Competitive reactions and diastereoselective Csingle bondH bond activation in the McLafferty rearrangement of photoionized 3-methyl valeramide
Résumé
Dissociative photoionization of 3-methyl valeramide is characterized by various degradations of the alkyl backbone, initiated by competitive intramolecular hydrogen migrations. The dominating pathway corresponds to butene elimination via the McLafferty rearrangement. At photon energies (Ehν) close to the ionization threshold, the McLafferty rearrangement is followed by a second hydrogen transfer, known as [McLafferty + 1] reaction. The methyl group at C(3) in combination with diastereospecific labeling at C(4) permits steric differentiation of the two γ-H(D)-atoms at C(4) according to the relative orientations of the stereogenic centers. Investigation of the syn- and anti-[4-D1]-diastereomers shows a strong preference for activation of the anti-γ-hydrogen in the McLafferty rearrangement. A straightforward analysis of the product distribution is impossible, because also C(4′) allows for a [1,5]-H shift, and the contributions of both sites are additionally superimposed by [McLafferty + 1] products. Photoionization studies of six isotopomers, employing tunable synchrotron radiation, combined with kinetic modeling enable a deconvolution of the branching ratios and a determination of the corresponding steric and kinetic isotope effects operative in the McLafferty rearrangement. The kinetic isotope effects (KIEs) are more or less independent of Ehν. The initiating [1,5]-H shifts feature very low KIEs, especially for the C(4)single bondH bond activation, whereas the subsequent hydrogen atom transfers in the course of the [McLafferty + 1] processes are affected by substantial KIEs. Interestingly, the steric effect (SE) decreases considerably at low Ehν (SE = 1.8, 2.6, and 2.8 at Ehν = 9.6, 10, and 11 eV, respectively), which can be explained by more pronounced epimerization prior to dissociation at lower energies.