Bis(oxazolinyl)phenylborane: A Lewis acid-containing ligand for methide abstraction-based coordination to aluminum(III)

A compound that contains a Lewis acidic boron center and coordinating oxazoline groups, bis(4,4-dimethyl-2-oxazolinyl)phenylborane (PhB(Ox^Me2)₂; 1), has been prepared and spectroscopically characterized. Solvent dependent ¹⁵N and ¹¹B NMR spectroscopic properties and solid-state ¹¹B NMR measurements provide support for intermolecular interactions involving Lewis acid and base sites. The bifunctional nature of oxazolinylborane 1 is demonstrated by its reaction with (AlMe₃)₂, which proceeds via methide abstraction by the boron and oxazoline coordination to aluminum to yield [(κ²-PhMeB(Ox^Me2)₂AlMe₂] (2). Compound 2 contains a planar six-membered chelate ring, in contrast to related bis(pyrazolyl)boratoaluminum compounds that are puckered. Additionally, compound 2 and related bidentate tris(oxazolinyl)phenylborato dimethylaluminum are inert toward aluminum-methyl bond protonolysis. This robust nature suggested the possibility of using these oxazolinylboratoaluminum compounds in catalytic reactions, as is demonstrated by lactide ring-opening polymerization.