The results show that 6 is generated both by hydrolysis
and photolysis of 3, and suggest that a common pathway is
involved in both processes.
at 570 nm decays in a first-order manner (Supporting
Information, Figure S1). The rate constant, kobs, increases
-
linearly with increasing [N3 ] (Figure 4).
Addition of N3- to the hydrolysis solution does not affect
the rate of decomposition of 3, but does significantly decrease
-
the yield of 6 at very low [N3 ] (Figure 2), demonstrating
Figure 4.
Plot of kobs from LFP experiments vs. [N3-]. Data were
fit by a weighted least-squares procedure to obtain ks and kaz. The
adjusted r2 ) 0.9967.
Figure 2. Results of azide trapping experiment in pH 7.1 phosphate
buffer at 30 °C. Key: 6 (2, 212 nm), apparent major azide adduct
((, 330 nm), and apparent minor azide adduct (1, 212 nm). The
kaz/ks is the average of the fit of all three materials to the standard
“azide clock” formulas.
The slope of that plot is kaz, the second-order rate constant
for reaction of N3- with the reactive intermediate, while the
intercept is ks, the pseudo-first-order rate constant for reaction
of the intermediate with the aqueous solvent. The ratio kaz/
ks of (2.64 ( 0.13) × 103 M-1 is identical with that obtained
from the azide-trapping experiments, demonstrating that both
experiments detect the same intermediate, 4, with a lifetime
(1/ks) of ca. 530 ns.
Scheme 3 summarizes the results of our experiments. This
scheme is similar to that previously demonstrated for the
decomposition of ester derivatives of carcinogenic aromatic
hydroxylamines.12 The cation 4 is about as selective as the
4-biphenylylnitrenium ion, 7 (kaz/ks ) 2.9 × 103 M-1), that
also yields a quinol, 8, as its major hydration product.12 The
intermediate detected after LFP is definitely 4, not the imine
5, because the kinetics performed by UV spectroscopy and
HPLC show that 5 has a lifetime of about 30 min at room
temperature, while the transient generated during the LFP
experiments has a lifetime of 530 ns.
-
that N3 traps a reactive intermediate produced in a rate
limiting step. As the yield of 6 decreases, the yields of two
-
new products, not generated in the absence of N3 , increase.
Application of the “azide clock” equations11 to the yields of
these three products generates the experimental kaz/ks shown
in Figure 2. Although the azide products have not yet been
characterized, the structure of 6 and the trapping results show
-
that N3 competes with the solvent for a selective cationic
intermediate, 4. The kinetics of the formation of 6 during
hydrolysis of 3 implicates 5 as a precursor, although 5 has
not yet been detected.
Laser flash photolysis (LFP) of 3 in O2-saturated pH 7.1
phosphate buffer at 308 nm generates a transient UV
spectrum with λmax ca. 570 nm (Figure 3). The absorbance
An apparent imine intermediate can be detected by HPLC
during the conversion of 7 into 8.12 This species has a
lifetime of ca. 6 h at room temperature, while 7 has a lifetime
of 560 ns under the same conditions.12 The quinol product
6 is also the hydration product of the related oxenium ion 9
(Scheme 4).10
The azide adduct identified in that study is 10, and kaz/ks
for 9 at 80 °C is 310 M-1.10 The structure of 10 demonstrates
that the charge in 9 is highly delocalized, and kaz/ks
comparisons to other oxenium ions show that the azide/
solvent selectivity of 9 is similar to that of the 4-bipheny-
(11) Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1982, 104, 4689–
4691. Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1982, 104, 4691–
4692. Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1984, 106, 1383–
1396.
Figure 3. Transient absorbance spectrum obtained 20 ns after 308
nm excitation of 3 in O2-saturated pH 7.1 phosphate buffer. The
(12) Novak, M.; Kahley, M. J.; Eiger, E.; Helmick, J. S.; Peters, H. E.
J. Am. Chem. Soc. 1993, 115, 9453–9460. McClelland, R. A.; Davidse,
P. A.; Hadzialic, G. J. Am. Chem. Soc. 1995, 117, 4173–4174.
spectrum was recorded with a 20 ns window.
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Org. Lett., Vol. 11, No. 21, 2009
Chakraborty, Mrinal
