Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides

5 years ago

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides

Hauke Kelch, Gernot Frenking, Theresa Dellermann, Florian Hupp, Lisa Mailänder, Jonas H. Müssig, Annika Ruppert, Ivo Krummenacher, Mehmet Ali Celik, Holger Braunschweig, Kai Hammond, Felix Lindl

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide–borole adduct formation and nitrene insertion is favored. Forging rings: The ring expansion of boracyclopentadienes (boroles) with organic azides is highly divergent, leading to three distinct 1,2-azaborinines, BN analogues of benzene (see graphic). To address the factors that determine the reaction outcome, the electronic and steric properties of the azide and borole substituents were systematically varied and DFT calculations were performed to elucidate the mechanism.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201700749