Langmuir 1999, 15, 2631-2634
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Cyclizations of 2-(ω-Bromoalkyloxy)phenoxide Ions in Dicationic Surfactants Giorgio Cerichelli* Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi de L’Aquila, Via Vetoio, 67010 Coppito Due (AQ), Italy
Luciana Luchetti* Dipartimento di Scienze e Tecnologie Chimiche, Universita` degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy
Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione c/o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, Box 34sRoma 62, P.le Aldo Moro 5, 00185 Roma, Italy
Gianfranco Savelli Dipartimento di Chimica, Universita` degli Studi di Perugia, Via Elce di Sotto, 06100 Perugia, Italy Received August 5, 1998. In Final Form: December 31, 1998 Cyclization of 2-(3-bromopropyloxy)phenoxide ion (PhBr7) in micelles is a model for SN2 reactions of nucleophilic anions at micelle-water interfaces. In this paper we report observed rate constants (kobs) in aqueous micelles of dimeric surfactants. kobs in 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1) is constant in a range of concentration, then increases at high [1], suggesting a transition to a different type of aggregate. A 1H NMR investigation confirms a phase transition at the [1] which corresponds to the reactivity change. (2S,3S)-2,3-Dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (2) forms very large aggregates and shows a modest catalytic effect. Cyclization of 2-(12-bromododecyloxy)phenoxide ion (PhBr16) in 1 under preparative conditions yields the intramolecular cyclic product only despite a low effective molarity and a relative high substrate concentration. This result reveals the presence of a well-organized substrate-surfactant aggregate.
Introduction Aqueous micelles can exert a medium effect both on spontaneous and nonspontaneous reactions.1-3 In bimolecular reactions, the concentration effect influences the observed rate constant (kobs) because the reactants can be brought together at the micellar surface or kept apart. This effect is independent of reaction medium, so micellar rate effects have to be treated in terms of models that estimate partitioning of the two reagents between water and micelles and second-order rate constants in each pseudophase. To apply these models we must estimate the reagent concentration in the micellar pseudophase, even if the volume available to the reactant is uncertain. Sometimes it is possible to measure reagent partitioning, but often it is calculated by using equations that contain parameters whose values are uncertain.2,4-7 For these reasons, second-order rate constants in water and micelles are very similar for many bimolecular reactions and comparison between second-order rate constants that differ by less than an order of magnitude
Scheme 1
is not very significant. On the other hand, comparing firstorder rate constants is more significant because they are independent of reaction volume. Cyclizations are intramolecular reactions and their rate constants only depend on medium effects because they are first-order processes. In particular, the cyclization reaction of 2-(3-bromopropyloxy)phenoxide (PhBr7), which gives a seven-member cyclic ether, reported in Scheme 1, has been studied both in homogeneous8 and in micellar solutions.9 Being an SN2 process where the charge in the transition state is dispersed relative to substrate, it is
* To whom the correspondence should be addressed. (1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (2) (a) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, pp 10151068. (b) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213-309. (3) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 18441850.
(4) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538-541. (5) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988, 92, 2896-2902. (6) Rodenas, E.; Ortega, F. J. Phys. Chem. 1987, 91, 839-840. (7) Neves, M. F. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome, F. J. J. Phys. Chem. 1989, 93, 1502-1505. (8) (a) Illuminati, G.; Mandolini, L.; Masci, B. J. Am. Chem. Soc. 1974, 96, 1422-1427. (b) Dalla Cort, A.; Illuminati, G.; Mandolini, L.; Masci, B. J. Chem. Soc., Perkin Trans. 2 1980, 1774-1777.
10.1021/la980989r CCC: $18.00 © 1999 American Chemical Society Published on Web 03/16/1999
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micelles of 1 under preparative conditions. This substrate, forming a sixteen-member cyclic ether, has an EM lower than that of PhBr7.
Chart 1
Experimental Section
affected by medium polarity; micellar-induced changes in the rate constant of the fully micellar bound substrate can give useful information on the properties of the micellar surface. Several papers have recently been published on dicationic surfactants, named gemini or dimeric, that have two hydrophobic tails and two polar headgroups, linked by a spacer group.10 The surfactant properties are very different from those of the corresponding monocationic compound and are strictly dependent on the spacer, whose nature can be very different. In particular, these surfactants show high surface activity, unusual viscosity changes with an increase in surfactant concentration, a low critical micelle concentration (cmc), and unusual micellar structures.11 In this paper we report results on the cyclization of PhBr7 in aqueous micelles of 1,4-bis(N-hexadecyl-N,Ndimethylammonium)butane dibromide (1) and (2S,3S)2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (2), reported in Chart 1. These gemini surfactants, in which the spacer is a butane chain, differ for the presence of two methoxy groups in 2, and these groups can strongly differentiate the surfactant properties. An aspect that has to be considered in the study of the reactivity of a bifunctional substrate is the competition between the intramolecular and the intermolecular reaction. The parameter that gives a measure of this competition is the kinetic effective molarity (EM). EM is described by eq 1, where kintra and kinter are the rate constants for the intramolecular and intermolecular reactions, respectively. This parameter is expressed in
EM )
kintra kinter
(1)
units of M and gives information on the yield of monomeric cyclic product as a function of the substrate concentration; a quantitative yield can be obtained if [substrate] , EM. PhBr7 EM in EtOH 75% at 25 °C is 25 M8a so under our kinetic experimental conditions ([PhBr7] ) 1 × 10-4) we could avoid the intermolecular dimerization. To verify if the aggregation should favor the intramolecular reaction despite a low EM, we studied the cyclization of 2-(12bromododecyloxy)phenoxide ion (PhBr16) in aqueous (9) (a) Cerichelli, G.; Luchetti, L.; Mancini, G.; Muzzioli, M. N.; Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1081-1085. (b) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. J. Phys. Org. Chem. 1991, 4, 71-76. (c) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85-92. (d) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G. Tetrahedron 1995, 51, 1028110288. (e) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. Langmuir 1996, 12, 2348-2352. (10) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083-10090. (b) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 11491153. (c) Fletcher, P. D. Curr. Opin. Colloid Interface Sci. 1996, 1, 101106 and references cited therein. (d) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 185, 84-93. (11) Rosen, M. J. CHEMTECH 1993, 30-33 and references cited therein.
Materials. Preparation and purification of 2-(3-bromopropyloxy)phenol (PhBr7), 2-(12-bromododecyloxy)phenol (PhBr16), 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (1), and (2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butanedibromide(2)havebeendescribed.8a,12-14 Kinetics. Reactions were followed in deionized, distilled, CO2free water as described.9a Solutions of 1 were not used until 24 h had passed after the preparation. The PhBr7 concentration was 1 × 10-4 M. Procedure for the Cyclization of PhBr16 under Preparative Conditions. PhBr16 (50 mg, 0.14 mmol) was added to 2.8 mL of an 0.1 M aqueous solution of 1 and 0.1 M KOH at 90 °C in order to have the ratio [PhBr16]/[1] ) 0.5. The solution was clear even if very viscous. After 3 h, the reaction mixture was diluted with water, allowed to reach room temperature, extracted with Et2O, and concentrated. The concentrated solution was analyzed by GCL, yielding benzo[2,3]-1,4-dioxacyclohexadecane (PhO216) (31 mg, 0.11 mmol). The identification was made by comparison with an authentic sample.12 NMR. 1H NMR measurements were carried out on a Bruker AC300P instrument operating at 300.13 MHz. GCL. Gas chromatographic analyses were performed with a Carlo Erba HRGC 5300 Mega Series Instruments, using a SPB35, 30 m × 0.25 mm capillary glass column. o-Didodecyloxybenzene was used as an internal standard.
Results and Discussion Kinetic Experiments. A monomolecular reaction in a micellar system is described by eq 2,15 where kobs depends
kobs )
k′w + k′mKS[Dn] 1 + KS[Dn]
(2)
on the first-order rate constants in micelles and water (k′m and k′w, respectively), on the binding constant of substrate to micellized surfactant (KS), and on the concentration of micellized surfactant ([Dn]). [Dn] can be obtained by subtracting the cmc, assumed to give the concentration of monomeric surfactant, from the surfactant analytical concentration. kobs generally increases monotonically with increasing surfactant concentration and becomes constant when the substrate is fully micellar bound.9a In the case of high binding constant and at increasing [D], kobs should remain constant provided that the micelle structure does not change, affecting k′m. The results, reported in Figure 1, show that kobs is constant over a range of concentrations and then increases at high [1]. Because at high [1] we were close to maximum solubility, we also ran the kinetics at 30 °C, obtaining an analogous pattern. At 25 °C, a catalytic effect is observed below the cmc, that is 3 × 10-5 M, which was determined by conductivity measurements and was in agreement with literature data,13 i.e., in a region where the surfactant is apparently not micellized and where its concentration is lower than that of the substrate. This can be due either to a (12) Mandolini, L.; Masci, B. J. Org. Chem. 1977, 42, 2840-2843. (13) Bunton, C. A.; Robinson, L.; Schaak, J.; Stam, M. F. J. Org. Chem. 1971, 36, 2346-2350. (14) Cerichelli, G.; Luchetti, L.; Mancini, G. Tetrahedron 1996, 52, 2465-2470. (15) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; Chapter 4.
2-(ω-Bromoalkyloxy)phenoxide Ions
Figure 1. Variation of kobs for the cyclization of PhBr7 with [1] at 25.0 (b) and 30.0 °C (9). The inset shows kobs at low [1].
micellization induced by the anionic substrate or to a catalytic effect by premicelles.16 kobs is approximately constant in a range of concentration (5 × 10-4 to 2 × 10-2 M) where the ratio k′m/k′w is 4.0. In this range, the catalytic effect is higher than that of N-hexadecyl-N,N,N-trimethylammonium bromide (CTABr) and intermediate between those of N,N,N-triethyl-Nhexadecylammonium and N-hexadecyl-N,N,N-tripropylammonium bromides (1.8, 2.6, and 5.6, respectively).9a This effect is in agreement with a reaction medium less polar than water. In general, dicationic surfactants are better catalysts than the corresponding monoanionic one, probably because the spacer decreases the extent of water penetration at the aggregate surface and the cyclization rate is increased in a less polar reaction site.13 The decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (3) is faster in cationic micelles than in water, and the ratio k′m/k′w ranges up to 103. Plots of log k′m for cyclization of PhBr7 against log k′m for decarboxylation of 3 are linear with slope of 0.46.9b Deviations are observed for surfactants that form relatively large assemblies, e.g., N,N-didodecyl-N,N-dimethylammonium chloride and 1,3bis(N-hexadecyl-N,N-dimethylammonium)propane dibromide (4). Using the value of k′m for decarboxylation of 3 in this range of concentration, reported in the literature,17 1 fits the LFER quite well, showing that the butane spacer is flexible enough to accommodate conformational changes in bound solutes. This observation is in agreement with the results of Danino et al.18 They have examined the behavior of a series of dimeric m-s-m surfactants, where m and s are the carbon numbers of the alkyl chains and of the alkanediyl spacer, respectively. For m ) 16, the surfactant has a tendency to form vesicles or membrane fragments when the spacer is short while only spheroidal micelles are present for s ) 8. In particular, 1.2 × 10-4 M 4 shows threadlike micelles, vesicles, and bilayer membrane fragments while 4.5 × 10-4 M 1 shows entangled threadlike micelles, some open membranes, and spheroidal micelles. Another difference between 1 and 4 is that 4 is much less soluble than 1, confirming the point that the spacer greatly influences the surfactant properties. At high [1], when the substrate is fully bound, we find variations of kobs with increasing [1] that correspond to (16) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982-3987. (17) Bunton, C. A.; Minch, M.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262-3272. (18) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 14481456.
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changes in the aggregate structure. This experimental pattern is observed also at higher temperature, if we exclude the point that this effect derives from solubility problems. kobs increases very sharply and does not reach a plateau. We cannot ascribe this behavior to a sphere to rod transition, even if at high [1] the reaction site is less polar. In fact, in this case, the kobs increase should be smaller and about 10%.9a To study this peculiarity deeper, we performed an NMR investigation. 1H NMR spectra of 1 in D2O show very broad signals. Small micelles have narrow signals,19 so we are in the presence of large assemblies, even at [1] ) 5.7 × 10-4, i.e., at about a 20-fold concentration higher than the cmc. We did not observe any significant variation of chemical shift with respect to [1], but because resonance broadening is associated with a transition to larger aggregates,19 we observed the variation of line width at half-height (lw) of the signal relative to N-CH3 group vs [1] at 25 °C. In a range of concentration (5.7 × 10-4 to 1 × 10-2 M) lw increases slowly from 14 to 23 Hz, and then at [1] ) 2.77 × 10-2, there is a sudden change and lw ) 60 Hz. Note that these values are much higher than that of a conventional surfactant such as CTABr, whose value is 1.2 Hz for an aqueous solution 5 × 10-2 M at 25 °C.9c This sharp variation has to be ascribed to a phase transition to a larger aggregate. Note that the lw increase is observed at the same [1] at which kobs increases. This confirms our statement that the cyclization of a 2-(ωhaloalkyloxy)phenoxide ion is a relatively simple but, nevertheless, a very sensible tool to investigate an aggregate structure. Another experimental observation indicates that these aggregates should be well-organized. In fact, when we solubilized 1 in water, the mixture initially is very viscous and cloudy. After 24 h, the solution becomes transparent and slightly less viscous. This indicates that the organization in the aggregate is a slow phenomenon, typical of a highly ordered structure. A similar feature was reported by Menger and Littau,10a when they observed that the surface tension of aqueous solutions of the gemini surfactant (p-phenylendimethylene)bis(hexadecyl hydrogen phosphate) was time dependent. They interpreted this peculiarity by considering that a gemini surfactant, differently from a monotailed one, organized with difficulty at the air/water interface and within the aggregate. The other surfactant under investigation, 2, has a very low solubility and we could only run kinetics up to [2] ) 8.9 × 10-4. The results, reported in Figure 2, show that kobs is constant for [2] > 6 × 10-4, with a modest catalytic effect. We were not able to measure the cmc, but it is certainly
