Ketone coordination chemistry of the diborane t-BuCH=C[B(C6F5)2]2

Reactions of the bifunctional borane t-BuCH=C[B(C6F5)2]2, 1, with simple Lewis bases were investigated to probe its Lewis acidity. Ethereal bases (Et2O, THF) bind very weakly, if at all, as no adducts were detectable in solution. Certain ketones, however, were observed to bind to 1. Acetone forms an adduct, 2, reversibly in solution (Keq at 293 K = 69.1), and the temperature dependence of this equilibrium yield thermodynamic parameters of H° = -59.5 ± 2.0 kJ mol-1; S° = -167 ± 4 J K-1 mol-1. Low temperature NMR spectroscopy suggests that the acetone binds to the more sterically accessible boron center trans to the t-Bu group. Ketone binding is strengthened when 2,3-diphenylcyclopropenone is reacted with 1, irreversibly forming an isolable adduct, 4, in high yield (95%). The availability of a resonance form incorporating the aromatic cyclopropenyl cation contributes to the added stability of this adduct in comparison to 2.