Langmuir 1996, 12, 2200-2206
Comparative Reactivities of Phosphotriesters toward Iodosocarboxylates in Cationic Micelles Robert A. Moss,*,† Ann T. Kotchevar,† Byeong D. Park,† and Paolo Scrimin*,‡ Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08903, and Department of Chemical Sciences, University of Trieste, Via Giorgieri, Trieste, Italy Received November 29, 1995. In Final Form: February 8, 1996X Comparative reactivities have been determined for the cleavages of p-nitrophenyl phosphotriester substrates 4-9, and 12, and 13 by iodosobenzoate (1) and iodosonaphthoate (11) in micellar cetyltrimethylammonium chloride at pH 8. The aggregate substrate reactivities can be manipulated by altering the phosphate substituents, with each PhO-to-MeO or -EtO change worth ∼1 order of magnitude decrease in the rate constant. With 2 PhO/EtO mutations, the reactivity of 4 is 470 times greater toward 1 than that of 5, and the differential increases to 1080 with nucleophile 11. Phosphodiester substrates such as 10 are inert to micellar 1. Reactivity differences can be amplified by covalent attachment of the substrate moiety to cationic surfactants (12 and 13), where the PhO/EtO changes induce 600-700-fold reactivity diminutions with nucleophiles 1 and 11. The differential reactivities of the various substrates reflect both innate reactivity differences and differences in binding to the micellar pseudophase.
Introduction The destruction (decontamination) of toxic phosphonates and phosphates unfortunately continues to be topical, particularly in light of the March, 1995, sarin attack in the Tokyo subway system. Iodosobenzoate (IBA, 1) was first introduced as a catalyst for the micelle-
mediated hydrolysis of reactive phosphates, phosphonates, and esters in 1983.1 Subsequently, IBA and its derivatives and analogues were shown to efficiently cleave a variety of phosphotriesters and phosphonates in aqueous micellar cetyltrimethylammonium chloride (CTACl) solutions.2-5 Most importantly, these reagents were also found to be catalytically active against the nerve agent fluorophosphonates sarin (2) and soman (3), as well as the toxic phosphoramidocyanidate tabun and the phosphotriester insecticide paraoxon.6 Given the high toxicity of (e.g.) 2 and 3, most research with phosphorolytic decontaminants utilizes models or simulants in place of these dangerous substrates. pNitrophenyl diphenyl phosphate (PNPDPP, 4) has become the unofficial “standard” simulant since its introduction †
Rutgers University. University of Trieste. X Abstract published in Advance ACS Abstracts, April 15, 1996. ‡
(1) Moss, R. A.; Alwis, K. W.; Bizzigotti, G. O. J. Am. Chem. Soc. 1983, 105, 681. (2) Moss, R. A.; Alwis, K. W.; Shin, J.-S. J. Am. Chem. Soc. 1984, 106, 2651. (3) Moss, R. A.; Kim, K. Y. J. Am. Chem. Soc. 1986, 108, 788. (4) Moss, R. A.; Chatterjee, S.; Wilk, B. J. Org. Chem. 1986, 51, 4303. (5) Katritzky, A. R.; Duell, B. L.; Durst, H. D.; Knier, B. L. J. Org. Chem. 1988, 53, 3972. (6) Hammond, P. S.; Forster, J. S.; Lieske, C. N.; Durst, H. D. J. Am. Chem. Soc. 1989, 111, 7860.
S0743-7463(95)01093-6 CCC: $12.00
by Bunton and Robinson in 1969,7,8 although its simulation of nerve agents 2 and 3 is not exact. Thus, the agents are less hydrophobic than PNPDPP and will partition more hydrophilically between aqueous and micellar phases. Additionally, the reactivity of PNPDPP toward IBA is somewhat greater than that of 2 and 3, at least in CTACl micellar solution.9 On the other hand, paraoxon (pnitrophenyl diethyl phosphate, 5) would be a better simulant than PNPDPP for 2 or 3 in terms of its hydrophilic/hydrophobic balance, but paraoxon is somewhat less reactive than the nerve agents.6,9
These considerations suggest that a comparative study of a series of closely related phosphotriester substrates would be helpful in creating “designer simulants” that could be selected for the close modeling of particular decontamination problems. A central question is how strongly the aggregate reactivity10 of a p-nitrophenyl phosphotriester simulant depends on its “other” substituents. In response, we examine here the micellar iodosocarboxylate cleavages of phosphotriesters 4-9, which carry varying arrays of phenyl, p-nitrophenyl, and alkyl substituents. For purposes of comparison, we also include the phosphodiester substrate bis(p-nitrophenyl) phosphate, 10, BNPP. (7) Bunton, C. A.; Robinson, L. J. Org. Chem. 1969, 34, 773. (8) For brief reviews, see: Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975; especially pp 150-161. And: Moss, R.A.; Ihara, Y. J. Org. Chem. 1983, 48, 588. (9) Agents 2 and 3 are 40 and 137 times more reactive than paraoxon toward IBA in 0.01 M CTACl at pH 7.5,6 whereas PNPDPP is ∼474 times more reactive than paraoxon under approximately comparable conditions; see below. (10) “Aggregate reactivity” refers here to the maximum reactivity of a substrate, as observed in aqueous micellar CTACl solution, and includes a component due to the intrinsic reactivity of the phosphotriester, as well as a factor reflecting its ability to bind to or partition into the micellar phase.
© 1996 American Chemical Society
Comparative Reactivities of Phosphotriesters
The 3-methyl substituent, introduced in 6, models the 3-CH2 covalent link present in the surfactant substrates (described below) and is meant to test whether steric interactions will perturb (twist) the adjacent nitro group enough to significantly affect the reactivity of the pnitroaryl leaving group (relative to the p-nitrophenylate [PNPO] leaving group of 4). The kinetics of substrate cleavage were determined in aqueous micellar CTACl solution using both IBA (1) and its somewhat more reactive relative 2,3-iodosonaphthoate (INA, 11).11 Recently, we studied the IBA and INA
cleavages of the covalently-linked, surfactant-PNPDPP substrate, 12, observing ∼20-fold rate enhancements in the phosphorolysis of 12, relative to 4, which binds to CTACl micelles only by noncovalent forces.12 In the present study, we also include the p-nitrophenyl diethyl phosphate surfactant-substrate 13. The availability of this surfactant-paraoxon permits reactivity comparisons of PNPDPP (4) and paraoxon (5) with each other and also with their surfactant-bound analogues 12 and 13, in addition to direct comparisons of the two surfactantsubstrates. The latter matchup provides a truer reflection of the innate reactivity differences between diphenyl and diethyl p-nitrophenyl phosphates because 12 and 13 are likely to be fully bound to their CTACl comicelles. Results Small Substrates. PNPDPP (4) was prepared and purified by literature methods,13 whereas paraoxon (5) and BNPP (10) were commercially available from Aldrich and Sigma, respectively. Substrate 6, 3-methyl-PNPDPP, was prepared in 90% yield by phosphorylation of ethereal 3-methyl-4-nitrophenol with diphenyl chlorophosphate in the presence of triethylamine.12 Methyl phenyl p-nitroaryl (11) Moss, R. A.; Zhang, H.; Chatterjee, S.; Krogh-Jespersen, K. Tetrahedron Lett. 1993, 34, 1729. (12) Kotchevar, A. T.; Moss, R. A.; Scrimin, P.; Tecilla, P.; Zhang, H. Tetrahedron Lett. 1994, 35, 4927. (13) Gulick, W. M., Jr.; Geske, D. H. J. Am. Chem. Soc. 1966, 88, 2928.
Langmuir, Vol. 12, No. 9, 1996 2201
phosphate 7 was made by sequential reaction of phenyl dichlorophosphate with 1 equiv each of methanol and 3-methyl-4-nitrophenol in CH2Cl2, each reaction carried out with an equivalent of triethylamine present. In an analogous procedure, dimethyl p-nitroaryl phosphate 8 was obtained from methyl dichlorophosphate, methanol, and 3-methyl-4-nitrophenol. Finally, bis(4-nitrophenyl) methyl phosphate (9) was prepared by reaction of methyl dichlorophosphate with 2 equiv of p-nitrophenol in CH2Cl2-Et3N. The structures of the new substrates were substantiated by 1H and 31P NMR spectroscopy and by elemental analysis (6 and 9). Substrates 7 and 8 were oils that could not be chromatographically purified to better than 95% HPLC purity without some decomposition. In these cases, the structures rest on 1H and 31P NMR spectroscopy and mass spectrometric identification of the appropriate parent molecular ions. Surfactant Substrates. The surfactant-PNPDPP 12 was made from 6 by bromination of the methyl group (NBS, CCl4, Bz2O2, reflux 18 h, 88%), followed by quaternization with N-(n-octadecyl)-N,N-dimethylamine (acetone, 25 °C, 6 days, purified yield 27%).12 It was purified by silica gel chromatography and characterized by NMR spectroscopy and elemental analysis. Preparation of the surfactant-paraoxon substrate 13 began with surfactant-PNP-benzoate 14, which was made by the quaternization of n-hexadecyldimethylamine with 4-nitro-3-(bromomethyl)phenyl benzoate14 (acetone, 25 °C, 4 days, 58%). Surfactant benzoate 14 was hydrolyzed to
the corresponding p-nitrophenylate surfactant (NaOH, MeOH, 25 °C, 1 h, 100%), and the latter was phosphorylated to 13 with diethyl chlorophosphate (CH2Cl2, Amberlite IRA-35, 25 °C, 3 h, 15%). Washing with dilute HCl and chromatography over silica gave 13 as the chloride salt, which was characterized by standard methods. Kinetics. The phosphorolytic cleavages of substrates 4-10, 12, and 13 by IBA (1) and INA (11) were studied kinetically in CTACl solutions at pH 8. To provide continuity with previous work,1,2,4,11,15 similar reaction conditions were adopted: the CTACl solutions contained 1 × 10-4 M IBA or INA, 1 × 10-5 M substrate, 5 × 10-4 to 5 × 10-2 M CTACl, 0.02 M NaH2PO4 buffer, and 0.01 M KCl. Solutions also contained 0.5% CH3CN and 0.51.0% DMF from the additions of substrate and catalyst, respectively. Kinetics were followed by monitoring the appearance of p-nitrophenylate ion at 400 nm (25 °C), and pseudo-first-order rate constants were calculated from the absorbance/time data by standard computational methods. In the case of surfactant-substrate 12, the required stopped-flow conditions led to slightly different reactant concentrations: for 12 + IBA,  ) 2.0 × 10-5 M and [IBA] ) 1.5 × 10-4 M; for 12 + INA,  ) 2.44 × 10-5 M and [INA] ) 1.25 × 10-4 M. For each substrate-catalyst combination, the concentration of CTACl was varied over (usually) seven to nine (14) Moss, R. A.; Bhattacharya, S.; Chatterjee, S. J. Am. Chem. Soc. 1989, 111, 3680. (15) Moss, R. A.; Wilk, B.; Krogh-Jespersen, K.; Blair, J. T.; Westbrook, J. D. J. Am. Chem. Soc. 1989, 111, 250.
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Figure 1. Pseudo-first-order rate constants (100kψ, s-1) for the cleavage of paraoxon (5) by INA (11, [) and IBA (1, ~) as a function of [CTACl] at pH 8.0. See text for reaction conditions values. and Table 1 for kmax ψ
Moss et al.
Figure 3. Pseudo-first-order rate constants (100kψ, s-1) for the cleavage of surfactant-paraoxon 13 by INA ([) and IBA (~) as a function of [CTACl] at pH 8.0. See text for reaction values. conditions and Table 2 for kmax ψ Table 1. Cleavage of Small Substrates by IBA (1) and INA (11)a case substrate 1e 2 3 4 5 6 7 8i 9 10 11
Figure 2. Pseudo-first-order rate constants (100kψ, s-1) for the cleavage of substrate 7 by INA ([) and IBA (~) as a function of [CTACl] at pH 8.0. See text for reaction conditions and Table values. 1 for kmax ψ
values, affording a rate constant-[surfactant] profile from which the maximum rate constant, kmax ψ , could be obtained at the optimal CTACl concentration. Representative examples of these profiles appear in Figures 1-3 for reactions of IBA and INA with substrates 5, 7, and 13, respectively; profiles for the cleavage of substrates 42 and 1212 have been published. Tabulations of kmax appear in ψ Tables 1 and 2. Kinetic data were reproducible to (5%. The correlations of Figures 1-3 pass through maxima because when we increase [CTACl] while keeping [IBA] or [INA] constant, we “dilute” the reagents in the aggregates. At high [CTACl], the “dilution” effect (which leads to a rate decrease) dominates. If the rate constants are corrected for dilution by dividing them by the mole fraction of the nucleophile in the micelles, well-behaved binding profiles can be obtained. This is illustrated for the reaction of 5 and INA in Figure 4, where data are reconfigured from Figure 1, and the mole fraction of INA [X(INA)] is [INA]/([INA] + [CTACl]). A very similar result (not shown) can be obtained for the reaction of 7 with INA (Figure 2).
4 6 7 8 5 9 10 4 7 8 5
Rb PhO PhO PhO MeO EtO PNPOf PNPO PhO PhO MeO EtO
R′ b catalyst 102[CTACl],c M 102kψmax,d s-1 PhO PhO MeO MeO EtO MeO O- g PhO MeO MeO EtO
IBA IBA IBA IBA IBA IBA IBA INA INA INA INA
0.10 0.050 0.50 1.0 0.50 0.25 h 0.05 0.05 0.50 0.25
6.40 3.58 0.136 0.00867 0.0135 7.36 h 26.0 0.982 0.0230 0.0240
a Conditions: pH 8.0, 0.02 M phosphate buffer, 0.01 M KCl, 25 °C, [substrate] ) 1.0 × 10-5 M, [catalyst] ) 1.0 × 10-4 M; see text. b R and R′ are the non-p-nitrophenyl substituents on the phosphate substrate. c [CTACl] at which kmax was observed. d Maximum ψ observed pseudo-first-order rate constant. These data are not corrected for the extent of ionization of IBA or INA (pKa’s are ∼7.2 and 7.1, respectively11). e From ref 2. f p-Nitrophenoxy, the substrate is bis-(p-nitrophenyl) methyl phosphate. g The substrate is bis(p-nitrophenyl) phosphate. h No cleavage was observed at various [CTACl] over 4 days. i From ref 11.
Table 2. Cleavage of Surfactant Substrates by IBA and INAa case substrate 1e 2 3e 4
12 13 12 13
PhO EtO PhO EtO
PhO EtO PhO EtO
catalyst 102[CTACl],c M 102kψmax,d s-1 IBA IBA INA INA
0.10 0.10 0.050 0.050
66 0.0941 524 0.893
a-d See the corresponding notes in Table 1. e From ref 12. The concentrations of 12 and catalysts vary from the “standard” conditions; see text.
We also conducted a brief study of the kinetic advantages associated with IBA-CTACl micellar catalysis; cf. Table 3. In one series of kinetic runs, several of the small substrates were subjected to IBA cleavage in the absence of CTACl micelles; in a second series, the cleavages were carried out in CTACl micelles but in the absence of IBA. Other conditions conformed to those described above and in Table 1. Discussion The data in Tables 1 and 3 indicate that both IBA and CTACl micelles contribute importantly to the efficient
Comparative Reactivities of Phosphotriesters
Langmuir, Vol. 12, No. 9, 1996 2203
Table 3. “Partially Catalyzed” Cleavages of Small Substratesa substrate
c -1 102kmax ψ , s
d -1 102kIBA 0 , s
102kCTACl ,e s-1 0
IBA f [kmax ψ /k0]
CTACl g [kmax ψ /k0]
4 7 8 5
PhO PhO MeO EtO
PhO MeO MeO EtO
6.40h 0.136 0.008 67 0.013 5j
0.0085 0.0015 i i
0.066h 0.006 0 0.000 42 0.000 34j
97h 23 21 40
a pH 8.0, 0.02 M phosphate buffer, 0.01 M KCl, 25 °C, [substrate] ) 1.0 × 10-5 M, [IBA] ) 1.0 × 10-4 M, if present. [CTACl], if present, is set at the concentration shown for the analogous substrate-IBA run in Table 1. b R and R′ are the non-p-nitrophenyl substituents on the phosphate substrate. c In the presencve of both CTACl and IBA; see Table 1. d IBA only, no CTACl. e CTACl only, no IBA. f Rate constant ratio in the presence of IBA. g Rate constant ratio in the presence of CTACl. h From ref 2. i No reaction after 7 days. j These values are given as 5.9 × 10-5 s-1 and 4.5 × 10-6 s-1 in ref 6, where the conditions differ slightly.
Table 4. Comparative Rate Constants
Figure 4. Rate constants multiplied by the inverse mole fraction of INA for the cleavage of paraoxon (5) by INA (11) as a function of [CTACl] at pH 8.0. The data are recalculated from Figure 1. See text for discussion.
catalysis of p-nitrophenyl phosphotriester cleavage. The IBA provides the potent O nucleophile essential to the cleavages, whereas the CTACl micelles bind and concentrate the IBA and phosphotriester reactants into a lowvolume “pseudophase” where the actual reaction occurs.16,17 Omission of either IBA or CTACl leads to a 1-2 order of magnitude decrease in the maximum rate constant for IBA micellar cleavage of the phosphotriesters. With the less reactive dialkyl p-nitrophenyl phosphates 5 and 8, 1 × 10-4 M IBA does not cleave 1 × 10-5 M substrate at all, unless CTACl micelles are present. The data in Table 3 also reveal the existance of intrinsic reactivity differences between the small molecule substrates in the absence of CTACl. In particular, IBA alone cleaves the bis-PhO,PhO substrate 4 ∼6 times faster than it cleaves the PhO,MeO substrate 7, while the dialkyl p-nitrophenyl phosphates 5 and 8 are inert to IBA under nonmicellar reaction conditions. To conveniently compare the dependence of the maximum rate constants for the iodosocarboxylate-CTAClcatalyzed cleavages on the substrate and catalyst structures, the data of Tables 1 and 2 have been used to generate Table 4, in which rate constant ratios or “comparative rate constants” are displayed. Entries 1-6 compare small substrate structural dependences for IBA cleavages, while entries 11-14 present related data for INA reactions. Comparisons of the catalytic potencies of IBA and INA toward identical small substrates appear in entries 7-10. Rate constant ratios for the surfactant substrates with (16) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. R. Acc. Chem. Res. 1991, 24, 357. (17) Surface tension measurements indicate that the critical micelle concentration of CTACl is 1.0 × 10-5 M under the conditions of Table 1.
rate const ratio
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1-1/1-2 1-2/1-3 1-3/1-4 1-2/1-4 1-1/1-5 1-6/1-7 1-8/1-1 1-9/1-3 1-10/1-4 1-11/1-5 1-8/1-9 1-9/1-10 1-8/1-11 1-8/1-10 2-1/2-2 2-3/2-4 2-3/2-1 2-4/2-2 2-1/1-1 2-2/1-5 2-3/1-8 2-4/1-11
4:6 6:7 7:8 6:8 4:5 9:10 4 7 8 5 4:7 7:8 4:5 4:8 12:13 12:13 12 13 12:4 13:5 12:4 13:5
IBA IBA IBA IBA IBA IBA INA/IBA INA/IBA INA/IBA INA/IBA INA INA INA INA IBA INA INA/IBA INA/IBA IBA IBA INA INA
1.8 26 16 410 470 ∞ 4.1 7.2 2.7 1.8 26 43 1080 1130 700 590 7.9 9.5 10 7.0 20 37
IBA or INA are found in entries 15-18. Finally, comparisons of surfactant substrates with their small substrate analogues are shown in entries 19-22. For each entry, Table 4 indicates the table and case reference for the rate constants that are to be compared, the substrates and catalysts involved, and the derived comparative rate constant. Entry 118 indicates that introduction of a methyl group at the 3 position of the p-nitrophenyl substituent of PNPDPP (substrate 6) brings about a minimal diminution of this substrate’s reactivity toward micellar IBA, relative to that of PNPDPP; the comparative rate constant is <2. Entries 2-5 deal with successive “mutations” of the PhO substituents of 6 to one (7) and then two RO substituents (8 or 5). Each PhO to MeO change reduces the aggregate substrate reactivity10 in excess of 1 order of magnitude (entries 2 and 3). Together, 2 PhO-to-MeO (or EtO) alterations cost factors of 410 (6:8, entry 4) or 470 (4:5, entry 5). These rate constant differences can be partially attributed to differential substrate binding; i.e., the more hydrophobic bis-phenoxy substrates (4 and 6) should partition more completely into the micellar phase, reacting more rapidly with the micelle-bound IBA, relative to the less hydrophobic PhO,MeO or bis-MeO (bis-EtO) substrates (7, 8, or 5). However, since poorer binding is also associated with a less pronounced dilution of the substrate as the surfactant concentration increases, these two factors are likely to cancel each other. This is indirectly confirmed by the observation that reactivity differences of similar magnitude persist with the surfactant substrates 12 and (18) References to “entries” in the following discussion refer to Table 4.
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13 (see below), which should be nearly completely bound to CTACl comicelles under our reaction conditions.17 This reinforces the conclusion drawn from Table 3 that significant portions of the PhO > MeO (EtO) reactivity differences observed in the reactions of the small substrates with micellar IBA reflect innate reactivity differences of the substrates. Perhaps the greater inductive withdrawing effect of Ph, relative to Me or Et, provides greater stabilization of a rate-determining (anionic) transition state, either for the IBA displacement of p-nitrophenylate or for the rate-determining addition of IBA to PdO with the formation of a metastable pentavalent trigonal bipyramidal intermediate.19,20 Entry 6 demonstrates that micellar IBA, however potent it may be toward phosphotriesters, is unreactive toward phosphodiesters. Thus, although bis(p-nitrophenyl) methyl phosphate 9 is the most reactive substrate in Table 1 toward micellar IBA (case 6), “removal” of the methyl group affords the phosphodiester 10, which is completely unreactive toward IBA. Efficient cleavage of the anionic phosphodiesters requires electrophilic assistance at PsO-, as provided by lanthanide cations,21 in addition to nucleophilic attack at PdO. Entries 11-14 indicate that the micellar INA cleavages of the small substrates manifest a comparative rate constant pattern that parallels the one elicited by IBA, although the INA values are somewhat larger. In particular, bis-PhO/bis-RO ratios exceed 1000 with INA (entries 13 and 14), slightly more that twice the comparable values obtained with IBA (entries 4 and 5). That INA is somewhat more reactive than IBA is reinforced by the direct comparisons presented in entries 7-10, where the aggregate INA reactivity is larger by factors of ∼2-7 with bis-PhO, PhO,MeO, bis-MeO, or bisEtO p-nitrophenyl phosphates. Most likely, the reactivity advantage of INA derives from its greater hydrophobicity, relative to IBA, which leads to enhanced binding to the CTACl micelles and more efficient reaction with micellebound substrate.11 The surfactant substrates 12 and 13 exhibit reactivities with IBA and INA that parallel those observed with their small substrate analogues, 4 (6) and 5. The principal observation is the persistence of the bis-PhO/bis-RO reactivity advantage: factors of 700 (IBA) or 590 (INA) for the surfactant substrates (entries 15 and 16) and factors of 470 or 1080 for the analogous small substrate reactions. Although octadecyl bis-PhO surfactant-substrate 12 is more hydrophobic than hexadecyl bis-EtO substrate 13, the latter should be extensively comicellized with CTACl. At an overall surfactant concentration of 1 × 10-3 M (Table 2) and a cmc of 1 × 10-5 M,17 we estimate that g99% of 13 (or 12) should be bound to comicelles during the IBA or INA cleavages. Thus, the observed aggregate reactivity differences between the PhO and RO substrates, surfactants and small molecules too, must have a strong intrinsic differential reactivity component. Entries 17 and 18 indicate that the reactivity advantage of INA over IBA, encountered with the small substrates (entries 7-10), persists with the surfactant substrates. Its value here is ∼8-10, slightly larger than observed above. (19) It is unclear whether the IBA/p-nitrophenyl phosphotriester cleavage involves a direct SN2(P) displacement at P or an additionelimination sequence at PdO. (20) See: Thatcher, G. R.; Kluger, R. Adv. Phys. Org. Chem. 1989, 25, 99. Williams, A. Adv. Phys. Org. Chem. 1992, 27, 1. (21) For leading references, see: Takasuki, B. K.; Chin, J. J. Am. Chem. Soc. 1995, 117, 8582. Tsubouchi, A.; Bruice, T. C. J. Am. Chem. Soc. 1995, 117, 7399. Moss, R. A.; Park, B. D.; Scrimin, P.; Ghirlanda, G. J. Chem. Soc., Chem. Commun. 1995, 1627. Breslow, R.; Zhang, B. J. Am. Chem. Soc. 1994, 116, 7893.
Moss et al.
Entries 19-22 deal with the comparative reactivities of surfactant and small molecule phosphotriesters. The bis-PhO substrates 12 and 4 have been discussed previously,12 with the 10-20-fold reactivity advantage of the surfactant-substrate attributed to enhanced reactivity of its phosphoryl center induced by the surfactant’s nearby cationic charge22 and from better binding to the CTACl comicellar host. Allowing for the steric hindrance added to the surfactant-substrate by dint of the covalent linkage at the 3 position of its p-nitrophenyl phosphate moiety would increase its reactivity advantage in the IBA cleavage from 10 to a factor of at least 18.23 Entry 20 indicates that the surfactant/small molecule substrate kinetic advantage for the bis-EtO (paraoxon) substrates is ∼7, parallel to the bis-PhO case. Interestingly, the kinetic advantages of the surfactantsubstrates increase with the INA catalyst, reaching ∼37 in the cleavage of the paraoxons (entry 22). INA cleavage of the bis-PhO substrates (entry 21) is also more discriminating than the comparable IBA reactions (entry 19). Conclusions The p-nitrophenyl phosphotriester substrates exhibit both innate and aggregate reactivity differences toward micellar iodosocarboxylate nucleophiles. These differences can be controlled by manipulation of the phosphotriester’s substituents. For p-nitrophenyl phosphotriesters such as PNPDPP (4), each PhO/MeO or PhO/EtO substituent change decreases substrate reactivity by ∼1 order of magnitude; the reactivity of PNPDPP toward IBA (1) is 470 times greater than that of the diethoxy substrate paraoxon (5). Cleavage reactions with INA (11) follow reactivity patterns that resemble those of IBA, but the INA reactions are faster by factors of ∼2-10. Phosphodiester substrates (e.g., 10) are inert to micellar IBA. Substrate reactivity differences can be amplified by covalent attachment of the phosphotriesters to cationic surfactants (as in 12 and 13), which facilitates comicellar binding. This reactivity enhancement is worth ∼1 order of magnitude, and factors are larger with INA (20-37) than with IBA (7-10). The bis-PhO/bis-EtO substituent alteration results in a reactivity decrease of ∼600-700 for the covalently bound surfactant-substrates. By appropriate choice of substrate substituents and surfactant attachment, it should be possible to construct “designer substrates” or simulants of varied reactivity and hydrophobicity to model differing decontamination scenarios. Experimental Section Materials and Methods. Iodosobenzoate (1) and p-nitrophenyl diethyl phosphate (5) were obtained from Aldrich and used without further purification. Bis(p-nitrophenyl) phosphate (10) was purchased from Sigma and used as received. PNPDPP (4)13 and iodosonaphthoate (11)11 were prepared according to literature procedures. CTACl was obtained from Eastman and purified by recrystallization from methanol/ether. Melting points are uncorrected. 1H NMR spectra (in ppm) were obtained on a Varian VXR-200 instrument at 200 MHz; chemical shifts are reported in values relative to internal Me4Si in CDCl3. 31P NMR spectra (in ppm) were determined on the same equipment at 81 (22) Relative to uncharged small substrates 4 or 5, the electrostatic interaction between N+ and PdO in surfactant-substrates 12 or 13 will activate the latter toward nucleophilic attack and, depending on the precise cleavage mechanism,19,20 either stabilize a pentacoordinate phosphorous oxyanion intermediate or enhance the leaving group ability of the (surfactant) p-nitrophenylate fragment. (23) This derives from comparing 12 with 6, rather than 4; i.e., the comparison is 2-1/1-2.
Comparative Reactivities of Phosphotriesters MHz and are reported relative to external H3PO4. Microanalyses were performed by Quantitative Technologies, Whitehouse, NJ. 3-Methyl-4-nitrophenyl Diphenyl Phosphate, 6. In 150 mL of anhydrous ether were dissolved 5.0 g (33 mmol) of 3-methyl4-nitrophenol (Aldrich) and 3.6 g (36 mmol) of triethylamine. Diphenyl chlorophosphate (9.0 g, 34 mmol, Aldrich) was added with stirring over 20 min, and the reaction mixture was then stirred for 4 h at 25 °C. The precipitate of Et3N‚HCl was filtered and solvent was removed under reduced pressure to give 11.4 g (29.6 mmol, 90%) of phosphate 6 as a tan oil. 1H NMR (CDCl3): 2.57 (s, 3 H, Me), 7.27-8.08 (m, 13 H, aryl). 31P NMR (CDCl3): -17.99 (s). Anal. Calcd for C19H16NO6P: C, 59.2; H, 4.19; N, 3.64. Found: C, 58.8; H, 4.11; N, 3.51. 3-Methyl-4-nitrophenyl Methyl Phenyl Phosphate, 7. A 3-neck, 500-mL, round-bottom flask was equipped with a nitrogen inlet, a reflux condenser, an addition funnel, and a magnetic stirring bar and charged with 4.24 g (20 mmol) of phenyl dichlorophosphate (Aldrich) in 20 mL of CH2Cl2 (dried over P2O5). The flask was cooled in an ice-water bath. A solution of 0.64 g (20 mmol) of dry methanol and 2.02 g (20 mmol) of dry triethylamine in 20 mL of dry CH2Cl2 was added dropwise, with stirring, over 10 min. After an additional 5 min, a solution of 3.1 g (20 mmol) of 3-methyl-4-nitrophenol and 2.02 g (20 mmol) of triethylamine in 80 mL of dry CH2Cl2 was added dropwise to the stirred reaction mixture over 30 min. The ice bath was removed, and the reaction mixture was refluxed for 2 h. Solvent was stripped on the rotary evaporator to give a brown oil and a solid. After the addition of 250 mL of ether, the solid (Et3N‚HCl) was removed by filtration, and the ether phase was washed 3 times with 100-mL portions of pH 2-3 dilute aqueous HCl. The ether phase was dried and stripped to afford 3.2 g (9.9 mmol, 49%) of phosphate 7 as a slightly brownish oil. Purification of 1.5 g of this material on silica gel (CHCl3/MeOH) afforded 1.2 g of 7 that was 95% pure by HPLC. 1H NMR (CDCl3): 2.61 (s, 3 H, 3-Me), 4.00 (d, JP-H ) 12 Hz, 3 H, OMe), 7.1-7.6, 8.0-8.1 (m, 8 H, aryl). 31P NMR (DMSO-d6): -10.43 (s). GC-MS: m/e 323 (M+).24 3-Methyl-4-nitrophenyl Dimethyl Phosphate, 8. The equipment used in the preparation of 7 was charged with a solution of 3.1 g (21 mmol) of methyl dichlorophosphate (Aldrich) in 20 mL of CH2Cl2 (dried over P2O5). The flask was cooled in an ice-water bath, and a solution of 0.68 g (21 mmol) of dry methanol and 2.1 g (21 mmol) of triethylamine in 20 mL of dry CH2Cl2 was added dropwise, with stirring, over 10 min. After an additional 5 min, a solution of 2.9 g (19 mmol) of 3-methyl4-nitrophenol and 2.1 g (21 mmol) of triethylamine in 100 mL of dry CH2Cl2 was added dropwise over 30 min. The ice bath was removed, and the reaction mixture was refluxed for 2 h. Solvent was stripped on the rotary evaporator to give a solid and brown oil. Workup as for 7 afforded 1.2 g (4.6 mmol, 24%) of 8 as a slightly brownish oil. 1H NMR (CDCl3): 2.58 (s, 3 H, 3-Me), 3.88 (d, JP-H ) 11.4 Hz, 6 H, OMe), 7.14-7.17, 7.96-8.06 (m, 3 H, aryl). 31P NMR (DMSO-d6): -3.79 (s). GC-MS: m/e 261 (M+).24 Bis(p-nitrophenyl) Methyl Phosphate, 9. A 500-mL, round-bottom flask, equipped as above but lacking the condenser, was charged with a solution of 3.1 g (21 mmol) of methyl dichlorophosphate in 10 mL of dry CH2Cl2 and cooled in an icewater bath. Next, a solution of 6.2 g (44.6 mmol) of p-nitrophenol and 4.5 g (44.5 mmol) of triethylamine in 100 mL of dry CH2Cl2 was added dropwise over 30 min. The ice bath was removed, and the reaction mixture was stirred for 2 h at ambient temperature. Solvent was stripped on the rotary evaporator to afford a yellow solid which was taken up in 600 mL of ether. Et3N‚HCl was removed by filtration, and the ether phase was washed 3 times with 200 mL of dilute aqueous HCl (pH 2-3), dried, and stripped to yield a white solid that was recrystallized from ether and then washed with cold petroleum ether to give 3.2 g (9.0 mmol, 43%) of 9 as a white solid, mp 124-126 °C. 1H NMR (CDCl3): 4.05 (d, JP-H ) 12 Hz, 3 H, OMe), 7.3-7.5, 8.28.4 (2A2B2, 8 H, aryl). 31P NMR (DMSO-d6): -12.06 (s). Anal. Calcd for C12H11N2O8P: C, 44.1; H, 3.13; N, 7.91. Found: C, 44.6; H, 3.43; N, 7.67. (24) This material could not be chromatographically purified to better than 95% HPLC purity; see Results section. HPLC conditions included a 3.9- × 300-mm µ-Bondapak C18 column with 7:3 CH3CN-H2O as eluent and detection at 254 nm.
Langmuir, Vol. 12, No. 9, 1996 2205 N-Octadecyl-N,N-dimethyl-N-[4-nitro-1-(diphenylphosphato)-3-benzyl]ammonium Bromide, 12. The phosphate 6 (5.0 g, 13 mmol) and 2.2 g (12.4 mmol) of freshly purified N-bromosuccinimide were added to 25 mL of CCl4 (distilled from K2CO3). About 50 mg of benzoyl peroxide was added as an initiator, and the mixture was stirred, refluxed, and irradiated with an IR heating lamp for 18 h. The hot mixture was filtered free of succinimide, and the CCl4 was stripped to give 4.5 g (9.7 mmol, 78%) of 3-(bromomethyl)-4-nitrophenyl diphenyl phosphate as a tan oil. 1H NMR (CDCl3): 4.84 (s, 2 H, CH2Br), 7.28.13 (m, 13 H, aryl). Without further purification, the bromomethyl compound was used to quaternize N,N-dimethyl-N-octadecylamine (Pfaltz and Bauer). Thus, 0.40 g (1.3 mmol) of the amine and 0.6 g (1.3 mmol) of the bromomethyl compound were dissolved in 25 mL of freshly distilled acetone and stirred at 25 °C in the dark for 6 days. Then, the solvent was stripped to give an oil that was purified by chromatography on silica gel using 20:1 CH2Cl2MeOH (acidified with 2 drops of HCl per 100 mL) as the eluent. Fractions with Rf ∼ 0.3 were collected to yield 6 as a tan oil, 0.25 g (0.33 mmol, 25%). 1H NMR (CDCl3): 0.87 (t, 3 H, CH2CH3), 1.25 (br s, 30 H, (CH2)15), 1.75 (m, 2 H, NCH2CH2R), 3.20 (s, 6 H, 2NCH3), 3.58 (m, 2 H, NCH2R), 5.71 (s, 2 H, NCH2Ar), 7.248.31 (m, 13 H, Ar). 31P NMR (CDCl3): -17.50 (s). Anal. Calcd for C39H58BrN2O6P‚0.5H2O: C, 60.7; H, 7.7; N, 3.6. Found: C, 60.7; H, 7.8; N, 4.0. N-Hexadecyl-N,N-dimethyl-N-[4-nitro-1-(diethylphosphato)-3-benzyl]ammonium Chloride, 13. This compound was prepared from the analogous benzoate surfactant 14, which was made by stirring 0.75 g (3.2 mmol) of N,N-dimethylhexadecylamine (Pfalz and Bauer) with 1.0 g (3.0 mmol) of 3-(bromomethyl)-4-nitrophenyl benzoate14 in 25 mL of freshly distilled acetone at 25 °C in the dark for 4 days. The precipitate was collected and washed with ether to give 0.93 g (1.6 mmol, 53%) of 14. 1H NMR (CDCl3): 0.85 (t, 3 H, CH2CH3), 1.23 (br s + sh, 26 H, (CH2)13), 1.80 (br s, 2 H, NCH2CH2R), 3.28 (s, 6 H, 2NCH3), 3.6-3.8 (m, 2 H, NCH2R), 5.71 (s, 2 H, NCH2Ar), 7.4-7.8, 8.18.3 (m, 8 H, Ar). The ester-surfactant 14 (0.64 g, 1.1 mmol) was dissolved in the minimum amount of methanol and stirred with 1.5 mL of 1 M aqueous NaOH for 1 h at 25 °C in order to cleave the PNP ester. The yellow product was extracted into CH2Cl2 and washed 3 times with water. The CH2Cl2 phase was dried over Na2SO4, solvent was stripped, and 0.53 g (1.1 mmol, 100%) of the yellow sodium salt of the p-nitrophenylate surfactant derived from 14 was isolated. NMR (CDCl3) demonstrated the absence of the benzoate moiety of 14. The phenylate surfactant (0.30 g, 0.61 mmol) was dissolved in 10 mL of dry CH2Cl2, cooled in an ice bath, and stirred with 1.4 g of dry Amberlite IRA-35 free base resin (Schweizerhall, 5 mequiv/g). To this was added 0.36 g (2.1 mmol) of diethyl chlorophosphate (Aldrich) dropwise over 20 min. The reaction mixture was allowed to warm to 25 °C and stirred for an additional 3 h. The resin was filtered off, and the filtrate was washed 3 times with dilute aqueous HCl (to remove diethyl phosphate). The CH2Cl2 phase was dried over Na2SO4, and the solvent was stripped. The residual oil was purified on a silica gel column that had been pretreated with a 7% methanol-0.5% aqueous HCl-CH2Cl2 mixture. The same solvent was used for elution. We obtained 50 mg (0.089 mmol, 15%) of paraoxon surfactant 13 as a tan oil. 1H NMR (CDCl3): 0.87 (t, 3 H, CH2CH2CH3), 1.24 + 1.38 (s + m, 32 H, (CH2)13 + 2OCH2CH3), 1.80 (br s, 2 H, NCH2CH2R), 3.23 (s, 6 H, 2NCH3), 3.6 (m, 2 H, NCH2R), 4.2-4.4 (m, 4 H, 2OCH2CH3), 5.78 (s, 2 H, NCH2Ar), 7.65 (d, J ) 6 Hz, 1 H), 8.15 (d, J ) 6 Hz, 1 H, Ar), 8.28 (s, 1 H, Ar). 31P NMR (CDCl3): -6.88 (s). Anal. Calcd for C29H54ClN2O6P‚1.5H2O: C, 56.1; H, 8.8; N, 4.5. Found: C, 56.2; H, 9.0; N, 4.4. Kinetic Studies. CTACl solutions were prepared with “steam-distilled” water (distilled, U.S.P., Electrified Water Co., East Orange, NJ) in 10 mM KCl, 20 mM NaH2PO4, pH 8.0 buffer. The CTACl concentration typically ranged from 5 × 10-4 to 5 × 10-2 M. Solutions of small molecule phosphate substrates were generally prepared in MeCN at a concentration of 2 × 10-3 M. Addition of 15 µL to 3 mL of CTACl solution gave a final [substrate] of 1 × 10-5 M with 0.5% MeCN. The catalyst was dissolved in DMF at a concentration of 2 × 10-2 M (IBA) or 1 ×
2206 Langmuir, Vol. 12, No. 9, 1996 10-2 M (INA), and 15 or 30 µL, respectively, was added to the 3 mL CTACl-substrate solution to initiate the reaction. The final [catalyst] was 1 × 10-4 M with 0.5 or 1% DMF. For reactions of surfactant-substrate 12 with IBA or INA, stopped-flow methods were needed.12 Micelles were prepared by dissolving both 12 and CTACl in CHCl3 at appropriate ratios. The CHCl3 was evaporated and the lipid film dried under vacuum for 2 h. A 10 mM KCl solution (pH 4) was added to make the correct concentration just before the reaction was carried out in order to minimize prehydrolysis. The sample was homogenized by brief sonication in a bath-type sonicator. The substrateCTACl solution was loaded in 1 syringe of the stopped-flow apparatus; the second syringe was charged with the catalyst in 5-fold excess over 12 and a matching CTACl concentration, in 20 mM phosphate buffer, pH 8.10, made up in 10 mM aqueous KCl. Equal volumes of this catalyst-CTACl-buffer solution and the substrate-CTACl solution (at pH 4) were combined in the stopped-flow reaction to give a product solution with 10 mM phosphate buffer at pH 8.0. Slower reactions were followed on a Gilford Model 250 spectrophotometer coupled to a Gilford Model 6051 recorder. Faster reactions were followed on a Durrum Model D-130 stoppedflow spectrometer coupled to a Tektronix Model 5103N/D15
Moss et al. storage oscilloscope; kinetic traces were recorded with photographs. Rate constants were obtained from computer-generated correlations of log(A∞ - At) with time for the appearance of p-nitrophenylate ion at 400 nm. Conditions for the kinetic runs are described above, in the Results section, and in the notes to Tables 1 and 3. Data appear in Tables 1-3 and in Figures 1-3. Kinetic runs were generally followed to 90% completion and showed good first-order behavior (r > 0.99) with rate constants reproducible to (5%.
Acknowledgment. We gratefully acknowledge financial support from the U.S. Army Research Office (Rutgers University), from the Ministry for University, Scientific, and Technical Research (University of Trieste), and from NATO (R.A.M. and P. S.). B.D.P. acknowledges a postdoctoral stipend from Aekyung Industrial Co., Korea. We thank Ms. S. Bose for assistance in determining the data in Table 3. We are grateful to Dr. H. Zhang, who provided a sample of INA and assistance with stopped-flow kinetic runs. LA951093E