Triton-B Adsorbed on Flysh: An Efficient Support for the Base Catalysed Reactions under Microwave Irradiations

Introduction

Usually the base catalysed reactions are done in presence of pyridine or piperidine whch are hazardous in nature.  The well known example include synthesis of cinnamic acids by condensation of various aromatic aldehydes with malonic acid by Deobner reaction¹ under conventional² as well as under microwave irradiations³.  Likewise 3-carboxycoumarins are synthesized using Meldrum’s acid⁴ by knoevenagelreaction using malanonitrile⁵.

Herein we wish to report the use of a mild base, triton-B adsorbed on flyash (25% composition) for the base catalysed reactions under microwave irradiations.  In recent years, microwave dielectric heating technology combined with solvent free conditions, has been used in many organic reactions, leading to shorter reaction times, higher yields, cleaner reaction products and environmentally more benign conditions compared with the classical heating^6,7.

Result and discussion

The main goal of this work is to study the use of a mild base Triton-B adsorbed on flyash for studying a number of base catalysed reactions so as to reduce the reaction time and the use of hazardous reagents.  When the same reactions were repeated under ordinary thermal conditions these took much longer time (55-80min) and yields were also not good.

The above said support (Triton-B flyash) has been used for the rapid synthesis of a number of cinnamic acids (la-k).  These acids are synthesized by irradiating a mixture of aromatic aldehyde, malonic acid and triton-B adsorbed on flyash (Scheme 1).

Scheme 1 Click here to View scheme

 

Also triton-B has been found useful for the synthesis of a number of 3-carboxycoumarins (2a-d).  These coumarins are synthesized by irradiating a mixture of salicylaldehyde, malonic acid and triton-B adsorbed on flyash (Scheme-2).

Scheme 2 Click here to View scheme

 

This base has further been used successfully for the preparation of  1-(2-hydroxyphenyl)-5-phenylpent-4-ene-1, 3-diones (3a-e), the compounds required for the synthesis of naturally occurring chromone derivatives with a styryl (2-phenylethenyl) or a dihydrostyryl (2-phenylethyl) substituent at 2-position, by Baker-Venkataraman rearrangement.  This rearrangement is generally carried out by heating 2-cinnamoyloxyacetophenones with pulverized potassium hydroxide in pyridine medium.  Herein we wish to report their synthesis in a simple way by irradiating  2-cinnamoyloxyacetophenones (the esters of cinnamic/p-methoxycinnamic acid with substituted o-hydroxyacetophenones) in the presence of triton-B adsorbed on flyash (Scheme-3).

 

Scheme 3 Click here to View scheme

 

The structure of all the products have been confirmed by the elemental analysis as well as by their IR and NMR spectra.  The results are reported in Table 1.

Table 1: Characterization data, reaction time and yields of compounds (1a-k), (2a-d) and (3a-e) under microwave irradiations and  classical methods. 

Compd	R1	R2	R3	Method A		Method B			M.p.	lit MP (oC)
				Time (sec)	Yield (%)	Time (min)		Yield (%)
1a	H	H	H	30	97	55		70	133	1338
1b	H	H	CH3	45	96	55		60	196-97	195-2038
1c	CH3	H	H	45	94	60		65	179-80	185-868
1d	H	H	Cl	50	93	70		60	248-49	249-508
1e	H	H	OCH3	50	94	75		70	171	1728
1f	H	H	NO2	50	96	75		60	273	2808
1g	H	H	OH	55	96	70		70	203	2068
1h	H	NO2	H	60	97	80		75	197	1978
1i	H	OCH3	OH	60	94	80		65	166	1688
1j	H	OCH3	OCH3	70	96	80		60	178	1808
2a	H	H	—	35	96	55		60	185	186-885
2b	H	OH	—	40	94	55		70	280	2835
2c	H	Cl	—	50	94	60		75	238-39	2405
2d	OH	H	—	40	95	60		65	246-47	248-505
3a	H	H	H	30	93	60		75	132-33	133-349
3b	H	CH3	H	45	95	70		75	130-31	1339
3c	CH3	H	H	45	96	70	60		148	1499
3d	H	H	OCH3	60	97	75	65		137-38	1409
3e	CH3	H	OCH3	60	92	80	65		150-51	1529
Method A – Microwave irradiation Method B – at 100oC on a waterbath

 

Experimental

All the melting points were taken in open capillaries and are uncorrected.  Homogeneity of compounds was routinely checked on silica gel-G TLC plates using benzene: ethyl acetate (9 : 1) as eluent.  The reactions were carried out in a domestic microwave oven (Samsung, Model No. CE118KF, output energy 900W, frequency 2450 MHz with temperature control arrangement) using 30% power for all experiments maintaining the oven temperature at 40^oC.

General procedure for preparation of cinnamic acids (la-j)

A mixture of aromatic aldehydes (5 mmol), malonic acid (6 mmol) and triton-B adsorbed on flyash (25% composition) was subjected to microwave irradiation.  After the completion of reaction (TLC), the reaction mixture was dissolved in chloroform and filtered to remove the flyash.  Distilled off the solvent and added saturated solution of sodium bicarbonate to the residue to remove the unreacted acid.  Filtered the solid separated, washed with water, dried and  crystallized from alcohol to get the desired product.

General procedure for preparation of 3-carboxycoumarins (2a-d)

Salicylaldehyde (5 mmol), malonic acid (6 mmol) and triton-B adsorbed on flyash (25% composition) was subjected to microwave irradiation.  After the completion of reaction (TLC) the reaction mixture was dissolved in chloroform and filtered to remove the flyash.  Distilled off the solvent and added saturated solution of sodium bicarbonate to remove the unreacted acid.  Filtered the solid obtained, washed with water, dried and crystallized from alcohol to get the desired product.

General procedure for preparation of 1-(2-hydroxyphenyl)-5-phenylpent-4-ene-1, 3- diones (3a-e)

2-Cinnamolyoxyacctophenone (5mmol) and triton-B adsorbed on flyash (25% composition) was subjected to microwave irradiation.  After completion of reaction (TLC) the reaction mixture was dissolved in chloroform and filtered to remove the flayash.  Distilled off the solvent and the solid separated was filtered, washed with water, dried and recrystallized from alcohol to get the desired product.

Conclusion

The use of triton-B adsorbed on flyash for the synthesis of cinnamic acids, 3-carboxycoumarins and 1-(2hydroxyphenyl)-5-phenylpent-4-ene-1, 3-diones under microwave irradiation proved to be highly efficient and convenient method.  This method is superior to conventional heating procedure, where the reaction takes much time (55-80 min) and the yields are significantly lower.

Acknowledgement

The author is thankful to the Head, Department of Chemistry, M.D. University, Rohtak for providing necessary facilities.

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