Synthesis, structure of borylmagnesium, and its reaction with benzaldehyde to form benzoylborane

The reaction of boryllithium 2 with 1.0 or 0.5 equiv of MgBr2 center dot OEt2 provided boryl Grignard reagents, borylmagnesium bromides 3 and 4, or bis(boryl)magnesium 5. Structures of 3, 4, and 5 in the crystals and solutions indicated the ionic character of the B-Mg bonds. The reactivity of borylmagnesium 3 with benzaldehyde was different from that of boryllithium 2. Benzoylborane 7 was fully characterized as the first acylborane.