© Copyright 2001 American Chemical Society
NOVEMBER 13, 2001 VOLUME 17, NUMBER 23
Letters Poly(aniline boronic acid): A New Precursor to Substituted Poly(aniline)s Eiichi Shoji and Michael S. Freund* Molecular Materials Research Center, Beckman Institute, California Institute of Technology, Mail Code 139-74, Pasadena, California 91125 Received September 11, 2001 A novel strategy for preparing a wide range of substituted poly(aniline)s from a single precursor is described. This approach relies on a boron activation/electrophilic displacement reaction resulting in ipsosubstitution. Specifically, we demonstrate this strategy to prepare a few of the many possible structures that can be generated from poly(aniline boronic acid), including poly(hydroxyaniline) and halogenated poly(aniline)s. The ability to tune the properties of poly(aniline) through the generation of these new structures will be useful in diverse fields where poly(aniline) is used ranging from polymer-based electronics to sensors.
Substituted poly(aniline)s are of great interest for a variety of applications ranging from electronics to sensors.1 As a result, there has been considerable interest in developing new synthetic approaches for their production.2 Typically, the substituted polymer can be generated via oxidative polymerization of the corresponding monomer.3 However, in many cases the desired moiety is either too difficult to oxidize or sensitive to oxidative or acidic conditions. As a result, new synthetic strategies are required. For example, an alternate strategy would be to use a monomer containing a reactive substitutent group to synthesize a precursor polymer that subsequently can (1) For examples, see: (a) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545-2555. (b) Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247-252. (c) Chan, H. S. O.; Ng, S.-C.; Wong, P. M. L.; Neuendorf, A. J.; Young, D. J. Chem. Commun. 1998, 1327-1328. (d) Varela, H.; Torresi, R. M.; Buttry, D. A. J. Electrochem. Soc. 2000, 147, 4217-4223. (e) Shoji, E.; Freund, M. S. J. Am. Chem. Soc. 2001, 123, 3383-3384. (2) For examples, see: (a) Zheng, W. Y.; Levon, K.; Laakso, J.; O ¨ sterholm, J. E. Macromolecules 1994, 27, 7754-7768. (b) McCoy, C. H.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 69336934. (c) Liu, G.; Freund, M. S. Chem. Mater. 1996, 8, 1164-1166. (d) Zhang, X.-X.; Sadighi, J. P.; Mackewitz, T. W.; Buchwald, S. L. J. Am. Chem. Soc. 2000, 122, 7606-7607. (3) For example, see: Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247-252.
be modified to form the desired structure.4 Herein, we report a novel strategy for synthesizing a wide range of substituted poly(aniline)s using poly(aniline boronic acid) (PABA) as the precursor. Boronic acids provide a versatile chemical precursor from which to build a wide range of groups (see Scheme 1).5 For example, aromatic boronic acid groups can be used for a variety of transformations via ipso-hydroxylation,5a ipso-halogenation,5b and ipso-nitration5i under mild conditions. In these particular cases, a boron activation/ (4) Tsuchida, E.; Yamamoto, K.; Shouji, E. Macromolecules 1993, 26, 7389-7390. (5) For examples of possible reactions, see: (a) Simon, J.; Salzbrunn, S.; Prakash, G. K. S.; Petasis, N. A.; Olah, G. A. J. Org. Chem. 2001, 66, 633-634. (b-i) Nesmeyanov, A. N.; Sazonova, W. A.; Drozd, V. N. Chem. Ber. 1960, 93, 2717. (b-ii) Kuivila, H. G.; Hendrickson, A. R. J. Org. Chem. 1952, 74, 5068-5070. (b-iii) Kuivila, H. G.; Williams R. M. J. Org. Chem. 1954, 19, 2679-2682. Thiebes, C.; Prakash, G. K. S.; Petasis, N. A.; Olah, G. A. Synlett 1998, 141-142. (c) Herradura, P. S.; Pendola, K. A.; Guy, R. K. Org. Lett. 2000, 2, 2019-2022. (d) Michaelis, A.; Becker, P. Ber. Dtsch. Chem. Ges. 1882, 15, 180. (e) Savarin, C.; Srogl, J.; Liebeskind, L. S. Org. Lett. 2001, 3, 91-93. (f) Savarin, C.; Srogl, J.; Liebeskind, L. S. Org. Lett. 2000, 2, 3229-3231. (g) Suzuki, A. Pure Appl. Chem. 1991, 63, 419. Badone, D.; Baroni, M.; Cardamone, R.; Ielmini, A.; Guzzi, U. J. Org. Chem. 1997, 62, 7170-7173. (h) Vogels, C. M.; Wellwood, H. L.; Biradha, K.; Zaworotko, M. J.; Westcott, S. A. Can. J. Chem. 1999, 77, 1196-1207. (i) Salzbrunn, S.; Simon, J.; Prakash, G. K. S.; Petasis, N. A.; Olah, G. A. Synlett 2000, 1485-1487.
10.1021/la0114272 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001
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Scheme 1
electrophilic displacement mechanism giving ipso-substitution has been proposed. A recent report also describes the regioselective oxidation of arylboronic acids to phenols and their one-pot conversions to symmetrical diaryl ethers under mild conditions (room temperature).5a Since conventional methods producing diaryl ethers such as Ulman reactions require activated substrates6 and high-temperature conditions (140-160 °C),7 the new route is a key breakthrough in this area. Reaction example g in Scheme 1, widely known as the Suzuki cross-coupling reaction, is an extremely powerful route to produce C-C bonds under mild conditions.5g Scheme 1 illustrates that boronic acid groups are reactive and can be used as a precursor for various transformations with isolated yields typically greater than 90%. Recently, it has been demonstrated that boronic acid substituted poly(aniline) 6 can be electrochemically1e,8 polymerized from 5 in the presence of fluoride.9 The resulting polymer exhibits conductivities and redox behavior similar to those of poly(aniline). In addition, the boronic acid groups remain reactive1e and in turn provide a chemical handle for further transformations. An important example of a substituted poly(aniline) whose structure is complicated by side reactions occurring during oxidative polymerization of its monomer is poly(hydroxyaniline) (see Scheme 2). Several reports exist in Scheme 2
the literature regarding whether a ladder 310 or linear 410a-c,11 polymer is formed; however, evidence has been presented that suggests both forms are possible. By using 6 as the starting material, it is possible to generate 4 exclusively in the presence of peroxide. Parts
Figure 1. Cyclic voltammograms of PABA in 0.5 M HCl (a) before and (b) after exposure to H2O2, (c) after iodination, and (d) after bromination conditions. Scan rate: 100 mV s-1.
a and b of Figure 1 show the redox behavior of 6 before and after the reaction with peroxide, respectively, showing the conversion of 6 to 4. The observed voltammetry is similar to that reported for the polymer produced by the electrochemical oxidative polymerization of 1.7,8 However, the structure of the polymer produced by the electrochemical oxidation of 1 (Scheme 2) is complicated due to the similar oxidative reactivity of both -NH2 and -OH groups as well as the loss of -NH2 to form 2. The formation of 4 from electrochemically generated thin films of PABA 6 is further supported by FT-IR results,12 which show the growth of an intense peak at 1220 cm-1 assigned to the C-O stretching associated with the formation of phenol. A broad peak at approximately 3300 cm-1 indicates O-H stretching with strong intermolecular hydrogen bonding, observed with polyhydroxy compounds.13 According to the ipso-substitution mechanism14 and spectroscopic analysis, our results suggest that (6) Patai, S. The Chemistry of the Hydroxyl Group; Wiley: New York, 1971; pt. 1. (7) (a) Cohen, T.; Wood, J.; Dietz, A. G. Tetrahedron Lett. 1974, 35553558. (b) Cohen, T.; Lewin, A. H. J. Am. Chem. Soc. 1966, 88, 45214522. (8) Nicolas, M.; Fabre, B.; Marchand, G.; Simonet, J. Eur. J. Org. Chem. 2000, 9, 1703-1710. (9) Valeur, B.; Pouget, J.; Bourson, J.; Kaschke, M.; Ernsting, N. P. J. Phys. Chem. 1992, 96, 6545-6549. (10) (a) Kunimura, S.; Ohsaka, T.; Oyama, N. Macromolecules 1988, 21, 894-900. (b) Ohsaka, T.; Kunimura, S.; Oyama, N. Electrochim. Acta 1988, 33, 639-645. (c) Barbero, C.; Silber, J. J.; Sereno, L. J. Electroanal. Chem. 1989, 263, 333-352. (d) Barbero, C.; Silber, J. J.; Sereno, L. J. Electroanal. Chem. 1990, 291, 81-101. (e) Goncalves, D.; Faria, R. C.; Yonashiro, M.; Bulhoes, L. O. S. J. Electroanal. Chem. 2000, 487, 90-99. (11) Zhang, A. Q.; Cui, C. Q.; Chen, Y. Z.; Lee, J. Y. J. Electroanal. Chem. 1994, 373, 115-121. (12) See Supporting Information. (13) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (14) (a) Taylor, R. Electrophilic Aromatic Substitution; Wiley: Chichester, 1990. (b) Perrin, C. L. J. Org. Chem. 1971, 36, 420. (c) Perrin, C. L.; Skinner, G. A. J. Am. Chem. Soc. 1971, 93, 3389.
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a single, linear structure 4 can be generated using boronic acid as the precursor. Another important example of a class of substituted poly(aniline)s which are difficult to synthesize using standard approaches are halogen-substituted poly(aniline)s 8.15,16 For example, during oxidative polymerization conditions, halogen-substituted anilines undergo elimination, resulting in the loss of a significant amount of halogen (4-48% for Cl and Br) in the resulting polymer.17 A range of halogenated poly(aniline)s 8 can be formed simply by exposing 6 to the corresponding molecular halogen in solution (see Scheme 1, reaction b). Parts c and d of Figure 1 show the voltammetry of iodo- and bromosubstituted poly(aniline)s, respectively. The voltammetry exhibited by bromo-substituted poly(aniline) is similar to that reported for the same polymer created electrochemically from 2-bromoaniline.18 High-resolution X-ray photoelectron spectra in the N1s region12 exhibit a peak near 400 eV with the major component at a binding energy of 399.7 eV, which is characteristic of amine -NH- nitrogen. An additional component is observed at 400.9 eV that corresponds to the positively charged N+ nitrogen typically observed for poly(aniline).19 A clear doublet is observed at (15) Dao, L. H.; Leclerc, M.; Guay, J.; Chevalier, J. W. Synth. Met. 1989, 29, E377-E382. (16) Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247-252. (17) Snauwaert, P.; Lazzaroni, R.; Riga, J.; Verbist, J. J. Synth. Met. 1986, 16, 245-255. (18) Prasad, B. M.; Singh, D.; Misra, R. A. J. Polym. Mater. 1996, 13, 305-311.
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71.9 and 70.8 eV,12 that corresponds to Br 3d3/2 and 3d5/2, respectively, and is attributed to the presence of C-Br.20 In summary, we report a new, versatile approach that can be used to synthesize a range of substituted poly(aniline)s. This approach was demonstrated for the synthesis of poly(hydroxyaniline), poly(iodoaniline), and poly(bromoaniline). These reactions represent just a few of the many possible reactions accessible through the boronic acid group present in PABA (see Scheme 1). Further studies of other boronic acid reactions as well as the yields and electronic and chemical properties of the resulting polymers are underway. In addition, uses of these materials for sensor and electrocatalytic applications are currently being investigated. Acknowledgment. This work was supported by the Arnold and Mabel Beckman Foundation. Supporting Information Available: Detailed experimental procedures, spectroscopic data, and supporting experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0114272 (19) (a) Lim, S. L.; Tan, K. L.; Kang, E. T. Langmuir 1998, 14, 53055313. (b) Neoh, K. G.; Young, T. T.; Looi, N. T.; Kang, E. T. Chem. Mater. 1997, 9, 2906-2912. (20) From data observed for poly(4-bromostyrene) in: Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley and Sons: Chichester, 1992; p 274.
