Titanium-Mediated Rearrangement of Bis(alkynyl)boranes: B─C Activation versus C─H Activation

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─C_alkynyl bond activation or C_Me─H activation of the Cp* ligand, leading to the formation of two distinct types of products.