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

摘要

The mechanism of copper(I)-catalyzed olefin cyclopropan- ation with diazomethanes has been studied at the BP86/ AE 1 level of density-functional theory. For the model system Cu(diazabutadiene)+ + ethene + diazomethane, copper car- bene complexes are confirmed as viable intermediates, with rate-determining barriers of the order of 25 kcal/mol (ener- gies including zero-point corrections) or 14 kcal/mol (when entropic contributions are included). For another model sys- tem' Cu(2,9-dimethyl-1,10-phenanthroline)+ + styrene + di- azoacetate, very small anti/syn selectivities (resulting in trans/cis-cyclopropanes) have been found both computa- tionally and experimentally. Cu(carbene) complexes with macrocyclic phenanthroline-based ligands 1 (aryl bridge- heads 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 lo(CuCHCO2Me)+, is put forward. Similar conformations are found in structures of related acyclic mono- and diarylphenanthrolines (either free or com- plexed 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 2o(CuCHCO2Me)+ the calixarene macrocycle effectively blocks one hemisphere of the catalyst.