O-Phenyloximes tethered to alkenes undergo 5-exo-trig iminyl radical cyclizations upon
microwave irradiation. Trapping of the resulting cyclic radicals results
in C–C, C–N, C–O, C–S, or C–X bond
formation. Allylic sulfides undergo a tandem cyclization–thiyl
radical β-elimination, affording terminal alkenes. The cyclizations
exhibit a broad scope, and in some cases they are highly diastereoselective.
The pyrroline adducts are versatile intermediates that can be transformed
into a range of different species.
A detailed study of iminyl radical cyclizations of O-aryloximes tethered to alkenes is reported. The reactions
can be
triggered by either microwave irradiation or conventional heating
in an oil bath. A variety of radical traps can be employed, enabling
C–C, C–N, C–O, C–S, or C–X bond
formation and producing a diverse array of functionalized pyrrolines.
Substrates containing an allylic sulfide furnish terminal alkenes
by a tandem cyclization–thiyl radical β-elimination pathway.
Cyclizations of hydroxylated substrates exhibit moderate diastereoselectivity
that in some cases can partially be attributed to intramolecular hydrogen
bonding. Computational studies suggested a possible role for thermodynamics
in controlling the stereochemistry of cyclizations. The reaction temperature
can be lowered from 120 to 100 °C by employing O-(p-tert-butylphenyl)oximes instead
of O-phenyloximes as substrates, and these second-generation
iminyl radical precursors can be used in a one-pot oxime ether formation–cyclization
that is promoted by conventional heating. The functionalized pyrrolines
obtained from these reactions can be conveniently transformed in several
different ways.
Microwave irradiation of O-phenyloximes triggers
N–O homolysis and 1,5-hydrogen atom transfer (HAT), resulting
in formal γ-C–H functionalization of ketones after trapping
of the radical intermediate and in situ imine hydrolysis.
The Lewis acid InCl3·H2O facilitated HAT,
enabling functionalization of benzylic and nonbenzylic secondary carbon
atoms. Functionalization of primary carbons was feasible but afforded
low yields, requiring ClCH2CO2H instead of InCl3·H2O as an additive. C–O and C–C
bond formation could both be accomplished by this method.
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