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

Bis(propynyl)zirconocene (6a) reacts with tris(pentafluorophenyl)borane to yield the carbon-carbon coupled Cp(2)M(mu-RC(4)R)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, SiMe(3)). 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 H-1 NMR spectroscopy, with Delta G double dagger 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.