Synthesis of a C(sp2)-bridged Phosphine-Borane by Ionic Coupling

The vinyl carbenoid H2C=CBr(Li) has been used as key precursor to prepare a geminal C(sp(2))-bridged phosphine-borane. Starting from bromoethene, two sequences of lithiation/electrophilic trapping, with ClPiPr(2) and FBMes(2) respectively, affords iPr(2)P-C(=CH2)-BMes(2) 3 [Mes = 2,4,6-(H3C)(3)C6H2]. This new phosphine-borane 3 was characterized by multi-nuclear NMR and mass spectroscopy. It adopts a monomeric open structure without P -> B interaction. A few crystals of a secondary product 4 were analyzed by X-ray diffraction, revealing an unusual dimeric structure.