Iron salts catalyze the allylic amination of alkenes by
arylhydroxylamines in moderate to good yields and
with high regioselectivity resulting from double-bond transposition.
The iron-catalyzed reaction of phenylhydroxylamine with representative alkenes in the presence of
2,3-dimethylbutadiene, an effective PhNO trap, produces
allyl
amines exclusively, excluding the intermediacy of free PhNO in the
amination reaction. The reaction of
FeCl2,3
with PhNO or PhNHOH produces a novel azo dioxide iron complex,
{Fe[Ph(O)NN(O)Ph]3}[FeCl4]2
(1a), whose
structure has been established by X-ray diffraction. The structure
of 1a features essentially tetrahedral
Fe(III)Cl4
-
anions and a novel six-coordinate dication having iron(II) bound
through the oxygens of three azobenzene
N,N-dioxide ligands. Evidence that 1a is the active
aminating agent in the catalytic reactions includes (1) its
isolation
from the catalytic reaction; (2) its facile reaction with alkenes to
produce allyl amine in high yield and regioselectivity;
(3) its amination of alkenes without the intervention of free PhNO; and
(4) its efficient catalysis of amination by
PhNHOH. The reaction of 2-methyl-2-pentene (2-MP) with
1a (dioxane, 70 °C) was examined kinetically;
the
appearance of allylamine was found to be first order in 1a
and first order in alkene. Rate constants determined
for
the reactions of 1a with a set of
para-substituted α-methylstyrenes lead to a Hammett ρ
value of −3.0. A small
kinetic D-isotope effect, 1.4 ± 0.2, is found for the intermolecular
amination of α-(trideuteriomethyl)styrene by
1a.
Low-temperature reactions of 1a with 2-MP,
β-methylstyrene, and styrene produce isolable alkene adducts
3a−c.
Thermolysis of 3a in dioxane gives the corresponding
allyl amine while treatment of 3a−c with
nitrosoarenes
regenerates the respective alkenes. IR, NMR, and UV−vis
spectroscopic data also support the formulation of
3a−c
as alkene complexes. Evidence that azo dioxide complex
1 transfers a PhNO (rather than PhN) unit to
alkene,
producing an intermediate allylhydroxylamine which is subsequently
reduced to the ultimate allyl amine, is provided
from model reaction studies and GC/MS monitoring. Various
mechanistic pathways are presented and analyzed.
The mechanism most consistent with all of the accumulated evidence
involves alkene coordination to 1 via
dechelation
of an azo dioxide ligand, intramolecular RNO transfer to coordinated
alkene to produce the allylhydroxylamine,
reductive deoxygenation of the allylhydroxylamine to allylamine, and
regeneration of azo dioxide complex 1 by
oxidation of another PhNHOH molecule by iron(III).