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

The
thermal elimination of α-olefin from tri-n-alkylborane
(dehydroboration) has been studied both experimentally
and theoretically. High-temperature 1H NMR spectroscopy
and temperature quench experiments revealed no evidence for practically
significant concentrations of free olefin at temperatures up to 200
°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 °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.