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A Combined Density Functional and ab initio Quantum Chemical Study of the Brandi Reaction
Author(s) -
Ochoa Estael,
Mann Matthias,
Sperling Dirk,
Fabian Jürgen
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200111)2001:22<4223::aid-ejoc4223>3.0.co;2-n
Subject(s) - cyclopropane , chemistry , homolysis , ab initio , computational chemistry , bond cleavage , density functional theory , ab initio quantum chemistry methods , concerted reaction , ring (chemistry) , photochemistry , radical , organic chemistry , molecule , catalysis
The Brandi reaction is the transformation of spiro[cyclopropane‐1,5′‐isoxazolidines] into tetrahydropyridones under thermal conditions. According to calculations performed by the restricted and unrestricted density functional theory and post‐Hartree−Fock single‐ and multireference methods of ab initio quantum chemistry, the reaction proceeds through 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 the spiro‐fused cyclopropane in the second. The activation energy of the rate‐determining first step of the parent reaction amounts to about 40 kcal mol −1 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 of the same order of magnitude. The effects of structural modification of spiro[cyclopropane‐1,5′‐isoxazolidines] 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 spiro‐cyclopropane by spiro‐cyclobutane are discussed. The theoretical results reflect Brandi’s experimental findings on the reactivity of the compounds under conventional thermal or flash vacuum thermolysis conditions and his hypothesis about the reaction mechanism reasonably well.