Hydration/elimination reactions of trapped protonated fluoroalkyl triazines

Abstract

Hydration/elimination reactions of trapped protonated fluoroalkyl triazinesFluorinated heterocycles represent a class of interesting compounds that find their applications in several fields, such as pharmaceutical chemistry, agrochemistry, and biology. 1,2 In the last decades different synthetic approaches have been set up for obtaining new interesting compounds. 3 -8 Among methods that allow to study heterocyclic compounds, mass spectrometry (MS), and MS n play a relevant role. In fact, by using different MS techniques, it is possible to study not only their structural features, but also reactivity, thermochemistry, kinetics, and many others properties of gas-phase reactions occurring inside a mass spectrometer.By continuing our research on gas-phase ion chemistry of organic molecules, 9 we wish to report here on the positive electrospray ionization (ESI) MS n n D 1-3 spectra, obtained inside an ion trap, of fluoroalkyl triazines 1-3 (Scheme 1) whose synthesis has been published recently. 10 ESI measurements have been carried out on a LCQ-DECA (ThermoFinnigan, San Jose, CA). Water, water/methanol (50% v/v) or deuterium dioxide have been used as solvents (see text). Operating conditions for the ESI source were as follows: spray voltage, 4.5 kV; capillary temperature 200°C; sheath gas (nitrogen) flow rate, ca. 0.75 l/min. Ultra pure helium He was the collision gas. CID collision energy: 0.5-1.0 eV (laboratory frame). The automatic gain control with a target value of 2 ð 10 7 in MS/MS measurements was used. The ESI mass spectra of compounds 1-3 are characterized by protonated molecules, sodium adducts and no fragment ions, while the MS 2 spectra show the presence of puzzling ions that differ 2, 22, 24 u from their precursors, and clusters of ions that differ 2 u with each other.When the protonated molecules of 1 (m/z 197) are selected as precursor ions and submitted to low-energy collision-induced dissociations (CIDs) inside the ion trap, the MS/MS spectrum reported in Fig. 1 (top) is obtained. Quite surprisingly, ions at m/z 195, differing 2 u from the precursor ion, are detected with high abundance. By considering the low number of hydrogen atoms in this molecule, the direct loss of H 2 from [M C H] C seems difficult to occur. Other groups of ions differing 2 u with each other are also present at m/z 173, 175, 177 and at m/z 155, 157 (Fig. 1, top). Among them, ionic species at m/z 173, formally due to loss of 24 u from [M C H] C , are the most intense in the MS/MS spectrum.To get light on the origin and structure of these ions, and in particular of those at m/z 195, different MS 3 experiments have been carried out. When ionic species at m/z 195 are selected as precursor ions, their MS 3 spectrum (Fig. 1, bottom) shows the formation of ions at m/z 173 and 175, already observed in the MS/MS spectrum of ions at m/z 197. While it is well explainable the loss of 20 u, due to elimination of HF, yielding ions at m/z 175, it is difficult to rationalize an eventual loss of 22 u that should yield ions at m/z 173.

Effect of protonation and deprotonation on the gas-phase reactivity of fluorinated 1,2,4-triazines

“…The presence of puzzling ions that differ 2, 22, and 24 u from their precursors and that of clusters of ions that differ 2 u from each other [14] is noteworthy. As an example, the product ion MS/MS spectrum of .…”

“…As an example, the product ion MS/MS spectrum of . This behavior can be explained by the occurrence of hydration/elimination reactions of trapped protonated molecules, mainly consisting of successive eliminations of HF followed by nucleophilic addition of water present in the ion trap [14].…”

Gas phase behavior of radical cations of perfluoroalkyl‐1,2,4‐triazines: an experimental and theoretical study

“…It is noteworthy that a species at m/z 219 is also formed by the protonated molecules of 2 under ESI-MS/MS conditions and it is due to consecutive losses of water and HF from the protonated molecule. [20,21] This pathway has been attributed to the proximity effect between the exocyclic H-(N) and the pendant phenyl group, which favours the formation of a fused tricyclic system. [21] Owing to electron ionization and low-energy CID, the formation of the ions at m/z 219 from the species [ …”

“…[19] In particular, compounds 1-4 (Scheme 1) are particularly suitable and interesting for studying their gasphase ion chemistry: they are sufficiently polar to be protonated or deprotonated by electrospray ionization [20] and show an interesting reactivity due to gas-phase hydration/elimination reactions. [21] The decomposition pathways of their protonated molecules consist mainly in the elimination of neutral molecules, such as H 2 O, CO and HF, while the ESI spectra of deprotonated species are dominated by radical anions. [20,21] On the other hand, compounds 1-4 are sufficiently volatile to be ionized by electron ionization.…”

“…[21] The decomposition pathways of their protonated molecules consist mainly in the elimination of neutral molecules, such as H 2 O, CO and HF, while the ESI spectra of deprotonated species are dominated by radical anions. [20,21] On the other hand, compounds 1-4 are sufficiently volatile to be ionized by electron ionization.…”