On the mechanism and stereoselectivity of the copper(I)-catalyzed cyclopropanation of olefins - A combined experimental and density functional study

The mechanism of copper(I)-catalyzed olefin cyclopropanation with diazometh anes has been studied at the BP86/AE1 level of density-functional theory. F or the model system Cu(diazabutadiene)(+) + ethene + diazomethane, copper c arbene complexes are confirmed as viable intermediates, with rate-determini ng barriers of the order of 25 kcal/mol (energies including zero-point corr ections) or 14 kcal/mol (when entropic contributions are included). For ano ther model system, Cu(2,9- dimethyl-1,10-phenanthroline)(+) + styrene + dia zoacetate, very small anti/syn selectivities (resulting in trans/cis-cyclop ropanes) have been found both computationally and experimentally, Cu(carben e) complexes with macrocyclic phenanthroline-based ligands 1 (aryl bridgehe ads and ether linkages) and 2 (calix(6]arene) have been optimized at the BP 86/SDD level. A qualitative explanation for the trans selectivity observed with I, based on the tilted, cleft-like conformation of 1 . (CuCHCO2Me)(+), is put forward. Similar conformations are found in structures of related a cyclic mono- and diarylphenanthrolines (either free or complexed with Cu2or Pd2+), which have been determined by X-ray crystallography. The observed cis selectivity of 2 is probably related to the fact that in 2 . (CuCHCO2M e)(+) the calixarene macrocycle effectively blocks one hemisphere of the ca talyst.