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The 1 Δ g Dioxygen Ene Reaction with Propene: A Density Functional and Multireference Perturbation Theory Mechanistic Study
Author(s) -
Maranzana Andrea,
Ghigo Giovanni,
Tonachini Glauco
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200204522
Subject(s) - diradical , chemistry , density functional theory , computational chemistry , potential energy surface , radical , exergonic reaction , transition state , propene , photochemistry , ab initio , excited state , atomic physics , catalysis , physics , organic chemistry , singlet state
This study aims to determine whether a balance between concerted and non‐concerted pathways exists, and in particular to ascertain the possible role of diradical/zwitterion or peroxirane intermediates. Three non‐concerted pathways, via 1) diradical or 2) peroxirane intermediates, and 3) by means of hydrogen‐abstraction/radical recoupling, plus one concerted pathway (4), are explored. The intermediates and transition structures (TS) are optimized at the DFT(MPW1K), DFT(B3LYP) and CASSCF levels of theory. The latter optimizations are followed by multireference perturbative CASPT2 energy calculations. 1) The polar diradical forms from the separate reactants by surmounting a barrier (Δ E ${^{\ne }_{{\rm MPW}1{\rm K}}}$ =12, Δ E ${^{\ne }_{{\rm B}3{\rm LYP}}}$ =14, and Δ E ${^{\ne }_{{\rm CASPT}2}}$ =16 kcal mol −1 and can back‐dissociate through the same TS, with barriers of 11 (MPW1K) and 8 kcal mol −1 (B3LYP and CASPT2). The diradical to hydroperoxide transformation is easy at all levels (Δ E ${^{\ne }_{{\rm MPW}1{\rm K}}}$ <4, Δ E ${^{\ne }_{{\rm B}3{\rm LYP}}}$ =1 and Δ E ${^{\ne }_{{\rm CASPT}2}}$ =1 kcal mol −1 ). 2) Peroxirane is attainable only by passing through the diradical intermediate, and not directly, due to the nature of the critical points involved. It is located higher in energy than the diradical by 12 kcal mol −1 , at all theory levels. The energy barrier for the diradical to cis ‐peroxirane transformation (Δ E ≠ =14–16 kcal mol −1 ) is much higher than that for the diradical transformation to the hydroperoxide. In addition, peroxirane can very easily back‐transform to the diradical (Δ E ≠ <3 kcal mol −1 ). Not only the energetics, but also the qualitative features of the energy hypersurface, prevent a pathway connecting the peroxirane to the hydroperoxide at all levels of theory. 3) The last two‐step pathway (hydrogen‐abstraction by 1 O 2 , followed by HOO‐allyl radical coupling) is not competitive with the diradical mechanism. 4) A concerted pathway is carefully investigated, and deemed an artifact of restricted DFT calculations. Finally, the possible ene/[π2+π2] competition is discussed.