Synthesis, aromatic character and reaction of Triazines

This presentation outlines the Synthesis, aromatic character and reaction of Triazines

1. Synthesis, reactivity, aromatic character and importance of Triazines Prepared by Dr. Krishna swamy Faculty DOS & R in Organic Chemistry Tumkur University

2. Triazines are class of nitrogen containing six membered heterocycles with molecular formula C 3 H 3 N 3 . Triazines are derived from benzene by replacement of three –CH group by three nitrogen atoms. This replacement results in three isomeric forms 1, 2, 3- isomer 1, 2, 4-isomer 1, 3, 5-isomer

3. 1, 2, 3- triazine is also called v- triazine 1, 2, 4-triazine is also called as- triazine 1, 3, 5-triazine is also called sym / s- triazine

4. All the three triazine isomers are aromatic in nature. 1, 2, 3- and 1, 2, 4-triazines has almost same basicity as pyrazine whereas 1, 3, 5-triazine less basic. Among the three triazine isomers, 1, 2, 3-triazines are least studied compared to 1, 2, 4 and 1, 3, 5-triazine isomers because of fragility of chain and rings with contiguous nitrogen atoms.

5. Till date, no compound containing the 1, 2, 3-triazines system has been isolated from natural source. Derivatives of 1, 2, 3-triazines are important class of heterocyclic compounds that are useful in organic synthesis since they act as diene in inverse-demand Diels-Alder cycloaddition. 1, 2, 3-triazines can be prepared from Tricycles Pentacycles 1, 2, 3-triazines

6. Tricycles The rearrangement of cyclopropenyl azide is a method used for the synthesis of monocyclic 1, 2, 3-triazine. This reaction occurs via [3+2] cycloaddition followed by ring expansion.

7. The more general synthetic method to obtain various mono-, di - and tri-substituted alkyl and aryl triazines is by oxidation of N- aminopyrazole with lead tetra acetate ( Pb ( OAc ) 4 ). The amino nitrogen incorporated into triazine ring probably via insertion of nitrene moiety to the N-N bond of the pyrazole ring.

8. Reaction with electrophilic reagents Electrophilic reaction on the ring carbon of 1, 2, 3-triazine can not proceed because of intensive pi-electron deficiency of the ring system. However, electrophilic reaction proceed on the ring nitrogen of substituted 1, 2, 3-triazines.

9. Reaction with nucleophilic reagents 1, 2, 3-triazine are highly π electron deficient and are readily attacked by nucleophiles. The reaction occurs exclusively at the C-4 position, even in the presence of a substituent at C-4.

10. Cycloaddition reaction 1, 2, 3-triazine are useful compounds that participate in cycloaddition reactions. These compounds behaves as π -deficient dienes and undergo inverse demand Diels-Alder addition with electron rich dienophiles .

11. 1, 2, 4-triazine 1, 2, 4-triazine are biologically very active compounds. Many have been tested for use in agrochemistry or medicines. 1, 2, 4-triazines can be prepared from Other heterocycles Condensation reactions

12. The transformation of 1-benzohydrazonoyl-1, 2, 3-triazoles into 1, 2, 4-triazine by heating with a slight excess of sodium hydride. This reaction proceed via Bamford -Stevens reaction followed by the ring enlargement by insertion of carbene gives 1, 2, 4-triazines. Other heterocycles

13. The synthesis of 1, 2, 4-triazine via condensation of 1, 2-diketones with acyl hydrazides and ammonium acetate under thermal condition. Condensation reaction

14. Reaction with electrophilic reagents The salt of simple 1, 2, 4-triazines are obtained by addition of dry acids to a solution of the triazine in organic solvents. Alkylation and acylation of 1, 2, 4-triazine systems have been studied extensively.

15. Reaction with nucleophilic reagents The most reactive position of the 1, 2, 4-triazine is position-5. This position is easily attacked by nucleophilic agents. Most leaving groups in 3-, 5- and 6-positions of 1, 2, 4-triazines can be easily substituted by carbon nucleophiles prepared from –CH acidic compounds.

16. Cycloaddition reaction 1, 2, 4-triazines behaves as electron deficient dienes and react with dienophiles such as electron rich alkenes and alkynes.

17. 1, 3, 5-triazine 1, 3, 5-triazines have been studied extensively both theoretically and experimentally. One of the most important applications of 1, 3, 5-triazines is in the field agricultural field as fungicides, insecticides and herbicides. Some representative examples includes

18. 1, 3, 5-triazines are often referred by trivial names such as cyanuric acid, cyanurates , cyanuryl chloride and melamines .

19. Synthesis The best method to obtain 1, 3, 5-triazines is the reaction of ammonium acetate and triethylformate .

20. Pinner triazine synthesis Reaction of aryl amidines and phosgene results in the formation of 2-hydroxy-4, 6-diaryl-s-triazines.

21. Trimerization of cyanogen chloride in the gas phase on activated charcoal is probable most useful industrial route to obtain cyanuric chloride.

22. Reaction with electrophilic reagents Do not undergo electrophilic substitution reaction. However, under vigourous conditions chlorination and bromination probably via nucleophilic addition reaction.

23. Reaction with nucleophilic reagents 1, 3, 5-triazine are extremely sensitive to nucleophilic substitution, particularly hydrolysis in presence of even trace of atmospheric water. Nucleophilic attack at the aromatic 1, 3, 5-triazine results in ring cleavage. This ring opening leads to the formation of important heterocycles like triazole and benzimidazole .

24. Formylating agents In Gatterman’s aldehyde synthesis, hydrogen cyanide can be substituted by 1, 3, 5-triazine. In presence of HCl, triazine undergoes electrophilic attack on aromatic compounds like furan, pyrroles and polyhydroxy benzenes yielding an intermediate aldimine hydrochloride which on hydrolysis afford aldehyde.

25. Several nucleophilic substitutions of halogen derivatives of 1, 3, 5-triazine take place by displacement of chlorine atom of cyanuric chloride with different nucleophiles. It is possible to obtain mono-, di - (or) tri-substituted-1, 3, 5-triazines by controlling the nature of nucleophiles (or) the reaction temperature.