Mechanisms of reaction of aminoxyl (nitroxide), iminoxyl, and imidoxyl radicals with alkenes and evidence that in the presence of lead tetraacetate, N-hydroxyphthalimide reacts with alkenes by both radical and nonradical mechanisms.
1,2-dideuterio-cyclohexene, 1,2-dideuterio-cyclooctene, and trans-3,4-dideuterio-hex-3-ene 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. The 2H NMR spectra imply that 88-92% of monoadducts were formed by the initial abstraction of an allylic H-atom, followed by capture of the allylic radical by a second >NO*, while the remaining 12-8% appear to be formed by an initial addition of >NO* to the double bond followed by H-atom abstraction by a second >NO*. A substantial and sometimes the major product formed with the NHPI/Pb(OAc)4 system has two PINO moieties added across the double bond. Since such diadducts 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 monosubstitution occurs by both homolytic and heterolytic processes.
Coseri
Sergiu
S
National Research Council, Ottawa, ON, K1A 0R6, Canada. coseris@icmpp.ro
Mendenhall
G David
GD
Ingold
K U
KU