Boga et al.
JOCArticle
SCHEME 6. Equilibrium between Valence Tautomers 3 and 3A
bands in the FT-IR spectrum of the azido group (in the
2100-2170 cm-1 region) were detectable in spectra recorded
in different solvents. Structures 7-9 are more populated
forms (the only detectable forms) with respect to 7A-9A.
Regarding the formation of complexes 7-9, there are two
main possibilities: (1) The azido form 3A is the reactive specie
and the ring closure occurs (by a ring-chain tautomeric
equilibrium) after the coupling of the reagents by a carbon-
carbon bond formation. (2) The less populated tetrazolic
isomer 3 reacts and remains in W-M complexes 7-9 in the
ring closed form.
Owing to the electronic effect of the azido group, the
second possibility represents the more probable reaction
pathway. In fact, the electron-withdrawing effect of the azido
group in 3A is moderate,14 as expressed by σmeta value (0.27)
of the azido substituent, while the resonance electronic effect
is an electron donating effect, as expressed by σR value
(-0.35).
With this consideration in mind, we expected that also the
coupling between 1,3,5-tris(N,N-dialkylamino)benzenes and
4,6-dinitrotetrazolopyridine (3) could produce stable W-M
complexes. In fact, within the electrophilic scale developed
by Mayr,11 compound 3 is considerably more powerful
electrophile than DNBF.12 4,6-Dinitrotetrazolopyridine
shows an equilibrium with the azido form 3A. The position
of the ring-chain tautomerism of Scheme 6 strongly de-
pends13 on the used solvent; however, 3 is a largely better
electrophilic reagent than 3A.
Herein we report about the reactions between some 1,3,5-
tris(N,N-dialkylamino)benzenes and 4,6-dinitrotetrazolo-
pyridine (3) that provide evidence of formation of the
corresponding carbon-carbon W-M complexes, giving us
not only a further example of these very rarely observed
intermediates but also the possibility to deepen and eluci-
date, through NMR spectroscopy, their behavior observed
in VT NMR experiments and to gain precious information
on the reversibility of the reaction.
This is a case which concerns a less populated form (the
tetrazolo form) as the more reactive form.
Dynamic Behavior of Complexes 7-9. Investigation on
compounds 7-9 regarding the effect of the change of the
temperature by NMR spectroscopy reveals some interesting
features.
1H NMR spectra, recorded at low temperature (below -
30 °C), of compounds 7-9 showed four signals in the region
between δ = 4.33 ppm and 6.56 ppm (see Table 1). Three of
these signals belong to the dialkylaminobenzene moiety,
while the last is due to the H-7 proton of the dinitrotetra-
zolopyridine moiety that presents the other signal (H-5) at
low field, in the characteristic region of aromatic protons.
Direct proton to carbon correlation experiments (gHSQC
sequence), performed at the same temperature, confirmed
the structure of complexes 7-9 (see Figure 1 and Figure
SI-2, Supporting Information). In the case of complex 7 in
CD2Cl2, chosen as an example, the correlation experiment
shows that the proton signals at 5.01 and 5.35 ppm are
connected to the two carbon atoms indicated as C-12 and
C-14, whose 13C signals fall at 91.5 ppm, in a range typical
of the sp2 carbon signals of 1,3,5-tris(N,N-dialkylamino)
benzene. The two remaining proton signals at 6.47 and 4.76
ppm show direct correlation with carbon signals at 63.38
and 41.63 ppm, respectively, providing clear evidence for
the sp3 hybridization of these carbon atoms of the two
partners.
Results and Discussion
Addition of a solution, cooled at -30 °C, of 4,6-dinitrote-
trazolopyridine (3) in CD2Cl2 (or in CDCl3, as well as in
CD3CN) to a solution of an equimolar amount of 1,3,5-tris
(N,N-dialkylamino)benzenes (4-6), dissolved in the same
solvent and precooled at the same temperature, produced an
1
orange solution and the appearance (in the H NMR spec-
trum) of new signals which agree with the W-M structure of
complexes 7-9 or of their parents 7A-9A. When the reac-
tion was carried out in acetone at - 30 °C, the zwitterionic
complexes were separated, as solid, from the solution.
In CDCl3 the azido form 3A is reported13 to be a largely
more populated form (about 100%) with respect to the
tetrazole form 3, which may be considered present in a very
low percent. In order to discriminate between forms 7-9 and
7A-9A we analyzed the complexes by IR spectrophotometry.
FT-IR spectral data of W-M complexes were in accor-
dance with the ring-closed form. In fact, no absorption
The apparent incongruence of two anisochronous hydro-
gens (and carbons) for the two CH signals C-12 and C-14 of
the complexes 7-9 can be easily explained because of the
presence of an asymmetric carbon center (C-7) and a “C2-
centre” (C-10):15 under these conditions, the two CH are
diastereotopic, so displaying anisochronous signal both in
1H and 13C spectra. The same effect can be observed on the
aromatic quaternary carbons that show three separated
signals, as well as on the piperidinyl, morpholinyl, and
pyrrolidinyl rings (see Tables 1 and 2).
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On raising the temperature, 1H NMR spectra of com-
pounds 7-9 in CD2Cl2 showed a gradual line broadening of
(14) (a) Exner, O. Correlation Analysis in Chemistry. Recent Advances;
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5570 J. Org. Chem. Vol. 74, No. 15, 2009