Mass Spectrometry of Di- Tert -Butylnaphthalenes and Benzyl- Tert -Butylnaphthalenes: Hydrogen Migration and Intermediate Ion—Molecule Complexes in Tert -Butylated Naphthalene Ions

The mass spectrometric fragmentations of isomeric di-tert-butylnaphthalenes 1–6 and benzyl-tert-butylnaphthalenes 8 and 9 have been studied using deuterated derivatives and the methods of tandem mass spectrometry. The molecular ions of 1–6, 8 and 9 decompose almost exclusively by the loss of a methyl radical. The unimolecular fragmentations of the resulting substituted 2-naphthyl-2-propyl cations 1a–6a, 8a and 9a show analogies to the fragmentation of unsubstituted 2-naphthyl-2-propyl cations but exhibit an interesting dependence of the reactions on the relative orientation of the tert-butyl or the benzyl substituent. Thus, the losses of ethene and of a methyl radical are observed as for the unsubstituted 2-naphthyl-2-propyl cation ions by rearrangements of the ionized propyl side chain. However, in the case of tert-butyl substituted 2-naphthyl-2-propyl cations 1a–6a, the loss of methyl increases distinctly if a naphthoquinodimethane radical cation can be formed by loss of methyl from the tert-butyl substituent. This is proven by D-labeling and indicates a particular stability of the quinoid radical cations. Further, the mass-analyzed ion kinetic energy spectra of the cations 1a–6a, 8 and 9 are characterized by intense signals owing to the tert-butyl cation or benzyl cation. The exception is metastable 4a which expels almost exclusively propene. This latter fragmentation is attributed to reactive interactions between the two side chains in a crowded 2,3-position at the naphthalene ring of 4a. The release of cations corresponding to the tert-butyl or benzyl substituent from metastable substituted 2-naphthyl-2-propyl cations occurs by proton transfer from the ionized propyl side chain to the naphthalene ring, followed by protolytic cleavage of the substituent and involves the formation of intermediate ion–molecule complexes. This is shown in the case of tert-butylated ions 1a–6a by proton transfer from the tert-butyl cation to the naphthalene moiety and subsequent elimination of isobutene. In the case of the benzylated ions 8a and 9a, the benzyl cation within the complex abstracts a hydride and dissociates as a toluene molecule. Independent proof of intermediate ion–molecule complexes, consisting of a tert-butyl or a benzyl cation and a propenylnaphthalene molecule, comes from the observation of unexpected fragmentations which require addition of the carbenium ion within the complex to the double bond of the propenyl side chain. Both for the tert-butyl cation and for the benzyl cation, the abundance of these intermolecular reactions exhibits a hitherto unknown dependence on the orientation of the two side chains and are most intense in the case of a 1,4-orientation. A possible explanation is a dependence of the excess energy and life time of the intermediate ion–molecule complex on the structure of the precursor.