title: On the mechanism of the copper-catalyzed cyclopropanation reaction.
abstract: The selectivity-determining step in enantioselective copper-catalyzed cyclopropanation with diazo compounds has been studied by experimental and computational methods. The addition of the very reactive metallacarbene intermediate in an early transition state to the substrate alkene is concerted but strongly asynchronous, with substantial cationic character on one alkene carbon in the neighborhood of the transition state. Evidence from isotope effects and Hammett studies supports the nature of the transition state. Formation of a metallacyclobutane intermediate by a [2+2] addition is kinetically disfavored. Ligand-substrate interactions influencing the enantio- and diastereoselectivity have been identified, and the preferred orientation of the alkene substrate during the addition is suggested.
authors.affiliation: Department of Chemistry, Technical University of Denmark, Lyngby.
authors.forename: Torben
authors.initials: T
authors.lastname: Rasmussen
authors.forename: Jakob F
authors.initials: JF
authors.lastname: Jensen
authors.forename: Niels
authors.initials: N
authors.lastname: Ostergaard
authors.forename: David
authors.initials: D
authors.lastname: Tanner
authors.forename: Tom
authors.initials: T
authors.lastname: Ziegler
authors.forename: Per-Ola
authors.initials: PO
authors.lastname: Norrby