An Organogel Formed by the Addition of Selected

Tulane University, New Orleans, Louisiana 70018. Received June 21, 1999. In Final Form: December 31, 1999. A clear organogel is formed upon the additi...
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An Organogel Formed by the Addition of Selected Dihydroxynaphthalenes to AOT Inverse Micelles Yan Y. Waguespack,† Sukanta Banerjee,‡ Premachandran Ramannair,‡ Glen C. Irvin,‡ Vijay T. John,*,‡ and Gary L. McPherson*,§ Department of Natural Science, University of Maryland Eastern Shore, Princess Anne, Maryland 21853, and Departments of Chemical Engineering and of Chemistry, Tulane University, New Orleans, Louisiana 70018 Received June 21, 1999. In Final Form: December 31, 1999 A clear organogel is formed upon the addition of small quantities of 2,6-dihydroxynaphthalene (2,6DHN) to sodium bis(2-ethylhexyl)sulfosuccinate (AOT) inverse micelles in isooctane. The phenomenon of organogel formation appears to be generally applicable for dihydroxynaphthalenes with hydroxyl groups at opposite ends of the molecule. The gel forms at concentrations of 2,6-DHN as low as 0.002 M and AOT/2,6-DHN molar ratios in the range of 50-70. The gel displays the photoluminescence characteristics of the dopant aromatic species. NMR characterizations of the gel structure reveal significant restrictions on the motion of 2,6-DHN on the NMR time scale. FTIR reveals the presence of hydrogen-bonding interactions between 2,6-DHN and AOT that may play a significant role in gel formation. A microstructure for the gel is proposed wherein 2,6-DHN bridging between micelles leads to rigidification and the formation of a novel partially frozen micellar structure.

Introduction The gelation of organic liquids by adding small quantities of low molecular weight additives is an interesting phenomenon that leads to fundamental questions on the nature of molecular self-assembly and network formation.1 Organogels represent a relatively recent class of compounds where molecular interactions through hydrogen bonds, metal coordination bonds, or dipolar interactions lead to network formation and the compartmentalization of an organic solvent. We describe here, a new organogel that is formed by linking micelles in an organic solvent into a cross-linked network. The surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) has two alkyl tails and a relatively small headgroup (Figure 1). When dissolved in nonpolar solvents, it selfaggregates to inverse micelles that can incorporate small amounts of water to form water-in-oil microemulsions. These systems have been studied extensively for their applications to materials synthesis,2 biocatalysis,3 and extraction processes.4 In earlier research, we have reported a remarkable transformation from inverse micelles to an organogel upon the addition of suitable phenols to AOT dissolved in nonpolar solvents.5,6 Unlike other surfactant or polymer-based gels,7-10 the novelty of these gels is the fact that very small concentrations of low molecular weight species (AOT and phenol) are often sufficient to gelate the whole solvent. For example, it is possible to gelate the whole solvent body with as little as 0.2% (w/v) total solute * To whom correspondence may be addressed. E-mail: vj@ mailhost.tcs.tulane.edu. † University of Maryland Eastern Shore. ‡ Department of Chemical Engineering, Tulane University. § Department of Chemistry, Tulane University. (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (2) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (3) Martinek, K. Biochem. Int. 1985, 24, 871. (4) Rahaman, R. S.; Chee, J. Y.; Cabral, J. M. S.; Hatton, T. A. Biotechnol. Prog. 1988, 4, 218. (5) Xu, X.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J. Phys. Chem. 1993, 97, 11350. (6) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809.

Figure 1. Chemical structure of (a) sodium bis(2-ethylhexyl)sulfosuccinate and (b) 2,6-dihydroxynaphthalene.

(surfactant + phenol) with the proper choice of phenol and the solvent. The phenomenon of gel formation is quite general and occurs in several nonpolar solvents such as isooctane, benzene, and CCl4. Usually, the optimum gelforming ratio of AOT/phenol is unity.6,11 Hydrogen bonding between the phenolic hydroxyl groups and the surfactant sulfosuccinate headgroups plays an important role in gel formation.5 A combination of NMR and FTIR characterizations of these gels suggests through intuitive interpretation that the microstructure consists of strands of stacked, motionally restricted phenol molecules with the surfactant adsorbed externally.6,11 We have also shown that it is possible to form organogels with selected resorcinol derivatives (3,5-dihydroxytoluene and (3,5dihydroxypentyl)benzene).12 Interestingly, these gels are (7) Haering, G.; Luisi, P. L. J. Phys. Chem. 1986, 90, 3500. (8) La Mesa, C.; Coppola, L.; Ranieri, G. A.; Terenzi, M.; Chidichimo, E. Langmuir 1992, 8, 2616. (9) Gruner, S. M. J. Phys. Chem. 1989, 93, 7562. (10) Capitan, D.; Segre, A. L.; Haering, G.; Luisi, P. L. J. Phys. Chem. 1988, 92, 3500. (11) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (12) Tata, M.; Waguespack, Y.; John, V. T.; McPherson, G. L. J. Mol. Liq. 1997, 72, 121.

10.1021/la990797b CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000

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much stronger than simple phenolic gels. A combination of NMR and FTIR spectroscopic evidence suggests that these gels derive their strength from binding of both the carbonyl groups of the surfactant, whereas in the gels made with simple phenols, only one is bound. In an effort to extend our investigations to other dihydroxyaromatics, 2,6-dihydroxynaphthalene (2,6-DHN) was added to solutions of AOT in isooctane. The dihydroxynaphthalene is virtually insoluble in isooctane and is only slightly soluble in the presence of AOT. Nonetheless, as little as 0.002 M 2,6-DHN was found to cause AOT/ isooctane solutions to form gels. These gels form at an AOT/2,6-DHN ratio of roughly 50 to 1, which strongly suggests that the AOT/2,6-DHN gels are fundamentally different from the 1:1 AOT/phenol gels previously studied.4,5,10,11 The AOT/2,6-DHN gels are clear and colorless and exhibit fluorescence. This paper presents a spectroscopic analysis and proposed structure for this new AOT gel system. Materials and Methods All chemicals were analytical grade obtained from Aldrich and were used as received. The surfactant AOT has a variable moisture content when purchased and was therefore dried at 70 °C for at least several hours to bring the water content to a w0 ([H2O]/[AOT]) value of less than 0.4. The clear gel is formed by dissolving calculated amounts of AOT in isooctane, followed by the addition of 2,6-DHN to the solution to maintain the AOT/ 2,6-DHN molar ratio of 50-60. The vial containing the mixture is sonicated at a slightly elevated temperature (30-50 °C) for about 1 h to facilitate dissolution of 2,6-DHN. The solution is then allowed to settle and cool, upon which the system gradually gels. At the lower concentrations of 2,6-DHN, gelation takes several hours, while at the higher concentrations, gelation occurs within 1 h. The gel melting points were initially determined by visual inspection. Typically 10 mL of the gel sample was taken in a small glass vial that was kept tightly closed. The vial was then immersed in a constant-temperature oil bath and allowed to equilibrate for 10-15 min. Following this, the vial was taken out and the contents visually examined for the presence of a liquid phase. If present, the oil bath temperature was recorded as the gel melting point; otherwise, the procedure was repeated at the next temperature. Typically the temperature of the bath was increased in 0.2 °C increments. Each melting point datum reported here is an average of at least two replicates. High-resolution NMR experiments were conducted on a General Electric model GE 500 Omega FT-NMR spectrometer operating at 500.05 MHz for 1H NMR. Data acquisition was continued until 64 scans were accumulated to get a reasonable signal/noise ratio. C6D6 was used to lock the signal. The T1 (longitudinal relaxation time) measurements were carried out by the inversion-recovery technique. The half-life for the recovery process was determined by the so-called “null point” method. FTIR spectra were recorded on an ATI-Mattson Galaxy 6021 FTIR spectrometer. The samples were placed in a CaF2 liquid cell of (Spectra Tech) with a path length of 0.3 mm. Gel samples were melted by warming, loaded into the cell in the liquid state, and then allowed to regelate. Routinely, a resolution of 4 cm-1 was employed. Results for 100 scans were accumulated and averaged. The fluorescence spectra were recorded using a PerkinElmer luminescence spectrophotometer (model LB-50) equipped with a Xe lamp as the excitation source. The emission slit was 2.5 nm in all measurements. Typical scan rates were 200 nm/ min. Viscosity measurements were conducted on a Rheometrics Scientific stress controlled SR-5000 instrument using parallelplate geometry (25 mm circular plates). A solvent trap setup was used to minimize water vapor incursion and solvent evaporation. The measurement involved stress sweep tests from 0.1 to 50 Pa over the shear rate regime of 10-5-103 s-1.

Figure 2. Melting points of AOT/2,6-DHN gels with various concentrations of AOT. The ratio of AOT/2,6-DHN is kept constant at 53.

Results and Discussion Although 2,6-DHN is virtually insoluble in isooctane, the presence of AOT in isooctane facilitates incorporation of 2,6-DHN into the solution, evidently through hydrogenbonding associations between the hydroxyl donor groups of 2,6-DHN and the carbonyl and sulfonate acceptor groups of AOT. This incorporation of 2,6-DHN into the AOT/ isooctane solution has to be accomplished with sonication and mild heating. Even so, only a small amount of the species becomes dissolved, typically with an AOT/2,6-DHN molar ratio greater than 50. However, the interesting observation is that even with such a low concentration of 2,6-DHN, a strong gelation of the system occurs with AOT levels of 0.2 M or higher. Thus, these gels are significantly different from the gels observed in our earlier work with monohydroxy aromatics where a 1:1 AOT/phenol is a requirement for gel formation, and significant deviations from this ratio result in the system remaining a low viscosity liquid.5,6,11,12 Specific characterizations of the gel are reported next. Studies on Gel Formation Melting and Viscosity Data. For the gel formation studies, various concentrations of AOT/isooctane (0.20.6 M) solutions were prepared. 2,6-DHN was then added to these solutions to maintain a AOT/2,6-DHN molar ratio of ∼53. After complete dissolution of 2,6-DHN, the sample was left to equilibrate at ambient temperature. The time for gelation ranged from less than 1 h (when high AOT concentrations were used) to a few hours (with low AOT concentration). Figure 2 illustrates the melting point data, with increasing AOT concentrations indicating gels stable at higher temperatures and requiring lower gelation times (at room temperature). Gels are realized by the compartmentalization of system volume by chains that cross-link or entangle. Gels with higher melting points must therefore have a greater cross-link and chain density. The solution before addition of 2,6-DHN behaves as a Newtonian fluid with a constant viscosity of 5.8 × 10-3 Pa‚s. Upon addition of 2,6-DHN and conversion to the gel, the viscosity increases dramatically to 2.5 × 104 Pa‚s over the low shear rate range 10-5-10-3 s-1. This is an increase

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of about 7 decades over the viscosity of the liquid state. Structural breakdown of the gel begins at an applied stress of 25 Pa and a shear rate of approximately 5 × 10-3 s-1. The gels vibrate upon knocking the sample vial which indicates elastic behavior. A full range of viscoelastic measurements is in progress and will be reported in a subsequent work. Here we focus on the spectroscopic investigations of molecular arrangements in the gel. NMR and FTIR Characterizations. The novelty of the AOT/2,6-DHN gels is the fact that the gels can be formed by the addition of very small quantities (0.002 M) of 2,6-DHN to low viscosity solutions of AOT in isooctane. Because FTIR and NMR are powerful tools to investigate the structure of inverse micelles,13,14 we have used these techniques to investigate the structure of these luminescent gels. Figure 3a illustrates that gelation leads to subtle but observable IR spectral shifts in AOT. Upon gelation, the AOT carbonyl bands indicate a small shift to lower frequencies, a consequence of hydrogen bonding to 2,6DHN. Although the frequency shift is small, it is significant considering the compositions used. In other words, AOT at a concentration of 0.4 M is 53 times in excess of 2,6DHN at a concentration of 0.0075 M; the corresponding ratio of CdO groups to OH groups is also 53/1. Thus, it is not unexpected that the shift observed is small because the vast majority of CdO groups are not perturbed. Gel formation also has a significant effect on the 1H NMR spectra of the system (Figure 3b,c). In general, NMR resonances associated with AOT and the solvent molecules remain well-resolved in the gel phase (Figure 3c), while resonances associated with 2,6-DHN are significantly broadened, indicating restricted motion of 2,6-DHN on the NMR time scale (Figure 3b). In such a situation, the magnetic field sensed by 2,6-DHN is not orientationally averaged, thereby giving rise to the observed spread of the resonance frequencies for each of the NMR-active nuclei in the molecule. The line broadening effect is shown clearly in Figure 3b. Once the gel is heated to a temperature of 40 °C (the visually observed melting point is ∼41 °C), the 2,6-DHN proton resonances appear sharp and indicative of unrestricted motion of 2,6-DHN. In 1H NMR, the value of the longitudinal relaxation time (T1) depends on the mobility of the segment containing the nucleus in question and also the proximity of the neighboring atomic nuclei. Usually T1 increases with increasing mobility. Figure 4 illustrates the T1 values for the various aromatic protons of 2,6-DHN. The data uncertainties are very large here because of the poor signal-to-noise ratios resulting from the very low concentrations of 2,6-DHN (0.0075 M). Higher concentrations of 2,6-DHN are not attainable because of the low solubility of the species in isooctane. Nevertheless, the composite data for protons 1-4 do appear to show an increase in the T1 values around the melting point for the gel (∼40.5 °C). A Hypothesis for Gel Microstructure. On the basis of the compositional data and the FTIR and NMR results, we can put together a tentative picture of the AOT + 2,6DHN gel structure. In dry isooctane, AOT aggregates to form inverse micelles that contain roughly 25-30 AOT molecules.15 Although, the gel forms at relatively high ratios of AOT/2,6-DHN (53), it does not form when the AOT/2,6-DHN ratio is increased to significantly higher values (>100). The observation implies that at least one 2,6-DHN molecule per AOT micelle is necessary for gel (13) Heatley, F. J. Chem. Soc., Faraday Trans. 1 1988, 84, 343-354. (14) Maitra, A. N.; Eicke, H.-F. J. Phys. Chem. 1981, 85, 2687. (15) Manoj, K. M.; Jayakumar, R.; Rakshit, S. K. Langmuir 1996, 12, 4068.

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Figure 3. FTIR and NMR characterizations of the AOT/2,6DHN gel. (a) FTIR of the carbonyl stretch region for (i) 0.4 M AOT in isooctane and (ii) 0.4 M AOT + 0.0075 M 2,6-DHN in isooctane. (b) Resonances arising from the protons of 2,6-DHN in an organogel [0.4 M AOT + 0.0075 M 2,6-DHN in C6D6/ isooctane (2% v/v)]: (i) 20, (ii) 25, (iii) 30, (iv) 33, (v) 36, and (vi) 40 °C. (c) Resonances arising from the protons of the AOT headgroup region in the gel state [0.4 M AOT + 0.0075 M 2,6DHN in C6D6/isooctane (2% v/v)]: (i) 20, (ii) 25, (iii) 30, (iv) 33, (v) 36, and (vi) 40 °C. The molar ratio of AOT/2,6-DHN is 53.

formation. At the gel-forming ratios of AOT/2,6-DHN, it is also unlikely that micelle structure is perturbed by the dihydroxynaphthalene dopant species. It is thus perhaps intuitive that the gel is simply a consequence of 2,6-DHN bridging the micelles and forming a chain network that can compartmentalize the solvent. Figure 5 illustrates the intuitively proposed gel structure. The hydroxyl groups at the 2 and 6 positions form hydrogen bonds with separate micelles. When small amounts of water are added to the system, the gel breaks down and a liquid water-in-oil microemulsion solution is obtained. Qualitatively, we also observe that upon inversion of the sample vial, the gel does not flow but may deform a little under the action of gravity. The gel surface also vibrates upon knocking the sample vial. These observations suggest that the gel is not an entangled chain network, but there are full cross-links between the chains

Addition of Dihydroxynaphthalenes to AOT Micelles

Figure 4. Longitudinal relaxation times of the various protons on 2,6-DHN as a function of temperature. The gel has the composition 0.4 M AOT + 0.0075 M 2,6-DHN in C6D6/isooctane (2% v/v). Proton designations on 2,6-DHN are as shown in Figure 3b.

Figure 5. Proposed microstructures for the AOT/2,6-DHN gels.

at locations where two (or more) 2,6-DHN molecules are attached to a single micelle (Figure 5). The spectroscopic results are valuable in providing an insight into the interactions of 2,6-DHN with AOT at the molecular level and allow us to develop a model for the gel microstructure. In addition, specific experiments on chemical variations of the gel-forming species indicate that the proposed microstructure of Figure 5 may indeed

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be correct. For example, the substitution of hydroquinone for 2,6-DHN does not lead to gel formation, indicating that the bridging species must be able to span the tail regions of two micelles. Small-angle neutron scattering studies of inverse micelles reveal a tail region 0.9 nm thick with intermicellar tail penetration distances of 0.25 nm.16 Thus, the shortest core-to-core distance is 1.3 nm. The oxygen-oxygen end-to-end distance in 2,6-DHN is 1.2 nm, a little smaller than the core-to-core distance but yet comparable. Thus, it is highly likely that hydrogen bonding is primarily between AOT CdO groups (the acceptor) and the 2,6-DHN hydroxyls (the donor). The sulfonate groups, while intrinsically capable of hydrogen bonding with 2,6DHN, may not be involved to the same extent simply because they are located further away from the tail. In addition to the observation that hydroquinone is not a gel-inducing species, screening with various dihydroxynaphthalenes offers strong evidence that gelation occurs only when the two hydroxyl groups lie at opposite ends of the molecules. For dihydroxynaphthalenes available commercially, we have found that low concentrations of 1,2-DHN, 1,3-DHN, 2,3-DHN, and 1,5-DHN (insoluble) do not produce gels with AOT in isooctane, while low concentrations of 2,6-DHN, 2,7-DHN, and 1,6-DHN do produce gels. From the gel formation studies it is evident that there is a percolation threshold for the bridged inverse micellar clusters above which the gel forms. It is intuitively apparent that parameters such as chain and cross-link densities should increase with the increase in AOT and 2,6-DHN concentrations. Thus, the observed rise in the melting point of the gels with an increase in the AOT concentration (Figure 2) is also in intuitive agreement with the network structure proposed. Some order of magnitude calculations might be of interest. At a concentration of 0.4 M, the close-packed volume occupied by AOT micelles would be about 25% of the total volume. The gel strands thus hold up 4 times as much solvent as the strand volume. If one assumes an aggregation number of about 30 and a micellar diameter of 2.5 nm, a concentration of 0.4 M AOT would imply strands with a total length on the order of 1013 m/L of gel. Other Characteristics of the Dihydroxynaphthalene-Based Gels. Figure 6 illustrates results of another doping experiment where the AOT/2,6-DHN gel is doped with a small amount of the monohydroxy aromatic p-cresol (0.038 M) and the system gradually heated to melt the gel. First we note that, although the resonances of 2,6DHN are broadened, the resonances of p-cresol are wellresolved, indicating that p-cresol is not incorporated into the gel structure. As expected, the resonances of 2,6-DHN show gradual sharpening above 38 °C, while the p-cresol resonances stay well-resolved. The fact that the resonances of the hydroxyl protons of the two species remain separated and distinct indicates that these protons do not exhange on the NMR time scale. It is also observed that the peak at about 3.5 ppm due to residual moisture remains distinct from the hydroxyl protons of 2,6-DHN and p-cresol. Again, these protons do not therefore exchange on the NMR time scale. It is not easy to say whether the residual water is just water dissolved in isooctane or minimal water entrapped in the immobilized micelles. Figure 7 illustrates a continuation of this experiment where the AOT/2,6-DHN gels were doped with increasing concentrations of p-cresol. While the resonances of 2,6DHN are broadened in all spectra, the p-cresol resonances (16) Kotlarchyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382.

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Figure 7. NMR spectra of a 0.4 M AOT + 0.0075 M 2,6-DHN gel doped with (i) 0.039, (ii) 0.15, and (iii) 0.38 M p-cresol. Spectra are taken at 22 °C in a C6D6/isooctane solvent (2% C6D6 by volume). Proton designations are as shown in Figure 6. The inset shows the stacked structure obtained at the p-cresol concentration of 0.38 M.6 Figure 6. Temperature effect on the NMR spectra of AOT (0.4 M) + 2,6-DHN (0.0075 M) gels doped with 0.038 M p-cresol. Spectra are taken in a C6D6/isooctane solvent (2% C6D6 by volume). The ratio of AOT/p-cresol is 10.5/1. Proton designations are shown in the inset. The 3.0-4.5 ppm spectral region covers AOT headgroup proton resonances. The temperatures are (i) 22, (ii) 28, (iii) 32, (iv) 38, and 44 °C.

are well-resolved at the lower concentrations of 0.039 and 0.15 M. At these concentrations, p-cresol is therefore not incorporated into the motionally restricted region of the strands that hold the 2,6-DHN/AOT gel structure. At 0.38 M p-cresol, however, the p-cresol resonances broaden. This is not an indication of p-cresol incorporation into the gel strands. Rather, the presence of p-cresol and AOT in a 1:1 ratio indicates the formation of the stacked gel structure we have reported in our earlier work.5,6,11 For reference, the stacked gel structure is shown in the inset of Figure 7. Because 2,6-DHN is a fluorophore, we measured the fluorescence characteristics of the AOT/2,6-DHN gel. The excitation and emission maxima of 300 and 380 nm, respectively, are similar to those found in the liquid state. It should be mentioned that the fluorescence characteristics of the gel furnishes no further information about the gel microstructure. However, from an applied perspective this opens up a number of possible novel applications for the organogels. For example, it might be possible to dope the gel with significant quantities of other luminescent species, which might then allow novel pathways for photoexcited energy transfer from one luminescent center to the other.

Conclusions Thus, the addition of selected dihydroxynaphthalenes in small amounts to fluid AOT inverse micellar solutions serves to “freeze-in” the micellar solution at ambient conditions through the formation of a novel organogel. It is proposed that the gel structure is made up of strands of interconnected inverse micelles of dihydroxynaphthalenes bridging the micelles, and perhaps cross-linking the strands. Hydrogen bonding between the surfactant (AOT) and the dihydroxynaphthalenes is responsible for positioning the dihydroxynaphthalenes as a bridge between micelles. The proposed microstructure of the gel is deduced from three key observations: (1) the low concentrations of selected dihydroxynaphthalenes that indicate minimal perturbations of the AOT micellar structure, (2) the spectroscopic results primarily through 1H NMR that reveal the selective motional rigidity of the dopant dihydroxynaphthalene species, and (3) the chemical experiments that show that hydroxyl groups on opposite ends of the dihydroxynaphthalenes are necessary for the gel to form. Scattering techniques may provide additional knowledge on the microstructure, but the simple observations listed do give us useful information. The gel displays luminescence characteristics of the dihydroxynaphthalenes. Some very interesting applications may be realized as a consequence of these novel “frozen-in” micellar structures. Water-in-oil microemulsions have the remarkable property of restricting particle growth to the nanometer range, and hence these materials have been extensively

Addition of Dihydroxynaphthalenes to AOT Micelles

studied for their applications to nanoparticle synthesis. It therefore becomes possible to synthesize magnetic, semiconductor, or other nanoparticles in water-in-oil microemulsions, dry the system to remove all water and the hydrocarbon solvent (isooctane), and reconstitute the system with isooctane to form an inverse micelle system that contains nanoparticles. These systems can then be converted to the organogel form by the addition of DHN. Thus, it is possible to confer interesting properties (semiconductor luminescence, superparamagnetism, etc.) to these gels by the incorporation of appropriate nanoparticles. Very interesting new classes of organogels have been described in the recent literature,17-19

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and the work described here adds to this class of materials that may have unique functional properties. Acknowledgment. We thank Mr. Ning Sun for assistance with the viscosity measurements. Support from DOD/DARPA through Grant MDA 972-97-1-003 is gratefully acknowledged. LA990797B (17) Terech, P.; Ostuni, E.; Weiss, R. G. J. Phys. Chem. 1996, 100, 3759. (18) Feng, K.-I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17207. (19) Yu, Z.-J.; Neuman, R. D. J. Am. Chem. Soc. 1994, 116, 4075.