Langmuir 1994,10, 1000-1002
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Nitrogen Analogs of AOT. Synthesis and Properties A. Leydet, B. Boyer, G. Lamaty, and J. P. Roque' Laboratoire de Chimie Organique Physique, Universite de Montpellier 11, Place E. Bataillon, 34095 Montpellier cedex, France
K. Catlin and F. M. Menge? Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received December 7, 1993. In Final Form: February 22,1994' The synthesis of AOT analogs, in which the two esters are replaced by more chemically stable amides, is described. The nitrogen analogs of AOT form reverse micelles in chloroform with w- values similar to that of AOT. The compounds are, however, too insoluble to form reverse micelles in heptane. Various alkyl groups can be placed on the amide groups of the AOT analogs in order to modulate the hydrophilic/ lipophilic balance.
Introduction Reverse micelles are self-assemblingsystems' comprised of a surfactant monolayer surrounding a spherical core of water. The particles are dispersed in a nonpolar solvent, with the surfactant tails projecting into the continuous phase. The system resembles a water-in-oilmicroemulsion except that no cosurfactant is required. Solutions of reverse micelles are transparent and thermodynamically stable. Perhaps the surfactant most commonly used to prepare reverse micelles is a remarkable compound called "AOT" (bis(2-ethylhexyl)sodium sulfosuccinate). More than 50 water molecules per AOT can be solubilized in a nonpolar solvent such as heptane. The literature describes many potential applications of AOT-stabilized water pools. Among these we might note a few recent publications on their use in enzymatic catalysis,2for the separation of peptides3 and proteins? for peptide synthesis: for the preparation of microparticles with controlled size composed of metal! semiconductors,' oxides! and polymers? and for the synthesis of porous polystyrenes with chemically active surfaces.1° AOT has a disadvantage in cases where the water pool is made basic: its two ester linkages are susceptible to hydrolysis.ll I t seemed useful, therefore, to prepare analogs of AOT that are more stable to basic conditions. For this reason, we set out to prepare AOT in which the two ester functionalities have been replaced by amides (the latter known to be far more impervious to hydrolysis than esters12). Amides have another advantage over esters, namely, that the amide nitrogen (in contrast to the corresponding ester oxygen) can be provided with various aliphatic groups, thereby modulating the hydrophilic/ @Abstractpublished in Advance ACS Abstracts, April 1, 1994. (1)Menger, F. M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1086. (2) b o , A. M.; Murray, M. A.; John, V. T.; Abraham, G. Biocatalysis 1991, 4, 253. Schlatmaun, J.; Aires-Barros, M. R.; Cabral, J. M. S. Biocatalyuis 1991,5, 137. (3) Kishi, K.; Furueaki, S. Kagaku Kogaku Ronbunshu 1991,17,614. (4) Lye, G. J.; Asenjo, J. A,; Pyle, L. Biochem. SOC.Trans. 1991,19, 275. (5) Fadnavis, N. W.; Deshpande, A.; Chauhan, S.; Bhalerao, U. T. J . Chem. SOC.,Chem. Comm. 1990,21,1548. (6) Lieiecki, I.; Pileni, M. P. J. Am. Chem. SOC.1993, 115, 3887. (7) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990,94,1598. (8) Oaeeo-Asare, K.; Arriagada, F. J. Colloids Surf. 1990,50, 921. (9) Candau, F.; Zekhnini, Z.; Durand, J.-P. J. Colloid Interface Sci. 1986,114,398. (10)Menger, F. M.; Tsuno, T.J. Am. Chem. SOC.1990,112,6723. (11) Boyer, B.; Lamaty, G.; Mouanga, J.; Roque, J.-P. New J. Chem. 1992, 16, 381. (12) Biechler, S.S.; Taft,R. W., Jr. J. Am. Chem. SOC.1957,79,4927.
lipophilic balance. Thus, nitrogen analogs of AOT are interesting from both practical and theoretical standpoints. 0 cIH,
0
-~J-CH~--CH-L--~H~,
0 c8H,, - N - L H ~ - - C H L - - C ~ H ~ ,
I
I
R
SO; Na'
'Na%,S
I
I
R
AOT analog-
AOT
Preparation of NP-bis(2-ethylhexy1)fumaramide (2) was achieved in 827% yield by reacting (2-ethylhexy1)amine with fumaroyl chloride (1) in dichloromethane in the presence of triethylamine and 4-(N,N-dimethylamino)pyridine (eq 1). The product, which is insoluble in most organic solvents except OMSO and DMF, was identified from its IRspectrum (3290cm-l for the NH, 1630cm-1 for the carbonyl) and from NMR (vinyl singlet a t 6.84 ppm and a-methylene triplet centered at 3.12 ppm).
Alkylation of the nitrogens by an ethyl or octyl group
was effected by first treating 2 in DMF with NaH while assisting the reaction with ultrasonic irradiation.13J4 Addition of an alkyl bromide at 0 "C gave the expected product (eq 2). The monoalkylation product (indicated by the NH band at 3290 cm-1 and by low-MW parent peaks in the mass spectra) was removed by column chromatography (silica gel, CHCldMeOH). 0
+bT
I
a) Nan. I) R.B., DMP OT. 1))
( 2 )
B
Direct sulfonation of 3 and 4 to the desired AOT analogs, using NaHS03 and Fen/S20s2- redox catalyst,16J6 failed (13)Fones, W. S. J. Org. Chem. 1949,14,1099. (14) Marvel, C. S.; Moyer, W. W., Jr. J. Org. Chem. 1957,22, 1065. (15) Corkill, J. M.; Goodman, J. F.;Walker, T.Trans. Faraday SOC. 1965,61, 589.
0743-7463/94/2410-1OOO$04.50/0 0 1994 American Chemical Society
Letters
Langmuir, Vol. 10, No. 4, 1994 1001
Table 1. Values of cy,for Nitrogen AOT Analogs (8-10) in Chloroform at 26 [l-octanol], M 8 (NH) 9 (N-ethyl) 10 (N-octyl) AOT 0 2.8 5.53 8.2 5.6 (68)b 0.1 19 8.2 (17.5)b 11.2 0.2 5.5 8.3 24.5 a [Surfactant] = 0.1 M. Values measured in heptane. in our hands. We thus resorted to an indirect approach in which thioacetic acid, plus benzoyl peroxide as a radical initiator, was reacted with 2-4 (eq 3).17-19Characterization of the thioacetates depended heavily on lH NMR (e.g., disappearance of the vinyl protons and appearance of an ABX system; see the Experimental Section). For thioacetates 6 and 7 we noted a splitting of the thioacetyl methyl owing to cis/trans isomerization about the "frozen" amide linkages. Thus, the methyl of 6 showed two peaks in a 32/68 ratio, whereas the methyl Of 7 gave a 40/60 ratio.
( 3 )
Oxidation of the thioacetates to sulfonates was carried out via addition of performic acid (HCOOH/30% HzO2) a t 60 "C over 2 h (eq 4).20-23Evidence for the structure of AOT analogs 8-10 includes disappearance of the thioacetyl methyl singlet and deshielding of the succinamide protons in proximity to the sulfur. FAB mass spectrometry on the anions gave the correct molecular weights. Mention should be made of a minor impurity (ca. 5 % ), believed to be the sulfur-free succinamide, that is difficult to remove entirely.
( 4 )
8 9
10
X-E B Ethyl R
-
_n-Octpl
Since the three nitrogen analogs of AOT (8-10) are sparingly soluble (
