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.