Mechanisms and Stereochemistry of Heterolytic Fragmentation

Heterolytic fragmentation can proceed by several mechanisms. A study of the solvolysis of γ‐aminoalcohol derivatives NCCCX, where X equals halogen or OSO2R, shows that fragmentation may occur by a one‐step mechanism or by several multi‐step mechanisms depending on structural, electronic, and steric factors. The one‐step synchronous mechanism has rigorous stereoelectronic requirements. It is associated with an increased reaction rate as evidenced by the “frangomeric” effect. Ketoxime derivatives RR′CNX may undergo Beckmann fragmentation by the synchronous mechanism or by prior rearrangement depending on their α‐substituents. Fragmentation is frequently accompanied by conventional substitution, elimination, and rearrangement reactions. Under such circumstances knowledge of all possible mechanisms of a given substrate is essential to bring about a desired transformation.