Dissoziative Ringöffnung halogensubstituierter Methylcyclopropan-Radikalkationen in der Gasphase

The mechanism of alkene elimination from protonated toluenesulphonamides generated by electrospray ionisation

The positive ion electrospray mass spectra of a range of sulphonamides of general structure CH₃C₆H₄SO₂NHR₁ [R₁ = C_nH_2n⁺¹ (n = 1-7), C_nH_2n-1 (n = 3, 4), C₆H₅, C₆H₅CH₂ and C₆H₅CH(CH₃)] and CH₃C₆H₄SO₂NR₁R² [R¹, R² = C_nH_2n+1 (_n = 1-8)] are reported and discussed. The protonated sulphonamides derived from saturated primary and secondary aliphatic amines generally fragment to only a limited extent unless energised by collision. Two general fragmentations are observed: firstly, elimination of an alkene, C_nH_2n, obtained by hydrogen abstraction from one of the C_nH_2n+1 alkyl groups on nitrogen; secondly, cleavage to form CH₃C₆H₄SO₂₊. The mechanism by which an alkene is lost has been probed by studying the variation of the intensity of the [M + H - C_nH_2n]⁺ signal with the structure of the alkyl substituent(s) on nitrogen and by monitoring the competition between the loss of different alkenes from protonated unsymmetrical sulphonamides in which two different alkyl groups are attached to nitrogen. This fragmentation is favoured by branching of the alkyl group at the carbon atom directly attached to nitrogen, thus suggesting that it involves a mechanism in which the stability of the cation obtained by stretching the bond connecting the nitrogen atom to the alkyl group is critical. This interpretation also explains the competition between alkene elimination and cleavage to form CH₃C₆H₄SO₂⁺ (and, in some cases, cleavage to form C₆H₅CH₂⁺ or [C₆H₅CHCH₃]⁺).