Silva et al.
JOCArticle
SCHEME 1. Self-Destruction of Benzohydroxamic Acid via
Lossen Rearrangement
However, alkaline hydrolysis of BDNPP is much faster than
that of DNPP, which complicates development of efficient
reagents that fully degrade phosphodiesters because initially
formed phosphate monoester dianions are unreactive to
anions due to the considerable electrostatic barrier to nu-
cleophilic attack.
FIGURE 1. Specific conductance as a function of [CTABr] in the
absence of LHA (O) and in the presence of molar fractions of LHA
of 0.048 (2), 0.091 (9), 0.11 (0), 0.14 ()), and 0.20 (b), at 25.0 ꢀC, in
the absence of buffer, pH 10.0.
cetyltrimethylammonium bromide, CTABr, accelerate the
spontaneous hydrolysis of BDNPP up to 30-fold and high
pH accelerates the reaction by concentrating nucleophiles,
7
8
R-Effect nucleophiles such as hydroxylamine, hydrazine,
4
,9,10
and hydroxamic acid
sphorylating agents. In an earlier investigation, products
are especially effective depho-
-
21,22
e.g., OH ,
tion of hydroxamates and phosphate triesters,
and (ii) cationic micelles accelerate the reac-
we
18,19,21
and intermediates of dephosphorylation of BDNPP anion
-
examined micellar effects on reactions of the less reactive
phosphodiesters with simple hydroxamates. We selected
lauryl hydroxamate because its long alkyl chain should
facilitate mixed micelle formation, effectively increasing
nucleophilic concentration in the micellar pseudophase,
and accelerating dephosphorylation of BDNPP by the
R-nucleophile. We monitored via several spectroscopic and
mass spectrometric methods the dephosphorylation pro-
ducts of BDNPP in comicelles of the amphiphilic R-nucleo-
phile, lauryl hydroxamic acid, as a functional surfactant, and
dodecyl- or cetyltrimethylammonium bromide.
by benzohydroxamate anion (BHO ) were identified, show-
ing that the reaction follows parallel paths with hydroxamate
4
attack on phosphorus and on the aromatic carbon. The
attack on carbon gives an intermediate, which was detected,
but slowly decomposes to aniline and 2,4-dinitrophenol,
whereas attack on phosphorus gives an unstable intermedi-
ate that undergoes a Lossen rearrangement to urea, amine,
1
1-16
-
isocyanate, and carbamyl hydroxamate.
Thus, BHO
behaves as a self-destructive intramolecular scissor which
reacts and loses nucleophilic ability (Scheme 1).
Reactivities of hydroxamate ions are increased by incor-
poration in functional polymers or by comicellization with
1
7-19
Results and Discussion
inert surfactants in water.
of pseudophase treatments of micellar rate effects,
As expected on the basis
2
0,21
Micelle Formation and Fractional Ionization. Critical mi-
celle concentrations, CMC, and fractional micellar ioniza-
tions, R, for mixed micelles of LHA and CTABr were
estimated from plots of conductance against [surfactant],
Figure 1. Breaks in these plots give CMC values that change
high local concentrations of hydroxamate ions in the inter-
facial micellar region are largely responsible for these
1
8
rate increases. Considering that (i) cationic micelles of
(
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little with increasing χLHA but approximate values of R, from
23-26
the ratio of slopes,
increase with increasing χLHA
,
(
indicating that addition of dodecyl hydroxamate ion effec-
tively neutralizes the cationic micelles, decreasing the micel-
lar surface potential and affinity for counteranions. This
effect is very sensitive to χLHA (Table 1).
(
4
(
4
-2
The CMC of DTABr in water is 1.5 ꢀ 10 M, but for a
(
(
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J. Org. Chem. Vol. 74, No. 21, 2009 8255