Solvent-Free Preparation of 1,8-Dioxo-Octahydroxanthenes Employing Iron Oxide Nanomaterials

In this study, 1,8-dioxo-octahydroxanthenes were prepared employing a simple, effective and environmentally sound approach utilizing an iron oxide nanocatalyst under solventless conditions. The proposed iron oxide nanomaterial exhibited high product yields, short reaction times and a facile work-up procedure. The synthesized catalyst was also found to be highly stable and reusable under the investigated conditions (up to twelve consecutive cycles) without any significant loss in its catalytic activity.


Introduction
All the natural reactions have at least one catalyst to improve its performance.Nowadays, catalysis is considered as a fundamental pillar in chemistry.Due to the needs of selecting environmentally friendly catalysts to reduce cost issues of the chemical industry [1], the selection of green catalysts has become a key challenge in modern society.Nanocatalysis is an emerging field in catalytic organic transformations.A number of chemical reactions employ nanocatalytic systems due to the larger surface area of nanoparticles compared to their bulk counterparts, giving rise to numerous catalytically active sites which lead the chemical transformations of the adsorbed reactive molecules.For these reasons nanoparticles are considered as suitable heterogeneous catalysts for a wide range of reaction.
Herein, we report on an evaluation of the catalytic activity of an iron oxide nanomaterial based on SBA-15 (FeNP@SBA-15) as active, stable and recyclable heterogeneous catalysts for the preparation of 1,8-dioxooctahydroxanthene and substituted compounds via solventless reaction between aromatic aldehydes and dimedone (Scheme 1).

Synthesis of Iron Oxide Nanocatalyst
A suspension of aminopropyl-functionalized SBA-15 materials (2.35 g, NH2 loading 0.85 mmol g −1 ) in an excess of absolute MeOH was combined with Salicylaldehyde (2 mmol, 0.244 g).The mixture color became yellow by imine formation in 6 h, after which Fe(NO)3•9H2O, (1 mmol) was added.The resulting mixture was slightly heated for 24 h, followed by formation of metal oxide nanoparticles indicated by the formation of a dark red color in the solution.The final material was filtered off, rinsed with methanol and water until colorless washings and subsequently oven-dried overnight at 80 °C.FeNP@SBA-15 exhibited 620 m 2 •g −1 of surface area and a pore size of 4.8 nm (5-7 nm iron oxide nanoparticle sizes).Typical Fe 3+ bands at BE 714 eV (Fe2p3/2) and 725 eV (Fe2p1/2) were observed by XPS for the synthesized catalyst, with only traces (<1%) of zerovalent Fe.

Preparation of 1,8-Dioxo-Octahydroxanthenes
The model reaction comprised the multicomponent reaction between an aldehyde (5 mmol), dimedone (10 mmol) and FeNP@SBA-15 (0.165 g, 0.5 mol%).In a typical reaction run, the mixture of the three components was heated at 80 °C under continuous stirring for a certain time.Reaction completion was monitored by TLC, after which the mixture was left to cool down at room temperature, followed by dissolution in dichloromethane (50 mL) and rotary evaporation to yield the final xanthene product (upon recrystallization in ethanol).The catalyst was recovered from the mixture via filtration, washed with hot ethyl acetate, oven-dried and reused in subsequent reaction runs.All products are well known and were fully characterized by IR and NMR.Scheme 1. Schematic illustration of the solventless multicomponent synthesis of xantheses catalyzed by FeNP@SBA-15.

Synthesis of Iron Oxide Nanocatalyst
A suspension of aminopropyl-functionalized SBA-15 materials (2.35 g, NH 2 loading 0.85 mmol g −1 ) in an excess of absolute MeOH was combined with Salicylaldehyde (2 mmol, 0.244 g).The mixture color became yellow by imine formation in 6 h, after which Fe(NO) 3 •9H 2 O, (1 mmol) was added.The resulting mixture was slightly heated for 24 h, followed by formation of metal oxide nanoparticles indicated by the formation of a dark red color in the solution.The final material was filtered off, rinsed with methanol and water until colorless washings and subsequently oven-dried overnight at 80 • C. FeNP@SBA-15 exhibited 620 m 2 •g −1 of surface area and a pore size of 4.8 nm (5-7 nm iron oxide nanoparticle sizes).Typical Fe 3+ bands at BE 714 eV (Fe2p 3/2 ) and 725 eV (Fe2p 1/2 ) were observed by XPS for the synthesized catalyst, with only traces (<1%) of zerovalent Fe.

Preparation of 1,8-Dioxo-Octahydroxanthenes
The model reaction comprised the multicomponent reaction between an aldehyde (5 mmol), dimedone (10 mmol) and FeNP@SBA-15 (0.165 g, 0.5 mol%).In a typical reaction run, the mixture of the three components was heated at 80 • C under continuous stirring for a certain time.Reaction completion was monitored by TLC, after which the mixture was left to cool down at room temperature, followed by dissolution in dichloromethane (50 mL) and rotary evaporation to yield the final xanthene product (upon recrystallization in ethanol).The catalyst was recovered from the mixture via filtration, washed with hot ethyl acetate, oven-dried and reused in subsequent reaction runs.All products are well known and were fully characterized by IR and NMR.

Results and Discussion
The catalytic performance of nanocatalysts is well known to depend on morphology, particle size and structure of nanoparticles [44].A number of conventional techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and inductively coupled plasma/mass spectrometry (ICP/MS) have been used to study textural and morphological properties of FeNP@SBA-15 catalysts [44].
We have previously reported in our earlier papers about the catalytic performance of FeNP@SBA-15 in various types of organic transformations including oxidation of sulfides to sulfoxides [44], esterification of carboxylic acids [45], oxidation of styrene derivatives [46] and oxidative esterification of alcohols and aldehydes (Table S1) [47].The results of the mentioned reports confirmed the high catalytic activities of supported FeNP in different conditions.
Table 2 shows that this system can be easily applied to various structurally different benzaldehyde containing electron-releasing or withdrawing group.The results of the optimized reaction in Table 2 shows that rates of reaction can be affected by different substituents in the aromatic rings.It is obvious that electron-withdrawing groups improved both yield and the rate of reaction through the activation of aromatic rings (Table 2, entries 2-4).On the other hand, the presence of electron-donating groups led to slower reaction rates (and reduced yields) as compared to electron-withdrawing groups (Table 2, entries 8 and 9).The efficiency of FeNP@SBA-15 as catalyst in the proposed synthesis was further compared with a range of literature reported data for the same chemistries (Table 3) [48][49][50][51][52][53][54][55].Results demonstrated that our method can provide excellent yields at moderate times of reaction with respect to reported procedures.Furthermore, the stability of the Fe-containing catalyst under the investigated reaction conditions was subsequently explored under optimized conditions.As Table 4 indicates, iron nanoparticles supported on SBA-15 could be recycled and reused twelve times without any appreciable reduction in catalytic activity.No iron leaching was detected in solution (<0.01 ppm, ICP-AES analysis), strongly supporting the stability of the proposed system under the optimized reaction conditions.a Reaction conditions: benzaldehyde (5.0 mmol) and dimedone (10.0 mmol), supported FeNP@SBA-15 (0.005 mmol, 0.167g) at 80 • C for 30 min; b Isolated yields.
Figure 1 also depicts a uniform distribution of particle sizes, which can also be observed in the used catalysts, and the high activity of catalysts is preserved well for up to ten runs.nal publication [1] contained a duplicated picture that was mistakenly included in the paper.The authors acknowledge that this error occurred due to the similarity in the file names, which went unnoticed during the preparation of the figures.The corrected Figure 1 appears below.The authors state that the scientific conclusions are unaffected.This correction was approved by the Academic Editor.The original publication has also been updated.Figure 1 also depicts a uniform distribution of particle sizes, which can also be observed in the used catalysts, and the high activity of catalysts is preserved well for up to ten runs.

Figure 1 .
Figure 1.Transmission electron microscopy image of spent FeNP@SBA-15 (after 10 runs).The reaction mechanism is shown in Scheme 2 in which the acidity of the Fe-containing material plays a key role in activating the carbonyl group in the first step as well as in the generated intermediate to close the catalytic circle (Scheme 2), generating the xanthene derivatives via final dehydration at 80 • C. A similar reaction mechanism based on similar acid-base carbonyl activation reactions has been recently described for Cirujano et al. using acidic H-USY or Al-MCM supports of metal oxide nanoparticles [56].

Table 1 .
Optimization of synthetic conditions for the synthesis of xanthenes a .

Table 3 .
Comparative performance of FeNP@SBA-15 with literature reported catalytic systems.

Table 4 .
Reuses of the supported FeNP catalyst in the reaction of benzaldehyde with dimedone.