An unusually reactive phosphodiester

Article abstract of DOI:10.1016/S0040-4039(00)00385-3

4-Nitro-1,8-naphthyl phosphate (4, NNP) is 2-3 orders of magnitude more reactive to basic and metal cation-mediated hydrolysis than its acyclic analogue (7); origins of the rate enhancements are discussed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00385-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3275–3278
An unusually reactive phosphodiester
Robert A. Moss ∗ and Kaliappa G. Ragunathan
Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA
Received 31 January 2000; accepted 2 March 2000
Abstract
4-Nitro-1,8-naphthyl phosphate (4, NNP) is 2–3 orders of magnitude more reactive to basic and metal cation-
mediated hydrolysis than its acyclic analogue (7); origins of the rate enhancements are discussed. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: phosphoric acid diesters; kinetics; catalysis; metal cations.
Phosphodiesters are famously resistant to hydrolysis: for example, dimethyl phosphate (DMP) hy-
drolyzes with Me–O scission and k=1.6×10−13s−1 (t ∼137 000 years) at pH 7, 25°C. P–O cleavage
1
1
2
of DMP is even slower, with k≤10−15s−1. Accordingly, bis(p-nitrophenyl) phosphate 1 (BNPP), with
its more reactive p-nitrophenylate leaving groups, has become the most commonly employed substrate
to gauge the potency of phosphodiester-lysing reagents and enzyme models.2 We have used BNPP to
study phosphodiester hydrolyses mediated by Ce4+,3 Th4+,4 and Zr4+.5 If k=1.1×10−11s−1 for the pH 7,
25°C (presumably P–O)2m hydrolysis of BNPP,2d then the 2 p-nitrophenyl for methyl substitutions that
‘convert’ DMP into BNPP are worth ∼2 orders of magnitude in actual hydrolytic reactivity at neutrality,
or ≥4 orders of magnitude comparing only P–O cleavages.
Recently, we reported that the reactivity toward a variety of nucleophiles of the commonly employed
phosphotriester substrate, diphenyl p-nitrophenyl phosphate (2, PNPDPP),6 was enhanced 40–300 times
by ‘mutation’ to the cyclic analogue, p-nitrophenyl 1,8-naphthyl phosphate (3, PNPNP).7 A similar
diphenyl→naphthyl fusion (with removal of a nitro group) transforms BNPP into 4-nitro-1,8-naphthyl
phosphate (4, NNP). Will this parallel structural change elicit similar activation in the intrinsically
unreactive phosphodiester series? And, if it does, what are the origins of the enhanced reactivity?
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00385-3
tetl 16665

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NNP was prepared from 1,8-naphthalene diol8 by phosphorylation to 5 (POCl3, Et3N,−Et2O; then
+
H2O, KHCO3, MeCN, 65% yield), followed by mononitration to 4 (1.3 equiv. of NO2 BF4 in MeCN,
4°C, 90 min). The potassium salt of NNP was purified by sevenfold recrystallization from ethanol (7.5%
yield), and characterized by IR, NMR, and elemental analysis (C, H, N, P).9a We prepared the partial
cleavage product of NNP, 6, by controlled hydrolysis of 4 (aq. KOH, pH 13, 37°C, 12 h), followed
by neutralization, and acidification to precipitate 6 in 40% yield. Finally, 7 (MNPP), which is both the
mononitro analogue of BNPP and the acyclic analogue of NNP, was prepared by reaction of (1:1) p-
nitrophenyl phosphorodichloridate and phenol (Et3N, Et2O, 25°C, 24 h; then aqueous NaHCO3, MeCN),
followed by crystallization from warm ethanol (72% yield). Both 6 and 7 were characterized by NMR
and elemental analysis.
The basic hydrolysis of NNP was first examined at pH 13 and 45°C, following the appearance of 6 at
480 nm.10 khydrol for NNP was 7.5×10−4
s
−1, which is 160 times larger than khydrol (BNPP) [4.7×10−6
s
−1],11 and 1700 times larger than khydrol (MNPP) [4.4×10−7 s−1).11 The basic hydrolysis of NNP at pH
13 is thus 1700 times faster than that of its acyclic relative, 7 (MNPP).
Metal cation mediated hydrolyses of NNP were studied with Eu3+, Th4+ and Zr4+. Rate constants were
determined for the NNP→6 cleavage and for the (slower) 6→4-nitro-1,8-naphthalene diol (8) cleavage.
NNP hydrolysis was monitored by following the decreases in NNP absorptions at 259 and 383 nm; while
hydrolysis of 6 was tracked by observing the appearance of absorptions at 476 nm (Eu3+), 480 nm (Zr4+),
or 475 nm (Th4+) due to formation of the metal complexes of 8,9b the hydrolysis product of 6. These
complexes were separately prepared for spectroscopic characterization. Rate constants were reproducible
to ±10%, and all reactions were followed for more than eight half-lives. For reactions with k>0.05 s−1
,
stopped-flow methods were employed. Rate constants and reaction conditions appear in Table 1; note the
use of the micellar polyoxyethylene Brij 35 surfactant with the Eu3+ and Th4+ cations.
NNP is 1700 times more reactive toward OH− at pH 13 than acyclic analogue MNPP, whereas the
metal cation mediated hydrolyses of NNP at pH ≤7 are ∼50–100 times more rapid (Table 1). Clearly, the
‘fusion’ of BNPP to NNP substantially enhances phosphodiester reactivity, just as the parallel structural
change does in the phosphotriester series.7 What are the origins of these accelerations? We believe that
several factors are at work.
Westheimer cited the ring strain in cyclic phosphodiesters, particularly the five-membered ethylene
phosphate (endocyclic O–P–O angle, ∼98°), as a major factor in its very rapid base-catalyzed hydrolysis,
which is ∼107 times faster in 1N NaOH than the hydrolysis of acyclic DMP (O–P–O angle, 104.8°).12,13
In contrast, the six-membered cyclic phosphate, trimethylene phosphate (O–P–O angle, ∼104.6°) is
essentially strain-free and only 7.5 times more reactive than DMP.12a An X-ray crystal structure of 514

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Table 1
Metal cation mediated hydrolyses of phosphodiestersa
gives the endocyclic O–P–O angle as 100.4°, significantly strained relative to either DMP or trimethylene
phosphate. If the O–P–O angle of NNP (4) is similar to that of 5, then ground state strain will contribute
to NNP’s high reactivity vs 7 in basic hydrolysis. Additionally, as we noted for phosphotriester 3
(unstrained, with O–P–O=105.8°),7 the P center is ideally suited for nucleophilic attack because the
aromatic rings are ‘tied back’ into the naphthyl unit.7 This feature, carried over to NNP, should contribute
to its kinetic advantage over acyclic 7.
Similar factors underlie the enhanced reactivity of NNP toward metal cations. In these reactions, the
cations bind the substrate’s P–O−, lessen the negative charge, and simultaneously provide a metal-bound
OH nucleophile to attack the P atom.2–5 The structure based enhanced reactivity of NNP operates in
both the metal-mediated and basic hydrolyses. Additionally, we expect a small contribution of micellar
binding in the Eu3+ and Th4+ cleavages, where Brij micelles are used.4
Recently, the correlation of enhanced phosphorolytic reactivity with ground state ‘angle strain’12 has
been questioned. Karplus presented computational evidence that much of the observed rate acceleration
of the five-membered cyclic phosphodiester stemmed from preferential solvation of its hydrolysis
transition state.15 A similar proposal was also offered in the phosphotriester series.16 Regardless of
the ultimate weightings assigned to the ring strain or preferential solvent explanations, the enhanced
reactivity of NNP vs MNPP (or BNPP) remains. NNP is an attractive, sensitive phosphodiester that
should prove useful in the quantitative evaluation of new phosphorolytic reagents.
Acknowledgements
We are grateful to Mr. Paul Gong for technical assistance and to both the U.S. Army Edgewood
Research, Development, and Engineering Center and the U.S. Army Research Office for financial
support.
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9. (a) IR; 1516, 1328 cm−1 (NO2). NMR (400 MHz, D2O, δ): 7.09 (d, J=8.0 Hz, H7), 7.18 (d, J=8.0 Hz, H2); 7.70 (t, J=8.0
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