Mass spectrometry of di-tert-butyl naphthalenes 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 deut erated derivatives and the methods of tandem mass spectrometry. The molecul ar ions of 1-6, 8 and 9 decompose almost exclusively by the loss of a methy l radical. The unimolecullar fragmentations of the resulting substituted 2- naphthyl-2-propyl cations 1a-6a, 8a and 9a show analogies to the fragmentat ion 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 radic al 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 t ert-butyl substituted 2-naphthyl-2-propyl cations 1a-6a, the loss of methyl increases distinctly if a naphthoquinodimethane radical cation can be form ed by loss of methyl from the tert-butyl substituent, This is proven by D-l abeling and indicates a particular stability of the quinoid radical cations . Further, the mass-analyzed ion kinetic energy spectra of the cations 1a-6 a, 8 and 9 are characterized by intense signals owing to the tert-butyl cat ion or benzyl cation. The exception is metastable 4a which expels almost ex clusively propene, This latter fragmentation is attributed to reactive inte ractions between the two side chains in a crowded 2,3-position at the napht halene ring of 4a, The release of cations corresponding to the tert-butyl o r benzyl substitueut from metastable substituted 2-naphthyl-2-propyl cation s occurs by proton transfer from the ionized propyl side chain to the napht halene ring, followed by protolytic cleavage of the substituent and involve s the formation of intermediate ion-molecule complexes. This is shown in th e case of tert-butylated ions 1a-6a by proton transfer from the tert-butyl cation to the naphthalene moiety and subsequent elimination of isobutene, I n the case of the benzylated ions 8a and 9a, the benzyl cation within the c omplex abstracts a hydride and dissociates as a toluene molecule. Independe nt proof of intermediate ion-molecule complexes, consisting of a tert-butyl or a benzyl cation and a propenylnaphthalene molecule, comes from the obse rvation of unexpected fragmentations which require addition of the carbeniu m ion within the complex to the double bond of the propenyl side chain. Bot h for the tert-butyl cation and for the benzyl cation, the abundance of the se intermolecular reactions exhibits a hitherto unknown dependence on the o rientation 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 a nd life time of the intermediate ion-molecule complex on the structure of t he precursor.