Reaction of Gas-Phase Iron(I) with Alkynes and Dienes: Mechanism of Rearrangement after Insertion into Propargylic or Allylic Carbon-Carbon Bonds

The structures of FeC_nH_2n-2⁺species formed by reaction of FeCO⁺with alkynes (n = 3–8) and dienes (n = 3–6, 8) in a high-pressure source of a tandem mass spectrometer were determined by collisionally activating the adducts and examining the spectra of product ions. The alkyne complexes rearrange by insertion of Fe⁺into the propargylic C-C bond followed by abstraction of a β-H atom from the resulting alkyl fragment. New insight into the details of the H atom transfer process is provided by determining the structures of ions formed by reaction of Fe⁺with an isomeric set of octynes and octadienes. In contrast to the mechanism previously proposed for rearrangement of Fe(olefin)⁺, the H atom is transferred primarily to C₃of the propargyl fragment to produce a (1,2-diene)Fe(1-alkene)⁺complex. For diene adducts, transfer of a H atom occurs to C₁of the π-allyl moiety to produce a smaller diene ligand. The structures of product ions were determined by comparing the collisionally activated decomposition (CAD) spectra of these ions with those of reference alkyne and diene complexes.