13 - Transition Metal-Mediated Alkenylations, Arylations, and Alkynylations

Abstract

As a rule, halogens, OH groups, and all other O substituents that are attached to a sp2-hybridized C atom of an alkene or an aromatic compound cannot be substituted by nucleophiles alone. One exception has already been discussed, and that was the addition/elimination mechanism of the nucleophilic substitution. This mechanism occurs, among other substrates, with alkenes that carry a strong electron acceptor on the neighboring C atom. The same mechanism is known to also operate for aromatic compounds—under the condition, again, that they carry an acceptor substituent at an appropriate position. The benzyne mechanism of nucleophilic substitution, that is, an elimination/addition mechanism, presents a second mode of nucleophilic substitution of a halogen at a sp2-hybridized C atom. Halogens or O-bound leaving groups, however, can also be detached from the sp2-hybridized C atom of an alkene or an aromatic compound, if organometallic compounds act as nucleophiles or if a transition metal is present in at least catalytic amounts. The most important substrates for substitutions of this type are alkenyl and aryl triflates, bromides, or iodides. The most important organometallic compounds to be introduced into the substrates contain Cu, Mg, B, or Zn.