2
700
The work of Wentrup was particularly significant in this field.5,7–11 In fact, he demonstrated that
nitreno-azines and -diazines, generated by gas-phase pyrolysis of the corresponding tetrazolo-azines and
diazines, may produce the cyano derivatives of the corresponding five-membered heterocycles. However,
these cyano heterocycles were obtained, as the main products, only in the case of the tetrazolo[1,5-
-
7
7
c]pyrazines and tetrazolo[1,5-c]pyrimidines. In the other cases gas-phase pyrolysis produced mixtures
of compounds and the cyano components were present only in small amounts.
We were very surprised to ascertain that little is known about the ring contraction of heterocyclic
azides when the reaction is performed at the melting temperature. In this regard, we anticipated that
pyrolysis of the tetrazolo[1,5-c]pyrimidines 4 (Scheme 2) may be obtained by warming the starting
12
tetrazole derivatives at about 10–20°C over the melting temperature, and we described this reaction
as a practical means to achieve the preparation of many 1-cyanoimidazole heterocycles. In addition,
alkyl- and arylimidazoles were also obtained in high yield by pyrolysis of azidopyrazines.1
3,14
Scheme 2.
Thus, our ongoing interest in the thermolysis of azido-azines and -diazines prompted us to extend the
above cited ring contraction to other heterocyclic azides. In this letter we wish to report our preliminary
results concerning the thermolysis of the tetrazolo[1,5-a]pyrimidines 7 and tetrazolo[1,5-b]pyridazine
10 which allowed a facile preparation of 1-cyanopyrazoles. The reaction was also attempted with the
tetrazolo[1,5-a]pyridines 13a thus providing an easy preparation of 1-cyanopyrroles which is of great
synthetic utility. Therefore, unlike Wentrup, our methodology affords higher yields of the corresponding
cyano heterocycles by means of simpler operational conditions. Since the cyano group is a versatile
precursor of other functionalities, the reaction appears to be of particular interest for the construction of
a variety of pyrazoles and pyrroles.
Pyrolyses of the tetrazolo[1,5-a]pyrimidines 7 and tetrazolo[1,5-b]pyridazines 10 (Schemes 2 and 3)
were performed in a standard pyrex tube (35/5 cm) joined to a condenser and plunged (about 5 cm) in
1
5
an oil bath. About 20 mmoles of the opportune tetrazole were put on the bottom of the tube and the
system was carefully heated at about 10–20°C over the melting temperature of the sample. As soon as the
compound melted evolution of nitrogen was observed. The reaction was particularly fast (few minutes),
then the tube was allowed to cool to room temperature and the desired cyano heterocycles were in general
collected from the walls of the tube. Both the tetrazolo[1,5-a]pyrimidine and tetrazolo[1,5-b]pyridazine
heterocycles gave facile thermolysis and the ring contraction produced the pyrazole derivatives 9 and
12 as the only detectable products (Table 1: a selected list of representative results). On the contrary,
when pyrolysis of the tetrazolo[1,5-b]pyridazines was performed at 380°C/0.02–0.03 mm, as described
7
by Wentrup, a mixture of cyanoallene, tetralonitrile, propargyl cyanide and 2-cyanocyclopropene
was obtained. Surprisingly, there was no evidence of the presence of the 1-cyanopyrazole. Moreover,