Elimination of H₂ from CH₃CH=N⁺HCH₃: A synchronous, concerted 1,4-H₂ elimination

Abstract

Most H 2 eliminations from cations in the gas phase are formally 1,1-or 1,2-processes. Larger ring size H 2 eliminations are rare and little studied. Thus, whether the 6-center, 1,4-elimination CH 3 CHϭN ϩ HCH 3 ¡ CH 2 ϭCHN ϩ HϭCH 2 ϩ H 2 is concerted and synchronous, as indicated by isotope effects and predicted by conservation of orbital symmetry, is a significant question. This reaction is characterized here by application of QCI and B3LYP theories. CH bond-breaking and H-H bond-making in this reaction are found by theory to be highly synchronized, consistent with previously established isotope effects and in contrast to "forbidden" 1,2-eliminations from organic cations in the gas phase. This reaction is made feasible by its conservation of orbital symmetry, the energy supplied by formation of the H-H bond, and a favorable geometry of the ion for eliminating H 2 . (J Am

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

“…However, whereas we attribute this to the avoidance of orbital symmetry constraints, Dewar concluded that multiple bonds do not break simultaneously because that would require an amount of energy roughly equal to the sum of the energies required to break each bonds individually [1]. Given that we recently found that H 2 elimination from CH 3 CHϭNH ϩ CH 3 , an orbital symmetry allowed 1,4-process with a high reverse critical energy, is highly synchronous [45], we believe that orbital symmetry restraints significantly influence the mechanisms of 1,2-eliminations by preventing synchronized trajectories from being taken. This is not necessarily incompatible with the ideas of Dewar.…”