Indian Journal of Chemistry Vol. 46B, September 2007, pp. 1470-1474 Studies in microwave mediated solvent-free synthesis of 5-aryl-2,3-diphenylpyrroles from 4-aryl-1,2-diphenyl-2-butene-1,4-diones H Surya Prakash Rao*, Bala Kishan Gorityala & K Vasantham Department of Chemistry, Pondicherry University, Pondicherry 605 014 E-Mail: hspr.che@pendiuni.edu.in Received 7 August 2006; accepted (revised) 4 June 2007 4-Aryl-1,2-diphenyl-2-butene-1,4-diones and ammonium formate when irradiated with microwaves furnish 5-aryl-2,3- diphenylpyrroles in good yield under solvent-free conditions. Keywords: Triarylpyrroles, microwave, solvent-free synthesis The pyrrole ring is a fundamental heterocycle found as a core structure in many natural and non-natural products 1 . In view of its importance, there is a continuous effort to develop expedient synthesis of pyrroles 2 . It is of interest to synthesize 5-aryl-2,3- triphenylpyrroles because triarylpyrroles, in general, have been evaluated as estrogenase blockers 3 . A convenient synthesis of some 5-aryl-2,3-diphenyl- pyrroles starting from readily available 4-aryl-1,2- diphenyl-2-butene-1,4-diones and ammonium formate under solvent-free microwave irradiation conditions 4 is reported here. A sample of 1,2,4-triphenyl-2-butene-1,4-dione (dibenzoyl styrene) 1a, a condensation product of acetophenone and benzil, loaded on ammonium formate, was subjected to microwave irradiation at 270W for 2 min. by which time reaction was complete (TLC). From the crude product 2,3,5- triphenyl-1H-pyrrole 2a was isolated in 65% yield (Scheme I). A small amount (28%) of 3,3,5-tri- phenyl-2,3-dihydro-1H-2-pyrrolone was also formed in this reaction. It is already shown that 2,3,5-tri- phenylpyrrole could be prepared conveniently from 1,2,4-triphenyl-2-butene-1,4-dione using bench top reagents, ammonium formate and palladium on carbon 5 . The present finding shows that the transformation can be achieved conveniently under solvent-free microwave irradiation, as a load on ammonium formate, without expensive palladium on carbon. It was shown earlier 6 that 3,3,5-triphenyl-2,3- dihydro-1H-2-pyrrolone was the only product formed when the microwave irradiation 1a and ammonium formate was carried in polyethylene glycol (PEG- 200). Thus, the present study shows that under solvent-free conditions pyrrole 2a is the major product, whereas under solution phase conditions pyrrolone was the major product. The transformation of 1a to 2a was taken as a test case to optimize conditions required for the formation of pyrrole. The conversion of dione 1a to pyrrole 2a did not take place when aqueous ammonia was used instead of ammonium formate. This result indicated that in addition to ammonia, which is released by disintegration of ammonium formate, formic acid is also required to promote transformation of 1a to 2a. By varying power input of the microwave oven it was found that the transformation of 1a to 2a worked well at 270W. Generality of the transformation of 2-butene-1,4-dione 1a to triphenylpyrrole 2a was proved by subjecting butenediones 1b-g having electron-withdrawing Cl (1b) or electron-donating CH 3 (1c), 4-OCH 3 (1d), 3,4-(OCH 3 ) 2 (1e) groups in the aryl ring to the reaction with ammonium formate under solvent-free conditions. In all the cases pyrroles 2b-e were formed in good yield (59-66%). There was no correlation between electron-withdrawing or electron-donating nature of aryl substituent to the yields of pyrrole 2, or time taken for completion of the reaction. The triarylpyrroles 2b-e are new compounds and they were characterized on the basis of analytical and spectra (IR, 1 H NMR, 13 C NMR and DEPT) which compared well with the spectra of the parent compound 2a. In continuation, 2-butene-1,4- diones 1f and 1g having biphenyl and 2-naphthyl groups on C-4 respectively were subjected to microwave irradiation as a load on ammonium formate. In both the cases triarylpyrroles, viz, 5-(biphen-4yl)-2,3-diphenyl-1H-pyrrole 2f and 5-(2-