Langmuir 1996,11, 4653-4660
4653
Synthesis and Esterolytic Chemistry of Some (Dialky1amino)pyridine-FunctionalizedMicellar Aggregates. Evidence of Catalytic Turnover Santanu Bhattacharya" and Karnam Snehalatha Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India Received April 17, 1995. In Final Form: August 2, 1995@ Four new (dialkylaminolpyridine-functionalizedsurfactants have been synthesized. Micelles were generated either from the surfactant alone in aqueous buffer (pH 8.5 or 9.0) or by comicellization in 1. x 10-3-1 x M aqueous micellar cetyltrimethylammoniumbromide (CTABr)solution at pH 8.5 or 9.0. Such aggregates were used to cleavep-nitrophenyl alkanoates or p-nitrophenyl diphenylphosphate. The nucleophilic reagents and the second-order "catalytic" rate constants toward esterolysis of the substrate p-nitrophenyl octanoate (at 25 "C, pH 9.0) were [cat.] = 1 x M, [CTABrl = 1 x M, and kcat. = 440.13 M-l s-l for lb, [cat.] = 5 x M, [CTABrl = 5 x M, and kcat, = 30.8 M-l s-l for IC, [cat.] =5 x M, [CTABrl = 5 x M, and kcat. = 183.64 M-l s-l for 2a, and [cat.] = 3 x M and kcat. = 54.1M-l s-l for 2b. The catalytic systems,especially lb/CTABrand 2a/CTABr, also conferredsignificantly greater reactivity toward the esters derived from alkanoic acids of moderate chain length (C6-clO) during hydrolytic cleavages relative to their shorter and longer counterparts. Importantly, the catalytic systems comprisingthe coaggregatesof either neutral l b and CTABr ( 1:10)or anionic 2a and CTABr ( 1:10)conformed to the Michaelis-Menten kinetic scheme and demonstrated turnover behavior in the presence of excess substrate.
Introduction 4-(Dialkylamino)pyridine(DAAP, 1)and its derivatives are remarkably powerful acylation catalysts. A wide variety of other reactions, which include hydrolysis, alkylation, phosphorylation, silylation, and polymeri z a t i ~ n , have ~ - ~ also been catalyzed by DAAP. Very high nucleophilicity5 associated with DAAPhas been attributed to such catalytic reactivities. Development of various catalysts with a high level of reactivity combined with elements of selectivity is one of the prime targets of much contemporary research.6 The simultaneous goals of mimicking enzymic reactivities7 and generating reaction specific catalysts8 have thus resulted in the preparation of a large number of nucleophile-functionalizedpolymeric and surfactant systems. About a decade ago, Mathias and co-workers reported water-soluble homopolymers of 4-N,N-(dialkylamino)pyridine consisting of linear 4-(pyrrolidino)pyridineunits
* To whom correspondence should be addressed. Telephone: 9180-309-2664.Fax:91-80-3341683. E-mail:
[email protected]. in. Abstract published in Advance ACS Abstracts, November 1, 1995. (1) (a) Hofle, G.; Steglich, W.; Vorbrungen, H. Angew. Chem., Int. Ed. Engl. 1978,17,569. (b) Scriven, E . F. V. Chem. SOC.Rev. 1983,12, 129. (2)Litvinenko, L. M.; Savelova, V. A,; Solomoichenko, T. N.; Zanslavskii, V. G.; Ved, T. V. Org.React. (Tartu) 1986,22, 162. (3) Cypryk, M.; Rubinsztajn,S.; Chojnowski, J.J.Organomet. Chem. 1989, 377, 197 and references cited therein. (4) (a) Menger, F. M.; Mecann, D. J. J. Org. Chem. 1986,50, 3928. (b)Fendler,J. H. Membrane Mimetic Chemistry;Wiley, New York, 1982. (5) (a) Jencks, W. Catalysis in Chemistry and Enzymology; Mcgraw-Hill: New York, 1969;Chapter 2, particularlypp 105-111. (b) England, W.; Kovacic, P.; Hanrahan, S.; Jones, M. J. Org. Chem. 1980, 45, 2057. (6) (a) Breslow, R. Pure Appl. Chem. 1990,62, 1859. (b) Weisz, P. B. In Biocatalysis and Biomimetics; Burrington, J. D., Clark, D. S., Eds.; ACS Symposium Series 392; American Chemical Society: Washington, DC, 1989; cf. p 6. (c) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1988,27, 89. (7) (a) Menger, F. M. Acc. Chem. Res. 1993,26,206. (b) Reichwein, A. M.;Verboom,W.;Reinhoudt,D. N. Recl. Trav. Chim. Pays-Bas 1994, 113, 343. (8) (a) Ford, W. T. In Polymeric Reagents and Catalysis; American Chemical Society: Washington, DC, 1986. (b) Bunton, C. A,; Savelli, G. Adv. Phys. Org. Chem. 1987,23, 187. @
along the b a ~ k b o n e .More ~ recently, due to their more desirable physical-chemical and dynamic-mechanical properties, polysiloxane-based DAAPs were also prepared.1° Notably, Fife and co-workers were able to synthesize a DAAP-based water-soluble oligomer that exhibited catalytic behavior with substantial substrate preference.'l However, while proving the utility of the polymeric catalysts, many of the investigations involving polymersupported DAAPs shed little light on the specific details of the process by which catalysts work. Thus, although the preparation of most of the polymer-supported DAAPs required elaborate synthetic efforts, many polymeric catalysts have exhibited lower levels of activity than (dimethylaminolpyridine (DMAF? itself. Perhaps these considerations and the desire to make catalytically more effective DAAP systems led to the development of surfactant-bound DAAP systems. Afew years ago, Katritzky and co-workers examined the reactivities of two different types of surfactant analogues of 4-DAAP, 3a,b and 4ad.12 While catalysts 3 were either cationic 3a or zwitterionic 3b and possessed more conventional surfactant skeletons, the catalysts 4 had interesting structural (bolaform) features. These surfactants contained a 4-DAAP moiety a t one end and a suitable polar group located some ten CH2 units away a t the other end. Importantly, addition of CTACl (cetyltrimethylammonium chloride) to micellar cationic 3a and zwitterionic 3b had not much effect on increasing the hydrolysis rate of activated esters. On the other hand, the presence of CTACl nearly doubled the rate with anionic 4b and enhanced the hydrolysis rate by a factor of -28 with neutral 4a.lZa The lack of remarkable rate enhancements with either of the previous sets of DAAP surfactants (3or 4) could be (9) Vaidya, R. A,; Mathias, L. J. J.Am. Chem. SOC.1986,108,5514. (10)(a)Rubinsztajn, S.;Zeldin, M.; Fife, W. K.Macromolecules 1990, 23, 4026. (b)Rubinsztajn, S.; Zeldin, M.; Fife, W. K. Macromolecules
1991,24, 2682. (11) Fife, W. K.; Rubinsztajn,S.; Zeldin, M. J . Am. Chem. SOC.1991, 113, 8535. (12) (a) Katritzky, A. R.; Duell, B. L.;Seiders, R. P.; Durst, H. D. Langmuir 1987,3,976. (b) Katritzky, A. R.; Duell, B. L.; Knier, B. L.; Durst, H. D. Langmuir 1988,4, 192. (c) Katritzky, A. R.; Duell, B. L.; Rasala, D.; Knier, B. L.; Durst, H. D. Langmuir 1988, 4, 1118.
0743-746319512411-4653$09.00/00 1995 American Chemical Society
Bhattacharya and Snehalatha
4654 Langmuir, Vol. 11, No. 12, 1995
Results and Discussion
-1
5,
(a) R (b) R
--
R
CHI
(a) X (b) X (e) Y
CHzCOzH
(d) X
..
----
p0u OH 0SO.H OPOsHg
COzH
due to the following reasons. Surfactant 3 had DAAP units attached to the headgroup region through a rather "rigid" piperazinium group. Presumably, the DAAPs attached through inflexible piperazinium headgroups might tend to stay in a fixed orientation such that the catalytic sites are less accessible to the substrate esters. On the other hand, the surfactant 4 contained D M moieties far separated from the charged ends. Long hydrophobic polymethylene spacers13between the DAAP unit and the polar headgroup in 4 could allow looping of DAAP units into the micellar interior making it again somewhat buried from the reaction site. We thought that bringing DAAP moieties closer to the polar headgroup region through a shorter yet flexible spacer13might lead to better catalytic performance. We also thought that the presence of amide groups as connectors between the hydrophobic chain and the DAAP unit in lb, IC,and 2b might assist in keeping DAAP moieties around the stern layer region because amide groups could participate in hydrogen bonding with the interfacially adhering water molecules. A COO- ion at the headgroup-bearing DAAP unit in 2a should also confine catalytic groups a t the reactive interface by ion pairing when comicellized with cationic surfactants. Herein, we present a detailed description of (a) the synthesis of four new DAAP-attached surfactants and (b) the kinetic characterization of their supramolecular assemblies and summarize relevant catalytic properties toward p-nitrophenyl alkanoates 5 and p-nitrophenyl diphenylphosphate (PNPDPP) 6 in mildly alkaline aqueous media. As is evident from the results obtained in this
-5
-6
study, the present catalytic systems possess superior esterolytic and catalytic activities to any of the existing DAAP-based surfactant systems developed so far. (13)Frechet et al. found that catalytic effectiveness is strongly influenced by the specific microenvironment at the reactive sites in the case of polymer-based systems. Thus, the presence of a short, threecarbon spacer between the polymer chain and the DAAPgroup increased the reactivity of the polymeric catalysts. See, for example, Deratani, A,; Darling, G. D.; Horac, D.; Frechet, J. M. J. Macromolecules 1987, 20,767. (b)Frechet, J. M. J.;Darling, G. D.; Hsuno, S.;Lu, P. Z.; Vivas de Meftahi, M.; Rolls, W. A., Jr. Pure Appl. Chem. 1988,60,353.
Synthesis. Our synthesis of (dialky1amino)pyridinefunctionalized surfactants began with the preparation of the neutral amphiphile lb. First, 4-chloropyridine hydrochloride was converted to 4-(methylamino)pyridineby heating with excess 40% aqueous MeNHzsolution a t 175 "C under pressure. From this, 4-(methylamino)pyridine was generated in pure form by sublimation (90%) and then reacted with methyl acrylate, and the resulting Michael adduct was hydr01yzed.l~This gave 3-[N-methylN-(4-pyridyl)amino]propionicacid which was coupledwith n-tetradecylamine through the acid chloride to give solid lb, 3-[methyl(4-pyridyl)amino]N-tetradecylpropanamide, in 56% yield. The cationic surfactant IC,N,N-dimethylN-octadecyl-N-[ [3-[methyl(4-pyridyl)aminolpropanamidolpropyllammonium bromide, was prepared by the following sequence. First, N,N-dimethyl-N'-acetylpropanediamine ( 8 )was quaternized with n-ClaH3TBr in dry EtOH (66%) followed by deacetylation with aqueous HBr, and then OH- treatment gave the free amino surfactant 9 (82%). This was then coupled with 3-[N-methyl-N-(4-pyridyl)aminolpropionyl chloride (dry CHC13, room temperature) to afford IC(61%). 4-Aminopyridine was acylated with stearic acid using dicyclohexylcarbodiimide,in CHCl3 a t ambient temperature (93%). The N-(4-pyridy1)octadecanamide (7a)thus obtained was reduced with LiAlH4 in THF in a Nz atmosphere under refluxing conditions to afford 4-(octadecy1amino)pyridine(7b)which on reaction with methyl acrylate followed by hydrolysis gave 3-[octadecyl(4-pyridyl)amino]propionicacid (2a). When 4-aminopyridine was subjected to Michael addition with excess methyl acrylate under refluxing conditions, dimethyl 3 3 [N-(4-pyridy1)iminold i p r ~ p i o n a t ewas l ~ produced in 48% yield. This on hydrolysis followed by careful acidification gave the corresponding diacid. This on conversion to the acid chloride and upon treatment with 1equiv of 9 followed by workup and purification afforded 52% 2b. All the final products lb, IC,2a, and 2b and the appropriate intermediates showed expected 'H-NMR (200 MHz) and IR spectra and elemental analysis; cf. Experimental Section. At this stage it might be useful to also describe the specific molecular features of the D M - b a s e d surfactants synthesized herein. l b is a neutral surfactant as it does not have any ionizable residue since the catalytic forms ofthis surfactant bear a deprotonated DAAP moiety. Note that, however, IC itself is cationic since its synthesis involved coupling of 3-[methyl(4-pyridyl)amino]propionic acid with a cationic surfactant, N,N-dimethyl-N-octadecylN-(aminopropyllammonium bromide) 9 through an amide bond formation. On the other hand, 2a is an anionic surfactant under the conditions of our esterolytic studies, pH =- 8.5, where the COOH residue near the DAAP unit ionizes to form carboxylate anion. Therefore, while 2a itself is anionic, the comicellar 2dCTABr (1:lO)is cationic due to the inclusion of a 10-fold molar excess of cationic CTABr in the mixed micellar system (see below). 2b is zwitterionic since it contains both the cationic quaternary NMe2+ and anionic COO- residues under the conditions of the esterolysis reactions. pKaDeterminations. The reactive forms of catalysts lb, IC,2a, and 2b are the free unprotonated forms of the respective (dialky1amino)pyridine moieties. Therefore, the pK, for the conversion of the conjugate acid (dialkylaminolpyridinium to the unprotonated (dialkylaminolpyridine form is an important datum in each case. A pHM rate constant profile for the cleavage of 2.5 x (14) (a)Hierl, M.A.; Gamson, E. P.; Klotz, I. M. J.A m . Chem. SOC. 1979,101, 6020. (b) Delaney, E. J.;Wood, L. E.; Klotz, I. M. J.Am. Chem. SOC.1982,104, 799.
(Dialky1amino)pyridine-FunctionalizedSurfactants
Langmuir, Vol. 11, No. 12, 1995 4655
Scheme '1
R
-
-
2b ~ClaHa,N'Me2(CH2)lNH.
-0.1,
I
I
I
5.5
Br'
6.0 6.5 7.0
7.5
8.0 8.5
9.0 9.5
PH
Figure 1. pH-rate constantprofile for the comicellar cleavages of 2.5 x M 6 (n = 6),by lb (1 x M)/CTABr (1 x M) (A),and 2.5 x M 5 (n = 81, by 2a (5 x M)/CTABr (5 x M) in phosphate buffer (0);log kv (s-l) vs pH. The
1
R
-
discontinuitiesat pH 7.7and 7.55are taken as systemicvalues for the catalyticcomicellar coaggregates. See text for a detailed descriptionofkineticmethods and reaction conditions and Table 1 for pKa values.
".Cl4HzPNH
Lk
Table 1. Kinetic Parameters for Micellar Esterolysis of p-Nitrophenyl Octanoatea
R
MQN(CHZ)INH~-
-
entry cat.
1c
MezN(CH2)lNHAc
8
b -? !!i
n-ClaHl7N*MeZ(CH,)lNHz,
Br-
9
Conditions: (a) n-C17H3&02H, DCC, CHC13, room temperature, 24 h, 93%. (b) LAH-THF, reflux, Nz, 8 h, 80%. (c) Methyl acrylate, reflux, 72 h, 67%. (d)Aqueous base-MeOH, reflux, 0.5 h; H3+0,55%.(e)Methyl acrylate,reflux, 24 h, 48%. (0Hydrolysis in H20, OH-/MeOH, reflux, 0.5 h; H3+0,90%.(g) (i)SOC12,room temperature, 1 h, (ii)9,CHCl3,room temperature 2 h, 52%. (h)Aqueous CH3NH2, EtOH, autoclave, 175 "C, 8 h, 90%. (i) (i) Methyl acrylate, reflux, 16 h, 50%, (ii) Aqueous base-MeOH, reflux, 0.5 h; &+O, 52%. (i) (i) SOC12, room temperature, 1 h, (ii) n-CldH2sNH2, CHC13, Et3N, room temperature 12 h, 56%. (k)(i) SOC12, room temperature, 20 min, (ii)9,CHC13, room temperature, 2h, 61%. (l)AczO,40 h, reflux, 82%. (m) n-ClsHs~Br,EtOH, reflux, 45 h, 66%. (n) Aqueous HBr, reflux, 12 h; OH- treatment, 82%. p-nitrophenyl hexanoate by 1 x M l b in 1 x M CTABr gave a n apparent pK, of 7.70 for l b under comicellar conditions. We also determined pH-rate constant profiles for the ester cleavage reactions induced by comicellar net cationic lc/CTABr (1:l) and 2a/CTABr (1:10)and holomicellar zwitterionic 2 b (3 x M) under the following conditions: 2.5 x M p-nitrophenyl octanoate, in 0.02 M phosphate buffer adjusted to the appropriate pH. The solutions also contained 1.0 vol % of CH3CN to introduce substrates into the micellar reaction medium. Typically, the pseudo-first-order rate constants for p-nitrophenyl ester cleavages at 25 "C were determined spectrophotometrically by following the release of the p-nitrophenoxide ion at 400 nm a t different pH's between 6.0 and 9.5. Plots of log kv vs pH (Figure 1)gave sharp discontinuities at definite pH values, which were taken as systemic pKavalues for the catalysts under micellized reaction conditions. For clarity only pH-rate constant profiles for two surfactant systems were shown in Figure 1. The pK, values of the 4-(dialkylamino)pyridines covalently attached to the surfactants (Table 1) are
lOask_y;".
u;gt;)d
pKae
0.3 9.36[30.4%1 0.73 4.8 0.1 1.0 7.70[95.2%1 41.9 440.13 3 0.5 5.0 7.75[94.7%1 6.2 13.09 4 5 0.5 5.0 7.55[96.6%1 88.7 183.64 6 0.5 5.0 7.80[94.1%1 2.28 4.85 0.5 1.0 9.36[30.4%1 0.8 5.26 7 0.5 0.5 7.75[94.7%1 14.6 30.83 8 0.3 8.10[88.8%1 14.4 54.05 9 a Conditions: 0.02 M pH 9.0 (phosphate)buffer, p = 0.08 (KCl), 25 i: 0.1 "C, [p-nitrophenyl octanoate] = 2.5 x 10-5 M, 1 vol % CH3CN. * Concentration of CTABr at whichkvmanwas determined. See text for discussion of pKa)s. Values in [ I are % ionization at pH 9.0. kcat. = kvman/[catalystl, corrected for 100% ionization of catalytic system. e This value is taken as "ko" in micellar CTABr alone. 1 2
Br'
"-C,.H,,N'Me,(CH,),NH.
103[catl, 1O3[CTAl3r1, M M*
nonee la lb IC 2a 2b la IC 2b
1.0
0.5
5.0
invariably lower than that of DMAF' (la) molecules solubilized in CTABr micelles. The net cationic character of comicellar CTAl3r (1x M)/lb (1x M), CTAI3r (5 x MYlc (5 x MI, and CTABr (5 x MY2a (5 x M) assemblies produced a n electrostatic environment that weakened the acidity of the DAAF' nucleophilic moieties by a t least -1-2 pKa units. Under the same electrostatic environment, however, DMAP (la) solubilized (pKa 9.36) in micellar CTABr altered the pKavalue for l a (9.71,in the aqueous media in the absence of CTABr) only modestly. Thus, the importance of a long apolar alkyl chain is appreciable. Obviously, in these systems the presence of a long hydrocarbon chain favors the uncharged nonprotonated form of DAAP through strong hydrophobic association. Thus, even in the case of net zwitterionic holomicelles of 2b,the pKa value is -1.4 pKa units lower than that of DMAP solubilized in a cationic CTABr micellar solution. Kinetic Studiesunder Pseudo-First-OrderConditions. The ester-cleaving abilities of la-c, 2a, and 2 b were examined from rate constant vs [CTABrl profiles for the cleavage reactions of PNPDPP in micellar CTABr. All reactions were carried out under the following conditions: 0.02 M phosphate buffer, pH 9.0, p = 0.08 (KCl), 25 f0.1 "C, [PNPDPP] = 2.5 x M a t different CTABr
-
4656 Langmuir, Vol. 11, No. 12, 1995
[CTABr] ,10-3M
Figure 2. Pseudo-first-orderrate constants for the cleavage of 2.5 x M 6 (PNPDPP)by 5 x low4M cationic catalyst (IC)as a function of [CTABr]at pH 9.0. See text for reaction conditions and Table 1for Kqmax values. concentrations. The buffer solutions also contained 1.0 vol % CH3CN. Solubilization of l b , IC,and 2 a required stirring in the comicellar (CTABr)buffer solution. Pseudofirst-order rate constants, kv, were determined spectrophotometrically for PNPDPP cleavages at each [CTABrl for each catalyst, by following the release of p-nitrophenoxide ion a t 400 nm. The reproducibility of kq was generally of better than 4% with all the catalytic formulations. The case of catalyst ICis illustrated in detail. The resulting kv/[CTABr] profile appears in Figure 2. The plot of Kv vs [CTABr]gave a sharp maximum a t [CTABrl =5 x M. Consequently, all kinetic experiments using IC were done using [CTABrl = 5 x M, when esterolyses were examined under pseudo-first-order conditions. In the absence of CTABr, aqueous buffered suspensions of ICdid not catalyze the esterolytic reactions presumably because the solution started becoming turbid with time due to lack of adequate optical stability of the resulting aqueous suspensions. Similar studies gave concentrations of CTABr at which kvmaxfor individual reagent systems could be determined. Thus for lb, kqmax was obtained with a [CTABrl = 1 x low3M, and for 2a, kvmaxwas obtained with a [CTABrl = 5 x M (data not shown). 2 b did not require addition of CTABr to produce optically stable micellar suspensions, and thus most of the kinetic studies with 2 b were done under holomicellar conditions. In Table 1, we collect values of kqmaXfor the cleavages of p-nitrophenyl octanoate by each catalyst when the overall ratio ofeach catalyst to CTABr is f z e d (1:lO)from the kinetic profiles and also with the CTABr concentration necessary to obtain kqmaxa t pH 9.0. Entries 2-6 in Table 1 include the kinetic parameters under the conditions where the overall ratio of each catalyst to CTABr is fixed (1:lO) for all five DAAPcatalysts used in the present study. On the other hand, entries 3 , 5 , 7 , 8 , and 9 represent the kinetic data where maximum rate enhancement is achieved. It should be noted, however, that to get optimal kymax values for different catalytic formulations, the individual mixture must have a varying ratio of CTABr to catalyst. We have also included in Table 1the calculated second-order “catalytic” rate constants (kcat, = kvmax/ [catalyst]1. These have been corrected for 100%ionization of the protonated DAAP moieties to free neutral DAAP groups. The last column of Table 1therefore provides a qualitative comparison of the cleavage power of the various DAAP-based micellar reagents especially under pseudofirst-order conditions. With the exception of DMAF’, l a , Table 1reveals reagents lb,c and 2a,b to be reasonable to good catalysts for the cleavage of the test alkanoate
Bhattacharya and Snehalatha substrate p-nitrophenyl octanoate in dilute aqueous micellar CTABr solution a t pH 9.0. When corrected for differing extents of ionization a t pH 9.0, the potencies of various DAAP reagents toward the esterolysis of pnitrophenyl octanoate in dilute aqueous micellar CTABr solution can be compared a t a fned catalysWTABr ratio. It appears that comicellar lb/CTABr is the most potent reagent toward this hydrolytic reaction. The next most potent reagent formulation is derived from coaggregates of 2dCTABr. Interestingly, zwitterionic holomicellar 2 b (entry 9) showed more esterolytic reactivity toward p-nitrophenyl octanoate than that of cationic lc/CTABr coaggregates (entries 4 and 8). The inferior reactivity of comicellar lc/CTABr relative to holomicellar 2 b is surprising, for which we do not offer any explanation. The -92-fold superiority of the catalytic system l b / CTABr (entry 3, Table 1)over DMAP/CTABr (entry 2) is likely a reflection of “tighter”binding and the more apolar “local”environment due to the presence of octadecyl chains in the former. The presence of a long apolar chain in l b also provides tighter binding.14” Similar magnitude rate enhancements have been observed in the cases of few other functional micelles also and could be due to the “concentration’’ of nucleophilic groups and substrate molecules a t the micellar surfaces, a favorable pKa effect, or the introduction of a n “alternative” reaction path involving functional nucleophile^.'^ The appreciable rate enhancement provided by coaggregates lb/CTABr over other catalytic formulations could be due to better accessibility of the DAAP groups to substrates since advantages due to pKa effects are comparable in cationic coaggregates. To bring the present sets of micellar DAAP reagents into proper perspective, we have also compared their kcat. values against p-nitrophenyl hexanoate with that of the DAAP surfactant systems12developed by Katritzky and co-workers. Under similar kinetic conditions against the same substrate, the reagent system lb/CTABr (kcat.= 391.81) is >6.3-fold more potent than the best reagent formulation 4b/CTAC1 described in the previous studies. However, it may not be appropriate to compare the kinetic potencies of different catalytic formulations of DAAP/ CTABr (present study) vs DAAP/CTACl (previous study)12 that contain cosurfactants with different counterions. This is particularly relevant in light of the findings of Romsted and co-workers. Thus, there is a substantial understanding of specific counterion effects on the properties of micellar solutions using surfactants with a monovalent cationic headgroup, i.e., NMe3+, with halide ions.16 Aggregates with charged surfaces bind counterions selectively,16and their solution properties such as aggregate size, shape, the binding of ions and molecules, and their effects on the rates and equilibria of chemical reactions are sensitive to counterion concentration and type. We therefore decided to determine the kcat, values with p-nitrophenyl hexanoate as a substrate with the present DAAP catalytic system l b in a comicellar environment of CTACl and compare it with the one developed by Katritzky and co-workers. Under practically the same kinetic conditions against the same substrate, the reagent system lb/CTACl (kcat,= 344.6) is -5.6-fold more potent than the best reagent formulation 4b/CTAC1 (kcat,= 62) described in the previous studies. The other coaggregate system comprising 2dCTAC1 (kcat,= 123.2) is also -2-fold more potent than 4blCTACl in terms of esterolytic capacities toward p-nitrophenyl hexanoate. ~~
(15)(a)Brown, J.M.; Bunton, C. A.; Diaz, S.;Ihara,Y. J.Org. Chem. 1980,45,4169. (b) Bunton, C. A,; Hong, Y. S.; Romsted, L. S.; Quan, C. J.Am. Chem. Soc. 1981,103, 5785, 5788. (16)See, for example, an excellent review: Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991,24,357.
(Dialky1amino)pyridine-Functionalized Surfactants
Langmuir, Vol. 11, No. 12, 1995 4657
From an examination of the kdk14 ratio of the pseudofirst-order rate constants for hydrolyses of 6 (n = 8) to that of 6 (n = 14) in the presence of comicellar 2dCTABr (l:lO), we find that the esterolyses rate for 5 (n = 8) is -22-fold greater than that for 6 (n = 14). Similarly, the corresponding rate constant for the hydrolyses of 6 (n = 8) by comicellar lb/CTABr (1:lO)is -5-fold greater than the hydrolysis rate of 6 (n = 14)under comparable kinetic conditions. In contrast, under the same comicellar conditions, the DAAP catalyst having the lowest esterolytic activity, i.e., ldCTABr (l:lO), showed only a modestly (-1.6-fold) greater hydrolytic rate for 6 (n = 8) over 6 (n = 14). On the other hand, while for ldCTABr (1:lO)the kdkz ratio is 0.6, the corresponding ratio for lb/CTAEir (1:lO)is -4.6 and that for 2dCTABr is -8.3. Clearly, the 0 conclusions about observed substrate preference are not 0 5 10 15 Alkanoate chain length (n) based on the order of catalytic activity. The esterolytic behavior of several para substituted Figure 3. Variation of catalytic rate constants (kcat,M-' s-l) phenyl esters of n-alkanoic acids with various chain for esterolysis as a function of alkanoatechain length of5 under comicellar conditions at 25 f0.1 "C, in aqueous b;ffer (0.02 M, lengths was examined in differentbinary solvent mixtures HzPOd-/HF'OP). M. - ., DH = 9.0. 0.08 M KC1. 161 = 2.5 x by Jiang and co-workers.18 These studies reveal that the [lal/[CTABrl= 5 x &l x M,' (lb]/[CTABr]= 1 x aggregationand self-coilingof the n-alkanoate chains occur W1 x M, [lcl/[CTABrl = 5 x M/5 x M, under experimental conditions. Earlier, in the 1960s, [2al/[CTABrl = 5 x M/5 x M, [2b] = 3 x M. Menger reported that intermolecular aggregation was the Catalyst l a (O), l b (O), IC(0),2a(A),2b (x). Except 2b, a cause of the rate retardation of the hydrolyses of pholomicellar catalytic system, all other reactions are done in nitrophenyl dodecanoate in aqueous media.lg Subsecomicellar catalytic systems with CTABr. quently, Knowles also suggested that such retardations with increase in the chain length beyond CgH19CO could Influence ofSubstrateChain Length. Hydrophobic also be attributed to the coiling of the long alkyl chains.20 association of a catalyst to a substrate, leading to rate enhancements, is well documented in the 1 i t e r a t ~ r e . I ~ Since the homologues of 6 differ in the chain length only, the selectivity observed presumably originates as a However, a systematic examination of the dependence of consequence of hydrophobic-lipophilic interactions.21 hydrolysis rates as a function of substrate chain length in a comicellar catalytic environment is not available. In We, however, recognize that the criteria of selectivity view ofthis, a series ofp-nitrophenyl alkanoates 6 ranging can be best addressed by utilizing mixed substrates to from the acetate to the palmitate was examined to study demonstrate the preferential esterolysis. However, in the the hydrophobic effects in the hydrolysis catalyzed by the present study this strategy cannot be utilized because comicelles of la, lb, IC, and 2a with CTABr and either of the mixed substrates will liberate the same holomicellar 2b. The reactions were carried out in a p-nitrophenoxide ion and it is impossible to distinguish the substrate from which the p-nitrophenoxide ion is C,., Hz,., COO liberated under the present experimental conditions. Examination of True Catalytic Behavior: TurnMicellar Catalyst Cn.,H2n., COO+ over Experiments. To examine the true catalytic pH = 8 . 5 efficiencies of different DAAP-based systems, kinetic runs were also carried out in the presence of excess substrate. NO2 0 2N Several points emerge from these experiments. Impor-5 , n = 2 - 1 8 tantly, the catalytic effectiveness is still maintained in the hydrolysis of excess substrates. After the completion phosphate buffer a t pH 9.0 using the same substrate and of hydrolysis of the excess of substrate, the activity of the the catalyst concentrations optimized as described in the catalytic formulations remained largely unaltered (see Kinetic Studies Section. Since the variants of substrates below). ESen with the use of a 25- or 50-fold excess of differed only by the number of CH2 groups in the acyl substrate, complete reaction was achieved. chain, these studies offered the opportunity to probe the Either the formation of the acylpyridinium species or preferences of different micellar reagent systems toward its decay should be the rate-determining step in the the esterolysesreactions of the substrates ofvarying chain reaction. The latter has been demonstrated to be ratelength. determining in imidazole-catalyzedz2or DAAP-linked Figure 3 shows a plot of the second-order "catalytic" polymer14acatalyzed hydrolysis of activated esters. If the rate constants (kcat. = kym"x/[catalystl) as a function of the deacylation step is rate-determining, then the acylpyrichain length of the alkanoyl portion of the substrate. dinium intermediate should accumulate in the reaction Remarkably, under the same micellar esterolytic condimedium. According to this scheme, if there is a "burst" tions, cationic lb/CTABr and net cationic 2dCTABr of products then one should obtain biphasic kinetics and blends showed markedly higher esterolytic activities the initial rate should be in excess of the steady state toward 5 (n = 8). In contrast, whilezwitterionicaggregates rate. Thus, the cleavage ofthe ester and the accompanying comprising 2b showed only modest preference for 6 (n = release of p-nitrophenoxide should be linear with time 81, the cationic lc/CTABr blend showed insignificant selectivity toward 5 with any specific n value under comparable kinetic conditions. The control experiments (18)Jiang, X. K; Yong-Zheng, H.; Wei-Qiang, F. J . Am. Chem. SOC. 1984,106,3839. using micellar CTABr-doped DMAP (la)did not exhibit (19)Menger, F.M.;Portnoy, C . E. J.Am. Chem. SOC.1967,89,4698. any significant preference for any specific chain length. (20)Blyth, C.A.;Knowles, J. R. J . Am. Chem. SOC.1971,93,3021. I
@
-
(17)(a) Gitler, C.;Ochoa-Solano, A. J . Am. Chem. SOC.1988,90, 5004. (b) Menger, F.M.;Ladika, M. J.Am. Chem.Soc. 1987,109,3145.
(21)Jiang, X. K.Acc. Chem. Res. 1988,21,3629. (22)Jencks, W. P. Catalysis in Chemistry and Enzymology; McGraw-Hill: New York, 1969;p 67.
4658 Langmuir, Vol. 11, No. 12, 1995
0.00
0
500
1000 Time (sec)
Bhattacharya and Snehalatha
1500
1800
Figure4. Kinetics under excess substrateconditions. Reaction M, [CTABrl = 1 x conditions: 25 f 0.1 “C, [2a] = 2.5 x M, [51 (n.= 8) = 4.5 x M, 0.02 M phosphate buffer, pH = 8.5.
and occur a t a rate controlled by the rate of deacylation or dephosphorylation of the acylated or the phosphorylated pyridinium intermediate, respectively. Consequently the turnover capacities of one catalytic formulation 2dCTABr against 5 ( n= 8 )were examined using the kinetic methods described by Bender and c o - w o r k e r ~ .On ~ ~the ~ basis of this equation, 2 a should be rapidly acylated in the presence of excess of 5, releasing p-nitrophenoxide ions in a stoichiometrically equivalent concentration to the [2al. After that, the liberation of p-nitrophenoxide should be linear with time and related to the ability of the catalyst to regenerate (turnover). A typical run with 2 a against excess of 5, n = 8, is shown in Figure 4. Although 2a/ CTABr presented a “burst” kinetic profile, the true turnover rate cannot be evaluated with precision because the released octanoate ions absorb strongly to the cationic micellar interface and presumably reduce the effectiveness of the optimized surface charge at the catalytic sites of the host. Thus, with the large excess of substrate over catalyst a plateau region is reached toward the end of the esterolysis in the steady state rate region. Fornasier and co-workers have also encountered similar problems with their studies on the metallomicellar catalytic cleavages of p-nitrophenyl p i c ~ l i n a t e . ~ ~ It is indeed interesting to note that comicellar lb/CTABr is the most competent esterolytic reagent among all the DAAP catalysts developed in the present study. When it comes to the turnover experiments, however, the comicellar 2a/CTABr system showed better results. It should also be mentioned that superior esterolytic performance does not ensure efficient turnover behavior. There are examples in the l i t e r a t ~ r eof~highly ~ , ~ ~potent esterolytic agents that do not show turnover behavior. To bring the catalytic systems into the perspectives of putative “p-nitrophenylesterases”,we also examined the kinetic data for the esterolysis in the presence of excess substrate in terms of the Michaelis-Menten kinetic scheme. The properties of many enzymes are adequately explained by Michaelis-Menten kinetics, i.e. ( W =W,, + Z&/V,[S]), where V,, is the maximal rate attained when enzyme sites are saturated with substrate, S, and KM= Michaelis c o n ~ t a n t . ~This ~ ~ predicts -~ that a plot of 1Nversus l/[S] (Lineweaver-Burk plot)23dwill yield a straight line with an intercept of lN,, and a slope of (23)(a) Bender, M. L.; Kezdy, F. J.; Wedler, F. C. J . Chem. Educ. 1967,44,84. (b) Michaelis, L.;Menten, L. M. Biochem. Z . 1913,49, 333. ( c ) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman: New York, 1988;Chapters 8-10, (d) Lineweaver, H.; Burk, D. J . Am. Chem. SOC. 1934,56,658. (24)Fornasier, R.;Scrimin, P.; Tecilla, P.;Tonellato, U. J . A m . Chem. SOC.1989,111, 224. (25)Fomasier, R.;Milani, D.; Scrimin, P.; Tonellato, U.J. Chem. SOC.,Perkin Trans 2 1986,233. Fei, 2. X. Angew. Chem., Int. Ed. Engl. 1994,33, (26)Menger, F. M.; 346.
KMN,,. We have employed the two most potent catalytic formulations, lb/CTABr and 2a/CTABr, for the following experiments. A series ofreactions a t pH 8.5 using a fmed catalyst concentration of l b or 2 a = 2.5 x M, in the presence of a comicellizingsurfactant [CTABr]= 1x M (to ensure micelle formation) was carried out either with varying (excess) concentrations of p-nitrophenyl octanoate, 5 ( n = 8) orp-nitrophenyl diphenylphosphate, 6. Znitial rates were computed by fitting a straight line through data points in the initial linear regime of the reaction profile. The data from all the esterolysis reactions of5 ( n= 8)or 6 were subjected to the above kinetic analysis. The corresponding plots (Lineweaver-Burk plot) were indeed found to be linear (Figure 5). The MichaelisMenten parameters, V,, kcatIKM where kcat. = VmaJ [cat.lbtalwere determined for 2 a (2.5 x M)/CTABr (1 x M) coaggregates against substrates 5 ( n = 8 ) and 6 a t pH 8.5 or 9.0. For 5 ( n = 81, we obtained V,, = 4.45 f 0.08 x M s-l, KM= 1.18 i 0.07 x M, kcat.= 0.018 s-l, and kcat/KM = 1.53 x lo3 M-I s-l. The corresponding values for 6 were 1.21f0.07 x 10+M s-l, 6.7 f 0.5 x M, 4.84 x s-l, and 72.13 M-I s-l, respectively. The Michaelis-Menten parameters for l b ([lbl/[CTABrl, 2.5 x M/1 x M) against 5 ( n = 6) at pH 9.0 are 8.64 f 0.04 x M s-l, 6.41 f 0.05 x M, 0.035 s-l, and 546 M-’ s-l, respectively. At least the kinetic criterion kcaJKM and selectivities observed in these comicellar organic aggregates seem to simulate those of enzyme reactions. It appears that after an initial burst ofproducts, a steady state rate is achieved which is controlled by the rate of release of product molecules from the catalytic surface of the aggregates. We thought that it was worthwhile to compare the ability of 2a/CTABr as a p-nitrophenyl esterasez7with that of chymotrypsin,27awhich has a kcatIKMvalue of 7.6 x lo4 M-’ s-l for 5 ( n = 7). This comparison reveals that chymotrypsin has a ca. 50-fold higher kcat& than that of 2dCTABr. It is striking that the catalytic system 2a/ CTABr shows the highest preference for 5 (n= 81, whereas the natural enzymes, e.g., cholesterol esteraseZlband chymotryp~in,~~“ show the maximum preference for 5 with n = 6 and n = 7, respectively. Natural enzymes are perfected by evolution and, therefore, are kinetically more competent than 2dCTABr. Nevertheless, it is remarkable that even relatively simple mixtures of oppositely charged amphiphiles with few features (as compared to protein) can mimic natural enzymic behavior so closely. Since we have utilized a reactive ester 5 as substrate, the kinetic data from natural enzymes were also taken against the same activated substrate to make the comparison meaningful. In summary, DAAP-based surfactants have proven to be efficient and quite versatile catalysts for the micellar cleavages of both reactive carboxylate esters and phosphotriesters (persistent toxic phosphate simulantzs)substrates. These systems are truly catalytic in character and mimic a t least some aspects of a n enzyme. Consequently, such systems could afford practical control of reaction rates and selectivity. Enantio- and diastereoselective processes can be designed based on such examples, and construction to exploit and demonstrate such elements is underway in related and other aggregate me so phase^.^^ (27)(a) Marshall, T.H.; Akgun, A. J.Biol. Chem. 1971 246,6019. (b) Sutton, L.D.; Quinn, D. M. J . Am. Chem. SOC.1990,$12,8404. (28)(a)Moss, R.A,; Kim, K. Y.; Swarup, S. J.Am. Chem. SOC.1986, 108,788.(b)Moss,R.A,;Zhang, H. M.; Chatterjee, S.;Korgh-Jespersen, K. Tetrahedron Lett. 1993,34,1729.( c )Yang, Y.-C.;Baker, J. A,;Ward, J. R. Chem. Rev. 1992,92,1729.
Langmuir, Vol. 11, No. 12, 1995 4659
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Figure 5. (a) Plot of W vs l/[S]for excess substrate 5 (n = 8) in the presence of 2dCTABr. Reaction conditions: [2al = M, pH = 8.5. (b) Plot of W M, [CTABrl = 1 x 2.5 x vs l/[Sl for excess substrate 6 in the presence of 2dCTABr. Reaction conditions: [2a] = 2.5 x lov6M, [CTABr] = 1 x M, pH = 9.0. (c) Plot of W vs l/[Sl for excess substrate 5 (n = 6) in the presence of lb/CTABr. Reaction conditions: [lbl = 2.5 x M, [CTABrl = 1 x M, pH = 9.0.
General Methods. Melting points were taken on a melting point apparatus using open capillary tubes and were uncorrected. 1H-NMR spectra were obtained on either Bruker 200 (200 MHz) or Jeol(90 MHz) instruments. Chemical shifts ( 6 ) are reported in ppm downfield from the internal standard. IR spectra were recorded on a Perkin Elmer Model 781 spectrometer. UV-visible spectra were recorded on a Shimadzu Model 2100 UV-visible recording spectrophotometer. Microanalyses were performed on a Carlo Erba elemental analyzer Model 1106. All the reagents and solvents were the highest grade available commercially and were purified, dried, or freshly distilled as required, according to literature procedures. Steam distilled water was used for kinetic studies, and pH measurements were made with Systronics Digital pH-meter 335. Kinetic Measurements. Reaction mixtures were generated in a 1cm quartz cuvette. The cuvette was filled with 2.97 mL of aqueous buffer (0.02 M phosphate, pH 9.0, 0.08 M KC1) containing a known concentration of the catalytic system. The micellar solution was equilibrated for 10 min in a thermostated cell compartment (TCC-60)(25 =k 0.2 "C) of a Shimadzu Model UV 2100 spectrophotometer. An appropriate aliquot ca. 10 pL of a stock solution of 5 or 6 in CH3CN was added by a Hamilton syringe. The reaction was initiated by quick but careful stirring, and the absorbance at 400 nm was recorded as a function of time. The esterolysis followed pseudo-first-order kinetics and the rate constants were obtained by nonlinear fit of the equation (A, Ao)/(A, -At) = ektwhereA, andAt are the absorbances at infinite time and time t , respectively. Michaelis-Menten treatment of kinetic data was done using ENZFITTER program, R. J. Leatherbarrow, Elsevier, Amsterdam, 1987. Duplicate runs generally showed a measurement discrepancy of less than &4%. Synthesis. 3-[N-Methyl-N-(4-pyridyl)aminolpropionic acid and 3,3'-[N-(4-pyridyl)imino]dipropionicacid were prepared by literature p r 0 ~ e d u r e s . l ~ ~ N-(4-Pyridyl)octadecanamide(7a). To a solution of 4-aminopyridine (1.0 g, 0.011 mol) in anhydrous CHC13 were added stearic acid (3.32 g, 0.012 mol) and DCC (2.40 g, 0.012 mol) at 0 "C, and the mixture was stirred a t ambient temperature for 24 h. The reaction mixture was filtered, the filtrate was evaporated, and the residue was chromatographed on silica gel with 1%MeOH in CHC13. The fractions corresponding to Rf0.7 were collected. Solvent evaporation afforded a white solid, 7a (3.54 g, 93%): mp 79 "C; lH-NMR (CDC13)0.90 (t, 3H), 1.20 (br m, 26H), 1.70 (br t, 4H), 2.40 (t,2H), 7.3 (br s, lH), 7.5 (apparent d, 2H), 8.50 (apparent d, 2H); IR 1683 ~ m - l . l * ~ 1-[(4-Pyridyl)amino]octadecane(7b). A solution ofN-(4pyridyl) octadecanamide 7a (3.54g, 9.8 mmol) in anhydrous THF was added to a suspension of LiAlH4 (0.73 g, 19.2 mmol) in anhydrous THF, at 0 "C. The mixture was refluxed under Nz for 8 h. Excess LiAlH4 was decomposed carefully by addition of EtOAc. The resulting mixture was extracted with CHC13, which upon solvent removal and chromatographicpurificationover silica gel using 5% MeOH in CHC13 gave compound 7b (2.85 g, 80%): mp 95 "C; lH-NMR (CDC13)0.9 (br t, 3H), 1.22-1.7 (br m, 32H), 3.10 (q,2H),4.10 (brm, lH), 6.40 (apparentd, 2H), 8.20 (apparent d, 2H); IR 3300 cm-l (br peak).14b 3-[Octadecyl(4-pyridyl)aminolpropionicAcid (2a). ~ ( O C tadecylamino)pyridine, 7b, (0.49 g, 1.42 mmol) was dissolved in methyl acrylate (7 mL), and the mixture was refluxed for 72 h. At the end of this period excess unreacted methyl acrylate was evaporated to yield a crude product. This was chromatographed on silica gel with 1%MeOH in CHC13 to give methyl 3-[octadecyl(4-pyridy1)aminolpropionate (0.41 g, 67%): mp 59-60 "C; 'HNMR (CDCls)0.9 (t, 3H), 1.22-1.97 (br m 1- t, 35H), 2.6 (t, 2H), 3.30 (t, 2H), 3.70 (m, 5H), 6.50 (apparent d, 2H), 8.20 (apparent d, 2H);IR 1743 ~ m - ~ . ~ ~ ~ Methyl 3-[Octadecyl(4-pyridyl)aminolpropionate (0.1 g, 2.39 mmol) was dissolved in 1:l aqueous MeOH and refluxed with 2 equiv of NaOH for 0.5 h. This produced a clear, light yellow solution, which was neutralized with concentrated HC1, concentrated in vacuo, and extracted with CHC13. The solvent from (29) (a) Bhattacharya, S. Proc. Ind. Acad. Sci. (Chem. Sci.) 1994, 106,1253. (b)Bhattacharya, S.;De,S. J . Chem. Soc., Chem. Commun. 1996,651. ( c ) Ragunathan, K.;Bhattacharya, S. Chem. Phys. Lipids
1996,77, 13.
4660 Langmuir, Vol. 11, No. 12, 1995 the organic layer was evaporated to give a white solid, 2a,which was recrystallized several times from EtOAdMeOH (5/1)(0.053 g, 54.6%): mp 98 "C; 1H-NMR (CDC13) 0.90 (br t, 3H), 1.10 (br m 38H), 2.70 (t, 2H), 3.40 (t, 2H), 3.9 (t, 2H), 7.0 (d, 2H), 8.1 (d, 2H); IR 1705 ~ m - ~ . ~ ~ ~ NJV-Dimethyl-A"-acetylpropanediamine (8). To ice cold N,Ndimethyl-l,3-propanediamine (8.0 g, 0.078 mol) was added dropwise acetic anhydride (15 mL, 0.16 mol). After the addition was complete, the mixture was refluxed to 100 "C for 40 h. Then, the reaction mixture was dissolved in water and allowed to boil to decompose excess acetic anhydride. The resulting solution was cooled and neutralized with aqueous NaHC03. The aqueous solution was concentrated and extracted with CHC13. The solvent from the CHC13extract was evaporated to give a brownish liquid, 8 (9.17 g, 81.6%): 1H-NMR(CDC13)1.30 (m, 2H), 1.9 (s,3H), 2.0 (m, BH), 2.90 (m, 2H), 7.40 (br s, 1H); IR 1640 c ~ - ' . ~ O NJV-Dimethyl-N-octadecyl-N-(aminopropy1)ammoniu m Bromide (9).W-acetyl-Nfl-dimethylpropanediamine(8) (4.2 g, 0.029 mol) was dissolved in 30 mL of anhydrous EtOH. n-Octadecyl bromide (12 g, 0.035 mol) was added to it, and the mixture was refluxed for 45 h. The solvent was removed, and the reaction mixture was purified by column chromatography on silica gel with 4% MeOH in CHC13 to obtain N,N-dimethyl&octadecyl-N-((acety1amino)propyl)ammonium bromide (9.22 g, 66%): mp 90 "C; 'H-NMR (CDC13)0.82 (t, 3H), 1.20 (br s, 32H), 2.0 (s, 3H), 3.1 (br m, 12H), 8.0 (br s, 1H); IR 1640 cm-l. To N~-dimethyl-N-octadecyl-N-((acetylamino)propylammonium bromide (10.2 g, 0.02 mol) was added 2 N HBr (30 ml), and the mixture was refluxed for 12 h. The reaction mixture was kept under vacuum to remove HBr and HzO. This produced a white solid which was dissolved in CHC13 and treated with aqueous Na2C03 solution until basic and extracted with excess CHC13, washed with water, and dried over anhydrous Na~S04. The solvent was evaporated to produce a highly hygroscopic solid, 9 (7.71g, 82%): mp 52 "C (softens) 110 "C (clear melt); 'H-NMR (CDC13)0.90 (t, 3H), 1.2 (br m, 35H), 2.9 (br t, 2H), 3.30 (br m, 10H); IR 3300 c ~ - ' . ~ O
Bhattacharya and Snehalatha (softening), 189 "C (clear melt); 'H-NMR (CD30D) 0.90 (t, 3H), 1.20 (br m, 36H), 1.70 (br s, 3H), 2.50 (t, 3H), 2.2 (br s, 2H), 3.0 (m, 15H), 4.0 (m, 6H), 6.70 (br s, 3H), 8.0 (d, 2H); IR 1635 cm-l; C, H, N analysis, calcd. for C32H63N40ClzBr.3HzO(C = 52.38%, H = 9.62%, N = 7.63%), found (C = 52.54%, H = 9.19%, N = 7.59%). The material was found to be very hygroscopic, and it crystallized as trihydrate (3Hz0)despite drying under vacuum. Repeated elemental analyses confirmed it.
S-[Methyl(4-pyridyl)aminol-N-tetradecylpropanamide (lb). To a solution of 3-[methyl(4-pyridyl)aminolpropionoyl chloride (0.10 g, 0.50 mmo1)inanhydrous CHC13was slowly added a solution ofn-tetradecylamine (0.106 g, 0.50 mmol) in anhydrous CHC13 and triethylamine (0.28 g, 2.75 mmol) at 0 "C. After the addition was complete, the reaction mixture was allowed to come to room temperature and stirred at ambient temperature for 12 h. The reaction mixture was washed three times with water, and the solvent was evaporated to get a crude solid, which was purified by column chromatography on silica gel with 15%MeOH in CHC13. Recrystallization from EtOAdMeOH (3/1) gave compound l b (0.12g, 56%): mp 62-64 "C; 'H-NMR (CDC13)0.90 (t, 3H), 1.30 (br m, 22H), 2.45 (t, 2H), 3.0 (s t, 5H) 3.70 (t, 2H), 6.50 (apparent d, 2H), 7.70 (apparent d, 2H); IR 1640 cm-l; C, H, N analysis, calcd. for C23H41N30-1.75HzO (C = 67.86%, H = 11.02%, N = 10.32%), found (C = 68.09%, H = 10.98%, N = 9.85%).
+
NJV-Dimethyl-N-octadecyl-N-[[[((hydro~c~bonyl)ethyl)(4-pyridyl)aminolpropanamidolpropyllam1noniumBromide (2b). To solid 3,3'-[N-(4-pyridyl)imino[dipropionic acid (0.30 g, 1.26 mmol) was added an excess of SOClz (1.2 g, 10.08 mmol), and the mixture was stirred at room temperature for lh. After this the solid was dissolved to form a clear yellow solution. Excess SOClz was removed by evaporation. Anhydrous ether was added to it, and the residual SOClz washings were removed with a syringe. The solid was allowed to settle down, and the supernatant solution was decanted. The amino surfactant 9 (0.56 g, 1.26 mmol) was dissolved in anhydrous CHC13 and was added to the above acid chloride solution in CHC13at 0 "C. The mixture NJV-Dimethyl-N-octadecyl-N-[[3-[methyl(4-pyridyl)ami- was allowed to stir at room temperature for 2 h. The solvent was no]propanamido]propyl]ammonium Bromide (IC). To evaporated under vacuum to get a white solid, which was 3-[methyl(4-pyridyl)amino]propionic acid (0.083 g, 0.46 mmol) recrystallized several times EtOAdMeOH (2/1),(0.478 g, 52%): was added excess SOClz (0.22 g, 1.84 mmol) in a flask equipped mp 120 "C (softening), 182 "C (clear melt); lH-NMR (DzO)0.9 (t, with a septum, and the mixture was stirred a t room temperature 3H), 1.2 (s, 30H), 1.9-2.1 (br m, 5H), 2.5 (t, 3H), 3.0 (s, 6H), 3.2 for 20 min. To this was added 10 mL of anhydrous ether, and (br m, 4H), 6.90 (apparent d, 2H), 8.0 (apparent d, 2H); IR 1707 the resulting mixture was stirred for another 5 min. A white and 1641 cm-l; C, H, N analysis, calcd. for C34Hs5N403Clzsolid precipitated out of the reaction mixture. The solid was Br.1.5Hz0 (C = 54.04%, H = 9.07%, N = 7.41%), found (C = allowed to settle down, and the ether was removed by a 53.90%, H = 9.49%, N = 7.33%). The compound was found to hypodermic syringe. The solid was washed twice with ether, be hygroscopic, and it crystallized as hydrate (1.5HzO)despite and dry CHC13 was added to it. The amine surfactant 9 (0.20 prolonged drying under vacuum. Repeated elemental analyses g, 0.46 mol) was dissolved in CHC13, and this solution was added confirmed it. to the ice cold acid chloride in a dropwise manner. After the addition, the contents of the flask were cooled and the mixture Acknowledgment. Financial support for this research was allowed to stir at ambient temperature for 1 h. Upon was available from the Grants-in-Aid Scheme of DRDO, evaporation of solvent a white solid was obtained and was recrystallized from EtOAdMeOH (211)(0.20 g, 61%): mp 142 "C Government of India. K.S. thanks University Grants Commission for a senior research fellowship. (30)Moss, R. A,; Lukas, T. J.;Nahas, R. C. Tetrahedron Lett. 1977, LA950303K 3851.
