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Download fileCatalyst-Controlled C–C σ Bond Cleavages in Metal Halide-Catalyzed Cycloisomerization of 3‑Acylcyclopropenes via a Formal 1,1-Halometalation Mechanism: Insights from Quantum Chemical Calculations
journal contribution
posted on 2015-02-06, 00:00 authored by Genping Huang, Yuanzhi XiaThe ring-opening cycloisomerization
reactions of cyclopropenyl
ketones developed by S. Ma et al. [J. Am. Chem. Soc. 2003, 125, 12386–12387] provided an efficient
method for the constructions of trisubstituted furans in which an
elegant control of the regiochemistry was achieved by using CuI or
PdCl2 catalyst. In the current report we aimed at uncovering
the origin of the divergent regiochemistry of the reactions with different
metal halide catalysts using quantum chemical calculations. By comparing
the energies of all possible pathways, we found that a novel mechanism
involving a formal 1,1-halometalation is the energetically most favorable
one. In this pathway, an organometallic intermediate is involved from
addition of the metal atom and the halide ligand to the same sp2 carbon of the cyclopropene moiety by sequential 1,5-addition
and 1,5-rearrangement steps, and the furan product is finally formed
via an asynchronous intramolecular substitution/metal halide elimination
process. The initial 1,5-addition was found to be the rate- and regiochemistry-determining
step. The calculations reproduced well the experimentally observed
selectivity. By analyzing the divergence of the Pd(II) and Cu(I) halides
using the distortion/interaction model, it was found that the interaction
energy plays a more important role in determining the selectivity.
The strong π-affinity of PdCl2 enables its strong
coordination with the C1C2 double bond
in the TS, and the opening of the more substituted C1–C3 single bond is favored. On the other hand, the harder Lewis
acid CuI is more sensitive to the steric effect and the opening of
the less substituted C2–C3 single bond
thus becomes predominant.