UNIMOLECULAR DISSOCIATIONS OF KETONE/FE+ COMPLEXES AND EVIDENCE FOR SUCCESSIVE CH/CC BOND ACTIVATION OF DIFFERENT SITES OF FLEXIBLE MOLECULES

Extensive labeling studies in conjunction with tandem mass spectrometry provide a detailed picture on the unimolecular gas-phase ion chemistry of several unbranched ketone/Fe+ complexes. All processes can be described in terms of the “remote functionalization” concept. From the investigation of isotopically labeled 4-heptanone/Fe+ complexes, intramolecular kinetic isotope effects are determined which demonstrate that the oxidative addition of the (complexed) Fe+ to a CH bond is not associated with a kinetic isotope effect. In contrast, for the generation of molecular hydrogen it is the reductive elimination step that is subject to a sizable isotope effect; the β-hydrogen transfer, however, shows only a minor effect. For ethylene loss from 4-heptanone/Fe+ both the olefin detachment and the β-CC cleavage are associated with secondary isotope effects of comparable size. The study of the Fe+ complexes of 5-nonanone (6a), 6-undecanone (7a), 4-octanone (8a), and 4-nonanone (9a) establishes for the first time the operation of successive CH/CC bond activation of different alkyl chains of flexible ketones. Evidence is presented that the neutral molecules with Δm = 30 and Δm = 44 do not correspond to authentic alkanes (i.e., ethane and propane); rather, the mass differences Δm are due to the consecutive elimination of CnH2n and H2 from the Fe+ complexes. The reverse sequence does not contribute to the overall reaction. The labeling data clearly establish that the olefin originates from the terminal part of one alkyl chain; molecular hydrogen is subsequently generated from the [M-CnH2n]+ intermediates by regiospecific CH-bond activation of the other alkyl chain. The trajectory of insertion of Fe+ into the CH bond shows a strong preference for the generation of eight-membered metallacycles. Evidence is also presented that the metallacycle generated by ethylene loss from the ω/(ω-1) positions of the n-pentyl group of 4-nonanone (9a), prior to further reaction (loss of molecular hydrogen), rapidly interconverts with the acyclic form of an Fe+-complexed 4-heptanone, thus making the two alkyl chains indistinguishable. © 1990, American Chemical Society. All rights reserved.