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Catalyst-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

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journal contribution
posted on 06.02.2015, 00:00 by Genping Huang, Yuanzhi Xia
The 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 C1C2 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.