Experimental Modeling of Selective Alkene Oligomerization: Evidence for Facile Metallacyclopentane Dehydrogenation Mediated by a Transannular β-Hydrogen Agostic Interaction

Ring-expansion of the zirconacyclopropane (η⁵-C₅Me₅)Zr[N(i-Pr)C(Me)N(i-Pr)](η²-CH₂CHC₆H₅) (1) was accomplished through insertion of ethene, propene, and styrene into a metal−carbon bond to yield the series of crystalline zirconacyclopentane derivatives 2−4, respectively, for which solid-state molecular structures have been obtained through single-crystal X-ray analyses. The metallacyclopentane rings of 2 and 4 adopt half-chair conformations that place all β-hydrogens distant from the metal center (cf. nonbonded distances of 3.2−4.0 Å). On the other hand, a twisted-envelope conformation for the zirconacyclopentane ring of 3 features a β-hydrogen agostic interaction with the metal center [cf. a Zr1−H21B distance of 2.38(2) Å]. Unlike 2 and 4, compound 3 decomposes rapidly in solution at 25 °C to provide the structurally characterized zirconacyclopent-3-ene derivative 5. Ring-expansion of 1 with 1-(trimethylsilyl)ethene directly provides the zirconacyclopent-3-ene derivative 7. A mechanism for this facile metallacyclopentane dehydrogenative process is proposed that features an intramolecular allylic β-hydrogen abstraction within a 3-butenyl metal hydride intermediate as a key step. It is further proposed that similar metallacyclopentane to metallacyclopent-3-ene dehydrogenations may be more common than previously thought.