Dynamic features of the zirconocene-boron-betaine complexes obtained by treatment of bis( alkynyl) zirconocenes with the tris( pentafluorophenyl) borane Lewis-acid

Article abstract of DOI:10.1016/S0022-328X(96)06610-7

Bis(propynyl)zirconocene (6a) reacts with tris(pentafluorophenyl)borane to yield the carbon-carbon coupled Cp₂M( μ-RC₄R)B(C₆F₅)₃ betaine product 4a (M = Zr, R = CH₃). A variety of differently substituted analogs was prepared (M = Zr, R = n-butyl, phenyl, cyclohexyl; M = Ti, R = CH₃; M = Hf; R = CH₃, phenyl, SiMe₃). These complexes 4 are chiral due to the presence of a rather stable propeller-like R-B(aryl)₃ conformation. The activation barrier of the intramolecular enantiomerization process of many examples of this series of complexes was determined by dynamic ¹H NMR spectroscopy, with ΔG? values ranging from ca. 13 to 16 kcalmol^-1, depending on the substitution pattern. Complex 4a reacts with 2,6-dimethylphenylisocyanide to yield the methylenecyclopropene derivative 1 that was characterized by X-ray diffraction.