4-Phenyl-1,2,4-triazole-3,5-dione

• 4-phenyl -1 ,2,4- triazoline -3 ,5- dione
• 3,5- dihydro -4-phenyl -4H- 1 ,2,4-triazole -3 ,5- dione

Red solid

Fixed

165 ° C

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PTAD ( IUPAC name: 4-phenyl -1 ,2,4- triazoline -3 ,5- dione ) is a Azodicarbonylverbindung. It was first shown in 1894 by Johannes Thiele and O. rod by oxidation of 4- phenylurazole with red lead in sulfuric acid. For a characterization of the amount of substance shown, however, was not enough. A preparatively useful synthesis was published by Richard Clive Cookson in 1971.

Extraction / representation

The synthesis starts from hydrazine and diethyl (1). These react to Ethylhydrazincarboxylat ( 2), which is reacted with phenyl isocyanate (3). The resulting 4 - phenyl-1- carbethoxysemicarbazid ( 4) is cyclized with base to give 4- phenylurazole (5) is formed from the oxidation finally by the PTAD (6).

Chemical Properties

PTAD is only stable in the absence of light and moisture, contact with alkalis occurs immediate decomposition. Water, alcohols and acids lead to slow decomposition. A melting or boiling point can not be determined because the compound from 160 to 180 ° C, decomposes without melting.

It is currently the most reactive known dienophile for Diels- Alder reactions. It is a thousand times more reactive than tetracyanoethene and even two thousand times more reactive than maleic anhydride. So simple cycloaddition reactions take place at very low temperatures and even with cyclooctatetraene, which is not an appropriate 1,3-diene for Diels -Alder reaction because of its tub shape, it can respond to the expected cycloadduct. It is believed that the reaction does not in this case performs, but occurs over a homotropylium zwitterion.

Use

In addition to the use in the Diels -Alder reaction, and other cycloaddition reactions, it is used because of its high reactivity and reaction rate as the capture reagent for unstable molecules with conjugated double bonds. With PTAD for the first time, the synthesis of highly strained prismane.