Titanium-Mediated Rearrangement of Bis(alkynyl)boranes: B─C Activation versus C─H Activation
Chenchang Ma1, Alexander Matler1, Shuai Zhu2
1Institute for Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität Würzburg, 97074, Würzburg, Germany.
Abstract:
Reactions between Rosenthal's titanocene (Cp2Ti) and decamethyltitanocene (Cp*2Ti) synthons with various bis(alkynyl)boranes were investigated. A series of titanium-fused boracyclobutenes were obtained through the reaction of the Cp*2Ti synthon with (Me3Si)2NB(CCR)2 or PhB(CCPh)2. This represents the first realization of approaching this structural motif via the rearrangement of bis(alkynyl)boranes within the coordination sphere of a d-block metal. These findings break the previous limitation that the boron substituent must be an amino group. Furthermore, the reaction of the Cp*2Ti synthon with (Me3Si)PhNB(CCPh)2 or (Mes2B)PhNB(CCPh)2 (Mes = 2,4,6-trimethylphenyl) led to the formation of novel tethered metallocene complexes, in which the titanacyclopentadiene structure is linked to one of the cyclopentadienyl groups via a ─CH2─BR─ bridge. Both experimental and theoretical studies provided insights into an unprecedented reaction mechanism. The process involves the initial formation of an η2-coordinated bis(alkynyl)borane intermediate, which was detected and analyzed by NMR spectroscopy and X-ray diffraction analysis. This intermediate subsequently undergoes either B─Calkynyl bond activation or CMe─H activation of the Cp* ligand, leading to the formation of two distinct types of products.
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