Methylidyne Transfer as a Plausible Deactivation Pathway for Ynene Metathesis
Takashi Kurogi, Daniel J. Mindiola
Abstract
Niobium methylidyne [(PNP)Nb≡CH(OAr)] (1) (PNP– = N[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3) reacts with excess ethylene to afford the propenyl-ethylene complex [(PNP)Nb(HC═CHCH3)(η2-H2C═CH2)(OAr)] (2), which, upon gentle heating, extrudes ethylene to yield a low-spin Nb(III) allyl complex [(PNP)Nb(η3-H2CCHCH2)(OAr)] (3). Isotopic labeling studies using [(PNP)Nb≡13CH(OAr)] (1-13C) have allowed us to not only observe and propose the formation of various intermediates in the conversion of 1 to 2 and then 3 but also demonstrate scrambling of the CH2 groups to only the terminal positions of the allyl moiety. The homologation of ethylene to a propenyl or isomeric allylic group might help explain why complex 1 is an inefficient catalyst for the ring-opening expansion polymerization of 2-norbornene to cyclic polynorbornene.