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A Combined Density Functional and ab initio Quantum Chemical Study of the Brandi Reaction
 
Eur. J. Org. Chem. 2001, 4223-4231
E. Ochoa, M. Mann, D. Sperling and J. Fabian

 
Abstract
The Brandi reaction is the transformation of 5-spirocyclopropaneisoxazolidines into tetrahydropyridones under thermal conditions. According to calculations performed by the restricted and unrestricted density functional theory and by post-Hartree-Fock single- and multireference methods of ab initio quantum chemistry the reaction proceeds via two biradical intermediates. These intermediates result from the homolytic cleavage of the N-O bond of the isoxazolidine ring in the first step, and the homolytic cleavage of one of the C-C bonds of spiro-fused cyclopropane in the second step. The activation energy of the rate-determining first step of the parent reaction amounts to about 40 kcal/mol at the RDFT/UDFT level of theory. This energy is not much higher than the energy of the first biradical intermediate relative to the reactants. The relative energies calculated at the quadratic CI and coupled cluster ab initio level were in the same order of magnitude. The effect of structural modification of 5 spirocyclopropaneisoxazolidines by substitution at carbon or nitrogen in the five-membered ring, introduction of a double bond into the five-membered ring and replacement of the spirocyclopropyl by spirocyclobutyl is discussed. The theoretical results reflect reasonably well Brandi´s experimental findings on the reactivity of the compounds under conventional thermal conditions or by flash vacuum thermolysis and his hypothesis about the reaction mechanism.

 

  
 

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TU Dresden
Fakultät Mathematik und
Naturwissenschaften
Fachrichtung Chemie
und Lebensmittelchemie
01062 Dresden

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TU Dresden
Fakultät Mathematik und
Naturwissenschaften
Fachrichtung Chemie
und Lebensmittelchemie
Helmholtzstraße 10
01069 Dresden

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