Microwave Assisted Synthesis of 2 , 4-Diphenyl-4 H-chromen-5-one Using ZnCl 2 / Montmorillonite K-10

Methods used for the synthesis of title compounds were reported 3 in acetic acid and phosphorous pentaoxide at 120 0 C, in methanol triethylamine at reflux temperature, in toluene n-heptane using 4 anhydrous ZnCl2, in benzene n-heptane and anhydrous ZnCl2 at reflux temperature 5 for 28-30 h, BF3 etherate 6 etc . All the above-mentioned methods have several limitations such as pro longer reaction time, strong acidic conditions and low yield. Consequently there is a scope for further work on this reaction towards mild reaction conditions, simple workup procedure, and better yield.


Introduction
2,4-Diphenyl-4H-chromen-5-one is an important class of organic compounds, which received a considerable attention due to their wide range of biological activities.They possess antijuvenile hormone activity 1 , antianaphylactic activity in trachea 2 as antiallergic, anti-inflammatory, anticancer agents and used in diabetic complications.
Methods used for the synthesis of title compounds were reported 3 in acetic acid and phosphorous pentaoxide at 120 0 C, in methanol triethylamine at reflux temperature, in toluene n-heptane using 4 anhydrous ZnCl 2 , in benzene n-heptane and anhydrous ZnCl 2 at reflux temperature 5 for 28-30 h, BF 3 etherate 6 etc .All the above-mentioned methods have several limitations such as pro longer reaction time, strong acidic conditions and low yield.Consequently there is a scope for further work on this reaction towards mild reaction conditions, simple workup procedure, and better yield.
In the continuation of our work 7 for synthesis of tetrahydro chromen-5-one using STO-DMF, the title compound synthesis was achieved by microwave irradiations in the presence of ZnCl 2 /montmorillonite K-10.The corresponding 2,4-diphenyl-4H-chromen-5-one forms in excellent yield.
Chemical and pharmaceutical industries are always under pressure to develop more environmentally friendly organic reaction methodologies using heterogeneous catalysis.Microwave irradiation is used for a variety of organic reactions due to decrease in reaction time, cleaner reactions, easier work-up and better yield.Thus the microwave oven procedure is now well established in MORE chemistry 8 .Faster rate of organic reactions under microwave irradiation has been also reported in our earlier work 9 .Solvent free microwave assisted chemical reactions in combination of solid supported reagents was used to carry out a wide range of reaction in shorter times as compare to other conventional reaction methods.
Industrial interest has been focused on the use of montmorillonite K10-supported zinc chloride (zinc montmorillonite) heterogeneous catalyst.The use of such remarkable material has been firstly reported in 1989.It is used in several reactions 10,11 , such as dehydration reaction, Beckmann rearrangement, synthesis of flavones, thiazoles, quinolones, nitrilies, etc. Considerable attention has been focused on the use of montmorillonite K10-supported zinc chloride for the synthesis of 5-nitrofufurylidine, fisher synthesis, in synthesis of alkyl halide from alkyl alcohol or alkyl ether and hydrogen halide, in a chloroformyl reaction of hydrogen chloride and formaldehyde, benzodiazepine 12 , in an olefin polymerization, alkylation, preparation of 1,1-diacetates from aldehydes 13 , hydroamination, 3-aza-cope rearrangement etc. 14 .

Experimental
ZnCl 2 /montmorillonite K-10 catalysts was prepared according to the literature procedure 13,14 .All reagents were obtained from commercial sources and used without further purification.Melting points were determined in open glass capillaries using electro thermal melting point apparatus and are uncorrected.The reactions were monitored by TLC using pre coated silica plates (Merck, silica gel 60F-254 on glass).Column chromatography was performed using Acme silica gel (100-200 mesh).A Samsung domestic microwave oven was used at 800watt power for all the experiments.

Catalyst preparation
A requisite amount of Zinc chloride is dissolved in water at room temperature.Impregnate the solid support as montmorillonite K-10 so that the aqueous solution of zinc chloride is adsorbed on the surface of or in porous portion of the solid montmorillonite K-10.Excess of water was evaporated by heating, to load zinc chloride on montmorillonite K-10.Since zinc chloride has a high deliquescence, it deteriorates during storage before or after the preparation of catalyst.Therefore the prepared catalyst should be stored in sealed bottle or in dry room.

Results and Discussion
In a model (Scheme 1) reaction α-β-unsaturated carbonyl compounds (1) and 1, 3cyclohexanedione (2) with ZnCl 2 /montmorillonite K-10 catalysts was irradiated in microwave oven for a specific time.After usual work up it offered the pure compound (3).The catalyst is recycled for several times in subsequent reactions without change in its efficiency.Synthesis of title compound using ZnCl 2 /montmorillonite K-10 catalysts under microwave irradiations is rapid & ecofriendly process.The reaction takes place in two steps.In first step Michael addition of cyclic 1, 3 diketone to chalcone followed by cyclization of resultant diketo intermediate and formation of the product.Most of the reaction was completed within 8-9 minutes giving 90-95% yield of products.However the earlier reported method required strong acidic conditions 3 and longer reaction time 5 .The suggested method is more suitable due to the shorter reaction time and easy workup.Use of ZnCl 2 catalyst alone in organic solvent required longer reaction time (Table 2).The best result was obtained with 10 mol% of ZnCl 2 /montmorillonite K-10 catalysts.High amount of the catalyst does not improve the yield of products.Recovery of catalyst was very easy so that it can be reused for consecutive reactions without any significant loss of efficiency (Table 3).ZnCl 2 /montmorillonite K-10 exhibited superior catalytic activity as compare to the unsupported salts such as ZnBr 2 , ZnCl 2 in terms of regeneration and easy disposal.Furthermore, the homogeneous ZnCl 2 catalyst is least preferred by the industry because environmental pollution, safety and corrosion problems.

Conclusion
In summary, we have been demonstrated an efficient and mild protocol for the synthesis of 2,4-diphenyl-4H-chromen-5-one using ZnCl 2 loaded on montmorillonite K-10 as a recyclable heterogeneous catalyst.Excellent yield of products were obtained and the catalyst is recycled and reused for several times.The procedure is simple, ecofriendly, solvent free and can be used as an alternative to the existing procedure.

b
Isolated and unoptimized yield.