Reaction pathway of aliphatic pinacol‐type rearrangement reexamined

Abstract

Pinacol rearrangement is often written to proceed via 1,2‐Me migration to the tertiary cationic center, followed by deprotonation to give pinacolone. Computational study was carried out for model reactions to clarify why the migration of the OH group is not involved in the mechanistic scheme despite the fact that OH is a better migrating group than Me. It was found that the migratory aptitude of X in both XCMe2‐CH2Cl and XCMe2‐CMe2Cl is in the order, NH2 > OMe > Ph > Me, indicating that a migrating group with n‐electrons has a larger aptitude than a π‐ or σ‐electron group. However, the reactivity differences became much smaller for XCMe2‐CMe2OMe 2+, a model compound for aliphatic pinacol rearrangement. Calculations of MeOCMe2‐CMe2OMe 2+ revealed that three initial ionization steps, CO heterolysis, concerted OMe migration and concerted Me migration, compete with each other. On the other hand, the ring‐opening step of the epoxide‐type intermediate formed via OMe migration was shown to have quite a large activation barrier. It was suggested that aliphatic pinacol rearrangement proceeds via the concerted Me migration route or the CO heterolysis‐Me migration‐deprotonation route. Epoxide may form by the concerted MeO migration, but it would not be an important intermediate of pinacol rearrangement. Copyright © 2010 John Wiley & Sons, Ltd.

Tuning the Selectivity: Study of Solvent‐Free Acid‐Mediated Pinacolic‐Pinacolone Rearrangement under Microwave Irradiation

“…The group R (phenyl or hydride) then undergoes an 1,2 shift via a transition state (Int 2a or Int 2b) to form a rearranged cation Int 3, which then deprotonates to yield the final rearrangement product (2a or 3a).The driving force for the 1,2‐shift comes from the formation of a more stable carbocation in which the positive charge can be delocalized on the remaining hydroxyl oxygen. Limited experimental data on the pinacol rearrangement suggests the order hydride > methyl > phenyl for relative migratory facility based on kinetic control of the reaction due to the instability of the carbocation intermediate species . Moreover, the capacity of the migrating group to carry positive charge during migration has been suggested as a helpful factor for nucleophilic migration within carbocation species .…”

Metal-responsive activation of C–O and C–C bonds of an aromatic vicinal diol in hydrothermal conditions: synthesis, structure, mechanism and magnetic properties

“…Furthermore, one of its reactions is also a textbook example of molecular rearrangement involving relatively stable tertiary carbocations upon protonation and C-O bond cleavage. 8 1,2-DiIJ1Hbenzimidazol-2-yl)ethane-1,2-diol (TDB, Scheme 1), as a related compound to pinacol, its benzimidazole rings being more electron rich and having a larger migratory aptitude than the methyl groups of pinacol, can serve as a chelating ligand for metal ions forming molecular clusters by different bridging modes. [9][10][11][12] On account of the two chiral carbon atoms, the coordinated structures have been synthesized and taken not only into chemical research but also biological fields.…”

Porphyrin metal–organic frameworks with bilayer and pillar-layered frameworks and third-order nonlinear optical properties

“…It should be noted that the BIE ligand is formed by in situ dehydration of the S,S-BED ligand. 55,56 The single crystal X-ray diffraction (XRD) analysis revealed that PMOF-1 crystallized in the triclinic space group P1 ˉ(Table S1 †). The asymmetric unit contains two Zn cations, one in situ formed TCPP-Zn ligand, one in situ formed BIE ligand, one coordinated water, one coordinated 1,4-dioxane and one 1,4-dioxane guest (Fig.…”