Boronyl Borinic Esters: Preparation as B pin / BuLi/TFAA Adducts, Structural Insights and Reactivity in Pd-catalyzed Allylic Borylation.

Novel boronyl borinic ester I was generated by quenching the B pin / BuLi-ate complex with trifluoroacetic acid anhydride (TFAA) via ring-opening in the 1,3,2-dioxaborolane moiety on ate-boron. Detailed NMR studies of the B pin / BuLi-ate complex in solution and in solid state allowed us to assume its oligomeric nature in solids with only ate-boron involved in the oligomerization process. The O-trifluoroacetyl pinacolate residue on borinic ester I initially formed on quenching with TFAA undergoes an unusual intramolecular transesterification with the carbonyl group of trifluoroacetyl forming othroester moiety in a few hours at r. t. to give boronyl borinic ester II. A solution of these reagents I/II was proved to be efficient for borylation of (2-fluoroallyl)pyridinium salts that are highly base sensitive.