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Can. J. Chem. Vol. 79, 2001
Scheme 1.
4-(N-Methylacetamido)phenyl azide (6, R = N(CH3)COCH3)
The 4-acetamidophenyl azide (2.5 mmol) and methyl io-
dide (3.0 mmol) were dissolved in anhydrous dimethyl
sulfoxide (50 mL) and sodium hydride hydride (95%,
3.0 mmol) was added under argon. After stirring for 1 h at
room temperature, the reaction mixture was diluted with
ether (150 mL) and the organic layer was washed with wa-
ter, dried over magnesium sulfate, and the solvent evapo-
rated under reduced pressure to give an oily residue which
solidified on standing. The crude product was purified by
column chromatography on silica gel eluting with 50:50
ethyl acetate:hexanes. 4-(N-Methylacetamido)phenyl azide
decomposed on heating above 100°C. HRMS m/z 190.0859;
C9H10N4O requires 190.0855. IR (cm–1): 2219. 1H NMR
(300 MHz, CDCl3) d: 7.62 (d, J = 8.5 Hz, 2H), 7.10 (d, J =
8.5 Hz, 2H), 3.26 (s, 3H), 1.94 (s, 3H). HPLC (see below)
showed only a single peak.
Laser flash photolysis experiments involved ca. 20 ns
pulses at 248 nm (60–120 mJ per pulse) from a Lumonics
excimer laser. A pulsed Xenon lamp providing monitoring
light. The sample was placed in a 4 × 1 × 1 cm cuvette, irra-
diated with the laser on the 4 × 1 face, and monitored per-
pendicular so that the path length was 4 cm. The cuvette was
replaced with a fresh solution after each irradiation. After
passing through a monochromator, the signal from the
photomultiplier tube was digitized and sent to a computer
for analysis.
Conventional flash photolysis experiments were per-
formed using an apparatus previously described (33) with
the sample being irradiated with a broad band flash lamp of
ca. 100 s duration.
4-ethoxyphenylnitrenium ion would be derived from the car-
cinogen phenacetin, an analgesic long removed from the
market. These cations were obtained essentially quantita-
tively upon irradiation of the appropriate azide in aqueous
solutions; they had lifetimes in water of the order of 1 ms.
In this paper, we report a study of the N-(methylacetamido)
phenylnitrenium ion, generated from the appropriate azide.
This represents the first spectroscopic detection of an arylni-
trenium ion bearing a 4-acetamido substituent. A para amido
substituent, on the ꢀ+ scale, is substantially electron donating
(ꢀ+ = –0.60) but it is not as good as p-methoxy (ꢀ+ = –0.78).
One of the objectives of our work was to see if the ꢀ+ order
was also observed when these substituents were present on
nitrenium ions.
Product analyses were performed with a Waters HPLC
system using a C18 column with 2 mL per minute flow rate
and the UV–vis detector set at 260 nm. Elution was carried
out with isocratic 60:40 acetonitrile:water. The 4-(N-
methylacetamido)phenyl azide was dissolved in water and
dilute aqueous buffers to give solutions of concentration 5 ×
10–4 M. Injection of these solutions before irradiation gave
only one peak in the chromatogram. As discussed in the Re-
sults section, irradiation of this aqueous solution at 254 nm
in a Rayonet reactor resulted in clean conversion to a peak
for an unstable species, which thermally converted over time
to a third peak with a retention time identical to that of p-
benzoquinone. That this peak did correspond to benzo-
quinone was verified from a scaled-up reaction where the
Experimental section
4-Acetamidophenyl azide (6, R = NHCOCH3)
4>-Aminoacetanilide (0.1 mol) was dissolved in 95% etha-
nol (100 mL) and, with cooling in an ice bath, 5.0 mL of
concentrated sulfuric acid was added. After 10 min, amyl ni-
trite (6.0 mL) was added in small portions. The reaction
temperature was raised to 35–40°C for 20 min, and after
cooling, the aryldiazonium salt was precipitated by the addi-
tion of ether (200 mL). The resultant precipitate was washed
repeatedly with ether until the ethereal washings gave no
coloration. The semiwet precipitate with ether was treated at
0°C with a solution of sodium azide (5.0 g) and sodium ace-
tate (10 g) in 50 mL of water. After stirring for 1 h, the
product was extracted into ether, which was washed with di-
lute sulfuric acid, water, 10% sodium carbonate, and water.
The ethereal extract was dried over magnesium sulfate and
the solvent was evaporated under reduced pressure. 4-
Acetamidophenyl azide was obtained as a solid with mp
114–118°C which was judged sufficiently pure to proceed to
1
product was isolated and its H NMR spectrum recorded.
The conversions of the azide to the benzoquinone were
quantitatively analyzed by determining the response factors
for the two, and using these to correct peak areas of the irra-
diated solutions.
Results and discussion
As shown in Fig. 1, 248 nm laser irradiation of an aque-
ous solution of 4-(N-methylacetamido)phenyl azide resulted
in a strong absorbance with a ꢁmax at 310 nm. This
absorbance was quite stable, showing only a small decay at
the longest times available with our laser apparatus (1 ms).
Addition of small concentrations of sodium azide acceler-
ated the decay, resulting in excellent first-order plots pro-
ceeding to the baseline above 290 nm. A plot of the decay
1
the next reaction stage. H NMR (200 MHz, DMSO-d6) d:
10.02 (s, 1H, disappeared in D2O), 7.63 (d, J = 8.8 Hz, 2H),
7.04 (d, J = 8.5 Hz, 2H), 2.04 (s, 3H).
© 2001 NRC Canada