Tropylium Tetrafluoroborate1

[27081-10-3]  · C7H7BF4  · Tropylium Tetrafluoroborate  · (MW 177.95)

(useful precursor to azulenes,2,3 cycloheptatrienes,4-15 heptafulvenes,4,9,16 8-azafulvenes,17 and tropones18,19)

Alternate Names: tropylium fluoroborate, cycloheptatrienocarbonium fluoroborate.

Physical Data: mp >200 °C (dec).

Solubility: sol polar organic solvents (e.g. MeCN, DMF), and H2O; reacts with DMSO.19

Form Supplied in: white powder; commercially available.

Analysis of Reagent Purity: elemental analysis; lmax 218 nm; (log ε 4.70), 274 nm; (log ε 3.61).20

Preparative Methods: most conveniently prepared by the reaction of cycloheptatriene with PCl3, followed by treatment with ethanolic HBF4 at 0 °C (0.24 mol scale, 80-89% yield).20 The reagent is obtained in 98-100% purity.

Purification: can be recrystallized from a large volume of ethyl acetate or ethyl acetate-acetonitrile; however, considerable material is lost and the purity of the reagent is not significantly improved.20

Handling, Storage, and Precautions: this material is a nonhygroscopic, stable source of the tropylium ion, making it a superior choice to either the halide (hygroscopic) or perchlorate (potentially explosive) salts. The reagent is a corrosive solid.

Synthesis of Azulenes.

Tropylium tetrafluoroborate (TpBF4) (2 equiv) reacts with a variety of allenylsilanes to generate 2-silylazulenes in fair to excellent yield (eq 1).2 This [3 + 2] annulation strategy allows for the synthesis of azulenes with complete regiochemical control over substituents on the newly formed five-membered ring. It is necessary to employ a nonnucleophilic acid scavenger to remove HBF4 generated in the reaction in order to obtain good yields of azulenes; poly(4-vinylpyridine) or methyltrimethoxysilane functions best in this capacity. An analogous reaction with propargylsilanes also gives rise to 2-silylazulenes.3 The 2-silylazulene products can be readily protodesilyated to yield 1,3-disubstituted azulenes in excellent yield. Certain substituted tropylium tetrafluoroborates will also undergo this reaction to generate azulenes. However, substituents are limited to groups bearing no a-hydrogens (to prevent heptafulvene formation,21 e.g. t-butyl or phenyl).

Synthesis of Substituted Cycloheptatrienes.

Tropylium salts react with a wide array of carbon- and heteroatom-based nucleophiles to provide 7-substituted cycloheptatrienes (7-CHTs). TpBF4 is best suited for this reaction since it is nonhygroscopic and indefinitely stable.20 For example, TpBF4 reacts with a variety of active methylene compounds such as diethyl acetonedicarboxylate,4 acetylacetone (eq 2),5 and malonic acid derivatives6 to give 7-CHTs. Alkyl-, alkenyl-, and akynyllithium reagents, as well as alkenylzinc and -aluminum derivatives, combine smoothly with TpBF4 to provide access to a host of 7-CHTs.7 a-Cycloheptatrienyl ketones are also easily prepared from TpBF4 by reaction with enolizable ketones8 or morpholinoenamines9 in the presence of an acid catalyst. Other nucleophiles which react with TpBF4 in this manner include isocyanides,10,11 inorganic cyanides,11 allylsilanes,12 stabilized phosphonium ylides,13 propargylic alcohols,14 and allylic amines.15

Synthesis of Heptafulvenes and 8-Azaheptafulvenes.

In a two-step procedure, TpBF4 can be used to synthesize a variety of heptafulvene derivatives.4,9,16 As previously discussed, TpBF4 reacts with nucleophiles to give cycloheptatrienes. These CHTs can then be used to produce heptafulvenes. For example, treatment of TpBF4 with 1,3-cyclopentanedione generates a 7-CHT. Reaction of this species with Phosphorus(V) Chloride yields a new tropylium cation, which when treated with base affords a heptafulvene derivative (eq 3).16a 8-Azaheptafulvenes can also be produced from TpBF4 by reaction with a variety of aniline derivatives (eq 4).17

Synthesis of Tropone.

TpBF4 is a convenient starting material for the synthesis of tropone in multigram quantities. This may be accomplished by treating tropyl ether (formed by the reaction of TpBF4 with aq NaOH) with acid-treated silica gel (eq 5).18 A more direct route involves heating TpBF4 in DMSO; this route provides tropone in 58% yield.19

1. For reviews of tropylium ion chemistry, see: (a) Asao, T.; Oda, M. MOC 1985, V/2c, 49. (b) Lloyd, D. Non-Benzenoid Conjugated Carbcyclic Compounds; Elsevier: Amsterdam, 1984; p 72.
2. Becker, D. A.; Danheiser, R. L. JACS 1989, 111, 329.
3. Danheiser, R. L.; Dixon, B. R.; Gleason, R. W. JOC 1992, 57, 6094.
4. Yagihara, M.; Kitahara, Y. CL 1972, 653.
5. Komatsu, K.; Tanaka, S.; Saito, S.; Okamoto, K. BCJ 1977, 50, 3425.
6. Vedejs, E.; Wilber, W. R.; Twieg, R. JOC 1977, 42, 401.
7. Picotin, G.; Faye, A.; Miginiac, P. BSF(2) 1990, 127, 245.
8. Reingold, I. D.; Trujillo, H. A.; Kahr, B. E. JOC 1986, 51, 1627.
9. Gierisch, S.; Daub, J. CB 1989, 122, 69.
10. (a) Ugi, I.; Betz, W.; Offermann, K. CB 1964, 97, 3008. (b) Ugi, I.; Fetzer, U.; Eholzer, U.; Knupfer, H.; Offermann, K. AG(E) 1965, 4, 472.
11. Vedejs, E.; Gabel, R. A.; Weeks, P. D. JACS 1972, 94, 5842.
12. Picotin, G.; Miginiac, P. TL 1988, 29, 5897.
13. Cavicchio, G.; D'Antonio, M.; Gaudiano, G.; Marchetti, V.; Ponti, P. P. G 1979, 109, 315.
14. Pryde, A.; Zsindley, J.; Schmid, H. HCA 1974, 57, 1598.
15. Krow, G. R.; Cannon, K. C.; Carey, J. T.; Ma, H.; Raghavachari, R.; Szczepanski, S. W. JOC 1988, 53, 2665.
16. (a) Bönzli, P.; Neuenschwander, M.; Engel, P. HCA 1990, 73, 1685; (b) Takahashi, K.; Nishijima, K.; Makino, N.; Takase, K.; Katagiri, S. CL 1982, 1895; (c) Bauer, W.; Betz, I.; Daub, J.; Jakob, L.; Pickl, W.; Rapp, K. M. CB 1983, 116, 1154.
17. Sanechika, K.; Kajigaeshi, S.; Kanemasa, S. S 1977, 202.
18. Ter Borg, A. P.; Van Helden, R.; Bickel, A. F. RTC 1962, 81, 177.
19. Garfunkel, E.; Reingold, I. D. JOC 1979, 44, 3725.
20. Conrow, K. OSC 1973, 5, 1138.
21. Nozoe, T.; Takahashi, K.; Yamamoto, H. BCJ 1969, 42, 3277.

John L. Kane, Jr. & Rick L. Danheiser

Massachusetts Institute of Technology, Cambridge, MA, USA

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