On the mechanism of allylic amination catalyzed by iron salts

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)Cl-4-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 degrees 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 alpha-methylstyrenes lead to a Hammett rho value of -3.0. A small kinetic D-isotope effect, 1.4 +/- 0.2, is found for the intermolecular amination of alpha-(trideuteriomethyl)styrene by 1a. Low-temperature reactions of 1a with 2-MP, beta-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).