Silica Chloride Nano Particle Catalyzed Synthesis of 2 , 2 '-( arylmethylene ) bis ( 5 , 5-dimethylcyclohexane-1 , 3-dione ) Derivatives

Silica chloride nano particle (nano SiO2–Cl), has been found to be heterogeneous catalyst for facile and simple condensation of dimedone with aromatic aldehydes into 2,2'-(arylmethylene)bis(5,5dimethylcyclohexane-1,3-dione) derivatives in dry CH2Cl2. (doi: 10.5562/cca1721)


INTRODUCTION
Knoevenagel condensation is one of the elemental reactions in organic chemistry.Treatment of aromatic aldehydes with 5,5-dimethyl-1,3-cyclohexanedione (1) under the same reaction conditions Knoevenagel condensation and Michael addition have taken place 2,2′-(arylmethylene)bis(5,5-dimethylcyclohexane-1,3-dione) (3)  are obtained in good yield.4][5][6] These reactions have been reported at catalyzed conditions for example SDS, 6b molecular iodine, 4 HClO 4 -SiO 2 and PPA-SiO 2 (Ref.3) in water or acetonitrile at reflux condition, solvent free conditions at 50-100 °C, (Ref.5) ultrasonic 7 and microwave. 8The above mentioned methods for the production of 2,2 ′ -(arylmet-hylene)bis(5,5-dimethylcyclohexane-1,3-dione), most are not general or use unconventional reagents and conditions which are not desirable for industrial purposes, however, suffer from some disadvantages involving long reaction time, harsh reaction conditions, alternative energy source like ultrasonic and microwave, unsatisfactory yields and use of organic solvents and catalysts.Therefore it was thought worthwhile to develop a new and mild methodology that overcomes the drawbacks of previous methods.But in our procedure this reaction was done with nano silica chloride in excellent yield.
The use of heterogeneous catalysts provides a perfect solution to overcome the above limitations.In recent years, the use of reagents and catalysts on solid supports has received significant attention. 9Nano Silica gel is one of the extensively used surface material supports for different chemical transformations in organic chemistry.One such modified nano silica is nano silica chloride (nano SiO 2 -Cl) that at the first time we reported to be an efficient catalyst for the synthesis of many organic compounds such as the amorphous silica chloride.
We wish to report an efficient method for one pot Knoevenagel condensation and Michael addition of dimedone with aromatic aldehydes catalyzed by nano silica chloride (nano SiO 2 -Cl) as a simple and highly effective heterogeneous catalyst.Nano Silica gel was surface-modified by using thionyl chloride according to a know procedure, which acts as a mild and efficient catalyst, can be a useful and cheap catalyst for the synthesis of 2,2′ -(arylmethylene)bis(5,5-dimethylcyclohexane-1,3-dione).Our studies have shown that thionyl chloride is a satisfactory chlorinating agent for silica, if used undiluted.The extent of reaction with thionyl chloride gives values for active silanols per unit area of silica surface, comparable to other method, for deter-mining available activities.Nano silica chloride was prepared by the readily available material and can also be easily removed from the reaction mixture.

Preparation of Nano Silica Chloride
To an oven-dried (120 °C) Silica gel nanoparticle (average size ≈ 20 nm) (5 g) in a round bottomed flask (250 mL) equipped with a condenser and a drying tube, was added thionyl chloride (100 mL) and refluxed for one week.The excess thionyl chloride was distilled off.The resulting white-grayish powder was flame-dried and stored in a tightly capped bottle.

Typical Experimental Procedure
To a mixture of dimedone (2 mmol) and aromatic aldehyde (1 mmol) in CH 2 Cl 2 (5 mL) was added nano silica chloride (10 mg) at 40 °C.The mixture was stirred for a specified period (Table 1).The progress of the reaction was monitored by TLC.After complete conversion of the starting material, as indicated by TLC, the reaction mixture was diluted with warm methylenedichloride (25 mL) filtered and concentrated under reduced pressure to obtain the crude solid, the crude solid was purified by recrystallization from 99% EtOH to give 2,2 ′ -(arylmethylene)bis(5,5-dimethylcyclohexane-1,3-dione) derivatives (Entry a-m).Product were characterized according to their data of 1 H NMR and 13 C NMR, mass spectra and melting point.
The effect of electron and the nature of substituents on the aromatic ring did not show strongly obvious effects in terms of yields under the reaction condition.b) All products were identified by comparison of their physical and spectral data with those of authentic samples.excellent yields.
The formation of compound 3 can be explained by the mechanism that presented in Scheme 2. The reaction sequence is one-pot Knoevenagel condensation, Michael addition and tautomerization.Dimedone adds easily to electron-poor alkenes in the Michael addition fashion.Thus, cascade reactions 10 of addition, elimination and addition could be achieved, but the intermediate alkenes 2 were not available even in 1:1 experiments according to the present reaction conditions. 11he electron-poor alkenes 2 are useful intermediates for Michael additions.
These are frequently performed in solution under base catalysis, 12 but even solvent free versions at base, 13 acid, 14 or Lewis acid, 15 catalysis were never quantitative and required chromatographic purification techniques.This poor situation did not change when dry solid supports and microwave heating were applied.
Inspection of the SEM and TEM images of a sample catalyst from this reaction indicates the involvement of silica chloride nanoparticles with a size distribution of 15-35 nm (average ≈ 20 nm), (Fig s. 1 and 2).A number of different solvents such as THF, Et 2 O, nhexane, toluene, CH 3 CN and CH 2 Cl 2 were also investigated and it turned out that CH 2 Cl 2 was better suited as solvent for this purpose.The work up of the reaction was performed using simple centrifuged, filtration and the recovered nano silica chloride was readily used in 7 subsequent reactions and exhibited consistent catalytic activity for the synthesis of 2,2′ -(arylmethylene)bis(5,5dimethylcyclohexane-1,3-dione) with out further purification.However, owing to extensive agglomeration, the activity of the catalyst dramatically decreased after the seven run.

CONCLUSION
In conclusion, a mild, efficient and simple method for the one pot synthesis 2,2'-(arylmethylene)bis(5,5dimethylcyclohexane-1,3-dione) derivatives has been described using nano silica chloride as a heterogeneous catalyst.The main advantage of this method is that it requires mild reaction conditions with operational simplicity and with excellent yields.

Figure 1 .
Figure 1.SEM image of silica chloride nano particle.

Figure 2 .
Figure 2. TEM image of silica chloride nano particle.