Aspects of mass spectra of organic compounds. Part X. Rationalization of a variety of electron-impact induced rearrangements of ions

Abstract

A wide variety of electron-impact induced rearrangements can be rationalized by one four-centre reaction scheme, [AB–C–D]⁺˙→[D–A–BC]⁺˙. In amides and derivatives where A is oxygen, B is carbon, and C is nitrogen, the migrating group D can be aryl, aroyl, methylene or substituted methylene, anilino, sulphonyl, or phosphorus. The behaviour of cyclic compounds where B is joined to D, including the loss of CO₂ from cyclic imides and the loss of SO₃ from saccharin and derivatives, is discussed. Competition between rearrangement and ‘simple’ cleavage processes is shown by a series unsubstituted and nitro- and methoxy-substituted N-methylbenzanilides.

Rearrangement and fragmentation of thionoesters and derivatives, where A is sulphur and C is oxygen, fits the general scheme and in this case the migrating group D can be alkyl. The corresponding rearrangement of thioloesters (A is oxygen and C is sulphur) was negligible, in contrast with a report in the literature. Some rearrangements of azoxy-compounds and thiocarbonates are incorporated into the scheme.

From the results we make the generalisation that: For rearrangement according to the above equation: (i) when A is oxygen, C must be nitrogen; (ii) when A is sulphur, C can be nitrogen or oxygen; (iii) B can be carbon, phosphorus, or nitrogen. Rearrangement is negligible when C is sulphur. The generalisation can be used as an aide-mémoîre for known rearrangement processes and as an indication of possible rearrangements of unexamined compounds.