CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

The transient intermediate
η2-cyclopropene/bicyclobutane
niobium complex [TpMe2Nb­(η2-c-C3H4)­(MeCCMe)] A, generated by
an intramolecular β-H abstraction of methane from the methyl
cyclopropyl complex [TpMe2NbMe­(c-C3H5)­(MeCCMe)] (1), is able to cleave
the CH bond of a variety of unsaturated hydrocarbons RH in a selective
manner to give the corresponding hydrocarbyl complexes [TpMe2NbR­(c-C3H5)­(MeCCMe)] (R =
2-furyl, 2-thienyl, 1-alkynyl, 1-cyclopentenyl, 1-ferrocenyl (Fc),
pentafluorophenyl). The activation of the C–H bond occurs stereospecifically
via a 1,3-CH addition across the Nb­(η2-cyclopropene)
bond of A. Full characterization of several of these
complexes includes multinuclear NMR spectroscopy, X-ray diffraction,
UV/vis spectroscopy, and electrochemical data. A charge transfer between
the ferrocenyl moiety and the niobium center is responsible for the
characteristic purple color of the bimetallic complex [TpMe2NbFc­(c-C3H5)­(MeCCMe)]. The
reactivity of these complexes with benzene follows qualitatively the
strength and the pKa of the CH bond that
is cleaved. The pentafluorophenyl complex [TpMe2Nb­(C6F5)­(c-C3H5)­(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling
to give two isomeric η4-butadienyl complexes, with
[TpMe2Nb­(C6F5)­(η4-CMeCMeCHCHMe)] as the major isomer.