Catalytic Alkyl Hydroperoxide and Acyl Hydroperoxide Disproportionation by a Nonheme Iron Complex

Alkyl hydroperoxides are commonly used as terminal oxidants because they are generally acknowledged to be stable toward disproportionation compared with H2O2. We show that alkylperoxide disproportionation is effectively catalyzed by the [Fe(tpena)](2+) (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate). A peroxidase-type mechanism, in other words, involvement of iron(IV)oxo species, is consistent with the rates and product distribution. Accordingly, O-2 tert-butanol, and cumyl alcohol are concurrently produced for substrates tert-butyl hydroperoxide and cumene hydroperoxide, respectively, in the presence of [Fe(tpena)](2+) with O-2 yields of 88% and 44%, respectively. Rate constants for initial O-2 production ([Fe] 0.005 mol %) were measured to 3.66(6) and 0.29(3) mM s(-1), respectively. Participating in the mechanism are spectroscopically detectable (UV-vis, EPR, resonance Raman) transient alkyl- and acylperoxide adducts, [(FeOOR)-O-III(tpenaH)](2+) [R = C(CH3)(3), C(CH3)(2)Ph, C(O)PhCl; T-1/2 = 30 s (5 degrees C), 20 s (5 degrees C), 1 s (-30 degrees C)] with their common decay product [Fe-I(tpenaH)](2+). Concurrently organic radicals proposed to be ROO center dot were detected by EPR spectroscopy. A lower yield of O-2 at 23% with an initial rate of 0.10(3) mM s(-1) for the disproportionation of mchloroperoxybenzoic acid is readily explained by catalyst inhibition by coordination of the product m-chlorobenzoic acid. Oxidative decomposition of the alkyl groups by a unimolecular beta-scission pathway, favored for cumene hydroperoxide, competes with ROOH disproportionation. Despite the fact that the catalytic disproportionation is effective, external C-H substrates when they are present in excess of ROOH-can be targeted and catalytically and selectively oxidized by ROOH-using [Fe(tpena)](2+) as the catalyst.