Fundamental studies of reactive organic intermediates including m-benzyne by using Fourier-transform ion cyclotron resonance mass spectrometry

摘要

Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry is a powerful tool that provides a means to study reactive intermediates that are not amenable to easy examination by other means. Neutral reaction intermediates may be studied by utilizing an approach wherein an inert, charged substituent is attached to a molecule containing the reactive moiety of interest. The first three studies use this approach to examine the intrinsic reactivity of m-benzyne. The first study involves two positively-charged chloro-substituted m-benzyne isomers and demonstrates that chloro substitution increases the rate of nucleophilic addition to the benzyne moiety. The relative energy of the two isomers is also measured and shown to agree qualitatively with predictions made by ab initio calculations. The second study involves the synthesis of a negatively-charged m-benzyne. The net effect of the carboxylate charge site is to bias the benzyne towards addition reactions with electrophiles rather than nucleophiles. Finally, the third study involves a hydroxy-substituted m-benzyne contained in a protonated quinoline ring system. This m-benzyne is shown to abstract hydrogen atoms more efficiently than any other m-benzyne studied by this distonic ion approach. The fourth ion-molecule reaction study involves the three dehydropyridinium isomers. Two of these isomers possess ionized carbene resonance structures while the other does not. The effect of the ionized carbene resonance structure is demonstrated by the addition of certain reagents, including tetrahydrofuran, to the radical site. Finally, the unusually facile hydrogen atom abstraction reactions of cytosine radical cation are explored. The properties of a radical cation that are conducive to such reactivity are examined and outlined. It is shown that the rate of hydrogen atom abstraction by a radical cation may be used to bracket its recombination energy.