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

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      https://www.ias.ac.in/article/fulltext/jcsc/116/02/0083-0091

    • Keywords

       

      Pinacol-pinacolone rearrangement; Wagner-Meerwein migration; relative migratory apti-tude; transition state geometry; Hammond’s postulate; PM3 SCF-MO method

    • 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

    • Author Affiliations

       

      Zodinpuia Pachuau1 R H Duncan Lyngdoh1

      1. Department of Chemistry, North-Eastern Hill University, Shillong - 793 022, India
    • Dates

       
  • Journal of Chemical Sciences | News

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