EXTENSIVE REARRANGEMENT REACTIONS OBSERVED FOR DIFFERENTLY N-SUBSTITUTED 2,5-DIMETHYLPYRROLES UNDER ELECTRON IONIZATION

The 70 eV electron ionization mass spectra of fourteen differently N-s ubstituted 2,5-dimethylpyrroles were studied, In accord with the aroma tic character of the pyrrole ring, all the compounds studied gave rise to an intense molecular ion peak Regardless of the functional group a t the 1-substituent, for all the compounds the principal fragmentation products were practically the same, although the relative peak intens ities varied depending on the structure of the substituent. In additio n, an amino group substituent prompted intense radical site initiated cleavages which were minor or totally absent with other compounds, The data showed that extensive rearrangement reactions took place. Some o f these were seen directly from the mass spectra, and some only from c loser examination of the ion structures, The structures of the ions at m/z 108 and 94, formally representing the [M-1](+) ions of 1,2,5-trim ethylpyrrole and 2,5-dimethylpyrrole, respectively, were carefully stu died using the collision induced dissociation (CID) technique. Measure ments showed that ring expansion reactions took place, protonated 2,5- lutidine and protonated 2-picoline being the most likely isomers forme d for the m/z 108 and 94 ions, respectively, The results obtained from the CID experiments were supported by semi-empirical calculations of heats of formation. (C) 1998 John Wiley & Sons, Ltd.