The Loss of Carbon Dioxide from Activated Perbenzoate Anions in the Gas Phase:  Unimolecular Rearrangement via Epoxidation of the Benzene Ring

Abstract

The unimolecular reactivities of a range of perbenzoate anions (X-C6H5CO3-), including the perbenzoate anion itself (X = H), nitroperbenzoates (X = para-, meta-, ortho-NO2), and methoxyperbenzoates (X = para-, meta-OCH3) were investigated in the gas phase by electrospray ionization tandem mass spectrometry. The collision-induced dissociation mass spectra of these compounds reveal product ions consistent with a major loss of carbon dioxide requiring unimolecular rearrangement of the perbenzoate anion prior to fragmentation. Isotopic labeling of the perbenzoate anion supports rearrangement via an initial nucleophilic aromatic substitution at the ortho carbon of the benzene ring, while data from substituted perbenzoates indicate that nucleophilic attack at the ipso carbon can be induced in the presence of electron-withdrawing moieties at the ortho and para positions. Electronic structure calculations carried out at the B3LYP/6-311++G(d,p) level of theory reveal two competing reaction pathways for decarboxylation of perbenzoate anions via initial nucleophilic substitution at the ortho and ipso positions, respectively. Somewhat surprisingly, however, the computational data indicate that the reaction proceeds in both instances via epoxidation of the benzene ring with decarboxylation resulting--at least initially--in the formation of oxepin or benzene oxide anions rather than the energetically favored phenoxide anion. As such, this novel rearrangement of perbenzoate anions provides an intriguing new pathway for epoxidation of the usually inert benzene ring.

Nucleophilic Aromatic Substitution

“…They also indicate that epoxides may be intermediates en route to the phenoxide products. 20 There has also been a theoretical study of the formation of trichlorinated dibenzo-p-dioxins by reaction of 2,4,5-trichlorophenolate ions with 2,4-dichlorophenol. 21…”

Ortho‐hydroxyl effect and proton transfer via ion–neutral complex: the fragmentation study of protonated imine resveratrol analogues in mass spectrometry

“…The substituted groups at different sites may also lead to significant differences in the unimolecular reactions . It is well known that the odd‐electron ions of ortho ‐compounds can be distinguished from those of related meta ‐ or para ‐isomers in electron impact ionization mass spectra .…”

“…The substituted groups at different sites may also lead to significant differences in the unimolecular reactions. [50][51][52][53][54][55][56][57][58][59][60][61][62] It is well known that the odd-electron ions of ortho-compounds can be distinguished from those of related meta-or para-isomers in electron impact ionization mass spectra. [50,55,57,58,60] Gas-phase unimolecular reactions of ortho-substitued compounds in ESI mass spectrometry have been recently studied, and they were found to give unique CID mass spectra as compared with those of the corresponding meta-or para-isomers.…”

“…[50,55,57,58,60] Gas-phase unimolecular reactions of ortho-substitued compounds in ESI mass spectrometry have been recently studied, and they were found to give unique CID mass spectra as compared with those of the corresponding meta-or para-isomers. [51][52][53][54][55][56]62] Imine resveratrol analogues are fundamental structures that contain a carbonnitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, and the presence of the hydroxyl group on the aryl group is very important for their radical scavenging and biochemistry activities. [63][64][65][66][67][68][69][70][71] Insights into unimolecule dissociation and ortho-effect are indispensable for illustrating the detailed mechanisms of gas-phase chemistry.…”

Negative charge induced dissociation: fragmentation of deprotonated N‐benzylidene‐2‐hydroxylanilines in electrospray ionization mass spectrometry

“…Various chromatographic methods in conjunction with tandem mass spectrometry are typically used to detect and quantify these imines . Although numerous studies have primarily focused on the mass spectrometry of cationic adducts derived from protonation or metal cation ionization, some of the investigations have initiated the use of anions generated under negative‐ion conditions . The hydroxyl group on the benzene ring of the imines gives birth to additional deprotonated ion in mass spectrometer, leading fragmentation to different pathways, and provides better understanding of the structural information of these compounds accordingly.…”