Dynamic NMR studies of cyclopropyldifluoroborane and vinyldifluoroborane

The two fluorines of cyclopropyldifluoroborane (1) and vinyldifluoroborane (2) show two chemical shifts at low temperatures, due to hindered rotation about the boron-carbon bonds. The rates of rotation were determined at high er temperatures, and the corresponding free-energy barriers at coalescence were found to be 4.85 +/- 0.1 and 4.56 +/- 0.1 kcal/mol, respectively. The two barriers indicate that the cyclopropyl and vinyl groups are comparable in electron donating ability. Calculated lineshapes suggest that determinat ion of the rotational barrier in phenyldifluoroborane by making use of the difference in coupling constants fur an ortho carbon and the two fluorines may not be possible. (C) 2000 Elsevier Science B.V. All rights reserved.