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Autori: Sergiu Coseri, G. David Mendenhall and K.U. Ingold
Editorial: J. Org. Chem., 70, p.4629-4636, 2005.
The two vinylic hydrogen atoms of cyclohexene, cyclooctene and trans-3-hexene were replaced by deuterium and the resulting dideuterio-alkenes were reacted with three >NO• radicals: 4-hydroxyTempo, di-tert-butyliminoxyl, both used as the actual radicals, and phthalimide-N-oxyl (PINO) generated from N-hydroxyphthalimide (NHPI) by its reaction with tert-alkoxyl radicals (t-RO•) and with lead tetraacetate. In all cases, except the NHPI / Pb(OAc)4 system, only mono >NO-substituted alkenes were produced. Their 2H NMR spectra indicate that 88-92 % were formed by the initial abstraction of an allylic H-atom, followed by capture of the allylic radical by a second >NO•. The balance of these mono-adducts were formed by an initial addition of >NO• to the double bond followed by H-atom abstraction by a second >NO•. A substantial, even major, product formed with the NHPI / Pb(OAc)4 system has two PINO moieties added across the double bond. Since such di-adduct are not formed with the NHPI / t-RO• system, a heterolytic mechanism is proposed, analogous to that known for the Pb(OAc)4-induced acetoxylation of alkenes. A detailed analysis of the NHPI / Pb(OAc)4 / alkene products indicates that mono-substitution occurs by both homolytic and heterolytic processes.
Cuvinte cheie: PINO, reaction mechanism, NHPI