Photoinduced rearrangement of aromatic N-chloroamides to chloroaromatic amides in the solid state. Inverted PI_N―SIGMA_N occupational stability of amidyl radicals (CROSBI ID 171988)
Prilog u časopisu | izvorni znanstveni rad | međunarodna recenzija
Podaci o odgovornosti
Naumov, Panče ; Topcu, Yildiray ; Eckert-Maksić, Mirjana ; Glasovac, Zoran ; Pavošević, Fabijan ; Manoj, Kochunnoonny ; Hara, Hideyuki
engleski
Photoinduced rearrangement of aromatic N-chloroamides to chloroaromatic amides in the solid state. Inverted PI_N―SIGMA_N occupational stability of amidyl radicals
We report a new solid-state photochemical rearrangement by which aromatic N-chloroamides exposed to UV light or sunlight are rapidly and efficiently converted to chloroaromatic amides in the crystalline state. The course, the intermediate (nascent chlorine vs. dichlorine) and the outcome of the reaction depend on the excitation (exposure time, wavelength and intensity) and on inherent structural factors (the directing role of the substituents and, as demonstrated by the different reactivity of two polymorphs of N-chlorobenzanilide, the supramolecular structure). The photolysis of the chloroamides provides facile photochemical access to arylamidyl radical intermediates, which in the absence of strong hydrogen bond donors are stabilized in the reactant crystals by C―H/N―Cl---pi interactions, thus providing insight into their structure and chemistry. Thorough theoretical modeling of the factors determinant to the stability and the nature of the spin-hosting orbital evidenced that although the trans-PI_|| state (Np spin) of the amidyls is normally preferred over the trans-SIGMA configuration (Nsp2 spin), stabilization by aromatic conjugation, steric and geometry factors, as well as by electronic effects from the substituents can decrease the PI―SIGMA gap in these intermediates significantly, resulting in similar and, in the case of the orthogonal amide-phenyl disposition―even reversed population of the unpaired electron in the two orbitals. Quantitative correlation established that the inverted spin stability and the PI_N―SIGMA_N crossover are collectively facilitated by the conformation, valence angle and disposition of the amide group relative to the aromatic system. The stabilization and detection of a trans-SIGMA radical was experimentally accomplished by steric locking of the orthogonal trans amide conformation with double ortho-tert-butyl substitution at the phenyl ring. The effects of the single para-phenyl substituents on the relative occupational stability of the arylamidyl radical states points out to non-Hammett behavior of the aromatic amidyl radicals. By including cumulative electronic effects from multiple substitutions, four distinct families of these species were identified. The PI state is the most stable structure of the N-phenylacetamidyl radical and of most of the substituted radicals, although the SIGMA and PI states can be also stabilized by introducing tert-butyl and nitro groups, respectively.
solid state photochemistry; amidyl radicals
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Podaci o izdanju
115 (26)
2011.
7834-7848
objavljeno
1089-5639
10.1021/jp203771c