The activation of η 5-pyrrole complexes toward nucleophilic attack

Abstract

Pyrrole is a five-membered aromatic heterocycle in which the nitrogen lone pair is delocalized over the π system of the ring. As a result of this π electron rich character, it reacts readily with electrophiles, but is not susceptible to nucleophilic attack. The η 5-coordination of pyrrole or of the pyrrolyl anion to certain transition metal fragments activates the heterocycle toward nucleophilic substitution or addition reactions. In the pyrrolyl complexes (C 4H 4N)Re(PPh 3) 2(H)(I) and (C 4H 4N)Ru(PEt 3) 2Cl, nucleophilic substitution reactions at the 2-position of the ring follow a pathway that involves hydrogen transfer from the ring to the metal ion with displacement of a labile halide ligand from the metal center. Nucleophilic addition of hydride and methoxide anions to the neutral pentamethylpyrrole ligands in [(MeNC 4Me 4)M(cymene)](OTf) 2, M=Ru and Os, has been found to occur at the α carbon atom of the heterocycle. However, reactions of other pyrrole sandwich complexes with nucleophiles may display competing reaction sites.