Chemistry of Heterocyclic Compounds, Vol. 38, No. 11, 2002
THERMAL RECYCLIZATION OF
5-R-2-ISOPROPENYLTETRAZOLES
INTO 5-R-3-METHYLPYRAZOLES
S. V. Voitekhovich, P. N. Gaponik, B. G. Kliaus, and O. A. Ivashkevich
Keywords: 2-alkenyltetrazoles, 3,5-disubstituted pyrazoles, thermolysis.
It is known that 2-alkenyl-5-phenyltetrazoles recyclize to 3-phenylpyrazine or its
4
-alkyl(aryl)derivatives on heating or irradiation with UV light [1]. We have observed for the first time the
thermally stimulated conversion of compounds of the given class, namely the formation of 5-substituted 3-
methylpyrazoles 1 from 5-substituted 2-isopropenyltetrazoles 2.
R
R
N
N
Me
o
1
40–150 C
–N2
N
R–C=N–N–
Me
_
N
+
HN
Me
N
2
a,b
3
1a,b
1
-3 a R = Ph, b R = Me
2
NCH
2
CH
2
The mechanism of the thermolysis apparently includes opening of the tetrazole ring with the elimination
of a molecule of nitrogen and the formation of an active nitrileimine 3, which is characteristic of 2,5-substituted
tetrazoles [2]. The nitrile imine 3 then undergoes an intramolecular cyclization into the pyrazole 1. The
observed transformation may provide a new method for the synthesis 3,5-disubstituted pyrazoles from the
relatively available tetrazoles 2 [3]. The procedure includes the prolonged heating of tetrazole 2 in a high
boiling organic solvent with subsequent removal of the solvent under vacuum. The yields of
5
-R-3-methylpyrazoles 1 is 45% (boiling of compound 2a in nonane for 8 h) and 88% (2b, DMF, 15 h). The
spectroscopic data and the physico-chemical characteristics of 3-methyl-5-phenylpyrazole (1a) correspond with
literature data [4]. Compound 1b was identified by comparison of its spectroscopic characteristics with those of
1
13
related 3,5-dialkylpyrazoles ( H NMR [4] and C NMR spectra [5]).
5
-N,N-Dimethylaminoethyl)-2-isopropenyltetrazole
(2b)
was
obtained
by
alkylation
5
-(N,N-dimethylaminoethyl)tetrazole with allyl bromide with subsequent dehydrobromination of 2-(2-bromo-1-
methylethyl)-5-(N,N-dimethylaminoethyl)tetrazole by a method described for the synthesis of 2a [3]. A viscous
oil, mp of the picrate 127-128°C (prepared by reaction of equimolar amounts tetrazole 2b and picric acid in
1
ethanol). H NMR spectrum (CD
3
CN, 100 MHz), δ, ppm: 5.93 (H, s, CHcis=C–N); 5.20 (1H, s, CHtrans=C–N);
2
.65-2.82 (2H, m, CH
2
); 2.98-3.17 (2H, m, CH
2
); 2.48 (3H, s, CH
3
); 2.26 (6H, s, 2 CH ). Found, %: C 52.83;
3
H 8.03; N 38.25. C
8
H
15
N
5
. Calculated, %: C 53.02; H 8.34; N 38.64.
_
2
_________________________________________________________________________________________
Scientific Research Institute for Physical Chemical Problems, Belorusian State University, Minsk
20050; e-mail: azole@bsu.by. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, 1607-1608,
November, 2002. Original articles submited July 29, 2002.
1
422
0009-3122/02/3811-1422$27.00©2002 Plenum Publishing Corporation