Catalyst-Controlled C-C sigma Bond Cleavages in Metal Halide-Catalyzed Cycloisomerization of 3-Acylcyclopropenes via a Formal 1,1-Halometalation Mechanism: Insights from Quantum Chemical Calculations

摘要

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 Cul 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 sp(2) 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 pi-affinity of PdCl2 enables its strong coordination with the C-1=C-2 double bond in the TS, and the opening of the more substituted C-1-C-3 single bond is favored. On the other hand, the harder Lewis acid Cu! is more sensitive to the steric effect and the opening of the less substituted C-2-C-3 single bond thus becomes predominant.