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

Abstract

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, O2, 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 O2 yields of 88% and 44%, respectively. Rate constants for initial O2 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 acyl-peroxide adducts, [FeIIIOOR(tpenaH)]2+ [R = C(CH3)3, C(CH3)2Ph, C(O)PhCl; T1/2 = 30 s (5 °C), 20 s (5 °C), 1 s (−30 °C)] with their common decay product [FeIVO(tpenaH)]2+. Concurrently organic radicals proposed to be ROO• were detected by EPR spectroscopy. A lower yield of O2 at 23% with an initial rate of 0.10(3) mM s–1 for the disproportionation of m-chloroperoxybenzoic acid is readily explained by catalyst inhibition by coordination of the product m-chlorobenzoic acid. Oxidative decomposition of the alkyl groups by a unimolecular β-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.