Nonclassical ammonium ions as intermediates in cinchona alkaloid rearrangements?

The classical/nonclassical nature of cationic ammonium intermediates proposed to be involved in cinchona alkaloid solvolysis and related reactions is investigated. While these intermediates are found to possess highly distorted geometries in which the central nitrogen atom and three of the attached groups are essentially coplanar, we do not find evidence of a nonclassical bonding array. Instead, we find evidence that the intermediate resembles a classical, albeit strained, aziridinium structure, which is still able to account for experimental observations.