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DFT Mechanistic Study of Functionalizations of ω‑Ene-Cyclopropanes and Alkylidenecyclopropanes via Allylic C–H and C–C Bond Cleavage Facilitated by a Zirconocene Complex

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posted on 09.11.2015, 00:00 by Chunyu Song, Yanfeng Dang, Yuan Tao, Zhi-Xiang Wang
A DFT mechanistic study has been performed to understand the [Zr]­C4H8 (Zr = ZrCp2)-mediated transformations of ω-ene-cyclopropane (ω-ene-CP) and alkylidenecyclopropane (ACP) to acyclic compounds. The transformations proceed via allylic C–H bond activation, hydride transfer, C–C bond cleavage of the three-membered ring, and additions of electrophiles. The energetic results indicate that, among the possible pathways, the one leading to the experimental products is most energetically favorable, rationalizing the selectivity of the reactions. The Zr-walk takes place via allylic C–H bond activation followed by hydride transfer, completing a 1,3-hydrogen transfer. In comparison, the Pd-walk involved in the Pd-catalyzed Heck-type relay coupling reactions proceeds via migratory insertion followed by β-H elimination, resulting in a 1,2-hydrogen transfer. The difference is due to the fact that the [Zr] active species does not have a Zr–H or Zr–C bond for CC bond migratory insertion, while the Pd–H or Pd–C bond in [Pd] active intermediates enables such an insertion. In addition, the preference of Pd­(II) over Pd­(IV) disfavors the allylic C–H bond activation involved in the Zr-walk process. We further explored if the three-membered ring in the ω-ene-CP and ACP could be enlarged to four- or five-membered rings for similar transformations. The energetic results indicate that it is promising to enlarge a three-membered to a four-membered ring, but the extension to a five-membered ring is inferior because of the endergonic ring-opening with somewhat high barrier.