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Weakly coordinating bulky anions designed by efficient use of polyfluoro-substitution
Journal of Fluorine Chemistry 105 (2000) 201±203
Weakly coordinating bulky anions designed by ef®cient use of poly¯uoro-substitution Hiroshi Kobayashi Institute of Advanced Material Study, Kyushu University Kasuga, Fukuoka 816-8580, Japan
Abstract The molecular design and applications of weakly coordinating tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) and tetrakis(penta¯uorophenyl)borate (FTPB) are reviewed. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Weakly coordinating bulky anions; Tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate; Tetrakis(penta¯uorophenyl)borate; Phase-transfer catalysis
Tetraphenylborate anions with many ¯uoro- and tri¯uoromethyl substituents on the phenyl parts have attracted wide-ranging interests at present as bulky and weakly coordinating anions (Fig. 1) [1±3]. The compounds of this group were designed originally in the mid-1970s so as to be useful as a phase-transfer catalyst for electrophilic and acidmediated reactions, and since then their chemistry have been taken over in our laboratory as a successful prototype, where many features characteristic of the poly¯uoro-substitution are multiply combined in the anionic species [4]. The studies of phase-transfer catalysis (PTC) had already attained to an established status at that time, however, most of the successful applications were con®ned within those to nucleophilic and base-mediated reactions by use of cationic catalysts.1 Under such circumstances we had an idea of extending the application of this technique to electrophilic and acid-mediated reactions, which was combined with a part of my formal task in the institute to develop new organic catalyses. Catalysts useful to the above-mentioned purpose had to satisfy the following requisites: (1) negatively charged permanent or stable ionic species; (2) three-dimensionally bulky molecular structure; (3) no or negligible nucleophilicity to cationic species; (4) high lipophilicity of its salts, irrespective of the kind of counter ion; and (5) chemical stability under acid and oxidative conditions. According to these requisites, our attention was focused to tetraphenylborate (TPB) anion as a prospective compound by simple analogy of the structural features of useful cationic counterpart, quaternary ammonium ion. TPB was already very
popular in the analytical use, and was known to be fatally labile against the above requisites, decomposing rapidly in the presence of acids or oxidants, even under aeration.2 To improve the drawbacks of TPB, we immediately conceived the use of ¯uorine-substitution. The peculiarities due to poly¯uoro-substitution impacted on me, once dated back to early 1960s, when 1,1,1,5,5,5-hexa¯uoro- and 1,1,1tri¯uoropentan-2,4-dionato-metal chelate complexes were veri®ed separable by gas-chromatography according to the kind of central metal ion [5]. Metal chelate complexes were thitherto regarded to be far from volatile matter and it sounded earthshaking for me that the poly¯uoro-substitution afforded them to dissolve in such an inert solvent as tetrachlorocarbon. Since then we had been dealing with the search for the peculiarities induced by poly¯uoro-substitution and some synthetic applications obtaining 1,2-bifunctional poly¯uorobenzene derivatives of analytical use. During such works, our group gained knowledge and experience on the chemistry of organo¯uorine compounds. Meanwhile some partly ¯uorinated tetraphenylborate ions appeared in literatures, and indicated a tendency that the introduction of ¯uoro- or tri¯uoromethyl substituents onto the phenyl groups actually improved the stability against acid and increased hydrophobicity of their salts [6±8]. Use of the ¯uorine-substitution for improving TPB seemed effective in one way to suppress its lability [9± 10],3 and in another way to prepare the required organo2
[4].
The lability of TPB was reported in some papers listed in Ref. [3] of
3
1
Some of the then current books concerning phase-transfer catalysis are listed in Ref. [1] of [4].
Effects induced by polyfluoro-substitution and some of their applications were reviewed previously. Some physical data were recently revised in [10].
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 2 7 4 - 8
202
H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203
Fig. 1. The structures of fluorinated tetraphenylborates, TFPB and FTPB.
magnesium intermediates on the synthetic sequence. Thus, the ®rst target was tetrakis(penta¯uorophenyl)borate (FTPB) ion, whose synthesis had been then reported, but few necessary physical properties were described [11]. In spite of the repeated attacks with various contrivances, the schemes going through a monohydro derivative from hexa¯uorobenzene failed in obtaining the target product, which was, however, found most recently to be successfully isolated in a stable form by an improved scheme starting with bromopenta¯uorobenzene [12]. Parallel to the unsettled works for FTPB, we turned our view to the use of tri¯uoromethyl groups for the improvement of TPB, especially in respect to durability against protic acids and oxidants and solubility in hydrophobic organic solvents. The conceivable phenyl moieties containing more than one tri¯uoromethyl group were so narrowed, that no choice except 3,5-bis(tri¯uoromethyl)phenyl one [13].4 The corresponding xylene derivative which we started with, was provided to us by the Daikin Industries, Osaka, who produced the compound as an intermediate of some agrochemicals. We were greatly indebted for their favor, which afforded us a successful synthetic scheme leading to tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) ion (Fig. 2).
The synthetic sequence and spectroscopic data of TFPB supported consistently the assigned structure [14],5 which was independently con®rmed by the single-crystal X-ray analysis of lithium TFPB tetrahydrate [15]. Synthetic scheme was further improved for a method affording a higher yield with easier processing, which was successfully applied to the syntheses of some higher homologues of tetraarylborate ions substituted with many tri¯uoromethyl groups [16]. Both ¯uoro- and tri¯uoromethyl-substitution afforded drastic improvements to TPB in the ef®cacy as a phasetransfer catalyst, though in somewhat different manner. The former favored to the suppression of lability under acid and oxidative conditions, while the latter to the increased solubility in hydrophobic organic solvents [12]. In the course of synthesis of TFPB, even before obtaining a de®ned specimen, we were impatient to examine its catalytic effect by a simple test of diazo-coupling of Nethylcarbazole with 4-nitrobenzene-diazonium tetra¯uoroborate in a dichloromethane±water two-phase system after the preceding examples.6 Encouraged by the positive results of the preliminary test, diazo-coupling reactions of arenediazonium ions with various diazophile components were investigated in two-phase systems in the presence of a catalytic amount of TFPB [17,18]. Kinetic investigations proved that TFPB catalyst turned over, as being expected, to promote the reaction under PTC conditions. The diazonium ions were dehydrated in the hydrophobic organic phase, to become more reactive than in the aqueous phase and meanwhile, retained free from the attack by various nucleophilic agents, so that complex side reactions were suppressed, affording a satisfactory yield of coupling products [19,20]. Sodium TFPB placed in a dichloromethane±aqueous sulfuric acid two-phase system, could incorporate oxonium ion into the hydrophobic organic phase quantitatively by exchanging sodium ion. The oxonium ion was regarded to stay in the form of ion-pair with TFPB. The analytical concentration of oxonium ion in the organic phase, there-
Fig. 2. Synthetic scheme of TFPB.
4 The disposition of two trifluoromethyl groups at 3- and 5-positions of the phenyl part had been known as one of those which induced the strongest electron-withdrawing effect, when we made our choice without noticing the important report concerned.
5 Dojin Chemical Laboratory, Kumamoto, offers reagent-grade TFPB with the trade name of `TFPB/The Kobayashi's reagent'. 6 Reports on diazo-coupling reactions in two-phase systems at that time are listed in Ref. [2] of [4].
H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203
fore, was de®ned by that of counter anion, TFPB. Its acidity changed over a wide range depending upon the aqueous acid concentration, conceivably due to the varied degree of hydration of oxonium. The TFPB-catalyzed two-phase system could be regarded as a quite unique one affording oxonium ions of high acidity even at a lower concentration [4]. The oxonium of high acidity in the organic phase served therein to generate electrophiles of higher order, E, by oxonium-catalyzed dehydration from the corresponding precursors, EOH, according to a general scheme as follows: EOH H3 O TFPBÿ ! E 2H2 O where TFPB carried oxonium ions catalytically to the reaction sites, to promote the overall electrophilic reactions of E or its equivalent in the two-phase systems. Such a scheme was exemplified by Friedel±Crafts alkylations [21], diazotization and nitrosation [22,23]. In the structures of TFPB and FTPB ions, the negative charge localized at the central boron is sterically shielded as well as stabilized by strong electron-withdrawing effect due to the shell of ¯uorine atoms arrayed on the surface of molecule. Therefore, the anions would interact with cationic species solely in an electrostatic manner at a longer distance off, no more in the covalent or the coordinate one. The cationic species in the hydrophobic solution phase, on the other hand, are retained under inert environments with least in¯uence by the counter ions and the solvent molecules. It can display therein its own inherent power in a case as it is, while in another case after converted to a cationic species of higher order [22,23]. Poly¯uoro-substitution successfully brought forth the weakly coordinating bulky anions such as TFPB and FTPB, which are prospective to develop novel ®elds of solution chemistry of cationic species. Extensive reports7 on their application as the effective counter ions in homogeneous
7 The applications reported independently from the groups other than ours are listed in Ref. [7] of [12].
203
olefin polymerization catalysts seem indicative of such prospects in near future.8 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M. Bochmann, Angew. Chem. Int. Ed. Engl. 31 (1992) 1181. K. Seppelt, Angew. Chem. Int. Ed. Engl. 32 (1993) 1025. S.H. Strauss, Chem. Rev. 93 (1993) 927. J. Ichikawa, H. Kobayashi, T. Sonoda, Rep. Inst. Adv. Mater. Study Kyushu Univ. 2 (1988) 189. R.W. Moshier, R.E. Sievers, Gas Chromatography of Metal Chelates, Pergamon Press, 1964 and references therein. C.E. Moore, F.P. Cassaretto, H. Posvic, J.J. McLafferty, Anal. Chim. Acta 35 (1966) 1. C.E. Meisters, C.E. Moore, F.P. Cassaretto, Anal. Chim. Acta 44 (1969) 287. J.T. Vandeberg, C.E. Moore, F.P. Cassaretto, H. Posvic, Anal. Chim. Acta 44 (1969) 175. H. Kobayashi, J. Synth. Org. Chem. Jpn. 45 (1987) 561. H. Kobayashi, Rep. Inst. Adv. Mater. Study, Kyushu Univ. 9 (1995) 1. A.G. Massey, A.J. Park, J. Organometal. Chem. 2 (1964) 245. K. Isshiki, H. Kobayashi, T. Sonoda, Rep. Inst. Adv. Mater. Study, Kyushu Univ. 9 (1995) 47. P.G. Gassman, A.F. Fentiman Jr., J. Am. Chem. Soc. 92 (1970) 2549. H. Nishida, N. Takada, M. Yoshimura, T. Sonoda, H. Kobayashi, Bull. Chem. Soc. Jpn. 57 (1984) 2600. J.H. Golden, P.F. Mutolo, E.B. Lobkovsky, F.J. DiSalvo, Inorg. Chem. 33 (1994) 5374. K. Fujiki, M. Kashiwagi, H. Miyamoto, A. Sonoda, J. Ichikawa, H. Kobayashi, T. Sonoda, J. Fluorine Chem. 57 (1992) 307. H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. (1981) 579 H. Iwamoto, M. Yoshimura, T. Sonoda, H. Kobayashi, Bull. Chem. Soc. Jpn. 56 (1983) 796. H. Iwamoto, H. Kobayashi, P. Murer, T. Sonoda, H. Zollinger, Bull. Chem. Soc. Jpn. 66 (1993) 2590. cf.H. Gilman, J.B. Honeycutt, J. Org. Chem. 22 (1957) 563. H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. (1982) 1185. H. Iwamoto, T. Sonoda, H. Kobayashi, Tetrahedron Lett. 24 (1983) 4703. H. Iwamoto, T. Sonoda, H. Kobayashi, J. Fluorine Chem. 24 (1984) 535.
8 Some of the reports on polymerization catalysts are listed in Ref. [5] of [12].
Weakly coordinating bulky anions designed by ef®cient use of poly¯uoro-substitution Hiroshi Kobayashi Institute of Advanced Material Study, Kyushu University Kasuga, Fukuoka 816-8580, Japan
Abstract The molecular design and applications of weakly coordinating tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) and tetrakis(penta¯uorophenyl)borate (FTPB) are reviewed. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Weakly coordinating bulky anions; Tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate; Tetrakis(penta¯uorophenyl)borate; Phase-transfer catalysis
Tetraphenylborate anions with many ¯uoro- and tri¯uoromethyl substituents on the phenyl parts have attracted wide-ranging interests at present as bulky and weakly coordinating anions (Fig. 1) [1±3]. The compounds of this group were designed originally in the mid-1970s so as to be useful as a phase-transfer catalyst for electrophilic and acidmediated reactions, and since then their chemistry have been taken over in our laboratory as a successful prototype, where many features characteristic of the poly¯uoro-substitution are multiply combined in the anionic species [4]. The studies of phase-transfer catalysis (PTC) had already attained to an established status at that time, however, most of the successful applications were con®ned within those to nucleophilic and base-mediated reactions by use of cationic catalysts.1 Under such circumstances we had an idea of extending the application of this technique to electrophilic and acid-mediated reactions, which was combined with a part of my formal task in the institute to develop new organic catalyses. Catalysts useful to the above-mentioned purpose had to satisfy the following requisites: (1) negatively charged permanent or stable ionic species; (2) three-dimensionally bulky molecular structure; (3) no or negligible nucleophilicity to cationic species; (4) high lipophilicity of its salts, irrespective of the kind of counter ion; and (5) chemical stability under acid and oxidative conditions. According to these requisites, our attention was focused to tetraphenylborate (TPB) anion as a prospective compound by simple analogy of the structural features of useful cationic counterpart, quaternary ammonium ion. TPB was already very
popular in the analytical use, and was known to be fatally labile against the above requisites, decomposing rapidly in the presence of acids or oxidants, even under aeration.2 To improve the drawbacks of TPB, we immediately conceived the use of ¯uorine-substitution. The peculiarities due to poly¯uoro-substitution impacted on me, once dated back to early 1960s, when 1,1,1,5,5,5-hexa¯uoro- and 1,1,1tri¯uoropentan-2,4-dionato-metal chelate complexes were veri®ed separable by gas-chromatography according to the kind of central metal ion [5]. Metal chelate complexes were thitherto regarded to be far from volatile matter and it sounded earthshaking for me that the poly¯uoro-substitution afforded them to dissolve in such an inert solvent as tetrachlorocarbon. Since then we had been dealing with the search for the peculiarities induced by poly¯uoro-substitution and some synthetic applications obtaining 1,2-bifunctional poly¯uorobenzene derivatives of analytical use. During such works, our group gained knowledge and experience on the chemistry of organo¯uorine compounds. Meanwhile some partly ¯uorinated tetraphenylborate ions appeared in literatures, and indicated a tendency that the introduction of ¯uoro- or tri¯uoromethyl substituents onto the phenyl groups actually improved the stability against acid and increased hydrophobicity of their salts [6±8]. Use of the ¯uorine-substitution for improving TPB seemed effective in one way to suppress its lability [9± 10],3 and in another way to prepare the required organo2
[4].
The lability of TPB was reported in some papers listed in Ref. [3] of
3
1
Some of the then current books concerning phase-transfer catalysis are listed in Ref. [1] of [4].
Effects induced by polyfluoro-substitution and some of their applications were reviewed previously. Some physical data were recently revised in [10].
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 2 7 4 - 8
202
H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203
Fig. 1. The structures of fluorinated tetraphenylborates, TFPB and FTPB.
magnesium intermediates on the synthetic sequence. Thus, the ®rst target was tetrakis(penta¯uorophenyl)borate (FTPB) ion, whose synthesis had been then reported, but few necessary physical properties were described [11]. In spite of the repeated attacks with various contrivances, the schemes going through a monohydro derivative from hexa¯uorobenzene failed in obtaining the target product, which was, however, found most recently to be successfully isolated in a stable form by an improved scheme starting with bromopenta¯uorobenzene [12]. Parallel to the unsettled works for FTPB, we turned our view to the use of tri¯uoromethyl groups for the improvement of TPB, especially in respect to durability against protic acids and oxidants and solubility in hydrophobic organic solvents. The conceivable phenyl moieties containing more than one tri¯uoromethyl group were so narrowed, that no choice except 3,5-bis(tri¯uoromethyl)phenyl one [13].4 The corresponding xylene derivative which we started with, was provided to us by the Daikin Industries, Osaka, who produced the compound as an intermediate of some agrochemicals. We were greatly indebted for their favor, which afforded us a successful synthetic scheme leading to tetrakis(3,5-bis(tri¯uoromethyl)phenyl)borate (TFPB) ion (Fig. 2).
The synthetic sequence and spectroscopic data of TFPB supported consistently the assigned structure [14],5 which was independently con®rmed by the single-crystal X-ray analysis of lithium TFPB tetrahydrate [15]. Synthetic scheme was further improved for a method affording a higher yield with easier processing, which was successfully applied to the syntheses of some higher homologues of tetraarylborate ions substituted with many tri¯uoromethyl groups [16]. Both ¯uoro- and tri¯uoromethyl-substitution afforded drastic improvements to TPB in the ef®cacy as a phasetransfer catalyst, though in somewhat different manner. The former favored to the suppression of lability under acid and oxidative conditions, while the latter to the increased solubility in hydrophobic organic solvents [12]. In the course of synthesis of TFPB, even before obtaining a de®ned specimen, we were impatient to examine its catalytic effect by a simple test of diazo-coupling of Nethylcarbazole with 4-nitrobenzene-diazonium tetra¯uoroborate in a dichloromethane±water two-phase system after the preceding examples.6 Encouraged by the positive results of the preliminary test, diazo-coupling reactions of arenediazonium ions with various diazophile components were investigated in two-phase systems in the presence of a catalytic amount of TFPB [17,18]. Kinetic investigations proved that TFPB catalyst turned over, as being expected, to promote the reaction under PTC conditions. The diazonium ions were dehydrated in the hydrophobic organic phase, to become more reactive than in the aqueous phase and meanwhile, retained free from the attack by various nucleophilic agents, so that complex side reactions were suppressed, affording a satisfactory yield of coupling products [19,20]. Sodium TFPB placed in a dichloromethane±aqueous sulfuric acid two-phase system, could incorporate oxonium ion into the hydrophobic organic phase quantitatively by exchanging sodium ion. The oxonium ion was regarded to stay in the form of ion-pair with TFPB. The analytical concentration of oxonium ion in the organic phase, there-
Fig. 2. Synthetic scheme of TFPB.
4 The disposition of two trifluoromethyl groups at 3- and 5-positions of the phenyl part had been known as one of those which induced the strongest electron-withdrawing effect, when we made our choice without noticing the important report concerned.
5 Dojin Chemical Laboratory, Kumamoto, offers reagent-grade TFPB with the trade name of `TFPB/The Kobayashi's reagent'. 6 Reports on diazo-coupling reactions in two-phase systems at that time are listed in Ref. [2] of [4].
H. Kobayashi / Journal of Fluorine Chemistry 105 (2000) 201±203
fore, was de®ned by that of counter anion, TFPB. Its acidity changed over a wide range depending upon the aqueous acid concentration, conceivably due to the varied degree of hydration of oxonium. The TFPB-catalyzed two-phase system could be regarded as a quite unique one affording oxonium ions of high acidity even at a lower concentration [4]. The oxonium of high acidity in the organic phase served therein to generate electrophiles of higher order, E, by oxonium-catalyzed dehydration from the corresponding precursors, EOH, according to a general scheme as follows: EOH H3 O TFPBÿ ! E 2H2 O where TFPB carried oxonium ions catalytically to the reaction sites, to promote the overall electrophilic reactions of E or its equivalent in the two-phase systems. Such a scheme was exemplified by Friedel±Crafts alkylations [21], diazotization and nitrosation [22,23]. In the structures of TFPB and FTPB ions, the negative charge localized at the central boron is sterically shielded as well as stabilized by strong electron-withdrawing effect due to the shell of ¯uorine atoms arrayed on the surface of molecule. Therefore, the anions would interact with cationic species solely in an electrostatic manner at a longer distance off, no more in the covalent or the coordinate one. The cationic species in the hydrophobic solution phase, on the other hand, are retained under inert environments with least in¯uence by the counter ions and the solvent molecules. It can display therein its own inherent power in a case as it is, while in another case after converted to a cationic species of higher order [22,23]. Poly¯uoro-substitution successfully brought forth the weakly coordinating bulky anions such as TFPB and FTPB, which are prospective to develop novel ®elds of solution chemistry of cationic species. Extensive reports7 on their application as the effective counter ions in homogeneous
7 The applications reported independently from the groups other than ours are listed in Ref. [7] of [12].
203
olefin polymerization catalysts seem indicative of such prospects in near future.8 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M. Bochmann, Angew. Chem. Int. Ed. Engl. 31 (1992) 1181. K. Seppelt, Angew. Chem. Int. Ed. Engl. 32 (1993) 1025. S.H. Strauss, Chem. Rev. 93 (1993) 927. J. Ichikawa, H. Kobayashi, T. Sonoda, Rep. Inst. Adv. Mater. Study Kyushu Univ. 2 (1988) 189. R.W. Moshier, R.E. Sievers, Gas Chromatography of Metal Chelates, Pergamon Press, 1964 and references therein. C.E. Moore, F.P. Cassaretto, H. Posvic, J.J. McLafferty, Anal. Chim. Acta 35 (1966) 1. C.E. Meisters, C.E. Moore, F.P. Cassaretto, Anal. Chim. Acta 44 (1969) 287. J.T. Vandeberg, C.E. Moore, F.P. Cassaretto, H. Posvic, Anal. Chim. Acta 44 (1969) 175. H. Kobayashi, J. Synth. Org. Chem. Jpn. 45 (1987) 561. H. Kobayashi, Rep. Inst. Adv. Mater. Study, Kyushu Univ. 9 (1995) 1. A.G. Massey, A.J. Park, J. Organometal. Chem. 2 (1964) 245. K. Isshiki, H. Kobayashi, T. Sonoda, Rep. Inst. Adv. Mater. Study, Kyushu Univ. 9 (1995) 47. P.G. Gassman, A.F. Fentiman Jr., J. Am. Chem. Soc. 92 (1970) 2549. H. Nishida, N. Takada, M. Yoshimura, T. Sonoda, H. Kobayashi, Bull. Chem. Soc. Jpn. 57 (1984) 2600. J.H. Golden, P.F. Mutolo, E.B. Lobkovsky, F.J. DiSalvo, Inorg. Chem. 33 (1994) 5374. K. Fujiki, M. Kashiwagi, H. Miyamoto, A. Sonoda, J. Ichikawa, H. Kobayashi, T. Sonoda, J. Fluorine Chem. 57 (1992) 307. H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. (1981) 579 H. Iwamoto, M. Yoshimura, T. Sonoda, H. Kobayashi, Bull. Chem. Soc. Jpn. 56 (1983) 796. H. Iwamoto, H. Kobayashi, P. Murer, T. Sonoda, H. Zollinger, Bull. Chem. Soc. Jpn. 66 (1993) 2590. cf.H. Gilman, J.B. Honeycutt, J. Org. Chem. 22 (1957) 563. H. Kobayashi, T. Sonoda, H. Iwamoto, Chem. Lett. (1982) 1185. H. Iwamoto, T. Sonoda, H. Kobayashi, Tetrahedron Lett. 24 (1983) 4703. H. Iwamoto, T. Sonoda, H. Kobayashi, J. Fluorine Chem. 24 (1984) 535.
8 Some of the reports on polymerization catalysts are listed in Ref. [5] of [12].