Ruthenium (III) Chloride as a Novel and Efficient Catalyst for the Synthesis of Substituted Pyrroles Under Solvent-Free
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Ruthenium (III) Chloride as a Novel and Efficient Catalyst for the Synthesis of Substituted Pyrroles Under Solvent-Free Conditions Surya K. De
Received: 3 January 2008 / Accepted: 13 March 2008 / Published online: 9 May 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Paal-Knorr condensation of 2,5-hexadione with primary amines in the presence of a catalytic amount of RuCl3 under solvent-free conditions has been accomplished with an excellent yield. This method is very easy, rapid, and high yielding reaction for the synthesis N-substituted pyrrole derivatives. Keywords Pyrroles Paal-Knorr reaction Ruthenium(III) chloride Solvent-free conditions
1 Introduction Pyrroles are an important class of heterocyclic compounds having different biological activities [1]. Members of this family have wide applications in medicinal chemistry, being used as antimalarial, antiinflammatory agents, antiasthamatic, antibacterial, antihypertensive, and tyrosine kinase inhibiting agents [2]. In addition, pyrroles are found in many naturally occurring compounds such as heme, chlorophyll, and vitamin B12 [3]. Despite their importance from a pharmacological, industrial, and synthetic point of view, comparatively few methods for their preparation have been reported [4]. Of the current methods such as Hantzsch [5], Knorr [6], and aza-Witting reactions, [7] the Paal-Knorr [8] reaction is one of the most simple and straightforward methods for the synthesis of N-substituted pyrroles. Many catalysts have been used to promote the Paal-Knorr reaction such as Ti(OiPr)4 [9], Al2O3 [10], Bi(NO3)3 [11], Bi(OTf)3 [12], Sc(OTf)3 [13], montmorillonite-KSF, [14] and others S. K. De (&) Burnham Institute for Medical Research, 10901, North Torrey Pines Road, La Jolla, CA 92037, USA e-mail: [email protected]
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[15]. No reaction is observed in the absence of catalyst. However, many of these methods have some drawbacks such as low yields of the products [11], long reaction times [11], harsh reactions conditions [10], tedious work-up leading to the generation of large amounts of toxic metal-containing waste [9], the requirement for an inert atmosphere or high temperatures [12], and the use of stoichiometric [11] or relatively expensive reagents. [9, 13] This reaction is usually carried in polar and toxic solvents such as DMSO, DMF, and other. Therefore, there is a need to develop new methods using less hazardous solvents or even better, those that do not need solvents at all. Therefore, the search continues for a better catalyst for the synthesis of pyrroles in terms of operational simplicity, economic viability, and greater selectivity.
2 Results and Discussion Recently, there has been growing considerable interest in the use RuCl3 as a catalyst in organic synthesis [16]. However, there are no reports in the use of ruthenium chloride as a catalyst for the synthesis of pyrroles. In continuation of our work to develop new synthetic methodologies [17], we report herein a facile method for the synthesis of pyrroles by the condensation of 1,4-di
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