Synthesis and investigation of isomerization reactions of terminal iron-alkenyl complexes

Article abstract of DOI:10.1021/om00121a018

The reaction of a variety of anionic nucleophiles (Nuc) with [CpFeCO[P(OPh)₃](η²-CH₂=C=CH ₂)]BF₄ leads to a new family of terminal alkenyl complexes, CpFeCO[P(OPh)](η¹-C(CH₂Nuc)=CH₂) (Nuc = H, Me, Ph, CH=CH₂, C≡CMe, SPh). Dissolving these complexes in CHCl₃ or other solvents in the presence of alumina (or upon alumina chromatography) causes a 1,3-hydrogen shift isomerization reaction that leads for the last four nucleophiles to CpFeCO[P(OPh)₃][η¹-(E)-C(Me)=C(H)Nuc] derivatives. For Nuc = Me, an equilibrium mixture of the terminal alkenyl complex (21%), CpFeCO[P(OPh)₃][η¹-(E)-C(Me)=C(H)Me] (72%), and CpFeCO[P(OPh)₃](η¹-(Z)-C(Me)=C(Me)H) (7%) is established. The (E)-2-butenyl isomer can also be prepared directly by the reaction of [CpFeCO[P(OPh)₃](η²-MeC≡CMe)]BF₄ with Li[HB-(sec-Bu)₃]. Reaction of this same η²-butyne complex with Li[HBEt₃] leads to a mixture of Z (57%) and E (43%) isomers. A similar reaction using Li[DBEt₃] shows that the Z isomer forms by direct trans addition to the alkyne whereas the E isomer forms from addition to the Cp ring followed by internal cis transfer from the ring to the alkyne. Both the pure E isomer and the mixture isomerize in CHCl₃ or upon chromatography to the same equilibrium mixture as the terminal butenyl isomer. A variety of deuterium labeling and protonation studies are reported that indicate that the intermediates in these isomerization reactions are the cationic carbene complexes formed by simple protonation of the alkenyl complexes.