The Ambident Nucleophilic Center. Stereochemical Consequences of HOMO‐LUMO and HOMO‐HOMO Dominant Processes

The nitrogen centers of pyridine, quinoline, pyrrole anion, carbazole anion, and acetonitrile possess, in each case, two energetically non‐equivalent electron pairs: one, a non‐bonding orbital, lies in the molecular plane or on the bond axis; the other is part of a π‐type orbital, and is perpendicular to the molecular plane or the bond axis. Simple molecular orbital considerations predict that the π‐type orbital will always be the higher lying and, therefore, the preferred donor orbital in a HOMO‐LUMO controlled frontier molecular orbital interaction with the HOMO and LUMO of an electrophilic reagent. This stereoelectronic effect seems to have been observed experimentally in the case of carbazole anion, but not in the cases of quinoline or acetonitrile (present work). The reasons for the different behavior of these nucleophiles have been analyzed using the semi‐empirical procedure AMI. It is pointed out that the lower lying non‐bonding orbital can control the stereochemical course of the reaction when HOMO‐HOMO interactions dominate. This will occur when the non‐equivalent electron pairs of the nucleophile lie close to the HOMO of the electrophilic reagent, as is the case in the reactions of pyridine, quinoline, and acetonitrile with methyl choride or trimethyloxonium cation. Transition structures for N‐methylation of acetonitrile and pyrrole anion have been located and are, respectively, linear and planar. However, a 45° bend of the pyrrole transition structure leads to only a 29‐fold rate retardation at 25°C, compared to the 300‐fold retardation calculated for acetonitrile.