The versatile Knorr pyrrole synthesis is an important route to pyrroles it involves the condensation of a β-keto ester with an α-amino ketone An important pyrrole synthesis, known as the Knorr synthesis, is of the cyclizative condensation type. An α-amino ketone furnishes a nucleophilic nitrogen and. Knorr Pyrrole Synthesis. Schematic representation of the Knorr Pyrrole Synthesis. Mechanism of the Knorr Pyrrole Synthesis. Mechanism of the Knorr Pyrrole.
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The Paal-Knorr Pyrrole Synthesis is the condensation of a 1,4-dicarbonyl compound with an excess of a primary amine or ammonia to give a pyrrole.
The reaction can be conducted under neutral or weakly acidic conditions. Venkataraman Amarnath has shown J. Any mechanism such as the following one that involves the formation of an enamine before the rate-determining step – the cyclization – must be ruled out.
If the ring is formed from an imine that is generated from a primary amine, a charged immonium ion must be an intermediate. Amarnath tried to stabilize or destabilize the immonium ion with different aryl groups as substituents:. syntehsis
Paal-Knorr Pyrrole Synthesis
The use of ammonia should give an uncharged intermediate and is therefore less affected by the choice of substitutents. The substituents also influence the basicity of the imine, with the nitro group leading to a more basic nucleophile.
The rates of cyclization have been compared using ammonia and methylamine. The nitro group has in every situation had a positive effect on the knnorr rate. The methoxy group has a negative effect on the cyclization rate in each case.
Comparison of the relative reaction rates of all substrates R: A mechanism that accounts for the influence of different substitution patterns mesodl and explains the influence of a p -nitrophenyl group making a nucleophile more reactive although not as the imine includes the cyclization of a hemiacetal which is followed by different dehydration steps:.
A more detailed description can be found in the work by Venkataraman Amarnath, and references cited therein J. Saidi, Synlett, Magnesium Nitride as a Convenient Source of Ammonia: Preparation of Pyrroles G. Ley, Synlett, Microwave mediated facile one-pot synthesis of polyarylpyrroles from butene- and butyne-1,4-diones H.
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