TiCl4-Catalyzed Hydroboration of Ketones with Ammonia Borane
P. Veeraraghavan Ramachandran*, Abdulkhaliq A. Alawaed and Henry J. Hamann
*Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States, Email: chandranpurdue.edu
P. V. Ramachandran, A. A. Alawaed, H. J. Hamann, J. Org. Chem., 2022, 87, 13259-13269.
A catalytic amount of titanium tetrachloride immensely accelerates the hydroboration-hydrolysis (reduction) of ketones with ammonia borane in diethyl ether at room temperature. The product alcohols are produced in very good yields within 30 min, even with ketones which typically requires 24 h or longer under uncatalyzed conditions.
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A 25 mL oven dried round bottom flask was charged with ketone (1 mmol, 1eq) and a magnetic stirring bar. The flask was sealed using a rubber septum. After purging the flask with nitrogen, distilled diethyl ether (3 mL) was added and the solution was cooled at 0 °C with an ice bath. Subsequently, titanium tetrachloride (TiCl4) (11 µL, 0.1 mmol, 0.1 eq) was added dropwise to the solution through the septum using a syringe. The septum was then carefully opened and ammonia borane (NH3BH3) (0.5 mmol, 0.5 eq) was added slowly to the reaction mixture. Upon complete addition the reaction flask was again sealed with a septum. After stirring at 0 °C for 1 minute, the reaction mixture was allowed to warm up to room temperature, stirred and monitored by TLC until completion. On completion of the reaction, the crude mixture was brought to 0 °C and quenched by slow addition of cold 1M of HCl, then transferred to a separatory funnel and extracted with diethyl ether (2 × 3mL). The combined organic layers were washed with brine (1 x 3 mL), dried over anhydrous sodium sulfate, filtered through cotton, and concentrated under rotary evaporation and the remain solvent was removed by applying high vacuum for 30 minutes.
P. V. Ramachandran, A. A. Alawaed, H. J. Hamann, Org. Lett., 2022, 24, 8481-8486.