Dediazonation of aromatic amines
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 1, January, 2012
207
Scheme 2
ratio of the reaction components is 1 : NaNO2 : 2 : Et3N =
= 1 : 6 : 1 : 1.
Scheme 1
Note that hydrodediazoniation in chloroform occurs
with high selectivity, since no even trace amounts of the
corresponding chloroarenes are observed in the products.
As in the case of using CCl4, the decomposition of salts
3c—f in chloroform is efficient only in the presence of
triethylamine. The reactions of reduction of diazonium
salts are significant for organic synthesis, and the discovꢀ
ered method of hydrodediazoniation with the Et3N/CHCl3
system is novel and also interesting from the preparative
point of view.
Thus, we found that the diazotization of aromatic
amines in the presence of lipophilic pꢀdodecylbenzeneꢀ
sulfonic acid for the first time allows this process to be
carried out in nonpolar solvents CCl4 and CHCl3. Under
the action of triethylamine, the intermediate diazonium
salts exhibit properties that are not typical of the known
diazonium salts, undergoing chlorodediazoniation in CCl4
or reducing in chloroform.
Ar = pꢀMeC6H4 (a), 45%; pꢀC6H13C6H4 (b), 88%; pꢀMeOC6H4 (c), 51%;
pꢀPhN=NC6H4 (d), 89%; pꢀNCC6H4 (e), 63%; pꢀNO2C6H4 (f),
35%; oꢀNO2C6H4 (g), 49%; mꢀNO2C6H4 (h), 54%.
As far as we know, the observed transformations have
no precedents in the chemistry of diazonium salts, and we
found no data on the use of acid 2 as an acidic component
of diazotization and of triethylamine as an agent inducing
halodediazoniation. The results of chlorodediazoniation are
general, since they involve amines with electronꢀdonating
substituents and different positions of the substituents in
the benzene ring.
The possibility of amine diazotization in nonpolar CCl4
is determined, of course, by the lipophilic properties of
acid 2 providing solubility in CCl4 that is not typical of the
known diazonium salts. Intermediate salts 3 were not isoꢀ
lated; however, their presence in the reaction mixture was
proved by the quantitative formation of the azocoupling
products with ꢀnaphthol, which is a typical reaction of
diazonium compounds. Heating of solutions of salts 3 (obꢀ
tained at the first stage of the reaction) in CCl4 in the
absence of triethylamine for 4 h did not result in their
complete conversion and gives low yields of the correꢀ
sponding chloroarenes 4. The addition of triethylamine
induces the instant decomposition of salts 3a—h with niꢀ
trogen evolving and the parallel formation of chloroarenes
4a—h. The effect of pyridine resembles that of triethylꢀ
amine but is less pronounced.
Although the detailed mechanism of the found chloroꢀ
dediazoniation reaction with CCl4 and the initiating role
of triethylamine were not studied, it is evident that these
transformations are freeꢀradical, which is confirmed by
the GC/MS observation of hexachloroethane as a couꢀ
pling product of trichloromethyl radicals in the reaction
mixture.
The diazotization of anilines 1c—f via Scheme 1 proꢀ
ceeds successfully in chloroform; however, the subsequent
decomposition of intermediate salts 3c—f proceeds via
different route than that in CCl4, and the complete reducꢀ
tion (hydrodediazoniation) occurs within 30—40 min inꢀ
stead of chlorodediazoniation to form the corresponding
arenes 5c—f as the single products (Scheme 2), which
were identified by gas chromatography.
Chloroform, CCl4, triethylamine (reagent grade) and 4ꢀdoꢀ
decylbenzenesulfonic acid (Aldrich) were used as received.
Chlorodediazoniation of amines 1a—h (general procedure). The
corresponding aniline (3 mmol) and NaNO2 (18 mmol) were
added to a solution of 4ꢀdodecylbenzenesulfonic acid (2) (0.98 g,
3 mmol) in CCl4 (10 mL). The solution was stirred at 20 С for
1.5—2 h until the initial aniline disappeared (TLC, eluent
C6H6—EtOH (9 : 1, vol.)). The reaction mixture was heated to
70 С, and triethylamine (0.16 mL, 3 mmol) was added, resultꢀ
ing in instant nitrogen evolving. Then the mixture was kept for
30—40 min to the negative reaction with ꢀnaphthol to the diꢀ
azonium salt. Anhydrous Al2O3 (0.9 g) was added to the reaction
mixture, which was stirred for 15 min and filtered off. The filꢀ
tered off Al2O3 was washed with CCl4 (20 mL), the combined
solutions of CCl4 was passed through a column with Al2O3, and
the solvent was evaporated in vacuo. The resulted chloroarenes
1
were identified by the H, 13C NMR and mass spectra and by
comparing with authentic samples.
Hydrodediazoniation of amines 1c—f (general procedure). The
reactions were carried out and treated as described above using
CHCl3 instead of CCl4. The obtained solutions of arenes in
CHCl3 were analyzed by GC/MS, and the chromatograms indiꢀ
cates peaks of compounds 5c—f only.
Methoxybenzene (5c). MS, m/z: 108 [M]+ (100), 93 (15),
77 (60), 65 (58), 51 (12), 39 (18).
1, 2ꢀDiphenyldiazene (5d). MS, m/z: 182 [M]+ (34), 152 (9),
105 (19), 77 (100), 44 (33).
Benzonitrile (5e). MS, m/z: 103 [M]+ (100), 77 (31), 50 (10),
44 (20), 40 (80).
Nitrobenzene (5f). MS, m/z: 123 [M+] (56), 107 (7), 93 (15),
77 (100), 65 (12), 51 (39), 43 (11).
Kutonova, K. V.
