Switching of (N,O)- to (C,N)-Chelation in π-Expanded Boron Quinolates

We report an intramolecular nucleophilic arylative dearomatization of diarylboron quinolates, leading to the formation of respective products where the boron atom is chelated by the 1,2-dihydroquinolate moiety. Kinetics and thermodynamics of this reaction were analyzed by density functional theory (DFT) calculations. Subsequent oxidation of obtained complexes with MnO₂ in refluxing CHCl₃ presumably gives rise to unstable, highly strained, rearomatized intermediates, which immediately undergo ethanolysis in situ, giving rise to EtO-substituted arylboron (C,N)-chelate complexes bearing a free OH group at the quinoline moiety. Treatment of these compounds with BF₃·Et₂O afforded two crystallographically authenticated diboron systems, where the 2-(8-oxidoquinolin-2-yl)aryl moiety is bound to the B(μ–OH)BF₂ fragment.