Langmuir 1994,10, 3982-3987
3982
Surfactant Effects upon Cyclization of o-(colHaloa1koxy)phenoxideIons. The Role of Premicellar Assemblies Giorgio Cerichelli* and Giovanna Mancini Centro CNR di Studio sui Meccanismi di Reazione, c / o Dipartimento di Chimica, Universita degli Studi di Roma "La Sapienza", P.le Aldo Mor0 5, 00185 Roma, Italy
Luciana Luchetti Dipartimento di Scienze e Tecnologie Chimiche, Universita degli Studi di Roma "Tor Vergata", Via della Ricerca Scientifica, 00133 Roma, Italy
Gianfranco Savelli Dipartimento di Chimica, Universita degli Studi di Perugia, Via Eke di Sotto, 06100 Perugia, Italy
Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, California Received February 7, 1994. In Final Form: June 30, 1994@ Aqueous cationic micelles of R'NR3Br (R'= C12H25, R = Me, DoTABr; R' = C16H33, R = Me, CTABr; R' = C16H33, R = Bu, CTBABr) and R ' N M ~ ~ ( C H Z ) W(~R~ =R C16H33) ~ B ~increase rates of cyclization of O--OC6&o(CH2)n-~(X = Br, I) designated PhXn. Rate increases are small, except for Ph17 in CTBABr,l
and activation parameters are similar to those in water or 75% aqueous EtOH. In very dilute surfactant there are large rate increases, by factors of up to 39 with PhI16 in CTABr, but CTBABr is less effective. These rate increases are ascribed to formation of substrate-premicellar complexes of PhBrlO, PhBrl2, and PhX16 with all the surfactants. These complexes dissolve into normal micelles at higher [surfactant], and the unusual rate effects disappear. Cyclizations of o-(whaloa1koxy)phenoxide ions, 1 (Scheme 1) are well studied,2 and factors such as ring strain and loss of rotational entropy in formation of the transition state have been explored in detail. Following precedent, we write substrate structure in terms of the size of the ring formed by cyclization.2 These cyclizations can be regarded as intramolecular sN2 reactions with varying effective molarities (EM).2 Reactions of the o-(3-bromopropyloxy)phenoxideand 0-(3iodopropy1oxy)phenoxide(PhBr7 and PhI7, respectively) in aqueous micelles are useful models for micellar effects upon rates of sN2 reactions in the interfacial region at micellar surfaces.' First-order rate constants in this region k", can be compared with those in water, k',, and a t 25.0 "C values of k',/k', range from 1.7 and 3.9 for PhBr7 and Ph17 in CTABr, CH3(CH2)15N(CH3)3Br,to 7.7 H&B~. and 34 in CTBABr, C H ~ ( C H Z ) ~ ~ N [ ( C H ~ ) ~ CThese relative rate constants decrease modestlywith increasing temperature.3 These values of k'm/Kw show that cationic micelles do not significantly control initial state conformations of PhX7 so as to increase rates of cyclization. We were therefore
* To whom the correspondence
should be addressed. Abstract published in Advance ACS Abstracts, September 1, 1994. (1)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. (2)(a) Illuminati, G.;Mandolini, L.; Masci, B. J. Am. Chem. SOC. 1974,96,1422. (b)Illuminati, G.;Mandolini, L.; Masci, B. J.Am. Chem. SOC.1977,99,6308.(c) Dalla Cost, A.; Illuminati, G.; Mandolini, L.; Masci, B. J . Chem. SOC.,Perkin Trans. 2 1980,1774. (3)Cerichelli, G.;Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J . Colloid Interface Sci. 1993,160,85. @
Scheme 1
1
X=Br,I
interested in the report by Wei et al. that, although micelles of CTABr did not significantly assist cyclization of PhBr7, they markedly speeded cyclization of the larger chain homolog^.^ However, rates were measured only at loe3 M CTABr, which is very close to the critical micelle concentration, cmc (ca. 8 x M in water at 25 0C),5so that micelles were in very low concentration. Under these conditions it is necessary to consider the possibility that these hydrophobic phenoxide ions associate with monomeric cationic surfactant or with premicelles, and that formation ofthe latter may be induced by the phenoxide ions. There is extensive evidence that substrates and surfactants interact at [surfactant] < cmc,6even considering decreases in the cmc induced by ionic reagents. For example, in many reactions, rate constants increase a t [surfactant] < cmc, especially with hydrophobicreactants.6 Generally, rate constants increase monotonically with increasing [surfactant], and it is then difficult to distinguish between reactant-induced micellization and reaction (4)Wei, L.; Lucas, A.; Yue, J.;Lennox, R. B. Langmuir 1991,7,1336. ( 5 ) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1970.
0743-7463/94/2410-3982$04.50/0 0 1994 American Chemical Society
Cyclization of o-(wHa1oalkoxy)phenoxide Ions in premicellar assemblies. There is evidence that premicellar assemblies are involved in some reactions; for example, double rate maxima are observed in some nucleophilic aromatic s u b s t i t ~ t i o n sand , ~ ~in attack of OHon thiophosphinates.7b Nonmicellizing, hydrophobic ammonium ions increase rates of a number of spontaneous and bimolecular, nonsolvolytic reactions, and in some reactions there is physical evidence for association of reactants and nonmicellized Cyclizations of 1could be mediated by normal cationic micelles, but, especially with the more hydrophobic substrates, e.g., n L 10, formation ofcatalytically effective nonmicellar assemblies might increase rates of formation of large rings in CTABr that would then not depend on micellar incorporation of substrate. In this context we consider aqueous micelles to be assemblies containing ca. lo2monomers Of Cl6 surfactants whose structures do not change sharply with [surfactant] =- C ~ C . ~In, a~ widely accepted model of micellization,nonmicellized surfactant although ~~J~ is considered to exist largely as m o n o m e r ~ , ~ small amounts of small n-mers may exist in equilibrium with monomer and micelles.
Results and Discussion The longer chain phenoxides are relatively unreactive, and they form cyclic ethers that are sparingly soluble in water,2 so it is convenient to follow reactions at higher temperatures and to compare calculated first-order rate constants, kobs, at 25 "C. Most of the rate constants in aqueous micelles and organic or aqueous solvents had been collected at 25 "C, although reactions of PhBr7 and Ph17 had been followed over a range of temperatures, and enthalpies and entropies of activation had been calculated.3 The first measurements were made with CTABr and CTBABr at concentrations well above the cmc, where PhBr7 and Ph17,193and therefore the higher homologs, are fully micellar-bound(Table 1). Temperatures for study of the longer chain compounds ranged from ca. 50 to ca. 74 "C. Activation plots were linear over this range and gave values of AH'+ and AS* (Table 2). For reactions of PhBr7 and PhI7, activation plots were linear from 25 to 60 "C and allowed extrapolation O f kobs to 25 "C for longer chain compounds. Values of AH'+ for cyclization of the higher homologs are similar to those for cyclizations of PhBr7 and Ph17 in micelles of CTABr and CTBABI.3 and in aqueous EtOH or water.2 Values of AS* become significantly more negative as the ring size is increased, as for cyclizations in aqueous EtOH,2 and for exactly the same reason, viz., the loss ofrotational degrees of freedom in formation of the transition state. The effect of ring size upon rate constants of cyclization, calculated for a range of temperatures, is shown in Table 3. It may be observed that the patterns of behavior are (6) (a)Martinek, K; Yatsimirski, A. M.; Levashov,A. V.; Berezin, I. V. In Micellization, Solubilization, and Microemultions, Mittal, K L., Ed., Plenum: New York, 1977; Vol. 2, p 489. (b) Romsted, L. 9.In Micellization, Solubilization, and Microemultiona; Mittal, K. L., Ed., Plenum: New York, 1977;Vol.2, p. 509. (c)Romsted,L. S. InSurfactunt in Solution; Mittal, K. L., Lindman, B., Eda.; Plenum: NewYork, 1984; Vol. 2, p 1015. (7) (a) Bacaloglu, R.; Bunton, C. A. J . Colloid Interface Sci. 1992, 163, 140. (b) Blask6, A.; Bunton, C. A.; Hong, Y. 9.;Mhala, M. M.; Moffatt, J. R.; Wright, S. J . Phys. Org. Chem. 1991, 4 , 618. (8) (a) Kunitake, T.; Okahata, Y.;Ando, R.; Shinkai, S.; Hirakawa, S. J . Am. Chem. SOC.1080,102, 7877. (b) Bunton, C. A,; Hong, Y. S.; Romsted,L. S.; Quan, C. J.Am. Chem. SOC.1081,103,5788.(c)Biresaw, G.; Bunton, C. A.; Quan, C.; Yang, Z.-Y. J . Am. Chem. Soc. 1084,106, 7178. (9) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (10)(a) Menger, F. M.; Portnoy, C. E. J . Am. Chem. Soc. 1967,89, 4698. (b)Menger, F. M. Pure Appl. Chem. 1070, 51, 999.
Langmuir, Vol. 10, No. 11, 1994 3983 Table 1. Temperature Effeots on Rate Constants of Cyelization of PhBrn in CTABr and CTBABP CTABP CTBABrC temp, "C PhBrlO PhBrl2 PhBrl6 PhBrlO PhBrl2 PhBrl6 49.5 55.1 58.8 59.7 62.6 64.2 66.7 67.0 69.9 73.8
0.953 1.54
0.531 0.917
0.263 0.403
2.46 3.62
1.28 1.91
0.620 0.946
4.90
2.80
1.38
6.98
3.56
1.67
3.07 4.27
1.88 2.44
0.916 1.11
6.30 6.58
3.85 4.01
1.91
9.17 11.73 16.59
5.34 8.50
Values of 104k0b, (8-l) with 0.01 M NaOH and 0.038 M. e 0.052 M.
2.70 3.35 4.58 M PhBrn.
Table 2. Activation Parametersa CTABP substrate
m
PhBr7* PhBrlO PhBrl2 PhBrl6
20.4 20.8 20.2 20.3
AS* -5.6 -12.6 -15.8 -16.9
CTBABP
75 % aq. EtOHd
m
AS*
m
19.4 19.8 18.7 19.7
-6.0 -14.8 -18.8 -17.2
22.0 21.5
ASs 1.2 -11.1
20.1
-19.6
Values of m ( k c a 1mol-1) and AS*(eu)with 0.01 M NaOH and lO-'M PhBrn. 3.8 x M except for PhBr7,2.4 x lom2M.C5.1 M. References 2a,b. e Reference 3. x
remarkably similar in micelles of CTABr and CTBABr. The patterns are also analogous to those observed in homogeneous solution, as in 75% aqueous EtOHzavband The values ofk& are similar in CTABr micelles and 75%aqueous EtOH, this effect is reported for a number of spontaneous reactionsS6 Therefore, whatever factors are controlling the activation parameters for reactions in aqueous EtOH, they are at work in micellar reaction regions which, as in other reactions, behave remarkably like homogeneous solvents with surface polarities slightly lower than that of water.ll We did not measure activationparameters in very dilute surfactant because temperature affects equilibria involving substrate and monomeric, premicellar and micellized surfactant. However, values of the cmc do not increase markedly with a temperature increase from 25 to 60 "C (see Experimental section) so changes in extents of micellization are not significantly perturbing our estimated activation parameters. Regardless of ring size, rate constants follow the sequence CTABr CTBABr < 75% aqueous EtOH. These comparisons show that rate constants follow the expected medium effects2The reaction site in aqueous micelles,the interfacial region, is less polar than water, based on spectral probedl and kinetic data.I2-l4 The polarity ofthe interfacial region of a CTABr micelle appears to be similar to that of EtOHl' and higher than that of a CTBABr mice1le.l The very high rates of cyclization in DMSO with respect to those observed in EtOH are as predicted for an intramolecular S Nreaction ~ of a phenoxide ion.2c The relations between ring size and rate constants of cyclization based on extrapolations of rate data to 26 "C do not change with increases in temperature to 60 or 76 (11)(a) Zachariasse, K A,;Van Phuc, N.; Kozankiewicz, B. J . Phys. Chem. 1981,85,2676. (b) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J . Phys. Chem. 1@82,86,3198. (12) Quina, F. H.; Chaimovich, H. J . Phys. Chem. 1070, 83, 1844. (13) Bunton, C. A,; Dorwin, E. L.; Savelli, G.; Si, V. C. Red. Trau. Chim. Pays-Bas. lOW),109, 64. (14) (a) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J.Am. Chem. SOC.1@82,104,6654.(b) Bunton, C. A.; Ljunggren, 9.J . Chem. SOC., Perkin Trans. 2 1084, 355. (c) Correia, V. P.; Cuccovia, I. M.; Stelmo, M.; Chaimovich, H. J.Am. Chem. SOC.1092,114, 2144.
3984 Langmuir, Vol. 10,No. 21, 1994
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Table 3. Cyclization of PhBrn in Fully-Formed Micelles or Homogeneous Solution@ CTABr
CTBABr
n
105koba
kJ k #
105kOb,
7f 10 12 16
40 0.59 0.36 0.17
0.015 0.0090 0.0043
181 1.3 0.94 0.37
7f 10 12 16
7g 10 12 16
580 9.7 5.5 2.6 5900 107 56 27
0.017 0.0095 0.0045
0.018 0.0095 0.0046
2500 18.5 11.7 5.2 23000 181 102 51
75% aq EtOHb
kJ k # T = 25.0 "C 0.0072 0.0052 0.0020 T = 50.0 "C 0.0074 0.0047 0.0021 T = 75.0 "C 0.0079 0.0044 0.0022
DMS@
kdkf
106kobs
kJ k f
77.3 0.33
0.0043
0.06
0.00078
370000 840 1000 800
0.0023 0.0027 0.0022
10'koba
1450 9.7 2.3 2.1 17000 75 9.5
0.0067 0.0016 0.0014
0.0044 0.0056
Values of 106kob,(8-l) interpolated or extrapolated from values of kobs in Table 1. Reference 2a,b. Reference 2c. Rate constants relative to those for reaction of PhBr7, based on mean values ofk,b, over a range of [surfactant]. e Rate constants relative to those for reaction of PhBr7. f Reference 3.8 Extrapolated from ref 3.
"C (Table 3). Therefore temperature effects on micellar structure are not having a significant effect on the role of micelles as reaction media, as shown earlier for reactions of PhBr7 and Ph17.3 Instead of comparing values of kobs for the various substrates in the different reaction media, we can compare rate constants relative to those ofPhBr7, at a series of temperatures (Table 3). All these observations are at variance with the conclusions of Wei et al. that micelles of CTABr increase rates of cyclization, provided that the forming ring is large, due to micellar-induced folding of the ~ u b s t r a t e .Micelles ~ do not have this effect, regardless of the ring size, so it is necessary to consider other explanations of the rate enhancements observed by Wei et al. As noted earlier, their experiments were made with M CTABr where the solution contains very few micelles and significant amounts of monomeric and perhaps premicellar surfactant. Cyclizations in Hexadecylic Cationic Surfactant Systems. Cyclizations of PhBr7 and PhI7 had been examined over a wide range of [surfactant] from well below to well above the cmc at 25.0 "C.lv3 There was a gradual, but small, increase of kobs above that in water, with increasing [surfactant] which was larger for Ph17 than for PhBr7, especially as the bulk of the surfactant head group was increased and the polarity of the interfacial region decreased. The same behavior was observed for reactions of these substrates in CTABr at 59.4 "C.3 The situation changes dramatically with an increase in the ring size. These experiments in dilute surfactant were made at 59.4"C, because of solubility problems at lower temperatures (Tables 4 and 5). For reactions of 1,with n = 10 and 12 (PhBrlO and PhBrl2, respectively),kobs in dilute surfactant is much higher than that in water. We could not measure the rate constant in water for the more hydrophobic substrates, n = 16 (PhBrl6 and PhIlG), but we predicted kobs for reaction in water of PhBrl6 by extrapolation of rate data in a mixed solvent. This estimated value is much lower than that in diluted surfactant. At high [surfactant] values, of kobs for both 1 and 2 decrease sharply to those characteristic of reactions in normal micelles (Tables 1, 4, and 5). The high rate constants that we observe in very dilute surfactant must be due to formation of reactive assemblies of substrate and monomeric or premicellar surfactant. These assemblies form only with the more hydrophobic substrates and not with PhBr7 or PhI7. We believe that the rate enhancements are not due to formation of 1:l
Table 4. Cyclizations in CTABP 1O3[CTABr1,M PhBrlO PhBrl2 PhBrl6 06 1.40 ca. le 0.2d 0.50 19.0 11.0 0.75 11.1 10.0 6.55 0.82 8.99 0.99 9.16 4.38 1.11 7.32 2.23 4.01 2.45 4.38 2.52 3.34 2.92 4.46 2.52 5.01 3.38 5.57 2.07 1.42 7.46 1.24 7.52 3.14 1.83 9.91 3.05 1.70 1.05 25.0 2.81 1.44 0.63 50.0 2.65 1.25 0.55 92.2 2.58 1.25 0.55 492 2.45 1.13 0.49 538
PhI16
13.8 7.09 3.38
2.00 1.46 1.27 0.76 0.59 0.49 0.36
Values of 104k,b, (8-l) a t 59.4 "C, with 0.01 M NaOH and M substrate unless specified. 10+ M PhBm. Approximatedvalue in water. Estimated from data for 75% aq. EtOH.
complexes of substrate and monomeric surfactant, i.e., to simple ion-pair formation, because in some experiments k o b s in dilute CTBABr is only slightly greater than in H20, but it then goes through a maximum with increasing [CTBABrl and finally decreases to a value for reaction in normal micelles (Table 5). Cyclizations in Short-Chainand Dicationic Surfactants. Experiments discussed thus far involve hexadecyl surfactants, but we also used a short-chain surfactant DoTABr, CH3(CH2)11N(CH3)3Br, and a dicationic surfactant (CDA)&2Br, 174-bis(N-hexadecyl-N,N-dimethy1ammonium)butanedibromide C H ~ ( C H ~ ) I ~ N ( C H ~ ) ~ ( C H Z ) ~ N ( C H ~ ) ~ ( C H ~ ) &Cyclizations H ~ ~ B ~ . of PhBr7 and PhI7 had been examined in a chemically similar propane surfactant and the rate effects compared with those on a de~arboxy1ation.l~ Cyclizations of PhBr7 and Ph17 in 0.5M DoTABr over a range oftemperaturebehave as expected. More complete data are in ref 3 for reactions at 59.4 "C, and there is essentially no rate enhancement for PhBr7 and an approximately 2-fold rate enhancement for PhI7. As with (15) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Phys. Org. Chem. 1991,4, 71.
Langmuir,Vol. 10,No.11, 1994 3986
Cyclization of o-(o-Haloa1koxy)phenoxideIons Table 5. Cyclizations in CTBABP lO~[C"BABr],M PhBrlO PhBrl2 PhBrl6 1.40 ca. le 0.2d Ob 4.37 0.50 0.57 3.01 1.48 1.52 3.45 1.98 6.04 8.64 5.34 13.6 2.16 5.90 2.27 3.18 2.32 4.49 2.53 3.17 3.45 4.96 5.55 3.62 2.65 7.47 5.21 3.58 2.19 9.52 4.74 3.51 10.91 4.85 3.45 1.93 23.0 1.39 42.0 4.88 49.2 4.88 3.00 1.36 75 4.87 2.61 1.25 245 4.93 3.14
PhI16
16.9 10.7 7.3 3.24 3.08
1.43
4 Values of 104&"b, (8-1) at 59.4 "C, with 0.01 M NaOH and lo-* M substrate unless specified. M PhBm. Approximatedvalue in water. d Estimated from data for 75% aq. EtOH.
Table 6. Cyclization in DoTABP 102[DoTABr], M PhBrlO PhBrl2 Ob 1.40 ca. le 1.56 8.16 6.06 2.49 2.52 1.70 4.98 2.10 1.02 7.39 1.97 0.98 9.89 2.07 0.89 71.5 1.79 0.73
PhBrl6 0.2d 6.49 0.47 0.85 0.45 0.42 0.23
Values of 1O4kOb, (s-l) at 59.4 "C,with 0.01 M NaOH and lom4 M substrate unlessspecified. b 10-6M PhBnt. Approximatedvalue in water. Estimated from data for 75% aq. EtOH.
behavior is strikingly similar to those of the single tail, monocationic surfactants. For example, hobl at high [(CDA)2CdBrl is not much larger than in water but is much lower than in very dilute surfactant. Nature of the ReactiveAssemblies. There are many examples of rate enhancements by nonmicellized cationic surfactants and by hydrophobic ammonium ions which, for steric reasons, cannot micellize. Typical examples are the NJVJV-trialkyl-N-octylammoniumsalts.8 There are linear free energy relations between cyclizations of PhBr7 and Ph17 and decarboxylation of 6-nitrobenzisoxazole-3-carboxylateion,16and this decarboxylation is catalyzed by nonmicellizing hydrophobic ammonium ions.8e It is therefore not surprising that premicellar cationic surfactants effectively catalyze cyclizations of the more hydrophobic substrates. Attempts to examine cyclizations in solutions of nonmicellizing hydrophobic ammonium salts were frustrated by solubility problems. Micelles have very mobile ~tructures,B~~ and the interfacial region, the site of polar and ionic reactions, is a fluid environment in which rate constants of many reactions are similar to those in water or in polar hydroxylicsolventa6Ja Rate enhancements of bimolecular reactions are due largely to concentration of reactants in the small volume of the interfacial region. Spontaneous reactions whose rates are strongly micellar-mediated are generally those sensitive to kinetic medium effects in homogeneous s o l ~ e n t s , ~ Jalthough ~ - ~ ~ there is some dependence upon the charge type of the reaction.14 Second-orderrate constants of sN2 reactions of nucleophilic anions are generally similar in water and at the micellar surface,14consistent with modest micellar effects upon rates of cyclization of the (haloalkoxy)phenoxideions (Tables4-6). This evidence does not mean that substrate folding is precluded in normal micelles, but if folding occurs, it is not kinetically productive, and normal aqueous micelles are unlikely to be synthetically useful in intramolecular cyclizations. They may, however, increase rates and regioselectivities of bimolecular cyclizations, e.g., Diels-Alder reactions.16 The next question concerns the factors that allow premicellar surfactant assemblies to increase reaction rates. Unless reagents are very hydrophobic, micelles should be more effectivethan premicelles in concentrating reagents and thereby speeding nonsolvolytic bimolecular r e a c t i o n ~ . ~ tThe ~ J ~situation is different for spontaneous reactions where rate effects do not involve concentration of more than one reagent in a submicroscopicregion and depend only upon the medium effects of the interfacial region, for fully bound substrates. Hydrophobic phenoxide ions should interact with amphiphilic cations to form ion pairs or clusters that would attract more hydrophobic solutes, especially if Coulombic attraction favors association. In this context we note that the longer chain (haloalk0xy)phenoxideion substrates are amphipbilicanions and should interact Coulombically and hydrophobically with surfactant cations to form tight ionpaired assemblies, in contrast to the much looser, fluid micelles,whose structures are not influencedby very dilute substrate. Rates in these small assemblies will be much faster than in water if pairing between ammonium and phenoxide ions even partially excludes water from the latter, or if the substrate coils to reduce water-hydrocarbon contact and therefore brings the oxide ion closer to the reaction center. Comparison of rates of reactions of
' 1 r @
7
8-
n
@
@ @
U ' I 0 6-
e 42 00 " " " " ' " " " " " ' ~
A
CTBABr. We examined cyclization of PhBrl2 in solutions of (CDA)2C&Br. We can observe (Figure 1) that kobo increases sharply in very dilute surfactant and then decreases to a constant value at higher [surfactant], as for reactions in monocationic surfactants. We know very little about the structure of micelles (or other association colloids) formed by assembly of (CDA)zC$Br, but its
(16)Jaeger, D. A; Shinozaki, H.;Goodeon, P.A. J. Org. Chem. 1991, 56, 2482.
3986 Langmuir, Vol. 10, No. 11, 1994
Cerichelli et al.
Table 7. Relative Rate Constants in Premicellar and in Micellar SurfactantO substrate PhBrlO PhBrl2 PhBrl6 PhI16
CTABr 7.8 11 13 39
CTBABr 1.2 5 11 11.8
a Values of kpr$kmicat 59.4 "C from data in 0.6-1.9 x CTABr and 1.5-2.2 x M CTBABr for kpre.
M
Table 8. Cyclizations in Micelles and PremicelletP bromides PhBr7" PhBrlO PhBrl2 PhBrl6 iodides PhI7" PhI16
104kOb,,8-1 micelles premicellesb 155 14od 2.5 19.0 ca. 1.2 12.0 ca. 0.6 6.55 1o4kob,, micelles premicellesb 194 ca. 0.5
15Bd 13.8
micelles 0.016 0.0077 0.0039
kJh premicellesb 0.14 0.086 0.047
kdk7 micelles premicellesb 0.0026
0.087
59.4 "C in CTABr. b Values are from maximum values of in Table 4. Reference 3. [CTABrl = 5 x M.
a At
kobs
bromide and iodide substrates in premicellar assemblies supports the concept that substrate in these assemblies is less hydrated than in, for example, normal micelles. In water, with no surfactant,' PhBr7 is more reactive than Ph17 because hydration of the leaving anion is more effective for Br- than for I-, but generally, in organic solvents, we have the usual order, iodide > bromide.' For reactions of PhBrl6 and PhI16 in CTABr, KI > kg,1' in dilute surfactant, but values of k&, decrease with increasing [CTABrl and at high [CTABrl k g , > k~ (Table 4). Therefore we have the paradoxical result that the transition state in premicelles is less hydrated than in normal micelles of CTABr. This result is consistent with a very strong interaction between substrate and surfactant in premicelles and a relatively loose organization of substrate in the water-rich interfacial region of a CTABr micelle. This behavior is different from that of PhBr7 and Ph17 in CTABr at 59.4 "C, where, as at 25.0 "C, kg, > KI in water and very dilute CTABr, but at higher [CTABrl, k~ > k g , , although relative rate constants are close to l.'a These substrates are not sufficiently hydrophobic to organize substrate-surfactant premicellar complexes. Formation of premicellar complexes seems to be related to the bulk of the surfactant head group, probably because tight pairing of an oxide anion and an ammonium cation is inhibited in the bulky Bu3N+ as compared with the MesN+ group. As a result, values ofkpr$kmic are sensitive to structures of substrate and surfactant. We take k,,, as the highest value of hobs in very dilute surfactant, so it is not known precisely, but kmio the value in fully formed micelles, is well established (Tables 4 and 5). Because of uncertainties in values of lapre,we can only use the trends of relative rate constants in Table 7, but there is clear dependence on substrate hydrophobicity and surfactant head group bulk. Our values of kpre are larger than kficby approximately 1order of magnitude in CTABr at 59.4 "C for the bromides with n 1. 10 (Table 8). The value for reaction of PhI16 is even larger. It is difficult to give firm values of kpre,but it is evident that differences between kpre and Kmic are observed only with n I 10. These differences in rate (17)karandkI are first-order rate constants for reactions of a bromide and an iodide in the specified conditions.
constants are very similar to the differences between our rate constants for reactions in micelles calculated at 25 "C and those of Wei et al. for reactions of PhBrlO and PhBrl2 in M CTABr at 25 "C.4 These comparisons show that our conclusions regarding reactions in premicelles at 59.4 "C apply also t o reactions at 25 "C. We can draw similar conclusions regarding the properties of premicellar complexes in CTBABr from values of kI/kgrforreactions ofPhBrl6 and PhI16, which are always greater than 1and increase slightly in going from 2.16 x to 1.06 x M CTBABr (Table 5). In this situation the bulky butyl groups tend to exclude water from micellar surfaces, in agreement with other results.'JsJg For spontaneous reactions, such as cyclizations, contributions of reactions in premicelles may lead to rate maxima in plots of kobs against [surfactant], although we know of only one other example.20 Frequently, rate constants increase at [surfactant] well below the cmc, under kinetic conditions, although here the assumption that the cmc is the concentration of monomeric surfactant This increase is is suspect if premicelles are pre~ent.~JO also seen with bimolecular reactions that occur in premicelles, but occasionally there are double rate maxima which are inconsistent with surfactant effects being due solely to This general behavior seems to be associated with sparingly water-soluble, hydrophobic substrates whose associationwith amphiphilicions decreases hydrocarbonwater contact and provides ion-ion or ion-dipole interactions. These substrates generally have polar, chromophoric groups at the reaction center, so they are also amphiphiles and should interact with both nonmicellar and micellar surfactants. The structures ofthe complexes so-formed are often uncertain, although anionic and cationic surfactants can spontaneously associate to form vesicles whose structures are well-established.21 The pseudophase model of micellar rate effects involves the implicit assumption that rate constants of fully micellar-bound substrates are, for spontaneous reactions, independent of surfactant concentration, unless an increase in concentration involves a major change in micellar structure. However, with the longer chain substrates, kobs decreases slightly with increasing [CTABrl > M (Table 4). We do not believe that this decrease is related to micellar growth, which should be small at 59.4 "C, and for reactions of PhBr7 and PhI7, micellar growth, at 25 "C, if anything, increases the rate.' At relatively high surfactant concentration, intermicellar distances are not large, relative to micellar radii, so it is possible that reaction of a substrate in one micelle is perturbed by proximity of another.
Conclusions Premicelles are much better catalysts than cationic micelles for cyclizations of 0-(o-haloalkoxy)phenoxideions, provided that the length of the alkyl group (n L 10)makes them sufficientlyhydrophobic to interact with nonmicellar surfactant. Micelles catalyze cyclization, but not to any large extent, except with Ph17 in CTBABr.' (18) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J.Phys. Chem. 1989,93, 1490. (19) (a) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331. (b) Bertoncini, C. R. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990,94, 5875. (20) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J . Chem. SOC.,Perkin Trans. 2 1989,1767. (21) M e r , E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989,245, 1371.
Cyclization of o-(wHaloa1koxy)phenoxide Ions
The high rates of cyclization in M CTABr reported by Wei et al. are also due to catalysis by premicelles rather than micelles. In particular, the statement by these authors that conditions were selected “to ensure that the probability of greater than single occupancy of a micelle is l o g 4 seems to be incorrect. The cmc of CTABr (ca. 8 x M in waterY will be lowered by addition of OH-, but if we assume that [micellized surfactant] = 6 x M, then with a micellar aggregation number of 100 we M micelles. Wei et al. did not specify the have 5 x concentration of phenoxide ion that they used, but they varied it over a 3-fold range. Average single occupancy of micelles requires the phenoxide concentration to be less than 5 x 10+ M, which in our experience is an impractically low concentration for kinetic study. For example, much higher concentrations were used in other kinetic studies. These considerations indicate that the surfactant catalysis observed by Wei et al. did not involve micelles but, as in our experiments, was due to formation of premicellar substrate-surfactant complexes. Most of our work involved (216 monocationic surfactants, but our conclusions regarding rate enhancements in premicellar assemblies apply also to reactions in DoTABr (Table 6) and a dicationic surfactant (Figure 1).It seems premature to assume that micelles control looping of substrates so as to increase rate constants of cyclizations and other reactions. In addition, bimolecular micellar rate enhancements in many reactions can be treated quantitatively on the assumption that they are due largely to increases in local concentration in the interfacial region whose properties, as a kinetic medium, are similarto those of polar aqueous or organic solvents, generally with no significant orientation effects.6~~~1~3 It appears that, although aqueous surfactants can increase rates of spontaneous cyclizations, they will not be synthetically useful for these reactions. In very dilute solution there is catalysis, but reactant concentrations are then very low. In micelles, where a useful amount of substrate can be solubilized, rate enhancements are very small. These generalizations are clearly related either to the “Catch 22” principle or to Murphy’s law.% (22)(a) van der Langkruis,G.B.; Engberts, J. B. F. N. J.Org.Chem.
1984,49, 4162.(b) Correia, V. P.; Cuccovia, I. M.; Chaimovich, H. J. Phys. Org. Chem. 1991,4, 13. (23)For a postulated orientation effect see: Broxton, T.J.;Christie, J. R.; Chung, R. Pa-T.J. Org. Chem. 1988,53, 3081.
(24)The provenanceofthie law is uncertain,but its predictive powers are well-established.
Langmuir, Vol.10, No. 11, 1994 3987
Experimental Section Materials. The preparation and purification of o-(o-bromoalkoxy)phenol,PhBrn, 0-(3-iodopropoxy)phenol,PhI7, and surfactants have been described.lI2 Prepamtion of0-(12-iodo~lexadecy2azy)phenol (PhZ16). PhBrl6 was converted to PhI16 with sodium iodide in acetone. 1HNMFt 6 (CDCla)7.05-6.78 (4H, m, C a ) , 5.96-5.53 (lH, 8, OH), 4.03 (2H,t, OCHZ),3.22 (2H,t, CHZI),1.95-1.73 (4H,mb,CHZ),1.681.20(16H,mb, CHZ).lacNMFt6 (CDCld 145.76(1-COH),145.90 (2-COC), 111.57 (3-CH),119.99 (4-CH), 121.21 (5-CH), 114.38 (6-CH),68.81(1’-OCH2)28.48(2’-CHz),30.44,29.45,29.34,29.30, 29.19 (3’-CH2-9’-CHz),33.61(10’-CH2),25.96 (11‘-CH2),7.27(12’CH2I). Reactionswere followed in deionized,distilled,COz-freewater. Kinetics. Reactionswere followed spectrophotometricallyas described.’ Substrate concentrations were generally M for reactions in water, where precipitation of the cyclic ethers is a major problem. We used 10-4 M substrate for reactions in surfactant solutions where precipitation was less of a problem but where light scattering by micelles may give an apparent absorbancechange and limit the sensitivityofthe measurements. We saw no systematic deviations from first-order kinetics. Reactions in 0.01 M NaOH were followed under conditions in which reactionswent to completion and intermolecularreactions were unimportant. Rates of cyclization are independent of
First-orderrate constants,kob(s-l),were generallycalculated from the absorbance data by taking infinity values, and values of hob,, at or near rate maxima in dilute surfactant by means of 2-3 independent determinationswhich agreed within 5%. The value ofkobfor cyclizationof PhBrl2 in water is only approximate becauseof precipitation of the cyclic ether, and the reaction could not be followed to completion. We could not follow reactions of the more hydrophobic substrates in water, and we estimated approximate values of kobfrom data in 76% aqueous EtOH on the assumptionthat solventeffectswould be unaffected by small changes in n. The rate constants in water are so much smaller than in premicelles that these assumptions do not affect interpretation of the data. Most of our experimentswith the longerchain substrates were at 59.4 ‘C or higher temperatures,and an increase in temperature increases the cmc. The changes, under our experimental conditions, are small; for example, for CTABr in water the cmc increases from ca. 8 x M at 25 “C to 9.5 and 11.5 x M at 40 and 60 “C, respectively. The corresponding values for at 25, 40, and 60 ‘C, CTBABr are 2.7, 4.8, and 5.4 x respectively, based on conductivity/surface tension data. Some values of hob for reactions in micelles (Table 3) were calculated from data over a range of temperatures (Table 1 and ref 3). Acknowledgment. Support of this work by CNR (Roma),the MURST, and the National Science Foundation (Organic Chemical Dynamics Program) is gratefully acknowledged.
