Molecular orbital studies on the Wagner-Meerwein migration in some acyclic pinacol—pinacolone rearrangements

Abstract

The semi-empirical PM3 SCF-MO method is used to investigate the Wagner-Meerwein migration of various groups during the pinacol-pinacolone rearrangement of some acyclic systems. Pinacol first protonates and dehydrates to form a carbocation that undergoes a 1,2-migration to form a protonated ketone, which then deprotonates to yield the pinacolone product. We study the Wagner-Meerwein migration of hydride, methyl, ethyl, isopropyl, t-butyl, phenyl and heterocylic 2-, 3-and 4-pyridyl groups in various acyclic 1,2-diol (pinacol) systems as they rearrange to pinacolones. This 1,2-migration involves a three-centred moiety in the cationic transition state. The migratory aptitude predicted here follows the order: hydride > t-butyl > isopropyl > ethyl > methyl > phenyl, which accords well with available experimental data and/or chemical intuition, reflecting also on the ability of the group involved to carry positive charge in the transition state. The structure of the migrating group (whether aliphatic or aromatic) within the transition state also supports the stabilising role of delocalisation of positive charge for reaction feasibility. Geometrical and thermodynamic considerations coincide in assigning the following order to relative "earliness" of the transition state along the reaction pathway: t-butyl > isopropyl > phenyl > methyl > 2-pyridyl > 4-pyridyl.

Übergangsmetallfreie oxidative Kreuzkupplung von Tetraarylboraten zu Biarylen mit organischen Oxidationsmitteln

“…Die Migration der elektronenreicheren Arylgruppe ist gegenüber der Migration der beiden trifluormethylierten Phenylgruppen favorisiert. [24] Gestützt durch Kreuzungsexperimente von Hirao und Mitarbeitern, erfolgt die Ligandenkupplung voraussichtlich intramolekular. [14] Für die weitere Reaktion des Intermediates II sind zwei leicht unterschiedliche Reaktionswege mçglich.…”

Isomerization of propargyl cation to cyclopropenyl cation: Mechanistic elucidations and effects of lone pair donors

“…Such an approach has been carried out using semi-empirical MO theory for alkyl group migration reactions in carbenes and carbocations. 33,34…”

Chemoselectivity of Pinacol Rearrangement Originate by Different Hexafluoroantimonate Oxidant

“…No matter in what mechanism, the migratory aptitude of groups, which may be determined by the stability of the product, has attracted considerable attention. Theoretical and experimental studies reveal the migration of groups occurs in accordance with the migratory aptitude, hydride > alkyl > phenyl , …”

“…However, sometimes the selectivity does not follow the expected migratory aptitude above , . The pinacol rearrangement has been subjected to much experimental researches pertaining to its chemoselectivity and the relative migratory aptitudes of various groups . It is difficult to generalize the migratory aptitude easily because there are many factors need consideration.…”