Photoinduced rearrangement of aromatic N-chloroamides to chloroaromatic amides in the solid state: Inverted ΠnN occupational stability of amidyl radicals

Panče Naumov, Yildiray Topcu, Mirjana Eckert-Maksić, Zoran Glasovac, Fabijan Pavošević, Manoj Kochunnoonny, Hideyuki Hara

Research output: Contribution to journalArticle

Abstract

We report a solid-state photochemical rearrangement reaction by which aromatic N-chloroamides exposed to UV light or sunlight are rapidly and efficiently converted to chloroaromatic amides. 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 radicals as intermediates, which in the absence of strong hydrogen bond donors are stabilized in the reactant crystals by C-H/N-Cl⋯π 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-Π|| state (Np spin) of the amidyls is normally preferred over the trans-Σ configuration (Nsp2 spin), stabilization by aromatic conjugation, steric and geometry factors, as well as by electronic effects from the substituents can decrease the Π-Σ 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 occupational spin stability and the ΠNN 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-Σ 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 point out to non-Hammett behavior. By including cumulative electronic effects from multiple substitutions, four distinct families of the aromatic amidyl radicals were identified. The Π state is the most stable structure of the N-phenylacetamidyl radical and of most of the substituted arylamidyls, although the Σ and Π states can also be stabilized by introducing tert-butyl and nitro groups, respectively.

Original languageEnglish (US)
Pages (from-to)7834-7848
Number of pages15
JournalJournal of Physical Chemistry A
Volume115
Issue number26
DOIs
StatePublished - Jul 7 2011

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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