Simanenko et al.
nucleophile is BrO-, which forms very rapidly when 1-4
are dissolved in water, and reactivities are independent
of the structures of the precursors. At pH ca. 10 there is
nucleophilic attack on phosphorus and oxidation of
phenoxide ions and organic sulfides, and these dibromo-
bromates, or the bromochloro derivative, are stable solids
which should be potentially useful in the development
of relatively mild, wide-spectrum, decontaminants, and
in preparative chemistry. As for reactions of ClO- with
phosphonofluoridates, the corresponding reactions with
BrO- are turnover processes,2 but reactions with sulfur
compounds, HD and VX, will be stoichiometric.
FIGURE 2. Variation of k′obs for the reaction of NPDEPN with
[BrO-] from 1 at pH 11.35.
Experimental Section
-1
TABLE 1. Second-Order Rate Constants (kBrO , M s-1
)
-
Materials. The nitrophenyl derivatives, NPDEPN and
NPOTos, were prepared from 4-nitrophenol and the acid
chloride in anhydrous dioxane/Et3N10 and NPDEP was a
commercial sample (Aldrich) or was prepared as above: NPO-
Tos was recrystallized (EtOH) and NPDEP and NPDEPN were
vacuum distilled.
The dibromobromates, 1, 2 and 3, have been described
earlier8 and were prepared by the general method for the
preparation of 3.8 N,N-Dimethylacetamide (3.0 mol, 275 mL)
was added with continuous stirring to a mixture of water-
cooled bromine (1.5 mol, 241 g) and 40% HBr (1.5 mol, 220
mL). The viscous red liquid crystallized with stirring and the
crystals were removed by filtration and air-dried. The product
was recrystallized, MeOH, with minimum heating, and can
be vacuum distilled by using a short path and a N2 trap.
Physical properties, mp 81-83 °C, and 1H and 13C NMR
spectra agreed with the literature.8,9 The crystal structure is
available in the Cambridge Structural Database (SEGMOG,
SEGMOG 01).
Bis(N-acetylpiperidine)hydrogen dibromobromate (1, mp
81-83 °C) and diacetylpiperazinehydrogen dibromobromate
(2, mp 183-184 °C) were prepared as described above.8
Bis(N,N-dimethylacetamide)hydrogen bromochlorobromate,
4, was prepared as described above,8 but from bromine (54
mmol 2.8 mL), 28% HCl (54 mmol, 6.1 mL), and N,N-
dimethylacetamide (108 mmol, 10 mL). The yellow-orange
precipitate was recrystallized, MeOH (mp 80-82 °C).8
Inorganic materials were commercial samples of the highest
purity available.
for Reaction of Hypobromite Ion at 25 °C in 1 M KCl
reagents
substrate
Br2-H2O
1
2
3
4
NPDEPN
NPDEP
NPOTos
0.16a
0.13a
0.12b
0.12a
0.12a
0.010a
0.017d
0.010c
0.015e
0.011c
0.015 f
0.011c
0.015 f
a pH 11.3. b pH 11.2. c pH 11.7. d pH 11.7 or 12.3. e pH 11.6. f pH
12.1.
generated from bromine water and from 1-4 as precur-
sors. Figure 2 shows a plot of the first-order rate
constants, k′obs (s-1), against [BrO-] for the reaction of
NPDEPN in solutions of 1, with correction for the
contribution of reaction with OH-. Similar linear fits of
k′obs as a function of [BrO-] were obtained for reactions
of the other substrates with all the BrO- precursors. Rate
constants of the reaction of BrO- formed from various
precursors, and monitored at pH where decomposition
could be neglected, are shown in Table 1. There are
contributions from reaction with OH-, especially at high
pH and with dilute BrO-.
The second-order rate constants are similar to those
for reactions with OH-, despite the large difference in
basicities. There are many examples of nucleophilic
attack on aryl, phosphoryl, or sulfonyl centers for which
rate constants vary with basicities, following Bronsted
relationships, but OH- and H2O are typically much less
reactive than expected from their basicities. There is
extensive discussion on these deviations and the source
of the so-called R-effect.12,13
Kinetics. Reactions were followed spectrophotometrically
at 400-440 nm, with freshly prepared aqueous solutions. The
pH was adjusted by using 0.01-0.05 M phosphate buffer and
KOH, and the ionic strength was 1.0 (KCl). Changes in [buffer]
did not affect kobs
.
All reactions were followed at 25 °C and hypobromite
concentrations were determined immediately before adding the
substrates in dioxane to the reaction solution. Substrate
concentrations were generally ca. 3 × 10-5 M and reaction
solutions contained 0.5 vol % of dioxane. Integrated first-order
kinetic plots were linear for up to 5 half-lives at the pH used
in following the nucleophilic reactions, and deviations from
first-order kinetics observed as the pH was decreased (Figure
1) are due to decomposition of 4-nitrophenoxide ion, as for
reactions with hypochlorite ion.10
The half-lives for decompositions with 0.1 M BrO- are
approximately 60, 700, and 400 s for NPDEPN, NPDEP,
and NPOTos, respectively, at 25 °C, and it will be easy
to obtain rapid decomposition of agents by going to higher
concentrations of the precursors.
Conclusions
The dibromobromate derivatives, 1-3, and the bromo-
chlorobromate derivative, 4, are potentially useful sources
of hypobromite ion, which is an effective R-nucleophile
and reacts very rapidly with 4-nitrophenyl phosphates
and phosphonates which are widely used as simulants
for the phosphonofluoridate nerve agents. The active
Acknowledgment. This research was made possible
in part by Award No. UC2-2489-DO-03 of the U.S.
Civilian Research & Development Foundation (CRDF).
Supporting Information Available: Tables S1 and S2
which show rate and pH data for NPDEPN and Tables S3 and
S4 which show data for NPDEP and NPOTos, respectively.
This material is available free of charge via the Internet at
(12) (a) Edwards, J. O.; Pearson, R. G. J. Am. Chem. Soc. 1962, 84,
16-24. (b) Colthurst, M. J.; Kanagasooriam, J. S. S.; Wong, M. S. O.;
Contini, C.; Williams, A. Can. J. Chem. 1998, 76, 678-685. (c) Um,
I.-H.; Buncel, E. J. Am. Chem. Soc. 2001, 123, 11111-11112.
(13) Pross, A.; Shaik, S. S. New J. Chem. 1989, 13, 427-433.
JO0402430
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