Factors affecting the relative stability of a series of iminium cation stereoisomers

The role of three structural variables on the stability of simple iminium cations has been examined via molecular orbital calculations. In general, the more stable conformation for the hydroxyl substituent is axial, and the more stable geometry for each of the two double bonds is E. These conclusions apply also to more complex iminium cations derived from acyl pyrrolidines that could be precursors for the aza-Cope rearrangement - Mannich cyclization reaction. For these systems, molecular orbital calculations are also used to determine the effect of pyrrolidine substituent steric bulk on the relative stability of the cations. Results indicate that larger groups favor the E geometry of the iminium cation. This holds true for either geometry (E or Z) of the second double bond. For all substituent groups except hydrogen, the E geometry of both double bonds leads to the most stable isomers. When both double bonds are in the E geometry, the axial position for the OR substituent is preferred for all substituents but R = phenyl.