Ion–dipole complexes in the unimolecular reactions of isolated organic ions. Fragmentation of protonated and cationated formate and acetate esters

Abstract

. Can. J. Chem. 66, 2403Chem. 66, (1988. The unimolecular fragmentation reactions occurring on the metastable ion time scale have been determined for protonated alkyl formates and acetates and for the adducts of CD3+ and C2H5+ with these esters. The protonated esters, R1C02RH+, fragment to form R+ + R1C02H or, by internal hydrogen transfer, R1C02H2+ + (R -H) with the relative yields depending on the identity of R' and R. Using deuterium labelling it has been shown that the added proton interchanges with hydrogens of the R group prior to fragmentation. The methyl-d3-cationated esters, R ' C O~R . C D~+ , mostly fragment to give R ' C O~C D~, H+ by elimination of (R -H), although R+ formation is observed in a few cases. The ethyl-cationated esters fragment primarily to give R ' C O~C~H S . H + by elimination of (R -H); specifically no elimination of C2H4 is observed. All of the results can be rationalized in terms of a step-wise mechanism of fragmentation involving initial formation of an ion-dipole complex of the alkyl ion R+ with a carbonyl compound. This complex may either fragment to form R+ or may proceed by way of an ion-dipole complex of a protonated carbonyl and an olefin to the protonated species plus (R-H). The results indicate that primary alkyl ions are not formed but rather that rearrangement to the more stable secondary or tertiary structure occurs. There is some evidence that s-butyl cations are not formed, but that these too rearrange to the t-butyl structure. Les rCsultats indiquent que les ions alkyle primaire ne se forment pas mais qu'ils se transposent de preference en ions secondaire ou tertiaire plus stable. On a la preuve que les cations s-butyles ne se forment pas mais qu'ils se transposent en cations t-butyle.[Traduit par la revue]

APCI low energy collision-induced dissociation fragmentation of protonated ortho silicates: mclafferty or ion-neutral complex rearrangement?

“…The ionizing deuterium is present on the ion throughout the sequential losses of ethylenes (m/z 182, 154, 126, and 98). Similar results were obtained by McMahon and Harrison [20] in a study of protonated ethyl esters where no exchange between the ionizing deuterium and the alkyl side chain took place when the fragmentation took place via ionneutral complexes.…”

“…Evidence that ion-neutral complexes are involved in the TEOS fragmentation can be drawn from the calculated (assuming complete randomization) versus observed hydrogen interchange data shown in Tables ion-neutral complexes [20]. In addition to side chain isomerization, the alkyl side chain (neutral part of the ion-neutral complex) may undergo an "end-over flip" [23] (where the neutral part of the complex rotates 180°), thus in the case of D 12-TEOS, placing the CD 3 deuteriums closer to the ether oxygen and the transfer of the deuteriums becomes preferentiaL This end-over flip may explain the observed high percentage of deuterated daughter ions in the D 12-TEOS MS/MS spectra (by placing the deuteriums in closer to the ionic part of the complex), yet allow for the hydrogen transfer during the ethylene losses as well if the chain flips an even number of times.…”