One-Pot Synthesis of Benzoxanthenes in Solvent Free Condition using Chloroaluminate Ionic Liquids as Catalyst

A mild and efficient method has been developed for the preparation of 14-aryl-14H dibenzo[a,j]xanthenes from one-pot condensation of aldehydes with β-naphthol using catalytic amount of Chloroaluminate ionic liquid (imidazolium chloride·3AlCl3 or pyridinium chloride·3AlCl3) under thermal solvent-free conditions. Excellent yields, short reaction times, easy workup and reusability of the catalyst as well as solvent free conditions are advantages of this procedure.


INTRODUCTION
Xanthenes related to specific class of oxygen heterocylic compounds which possess various biological and pharmacological activities such as antibacterial [1][2], anti-proliferative [3], anti-inflammatory [4], and antiviral activities [5]. Xanthenes especially benzoxanthenes are of interest as functional materials in many disciplines due to their useful spectroscopic properties. They are used as dyes [6][7], in laser technologies [8], as pH-sensitive fluorescent materials for visualization of biomolecules [9], as photostable dyes, polymerizable light emitting dyes [10][11], and white organic light emitting dyes [12]. Natural xanthene dyes may also be extracted from soil and plants such as Indigo feralongeracemosa [13]. Recently different methods have been reported for the synthesis of benzoxanthenes, including the reaction of βnaphthol with aldehydes in the presence of a catalyst, such as amberlyst-15 [14], sulfamic acid [15], molecular iodine [16], AcOH-H2SO4,in acidic medium [17], heteropoly acids (HPAs) [18], and HClO4-SiO2 [19]. However, some of these methods suffer from one or more disadvantages such as low yields, long reaction times, special apparatus, excess reagents, and the use of toxic catalyst and solvents. Thus, the development of an improved procedure for the synthesis of benzoxanthene derivatives would be highly desirable. In recent years, ionic liquids have attracted extensive interest as benign reaction media in organic synthesis because of their unique properties of non-volatility, non-flammability, recyclability, and ability to dissolve a wide range of materials [20]. As a result of their green credentials and potential to enhance reaction rates and selectivity, ionic liquids have been found increasing applications to organic synthesis. Previously reactions of xanthenes synthesis have been reported in ionic liquid in presence of catalyst, bronsted acid, and metal salts. Herein, we report one-pot synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes in Lewis acidic ionic liquids such as 1-butylpyridinium chloride and imidazolium chloride·3AlCl3.

Instrumentation:
The 13 C NMR and 1 H NMR spectrum were recorded on JEOL using TMS as internal standard. FT-IR of all synthesized xanthenes was recorded on a spectrometer Perkin Elmer Spectrum BX II from range 4000-400 cm -1 by making sample pallets with KBr. The melting points of synthesized compounds were determined on a Thomas Hoover Unimelt capillary melting point apparatus.

RESULTS AND DISCUSSION
The poor yield of 3a in 4b and 4d as compared to 4a IL could be attributed to the lower Lewis acidic nature of ZnCl2, which leads to lower yield of corresponding product. Further, the condensation reaction of (1a) in pyridinium ionic liquid 4c with same counter anion(AlCl4 -) found to be less efficient in comparison to imidazolium ionic liquid. The UV-visible spectrum of 3a showed an intense absorption band at 320 nm and 334 nm, inferring extended conjugation in 3a heterocycle ( figure 2). The appearance of a strong band at 1248 cm -1 in the IR spectrum of 3a was assigned to C-O stretching. In the 1 H NMR spectrum of 3a, aliphatic CH proton of 3a appeared as a singlet at 6.43 ppm and aromatic protons resonances were observed in 6.93-8.37 ppm region. The appearance of a peak at m/z 372 corresponding to [M] + in the electron impact-mass spectroscopy(EI-MS) spectrum further confirmed the formation of 3a in the reaction.

Entry
Ionic To check the feasibility of the optimized reaction condition, we carried out this reaction using different aromatic aldehydes (table 2). It has been found that the nature of the functional group on the aromatic ring of the aldehyde affects the reaction time and yield. The presence of electron withdrawing group at para position in comparison to the unsubstituted aromatic aldehyde shows increase of the yield while the presence of an electron donating group decreases the yield. Though meta and para-substituted aromatic aldehydes gave good results, ortho-substituted aromatic aldehydes (such as 2-nitro benzaldehyde) gave lower yields because of the steric effects [21]. Based on the results, a possible explanation for the reaction can be proposed.
The IL 4a was recovered from the reaction mixture by extracting the crude product with dichloromethane and reused for the synthesis of 3a. In the second run, 3a was obtained in 81% yield by condensation of β-naphthol with 1a in the presence of recovered 4a IL. Slight decrease in the yield of 3a was observed in third run, as 3a was isolated in 79% yield.