Thermal Dehydroboration: Experimental and Theoretical Studies of Olefin Elimination from Trialkylboranes and Its Relationship to Alkylborane Isomerization and Transalkylation

The thermal elimination of a-olefin from tri-n-alkylborane (dehydroboration) has been studied both experimentally and theoretically. High-temperature H-1 NMR spectroscopy and temperature quench experiments revealed no evidence for practically significant concentrations of free olefin at temperatures up to 200 degrees C. Furthermore, attempts to remove 1-octene from tri-n-octylborane by prolonged heating under vacuum were largely unsuccessful. The experimental findings were supported by high-level theoretical calculations, which predicted a very minor equilibrium concentration of dehydroboration products at temperatures less than 200 degrees C. The results revealed that a putative continuous dehydroboration process will most likely be a slow process and will be difficult to achieve selectively. At the same time, reversible dehydroboration was found to be the most likely route for alkylborane isomerization and transalkylation reactions, and theoretical findings were used to rationalize previous experimental findings. A range of alternative mechanistic proposals for isomerization and transalkylation were also evaluated and were found to be uncompetitive with dehydroboration.