NOVEL IONIC LIQUID [Et3N-SO3H][MeSO3]: SYNTHESIS, CHARACTERIZATION AND CATALYTIC PERFORMANCE FOR THE SYNTHESIS OF 14-ARYL-14H-DIBENZO[A,J]XANTHENES

The main aim of this work is introducing a novel protic acidic ionic-liquid catalyst for organic synthesis. Thus, N,N-diethyl-N-sulfoethanaminium methanesulfonate ([Et3N-SO3H][MeSO3]) was synthesized by the reaction of triethylamine with chlorosulfonic acid, and then with methanesulfonic acid. The structure of this novel protic acidic ionic liquid was identified by FT-IR, H NMR, C NMR and mass spectral data. Its catalytic activity was examined for the solvent-free reaction of 2-naphthol with arylaldehydes to provide 14-aryl-14Hdibenzo[a,j]xanthenes. [Et3N-SO3H][MeSO3] efficiently catalyzed the reaction, and the products were obtained in high to excellent yields and in short reaction times.

A useful and practical technique, which is in accordance with green chemistry protocols, is solvent-free conditions.This technique has several advantages with respect to the classical synthetic procedures, which consist of: (i) higher yield of product, (ii) shorter reaction time, (iii) safer reaction profile, (iv) easier workup, (v) higher selectivity in many reactions, (vi) fewer energy requirement to promote reaction, (vii) maximum incorporation of the starting materials into the aim product, (viii) prevention or minimization of waste/by-products, and (ix) avoidance of using harmful and volatile organic solvents [9][10][11].
In this research, we wish to introduce a novel protic acidic ionic-liquid catalyst for organic synthesis.To reach this goal, we have synthesized N,N-diethyl-N-sulfoethanaminium methanesulfonate ([Et 3 N-SO 3 H][MeSO 3 ]), and characterized it by means of FT-IR, 1 H NMR, 13 C NMR and mass spectra.Then, we have successfully applied the ionic liquid as an efficient catalyst for the solvent-free reaction of 2-naphthol with arylaldehydes to provide 14-aryl-14Hdibenzo[a,j]xanthenes.

EXPERIMENTAL
All chemicals were purchased from Fluka or Merck Chemical Companies.All known compounds were identified by comparison of their melting points and spectral data with those reported in the literature.Progress of the reactions was monitored by thin layer chromatography (TLC).The melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.The 1 H NMR (250, 300 or 400 MHz) and 13 C NMR (62.5, 75 or 100 MHz) were run on a Bruker Avance DPX, FT-NMR spectrometers, δ in ppm.Mass spectra were obtained with Shimadzu GC-MS-QP 1100 EX model.

General procedure for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes
A mixture of aldehyde (1 mmol), 2-naphthol (2 mmol, 0.288 g) and [Et 3 N-SO 3 H][MeSO 3 ] (0.25 mmol, 0.069 g) was stirred by a small glass rod at 110 °C.After the reaction was completed (as monitored by TLC), the reaction mixture was cooled to room temperature, and the resulting precipitate was recrystallized from ethanol (95%) to give the pure product.

RESULTS AND DISCUSSION
To develop our researches on the production and catalytic application of a new class of acidic ILs in which a SO 3 H group bonded to a positive tertiary nitrogen [5][6][7][8], we have prepared a novel member of this ILs, namely N,N-diethyl-N-sulfoethanaminium methanesulfonate ([Et 3 N-SO 3 H][MeSO 3 ]), according to Scheme 1.
Characterization of the ionic liquid was achieved by studying its FT-IR, 1 H NMR, 13 C NMR and mass spectral data; the corresponding data have been reported in the experimental section.The FT-IR data verified presence of the expected functional groups and bonds in [Et 3 N-SO 3 H][MeSO 3 ] (Table 1).], the peak corresponded to nine hydrogens of the three methyl groups was observed at 1.31 ppm as triplet.The peak appeared at 2.63 ppm as singlet is related to the methyl group of metanesulfonate.The peak related to six hydrogens of the three methylene groups appeared at 3.22 ppm as a quartet.The peak pertaining to the acidic hydrogen was seen at 9.19 ppm.
In the 13 C NMR spectrum, three peaks observed at 9.0, 40.1 and 46.3 ppm are related to the carbons of CH 3 of ethyl, CH 3 of methanesulfonate and CH 2 , correspondingly.
The mass spectrum of the [Et 3 N-SO 3 H][MeSO 3 ] showed the peak related to molecular mass [M + ] at m/z 277.In addition, the negative mode of mass spectrum showed the peaks corresponded to [M + ], [M + + 1] and [M + + 2] of the anion (methanesulfonate) at m/z 95, 96 and 97, respectively.
After that the ionic liquid was characterized, its catalytic activity was studied for the production of 14-aryl-14H-dibenzo[a,j]xanthenes.With the aim of optimizing the reaction conditions, the condensation of 2-naphthol with m-nitrobenzaldehyde (Scheme 2) was checked in the presence of various amounts (15-27 mol%) of [Et 3 N-SO 3 H][MeSO 3 ] at a range of 100-115 °C under solvent-free conditions and also in some solvents; the results are summarized in Table 2.As this Table indicates, the best results were obtained when the reaction was carried out using 25 mol% of the catalyst at 110 °C in the absence of solvent (Table 2, entry 3).The reaction was also examined in the presence of the reactants for the preparation of the catalyst (Table 2, entries 10-12); however, the reaction results were better when [Et 3 N-SO 3 H][MeSO 3 ] was applied as catalyst.Thus, our program for the synthesis of [Et 3 N-SO 3 H][MeSO 3 ] was logical.
Scheme 2. The model reaction.After optimizing the reaction conditions, effectiveness and scope of the catalyst were evaluated by examining the reaction of various arylaldehydes with 2-naphthol; the results are demonstrated in Table 3.As it can be seen in this Table, all aldehydes (benzaldehyde and arylaldehydes bearing electron-withdrawing, halogen or electron-donating substituents) afforded the relevant products in high to excellent yields and in short reaction times; these excellent results showed high efficacy and generality of [Et 3 N-SO 3 H][MeSO 3 ] to catalyze the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes.Step 2 Step 3 Step 4 Step 5 Scheme 3. The proposed mechanism for the synthesis of In fact, our novel ionic liquid (IL) is a dual-functional catalyst, because it has both acidic and basic sites (SO 3 H group is acidic, and methanesulfonate is basic); this subject has been reported for another member of this class of ILs [6].So, [Et 3 N-SO 3 H][MeSO 3 ] can particularly apply as a highly effectual and general catalyst for reactions which need to both acidic and basic catalysts simultaneously; e.g. the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes; this issue is shown in the mechanism (Scheme 3).The SO 3 H group activates the carbonyl groups (electrophiles) to accept nucleophilic attacks (steps 1, 3 and 4); the acidic group also accelerates removing H 2 O from the intermediates via hydrogen bonding with OH groups (steps 2 and 5).In the other hand, the methanesulfonate anion can speed up steps 1, 3 and 4 by activating the nucleophiles; this basic site also accelerates removal of H 2 O from the intermediates by assistance to abstracting a proton (steps 2 and 5).The mechanism is supported by the literature [6,21,26].

CONCLUSION
In summary, we have reported synthesis, characterization and catalytic performance of a novel protic acidic ionic liquid in organic synthesis.[Et 3 N-SO 3 H][MeSO 3 ] successfully catalyzed the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes; the benefits of this method include effectiveness, generality, high yields, short reaction times, easy purification of the products, simple synthesis of the catalyst from easy-available reactants, and application of solvent-free conditions in the reaction.

Table 2 .
Effect of the catalyst amount and temperature on the reaction of 2-naphthol with mnitrobenzaldehyde.
a Isolated yield.