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F. Bentiss, M. Lagrenée, H. Vezin, M. Bouanis and B. Mernari
Vol. 39
hypophosphorous acid. The 4H compounds are being tested
as corrosion inhibitors [11].
both intermediates, that the Mulliken charge values are
identical for the two structures with the more negative
charge centered at the 4H position. This finding suggests
that methylation at the 4 position should favored. However
Scheme 3 shows that HOMO is only distributed on the
nitrogen 1 and 2 and completely absent for the nitrogen 4.
Assuming a nucleophilic substitution reaction for the
methylation it should be governed by the HOMO distribu-
tion and indicate, despite of a higher negative charge on
the 4N position, that the methylation occurs on N1 position
to yield the 1-methyl-3,5-diphenyl-1,2,4-triazoles.
Results and Discussion.
The synthesis of the 1,2,4-triazoles is shown in Scheme
1. This method involves the diazotation of the 4-amino-
1,2,4-triazole in the presence of hypophosphorous acid, so
that the diazonium salt is reduced almost as rapidly as it is
formed. Consequently no large concentration of diazonium
salt is present. The yields obtained by this method
(Table 1) are better than those obtained in procedures
where the 1,2,4-triazole is diazotized, in aqueous hydro-
chloric or nitric acid solution, prior to reaction with hypo-
phosphorous acid. The latter method leads to significantly
lower yields of triazoles along with substantial amounts of
other compounds. Elemental analysis (Table 1) and mass
spectra were used to identify compounds 2a-m. The melt-
ing points of the known triazoles are in good agreement
with those reported elsewhere (Table 1). The new triazole
Scheme 2
1
derivative structures were confirmed by NMR data. H and
13
C NMR data are given in Tables 2 and 3. Chemical shifts
of signals are in accordance with the proposed structures.
The 3,5-symmetrically disubstituted 1,2,4-triazole ring has
three nitrogen atoms making possible two tautomeric forms
possible as shown in Scheme 1. It is known that in 3,5-disub-
stituted 1,2,4-triazoles, substituents in position 3 and 5 have
additive effects on the chemical shifts of N-methyl groups, so
that the chemical shifts of the N-methyl groups of such com-
pound can be calculated. In this way, it was possible to assign
the peaks of mixtures of N-methylated derivatives obtained
by methylation of 1,2,4-triazoles [12].
Mulliken distribution charge on the anion derived from 2a
Scheme 3
The methylation reaction was effectuated on 2a. A mix-
ture of methyliodide and ethanol was added to a solution of
2a in a dilute aqueous solution of sodium hydroxyde. After
24 hours the product was extracted with chloroform.
Evaporation of the solvent gave a product that was
1
recristallized from ethanol.The H nmr spectra of this
product gave a simple methyl peak at 4 ppm indicating that
no appreciable amount of 4-methyl derivative was
observed in methylation of 2a, which is in good agreement
with the results of S. Kubota et al. [12].
These two possible tautomeric forms have been studied
by theoretical calculations using semi-empirical PM3 for-
malism. The results show that these two forms have essen-
tially the same energy of formation that is 100.28 Kcal/mol
for the 1H position and 99.38 kcal/mol for the 4H position,
respectively. This result leads us to conclude that both tau-
tomeric forms should exist in a 50/50 ratio. Calculation
were also performed on the anionic intermediate in order
to explain the methylated isomer formed. Since this
methylation occurred from a nucleophilic substitution we
have analyzed the Mulliken charge population analysis of
the anion derived 2a (Scheme 2) and the HOMO distribu-
tion along the molecule 2a (Scheme 3). We can see, for
Distribution of the HOMO energy along 2a
EXPERIMENTAL
Melting points were determined with a Digital melting point
apparatus of IA 9000 series and are uncorrected. Elemental analy-
ses of C, H and N were performed by the Elemental Analysis ser-
1
13
vice of CNRS, Vernaison, France. H and C NMR spectra were
recorded with a Bruker F.T. AC 200 spectrometer in DMSO-d
6
with TMS as internal standard. Mass spectra were obtained using
a Finnigun MAT Vision 2000 MALDI-TOF spectrometer (Laser