We investigated several reactions to reduce the azo group, using azobenzene as example, with reagents
of the types indicated above. Diketones 1a,b in boiling acetic acid do not reduce azobenzene, 9-phenyl-
1,2,3,4,5,6,7,8-octahydro-9H-xanthene (8) does not reduce it also. On the other hand 9,10-diphenyl-
1,2,3,4,5,6,7,8,9,10-decahydroacridine (9) under the same conditions reduces azobenzene to aniline. The
reduction of azobenzene occurs even more rapidly on treating it with a mixture of diketone 1b and benzylamine
(i.e. N-benzyldecahydroacridine in statu nascendi). Therefore dihydropyridine, but not 4H-pyran, derivatives
are reducing agents at least in the first stage of the reduction of the azo group.
We then checked the possibility of reducing hydrazobenzene with the same reagents under the same
conditions. Both pyran 8 and dihydropyridine 9 reduce hydrazobenzene with the formation of 9,10-diphenyl-
1,2,3,4,5,6,7,8-octahydroacridinium salt (10) (isolated as the perchlorate). Formation of aniline was also fixed
on reduction with the aid of dihydropyridine 9. Diketones 1a,b also reduce hydrazobenzene on boiling in acetic
acid (evidently the reducing agents are 4H-pyran derivatives formed as intermediates). Salts 3a,b are the
reaction products. Derivatives of both dihydropyridine and of 4H-pyran may therefore act as reducing agents in
the subsequent stages of reduction of the azo group. Compounds 2b, 3a,b, 4b, and 10 were identified by
comparison with known samples. The remaining salts were not known previously.
There were absorption bands in the IR spectrum of salt 2a at 3360 and 3440 cm-1 for the primary amino
group. In the spectra of salts 6a,b and 7 there were absorption bands at 3320-3330 and 1655-1695 cm-1 for the
CO–NH group. In the spectra of the remaining new salts there was no absorption in the 1620-1800 region nor
above 3100 cm-1.
In the NMR spectra of the tricyclic pyridinium salts (Table 1) the aliphatic protons gave three groups of
signals. In the 2.95-3.30 and 2.45-3.00 ppm regions there were signals of protons linked respectively with the
α- and β-positions of the pyridinium ring, the remaining alicyclic protons gave signals at 1.80-2.35 ppm. The signal
TABLE 1. 1H NMR Spectral Characteristics of Pyridinium Salts
Chemical shifts, δ, ppm (J, Hz)
Com-
CH2
*
pounds
4-H, s
Ar–H
Other
2
3
*
*
3.21, t
2.91, t
2.30, q
8.1 7.57-7.68, m
8.46 8.08, s
2a
4a
5a
(J = 7.5)
(J = 7.5)
(J = 7.5)
3.18, t
2.93, t
2.22, q
(J = 7)
(J = 7)
(J = 7)
3.24, t
2.97, t
2.32, q
8.09 7.53-7.59, m,
7.78 d (J = 8),
7.95-8.00, m,
(J = 7.5)
(J = 7.5)
(J = 7.5)
8.15 d (J = 8)
3.00, br. d
2.47-2.56, m 1.85, q
7.92 7.44-7.56, m,
7.66 d (J = 7),
7.90-8.00 m,
5b
(J = 3)
8.17 d (J = 7)
3.19, t
2.89, t
2.27, m
8.01 7.40, d (J = 8), 2.26 s (СН3СО),
6a
6b
7
(J = 7.5)
(J = 7.5)
7.99, d (J = 8)
7.86 7.26, d (J = 8), 2.26 s (СН3СО),
8.02, d (J = 8) 9.09 s (СОNH)
9.22 s (СОNH)
2.97, br. s
2.50 br. s
1.81, br. s
1.82, br. s
2.97, br. s
2.50, m
7.88 6.86, d (J = 8), 2.25 s (СН3СО),
7.06, d (J = 8), 9.09 s (CONH)
7.26, d (J = 8),
8.02, d (J = 8)
_______
* Protons of CH2 groups linked to positions 2 and 6 of the pyridinium ring.
*2 Protons of CH2 groups linked to positions 3 and 5 of the pyridinium ring.
*3 Protons of remaining CH2 groups.
419