Fragmentation patterns in the electron impact mass spectra of 1,3,5-triazin-2-one derivatives

Fragmentation patterns in the electron impact (EI) mass spectra of a series of 5-substituted 1,3,5-triazin-2-ones, prepared from the reaction of dimethylolurea with selected primary amines, have been elucidated using a combination of high-resolution, comparative low-resolution and metastable peak analysis. In addition to characteristic fragmentations, in which the triazine nucleus remains intact, the 5-substituted 1,3,5-triazin-2-ones appear to exhibit a series of extrusion/ring-contraction processes, resulting in the formation of 3-and 4-membered ring fragments. Resonance-stabilised even-electron ions, arising, in each case, from loss of a hydrogen atom from the molecular ion, are considered pivotal in the formation of these heterocyclic fragments.


Introduction
The emission of formaldehyde from urea-formaldehyde (UF) resins 1 may be reduced by decreasing the formaldehyde content 2 and adding cross-linking agents such as ammonia or melamine during the resinification process.The inclusion of ammonia has been found to result in the formation of triazine-urea-formaldehyde resins, the presence of three reactive sites in the triazine nucleus permitting cross-linking. 3 While the mass spectra of 1,3,5-triazines have been investigated, 4 to our knowledge, no such studies of 1,3,5-triazin-2-ones have been reported.As part of a study of UF resins, 5 we have prepared a series of 5-substituted 1,3,5-triazin-2-ones and, here, we discuss the electron impact (EI) mass spectra of the latter systems.

Results and Discussion
The triazinones 3-8 were prepared, following Burke's method, 6 from dimethylolurea 1 and the corresponding primary amines 2 (Scheme 1).A combination of high-resolution, comparative low-resolution and metastable peak data was used to explore the fragmentation of these compounds.Selected MS data are summarised in Table 1 and the proposed fragmentation pathways are outlined in Schemes 2 and 3.In addition to the foregoing fragmentations, in which the triazine nucleus remains intact, the 5-substituted 1,3,5-triazin-2-ones 3-8 appear to exhibit a series of extrusion/ring-contraction processes, resulting in the formation of 3-and 4-membered ring fragments.The resonancestabilised cations B are considered pivotal in the formation of these heterocyclic fragments (E-I).
Thus, elimination of a neutral imine (R2C=NR) from the cation B would afford, in each case, the common, diazolium ion E (m/z 85) (see Figure 1a), dehydrogenation of which gives cation F (m/z 83).On the other hand, extrusion of the neutral molecules, CH2=NH or HN=C=O, from cations of type B would afford fragments which retain the 5-alkyl substituent and which have been tentatively formulated as the diazetinones G and their deoxo analogues H, respectively.
Access to these latter ions (G and H) is attributed to the similar, but fragment-specific eliminations depicted in Figures 1b and 1c; while there is metastable peak support for the fragmentation, BG, there is similar evidence, in some cases, for the loss of CH3NH$ from the molecular ion, i.e.AG.Elimination of a moiety containing the 5-substituent (Figure 1d) would Accurate masses (m/z) are followed, in parentheses, by calculated formula masses; an asterisk indicates a pathway supported by metastable peak data.
The 5-benzyl derivative 8, not surprisingly, exhibits a number of additional, characteristic fragmentations, all of which are supported by high-resolution data (Scheme 3).126-129 o C)] was prepared following a reported method. 7The synthesis of the 1,3,5triazone derivatives, of which compounds 3 8 and 7 [158-159 o C (lit. 6 158 o C)] are known, is illustrated by the following example.

a
Scheme 1 Scheme 2