Trimethylammonium Tetraphenylborate

Wittig and co-workers2 have shown that triphenylborane can be readily synthesized in high yield by simple thermal decomposition of trimethylammonium tetraphenylborate. They obtained the latter compound by collecting the precipitate which was formed on mixing aqueous solutions of trimethylammonium chloride and lithium tetraphenylborate. The lithium salt was isolated from the products of the reaction of boron trifluoride-etherate with 

Samples of triphenylborane are obtained readily by thermal decomposition of trimethylammonium tetraphenylborate prepared by the method described. Preparation of triphenylborane by this method eliminates the tedious and somewhat hazardous treatment of the reactive residues produced during the formation of the triphenylborane by the Grignard method.3 

The procedure gives satisfactory results on reduced scales, e.g., one-fourth or one-sixth the quantities given above. In addition, a saturated solution of boron trichloride in ether may be used in place of the solution described. A saturated solution may be prepared by allowing boron trichloride gas to bubble into ether at room temperature until excess boron trichloride-etherate settles out of the liquid phase. 

Trimethylammonium tetraphenylborate is a white, crystalline solid. It is very soluble in acetone and moderately soluble in dichloromethane. It is difficult to dry at room temperature. 

Taken in part from the B.S. thesis of Robert E. Sherman, St. Louis University, St. Louis, Mo., 1967 

---

Triphenylborane is best obtained from phenylmagnesium bromide and trifluoroborane-diethyl etherate,1 although it is conveniently available on a small scale by pyrolysis of trimethylammonium tetraphenylborate.2 According to the procedures described to date,1,3 the Grignard reaction gives yields of 50-60%. However, triphenylborane is obtained in 80-90% yield if the following points are observed in the preparation ... 

---