ionization potential of 70 eV. FT-IR spectra were recorded on a Shimadzu IR-460 spectrometer. The 1 H and 13 C NMR spectra were analyzed using a Bruker DRX-500 advance spectrometer at 500. General procedure for preparation of compounds 5a-f. A mixture of 1-(6-hydroxy-2-isopropenyl-1-benzofuran-yl)-1-ethanone 1 (2 mmol), aldehyde 2 (2 mmol) and 4 ml of WEWA catalyst (5%) was added to the magnetically stirred mixture of 4 ml of WEWA catalyst (5%), alkyl bromides 3 (2 mmol), and triphenylphosphine 4 at 80 °C. After completion the reaction by TLC monitoring, the catalyst was isolated by filtration, then the product was dissolved in ethyl acetate and purified by small column chromatography (CC) or paper chromatography (PC) (Hexane: EtOAc = 5:1). During the separation by CC or PC, remaining catalyst and triphenyl phosphine oxide were also removed from the product easily and finally washed with water to afford pure title compound 5.     General procedure for the preparation of compounds 12. A mixture of 2-hydroxyacetophenone 11