Homolytic, Heterolytic, Mesolytic - As You Like It: Steering the Cleavage of a HC(sp3)-C(sp3)H Bond in Bis(1H-2,1-benzazaborole) Derivatives.
A set of (3,3')-bis(1-Ph-2-R-1H-2,1-benzazaborole) compounds, in which R=tBu (Bab-tBu)2, R=Dipp (Bab-Dipp)2or R=tBu and Dipp (Bab-Dipp)(Bab-tBu), was synthesized and fully characterized using1H,11B,13C, and15N NMR spectroscopy as well as single-crystal X-ray diffraction analysis. The central HC(sp3)-C(sp3)H bond with restricted rotation at the junction of both 1H-2,1-benzazaborole rings displayed an intriguing reactivity. It was demonstrated that this bond is easily mesolytically cleaved using alkali metals to form the respective aromatic 1Ph-2R-1H-2,1-benzazaborolyl anions M+(THF)n(Bab-tBu)-(M=Li, Na, K) and K+(THF)n(Bab-Dipp)-. Furthermore, the central HC(sp3)-C(sp3)H bond of bis(1H-2,1-benzazaborole)s is also homolytically cleaved either by heating or photochemical means, giving corresponding 1Ph-2R-1H-2,1-benzazaborolyl radicals (Bab-tBu).and (Bab-Dipp)., which rapidly self-terminate. Nevertheless, their formation was unambiguously established by NMR analysis of the reaction mixtures containing products of the self-termination of the radicals after heating or irradiation. (Bab-Dipp).radical was also characterized using EPR spectroscopy. Importantly, it turned out that the essentially non-polarized HC(sp3)-C(sp3)H bond in (Bab-tBu)2is also cleaved heterolytically with 2 equiv of MeLi, giving the mixture of Li+(SOL)n(Bab-tBu)-(SOL=THF or Et2O) and lithium methyl-substituted borate complex Li+(SOL)n(Bab-tBu-Me)-in a diastereoselective fashion.
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic.
Martin
M
Hejda
Research Institute for Organic Syntheses, Rybitví 296, CZ-533 54, Pardubice, Czech Republic.
Antonín
A
Lyčka
Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic.
Tomáš
T
Mikysek
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic.
Roman
R
Jambor
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic.
Aleš
A
Růžička
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic.
Jaromír
J
Vinklárek
Institute for Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany.
Claudia
C
Wilfer
Institute for Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany.
Alexander
A
Hoffmann
Institute for Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany. sonja.herresec-pawlis@ac.rwth-aachen.de.
Sonja
S
Herres-Pawlis
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, Czech Republic. libor.dostal@upce.cz.
Libor
L
Dostál
C−C bond cleavage; atropisomerism; boron; heterocycles; hindered rotation; radicals