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

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.