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Control of Kinetics and Thermodynamics of [1,5]-Shifts by Aromaticity: A View through the Prism of Marcus Theory
 

Summary: Control of Kinetics and Thermodynamics of [1,5]-Shifts by
Aromaticity: A View through the Prism of Marcus Theory
Igor V. Alabugin,*, Mariappan Manoharan, Boris Breiner, and Frederick D. Lewis
Contribution from the Department of Chemistry and Biochemistry, Florida State UniVersity,
Tallahassee, Florida 32306-4390, and Department of Chemistry, Northwestern UniVersity,
EVanston, Illinois 60208-3113
Received April 21, 2003; E-mail: alabugin@chem.fsu.edu
Abstract: The effects of aromatic stabilization on the rates of [1,5]-hydrogen shifts in a series of carbo-
and heterocyclic dihydroaromatic compounds were estimated by B3LYP/6-31G** computations. The aromatic
stabilization energy of the product is directly translated into increased exothermicity of these reactions.
Relative trends for a significant range of endothermic and exothermic [1,5]-shifts with different intrinsic
activation energies are reliably described by Marcus theory. The effects of aromaticity or antiaromaticity
are very large and can lead to dramatic acceleration or deceleration of [1,5]-hydrogen shifts and even to
complete disappearance of the reaction barrier. Not only the activation energy but the shape and position
of the reaction barrier can be efficiently controlled by changes in the aromaticity of the products, making
these systems interesting models for studying hydrogen tunneling. Marcus theory can also be applied
successfully to other pericyclic shifts such as [1,5]-shifts which involve chlorine and methyl transfer.
Introduction
The [1,5]-sigmatropic hydrogen shift (Figure 1) is a textbook
pericyclic reaction1-3 which has stimulated many mechanistic

  

Source: Alabugin, Igor - Department of Chemistry and Biochemistry, Florida State University

 

Collections: Chemistry; Biology and Medicine