Triphenylcarbinol (97% purity, Purchased from Sigma-Aldrich)

Cycloheptatriene (Purchased from Sigma-Aldrich)

Tetrafluoroboric Acid (48% by weight aqueous solution, Purchased from Sigma-Aldrich)

Acetic Anhydride (Purchased from Sigma-Aldrich)

Diethyl Ether (Reagent Grade, Purchased from Sigma-Aldrich)

To a 1000 mL one-neck round bottom flask equipped with stir bar was added 280 mL of acetic anhydride followed by 30.91 g (0.1153 mol, 1 equiv., 0.415 M) of 97% triphenylcarbinol. The reaction mixture was stirred initially at room temperature. Some of the triphenylcarbinol dissolved but the majority remained as a white, flaky solid.  To ensure a homogenous reaction mixture, the solution was heated gently in a water bath (to about 50 ^oC) until the remainder of the carbinol dissolved. Upon complete dissolution and cooling to ~30 ^oC, 18.75 mL of HBF₄ (0.150 mol, 1.3 equiv.) was added slowly dropwise¹(CAUTION:  HBF₄ is corrosive and addition is exothermic). An ice bath was utilized to maintain a constant temperature of approximately 28 ^oC. Addition was performed over one hour. Upon complete addition, cycloheptatriene (~14 mL, 0.1345 mol, 1.17 equiv.)²  was added dropwise over the course of 5 minutes. After stirring for ~ 5 minutes, a white precipitate began to form. Immediately, ~ 300 mL of diethyl ether was added to the flask inducing rapid precipitation. The reaction mixture was then filtered through a medium fritted funnel. The fluffy white solid (tropylium tetrafluoroborate) was washed three times with ~50 mL portions of diethyl ether. The solid was removed and saved. 100 mL of diethyl ether was added to the filtrate and a second crop of tropylium tetrafluoroborate began to crash out of solution. The filtrate was re-filtered through the funnel and the solid was washed with three portions of 25 mL of diethyl ether. The combine solid was dried in vacuo to give 19.01 g (91%) of tropylium tetrafluoroborate.       

1. Immediately after the first drop of the acid, the solution turned a bright yellow color which indicated the formation of the trityl cation.
2. A deep red color was observed and, on complete of addition of the triene, the solution returned to the original yellow tinge.

¹H-NMR (400 MHz, CD₃CN): 9.26 ppm (Singlet, 7 H), ¹³C (100 MHz, CD₃CN): 156.8 ppm

Dauben, H. Jr.;  Honnen, L.; Harmon, K. J. Org. Chem., 1960, 25, 1442