Piloty-Robinson pyrrole synthesis

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The Piloty-Robinson pyrrole synthesis , also known as the Piloty-Robinson synthesis , Piloty-pyrrole synthesis or Piloty-Robinson reaction , is a name reaction in organic chemistry . The reaction was named after the German chemist Oskar Piloty , who published it in 1910, the English chemist and Nobel Prize winner Robert Robinson (1886–1975) and his wife Gertrude Maud Robinson (1886–1954). These investigated the reaction primarily with regard to the reaction mechanism . With the help of the reaction, pyrroles can be synthesized from azines .

Overview

In the presence of a catalytic amount of a Brønsted acid (e.g. HCl ), azines react with the supply of heat and ammonia split off to form pyrrole derivatives.

R 1 = H , alkyl group , aryl group ; R 2 = alkyl group, aryl group

Instead of a Brønsted acid, a Lewis acid can also be used. In the case of aliphatic azines as starting material, pyrazolines can be formed as a by-product in this reaction .

mechanism

Mechanism of the Piloty-Robinson pyrrole synthesis; R 1 = hydrogen, alkyl group, aryl group; R 2 = alkyl group, aryl group

First, both nitrogen atoms of azine 1 are protonated . Then two C = C double bonds are formed with splitting off of two protons 2 . A rearrangement of electrons 3 leads to the formation of a new C -C bond with simultaneous cleavage of the N -N bond. A diimine 4 is formed . By protonation of the imine functions, the iminium ion 6 , which also contains an enamine group , is formed via intermediate stage 5 . The amino group 6 now attacks the carbon atom of the iminium ion intramolecularly nucleophilically. In the five-membered ring 7 thus formed, ammonia (NH 3 ) is split off with the formation of a double bond . Finally, a proton is split off from the cation 8 and the pyrrole derivative 9 is formed with aromatization .

Individual evidence

  1. ↑ O. Piloty : Synthesis of pyrrole derivatives: pyrroles from succinylosuccinic acid ester, pyrroles from azines . In: Ber. German chem. Ges. 43, 1910 , pp. 489-498 ( doi : 10.1002 / cber.19100430182 ).
  2. ↑ GM Robinson, R. Robinson : LIV.-A new synthesis of tetraphenylpyrrole . In: J. Chem. Soc., Trans. 113, 1918 , pp. 639-645 ( doi : 10.1039 / CT9181300639 ).
  3. ^ A b Z. Wang: Comprehensive Organic Name Reactions and Reagents, 3 Volume Set . John Wiley & Sons, Hoboken, New Jersey 2009 , ISBN 978-0-471-70450-8 , pp. 2217-2218.