The Journal of Organic Chemistry
Article
The pH-rate profile indicates the reaction rates increases
with increasing the concentration of TAZ(−). The greatest
reactivity of the nucleophile is observed at the plateau at pH ∼
TAZ(−) and DEDNPP. The activation parameters are
summarized in Table 2. Additional kinetic studies also indicate
1
1.5 (Figure 1A), consistent with near complete deprotonation
Table 2. Activation Parameters for the Reaction of TAZ (1.0
−
1
−5
−1
of TAZ forming its anionic and reactive form, which is in
mol L ) with DEDNPP (5.2 × 10 mol L ) at pH 11.4
43
a
agreement with its pK of 10.1. Results indicate that in more
and 298.15 K
a
acidic solutions (pH < 6) the rate of formation of DNP
decrease severely, revealing that the neutral form of TAZ
ΔH (kcal mol−1)
‡
ΔS (cal K mol−1
‡
−1
)
ΔG (kcal mol−1)
‡
7
.9
−31.9
presents extremely low reactivity, consistent with its pK of
aH
4
3
a
2
.5. Indeed, we observed that at pH 3.58, the cleavage of
DEDNPP in the presence of TAZ is less than 2 times faster
cleavage of Paraoxon evaluated in pH range 8.4−11 also
indicates the anionic form of the nucleophile is responsible for
its expressive reactivity. The lower reactivity observed for
Paraoxon compared to DEDNPP emphasizes the influence of
the worst leaving group 4-nitrophenol (pK = 7.1) in the
former, compared to 2,4-dinitrophenol (pK = 4.1) in the
latter. Remarkable rate enhancements on the order of 10 and
a nucleophilic mechanism. First, a linear profile obtained for
k
obs versus TAZ(−) concentration is consistent with a single
nucleophile molecule involved in the rate determinant step.
Second, solvent kinetic isotope effect (SKIE) investigated in
D
2
O at pD 11.2 and 12, indicating no significant change on
reaction rate compared to kobs determined in H O at pH values
4
5
a
2
4
5
a
6
1
5
1
0 -fold compared to the neutral hydrolysis of DEDNPP and
Moreover, kinetic studies also suggest that TAZ(−) presents
no significant propensity to promote DEDNPP degradation via
plausible reaction mechanism considering one of two possible
nucleophilic centers of TAZ(−), consistent with the
nucleophilic attack of nitrogen lone pair on the phosphorus
atom, forming a phosphorylated intermediate and the nontoxic
diester diethyl phosphate (DEP). NMR analysis and DFT
calculations corroborate this proposition and also allowed to
suggest the main nucleophilic center, as discussed as follows.
Mechanistic Investigation by NMR. As a way to obtain
further insights on the mechanism of dephosphorylation of
Paraoxon were observed, respectively. It is worth mentioning
that even in weakly basic medium (pH = 8.4) rapid
detoxification of Paraoxon promoted by TAZ(−) was
3
observed, as noticed by the rate acceleration of >10 -fold
compared to spontaneous hydrolysis.
−
kobs = k + k [OH ] + k [TAZ]χ
0
OH
kN
TAZ
(1)
Thus, results suggest that the anionic form is responsible for
the pronounced reactivity of TAZ, compared to other iminic
heterocycles previously reported. For example, the degradation
of Paraoxon promoted by TAZ(−) is up to 190 times faster
40
than its analogue IMZ at similar reaction conditions, since
the more reactive species imidazolate is only accessible at
1
31
1
DEDNPP promoted by TAZ(−) sequential H and P{ H}
NMR spectra were acquired directly from the reaction
46
much higher basic medium (pK (IMZ) = 14.2). Regarding
a
3
1
1
the degradation of DEDNPP, results indicate that TAZ(−) is
four times more effective than alkaline hydrolysis and presents
similar reactivity to powerful alpha-nucleophiles (e.g.,
Pralidoxime), as illustrated in Table 1. A related point to
medium. As observed in Figure 2, P{ H} NMR spectra
showed three signals at 0.7, −7.5, and −9.0 ppm. The first two
signals were attributed, respectively, to DEP, which pro-
gressively increases during the reaction, and DEDNPP, which
slightly decreases but persists even after several minutes. The
signal of lesser intensity at −9.0 ppm was only visible at early
stages of the reaction and was assigned to the reaction
intermediate (INT1).
Table 1. Kinetic Parameters for the Reactions of Several
Nucleophiles Promoting DEDNPP Hydrolysis at 25° C
second-order rate constants
k
/
TAZ
a
−1
−1
These results corroborate the proposed mechanism
presented in Scheme 3, consistent with a nucleophilic attack
from a nitrogen lone pair to phosphorus atom of DEDNPP,
forming a quickly hydrolyzable species. However, the
remaining DEDNPP amount observed after 4120 min
indicates that a higher quantity of the nucleophile is necessary
for faster reaction completion. Similar dependence was also
detoxifying agent (N)
(k /L mol
s
)
kN
N
3
8
Alkaline hydrolysis
0.25
4.4
TAZ(−)
1.10 (0.03)
−
282.0
3
4
1
4
(5)-
3.9 × 10−
hydroxymethylimidazole
−
−
2
2
38
42
IMZ
1.77 × 10
2.77 × 10
62.1
39.7
3.8
4
(5)-methylimidazole
36
b
30
reported in recent work. Thus, a second reaction was
conducted, increasing the TAZ/DEDNPP concentration ratio
by ∼10 times. Again, the reaction progress was monitored by
NH OH
0.29
2
b
29
Pralidoxime
1.11
∼1
a
Considering second order rate constants obtained from pH profiles
3
1
1
b
P{ H} NMR analyses and, finally, a dephosphorylation
(6 < pH < 12). Suicide nucleophile.
process featuring nucleophile regenerationwith progressive
increasing of the DEP signal until DEDNPP peak was
completely vanishedwas observed, as shown in Figure 3.
consider is that alpha-nucleophiles behave as suicide
nucleophiles, as the product formed in the reaction with OPs
is not hydrolyzed, whereas regeneration of TAZ is observed
herein, as discussed in the following sections.
Regarding the reaction mechanism, highly negative entropies
of activation were obtained from Eyring plots at pH 11.4,
indicating a bimolecular nucleophilic reaction between
1
H NMR study was carried out to determine which nitrogen
of TAZ(−) was responsible for the nucleophilic attack. A
typical set of signals corresponding to DEDNPP, DEP, and
DNP was observed and consistent with previous works. The
most relevant data is presented in Table 3. As indicated by
kinetic studies, no evidence pointing to alkylation of TAZ(−)
4
029
J. Org. Chem. 2021, 86, 4027−4034
Fernandes, Juliano M.
