1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH2+: Guided Dissociations

1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH2=CH2 and CH3CH3 from (CH3)2NH+CH2CH3 (1) and of CH4 from (CH3)2NH2+ are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH3)2NH2+ + CH2=CH2 from 1. CH3CH3 elimination by H-transfer to C2H5+ to form CH3NH+=CH2 + CH3CH3 also takes place. (CH3)2NH2+ eliminates methane by CN bond extension followed by β-H-transfer to give CH2=NH+ + CH4. Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry.

Graphical Abstract1,2-Eliminations from (CH3)2NH+CH3 and CH3NH2+CH3 are characterized. These avoid violating conservation of orbital symmetry by dissociating through ion-neutral complexes, 1,1-like or highly asynchronous transition states.Figure optionsDownload full-size imageDownload high-quality image (107 K)Download as PowerPoint slide