ON THE MECHANISM OF ALLYLIC AMINATION CATALYZED BY IRON SALTS

Iron salts catalyze the allylic amination of alkenes by arylhydroxylam ines in moderate to good yields and with high regioselectivity resulti ng from double-bond transposition. The iron-catalyzed reaction of phen ylhydroxylamine with representative alkenes in the presence of 2,3-dim ethylbutadiene, an effective PhNO trap, produces allyl amines exclusiv ely, excluding the intermediacy of free PhNO in the amination reaction . The reaction of FeCl2,3 with PhNO or PhNHOH produces a novel azo dio xide iron complex, {Fe[Ph(O)NN(O)Ph](3)}[FeCl4](2) (1a), whose structu re has been established by X-ray diffraction. The structure of 1a feat ures essentially tetrahedral Fe(III)Cl-4-anions and a novel six-coordi nate dication having iron(II) bound through the oxygens of three azobe nzene N,N-dioxide ligands. Evidence that 1a is the active aminating ag ent in the catalytic reactions includes (1) its isolation from the cat alytic reaction; (2) its facile reaction with alkenes to produce allyl amine in high yield and regioselectivity; (3) its amination of alkene s without the intervention of free PhNO; and (4) its efficient catalys is of amination by PhNHOH. The reaction of 2-methyl-2-pentene (2-MP) w ith 1a (dioxane, 70 degrees C) was examined kinetically; the appearanc e of allylamine was found to be first order in 1a and first order in a lkene. 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. Lo w-temperature reactions of 1a with 2-MP, beta-methylstyrene, and styre ne produce isolable alkene adducts 3a-c. Thermolysis of 3a in dioxane gives the corresponding allyl amine while treatment of 3a-c with nitro soarenes regenerates the respective alkenes. IR, NMR, and UV-vis spect roscopic data also support the formulation of 3a-c as alkene complexes . Evidence that azo dioxide complex 1 transfers a PhNO (rather than Ph N) 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 pathw ays are presented and analyzed. The mechanism most consistent with all of the accumulated evidence involves alkene coordination to 1 via dec helation of an azo dioxide ligand, intramolecular RNO transfer to coor dinated alkene to produce the allylhydroxylamine, reductive deoxygenat ion of the allylhydroxylamine to allylamine, and regeneration of azo d ioxide complex 1 by oxidation of another PhNHOH molecule by iron(III).