Intramolecular electrophilic aromatic substitution in gas‐phase‐protonated difunctional compounds containing one or two arylmethyl groups

Abstract

A variety of dibenzyl esters and ethers undergo a rearrangement process upon isobutane chemical ionization and collision-induced dissociation of their MH(+) ions, whereby a new bond is formed between the two benzyl groups, giving rise to abundant [C(14)H(13)](+) (m/z 181) ions. This rearrangement has been explained as an intramolecular electrophilic substitution in the gas phase occurring in an ion-neutral complex formed by the cleavage of one of the benzyl-oxygen bonds. A similar highly efficient intramolecular electrophilic substitution takes place in di-alpha- and beta-naphthylmethyl adipates affording m/z 281 [C(22)H(17)](+) ions, but not in the sterically hindered di-9-anthracylmethyl adipate. An analogous efficient rearrangement occurs in benzyl alpha- and beta-naphthylmethylcyclohexane-1,4-dicarboxylates and in benzyl alpha- and beta-phenylethylcyclohexane-1,4-dimethanol ethers. The analogous rearrangement is much less efficient in benzylallyl, benzylpropargyl and benzyl-9-anthracylmethyl derivatives, even less in benzylisopropyl and benzylacetyl analogs, and it is absent in benzyltetrahydropyranyl derivatives. The distinctive behavior of the protonated difunctional benzyl derivatives is interpreted in terms of the energy requirements of the O-R bond heterolysis of the protonated functionalities, the ability of the neutral R' groups (non-dissociated from the oxygen atom) to play the role of the nucleophile in the intramolecular electrophilic substitution processes and the electrophilicity of the R(+) ions.

Gas-Phase Chemistry of Benzyl Cations in Dissociation ofN-Benzylammonium andN-Benzyliminium Ions Studied by Mass Spectrometry

“…The attack at the phenyl ring has been reported in previous studies [20][21][22]. A new case that INC-mediated electrophilic aromatic substitution reaction between benzyl cation and imidazole derivatives was observed during the fragmentation of N,N'-dibenzylimidazolium cations ( Table 2).…”

“…In other pathways, the benzyl cation can attack the phenyl ring via INC-7 resulting in the formation of product ions including m/z 185, m/z 181, and m/z 95, which depends on the shift of the replaced proton. It is noteworthy that electrophilic substitution by the benzyl cation can occur at different positions of the aromatic ring, so product ions at m/z 185 and 181 are probably mixtures of several isomers, which has been investigated and demonstrated in the studies of other multibenzyl-substituted compounds [20][21][22]. In this study, the isomerized product ions are no longer discussed in detail.…”

“…In electrospray ionization mass spectrometry (ESI-MS), fragmentation of benzylated cations prefers to generate benzyl cations [7,8,[20][21][22][23][24][25][26][27][28][29][30], though a few exceptions have been reported that tropylium ion can be co-produced in the fragmentation of benzylpyridinium ions [18,31,32]. Besides the formation of benzyl cations, other interesting ions resulting from hydride transfer [24,25] or electrophilic aromatic substitution reactions [20][21][22] via the nonconventional ion/neutral complex (INC) can also be generated in the dissociation of benzylated cations.…”

“…Consequently, a need for other examples of INC-mediated reactions is evident in order to better understand the fragmentation reactions in the contemporary ESI mass spectrometry, a method that has become one of the most popular analytical methods. Thus, renewed interest in studies on INC-mediated fragmentation reactions has emerged in recent years [8,[20][21][22][23][24][25][39][40][41][42][43][44][45][46][47].…”

N-Centered Odd-Electron Ions Formation from Collision-Induced Dissociation of Electrospray Ionization Generated Even-Electron Ions: Single Electron Transfer via Ion/Neutral Complex in the Fragmentation of ProtonatedN,N′-Dibenzylpiperazines and ProtonatedN-Benzylpiperazines

“…The well-documented reactions are isomerization, proton transfer and small alkyl carbocation transfer. In very recent years, several novel reactions were uncovered in ESI mass spectrometry, such as electrophilic aromatic substitution [18][19][20], hydride transfer [21,22], transacylation [23], formaldehyde loss [24], and ester group transfer [25].…”