On the Mechanism and Stereoselectivity of the Copper(I)-Catalyzed Cyclopropanation of Olefins − A Combined Experimental and Density Functional Study

Abstract

The mechanism of copper(I)-catalyzed olefin cyclopropanation with diazomethanes has been studied at the BP86/AE1 level of density-functional theory. For the model system Cu(diazabutadiene)+ + ethene + diazomethane, copper carbene complexes are confirmed as viable intermediates, with rate-determining barriers of the order of 25 kcal/mol (energies including zero-point corrections) or 14 kcal/mol (when entropic contributions are included). For another model system, Cu(2,9-dimethyl-1,10-phenanthroline)+ + styrene + diazoacetate, very small anti/syn selectivities (resulting in trans/cis-cyclopropanes) have been found both computationally and experimentally. Cu(carbene) complexes with macrocyclic phenanthroline-based ligands 1 (aryl bridgeheads and ether linkages) and 2 (calix[6]arene) have been optimized at the BP86/SDD level. A qualitative explanation for the trans selectivity observed with 1, based on the tilted, cleft-like conformation of 1·(CuCHCO2Me)+, is put forward. Similar conformations are found in structures of related acyclic mono- and diarylphenanthrolines (either free or complexed with Cu2+ or Pd2+), which have been determined by X-ray crystallography. The observed cis selectivity of 2 is probably related to the fact that in 2·(CuCHCO2Me)+ the calixarene macrocycle effectively blocks one hemisphere of the catalyst.