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Pyrrole is an extremely weak base for an amine, with a conjugate acid pK a of −3.8. The most thermodynamically stable pyrrolium cation (C 4 H 6 N +) is formed by protonation at the 2 position. Substitution of pyrrole with alkyl substituents provides a more basic molecule—for example, tetramethylpyrrole has a conjugate acid pK a of +3.7.
Below is one published mechanism for the reaction: [5] Mechanism for the Hantzsch Pyrrole Synthesis. The mechanism starts with the amine (1) attacking the β carbon of the β-ketoesters (2), and eventually forming an enamine (3). The enamine then attacks the carbonyl carbon of the α-haloketone (4). This is followed by the loss of H 2 O, giving ...
[1] [2] Although the Paal–Knorr synthesis has seen widespread use, the mechanism wasn't fully understood until it was elucidated by V. Amarnath et al. in the 1990s. [3] [4] The furan synthesis requires an acid catalyst: [5] Paal–Knorr furan synthesis. In the pyrrole synthesis a primary amine participates: Paal–Knorr Pyrrole Synthesis
The resulting product, diethyl 3,5-dimethylpyrrole-2,4-dicarboxylate, has been called Knorr's Pyrrole ever since. In the Scheme above, R 2 = COOEt, and R 1 = R 3 = Me represent this original reaction. Knorr's pyrrole can be derivatized in a number of useful manners. One equivalent of sodium hydroxide will saponify the 2-ester selectively.
The aldehyde and pyrrole are heated in this medium to afford modest yields of the meso tetrasubstituted porphyrins [RCC 4 H 2 N] 4 H 2. The reaction entails both condensation of the aldehydes with the 2,5-positions of the pyrrole but also oxidative dehydrogenation of the porphyrinogen [RCC 4 H 2 NH] 4.
Structures and names of common heterocyclic compounds Pyridine, a heterocyclic compound. A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). [1]
2-mesityl-3-methylpyrrole was synthesized in 2004 via the Trofimov reaction. The reaction of the ketoxime with acetylene yielded a mixture of products with the primary one being the N-H pyrrole. Small amounts of the N-vinyl product were also observed as well as O-vinylketoxime. The N-vinyl product was then used in the synthesis of a new BODIPY. [5]
Compound 1, a pyrrole, is coupled with aryl boronic acid, 2, to afford product 3, which is then carried forward to the target 4. The nitrile group of 2 does not poison the catalyst. Pyridine is the ligand used for the reaction. Although the reaction requires three days, it was carried out at room temperature in ambient air and resulted in a 93% ...