Triphenylcarbenium Tetrafluoroborate

Ph3C+ BF4-

[341-02-6]  · C19H15BF4  · Triphenylcarbenium Tetrafluoroborate  · (MW 330.15)

(easily prepared1 hydride abstractor used for conversion of dihydroaromatics to aromatics,2-4 and the preparation of aromatic and benzylic cations;5-8 oxidative hydrolysis of ketals9 and thioketals;10 conversion of acetonides to a-hydroxy ketones;9 oxidation of acetals11 and thioacetals;12 selective oxidation of alcohols and ethers to ketones;9,13-15 oxidation of silyl enol ethers to enones;16 hydrolysis of TBS and MTM ethers;17 oxidation of amines and amides to iminium salts;18-20 oxidation of organometallics to give alkenes;21-23 sensitizer for photooxidation using molecular oxygen;24 Lewis acid catalyst for various reactions;25 polymerization catalyst;26 other reactions27-30)

Alternate Name: trityl fluoroborate.

Physical Data: mp ~200 °C (dec).

Solubility: sol most standard organic solvents; reacts with some nucleophilic solvents.

Form Supplied in: yellow solid; commercially available.

Preparative Methods: the most convenient procedure involves the reaction of Ph3CCl with Silver(I) Tetrafluoroborate in ethanol.1b The most economical route employs the reaction of Ph3CCl with the anhydrous Tetrafluoroboric Acid-Et2O complex.1c

Purification: recrystallization of commercial samples from a minimal amount of dry MeCN provides material of improved purity, but the recovery is poor.1a

Handling, Storage, and Precautions: moisture-sensitive and corrosive. Recrystallized reagent can be stored at rt for several months in a desiccator without significant decomposition. This compound is much less light-sensitive than other trityl salts such as the perchlorate.1a

Preparation of Aromatic Compounds via Dehydrogenation.

Dihydroaromatic compounds are easily converted into the corresponding aromatic compound by treatment with triphenylcarbenium tetrafluoroborate followed by base.2 Certain a,a-disubstituted dihydroaromatics are converted to the 1,4-dialkylaromatic compounds with rearrangement (eq 1).3 Nonbenzenoid aromatic systems, e.g. benzazulene4a or dibenzosesquifulvalene,4b are readily prepared from their dihydro counterparts. Aromatic cations are also easily prepared by hydride abstraction, for example, tropylium ion (e.g. in the synthesis of heptalene (eq 2)),5 cyclopropenyl cation,6 and others, including heterocyclic systems.7 Some benzylic cations, especially ferrocenyl cations,8 can also be formed by either hydride abstraction or trityl addition.

Oxidation by Hydride Abstraction.

In the early 1970s, Barton developed a method for the oxidative hydrolysis of ketals to ketones, e.g. in the tetracycline series (eq 3).9 The same conditions can also be used to hydrolyze thioketals.10 Acetonides of 1,2-diols are oxidized to the a-hydroxy ketones in good yield by this reagent (eq 4).9 The hydrogen of acetals is easily abstracted (eq 5), providing a method for the conversion of benzylidene units in sugars to the hydroxy benzoates.11 The hydrogen of dithioacetals is also abstracted to give the salts.12 Since benzylic hydrogens are readily abstracted, this is also a method for deprotection of benzyl ethers.9,13 Trimethylsilyl, t-butyl, and trityl ethers of simple alcohols are oxidized to the corresponding ketones and aldehydes in good yield. Primary-secondary diols are selectively oxidized at the secondary center to give hydroxy ketones by this method (eq 6).14 2,2-Disubstituted 1,4-diols are oxidized only at the 4-position to give the corresponding lactones.15 Trimethylsilyl enol ethers are oxidized to a,b-unsaturated ketones, thereby providing a method for ketone to enone conversion (eq 7).16 t-Butyldimethylsilyl (TBDMS) ethers are not oxidized but rather hydrolyzed to the alcohols, as are methylthiomethyl (MTM) ethers.17 Benzylic amines and amides can be oxidized to the iminium salts,18 allylic amines and enamines afford eniminium salts,19 and orthoamides give triaminocarbocations.20

Generation of Alkenes from Organometallics.

Various b-metalloalkanes can be oxidized by trityl fluoroborate to the corresponding alkenes.21-23 The highest yields are obtained for the b-iron derivatives (eq 8), which are easily prepared from the corresponding halides or tosylates.21 Grignard reagents and organolithiums also undergo this reaction (eq 9),22 as do Group 14 organometallics (silanes, stannanes, etc.).23

Sensitizer of Photooxygenation.

Barton showed that oxygen, in the presence of trityl fluoroborate and ordinary light, adds to cisoid dienes at -78 °C in very high yields.24 For example, the peroxide of ergosterol acetate is formed in quantitative yields under these conditions (eq 10),24a,b which have been used also for photocycloreversions of cyclobutanes.24c

Lewis Acid Catalysis.

Trityl fluoroborate is a good Lewis acid for various transformations,25 e.g. the Mukaiyama-type aldol reaction using a dithioacetal and silyl enol ether (eq 11).25a It has also been used as the catalyst for the formation of glycosides from alcohols and sugar dimethylthiophosphinates (eq 12)25b and for the formation of disaccharides from a protected a-cyanoacetal of glucose and a 6-O-trityl hexose.25c Michael additions of various silyl nucleophiles to conjugated dithiolenium cations also proceed well (eq 13).25d,e Finally, the [4 + 2] cycloaddition of cyclic dienes and oxygenated allyl cations has been effected with trityl fluoroborate.25f

Polymerization Catalyst.

Several types of polymerization26 have been promoted by trityl fluoroborate, including reactions of orthocarbonates26a and orthoesters,26b-d vinyl ethers,26e-g epoxides,26h,i and lactones.26j,k

Other Reactions.

Trityl fluoroborate has been used often to prepare cationic organometallic complexes, as in the conversion of dienyl complexes of iron, ruthenium, and osmium into their cationic derivatives.27 It alkylates pyridines on the nitrogen atom in a preparation of dihydropyridines28a and acts as a tritylating agent.28b It has also been used in attempts to form silyl cations and silyl fluorides from silanes.29 Finally, it has been reported to be a useful desiccant.30

1. (a) Dauben, H. J., Jr.; Honnen, L. R.; Harmon, K. M. JOC 1960, 25, 1442. (b) Fukui, K.; Ohkubo, K.; Yamabe, T. BCJ 1969, 42, 312. (c) Olah, G. A.; Svoboda, J. J.; Olah, J. A. S 1972, 544.
2. (a) Müller, P. HCA 1973, 56, 1243. (b) Giese, G.; Heesing, A. CB 1990, 123, 2373.
3. (a) Karger, M. H.; Mazur, Y. JOC 1971, 36, 540. (b) Acheson, R. M.; Flowerday, R. F. JCS(P1) 1975, 2065.
4. (a) O'Leary, M. A.; Richardson, G. W.; Wege, D. T 1981, 37, 813. (b) Prinzbach, H.; Seip, D.; Knothe, L.; Faisst, W. LA 1966, 698, 34.
5. (a) Dauben, H. J., Jr.; Gadecki, F. A.; Harmon, K. M.; Pearson, D. L. JACS 1957, 79, 4557. (b) Dauben, H. J., Jr.; Bertelli, D. J. JACS 1961, 83, 4657, 4659. (c) Peter-Katalinic, J.; Zsindely, J.; Schmid, H. HCA 1973, 56, 2796. (d) Vogel, E.; Ippen, J. AG(E) 1974, 13, 734. (e) Beeby, J.; Garratt, P. J. JOC 1973, 38, 3051. (f) Murata, I.; Yamamoto, K.; Kayane, Y. AG(E) 1974, 13, 807, 808. (g) Kuroda, S.; Asao, T. TL 1977, 285. (h) Komatsu, K.; Takeuchi, K.; Arima, M.; Waki, Y.; Shirai, S.; Okamoto, K. BCJ 1982, 55, 3257. (i) Müller, J.; Mertschenk, B. CB 1972, 105, 3346. (j) Schweikert, O.; Netscher, T.; Knothe, L.; Prinzbach, H. CB 1984, 117, 2045. (k) Bindl, J.; Seitz, P.; Seitz, U.; Salbeck, E.; Salbeck, J.; Daub, J. CB 1987, 120, 1747.
6. (a) Zimmerman, H. E.; Aasen, S. M. JOC 1978, 43, 1493. (b) Komatsu, K.; Tomioka, I.; Okamoto, K. BCJ 1979, 52, 856.
7. (a) Yamamura, K.; Miyake, H.; Murata, I. JOC 1986, 51, 251. (b) Matsumoto, S.; Masuda, H.; Iwata, K.; Mitsunobu, O. TL 1973, 1733. (c) Yano, S.; Nishino, K.; Nakasuji, K.; Murata, I. CL 1978, 723. (d) Kedik, L. M.; Freger, A. A.; Viktorova, E. A. KGS 1976, 12, 328 (Chem. Heterocycl. Compd. (Engl. Transl.) 1976, 12, 279). (e) Reichardt, C.; Schäfer, G.; Milart, P. CCC 1990, 55, 97.
8. (a) Müller, P. HCA 1973, 56, 500. (b) Boev, V. I.; Dombrovskii, A. V. ZOB 1987, 57, 938, 633. (c) Klimova, E. I.; Pushin, A. N.; Sazonova, V. A. ZOB 1987, 57, 2336. (d) Abram, T. S.; Watts, W. E. JCS(P1) 1975, 113; JOM 1975, 87, C39. (e) Barua, P.; Barua, N. C.; Sharma, R. P. TL 1983, 24, 5801. (f) Akgun, E.; Tunali, M. AP 1988, 321, 921.
9. (a) Barton, D. H. R.; Magnus, P. D.; Smith, G.; Strecker, G.; Zurr, D. JCS(P1) 1972, 542. (b) Barton, D. H. R.; Magnus, P. D.; Smith, G.; Zurr, D. CC 1971, 861.
10. Ohshima, M.; Murakami, M.; Mukaiyama, T. CL 1986, 1593.
11. (a) Hanessian, S.; Staub, A. P. A. TL 1973, 3551. (b) Jacobsen, S.; Pedersen, C. ACS 1974, 28B, 1024, 866. (c) Wessel, H.-P.; Bundle, D. R. JCS(P1) 1985, 2251.
12. (a) Nakayama, J.; Fujiwara, K.; Hoshino, M. CL 1975, 1099; BCJ 1976, 49, 3567. (b) Nakayama, J.; Imura, M.; Hoshino, M. BCJ 1980, 53, 1661. (c) Nakayama, J. BCJ 1982, 55, 2289. (d) Bock, H.; Brähler, G.; Henkel, U.; Schlecker, R.; Seebach, D. CB 1980, 113, 289. (e) Neidlein, R.; Droste-Tran-Viet, D.; Gieren, A.; Kokkinidis, M.; Wilckens, R.; Geserich, H.-P.; Ruppel, W. HCA 1984, 67, 574. (f) However, azide abstraction is seen with azidodithioacetals: Nakayama, J.; Fujiwara, K.; Hoshino, M. JOC 1980, 45, 2024.
13. (a) Barton, D. H. R.; Magnus, P. D.; Streckert, G.; Zurr, D. CC 1971, 1109. (b) Doyle, M. P.; Siegfried, B. JACS 1976, 98, 163. (c) Hoye, T. R.; Kurth, M. J. JACS 1979, 101, 5065. (d) For simple ethers, see: Deno, N. C.; Potter, N. H. JACS 1967, 89, 3550.
14. (a) Jung, M. E. JOC 1976, 41, 1479. (b) Jung, M. E.; Speltz, L. M. JACS 1976, 98, 7882. (c) Jung, M. E.; Brown, R. W. TL 1978, 2771.
15. Doyle, M. P.; Dow, R. L.; Bagheri, V.; Patrie, W. J. JOC 1983, 48, 476; TL 1980, 21, 2795.
16. (a) Jung, M. E.; Pan, Y.-G.; Rathke, M. W.; Sullivan, D. F.; Woodbury, R. P. JOC 1977, 42, 3961. (b) Reetz, M. T.; Stephan, W. LA 1980, 533.
17. (a) Metcalf, B. W.; Burkhardt, J. P.; Jund, K. TL 1980, 21, 35. (b) Chowdhury, P. K.; Sharma, R. P.; Baruah, J. N. TL 1983, 24, 4485. (c) Niwa, H.; Miyachi, Y. BCJ 1991, 64, 716.
18. (a) Damico, R.; Broaddus, C. D. JOC 1966, 31, 1607. (b) Barton, D. H. R.; Bracho, R. D.; Gunatilaka, A. A. L.; Widdowson, D. A. JCS(P1) 1975, 579. (c) Wanner, K. T.; Praschak, I.; Nagel, U. AP 1990, 322, 335; H 1989, 29, 29.
19. Reetz, M. T.; Stephan, W.; Maier, W. F. SC 1980, 10, 867.
20. Erhardt, J. M.; Grover, E. R.; Wuest, J. D. JACS 1980, 102, 6365.
21. (a) Laycock, D. E.; Hartgerink, J.; Baird, M. C. JOC 1980, 45, 291. (b) Laycock, D. E.; Baird, M. C. TL 1978, 3307. (c) Slack, D.; Baird, M. C. CC 1974, 701. (d) Bly, R. S.; Bly, R. K.; Hossain, M. M.; Silverman, G. S.; Wallace, E. T 1986, 42, 1093. (e) Bly, R. S.; Silverman, G. S.; Bly, R. K. OM 1985, 4, 374.
22. Reetz, M. T.; Schinzer, D. AG(E) 1977, 16, 44.
23. (a) Traylor, T. G.; Berwin, H. J.; Jerkunica, J.; Hall, M. L. PAC 1972, 30, 597. (b) Jerkunica, J. M.; Traylor, T. G. JACS 1971, 93, 6278. (c) Washburne, S. S.; Szendroi, R. JOC 1981, 46, 691. (d) Washburne, S. S.; Simolike, J. B. JOM 1974, 81, 41. (e) However, organostannanes lacking a b-hydrogen afford alkyltriphenylmethanes in good yield. Kashin, A. N.; Bumagin, N. A.; Beletskaya, I. P.; Reutov, O. A. JOM 1979, 171, 321.
24. (a) Barton, D. H. R.; Haynes, R. K.; Leclerc, G.; Magnus, P. D.; Menzies, I. D. JCS(P1) 1975, 2055. (b) Barton, D. H. R.; Leclerc, G.; Magnus, P. D.; Menzies, I. D. CC 1972, 447. (c) Okada, K.; Hisamitsu, K.; Mukai, T. TL 1981, 22, 1251. (d) Futamura, S.; Kamiya, Y. CL 1989, 1703.
25. (a) Ohshima, M.; Murakami, M.; Mukaiyama, T. CL 1985, 1871. (b) Inazu, T.; Yamanoi, T. Jpn. Patent 02 240 093, 02 255 693 (CA 1991, 114, 143 907j, 143 908k); Jpn. Patent 01 233 295 (CA 1990, 112, 198 972r). (c) Bochkov, A. F.; Kochetkov, N. K. Carbohydr. Res. 1975, 39, 355; for polymerizations of carbohydrate cyclic orthoesters, see: Bochkov, A. F.; Chernetskii, V. N.; Kochetkov, N. K. Carbohydr. Res. 1975, 43, 35; BAU 1975, 24, 396. (d) Hashimoto, Y.; Mukaiyama, T. CL 1986, 1623, 755. (e) Hashimoto, Y.; Sugumi, H.; Okauchi, T.; Mukaiyama, T. CL 1987, 1691. (f) Murray, D. H.; Albizati, K. F. TL 1990, 31, 4109.
26. (a) Endo, T.; Sato, H.; Takata, T. Macromolecules 1987, 20, 1416. (b) Uno, H.; Endo, T.; Okawara, M. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 63. (c) Nishida, H.; Ogata, T. Jpn. Patent 62 295 920 (CA 1988, 109, 57 030h). (d) See also Ref. 25c. (e) Kunitake, T. J. Macromol. Sci., Chem. 1975, A9, 797. (f) Kunitake, T.; Takarabe, K.; Tsugawa, S. Polym. J. 1976, 8, 363. (g) Spange, S.; Dreier, R.; Opitz, G.; Heublein, G. Acta Polym. 1989, 40, 55. (h) Mijangos, F.; León, L. M. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 885; Eur. Polym. J. 1983, 19, 29. (i) Bruzga, P.; Grazulevicius, J.; Kavaliunas, R.; Kublickas, R. Polym. Bull. (Berlin) 1991, 26, 193. (j) Khomyakov, A. K.; Gorelikov, A. T.; Shapet'ko, N. N.; Lyudvig, E. B. Vysokomol. Soedin., Ser. A 1976, 18, 1699, 1053; DOK 1975, 222, 1111.
27. (a) For a review, see any basic organometallic text, e.g. Coates, G. E.; Green, M. L. H.; Wade, K. Organometallic Compounds; Methuen: London, 1968; Vol. 2, pp 136ff. (b) Birch, A. J.; Cross, P. E.; Lewis, J.; White, D. A. CI(L) 1964, 838. (c) Cotton, F. A.; Deeming, A. J.; Josty, P. L.; Ullah, S. S.; Domingos, A. J. P.; Johnson, B. F. G.; Lewis, J. JACS 1971, 93, 4624.
28. (a) Lyle, R. E.; Boyce, C. B. JOC 1974, 39, 3708. (b) Hanessian, S.; Staub, A. P. A. TL 1973, 3555.
29. (a) Sommer, L. H.; Bauman, D. L. JACS 1969, 91, 7076. (b) Bulkowski, J. E.; Stacy, R.; Van Dyke, C. H. JOM 1975, 87, 137. (c) Chojnowski, J.; Fortuniak, W.; Stanczyk, W. JACS 1987, 109, 7776.
30. Burfield, D. R.; Lee, K.-H.; Smithers, R. H. JOC 1977, 42, 3060.

Michael E. Jung

University of California, Los Angeles, CA, USA

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